DE SIGN CONSIDERATIONS TO IMPROVE PERFORMACE OF G EOSYNTHETIC REINFORCED MECHANICALLY STABILI ZED EARTH WALLS IN T RANSPORTATION INFRAST R UC T URE by BRADEN M. PETERS B.S., C olorado School of Mines, 2001 A thesis submitted to the Faculty of the Graduate School of the University of Colorado Denver in partial fulfillment of the requirements for the degree of Master of Science Civil Engineering Program 2017
ii This thesis for the Master of Science degree by Braden M. Peters h as been approved for the Civil Engineering Program b y Chengyu Li Chair Jo nathan Wu Advisor Peter Hoffman Date: May 13, 2017
iii Peters, Braden M. ( M.S., Civil Engineering Program) Design Considerations to Improve Performance of Geosynthetic Reinforced Mechanically Stabilized Earth Walls in Transportation Infrastructure Thesis directed by Professor Jo nathan Wu ABSTRACT For approximately 40 years, Mechanically Stabilized Earth (MSE) walls have become incre asingly popular and generally accepted as a standard earth retaining structures. MSE walls are an economic solution to grade separation needs and are being constructed at a rate of hundreds of thousands of square feet annually. National and International organizations have published guidelines for the design and construction of MSE walls. Although catastrophic failure of MSE walls is rare, many walls have exceeded the intended performance criteria leading to costly repairs or ongoing maintenance ( Samtani and Nowatzki 201 6 ) estimate a serviceability failure rate of 10 % and structural failure rate of 1 % Valentine (2013) estimated that the failure rate of Geosynthetic Reinforced MSE ( GMSE ) walls in the U.S. is as high as 5% and t he National Concrete Masonry Assoc iation (NCMA) has estimated a 2% to 8% failure rate nationally for GMSE walls. This is unaccep table for engineered structures in transportation facilities So why are failure rates so much higher than other earth retaining structures? The common conclusion amongst engineers is incompetent construction, ( e.g., inappropriate backfill, poor compaction, improper grading etc. ). By evaluating a variety of observed adverse wall performance this report concludes that the majority of failures could have
iv been avoided in design. Based on an understanding of important design considerations and applying an acceptable level of risk overall G MSE wall performance can be improved. The form and conte nt of this abstract are approved. I recommend its publication. Approved: J o nathan Wu
v TABLE OF CONTENTS CHAPTER I. MECHANICALLY STABILI ZED EARTH WALLS ............................................................. 1 Introduction .............................................................................................................. 1 Objective and Scope ................................................................................................. 3 Definition of Failure .................................................................................................. 4 Limitations ................................................................................................................. 5 II. GUIDANCE DOCUMENTS A ND CURRENT PRACTICE ................................................. 6 Introduction .............................................................................................................. 6 Summary of AASHT O (2014) Design Guidance ......................................................... 7 Summary of FHWA (2009) Design Guidance ............................................................ 8 Summary of NCMA (2010) Design Guidance ............................................................ 9 Overview of Current Practice .................................................................................. 10 Reinforced Backfill Material ............................................................................... 11 Geosynthetic Reinforcement ............................................................................. 14 Design Methods ................................................................................................. 17 Construction Inspection ..................................................................................... 23 III. LITERATURE REVIEW AN D EVALUATION OF GMSE WALLS .................................... 24 Introduction ............................................................................................................ 24
vi Literature Review of Compilation Studies .............................................................. 25 Geotechnical Engineers should Design MSE Walls and Prepare Plans and Specifications ..................................................................................................... 26 Disregarding Lessons Learned ........................................................................... 27 Geosynthetic Reinforced Earth Retaining Walls Poor Performance ................. 30 Frequent contributions to MSE wall Failures .................................................... 31 Geosynthetic Reinforced Structure Failures Forensic Studies ........................... 33 Nationwide Survey on MSE Wall Abutment Status ........................................... 36 Extensive GMSE Wall Failure Review ................................................................. 36 Coordination between the EPC/EPCM Contractor and MSE Engineer .............. 38 Literature Review of Individual Case Studies .......................................................... 39 Clayey Poorly Draining Soils Adequate Plans and Design Case Study ............ 40 MSE Wall Success Good Communication Case Study ...................................... 42 Under Designed Drainage Structure Surface Water Flow Case Study ........... 44 Inadequate Design Assumptions and Submit tal Review Settlement and Consolidation Case Study ................................................................................... 46 Incorrect Evaluation of Settlement Differential Settlement Case Study ......... 49 Inadequate Reinforcement Length Excessive Displacement Case Study ....... 51 Literature Review of Geosynthetics in MSE Walls .................................................. 53
vii Use of Multitiered Geosynthetic Reinforced Soil Walls ................................... 53 Success of Geosynthetic Reinforced Soil ........................................................... 56 Use of GRS Structures for Retaining Walls ........................................................ 57 Comparison of Modeling Methods for Geogrid Reinforced MSE Walls ............ 58 Comparison between GMSE and GRS Design Theories ..................................... 59 Quality Management for Successful GMSE Walls ............................................. 61 Design Criteria for Closely Spaced GMSE Walls ................................................. 62 Literature R eview of MSE Wall General Reports .................................................... 63 Reliability of MSE Internal Stability Modelling .................................................. 64 Owner Controlled Aspects ................................................................................. 64 Collaboration and Systematic Approach to Design and Construction .............. 67 Risk of Failure Probability .................................................................................. 68 Teaching Retaining Wall Design ......................................................................... 69 Sustainability Assessment of Cantilever Retaining Wall vs. MSE W all .............. 69 MSE State of the Practice Internal Design Review ............................................ 70 General Remarks Based on Literature Review ....................................................... 71 Discussion and Recommendatio ns ......................................................................... 75 Design Considerations ............................................................................................ 76 Design Methods ................................................................................................. 76
viii Site Characterization .......................................................................................... 78 Wall Component Selection ................................................................................ 78 Communication and Experience ........................................................................ 81 Construction ............................................................................................................ 82 Submittals .......................................................................................................... 83 Quality Control and Quality Assurance (QC/QA) ............................................... 84 Discussion on Risk ................................................................................................... 85 Design Risks that Effect Performance ................................................................ 85 Construction Risks that Effect Performance ...................................................... 86 IV. C ONCLUDING REMARKS ......................................................................................... 88 REFERENCES ................................................................................................................ 92
ix LIST OF TABLES TABLE Table 2 1 MSE Wall Select Granular Reinforced Fill Requirements (FHWA, 2009, Table 3 1) .............................. 12 Table 2 2 Recommended Limits of Electrochemical Properties for Reinforced Fills with Geosynthetic Reinforcements ( FHWA, 2009 Table 3 4 ) 14 Table 3 1 Generic Causes and Observed Technical Errors of MSE Failure (Wu and Chou 2013, Table 3) .. 35
x LIST OF F IGURES FIGURE Figure 2 1 Basic Geometry and Forces for MSE Walls (Samtani and Nowatzki (2016), Figure 2.2) 11 Figure 2 2 Long term geosynthetic reinforcement strength co ncepts (FHWA, 2009, Figure 3 6) .. 16 Figure 2 3 Potential External Failure Mechanis ms for an MSE Wall (FHWA, 2009) .. 20 Figure 2 4 Determination of Kr/Ka for the simplified Met hod (WSDOT, 2001) .... 23 Figure 2 5 Photo of Failure of a 45 foot GMSE Wall (from Ba chus and Griffin 2011, Figure 2. 29 Figure 3 1 Basic Failure Mechanisms (Koerner and Koerner 2013, Figure 5) .. 39 Figure 3 2 Photo of Failure of Wall A (Scarbo rough 2005, Figure 1) ..... 42 Figure 3 3 Surface water overtopping MSE Wall (Top) and Failed MSE wall ( Bottom), (Haramy, et al., 2010) .... 46 Figure 3 4 Vertical Settlement (Top Left), Severe Longitudinal Cracking (Top Right), and Bulging Baskets (Bottom) (Dodson, 2010. 49 Fig ure 3 5 MSE Wall and Caisson Configuration and Sloped Reinforcing Strip Detail (Kim, et al., 2010, Figure 3) 51 Figure 3 6 Model Predicted Total Displacement (291 mm) (from Hossain, et al. , Figure 10) .. 53
xi Figure 3 7 Basic Difference between a) MSE and b) GRS Wall Designs (VanBuskirk 2010, Figure 1) .. 61 Figure 4 1 Relationship among Fines, Water in the Reinforced Fill Zone and Design and C onstruction QC/QA (NCHRP, 2013) 77
xii LIST OF ABBREVIATION S ABBREVIATION % P ercent i Load Modifier i Load factor Resistance factor AASHTO American Association of State Highway and Transportation Officials ASD Allowable Stress Design ASTM American Society for Testing and Materials B.S. Bachelors of Science CMP Corrugated Metal Pipe C ulvert EH Lateral Forces from the Retained Soil EQ Seismic Loading ES Surcharges EV Weight of the Reinforced Fill F.S. Factor of Safety FHWA Federal Highways Administration GAI Geosynthetic Accreditation Institute GEC Geotechnical Engineering Circular GMSE Geosynthetic Reinforced MSE GSI Geotechnical Society Institute GRS Geosynthetic Reinforced Soil HDPE High Density Polyethylene EPC Engineering, Procurement and Co nstruction
xiii EPCM EPC construction management Ka Active Earth Pressure Coefficient Ko At Rest Earth Pressure Coefficient Kr Active Earth Pressure Coefficient within Reinforced Fill LAP Laboratory Accreditation Program LRFD Load Factor Resistance Design MDOT Montana Department of Transportation mm millimeter M.S. Masters of Science MSE Mechanically Stabilized Earth NC HRP National Cooperative Highway Research Program NCMA National Concrete Masonry Association No. Number PET Polyethylene Terephthalate PI Plastic ity Index PVC Polyvinyl C hloride QA Quality Assurance QC Quality Control Qi Nominal force effect RFCR Reduction Factor creep RFD Reduction Factor durability RFID Reduction Factor installation damage Rn Nominal resistance Rr n
xiv RSS Reinforced Soil Slopes SRW Segmental Retaining Wall Tal long term nominal tensile strength Tult ultimate tensile strength TxDOT Texas Department of Transportation WSDOT Washington State Department of Transportation
1 CHAPTER I MECHANICALLY STABILI ZED EARTH WALLS Introduction For approximately 40 years, Mechanically Stabilized Earth (MSE) walls have become increasingly popular and generally accepted as a standard earth retaining structures. MSE walls are an economic solution to grade separation needs and are being constructed at a rate of over one million square feet annually likely representing over half of all retaining walls on transportation facilities ( Federal Highways Administration [ FHWA ] 2009) MSE walls in transportation facilities have been used for sound walls, ro ck fall barriers, to support roadways/railroads/runways, to support bridges, as culvert or bridge headwalls, and to a lesser extent retain cut slopes MSE walls are highly desired because they are simple and rapid to construct, economical, more tolerable to deformations and perform better under seismic loading compared with other types of retaining walls (FHWA 2009). MSE walls are a relatively new technology, and as such, have experienced some growing pains. Modern day MSE walls were brought to the United States in the 1970s when a steel strip reinforcement system, developed by French Architect Henri Vidal, was built in California. Shortly after, geotextile reinforced walls were constructed followed by the first use of geogrid in 1981. By 1983, geogrid s were extensively used for earth reinforcement (FHWA 2009). The rapid development has led to the advancement of numerous systems (proprietary and nonproprietary ), differing analysis approaches (earth pressure, limit equilibrium, and continuum mechanics) and various design methods (Coherent Gravity, Tieback Wedge, Simplified, At Rest Earth Pressure ( Ko) Stiffness, and Structure Stiffness
2 methods) National and International organizations continually publish updated guidelines for the design and construction of MSE walls. In the United States American Association of State Highway and Transportation Officials (AASHTO), FHWA and National Concrete Masonry Association (NCMA) have published guidelines based on historical performance of MSE walls. T he se guidelines specify a practical design methodology that limits MSE wall deformations. The design methodology generally revolves around evaluating the stability of potential failure mechanisms. Failure mechanisms can be grouped into external and internal stability and primarily evaluated using either Allowable Stress Design (ASD) or more recently Load Factor Resistance Design (LRFD) Common failure mechanisms evaluated include limiting eccentricity, sliding, bearing, pullout and tensile resistance of the reinforcement, facing element s and global stability Although complete catastrophic failure of MSE walls is rare, many walls have exceeded the intended performance criteria leading to costly repairs or ongoing maintenance. ( Samtani and Nowatzki 201 6 ) estimate a serviceability failure rate of 10 percent (%) a nd structural failure rate of 1% Valentine (2013) estimated that the failure rate of Geosynthetic Reinforced MSE ( GMSE ) walls in the U.S. is as high as 5% and t he NCMA has estimated a 2% to 8% failure rate nationally for GMSE walls. This is much higher that other earth retaining structures and particularly concerning for transportation facilities. By evaluating a variety of observed adverse wall performance issues, overall MSE wall performance can be improved.
3 Discussions on high rates of poor MSE wall perforce are nothing new. Several publications have extensive lists of causes of failures including budget constraints, team communication, water infiltration, poor backfill, lack of inspection, corrosion, etc. This undertaking differs from past efforts in that it provides (a) thorough documentation of observed wall distresses specific to GMSE walls in transportation facilities (b ) quantifiable design considerations for the most common problems and ( c ) a risk base discussion for better communication between the owner, engineer, and contractor. Risk defined as being the probability of adverse performance. Because designs with very low probability of serviceability failure are very costly, a practical level of risk should be determined. The experience gained from the included case studies will allow the engineer to apply the recommendations with an understanding of the underlying risks. O bjective and Scope The objective of this thesis is to identify important design and construction considerations of GMSE walls that lead to adverse wall performance on transportation projects C onclusions and recommendations will be developed based on reports of observed adverse G MSE wall performance Th r ough a clear presentation of results of this study, a practitioner can learn from previous failure and develop an understanding of important considerations By being aware and applying these lessons learned, overall GMSE wall performance can be improved The scope of work to attain the stated objective is as follows:
4 Perform a literature review to collect information pertaining to G MSE wall guidance, current state of the practice, and reports of adverse wall performance Identify types of adverse perfo rmance and establish comprehensive conclusions pertaining to the cause of failure. Provide implementable design recommendations for better performance of G MSE walls. Identify and discuss risks associated with G MSE walls so that the owner, engineer, and co ntractor can better communicate. The focus of this study is on GMSE walls used in transportation facilities not MSE walls with inextensible reinforcements or designed following guidelines other than AASHTO or FHWA. However, brief discussions concerning these alternatives are included for comparison or context It is assumed that the reader has a basic comprehension of the site characterization, analysis, design, and construction of G MSE walls and is familiar with the guidance documents. The extensive r eference list is included for additional information. Definition of Failure For this document, failure is defined as unacceptable horizontal or vertical deformation of the wall face or supported zone that adversely affects the performance of the GMSE wall requiring a significant repair or continued maintenance. Deformations exceeding two inches are typical of GMSE walls considered to be undergoing failure
5 Limitations Recommendations are based on interpretations and conclusions drawn from the literat ure review and the authors own engineering judgement. These interpretations are not intended as a substitute for ones own engineering judgement. Nothing in this report is to be inferred as a replacement of the AASHTO, FHWA, or NCMA guidance documents.
6 CHAPTER II GUIDANCE DOCUMENTS A ND CURRENT PRACTICE Introduction A review of literature was conducted to determine the current practice for GMSE wall design and construction techniques on transportation projects in the United States AASHTO, FHWA, and NC MA have published guidelines based on historical performance of GMSE walls AASHTO and FHWA are widely accepted for both private and public projects, whereas NCMA is primarily used for private and commercial works. Because AASHTO and FHWA guidelines are sometimes considered conservative, especially with respect to fill requirements, a review of NCMA is included in this report for comparison. NCMA guidelines are not considered a suitable substitution on transportation p rojects. AASHTO was the first national standard for bridge design and construction dating back to 1931 utili zing the Working Stress Design philosophy. This guidance has since evolved to include the LRFD philosophy and a wide range of structures including GMSE walls. Revised editions a re typically published every four years with supplementary interim specifications of tentative revisions published each year. AASHTO (2014) is the frequent reference for this report citing a section, article, figure, table, or equation when needed. Sectio n 11.10 presents the specifications for MSE wall design in the left column with select commentary, designated by a C, in the right column. FHWA Geotechnical Engineering Circular ( GEC ) 011 is FHWAs primary technical guideline for selecting, designing, spe cify, monitor, and contract MSE walls and Reinforced Soil Slopes (RSS) on transportation facilities. The 2009 FHWA manual is a significant update
7 from A SD presented in the 2001 FHWA NHI 00043 manual, to LRFD philosophy. The referenced FHWA (2009) docume nt is presented as a series of chapters and subsections that are continuous across two volumes. Volume I primarily contains design considerations of MSE walls with Volume II presenting RSS considerations and the six especially useful appendixes, labeled A through F. NCMA organization originally published guidelines in 1993 specifically for GMSE walls with modular block facing and geosynthetic reinforcement with 100% coverage NCMA is an international trade organization that represents produces and suppliers and is dedicated to the promotion of manufactured concrete products. The current 3rd edition published in 20 10 and subsequent best practices guide published in 2016 is intended to provide and engineering approach for design and cons truction of segmental retaining walls (SRWs). A reinforced SRW is a MSE wall with a dry staked dry cast machined produced concrete unit. NCMA (2010) and NCMA (2016) are presented as a series of sections and subsections that are intended to co ver all c omponents and unique features of SRW systems There are significant differences between NCMA (2010) and AASHTO (2014 )/ FHWA (2009) that typically lead to less conservative construction requirements. Summary of AASHTO (2014) Design Guidance AASHTO (2014), Section 11.10 specifies that MSE walls may be used when traditional gravity walls are considered, especially where substantial settlement is anticipated, expect where access to utilities would disrupt reinforcements, scour may potentially undermine the wal l, or aggressive conditions may degrade wall components. ASSHTO (2014) specifies design of internal, external, compound, and facing stability; however, does not provide
8 geometrically complex guidelines and refers the reader to FHWA (2009) for tiered, back to back, and walls with trapezoidal sections. AASHTO (2014) requires a uniform minimum reinforcement length of 70 % of the wall height with an 8 foot absolute minimum, unless accurate site specific soil strengths are known amongst other conditions. AASHTO (2014) commentary states that this requirement has no theoretical justification but is based on historical practic e with accep table horizontal deformations. AASHTO (2014) designs evaluate the service limit state, soil failure (external stability), structural stability (internal stability), and seismic conditions. AASHTO (2014) also provides guidance on drainage, subsu rface erosion, and special loading conditions. Movement and stability at the service limit state includes evaluating for settlement, lateral displacement, and overall stability (limit equilibrium slope stability analysis). Safety against soil failure is evaluated at the strength limit state for limiting eccentricity, sliding bearing and overturning. Safety against structural failure is evaluated at the strength limit state for reinforcement pullout and tensile resistance of the reinforcement. Summary o f FHWA (2009) Design Guidance FHWA (2009) follow s LRFD methodology and generally agree s with AASHTO (2014) design approach by evaluating the internal, external, compound, and facing stability. Both guidelines have similar criteria for soil strength and re inforcement s, including follow ing the S implified M ethod for evaluating internal stability of both extensible and inextensible reinforcements developed in the late 1990s ( (Washington State Department of Transportation [WSDOT] 2001). However, some confusion remains between AASHTO and
9 FHWA design requirements in presentation, terminology, nomenclature, equations variables, and other idiosyncrasies (e.g. determination of soil aggressiveness ) FHWA (2009) presents specific recommend ations for MSE project criteria, site evaluation, wall selection, facing considerations, costs, and monitoring. The manual also provides specific guidelines for contracting, construction methods materials, and inspection o f MSE walls and RSS. FHWA (2009) lists general potential disadvantages of reinforced soil structures as requiring a relatively large space for excavation, the use of select granular fill, and often requires a shared design responsibility between material suppliers and owners. Summary of NCMA (2010) Design Guidance NCMA (2010) utilize s the ASD approach which contrasts with the AASHTO (2014)/FHWA (2009) LRFD approach. In addition, the guidelines present less stringent criteria for many of the wall components that typ ically lead to less co nservative construction requirements. Some of these criteria include a s maller minimum reinforcement length (0.6H), neglects long term creep, less strict backfill requirements, no soundness requirements on backfill, always uses the Coulomb Equ ation for la teral earth pressure, and specifies a lower Factor of Safety (F.S.) for overturning. NCMA (2010) does provide valuable detailed descriptions, mechanical properties, and engineering performance of SRW units including dimensions, mix designs, compressive st rengths, tolerances, connections, layout, and facing stability (e.g. crest toppling, surcharge loading, etc.). The guidelines also include extensive design considerations for surface and subsurface drainage and outline the roles and responsibilities of the owner,
10 architect, civil engineer, geotechnical engineer, SRW engineer, construction inspector, and structural engineer. O verview of Current Practic e MSE walls are an economic gravity structure constructed of compacted soil with horizontal reinforcement inclusions (reinforced zone) and various facing elements The inclusions add lateral stability and tensile capacity to form a composite mass. The inclusions are made of metallic or poly meric strips, grids, or sheets ty pically placed horizontally, perpendicular to the facing Fills primarily consisting of well graded, low plasticity, granular soils are recommended to limit deformations. The weight of the reinforced fill designated as load type EV resists the lateral f orces from the retained soil designated as load type EH surcharges ES and seismic loading EQ (AASHTO 2014) Gravity structures require competent foundation soils or deep foundations to limit total or differential settlements. MSE basic geometry an d forces from the retained soil in LRFD format are shown in Figure 2 1
11 Reinforced Backf ill Material AASHTO (2014) specifies that backfill materials should be granular, f r ee draining material. Because of the uncertainties associated with using empirical soil strength parameters and unk nown s with regard to quality of inspection or construction control many transportation agencies have adopted conservative re inforced fill criteria, generically referred to as select fills A ccording to AASHTO (2014) much of the experience with MSE walls with respect to internal stress, pullout, and failure surface shape is heavily influenced by granular soil properties. In add ition, cohesive soils are difficult to compact and are likely to experience creep over the life of the structure. Ongoing creep could result in excessive deformation s or even collapse, especially in situations where hydrostatic water pressures are likely to develop. Figure 2 1. Basic Geometry and Forces for MSE Walls (Samtani and Nowatzki ( 201 6), Figure 2.2)
12 According to FHWA (2009), MSE walls should be reasonably free from organic or other deleterious material and conform to the fill requirements presented in Table 2 1 Special considerations should also be given to low durability mate rial partials ( shale, mica, gypsum, etc.), rock fills, and drainage as even low fines content material may not be free draining. Compaction specifications should also specify a lift thickness and range of acceptable moisture contents To prevent facing mo vement, special compaction zone utilizing lighter equipment and thinner lifts should be considered (FHWA, 2009) According to AASHTO (2014) and FHWA (2009), thous ands of MSE walls with reinforced fills meeting the criteria above have performed excellently. A maximum effective friction angle of 34 degrees and cohesion of zero is usually assumed as the peak Table 2 1. MSE Wall Select Granular Reinforced Fill Requirements (FHWA, 2009 Table 3 1).
13 shear strength parameters (FHWA 2009). Project specific t esting may be used to justify higher internal friction angle values, but not to exceed 40 degrees This limit has been determined from comparison of full scale wall data with common design methods (e.g. simplified method). It is anticipated that assumed internal f riction angles in excess of the 40degree limit would greatly underestimate reinforcement loads (AASHTO, 201 4 ). Loosening the restrictions on select fills for MSE walls has been greatly debated especially with respect to the amount of f ines content ( % passing the number (No.) 200 sieve) The intent is to significantly reduce costs and/or environmental impacts by using lower quality fills or reusing on site native soils. NCMA (2010) suggests a 35% fines content criterion but allows up to 50% National Cooperative Highway Research Program ( NCHRP 2013) urges that AASHTO maximum fines content should be increased from 15 % to 25 % a few State Department of Transportations (DOTs) allow fines content greater than 15 % (NCHRP 2013), and Samtani and Nowatzki (2016 ) suggests that MSE walls for transportation facilities can be successfully constructed using e stablished design methods with fines content up to 50% The caveats of these suggestions being appropriate fill material selection, material design parameters, compaction control, and effective drainage. The electrochemical index properties of the reinforced fill and native soils must be correlated to the corrosion/degradation of the reinforcements to achieve the 75 year design life specified b y AASHTO. Limits for electrochemical criteria of reinforc ed fills used with geosynthetic r einforcements have been developed by FHWA (2009a) as shown in Table 2 2 The actual degradation of geosynthetic reinforcements with time will depend on the specific polymer amongst other factors (configuration, stress, etc.) leading to
14 uncertainties as to their durability (FHWA, 2009a). Research on corrosion of metallic reinforcements has led to more stringent requirements on re inforced fills but is not the focus of the report; therefore, are not presented. Geosynthetic Reinforcement MSE reinforcement types are generally classified as either extensible (geosynthetic) or inextensible ( metallic ). This report focus es on observed d istress of GMSE walls more specifically, geogrid reinforced walls. Geotextile, g lass fiber composites ultra high modulus polymers geocells, and other geocomposites are also excluded because they are not widely used in transportation projects. According to FHWA (2008), geosynthetic reinforcements are often a very economical alternative to metallic reinforcements, especially under certain environmental conditions, and allow for a wider variety of wall facings. The geosynthetic reinforcement cost is approximately 15 % to 20% of the total cost of the MSE wall; therefore, some conservatism in design (strength and spacing) is not excessively expensive (FHWA 2008). Despite the wide use of geosynth etic as soil reinforcement, there are unknowns with respect to long term performance. Specifically, with respect to time and temperature dependent creep under sustaine d loading leading to excessive deformations. Additional Table 2 2 Recommended Limits of Electrochemical Properties for Reinforced Fills with Geosynthetic Reinforcements ( FHWA, 2009 Table 3 4 ).
15 research is also recommended by FHWA (2008) to evaluate the strain incompatibility between the relatively stiff facing and the extensible geosynthetic reinforcement under seismic loading. Selection o f geosynthetic reinforcements depend on the durability and the long term geosynthetic soil stress transfer (FHWA, 2008). Each geosynthetic is different in its resistance to ageing and chemical attack due to varying polymer types, quality, additives, and pr oduct geometry (FHWA 2009). FHWA (2008) estimates there are more than 600 differ ent geosynthetic products available in North America. This variety presents a significant challenge for engineers to design and develop specifications for the considerable range of physical and mechanical properties The challenges have likely lead to the conservative approach of determining long term nominal design st r engths of geosynthetic reinforcements By using recommended reduction factors presented in FHWA (2009) i t is common to use a long term nominal tensile strength (Tal) 50 % to 9 7 % lower than the ultimate tensile strength (Tult), as determined from a wide wid th strip test per American Society for Testing and Materials ( ASTM ) D4595. Selection of the Tal is determined from reducing the Tult by all possible strength time dependent losses (FHWA, 2009 ). Reduction factors used to represent the strength loss over the design life of a GMSE wall is shown in Figure 2 2 The figure shows that there are immediate losses at the time of installation and additional losses over the design life of the reinforcement Reduction factors include installation damage (RFID), creep (RFCR), and durability (RFD). FHWA (2009) recommends that Tal values for specific products be
16 determined from extensive field and/or laboratory testing conducted by agency or an independent third party. In addition, to account for uncertainties in long term reinf orcement strength and potential local overstresses, a LRFD Strength I limit state resistance factor of 0.90 is applied when calculating soil reinforcement resistance under static loading FHWA (2009) This is a higher resistance factor, allowing for more r einforcement strength benefit, than those recommended for steel reinforcements The reasoning behind this is that confinement is not considered in developing Tal, observations indicate lower stress levels in reinforcements than used in design, and that strain of the reinforced fill is considerably less than the rupture strain of the geosynthetic reinforcement leading to visible deformations rather than sudden collapse. The most common reinforcement used in GMSE walls for transportation facilities are geogrids; ho w ever, geotextiles have gained some traction with extensive use in temporary structures and more recently, as Geosynthetic Reinforced Soil (GRS) technology for the Figure 2 2. Long term geosynthetic reinforcement strength concepts (FHWA 2009, Figure 3 6)
17 application Integrated Bridge Systems Geogrids are favorable because the y are marketed as a complete supplied package readily available from nationwide commercial suppliers. The most common geogrids for GSME walls include high density polye thylene geogrid (HDPE) and polyethylene terephthalate (PET) HDPE is considered to be a stiff geogrid, manufactured from drawing a perforated polymer sheet. PET geogrids are referred to as flexible geogrids that are formed from weaving polymer strands. Both geogrids can be manufactured as uniaxial (stronger in one direction) or biaxial (co mparable strengths in two directions); however, for efficiency uniaxial geogrids are typically used exclusively in GMSE walls. In addition to reinforcements, s ubsurface drainage and separation geosynthetics (mainly geotextiles) in accordance with AASHTO Sp ecification M 288 are used extensively in construction of G MSE walls. Design Methods The earth pressure design methodology generally revolves around evaluating the stability of potential failure mechanisms. Common failure mechanisms evaluated include limiting eccentricity, sliding, bearing, pullout and tensile resistance of the reinforcement, facing elements, and global stability. Failure mechanisms can be grouped into internal and external stability and evaluated by us ing established analytical methods base d on either ASD or more recently LRFD platforms LRFD become the required specification in federally funded transportation projects as of October 2010 following a transition period as stated in the FHWA ( 2000) policy memorandum. LRFD is defined by AASHTO (2014) as a reliability based design methodology in which force effects caused by factored loads are not
18 permitted to exceed the factored resistance components. LRFD methodology can be summarized as factored resistance must be equal to or exceed factored loads as defined by AASHTO Equation (220.127.116.11): iiQi n= Rr AASHTO Eq. (18.104.22.168 1) where: i = Load Modifier (a factor related to ductility, redundancy, and operational classification) i = Load factor Qi = Nominal force effect Rn = Nominal resistance Rr n MSE walls designed in the context of LRFD must satisfy the strength limit state, services limit state, and extreme event limit state as defined by AASHTO A limit state is a con dition beyond which a structural component ceases to satisfy the provisions for which it is designed (FHWA 2007). The strength limit state considers the stability of each structural element to relevant load combinations, whereas, service limit state provides restrictions on stress and deformation. Extreme event limit state of MSE walls ensures the overall, external, and internal stability requirements are meet during seismic loading or vehicular collision. Relevant load combinations for MSE walls include permanent and transient loads. Both, maximum and minimum load factors for permanent loads are evaluated to determine the critical combination. External S tability of GMSE Walls External stability of GMSE walls for transportation facilities is typically the responsibility of the owner agency or representative. The agency is proposing the need for the structure at a specific location ; therefore, the agency determines feasibility with respect
19 to site conditions s tructure selection, and anticipated loading Site characterization typically includes topography surveys, field reconnaissance, subsurface exploration, and laboratory testing to determine subsurface conditions and soil/rock parameters for use in design. S tructure selection includes such things as aesthetics, performance, environmental constraints, constructability, and cost. Loads that may be specific to a site include surcharges, traffic loads, hydrostatic pressures, seismic loads, etc. External stabilit y is evaluated assuming the reinforced zone acts like a rigid body similar to other gravity structures. Failure mechanisms evaluated c onsist of slidi ng limiting eccentricity (overturning), bearing resistance (including settlement) and global stability as shown in Figure 2 3 This evaluation determines the size of the r igi d body with respect to adequacy of the foundation soils. Greater reinforcement lengths and embedment beyond the minimums outlined in AASHTO (2014) may be required. In some cases, foun dation improvements such as deep foundations or soil treatments may be required to support the structure. Evaluating the external stability will be iterative with internal stability calculations to access compound stability and as assumptions become known, such as facing type, backfill materials, reinforcement type, final loading, and possibly geometry requirements from i nternal stability calculations
20 Internal S tability of GMSE Walls In transportation projects, the GMSE wall system manufacturer/supplier is typically responsible for internal stability design. The advantage is that the wall system supplier is intimately familiar with the wall system components and designs are optimized for the specific component properties According to NCHRP 290 (1987), t he internal failure mechanisms are reinforcement pullout and rupture The design parameters that need to be considered include backfill parameters, reinforcement propert ies (modulus, stiffness, and tensile strength), soil to reinforcement interaction parameters (soil density, reinforcement spacing, normal pressure, soil and reinforcement friction, etc.), reinforcement geometry, and construction parameters (compaction stre ss and reinforcement orientation) (NCHRP 290, 1987). Additionally, wall components must be designed, including facing elements, reinforcements, and reinforcement connections accounting for l ateral and vertical Figure 2 3. Potential External Failure Mechanisms for a n MSE Wall (FHWA, 2009)
21 deformations In some cases, agencies may pr e approve MSE wall systems for better efficiency of reviews. The original MSE design method was developed for inextensible reinforcements and came to be known as the Coherent Gravity Method. According to WSDOT (2001), t his method a ssum es that the MSE wall behaves as a rigid body and an overturning moment is transmitted through the reinforced soil mass The method was refined by several MSE specific research studies to include a bilinear envelope of maximum reinforcement tension and a variable state of stress. This method assumes that the failure plane does not actually develop, the active wedge does not displace, and the inextensibility of the steel reinforcements prevents structure deform ation (Anderson Gladstone, and Withiam, 2010). The Tieback Wedge Method and Structure Stiffness Methods w ere developed and verified using laboratory and full scale wall testing. Although different in approach with respect to how the lateral earth pressure is calculated ; both methods assume the failure plane devel op s along the Rankine failure surface for extensible reinforcements (WSDOT 2001) The Rankine failure surface is defined by a straight line oriented at an angle of ASHTO and FHWA have adapted the Simplified Method developed and presented in WSDOT (2001) for internal stability design. The Simplified Method was intended to unify and simplify the design methods above with respect to how vertical stresses are calculated and how reinforceme nt stiffness is considered in design (WSDOT 2001). The Simplified Method assumes t he wall is flexible and does not consider the overturning moment for internal vertical stress calculations (WSDOT 2001) To allow for various reinforcement
22 types, a r atio of earth pressure coefficient with the reinforced zone to active earth pressure coefficient ( Kr/ Ka) approac h is used as shown in Figure 2 4 The value of Kr is used to determine the maximum reinforcement tension assuming Ka is calculated per the Rankine or Coulomb equations Stability with respect to pullout failure is achieved when the factored effective length of the reinforcement in the resisting zone is greater than or equal to the factored tensile load at each reinforcement layer (FHWA 2009) Fig ure 2 4. Determination of K r /K a for the simplified Method (WSDOT, 2001)
23 Construction Inspection Construction inspection of MSE walls is similar to any other earthwork; it must verify proper site preparation, correct installation of components (level ing pads, facing systems, reinforcements, drainage networks, etc.), and placement and compaction of backfill soil (NCHRP 290, 1987). Quality Control (QC) in accordance with design specification is typically the responsi bility of the contractor while Quality A ssurance (QA) is the responsibility of the owner agency or representative. According to NCHRP 290 (1987) site preparation construction inspection is necessary to verify agreement between actual and specified base elevations, conformance of encou ntered soils with parameters considered in design, the absence of hard or soft foundation soils, and verify adequate drainage. Wall components need to be evaluated for conformance with the construction drawings, have been properly transported and stored, f ree of visible damage, and are properly installed (NCHRP 290, 1987). B ackfill type, placement, and compaction must be within limits and tolerances identified in the specifications.
24 CHAPTER III LITERATURE REVIEW AND EVALUATION OF G MSE WALLS Introduction In this Chapter, various p ublications relating to poor MSE wall performance were reviewed to identify types of wall distress and possible causes of failure. Publications concerning MSE walls with both extensible and inextensible reinforcements, as well as, performance of public and private projects were reviewed. T he literature review is intended to develop an understanding of the most common problems, important considerations, and underlying risks associated with GMSE walls used in transportation facilities Some of these causes of failures including budget constraints, team communication, water infiltration, poor backfill, lack of inspection, degradation, etc. The literature review in support of this thesis has been broken out into four categories: Comp ilation Studie s, Individual Case Studies, Geosynthetics in MSE walls, and MSE Wall General Reports. Compilation Studies include work that evaluates many sites of MSE walls or draws conclusions from a statistical evaluation of MSE wall failures. Individual Case Studies discusses one site or particular failure and presents details of the MSE wall in regard to design, construction, or failure mechanism. Geosynthetics in MSE walls include work that focuses specifically on t he behavior and/or performance of ex tensible reinforcement s in MSE walls MSE Wall General Reports include all other reports concerning various aspects of MSE performance with respect to planning, design, construction, maintenance, or repair. This section also summarizes the findings of the literature review in a general observations section.
25 Literature Review of Compilation Studies Compilation Studies include work that evaluates multiple MSE walls or draws conclusions from a statistical evalu ation to identify trends developments, or improvements. These studies tend to be more academic in nature and supplement or enforce guidance document recommendations and conclusions with respect to MSE wall performance The following documents were review ed and are summarized below: Applying Lessons Learned in the Past 20 Years of MSE Wall Design & Construction Harpstead, Schmidt, and Christopher (2010). A Perspective on Mechanically Stabilities Earth Walls Pushing the Limits or Pulling Us Down Bachus and Griffin (2011). An Assessment of the Factors that Contribute to the Poor Performance of Geosynthetic Reinforced Earth Retaining Walls Valentine ( 2013). Look Out Below!!! Potential Pitfalls and Suggested Improvements in Design and Construction of MSE Walls DiFiore and Strohman (2013). Forensic Studies of Geosynthetic Reinforced Structure Failures Wu and Chou (2013). Evaluation of Mechanically Stabilized Earth (MSE) Walls for Bridge Ends in Kentucky; What Next? Sun and Graves (2013). A data bas e, statistics and recommendations regarding 171 failed geosynthetic reinforced mechanically stabilized earth (MSE) walls Koerner and Koerner (2013).
26 Risk Evaluation and Mitigation for MSE Walls Perspectives from an EPC/EPCM Contractor Clemente, Lamon te, Davie, and Lewis (2016). Geotechnical Engineers should Design MSE Wall s and Prepare Plans and Specifications Harpstead, et al. (2010) report MSE structures are a useful technology for constructing cost effective and easily built retaining walls. Howev er, the MSE design is conservative, which does not necessarily lead to advancement in MSE wall technology. The authors state that MSE walls are not often treated with the same diligence as other engineered structures for the geotechnical investigation. Be cause MSE wall design is based on soil structure interaction, a geotechnical engineer should design MSE walls and prepare plans and specifications, given their understanding of the soil structure interaction. Significant issues have arisen due to lack of c ommunication between the various engineers and contractors leading to poor execution of MSE wall design, construction, and ultimately performance. Harpstead, et al. (2010) cite that 26 known failures were reported in 35,000 walls; however, the authors are aware of many failures go unreported, especially where litigation is involved. MSE wall designs contain elements of both structural and geotechnical engineering. Th e structural component of the wall facing, connections, and structural frames for obstruction avoidance need to be designed and reviewed by a structural engineer. Geotechnical design elements include soilstructure interaction. The authors believe that a geotechnical engineer should assume responsibility for designing MSE walls based on their understanding of the soil structure interaction. Thus, the geotechnical engineer should also
2 7 be responsible for preparation of the plans and specifications with colla boration of b oth a qualified structural and civil engineer, either directly, or as a minimum, in a review capacit y From review of cited wall failures, Harpstead, et al. (2010) identifies the critical wall components as drainage systems and quality of th e reinforced fill. The authors suggest that the majority of failures result from either the designer or the contractor not understating the importance of each of these wall elements and the requirement that each element perform as expected. Addi tionally, proper QA/QC should be completed during construction of the MSE wall. The authors suggest that full time inspection is warranted as opposed to occasionally checking backfill compaction. Proper QA/QC requires meticulous attention to design and constructio n details including assessing existing condition and verifying the soil strength parameters in addition to verifying all construction requir ements. Disregarding Lessons Learned In A Perspective on Mechanically Stabilities Earth Walls Pushing the Limits or Pulling Us Down Bachus and Griffin (2011) warns engineers designing MSE walls to heed warnings learned from previous failed projects. With the recent upsurge of MSE wall construction, the limits of design have been pushed; many of which are innovative i n terms of wall height or creative applications, others have been pushed to limits of failure. The authors note the continued failures of MSE structures from similar failure mechanisms, thus signaling the same mistakes. The use of MSE walls are an innovative, yet simple, method for reinforced soil slopes. However, the rate of failure has increased. Bachus and Griffin (2011) report that
28 these failures are often due to previously recognized issues. The goal of this article is to not only identify common desig n issues, but heighten the awareness of a robust design and diligent construction to reduce the likelihood of future failures. Bachus and Griffin (2011) initially presented MSE designs that were cutting edge and were successfully designed and constructed ( i.e., the good), then identified other MSE walls that had adverse impacts to the structure of the wall (i.e., the bad), and finally identified MSE walls that did not meet structural or aesthetic requirements (i.e., the ugly ). Figure 2 5 provides an e xample of neglecting to consider adverse impacts of foundation soil settlement in design. Figure 2 5. Photo of Failure of a 45 foot GMSE Wall (from Bachus and Griffin 2011, Figure 2) The authors identify that similar issues lead to repeated occurrences of MSE wall failures. The authors cite a 2009 Koerner and Koerner study of private project GMSE walls that 65% of wall failures were attributable to the design; 33% to the contractor; and 2% to facing failure. The same study suggested that technical factors leading to failures were fine
29 grained soil backfill, poor compaction, and/or the influence of water. The majority of failures were reported to have occurred within two years of construction. None of the wall failures observed was attribut able to the geosynthetic reinforcements. Bachus and Griffin (2011) ask, With all of this prior knowledge and experience, are their explanations as to why we still continue to have problems? Possible explanations include: inexperience, poor understanding and forgetting design details and principles; wishful thinking (i.e., use of probably acceptable materials and design criteria); and market pressure. The authors note that a desi gn of a small (less than 6 feet high) wall cannot be scaled to a mid size (1 2 to 40 feet high) or large (40 to approximately 140 feet high). In particular, design components should not be overlooked or underestimated includes drainage, facing strength, and compaction. Bachus and Griffin (2011) make f ive specific recommendations th at should be implemented and considered standard for MSE wall designs as follows: 1) The wall designer should assume responsibility for the engineered system. 2) Drainage design must be a component of the design (either freely draining or include drain lines). 3) Qualified quality assurance oversight shall be performed during construction to ensure that the design is followed. 4) Laboratory testing should be performed on soil and soil/geosynthetic interface materials to determine actual strength and creep characterist ics. 5) Consider long term maintenance requirements during the design.
30 Geosynthetic Reinforced Earth Retaining Walls Poor Performance Valentine ( 2013) discusses contributions to poor performance in GMSE walls in An Assessment of the Factors that Contribute to the Poor Performance of Geosynthetic Reinforced Earth Retaining Walls. Valentine ( 2013) investigated 45 geosynthetic reinforced soil structures with poor performance (e.g., deemed unusable or failure). The evaluation included both completely failed GMSE walls and ones that have not completely failed, but have degraded beyond its useful life. Valentine (2013) states that GMSE walls have gained popularity because of their economic and aesthetic values; however, the performance is unsatisfactory with failur e rates up to 5% and literature evaluating failed GMSE walls is sparse. Difficulties in forensic evaluations of GMSE walls arise from the inaccessibility of the MSE structure simply because it is buried, unidentified or uncertain failure factors, disagreement among professionals evaluating the failure, and professional reputations are at stake. Additionally, oftentimes failures are involved in legal claims that restrict the information available. Valentine ( 2013) notes that most failed GMSE walls are a result of multiple factors and are rarely attributed to a single cause. Factors identified in GMSE wall failure can include flawed engineering design, poor construction or inspections during construction, inclement weather or other natural conditions (e.g. sink hole), leaking utility line, poor soil conditions, and internal or external drainage. This paper specifically reviewed four projects with failed GMSE walls. The first GMSE wall failure was attributed to the contractor not following the reinforced s oil type or
31 reinforcement wall coverage ratios identified in design and the inspector not identifying this discrepancy, with failure initiat ing with a malfunctioning water pipe. The second GMSE wall failure was attributed to other construction at the toe o f the wall for landscaping that both undercut and saturated the wall toe. The third GMSE wall failure was attributed to poor design with an inadequate storm sewer size and incorrect shear strengths selected that contributed to the development of the global failure plane. The fourth GMSE wall failure was attributed to internal drainage measures in the design to account for perched water behind the wall crest and softening of the clay that was typical for the area. In conclusion, Valentine ( 2013) assessment o f 45 failed GMSE walls identifies the most common factors leading to failure. The primary factors for failure are a presence of water and incorrect soil type used. Recommendations made to owners in future GMSE walls are to require minimum experience for th e designer and contractor and require a thorough inspection. Frequent contributions to MSE wall Failures DiFiore and Strohman (201 3 ) discuss frequent contributors to MSE failures and common pitfalls in design and construction of MSE walls. MSE walls are t ypically more cost effective than conventional reinforced concrete retaining walls. The walls are generally aesthetically pleasing, and contractors can install them with little specialized construction skills or equipment. Common contributors to failed MSE walls include use of marginal soils, inadequate water management, poor coordination, and insufficient quality assurance/quality control. Using poor quality material, such as clay or silts, can lead to issues with compaction,
32 settlement, water retention, l ong term stability, soil migration, and freeze thaw potential. When MSE walls are designed without consideration of hydrostatic or seepage pressures, performance or stability of the wall can be compromised Common contributors in clude ineffective wall drainage; unknown water sources identifi ed during or after construction; and incidences with the potential for leaking water utility pipelines (e.g., irrigation lines used for vegetative growth on top of walls). Project coordination is an important process for a successful MSE structure to identify the various members roles and responsibility, such as the liability for site specific property evaluations (e.g., settlement, bearing capacity, and global stability) that can go unperformed. QA/QC is a fundamental step in ensuring that the as built MSE structure is constructed pursuant to the design plans and specifications. DiFiore and Strohman (201 3 ) present the following suggested improvements based on their experience: Hold a pre design meeting, a pre construction meeting, and regular site construction meetings. Provide wall drainage details in accordance with FHWA and NCMA design recommendations and MSE wall manufacturers design guidelines. Verify MSE wall stability and performance checks have bee n performed and reviewed. Verify that the contractor is trained and qualified to construct MSE structures. Perform QA/QC tests and inspections. Encourage open communications.
33 Geosynthetic Reinforced Structure Failures Forensic Studies Wu and Chou (2013) present a compilation of lessons learned from failed MSE walls that can be used for future successes. Nineteen cases of MSE failures of highway related infrastructures in mountainous areas were carefully examined and quantitatively studi ed. The results indicate that rainfall is the primary natural cause of failure and incorrect engineering practices are the cause of MSE malfunction. Human caused failures arise from inadequate project planning and site exploration, poor quality constructio n, lack of essential training for slope stability analysis. The results of this study intended to be used to improve engineering practices to ensure a more reliable MSE structure. Wu and Chou (2013) have observed that most MSE structures have performed wel l, leading to the conclusion that the design of them may be conservative. However, poor performance has been observed more recently. Failure studies are necessary to identify common mistakes to improve the safety of MSE structure failure. Wu and Chou (2013) intended to use descriptive statistics to interpret failure information to communicate specific results to practicing engineers. However, because each of the 19 structure s evaluated were designed and constructed for site specific condition, each had its own cause of failure, making the evaluation difficult to determine a concise cause of failure The generic causes of failure of MSE structures for technic al errors are provided in Table 3 1
34 Table 3 1. Generic Causes and Observed Technical Errors of MSE Failure ( Wu and Chou 2013, Table 3). Generic cause Observed technical errors Project planning and site exploration Lack of clear scope of the project Con fl icting client expectations Improper site planning for overall application Inadequate scope or extent of site explorations Misuse or misinterpret site related information Neglect the effects of unfavorable geological, hydrological, Ecological, and environmental, conditions Analysis Conceptual errors Misuse of analysis tools Misuse or misinterpret parameters Calculation errors Failure to identify all loads and load combinations Lack of redundancy Design Misuse or neglect of the importance of surface and subsurface drainage system Detailing de fi ciencies Speci fi cation defi ciencies Failure to consider surveillance, monitoring, and maintenance Material Misuse of geogrid materials Improper fi ll material Construction Improper sequencing Improper methods or timing of construction Insuf fi cient compaction or poor construction quality assurance and quality control Inappropriate site preparation Service and maintenance Structural alterations Neglecting routine cleanup of the drainage system Operation beyond the scope of the design Changes in structural use Inadequate surveillance, monitoring, and maintenance Unexpected construction disturbance adjacent to the site The trigger of failure was determined to be intense rainfall in 17 of the 19 cases. Wu and Chou (2013) concluded that r ainfall initiates instability by saturating soils and decreasing effective stress. An earthquake was the trigger of failure in the remaining two cases studied. An important note made was that MSE structures were found to exhibit better dynamic performance as compared to conventional concrete walls during an earthquake, substantiating that MSE designs lead to higher dynamic flexibility and ductility. Although the triggers of failure were rainfall and earthquakes, the cause of failure were generally attribut ed to inadequate project planning and site exploration and poor
35 construction quality assurance. In 10 of the 19 cases, the failure was highly likely from improper project planning and site exploration resulting in failures from differential settlement or b earing instability from encountering an unexpected soft stratum, deepseated slope instability or upward seepage pressure. Other issues derived from poor project planning included misinterpretation of strength parameters of the soil and insufficient boring information. Other technical errors arose from lack of clear project scope and insufficient communication. The materials used as backfill is another primary cause of failure. Most designs specify granular soils for backfill, low quality soils (e.g., silt clay, crushed shale) are often used for economic reasons. T his art icle cited a 2010, Koerner and Koerner study of 82 reinforced structure failures, where 76% used silt and clay as backfill soils. The safety of an MSE structure is highly dependent on the availability of qualified fill material. The frequency of failure because of improper analysis or design was 37%. These types of MSE structure failures may be due to the lack of experience, negligence, lack of education, incompetence, or the inability to communicate. Wu and Chou (2013) state the most common technical errors were from the misinterpretation of soil conditions and incorrect selection of soil strength parameters. Additionally, evidence indicates that the MSE failures have been the result of a lack of essential training in traditional slope stability analysis, rather than from any deficiency in the expertise of MSE professionals. Poor construction quality assurance is as high as 44%. Improvements of quality assurance are mandatory to ensure the safety of MSE structures and independent surveillance should be occurring to ensure correct compaction.
36 Nationwide Survey on MSE Wall Abutment Status Sun and Graves (2013) summarize a nationwide survey and inspection/rating results for 56 MSE wall abut ments in Kentucky. Survey invitations were sent to 49 states and 5 Canadian provinces with 39 states and 2 provinces responding. Survey questions were focused specifically on MSE abutments and considered type, geometry, reinforcement, backfill, foundation failures, and inspection. Relevant findings include 12 of 33 state/province responses (36%) report MSE abutment failures, with up to 10 failures reported in one state. Over half of these state/provinces reporting failures (7 out of 12) identified failure type as Settlement/washout of backfill material. Interesting findings include that the vast majority of state/provinces limit the maximum height of MSE wall abutments to 40 feet, as compared to the 50 foot height allowed by FHWA (2009). Also, only 2 4 states/provinces (73% of responses) had guidelines for building MSE abutments, three state/provinces had a formalized maintenance rating system, and only two states/provinces had maintenance inspectors guidelines. Extensive G MSE Wall Failure Review Koerner and Koerner (2013) performed an extensive review on 171 failed GMSE walls and created a database, statistics, and recommendations based on their investigation. The data base includes 44 cases of excessive deformation and 127 cases of collapse of at least part of the wall. The main statistical findings are as follows: 1) 96% were private (as opposed to public) financed walls 2) 78% were located in North America
37 3) 71% were masonry block faced 4) 65% were 412 m high 5) 91% were geogrid reinforced; the other 9% were geo textile reinforced 6) 86% failed in less than four years after their construction 7) 61% used silt and/or clay backfill in the reinforced soil zone 8) 72% had poor to moderate compaction 9) 98% were caused by improper design or construction (incidentally, none (0%) we re caused by geosynthetic manufacturing failures) 10) 60% were caused by internal or external water (the remaining 40% were caused by internal o r external soil related issues) Koerner and Koerner (2013) also presented critical issues associated with failure, as follows: f ine grained silt and clay soils used for the reinforced zone backfill, poor placement and compaction of these same fine grained backfill soils, drainage systems and utilities being located within the reinforced soil zone, nonexisting water control either behind, beneath or above the reinforced soil zone, and improperly determined and/or assessed design details.
38 In this article, Koerner and Koerner (2013) discuss the large number of MSE structure failures that include exc essive deformation and actual collapse. Inadequate or improper design (e.g., lack of drainage procedure and placement) and/or construction (e.g., use of poor soil materials and inadequate placement and compaction) are the primary causes of failure. Prime f ailure mechanisms evaluated with the number and % of cases observed is shown in Figure 3 1 The authors noted that were no cases of failure observed to have been due to poor geotextile or geogrid products. Figure 3 1. Basic Failure Mechanisms (Koerner and Koerner 2013, Figure 5) Coordination between the EPC/EPCM Contractor and MSE Engineer In Clemente et al. (2016), the interaction between the MSE Engineer and the engineering, procurement and construction/construction management (EPC/EPCM) was writte n about from the EPC/EPCM viewpoint. Several parties are engaged in the
39 procurement, design and construction of MSE walls, often under a design build contract under an EPC/EPCM performance. This interaction, or lack of interaction, contributes to the success or failure of an MSE structure. The authors developed an MSE wall survey to evaluate the performance of MSE walls that were completed by 20 geoprofessionals. The survey results indicate that MSE walls are generally well designed and constructed. Howeve r, the responses indicate a large variation in the design and construction management practices From these responses, the authors r ecommended the following best practices for a successful MSE design: Ensure technical specialists are involved in every aspe ct of procurement, design, and construction. Perform adequate and thorough planning with a clear division of responsibilities. Use project specific specifications. Incorporate internal and external drainage provisions. Ensure that the design is followed th rough construction with adequate oversight by qualified personnel. Literature Review of Individual Case Studies Individual Case Studies discusses details of specific MSE wall performance in regard to design, construction, or failure mechanism. The followin g documents were reviewed and are summarized below: A Tale of Two Walls: Case Histories of Failed MSE Walls (Scarborough, 2005) Case History of Two MSE Walls on Steep Slopes (Boyle and Perkins, 2007).
40 Heeding Natures Call: R eplacing MSE Wall with a Bridge (Haramy Anderson, and Alzamora, 2010). Lessons Learned from Settlement of Three Highway Embankment MSE Walls (Dodson, 2010). A Case History of MSE Wall Failure: Finite Element Modeling and Evaluation ( Kim, Bhowmik, and Willmer, 2010) Effects of Backfill Soil on Excessive Movement of MSE Wall ( M. Hossain Kibria, Khan, J. Hossain, and Taufiq 2012) Clayey Poorly Draining Soils Adequate Plans and Design Case Study Scarborough (2005) discusses two segmental block MSE wall failures (i.e., W all A and Wall B) using geogrid reinforcement within a clayey soil backfill in eastern Tennessee. One wall failed (Wall A) and the other (Wall B) experienced large deformation, but remains in service. Scarborough notes that MSE wall behavior is strongly dependent on the soil and rock characteristics used in their construction and that a knowledge of the interaction of the natural earthen materials having variable characteristics with the manufactured components of a n MSE wall is a critical component of a successful wall design. This paper states that, Factors contributing to the observed failures include not only the technical, but also professional issues. Wall A failed, reportedly due to poor drainage leading to a hydrostatic pressure buildup behind the wall facing as s een in Figure 3 2 Additional factors included issues during construction (e.g., poor compaction of fill, wall height extending higher than designed, and use of clay backfill). The author indicated that the designer of the wall may
41 have not understood that earth pressures for cohesive soils (e.g., clay) are not well predicted by conventional theories and that alternative use of equivalent fluid pressures may be useful when poorly draining soils are used for retaining walls. Figure 3 2. Photo of Failure of Wall A (Scarborough 2005, Figure 1) Wall B experienced large deformation. During the forensic evaluation of the wall failure, poorly consolidated backfill soils were originally thought to be the cause of failure. However, through 35 compaction tests performed from soil within the retained fill, the engineers determined that both the reinforced and retained fill were well compacted. Ultimately, the cause of failure was due to the lack of a global stability analysis being performed, which should have identified the type of soil to be used as backfill. Additionally, while internal, external, and facing stability may have been adequate as designed, consideration of global stability would have likely added more geogrid reinforcement into the design of the MSE wall. This paper reports that MSE wall designers may neglect important design considerations, possibly due to economic constraints, lack of design experience or
42 understanding of regional materials, ability to shift liability, or lack of control during construction. Scarborough (2005) noted that because of the high rates of problems experienced, a municipality in Tennessee temporarily placed a moratorium on all MSE Walls this ban was then lifted, but the municipality now requires an independent third party technical review of the design prior to approval. Scarborough (2005) concludes this paper with, The failures of these two retaining walls brings attention to the professional issues of what constitutes adequate plans and design for a n MSE wall, as well as to the technical issues as sociated with the use of clayey/poorly draining soils. MSE Wall Success Good Communication Case Study In 2005, the Beartooth Highway on the Montana Wyoming boarder had high snowmelt concurrently with a hea vy rainstorm that lead to debris flows and severe damage of the roadway (Boyle and Perkins, 2007). Because this road is critical to the local tourism industry, the State of Montana and the Montana Department of Transportation (MDT) expedited design and construction of the road reconstruction, which included two large MSE Walls under a Design Build Contract. Indeed, the designbuild team was selected within a month of the damaged roadway incident. Due to the depth of debris chutes and steep topography, the team decided to construct MSE walls at two locations where erosion was observed to up to 12 meters below the downslope shoulder of the roadway. This technically difficult project in a remote location was fully completed in four months because of the succes sful design and good communication between the design engineers and construction crew.
43 Through co locating the MDT and the designbuild team members in an office during design and on site during construction, the team was better able to communicate the des ign for successful implantation. Because the designers had a thorough understanding of readily available materials, the MSE walls were designed to be readily modified so that wall elements and the design could be modified to the field conditions in a short timeframe. Additional construction constraints came from limited laydown and work areas, and variable excavation and backslope conditions. To address these potential concerns, the designers opted for one type of high strength geogrid reinforcement. Altho ugh, some cost savings would have been incurred by identifying the lowest acceptable material, the overall cost savings was greater because of the simplification during design, construction, and oversight. Additionally, the MSE wall plans were purposefull y dynamic to be modified based on field excavations, cut slope stability, and foundation conditions. As noted in Boyle and Perkins (2007), Decisions on minimum reinforcement lengths, the need to excavate and replace native materials in the gully at Site 6 and to use a micropile supported concrete slab below the Site 7 MSE wall were not made until the excavations were completed and the foundation conditions exposed. The technically complex Beartooth Mountain roadway was reconstructed on an expatiated sche dule following severe damage during storm events because of experienced design teams performed a quality design, anticipated and planned for field variances, and interacted consistently with the team throughout construction.
44 Under Designed Drainage Structure Sur face Water Flow Case Study Haramy et al. (20 10 ) presents the collapse and proposed repair alternatives for an approximately 60foot segment of MSE w all along the General Hitchcock Highway in the Catalina Mountains n ear Tucson, Arizona. Prior to wall construction, at least one emergency repair for roadway damage and embankment failure resulting from debris flows was known at this location In addition, t he area experienced a large forest fire shortly after wall const ruction in 2003 and large amounts of sediment debris were observed during precipitation events. The wall failed during a series of monsoon storms between July 27th and 31st, 2006. The failed segment of the 20 foot high MSE wall at MP 9.8 was constructed using the welded wire MSE system manufactured by Hilfiker. The reinforcement lengths were designed to be 70% of the wall height and spaced 24 inches vertically. A 36 inch corrugated metal pipe culvert (CMP) to capture inboard drainage and an 18 inch outboard drop inlet were installed within this wall segment. The vertical CMP was tied into the 36 inch CMP at the bottom of the wall approximately 24 inches from the face.
45 Severe runoff and sedimentation plugged the culverts at the failure location resulting in overtopping of the roadway ditches and curbs as s ho wn in Figure 3 3 It was believed that overtopping surface water fl owed through the facing rock to the base of the wall below the embedded compacted fill that was intended to provide adequate bearing capacity Because the water could not easily flow out and down the slope, the authors believe the water turned an d flowed parallel to the roadway to the low point in the wall. Water built up in the facing column below grade until it either caused failure of the compacted fill in front of the wall or it overtopped causing failure by erosion. Either way, the foundation of the facing rock was lost or it was assumed the facing rock emptied out the bottom of the wall leaving the wall fill exposed to sloughing and erosion. Following the failure, hydraulic engineers d etermined a larger drainage structure Figure 3 3. Surface water overtopping MSE Wall ( Top ) and Failed MSE wall ( Bottom ), (Haramy et al. 20 10).
46 with a minimum opening width of 12 feet and height of 4 feet is required to provide adequate capacity and debris/sediment passage. This is significantly larger than the originally installed 36inch culvert. Considering emergency repair constraints, such as site geometry, environmental restrictions, and construction schedule, the selected drainage structure alternative was a bridge to span the entire 60 foot failure location. In addition, the design features were implemented including (1) paved conveyance ditches to improve the collection of surface water runoff and debris, (2) curb was removed from the top of the wall to allow for sheet flow, (3) the toe embedment material above the bottom of the wall was rep laced with riprap to protect the base and foundation of the wall from future erosion and prevent hydrostatic pressure built up in the wall facing. Inadequate Design Assumptions and Submittal Review Settlement and Consolidation Case Stud y Dodson (2010) pr ovided lessons learned from settlement and vertical deformation observed in three highway embankment MSE walls constructed in 2004 on a rural highway in northern California. Three to six inched of settlement was observed in the outbound roadway lane less than six months after construction following a rain on snow event Also, severe longitudi nal cracking with voids up to 2 meters deep was observed near the centerline of the roadway. Bulging of the facing baskets were also observed during construction but was considered a construction and quality control issue and repairs were immediately made by the MSE wall supplier. Photos of wall distress are presented in Figure 3 4 The bulging baskets were likely the first indication that some kind of vertical settle ment
47 in the wall fill was occurring post construction. Despite repeated repairs, the settlement continued and the road had to be closed to public traffic after the second winter season. The 25to 33 foot tall MSE walls consisted of wire face MSE walls with metal reinforcements; however, non select on site material was identified as suitable for reinforced zone backfill to provide significant cost savings with relatively low risk to the overall performance of the walls. N on select wall backfill was explored during the 30% design because the nearest borrow pit was approximately 45 miles away. Without the cost savings from using on site material, the scope of the project was in jeopardy of being reduced to stay within the al lotted construction budget. Initially, the risks of using the non select materials appeared low in design because the on site material contained less than 35 % fines and were generally non plastic. Design recommendations f ro m preliminary external stability calculations include minimum reinforcement lengths at 70% of the wall height but required special consideration for adequate drainage, soil reinforcement interaction, and higher corrosion potential. The use of non select material was reconsidered at the 95% design phase when settlement was estimated at 4 inches. Mistakenly, the recommendation to change the MSE wall backfill was not implemented and the project went to bid allowing a backfill with fines contents up to 45 %
48 Distress locations were correlated to surface drainage features that lead the construction and engineering staff to believe that piping of the MSE wall backfill was a major contributor to distress. Repairs consisted of a cutoff wall, paved ditches, Figure 3 4. Vertical Settlemen t ( Top Left), Severe Longitudinal Cracking ( Top Right ), and Bulging Baskets ( Bottom) (Dodson, 2010)
49 underdrains, geomembranes, increased curb heights, and sealing around guardrail posts. The author concludes that three main lessons were learned from this cas e study, (1) that the estimated repair costs far exceed the estimated cost of importing backfill materials despite the appearance of using onsite materials to be low risk, (2) long term behavior between a rigid temporary shoring system and the flexible MSE Wall system need s to be considered and (3) prevention of surface water infiltration into the MSE wall system would have prevented piping of the backfill material. Incorrect Evaluation of S ettlemen t Differential Settlement Case Stud y Kim, et al. (2010) evaluated an MSE wall failure of an 11.5 meter high wall comprised of reinforced concrete facing panels with steel reinforced strips. Some of the facing panels were installed over previously installed and abandoned concrete caissons e xtending to competent bedrock. The reinforced steel strips were ob served to have sheared near the wall facing connection resulting in collapse of the facing panels at ten caisson locations. The site is located in Georgia within a piedmont geologic setting where a large variation of soil conditions is typical. The subsurface soil is comprised of a layer of fill underlain by residual soils, weathered bedrock, and parent bedrock. The fill and residual soils ranged in thickness from 1 to 12.5 meters and consist of loose to medium dense clayey sand to firm sandy clay. In addition, the MSE wall was constructed in two phases. Some drilled caisson foundations were installed prior to the MSE wall construction; when the building orientation was realigned, the MSE wall was realigned to match the new building design. As a result of the mid construction modification, a portion of the MSE wall rested on previously placed caissons.
50 This forensic evaluation included finite element modelling and evaluated the failure mechanism. Additionally, parametric analysis was performed for the present and fin al loading conditions to provide corrective measures from a future failure of this MSE wall. Modeling results neglecting the caissons, accurately predicted a settlement rang e (150 to 610 millimeter [ mm] ) similar to the observed settlements (between 135 to 520 mm and 15 to 665 mm for the apron fill and the wall panels, res pectively); indicating that if no caissons were present below the wall, the M SE wall would not have failed It was concluded that the MSE wall facing panels placed over the caisson s experienced significant differential settlement compared to the reinforced fill zone ca using excessive tensile stress and ultimate failure of the facing panel connection. T he wall soil backfill and reinforced strips had settled due to the consolidation of the underlying compressible soils and the compression of th e wall backfill under its own weight. The facing panels placed over the caissons did not undergo deformation leading to a differential settlement condition as shown in Figure 3 5 Figure 3 5 MSE Wall and Caisson Configuration and Sloped Reinforcing Strip Detail (Kim, et al., 2010, Figure 3).
51 Kim, et al (2010) noted that MSE walls are flexible structures and can withstand significant differential settlement as long as the wall panels and reinforced strips can settle together. However, excessive differential settlement between the wall panel and the reinforced strips can cause a significant increase in tensile stresses and may result in wall failure. The paper summarized, when significant differential settlement is anticipated a detailed evaluation of deformation of the wall should be performed during design. Inadequate Reinforcement Length Excessive Displacement Cas e Study Hossain, et al. (2012) presents an MSE wall that has been observed to have bulging wall facings and deformations up to 450 mm in Texas. This paper presents the site and laboratory investigation testing program, as well as finite element modeling co nducted in PLASIX to evaluate the potential cause of the MSE wall movement. Hossain, et al. (2012) notes the construction of an MSE wall does not require special craftsmanship and skill. Nonetheless, important factors to evaluate prior to construction incl ude site specific material specification, construction quality control, and a performance monitoring plan. The MSE wall discussed underwent a comprehensive evaluation that included soil test borings Grain size analysis indicated backfill material was betw een 28.9 and 30.7%, moisture content ranged from 7.9 and 18 .9%, Atterberg limits indicate plasticity indices between 5.8 and 10.2 and soil resistivity was determined to be low. In this MSE wall instance, the backfill soil did not satisfy either FHWA or th e Texas Department of Transportation (TxDOT) requirements (TxDOT, 2014) The presence of perched water zones
52 behind the wall suggest the possible intrusion of water into the high fine content backfill producing pressure causing the MSE to shift. Another major cause of the MSE wall to shift noted was inadequate reinforcement length, which was constructed at 30% of the wall height. The as built reinforcement length covers the active zone, but does not cover the length needed for the resisting zone. Figure 3 6 illustrates the total and vertical displacement predicted using the PLAXIS model, which corresponds to the actual measurements of wall movement identified in the field. The model also indicated that the movement would be rotational, that also matches fiel d conditions and that inadequate reinforcement length in the upper portion of the MSE wall could have caused excessive movement. In summary, Hossain, et al. (2012) notes that excessive movement of the MSE wall may be a result of a high percentage of fines in the backfill soil not meeting FHWA requirements, pressure due to poor drainage derived from the fine backfill material, and inadequate reinforcement length. Figure 3 6. Model Predicted Total Displacement (291 mm) (from Hossain, et al. , Figure 10).
53 Literatu re Review of Geosynthetics in MSE Walls Geosynthetics in MSE walls subject include works that focus specifically on the behavior and/or performance of extensible reinforcements in MSE walls. This section includes research conducted on both GMSE walls and GRS. The following documents were reviewed and are summarized below: Geosynthetic Reinforced Multitiered Walls (Leshchinsky and Han, 2004). Geosynthetic Reinforced Soil Walls and Slopes: US Perspective (Christopher, Leshchinsky, and S tulgis, 2005) GRS A New Era in Reinforced Soil Technology (Barrett and Ruckman, 2007). Experimental and Analytical Investigation of Geogrid MSE Walls (Reddy and Navarrete, 2008). Adoption and implementation of GRS design concepts A Consult ants Perspective ( VanBuskirk 2010) Need for and Justification of Quality Management Systems for Successful Geosynthetic Performance Need for and Justification of Quality Management Systems for Successful Geosynthetic Performa nce (Koerner and Koerner, 2012 ). Load Carrying Capacity and Required Reinforcement Strength of Closely Spaced SoilGeosynthetic Composites (Wu and Pham, 2013). Use of Mu ltitiered Geosynthetic Reinforced Soil Walls Leshchinsky and Hans (2004) Geosynthetic Reinforced Multitiered Walls pre sents design considerations for the use of multitiered MSE walls where high retaining walls are required. Considering the fact that tensile stress in the reinforcemen t increases rapidly with
54 height, m ultitiered MSE walls are built with an offset betw een shorter tiered walls to reduce the tensile stress in the lower tier s Tiered walls are seen as an alternative to closer spacing reinforcement, which increases cost. The authors recognize that rational design me thodology for various configurations of multitier MSE walls is lacking; both AASHTO and NCMA rely on an empirical approach derived without theoretical or experimental basis. Also, current design guidelines are limited to two tiered walls with zero batter when walls with more than five tiers hav e been successfully and economically constructed. The purpose of this study was to evaluate multitiered wall stability by quantifying the effects of offset distance, fill quality, foundation soil, reinforcement length and stiffness, water, surcharge, and number of tiers. Leshchinsky and Hans (2004) article presents the theory that if results of multiple parametric studies performed independently produces similar results, then an acceptable level of confidence in the results can be assumed The authors pe rformed parametric studies assessing tensile strength as a function of reinforcement length and stiffness, offset distance, the fill and foundation strength, water, surcharge, and number of tiers, which may be used in multitiered wall analysis. The values of each parameter were investigated by changing its value from the baseline case while keeping other parameters unchanged. The modelling provided the results shown below. Many of which are intuitive and support the mode of thought for construction of GMSE walls: As offset distance increases, the required reinforcement strength decreases. Additionally, the required strength of three tiered walls is greater than two tiered walls. When offset is large, each tier performs independently.
55 Low quality fill requir es higher reinforcement strength. The offset distance for tiered walls to function independently is larger when the low quality fill is used, thus leading to the recommendation that an increase in the number of tiers results in a significant increase of th e required reinforcement strength. There was no significant effect of reinforcement length in two tiered walls; however becomes more apparent in three tiered walls. The longer reinforcement required less strength presumably because shorter reinforcement i ncurred pull out failure and did not contribute to compound stability. There was negligible difference evaluating the effect of reinforcement stiffness and reinforc ement type Therefore, wall systems comprised of either metal or geosynthetic can be evaluated using limit equilibrium analysis. The quality of foundation soil type is a dominant factor for wall stability. Decreasing the quality of foundation soil leads to the need for increased reinforcement length and strength. The authors noted that po or agreement between FLAC and limit equilibrium in this case suggests further investigation. Water present in the wall structure impacts wall stability. A sudden drawdown may decrease stability, thus requiring stronger and longer reinforcement.
56 Leshchinsk y and Han (2004) presents a preliminary approach to multitiered GMSE walls design, yet recognize further development is required. The study yielded that properly conducted limit equilibrium analysis can reliably provide reinforcement layout strength, and connection capacity for MSE walls with complex boundary conditions as compared to continuum mechanics based analysis Success of Geosynthetic Reinforced Soil In the Christopher, Leshchinsky and Stulgis (2005) article, Geosynthetic Reinforced Soil Walls and Slopes: US Perspective the authors present the cost advantages and successful performance of GRS w alls This paper reviews three different design methodologies with respect to their ease of use and accuracy of results. At the time this article was written, design of GRS walls in the US followed three guideline documents (AASHTO Standard Specifications for Highway Bridges 17th Edition, 2002; AASHTO LRFD Bridge Design Specifications 2nd Editions with 2002 Interim Revisions, 1998/2002; and FHWA Me chanically Stabilized Earth Wall and Reinforced Soil Slope Design & Construction Guidelines, 20 0 1). The three design approaches presented in Christopher, Leshchinsky, and Stulgis (2005) include lateral earth pressure analysis, limit equilibrium analysis an d continuum mechanics based analysis. Lateral earth pressure analysis assumes a coherent mass for the reinforced soil to evaluate the external stability and produces a conservative design with respect to reinforcement strength of MSE structures on firm foundations and reinforced fills with no positive pore pressures. Limit equilibrium analysis uses stabilizing forces contributed by equally mobilized reinforcement layers that require computer programs in
57 design The continuum mechanics based analysis is rigo rous and requires a thorough characterization of the design elements, and if selected incorrectly may lead to conservative results. This method is applicable for research and forensic tool, and is becoming more accepted in design if performed by a trained analysis. Christopher, Leshchinsky, and Stulgis (2005) predict that with the adoption of LRFD design platform, public sector design methods in the US will shift from the Bishop Method moment equilibrium method to the more rigorous Spencer Method limit equilibrium method. Another ongoing design development in the US identified by the authors is the need to calibrate design models from high quality granular fill to allow for the use of marginal soils with higher f ines content. The authors credit NCMA and Highway Innov ative Technology Center contributions in the adva ncement of GRS systems in the United States Use of GRS Structures for Retaining Walls Barrett and Ruckman (2007) present the case of using GRS structures instead of MSE Structures. MSE walls use widely spaced inclusions as quasi tiebacks with heavy and stiff facings, such as concrete blocks or panels, to stabilize the backfill, while GRS use closely spaced geosynthetic reinforcements with minimal fac ing material. The authors note that GRS is superior to MSE in resisting static and dynamic loads, as proven by the presented negative batter walls case studies. Additionally, the authors suggest deficiencies of the current guidelines for design and construction of MSE structures include : To some degree, our national guidelines are unc onservative, and lead to or allow highfailure rate MSE walls built with dirt, widely spaced, expensive reinforcement, huge expensive blocks, unnecessary embedment, impossible creep reduction factors, impossible overturning calculations, odd connections and hinge heights, and needless gravel behind facing blocks and counterproductive concrete pads under facing blocks. Problem is, simply stated, those guidelines are inherently w rong. They
58 allow walls that cannot tolerate high seismic forces, walls that poorly tolerate construction errors, wall designs that cannot be extended to true truncated bases, and that should not be used as bridge piers, rockfall barriers, negative batter and GRS arches. According to Barrett and Ruckman (2007), GRS structures are a superior system to MSE structures. GRS walls have been used in complex wall structures and have withstood strong earthquakes when they have been constructed with granular backfill soil and closely spaced reinforcements. The a uthors encourage engineers to gain a deeper understanding of GRS construction and believe the use of GRS is encumbered by current AASHTO, NCMA, and FHWA guidelines, which are artifacts from traditional tie back theory. The main component of a successful GR S is to have a closely spaced reinforcement, which confine the granular particles, preventing dilation. Comparison of Modeling Methods for Geogrid Reinforced MSE Walls Smallscale testing of an MSE wall reinforced with geogrids was evaluated by Reddy and N avarrete (2008). The testing was then compared to numerical modeling and full scale MSE wall prototype testing. The eventual purpose of the research was to provide rational methods to determine appropriate factors for use in LRFD design of MSE walls. Two types of geogrids (HDPE and polyethylene terephthalate [PET]) were tested. A full scale MSE prototype was tested measuring movements, reinforcement strains, and settlement, with one half of the wall using each type of geogrid. The authors determined that t he results of the pilotstudy suggested similar results for both types of geogrid and that strains encountered in the geogrids were very small, even at loads exceeding working conditions.
59 The pilot scale testing was then compared to both modeling and full scale MSE wall prototype testing. Large differences were observed in results of the small scale testing to each of the modeling and the full scale MSE wall prototype testing. These discrepancies were attributed to the material property discrepancies and ge ogrid distortion for modeling and lack of gravity for full scale wall behavior. Reddy and Navarrete (2008) concluded that while smallscale testing may be useful for preliminary results, computer modeling and/or prototype testing are necessary to accurate l y evaluate MSE wall behavior. Comparison between GMSE and GRS Design Theories VanBuskirk s (2010) Adoption and implementation of GRS design concepts A Consultants Perspective presents the fundamental differences between MSE and GRS design concepts. The author acknowledges that GRS designed wall are a robust and proven technology, but are not currently covered by any widely acce pted guideline or standard. Fundamental engineering diffe rences explored include compaction induced stresses, geotextile soil interaction, reinforcement spacing vs. aggregate size, stresses and strains in reinforcement, creep behavior, and QA/QC. VanBuskirk (2010) identifies the fundamental difference between stabilized earth of MSE walls and the composite behavior of GRS. MSE walls require the reinforcement to be secured to the facing, typically though proprietary design systems, to resist the soil between reinforc ement elements. The influence from the reinfor cement on internal shearing of the soil mass is ignored. GRS infers that with sufficiently tight spacing, the soil and reinforcement act as a composite, maintaining the confining stress in the soil. The author
60 points out the facing of GRS walls act more as a faade and only needs to confine the soil and resist constructioninduced loads. Basic differences between MSE and GRS design concepts are shown in Figure 3 7 Figure 3 7 Basic Difference between a) MSE and b) GRS Wall Designs ( VanBuskirk 2010, Fi gure 1) VanBuskirk (2010) presents that reinforcing spacing plays a much more significant role in internal stability than reinforcement strength or coverage. Thus, external stability governs walls with tight reinforcement spacing as compared to internal o r compound failures that govern MSE wall designs. The result, GRS walls designed as a composite requires significantly reduced rei nforcement lengths as compared to MSE. Other potential advantages of closely spaced reinforcement include suppression of soil dilation, reduction
61 or elimination of creep in reinforcements, reduced stresses in reinforcements, simplified designs, and advantageous QA/QC procedures. VanBuskirk (2010) concludes tha t it would be more appropriate to develop a separate GRS design standard to avoid confusion with the potentially unconservative and complicated MSE design methods. The author believes that b y following sound engineering principles it is possible to calc ulate reinforcement requirements as a function of reinforcement spacing and that the adoption of composite behavior of GRS will lead to a significant cost savings. Quality Management for Successful GMSE Walls In Koerner and Koerner (2012) Geotechnical Society Institute ( GSI ) White Pater #26 titled, Need for and Justification of Quality Management Systems for Successful Geosynthetic Performance, the authors present three groups of geosynthetic field failures and the necessity of implementing quality m anage ment. The first failure type is derived from holes created in geomembranes during construction that reduce strength and increase fluid flow. The second GMSE failure type is failed geotextile filters due to poor placement in the field including lack of contact of placed geotextile material and glued/blocked geotextile The third failure type is G MSE wall failures However, from the 141 failures evaluated, geosynthetic reinforcement failure was not considered to be a leading cause of failure The authors present the elements of a quality management system a nd its necessity in engineering and goes on to present the quality management specific to geosynthetics and geosynthetic systems. At the time of the white paper publication, there are 230
62 Geosynthetic A ccreditation Institute (GAI) Laboratory Accreditation Program (LAP) test methods available for accreditation. These GAI LAP tests are available for geosynthetic materials. Additionally GSI has two inspector certification programs available that focus on the QA/QC of field inspection of waste containment geosynthetics and compacted clay liners and the other is focused on G MSE wall, berm and slope field inspection. There are also two programs offered through the International Association of Geosynthetic Installers for certified welding and approved installation contractors. Implementation of a robust QA/QC program in the field for construction of GMSE structures can bot h identify design modifications or observe and correct field construction practices. The authors conclude that not only is the field QA/QC program imperative, but also a preparation of and following a QA document identifies potential field issues before they become a problem. Design C riteria for Closely Spaced GMSE Walls In Wu and Phams (201 3) article, Load Carrying Capacity and Required Reinforcement Strength of Closely Spaced Soil Geosynthetic Composites, the authors present that the role of reinforcement spacing is much more significant than reinforcement strength. In previous studies, these two parameters are considered to have an equal role on performance. The authors identify that c m) reinforcement significantly enhances the beneficial effects of geosynthetic inclusion, which is referred to as GRS. GRS walls are s imilar to G MSE walls in geometry; however, GRS walls use closely spaced geosynthetic reinforcement to improve the behavior of the soil mass, while a G MSE wall relies on the reinforcements as tiebacks to resist failure. Current design equations do
63 not properly account for relative roles of reinforcement spacing and strength in a GRS mass. Wu and Pham (2013) present an analytical model for accurately predicting the load carrying capacity and required reinforcement strength of a GRS mass where failure is as sumed to be controlled by rupture of the reinforcement. Literature Review of MSE Wall General Reports MSE Wall General Reports include all other reports concerning various aspects of MSE performance with respect to planning, design, construction, mainten ance, or repair. These reports may not be specific to adverse performance of MSE w alls, but provided valuable insight as to some of the challenges with using MSE walls in transportation facilities The following documents were reviewed and are summarized below: Stochastic Modeling of Redundancy in MSE Walls (Zevgolis and Bourdeau, 2008). Design and Procurement Challenges for MSE Structures: Options going forward (Simac and Fitzpatrick, 2010). The Geotechnical Engineers Role in Design/Construction of MSE Retaining Walls (Smith and Janacek, 2011). Evaluation of LRFD Resistance Factors and Risk for MSE Walls (Wasman McVay, Bloomquist, Harrison, and Lai, 2011) Teaching Retaining Wall Design with Case Histories (Sharma, 2011). Sustainability Ass essment of Two Alternative Earth Retaining Structures (Giri and Reddy, 2015).
64 State of the Practice of MSE Wall Design for Highway Structures (Anderson, Gladstone, and Sankey, 2012). Reliability of MSE Internal Stability Modelling Zevgolis and Bourdeau (2008) used stochastic modeling to assess the reliability of MSE walls internal stability based on the theory that MSE walls are inherently redundant (e.g., if a reinforcement element fails, the remaining elements of the wall assumes additional responsibi lities from the failed piece in terms of loads ) The modeling method presented evaluates both the reliability of an element (e.g., layer of reinforcement) and the reliability of a layer given another layer has already failed which has been omitted from pr evious MSE structure modeling efforts This paper also includes Zevgolis and Bourdeau (2008) modeling case example of a six meter MSE wall supporting a bridge abutment. Assuming the top layer of reinforcement to be the most critical, probabilities of failure corresponding to different states of failure were reported in a transition probability matrix. The model results indicated that probabilities of failure propagation exist for three different states of failure, with pullout mechanism be ing the highes t risk of failure. Assuming that the modeling with respect to internal stability, accounting for the redundant nature of loads that is inherent of this type of structure by propagating loads from one state of failure to another using transition probabili ties Owner Controlled Aspects In Simac and Fitzpatrick (2010), the authors address how owners (e.g., private, commercial, or government) can best contract the work to facilitate successful MSE
65 structures. This paper states that owner controlled aspects of the project influence both cost and performance and include the following: Contracting options; Integration/coordination with various site/civil, geotechnical and the MSE designers; Compatibility with geotechnical site investigation, testing, analysis and recommendations to MSE design methods, procedures, and guidelines; and Responsibility/liability for global stability. The contracting mechanism the owner selects is the first step towards overall success of the project. Rationally, the owners contracting selection criteria is strongly influenced by cost. However, a more advantageous cost may be associated with the most aggressive contractor supplied design and design assumptions, which inherently lead to a higher risk. To minimize these risks, an owner sh ould define requirements and conditions (i.e., design must meet AASHTO or NCMA structure design standards). The second step the owner can take to establish a successful project is to coordinate activities, responsibilities, and communication between the design professionals. As Simac and Fitzpatrick (2010) state, This coordination role is difficult because inte gration of the MSE system into the overall site design requires effective communication between three overlapping engineering disciplines, i.e., the site (civil) engineer, geotechnical engineer and MSE designer. The critical coordination is between MSE des igner and geotechnical engineer, who need to work together, but clearly understand their division of responsibilities to achieve a successful project and outcome for the public. Understanding each professionals responsibilities should clarify the owners coordination role for each contracting method.
66 The third step the owner can do to establish a successful project is to enable the MSE designer to be included in the geotechnical investigations to ensure adequate data is collected to design the MSE structur e. Investigation data collected include geologic and hydrogeologic conditions in the MSE construction area. Compatibility of the geotechnical site investigation, testing, analysis leads to successful recommendations in the MSE design The final step discussed in Simac and Fitzpatrick (2010) that the owner can perform to ensure a successful MSE Wall design is to reduce confusion over the responsibility of the global stability. The issue with global stability has been a major problem among failed MSE walls o n who is responsible the site (civil) engineer, geotechnical engineer, and/or MSE designer This confusion can be reduced through the owners identification of responsibility, which is dependent on the type of contract selected (e.g., a designbuild contr act is best coupled with the global stability responsibility resting on the MSE designer while a design bid build contract may be best coupled with this responsibility resting on the geotechnical engineer In summary, the paper by Simac and Fitzpatrick (2010) was written to guide owners to improve the likelihood of good performance of MSE structures. The authors emphasize contracting mechanisms, good communication, integration of the MSE engineer with the geotechnical investigations, and identifying responsibility of global stability. The authors also state that, At this stage of the market maturation process, the authors conclude that o wner provided designs appear to be the best way to improve both cost and quality of the finished product and should receive strong consideration.
67 Collaboration and Systematic Approach to Design and Construction In response to unsatisfactory performance of certain MSE walls, Smith and Janacek (2011) reviewed the design and construction practices used in t hese walls The specific focus of this paper was to address the role of the geotechnical engineer in the design and construction of MSE walls. The authors state that because of the nature of MSE wall design (i.e., small, specialized firms that use proprie tary design and construction procedures and techniques) ; industry wide understanding of design and implementation is limited. Smith and Janacek (2011) state that based on several forensic assessments, a lack of involvement from the geotechnical engineer from design through construction contributes to poor retaining wall performance. Additionally, poor performance of MSE walls is attributed to lack of communication between responsible parties, undefined roles of responsibility of MSE wall structures, use o f generalized site conditions used in designs rather than site specific soil conditions, among other issues. Additionally, this paper discusses the importance of geotechnical data and design into the project plans. Shear strength parameters, bearing layers foundation improvements and retained zone improvements must be clearly detailed in the typical cross sections, detailed cross sections, and the wall plan and profile sheets. Another important step in the design process is to have a joint review and reso lution of the final plans between the geotechnical engineer and the MSE wall engineer. The goal of this review is to identify potential conflicts that may adversely affect construction issues and long term performance of the wall. This review should consider items that may adversely impact the MSE walls stability including, but not limited to: utilities, construction
68 sequencing, grades within the influence zone, backfill volumes, drainage, undrained areas, consistency between plans, and incorporation of design aspects that may have been updated during design improvements in draft plans. Smit h and Janacek (2011) also discuss the importance of the geotechnical engineers role during construction. Construction oversight should include a pre construction meeting, material testing, and construction observations. These components assist in the proper implementation of the project plans. In summary, Smith and Janacek (2011) reports that the geotechnical engineer must take a proactive and substantial role in the design and construction of MSE walls for an improved performance. The geotechnical enginee r should also be responsible for the external stability design, while the wall system fabricator or engineer under contract to the contactor should be responsible for the internal stability design. Risk of Failure Probability Wasman, et al. (2011) present s a risk assessment that designers can use to select an acceptable risk, in terms of lives or cost and address or adjust the design associated with a specific component of a design. Current AASHTO LRFD codes are intended to be a reliability based approach but values were derived from ASD Factors of Safety and not on variability of soil design parameters (friction angle, unit weight, cohesion, etc.) The study investigates the influence of soil property variability with respect to bearing and sliding stabilit y, and includes a risk assessment of two typical MSE overpass scenarios. This risk assessment use s a probabilistic approach to determine the probability of failure in terms of consequence and allows designers and owners to identify where the risk
69 is pres ent and where to focus on bolstering the design. The analysis consisted of modeling soil unit weight and friction angle as lognormal with established mean and coefficient of variation values ( e.g., economic consequence value). The risk analysis process p resented in Wasman, et al. (2011) was used on two typical overpass MSE wall sections. The results identified the highest risk was bearing capacity when the foundation soil were characterized with low friction angle for both sites. In summary, Wasman, et al. (2011) notes that during the design stage, designers can use this risk assessment to evaluate the probability of failure and select acceptable risk in terms of dollars or lives. Teaching Retaining Wall Design Sharmas (2011) paper presents implementatio n of wall failure case histories within a class setting while teaching undergraduate civil engineering students. The presented project allows the students to determine probably cause of failure, design a new wall using a concrete cantilever approach, suggest alternatives using MSE or SRW designs, and identify problems due to time and cost constraints. Sharma (2011) states students have benefit ed from evaluating a case history to gain a greater understanding of lateral earth pressures for geotechnical w all designs Sustainability Assessment of Cantilever Retaining Wall v s. MSE Wall Giri and Reddy (2015) present the life cycle study of two alternate earth retaining structures (cantilever retaining wall and MSE wall) based on environmental, economic, and s ocial impacts. The environmental impact considers material acquisition, construct ion, maintenance, and demolition e fforts Economic impact considers direct costs incurred from
70 materials, transportation, and labor during construction. Social impacts are no t well quantified, but consider items such as employment, health and safety, aesthetics, cultural aspects, and quality of life. The modelled designs for both the cantilever and MSE walls use similar parameters (geometry, backfill soils, etc.) and assume a safe and reliable equivalent technical design. T his study concluded that the MSE wall is more sustainable than a cantilever retaining wall over the life cycle. It was observed that not only was the MSE wall more sustainable overall, it was more sustainable for each of the three categories analyzed (environmental, economic, and social) with the most noticeable variance being the environmental impacts of respiratory inorganics and fossil fuels associated with concrete and steel production. MSE State of the Pr actice Internal Design Review Anderson, Gladstone, and Sankey ( 2012) presents the difficulty of providing a reliable MSE wall design because design method s have become more abundant, more diverse, and more complex in nature. This paper was written to provide the state of the practice, basic principles of MSE wall designs and present reasons why a different design method should be used for each reinforcement material. The authors recognize that MSE wall designers must fully understand reinforcement behavior and use the appropriate design method t o obtain the structural performance and service life required The paper presents the various design methods used (coherent gravity, simplified, etc.) and explained that one must understand reinforcement prop erties and behavior between inextensible and extensible reinforcements. The authors also point out that designers are confronted with differences between FHWA and AASHTO guidance,
71 transitions between U.S. customary units and metric units, as well as a change from ASD to LRFD design platforms This evaluation s uggests that the Coherent Gravity Method should be used for MSE wall designs with inextensible reinforcements (e.g., steel) and the Simplified Method should be used for extensible reinforcements (e.g., geosynthetic). The paper also recommend s solely using AASHTO specifications for MSE wall design, and suggests that FHWA courses should teach material consis tent with AASHTO specification to mi nimize confusion and complexity General Remarks Based on Lite rature Review Compilation studies reviewed are valuable to confirm the seriousness of issues affecting G MSE wall performance. These reports provide insight into repetitive technical factors such as backfill selection and the influence of water. Many of t he compilation studies also develop conclusions considering intangible factors such as contacting methods, communication, and experience on G MSE wall performance. Literature review of the use of geosynthetics in MSE walls and various relevant MSE wall general repo rts provided valuable insight into considerations for G MSE wall selection, design, construction and possible deformations Of the case studies evaluated the authors generally tended to focus on the physical parameters (subsurface conditions, fines content of fills, groundwater, etc.) that lead to the wall failure with little discussion of design condition s except for acknowledging that a more detailed evaluati on would have been warranted. Not a single case history documented
72 restrictions placed on the design effort, such as budget, time constraints, access, etc. Nor were there any mention of coordination with owners, partners, sub contractors, etc. As in the case of Haramy et al. (2010), the amount of debris during runoff events was mis ev aluated that lead to plugging of the 36 inch culvert that is now constructed as a bridge. However, no mention was made as to the effort exerted to evaluate drainage/debris conditions during design or the coordination between hydraulic and geotechnical eng ineers. I n the case of Dodson (2010), it was acknowledged in hindsight that select backfill should have been used, differential settlements evaluated, and surface water controlled; however, the paper failed to include design constraints as lessons learned The paper only alluded to the pressure s to produce a project within initial scope is what lead to the improper selection of backfill or that other constraints and/ or a systemic design mentality lead to the improper evaluation of surface water flows inadequate quality assurance lead to mistakes in the plans, and /or improper evaluation and early detection of construction concerns coul d have prevented wall distress. In the case history by Hossain, et al. (2012), the authors were quick to investigate an d point out high fines content backfill and areas of heightened soil moisture as the leading causes of excessive deformation, as see in the article title Effects of Backfill Soil on Excessive Movement of MSE Wall. Only secondarily did the authors acknowl edge that the reinforcement length of about 30% of the wall height or the storm sewer obstruction contributed to deformation. Additionally, the backfill is generally within reasonable limits propo sed by NCMA (2010) and others. Although the article provide s precise details of investigation methods into physical wall parameters, no attempt was made to explain the
73 origin or reason for the shortened reinforcement lengths. Because the case study was a double sided embankment with sufficient space available and n o mention was made as to the determination of this reinforcement length during the investigation, leads to an assumption that this was the design reinforcement length and ultimatel y the leading cause of failure. Several case studies did identify inadequacies in the design, plans, and specifications as contributing factor s to adverse performance. Scarborough (2005) noted that designers may neglect important design considerations, possibly due to economic constraints, lack of design experience or understanding of regional materials, ability to shift liability, or lack of control during construction. In the case study presented by Scarborough (2005) t he hig h rates of problems experience caused a municipality in east Tennessee to require an independent third party technical review of the design prior to approval. In the case of Kim et al. (2010) it was recognized that a detail evaluation of deformation modes and stresses should have been performed in design; however, the author did not reveal why such a significant change was made during construction or the level of des ign effort that w as prop osed to make such change Lastly, there appears to be a lack in effort to document deformation and wall failures wall failures are plentiful but there very few case study reports This is not likely due to a lack of concern or evaluation, but more likely budgetary constraints time constraints, or legal concerns. Contingency or emergency funds used to address wall failures are typically i n short supply and shared with other threats on our transportation systems. The traveling publics expectation of open roadways leads to an urgency that
74 limits the time required to adequately investigate and evaluate failures, with the majority of the effort focused on repair alternatives. The concern over public scrutiny or risk of possible legal action also discourages practitioners from willingly present ing poor performance or providing a critical review
75 CHAPTER IV Discussion and Recommendations Th e literature review undertaken in this study has revealed seemingly countless combinations of causes of adverse MSE wall performance. Considerations identified by these reviews include observations such as geometry and wall layout, over simplified design, obstructions, seismicity, wall embedment, surface/subsurface drainage, backfill material, compaction, geotextile filters, leveling pads, durability of facing, contract type, design team communication, contractor experience, etc. In order to isolate the primary cause of adverse performance, t hese design and construction considerations are discussed systematically in the following sections. The bulk of the authors blame poor design practices or construction QA/QC as seen by the repetitive conclusions pointing to poor reinforced zone backfill placement (high fines, poor compaction, etc.) and ineffective drainage (surface drainage, erosion, inundation, etc.). However, in each instance there are underlying tones of poor communication that possibly surpass technical conside rations. For example, NCHRP 24 22 (2013) propose a seemingly equal relationship between fines content, pore pressure, and design and construc tion QA/QC as shown in Figure 4 1
76 Figure 4 1. Relationship among Fines, Water in the Reinforced Fill Zone and Design and Construction QC/QA (NCHRP, 2013). Design Considerations Design c onsiderations identified can be grouped into technical and intangible factors. Technical considerations include design methods, site characterization, and wall compon ent selection (backfill selection, facing type, etc.). Intangible factors include cont r acting methods, communication, experience, etc. The following sections focus on the primary influences with respect to adverse wall performance identified from the lite rature review. Design Methods Although FHWAmandated a transition from ASD to LRFD between 2000 and 2010, some DOTs are still working through the backlog of projects initiated before the transition. To complicate things further, designers have had to shift from U.S. customary units to metric units and then back to U.S. units. Taken together, these transitions require engineers
77 to toggle between two design platforms (ASD, LRFD), two sets of units (U.S., metric), and multiple MSE design methods (TxDOT 2014). Anderson et al. ( 2012) pres ents the argument that confusion resulting from various design methods, changes in design platforms, and inconsistent guidance has led to a difficulty of providing reliable MSE wall designs. This argument primarily revolves around conflicting internal stability methods (e.g. Coherent Gravity Method vs. Simplified Method). Although changes in guidance increase the difficulty of design, no case study of adverse performance reviewed identified internal stability design calculations, following AASHTO guidance, to be a leading cause of failure. To the contrary, several studies suggest that MSE wall design is conservative with Har pstead, et al. ( 2010) identifying this as a hindrance to the advancement in MSE wall technology and contributes to a lack in attentiveness during design. Unlike internal stability design methods, incorrect external stability evaluation appears to be a significant contributing factor to poor performance or wall failure. In the case history by Scarborough (2005), inadequate lengths of geog rid reinforcement, resulting from overlooking the global stability analysis, was concluded as the ultimate cause of failure. In the case history presented by Kim, et al. (2010), incorrect evaluation of differential settlement of a known foundation condition led to excessive tensile stress and ultimate failure of the facing panel connection. Although internal stability is essential to arriving at an appropriate MSE wall d esign, it appears that correct evaluation of external stability is more critical with respect to performance. The relative unknowns of site conditions and limitations of numerical
78 modeling likely contribute to incorrect evaluation of bearing resistance, s ettlement, hydrostatic conditions, and slope stability. Site C haracterization Through nineteen cases of MSE failures of highway related infrastructures in mountainous areas, Wu and Chou (2013) identified inadequate project planning and site exploration as a major contributor in 10 of the 19 cases. Poor site characterization failures consisted of differential settlement or bearing instability from unexpected soft stratum, unidentified slope instability, and upward seepage pressure. Wu and Chou (2013) state that the most common technical errors were from the misinterpretation of soil conditions and incorrect selection of soil strength parameters derived from insufficient boring information. Because MSE walls are considered flexible with a perception that the design is conservative, geotechnical engineers have tak en a lackadaisical approach to investigating site conditions for MSE walls. FHWA (2009) recommends that a minimum investigation consist of subsurface borings located every 100 feet along the front of the structure and every 150 feet along the back However it is common that an MSE wall be designed from a single subsurface boring where truck access permits. In an urban environment or steep terrain, this means the boring may not even be located withi n the proposed wall mass. Wall Component Selection Primary wall components specified in design include facing type, reinforcement, reinforcement zone backfill, and drainage system. The facing type, connections, and reinforcement alone were not identified as being significant contributors to wall failures. The Koerner and Koerner (2013) study of 171 failed MSE walls noted that, there are no
79 cases involving inadequate or improper manufactured geo textile or geogrid products (p.27). These components are well understood with respect to strength, can be tested, and not excessively expensive allowing for an acceptable level of conservatism in design (FHWA 2008) Reinforced zone backfill and adequate drainage systems were repetitively identif ied as the primary technical factors leading to adverse performance. Loosening the restrictions on select fills for MSE walls has been greatly debated, especially with respect to the amount of fines content ( % passing the No. 200 sieve). The intent is to significantly reduce costs and/or envir onmental impacts by using lower quality fills or reusing on site native soils. NCMA (2010) NCHRP ( 2013), and Samtani and Nowatzki (2016) suggest that MSE walls for transportation facilities can be successfully constructed using established design methods with fines content in the range of 25 % to 50% The caveats of these suggestions being appropriate fill material selection, material design parameters, compaction control, and effective drainage Wall failures identi fied in the literature review as resulting from low quality backfill did not sufficiently identify if materials used were within specification. Either way, until overall MSE wall performance improves, high fines content (>15%) and/or plastic ( plastic ity index [ PI ] > 6 ) soils should be used with caution and only when favorable conditions exist. Surface and subsurface water contribute to poor performance and must be accounted for in design. Haramy et al. (2010) presented a case history where and undersize d culvert lead to overtopping and ultimately a significant wall failure and Dodson (2010) concluded that surface water infiltration contributed to wall deformation. DiFiore and Strohman (201 3 ) identify that walls designed without consideration of hydrosta tic or
80 seepage pressure can adversely affect performance or stability of the wall. Identified conditions included ineffective wall drainage (clogging, pipping, etc.), unknown water sources identified during or after construction, and incidences with the p otential for leaking water utility pipelin es including irrigation line. GMSE wall design concepts consider the geosynthetic to act as a tieback to resist failure This approach does not take full advantage of the benefits observed when the soil geosynthet ic act as a composite, as in the case of closely spaced ( < 12 inches) reinforcement or GRS. Wu and Phams (2013) conclude that reinforcement spacing is much more significant than reinforcement strength. Barrett and Ruckman (2007) note that GRS is superior to MSE in resisting static and dynamic loads, as proven by the presented negative batter walls case studies and observations after a seismic event GRS research suggests simplified design, shorter reinforcement lengths, lower costs, and improved strength a nd stiffness performance over GMSE. Due to perceptions of increased costs it appears that e ngineer s have been reluctant to call for closer spaced reinforcement Leshchinsky and Hans (2004 ) choose to evaluate the effects of offset distance and number o f tiers in order to avoid costs associated with closer reinforcement spacing in tall walls. Barrett and Ruckman (2007) believe the use of GRS is encumbered by current AASHTO, NCMA, and FHWA guidelines, which the authors believe are artifacts from traditional, tie back theory. Wu and Phams (2013) recognize that current design equations do not properly account for relative roles of reinforcement spacing and strength in a GRS mass and have proposed an analytical model for predicting ultimate load carrying capacity.
81 Considering the high rates of failure observed, the cost savings are not worth the potential risk and GRS walls should be considered whenever GMSE is proposed, especially when boundary conditions are complex or not fully investigated. Add itionally, it appears that the adaption of GRS specific design guidance would actually offset the increased costs of closer reinforcement spacing or lead to additional cost savings Many of the GMSE wall failures reviewed may have been avoided had they bee n designed and constructed as GRS walls. Communication and Experience Although individual case studies tend to focus on technical factors such as soil type, soil compaction, and influence of water; compilation studies and general repor ts presented above ex tensively address intangible factors such as market pressure, inexperience, well defined roles and responsibilities, and poor communication leading to poor understanding as the origin of poor wall performance As Bachus and Griffin ( 201 1, pp 437) points o ut in an attempt to make the client happy, get the job, or to get a lower price, there may be an unconscious impact to overlook some aspects of design. This includes shifting certain aspects of the design to someone else where critical steps could easily be forgotten (e.g. adding notes to the plans indicating external stability is to be performed by others). In the case history of Boyle and Perkins (2007), the success of a highly complex project was attributed to good communication through co locating of an experienced design team, simplified designs, anticipation for field varia nces, and consistent interaction with the team throughout construction. Simac and Fitzpatrick (2010), states that in order to establish a successful project, the owner must coordinate activities, responsibilities, and
82 communication between the design professionals. This includes integration of the MSE wall engineer with the geotechnical investigations, compatibility of design, construction, and field conditions, and establishing r esponsibility/liability for global stability The bright side is that significantly lower risk of poor performance can be achieved with minimal commitment of resources by the owner. Construction MSE walls have long been publicized as simple, rapid, and require no special skills for construction. To navigate the multitude of wall systems, owner agencies commonly require a contractor furnished design (within some requirements set by the owner) in the form of a submittal process where contractor shop drawings are incorporated into the contract and used to build the structure. Construction c onsiderations are typically discussed in terms of QC, which is the contractors work, and QA, which is the own ers responsibility to enforce the contract, specifications, and plans. QC/QA must verify proper site preparation, correct installation of components (leveling pads, facing systems, reinforcements, drainage networks, etc.), and placement and compaction of backfill soil. According to NCHRP 290 (1987), site preparation QC/QA is necessary to verify agreement between actual and specified base elevations, conformance of encountered soils with parameters considered in design, the absence of hard or soft foundation soils, and verify adequate drainage. Wall components need to be evaluated for conformance with the construction drawings, have been properly transported and stored, free of visible damage, and are properly installed (NCHRP 290, 1987). Backfill type,
83 placement, and compaction must be within limits and tolerances identified in the specifications. The following sections discuss common or suspected defi ciencies in the submittal process, QC, and QA that contribute to a dverse wall performance identified fr om the literature review. Submittals The submittal required by owner agencies typically includes some description of contractor experience, material certifications, shop drawings, and supplemental special provisions. The submittal review is intended to ve rify that the contactor is properly trained and qualified to construct MSE walls, proposed materials meet specifications, stability and performance checks are complete and accurate, and design assumptions are appropriate for site conditions based on site c haracterization and external stability design conducted by the owners geotechnical engineer. Discussions on the process of submittal reviews were strangely absent during the literature review but nonetheless imp o r tant as the overla p from the design phase to construction. Because years may have passed between original design and construction, or availability of staff, the reviewer may not have been involved in the original design. Couple this with minimal review time periods typically less than two weeks as dictated in the special provisions, leaves little time for the reviewer to develop a thorough understanding of proposed materials, verify the proposed design, and evaluate contractor experience Bachus and Griffin (2011) termed t his wishful thinking that materials are probably acceptable and concepts generally adapted. In addition, owners commonly disregard the
84 importance and fail to budget for geotechnical engineers to conduct these reviews and relay on QA construction staff w ith little to no knowledge of the original intent or design assumptions. The limited time frame and lack of allocated resources discourages team interaction and even more surprisingly has resulted in a careless or uninformed submittal reviews. The case h istory provided by Dodson ( 2010) illustrated a mistake in the project plans that allowed up to 45 % fines in the backfill; the mistake was missed again during the submittal review signifying a lack in continuity from the design phase to construction. Qual ity Control and Quality Assura n ce (QC /QA ) Again, the perception that MSE walls are simple, rapid, and require no special skills has lead contractor and owners to not require full time QC/QA, trained staff, or pay attention to details outside of checking co mpaction However, as Barrett and Ruckman (2007) point out, it appears that MSE walls poorly tolerate construction errors Harpstead, et al. (2010) summarized the need for QC/QA as follows : The major lesson learned with regard to construction is the need for both the contractors quality control (QC) and the owners quality assurance (QA) in the field. Often the design and construction team confuse the roles and in many projects the authors find one or both activitie s being excluded from the construction process. The disconnect between the project design professionals and the wall engineer can also lead to the misunderstanding of QA and QC of key components of the MSE wall design, specif ically soil strength parameter ( p. 482). The importance of QA/QC in both the design, as well as the field, cannot be underestimated. During design, competent designers shall perform the modeling needed of GMSE structures and preparation of the drawings and specifications. Additionally the
85 design shall be reviewed for details of calculation and assumptions used and an overview review of practicality of design and constructability of design. Time and time again, professional papers (Boyle and Perkin 2007; Koerner and Koerner, 20 1 2 ; Va lentine, 2013 ) contribute poor GMSE wall performance with not verifying design assumptions (foundation materials, subsurface drainage, etc.) and construction not following the design parameters, typically with use of poor quality backfill and inappropriate geosynthetic protection, layout, connection, and spacing. Construction QA/QC plan and properly trained inspectors is imperative to successful performance of the GMSE wall. Discussion on Risk Because d esigns with very low probability of failure are very costly, a practical level of risk should be determined. But with an extensive list of causes of failures, how is the engineer able to communicate the probability of a dverse performance? The following discussion identifies the risk associated with each GMSE wall parameter so that the reader can better communicate design and construction considera tions associated with GMSE wall performance Design Risk s that Effect Performance Risks evalu ated in design consist primarily of choosing appropriate design strength parameters, long term aesthetics and defining the design life of the GMSE wall. The challenge with evaluating these risks is that they relate back to tolerable deformation, which is different for each wall component and not always easily determined (e.g soil shear strength). Therefore, s trength parameters should consider the consequence of failure
86 which is a project specific consideration relying heavily on experience of the desig ner, not a theoretical approach Based on the high failure rates observed for GMSE walls, optimistic strength values do not appear to be worth the economic savings. During the design process, assumptions need to be made concerning the aesthetics and estimating the design life from inevitable deterioration. Although guidance documents provide some estimate of design life by following limits of specific components (e.g. backfill electrochemical property limits), actual data on specific wall system durabil ity is nonexistent, requiring the engineer to forecast resiliency with limited technical knowledge. Based on the wall system or materials selected, maintenance protocols may need to be implemented to assure the assumed design life is realized. This will require a commitment from the owner agency of qualified personnel and budget. This commitment should be communicated and obtained during the wall selection process. Construction Risks that Effect Performance Potential c onstructability problems should be identified during design and continually evaluated and monitored throughout construction. Some of these problems relate to selection or availability of materials adequate construction space, appropriate traffic control, site conditions, weather, equipme nt, and experience of contractors and inspectors Underperformance of an individual wall component may lead to combined deformations resulting in serious problems. Performance risks associate with poor constructability primarily include increased deformat ions and poor drainage. For example, the use of fine grain soils during wet weather may lead to poor compaction control, resulting in increased vertical and horizontal deformations, generating adverse site grades
87 causing drainage issues, leading to furthe r deformations that overstress the facing connections resulting in costly repairs To alleviate constructability risks, the owner agency must commit qualified personnel and budget to train or vet contractors and inspectors capable of executing the plans and specifications.
88 CHAPTER V C ONCLUDING REMARKS G MSE walls are used frequently in transportation infrastructure as a standard earth retaining structure because of their relatively simple construction and low cost; however, the serviceability failure rates of G MSE walls are up to 10 % Consistent with the low cost of GMSE wall construction, geotechnical design of GMS E walls tend to have a small budget and may not be adequate to fully characterize the earth conditions (e.g., soil type, drainage, etc.). Additionally, the evaluations of GMSE failures are rarely evaluated through forensic studies. When evaluated, the com mon cause for failure attributes improper construction as the root cause of the failure. This thesis discusses the importance of performing an appropriate level of effort in the design stage for GMSE walls that will reduce the percentage of future failure s. Understanding the best use of wall components and correctly identifying the design criteria will limit failure. A significant cost savings can be made by bolstering design and reducing the GMSE wall serviceability failures or even collapses. The use of G MSE w alls is a beneficial retaining structure because of their ease of construction and cost effectiveness. Because of the high rates of failure, the cost savings are not worth the potential risk from inadequate designs ; as such there have even been morat oriums on building any MSE Walls (Scarborough, 2005). This thesis presents an argument that many of the issues observed in failed GMSE Walls can be alleviated through proper design.
89 The primary lessons learned include: Communication: T he owner must take re sponsibility to establish a successful project, coordinate activities, responsibilities, and communication between the design professionals. This includes integration of the GMSE wall engineer with the geotechnical investigations, compatibility of design, construction, and field conditions Site Characterization: T he owner is proposing to build the structure in the specified location and it is the owner's responsibility to investigate the feasibility of the proposed improvement, including the adequacy of the foundation soils to support the proposed structure. (Anderson et al ., 2012) Frequent technical errors from the misinterpretation of soil conditions and incorrect selection of soil strength parameters were derived from insufficient boring information. Geo professionals must take a more proactive role in evaluating site conditions as a whole, including maintaining responsibility for external design and taking a larger role in surface/ external drainage provisions. The relative unknowns of site conditions and limitations of numerical modeling likely contribute to incorrect evaluation of bearing resistance, settlement, hydrostatic conditions, and slope stability. Geosynthetic Reinforcement: GMSE wall design concepts consider the geosynthetic to act as a tieb ack. This approach does not take full advantage of the benefits observed when the soil geosynthetic act as a composite, as in the case of closely spaced ( < 12 inches) reinforcement or GRS. GRS research suggests improved strength and stiffness performance over GMSE. Many of the GMSE wall failures
90 reviewed may have been avoided had they been designed and constructed as GRS walls. Wall Backfill: Reinforced zone backfill and adequate drainage systems were repetitively identified as the primary technical fac tors leading to adverse performance. Until overall GMSE wall performance improves, high fines content (>15%) and/or plastic (PI > 6) soils should be used with caution and only when favorable conditions exist. Submittals: Submittal reviews are an important o verlap from the design phase to construction. Minimal review time periods leaves little time for the reviewer to develop a thorough understanding of proposed materials, verify the proposed design, and evaluate contractor experience. In addition, owners co mmonly disregard the importance and fail to budget for geotechnical engineers to conduct these reviews and relay on QA construction staff with little to no knowledge of the original intent or design assumptions. The limited time frame and lack of allocated resources discourages team interaction, and even more surprisingly has resulted in a careless or uninformed submittal reviews. QA/QC: T he owner agency must commit qualified personnel and budget to train or vet contractors and inspectors capable of execu ting the plans and specifications. Implement a consistent and clear line of communication between the design engineer through construction to ensure design assumptions are consistent with actual conditions and the design is being correctly followed and ade quate field testing is being performed.
91 In summary, GMSE w alls are a valuable resource to prov ide a cost effective means for grade separation needs and should continue to be used with proper design and construction. Although backfill and drainage manageme nt may dominate GMSE wall failures in the private sector insufficient site characterization and/or the lack of coordinated activities between the design professionals are leading causes of GMSE wall failures in transportation facilities. The good news is that thorough risk based discussions of the level of effort required; significantly lower risk of poor performance can be achieved with minimal commitment of resources by the owner.
92 REFERENCES American Association of State Highway and Transportation Officials ( AASHTO ) ( 2002). Standard Specifications for Highway Bridges 17th Edition American Association of State Highway and Transportation Officials, Washington, DC. AASHTO (1998/2002). LRFD Bridge Construction Specifications, 2rd Edition with 2002 Interim Revisions American Association of State Highway and Transportation Officials, Washington, DC. AASHTO (2014). AASHTO LRFD Bridge Design Specifications, 7th Edition with 2016 Interims. American Association of State Highway and Transportation Officials, Washington, DC. Anderson, P.L., R.A. Gladstone, and J.L. Withiam (2010). Coherent Gravity: The Correct Design Method for Steel Reinforced MSE Walls Proceedings of ER2010 Earth Retention Conference 3, ASCE, Bellevue, Washington Anderson, P.L., Gladsto ne, R.A., and Sankey, J.E. ( 2012), State of the Practice of MSE Wall Design for Highw ay Structures Geotechnical Engineering State of the Art and Practice: Keynote Lectures from GeoCongress 2012, 443 463. Bachus, R. C. and L. M. Griffin (2011). A Perspect ive on Mechanically Stabilities Earth Walls Pushing the Limits or Pulling Us Down. 2010 Earth Retention Conference (ER2010). Barrett, R. K. and A. C. Ruckman (2007). GRS A New Era in Reinforced Soil Technology. Geo Denver 2007: New Peaks in Geotechnics. Geosynthetics in Reinforcement and Hydraulic Applications. Boyle and Perkins (2007). Case History of Two Mechanically Stabilized Earth Walls on Steep Slopes. Geo Denver 2007: New Peaks in Geotechnics. ASCE. Christopher, Leshchinsky, and Stulgis (2005). G eosynthetic Reinforced Soil Walls and Slopes: US Perspective. ASCE 2005. GSP 141 International Perspectives on Soil Reinforcement Applications. Clemente, J. L. M., K. A. Lamonte, J. R. Davie, and M. R. Lewis (2016). Risk Evaluation and Mitigation for MSE Walls Perspectives from an EPC/EPCM Contractor. Geosynthetical and Structural Engineering Congress. ASCE. 2016. DiFiore, S. J. and B. P. Strohman (2013). Look Out Below!!! Potential Pitfalls and Suggested Improveme nts in Design and Construction of MSE Walls. Forensic Engineering 2012. ASCE 2013. Dodson, M. (2010). Lessons Learned from Settlement of Three Highway Embankment MSE Walls. 2010 Earth Retention Conference. ER2010.
93 Federal Highway Administration (FHWA) (200 0). Memorandum Load and Resistance Factor Design (LRFD), David H. Densmore, Director of Bridge Technology, Federal Highway Administration, U.S. Department of Transportation, Washington, DC. June 28, 2000 FHWA (2001). Mechanically Stabilized Earth Walls an d Rei nforced Soil Slopes, Design and Construction Guidelines Report No. FHWA NHI 00 043, Authors: Elias, V, Christopher, B.R. and Berg, R.R., Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA (200 7 ). LRFD for Highway Bridge Substructures and Earth Retaining Structures Reference Manual Report No. FHWA NHI 05094, Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA (2008). Geosynthetic Design and Construction Gui delines, Report No. FHWA NHI 07092, Authors: Holtz, R. D., Christopher, B. R., and Berg, R. R., Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA (2009). Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Report No. FHWA NHI 09083, Authors: Berg, R. R., Christopher, B. R. and Samtani, N. C., Federal Highway Administration, U.S. Department of Transportation, Washington, DC. FHWA (2009a). Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Soil Slopes FHWA NHI 09087, Authors, Elias, V., Fishman, K. L., Christopher, B. R. and Berg, R. R., Federal Highway Administration, U.S. Department of Transportation, Washington, DC. Giri, R. K. and K. R. Reddy (2015 ). Sustainability Assessment of Two Alternative Earth Retaining Structures. International Foundations Congress and Equipment Expo ASCE. 2015. Haramy, K., S. A. Anderson, and D. E. Alzamora. ( 2010) Heeding Natures Call: Replacing MSE Wall with a Bridge. ASCE 2010 Earth Retention Conference (ER2010). Harpstead, D. L., J. M. Schmidt, and B. R. Christopher (2010). Applying Lessons Learned in the Past 20 Years of MSE Wall Design & Construction. ASCE. 2010 Earth Retention Conference (ER2010). Hossain, M. S., G ., Kibria, M. S. Khan, J. Hossain, and T. Taufiq. (2012). Effects of Backfill Soil on Excessive Movement of MSE Wall. Journal of Performance of Constructed Facilities. ASCE. November/December 2012. Kim, D., S.K. Bhowmik, and J.L. Willmer (2010). A Case History of MSE Wall Failure: Finite Element Modeling and Evaluation. GeoFlorida 2010: Advances in Analysis, Modeling & Design. ASCE.
94 Koerner, R. M. and G. R. Koerner (2012). Need for and Justification of Quality Management Systems for Successful Geosynthe tic Performance. GSI White Paper #26. Geosynthetic Institute. October 2012. Koerner, R. M. and G. R. Koerner (2013). A data base, statistics and recommendations regarding 171 failed geosynthetic reinforced mechanically stabilized earth (MSE) walls. Geotext iles and Geomembranes 40 (2013) 2027. Leshchinsky D. and J. Han (2004). Geosynthetic Reinforced Multitiered Walls Journal of Geotechnical and Geoenvironmental Engineering. ASCE. December 2004. NCHRP 290 (1987). "National Cooperative Highway Research Pr ogram Report 290, Reinforcement of Earth Slopes and Embankments," Transportation Research Board, Washington, D.C. NCHRP 2422 (201 3 ). S electing Backfill Materials for MSE Walls, Final Report for National Cooperative Highway Research Project 24 22. Authors: Marr, W. A. and. Stulgis, R. P., Transportation Research Board of the National Academies, Washington, DC, 353 pp. NCMA (2010). Design Manual for Segmental Retaining Walls, 3rd Edition, National Concrete Masonry Association, Herndon, Virginia. NCMA (2016). Segmental Retaining Walls Best Practices Guide, National Concrete Masonry Association, Herndon, Virginia. Reddy, D. V. and F. Navarrete (2008). Experimental and Analytical Investigation of Geogrid MSE Walls. ASCE, from Research to Practice in Geotechnical Engineering Congress. 2008. Samtani, N.C. and E.A. Nowatzki, 2016 Draft. Mechanically Stabilized Earth (MS E) Wall Fills, A Framework for U se of Locally Available Sustainable Recourses (LASR). Central Federal Lands Highway Division, Federal Highway Administration, Lakewood, CO. Scarborough, J. A. (2005). A Tale of Two Walls: Case Histories of Failed MSE Walls. Proceedings of Sessions of the Geo Frontiers 2005 Congress, ASCE. 2005. Sharma, S. (2011). Teaching Retaining Wall Design with Case Histories. Proce edings of Sessions of the Geo Frontiers 2011 Congress ASCE. 2011. Simac, M. R. and B. J. Fizpatrick (2010). Design and Procurement Challenges for MSE Structures: Options going forward. ASCE 2010 Earth Retention Conference (ER2010). Smith, D. L. and N. W. Janacek (2011). The Geotechnical Engineers Role in Design/Constr uction of MSE Retaining Walls. Geo Frontiers ASCE. 2011.
95 Sun and Graves (2013). Evaluation of Mechanically Stabilized Earth (MSE) Walls for Bridge Ends in Kentucky; What Next? Kentucky Transp ortation Cabinet in cooperation with the U.S. Department of Transportation, Federal Highway Administration. Report Number KTC 13 11/SP443 12 1F. July 2013. Texas Department of Transportation ( TxDOT ) (2014). Design Parameters and Methodology for Mechanically Stabilized Earth (MSE) Walls. Texas Department of Transportation Research and Technology Implementation Office in cooperation with the Texas Department of Transportation and the Federal Highway Administrat ion. Report Number FHWA/TX14/06716 1. October 2014 Valentine ( 2013). An Assessment of the Factors that Contribute to the Poor Performance of Geosynthetic Reinforced Earth Retaining Wall. Proc., Design and Practice of Geosynthetic Reinforced Soil Struct ures, H. Ling, et al., DEStech, Lancaster, PA, 318 327. VanBuskirk, C. (2010). Adoption and implementation of GRS design concepts A Consultants Perspective, 19th Vancouver Geotechnical Society Symposium, Geosynthetic Reinforced Walls, Slopes, and Eart hworks, May 2010. Wasman S., M. McVay, D. Bloomquist, M. Harrison, and P. Lai (2011). Evaluation of LRFD Resistance Factors and Risk for MSE Walls. GeoRisk, ASCE. Washington State Department of Transportation ( WSDOT ) (2001). Development of the Simplified Method for Internal Stability Design of Mechanically Stabilized Earth Walls. Authors: Allen, T., Christopher, B., Elias, V., DeMaggio, J., Report Number WA RD 513.1, Washington State Department of Transportation in cooperation with the U.S. Department of Transportation, Federal Highway Administration, July 2001. Wu, J. Y. and N. N. Chou (2013). Forensic Studies of Geosynthetic Reinforced Structure Failures. Journal of Performance of Constructed Facilities. ASCE. September/October. Wu, J. T. H. and T. Q. Ph am (2013). Load Carrying Capacity and Required Reinforcement Strength of Closely Spaced Soil Geosynthetic Composites. Journal of Geotechnical and Geoenvironmental Engineering ASCE. September. Zevgolis, I. E. and P. L. Bourdeau (2008). Stochastic Modeling of Redundancy in MSE Walls. GeoCongress ASCE.