DESIGN RECOMMENDATIONS FOR ROCKFALL FENCE POST FOUNDATIONS BASED ON FULL SCALE TESTING by R OBERT D OUGLAS G ROUP B. A ., Hamilton College 200 6 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Science Civil Engineering 2015
ii This thesis for the Master of Science degree by Robert Douglas Group has been approved for the Civil Engineering Program by Brian Brady, Chair Nien Yin Chang Aziz Khan April 2 4 2015
iii Group, Robert Douglas (M.S., Civil Engineering) Design Recommendati ons for Rockfall Fence Post F oundations Based on Full Scale Testing Thesis directed by Professor Nien Yin Chang ABSTRACT This manuscript presents the analyses of full scale field testing of flexible rockfall barrier post foundations under horizontal loading. While significant research has been conducted and published regarding design and analysis of the net, post, and cable components of flexible rockfall barrier systems, little research has been conducted or published regarding the design of anchors and foundations to support these structures and transfer impact loads to the ground. To better understand past efforts at designing these summaries, review of past design g uides, case studies on rockfall barrier impacts, and past full scale barrier testing efforts that focused specifically on foundations. These chapters are followed by an overview of full scale foundation testing efforts that the author helped to coordinate with a group of geotechnical engineers in Colorado. Full scale testing provides the most accurate and representative data sets for developing design criteria for future systems. The latter chapters include a discussion of design considerations for post fou ndations of rockfall barriers based on the results of the testing, as well as design recommendations for specific barrier applications. The results presented in this research document a quantitative assessment of loading conditions experienced by rockfall barrier post foundations, as well as qualitative performance assessment of the various foundation types. The form and content of this abstract are approved I recommend its publication. Approved: Nien Yin Chang
iv TABLE OF CONTENTS CHAPTER 1. I n troduction ................................ ................................ ................................ ................. 1 1.1 Background ................................ ................................ ................................ .......... 1 1.2 Previous Research by the Author ................................ ................................ ......... 3 1.3 O bjectives of Current Research ................................ ................................ ............ 3 2. Literature Review ................................ ................................ ................................ ........ 5 2.1 Introduction ................................ ................................ ................................ .......... 5 2.2 Design Method and State of Practice Summaries ................................ ................ 6 2.2.1 R. Turner Duffy, and J. Turner (2009) ................................ ........................ 6 2.2.2 Kane and Shevlin (2012) ................................ ................................ .............. 7 2.2.3 Brunet, Giacchetti, and Grimod (2013) ................................ ........................ 9 2.2.4 Stelzer and Bichler (2013) ................................ ................................ ............ 9 2.3 Barrier Impact Case Studies ................................ ................................ ............... 10 2.4 Numerical Modeling ................................ ................................ .......................... 11 2.4.1 Cazzani, Mongiovi, and Frenez (2002) ................................ ...................... 11 2.4.2 Gentilini, Govoni, de Miranda, Gottardi, and Ubertini (2012) .................. 11 2.5 Full Scale Rockfall Barrier Testing ................................ ................................ ... 12 2.5.1 Muraishi, Samizo, and Sugiyama (2005) ................................ ................... 12 2.5.2 Buzzi, Spad ari, Giacomi ni, Fityus, and Sloan (2012) ................................ 14 3. Rockfall Characterization and Mitigation Theory ................................ .................... 15
v 3.1 Causes of Rockfall ................................ ................................ ............................. 15 3.2 Rockfall Characterization ................................ ................................ ................... 17 3.2.1 Rockfall Rating Systems ................................ ................................ ............ 17 3.2.2 Individual Site Characterization ................................ ................................ 22 3.3 Rockfall Mitigation Techniques ................................ ................................ ......... 27 3.3.1 Avoidance Measures ................................ ................................ .................. 28 3.3. 1.1 Realignment or Relocation ................................ ................................ 28 188.8.131.52 Tunnels ................................ ................................ ............................... 28 184.108.40.206 Elevated Structures ................................ ................................ ............. 28 3 .3.2 Stabilization Measures ................................ ................................ ................ 29 220.127.116.11 Rock Scaling ................................ ................................ ....................... 29 18.104.22.168 Trim Blasting ................................ ................................ ...................... 30 22.214.171.124 Rock Dowels ................................ ................................ ....................... 30 126.96.36.199 Rock Bolts ................................ ................................ .......................... 31 188.8.131.52 Shotcrete ................................ ................................ ............................. 31 3.3.3 Protection Measures ................................ ................................ ................... 32 184.108.40.206 Catchment Ditches ................................ ................................ .............. 32 220.127.116.11 Draped Netting ................................ ................................ .................... 35 18.104.22.168 Rigid Barriers ................................ ................................ ...................... 37 22.214.171.124 Rock Sheds ................................ ................................ ......................... 38
vi 3. 3.3.5 Flexible Barriers ................................ ................................ ................. 39 3.4 Flexible Rockfall Barrier Design and Evaluation ................................ .............. 39 3.4.1 Flexibl e Rockfall Barrier Components ................................ ....................... 39 3.4.2 Barrier Performance Assesment ................................ ................................ 42 126.96.36.199 Full scale Performance Tests ................................ .............................. 42 3 .4.2.2 ETAG 027 Guideline ................................ ................................ .......... 43 3.4.3 Flexible Barrier Design ................................ ................................ .............. 48 3.5 Flex ible Barrier Post Foundations ................................ ................................ ...... 49 3.5.1 Common Post Foundation Types ................................ ............................... 49 3.5.2 Post Foundation Loading ................................ ................................ ............ 50 3.5.3 C urrent Post Foundation Design ................................ ................................ 52 3.5.4 Direct Post Impacts ................................ ................................ ..................... 54 4. Full scale Test Data Collection and Results ................................ ............................. 56 4 .1 Full scale Test Site Setup ................................ ................................ ................... 56 4.2 Post base System Testing Configurations ................................ .......................... 59 4.3 Test Results ................................ ................................ ................................ ........ 69 4.4 Descriptive Summary of Test Results ................................ .............................. 76 4.4.1 A Series ................................ ................................ ................................ .... 76 4.4.2 B Series ................................ ................................ ................................ ...... 76 4.4.3 C Series ................................ ................................ ................................ ...... 77
vii 4. 4.4 D Series ................................ ................................ ................................ ...... 79 4.4.5 E Series ................................ ................................ ................................ ....... 80 4.4.6 F Series ................................ ................................ ................................ ....... 81 4.4.7 G Series ................................ ................................ ................................ ...... 82 4.4.8 Eval uating Loading and Deflection ................................ ............................ 83 4 .4.9 Study Limitations ................................ ................................ ....................... 84 5. Design Considerations and Recommendations ................................ ......................... 85 5.1 Introduction ................................ ................................ ................................ ........ 88 5.2 Design Considerations ................................ ................................ ........................ 85 5.2.1 System Flexibili ty vs. Foundation Flexibility ................................ ............ 85 5.2.2 System Strength ................................ ................................ .......................... 86 5.2.3 Constructability ................................ ................................ .......................... 88 5.2.4 System Maintenance ................................ ................................ ................... 89 5.3 Design Recommendations ................................ ................................ .................. 91 6. S ummary ................................ ................................ ................................ ................... 94 7. C onclusions ................................ ................................ ................................ ............... 98 R eferences ................................ ................................ ................................ ................... 100 Appendix A. Summary of Mitigation Measures ................................ ................................ .......... 103 B. Load Cell Test Data ................................ ................................ ................................ 106
1 1. I ntroduction 1.1 Background Rockfall hazards are common throughout mountainous areas, creating safety risks that affect various types of infrastructure including highways, railways, and other structures. Flexible rockfall barrier systems are commonly installed to protect against these safety risk s caused by falling rock. While significant research has been conducted and publish ed regarding the net, post, cable and anchor components of flexible rockfall barrier systems, a review of literature reveals that little research has been conducted or published regarding the design, construction or performance of barrier post foundation systems. Many current practitioners believe that the primary purpose of the supporting posts i n a flexible rockfall barrier is to maintain the height of the net panel components Additionally, with the development of more flexible systems, support posts a nd their associated foundations are increasingly viewed as a temporary and replaceable component of the overall barrier system and as such, have not been assessed as a critical element in the successful performance of the system (Kane and Shevlin, 2012) While these concepts are technic ally correct research of full scale impact testing on rockfall barriers and attenuator systems during rockfall events revealed that when fence posts remained standing for even an additional several seconds the likelihood of rockfall reaching the protecte d structure was greatly diminished (Arndt, Ortiz, and Turner, 2009). Additionally, the treatment of posts and foundations as e xpendable or replaceable
2 components is not in line with the expectations of many owners, who often anticipate years of successful performance out of barrier system. This resiliency benefit and expectation of performance longevity should be accounted for in the successful design of a barrier post foundation system. In addition to performance concerns the lack of published literature on the design of barrier post foundations has generated uncertainty among many engineers With the lack of a recognized standard or guideline, many engineers are left to refer to established building codes, which were not designed with flexible rockfall b arriers in mind Kane and Shevlin 2012). This has often led to overly conservative and robust systems, which are difficult and expensive to both construct and maintain, and typically do not provide any performance benefit to the system. Due to the complex interaction of system components, as well as the dynamic nature of rockfall events, dynamic full scale testing provides the most reliable results for determination of barrier post foundation behavior. While full scale testing of flexible rockfall barriers is commonplace (Stelzer and Bichler 2013), the results of these tests have not been widely considered or reported on with respect to post foundation design. In addition, the standard full scale test for rockfall barriers is somewhat limited in that it only considers impacts to the center of a net panel and not impacts to support posts.
3 1.2 Previous Research by the Author Based on full scale testing of direct impacts on rockfall barrier posts (Arndt, Ortiz, and Group, 2014) published observations, results, and summaries of maximum loading conditions and general performance of several different types of post foundation systems with various uphill retaining anchor systems The full scale tests included twenty nine post and post foundation direct impact tests t hat were conducted for energies up to 220 kJ. efforts, observing and documenting test results, data compilation and reduction, and review of written work prior to publishing. T he full publication can be reviewed for summaries on the general performance of various post foundation systems, as well as considerations. 1.3 Objectives of Current Research This work is written to satisfy a number of objectives that build on the research co nducted by (Arndt, Ortiz, and Group, 2014). The first objective is to review the available literature regarding full scale testing, behavior, and design of rockfall post foundations under loading conditions resulting from rockfall events that impact net panels as well as support posts The second is to analyze the data from (Arndt, Ortiz, and Group, 2014) utilizing a different framework than that used by the original authors. The new fram ework will account for displacement in addition to maximum load to relate the performance of post foundations to deflection within the foundation itself. The final objective is to use results from the data analysis and test observations to develop a set of
4 design recommendations for flexible rockfall barri er post found ations. The design recommendations will account for a variety of factors including system deflection, maximum loading, and system resiliency
5 2. Literature Review 2.1 Introduction This literature review is concerned with the design, construction, and performance of support post foundations on flexible rockfall barriers. Rockfall is a natural hazard that is affected by a variety of external factors such as atmospheric conditions, changes in slope conditions, and number of bloc ks involved in a single rockfall event, all of which can vary significantly even within an individual site. Due to the uncertainty produced by these varying conditions, an emphasis is placed on full scale testing results and case studies of field deployed barriers impacted by natural events representing a range of site conditions and impact energies. This coupled approach allows a quantitative assessment of the forces applied to post foundations, as well as a qualitative performance based assessment of the behavior of post foundations under varying site conditions and loading conditions. The following is broken up into four sections. The first section reviews efforts to summarize the current state of practice with regard to flexible rockfall barrier post foundation design. Some of these resources also offer suggestions on design methods for post foundations but do not provide explicit guidelines or recommendations The second section reviews case studies of flexible rockfall barriers and the effects on pos t foundations following impacts by rockfall events The third section reviews the manner in which numerical models for flexible rockfall barriers treat post foundation elements.
6 The fourth section reviews past full scale rockfall barrier testing efforts wi th respect to post foundation performance. 2.2 Design Method and State of Practice Summaries 2.2.1 R. Turner, Duffy, and J. Turner (2009) R. Turner, Duffy and J. Tur ner summarize the California Department of Transportation rockfall barrier post foundation design and construction practices, discuss foundation loading conditions during rockfall impacts, and present a design analysis for post foundation design in unconsolidated materials. These authors were some of the first to bring attention to the lack of literat ure and guidance on the design of post foundations for flexible rockfall barriers. They provide an overview of common foundation types in three categories of geomaterials: soil, rock overlain by a thin veneer of soil, and exposed bedrock. Foundation constr uction within the California DOT typically consists of either 0.5 m 2 reinforced concrete footings, or drilled and grouted anchor bars with a thin concrete leveling pad. To demonstrate that the 0.5 m 2 reinforced concrete footings are appropriate for a rang e of rockfall impact energies, the authors compare lateral and bearing resistance developed within the footings to axial and shear load data generated during a performance test of a 1000 kJ rockfall barrier. The analysis applies only to barriers with uphil l tie back anchors. It is implied that bearing resistance is either assumed or determined using a
7 standard bearing capacity equation. Lateral resistance is determined from sliding resistance along the base of the footing and passive soil resistance in fron t of the footing. Figure 2.1 depicts a free body diagram of forces applied to a barrier post foundation during a rock impact on a barrier net panel. The authors do not assess transfer of energy within bar anchor systems or direct impacts to posts within a system. Figure 2.1: Free body diagram of forces on post foundation by R. Turner, Duffy, and J. Turner (2009). 2.2.2 Kane and Shevlin (2012) Kane and Shevlin review the current standard of design practice, providing a summary, design considerations, and relative merits for available barrier foundation types; including reinforced and unreinforced concrete blocks, drilled shafts, and micropiles. For concrete block foundations, the authors describe the same method of the 0.5 m 2 block provided by (R. Turner, Duffy, and J. Turner 2009). They state that the design is appropriate for low energy barriers but not sufficient for high energy barriers due to limited frictional and passive resistance provided by the small foundation footprint and
8 the insufficient resis tance to anchor bolt breakout as recommended by the American Concrete Institute (2008). For high energy barriers, the authors recommend drilled shaft foundations designed using L PILE software with foundation loading data provided by barrier manufacturers and a specified pile displacement of 1 to 1.3 inches. Finally, the authors report that micropile foundations may be used as an alternative to concrete blocks or larger deep foundations. Figure 2.2 depicts a common micropile foundation for a barrier constru cted without uphill tieback anchors. The authors report that design issues arise with micropile foundations due to shear loading, as well as the interaction between concrete caps and anchors under moment loading conditions. Figure 2.2: Diagram from rockfall barrier manufacturer for micropile post foundation on soil pr ovided by Kane and Shevlin (2012 )
9 2.2.3 Brunet, Giacchetti, and Grimod (2013) Brunet, Giacchetti, and Grimod discuss rockfall barrier behavior and design as it relates to the Guideline for European Technical Approval of Falling Rock Protection Kits, commonly referred to as ETAG 027. ETAG 027 has become the main guideline, test, and construction framework used within the flexible rockfall barrier manufacturing industry. The authors indicate s everal significant limitations of the ETAG 027 guidelines; specifically that it does not address post foundation design, it does not consider direct impacts to support posts, and it does not account for impacts by multiple rocks during a single event. Whil e the authors point out these limitations, they do not seek to address them; rather they indicate further research and individual designer attention is necessary on the subject of foundation design and direct post impacts. 2.2.4 Stelzer and Bichler (2013) Stelz er and Bichler concur with Brunet, Giacchetti, and Grimod (2013) regarding the limitations of the ETAG 027 guidelines. They present a summary of a more comprehensive technical document that seeks to address areas in which the ETAG 027 guidelines fall short protection against rockfall Terms and definitions, effects of actions, design, monitoring specifies minimum constru ction standards for post foundations, as well as minimum design verification requirements. Design verification is separated into two components; an effect side and a resistance side. The effect side specifies the test load results that a
10 designer must acco unt for in a foundation design. The resistance side requires verification of two components; the cross section of steel reinforcing elements and bonding surface between anchor grout and underground. These design verifications appear to lead designers towar d micropile type foundations and may preclude the use of drilled shaft or poured concrete foundations. 2.3 Barrier Impact Case Studies Giacchetti Brunet, and Grimod (2014) present the only available case study of rockfall barrier impacts focusing specifical ly on the effects to the post foundation. The authors examine a range of foundation types from very large reinforced concrete plinths to foundation plates bearing directly on soil with light grouted bar anchor systems. Interestingly, several barriers with unusually light post anchor systems performed extremely well under impact, with the barriers exceeding their nominal impact energy capacity rating. The authors attribute this increase in energy capacity to the ability of the light post foundations to defle ct along with the system more readily than a very stiff foundation. Light post foundation systems did experience more significant settlement than large, stiff foundations but not to a degree that impacts the performance of the barrier. According to Europe an testing guidelines, a barrier that maintains 70% of its height when impacted is still considered effective. Light foundation systems on barriers that are 4 m high can settle multiple cm and still be considered functional. The authors even suggest renami ng
11 concerned with significant post base displacement either into the ground or down the slope. 2.4 Numerical Modeling 2.4.1 Cazzani, Mongiovi, and Frenez (2002) Cazzani, Mongiovi, and Frenez use ABAQUS/Explicit software to model the dynamic impact and large displacements associated with typical rockfall barrier performance. Support posts are modeled as three dimensional two noded beam elements. Posts are attached to a base plate by a hinged connection, so that they are free to rotate during impact. Similar to many other models, post foundations are treated as perfectly rigid. Direct post impacts are not modeled. 2.4.2 Gentilini, Govoni, de Miranda, Gottardi, and Ubertini (2012) The author s present a three dimensional non linear dynamic numerical model for design and verification of flexible rockfall barriers. The focus of this review is the manner in which the model treats anchorage components and the support structure. The authors employe d ABAQUS/Explicit to construct the model based on its ability to solve high speed dynamic events. The model is designed assuming a rock impact in the center of a net panel in the barrier.
12 Post impacts are not modeled. To determine anchorage forces, post b ases are connected by nodes to a supporting structure. The nodes do not allow for failure at the post bases. The authors report good agreement between experimental load cell data and model data for loads at post foundation anchorages, although it is not cl ear if these loads are shear loads, axial loads, or some combination of the two. Foundation and post failure does not appear to be modeled explicitly. 2.5 Full Scale Rockfall Barrier Testing 2.5.1 Muraishi, Samizo, and Sugiyama (2005) The authors examined the perf ormance of a relatively low energy rockfall barrier (less than 100 kJ) through full scale field testing conducted by rolling rocks down a controlled chute into barriers. Separately, they also performed static load tests on four different post foundation ty pes in four different soil conditions for a total of sixteen foundations tests. Static load tests were performed on foundations consisting of two shapes of structural steel, a small unreinforced concrete block, and a combination of structural steel and co ncrete block. The foundations were placed in different soil depths compacted to different densities and loaded using a hydraulic cylinder placed at the base of the post. The unreinforced concrete block foundation was selected for the full scale barrier tes t based on adequate performance and ease of construction. Figure 2.3 depicts the different types of foundations tested, while Figure 2.4 indicates the results of the foundation test.
13 The authors tested barriers on a variety of ground conditions with a var iety of foundation types to determine the most appropriate foundation type that is deployable in a wide variety of ground conditions, including both talus cones and solid bedrock. The authors found a relationship between ground conditions and barrier perfo rmance, namely that foundations installed in softer ground conditions led to poor barrier performance. Figure 2.3: Foundation types tested by Muraishi, Samizo, and Sugiyama (2005) Figure 2.4: Foundation test results produced by Muraishi, Samizo, and Su giyama (2005)
14 2.5.2 Buzzi, Spadari, Giacomini, Fityus, and Sloan (2012) The authors performed full scale testing on several different rockfall barriers rated for impact energies of up to 35 kJ. Their research focused on the overall stiffness of the systems and its influence on the transmission of load to other components of the barrier such as posts and cables. Overall, the authors determined that a decrease in the stiffness k impacts. They discuss the effect of stiffness on a micro scale, meso scale, and macro scale. Micro scale stiffness refers to the ductility of steel wire that comprises the mesh itself. Meso scale stiffness refers to deformability resulting from the weave pattern of specific mesh types. Macro scale refers to the interaction of different system components, specifically along the boundary conditions. These components can include flexible posts, braking elements, or other types of energy dissipating devices.
15 3. R ockfall Characterization and Mitigation Theory 3.1 Causes of Rockfall Rockfall is generally defined as the gravitationally driven rapid downslope movement of detached bedrock material (Turner and Schuster 2012). Th is downslope movement may occu r through some combination of free falling, bouncin g, sliding, or rolling. Rockfall is typically considered independently from l arger slope movements such as landslides and rotational failures (Wyllie and Mah 2004). The mechanics governing the development of rockfall are distinct from those that lead to global slope instability Additionally, the impacts associated with rockfall are typically less significant on a per event basis and therefore, rockfall is treated as a condition that may be managed, rather than as an inherent failure of the slope itself. Many efforts have been made to characterize and catalogue the causes of rockfall events. The state of California conducted a comprehensive study of rockfall events along California highways in an effort to assess both the causes of rockfalls and the effectiveness of various mitigation methods (McCauley et al., 1985). Due to the variety in rock type, climate, and topography found throughout California, the study provides useful broad guidance that should tra nslate well to other regions. The study identified 14 different event triggers based on 308 different failure events. Table 3. 1 indicates the various rockfall causes identified by the study. Of these causes, 68% are related to the effects of water on or wi thin the slope and 17% are related to geologic conditions,
16 indicating that water and geology have the most significant influence on rock slope stability of any other factors (Wyllie and Mah 2004). While the study only attributes a single cause to each even t, in reality it is likely that a combination of these two significant factors accounts for the majority of slope instability. Water works into and through geologic structure over time and along with changes in temperature, facilitate s the weathering proc ess destabilizing portions of the slope. Table 3.1: Causes of Rockfalls on California Highways ( adapted from Wyllie and Mah 2004) Rockfall Cause Percentage Rain 30 Freeze thaw 21 Fractured rock 12 Wind 12 Snowmelt 8 Channeled runoff 7 Adverse planar fracture 5 Burrowing animals 2 Differential erosion 1 Tree roots 0.6 Springs or seeps 0.6 Wild animals 0.3 Truck vibrations 0.3 Soil decomposition 0.3
17 3.2 Rockfall Characterization Rockfall site characterization may fall into one of two categories: classification relative to other rockfall sites in an established system or identification and assessment of individual site conditions (Turner and Schuster 2012). Agencies that are tasked with management of a large number of rockfall sites, su ch as transportation departments, railways, or municipalities in mountainous areas, often rely on the relative classification of rockfall sites as a means of prioritizing mitigation efforts. This type of assessment is typically referred to as a rockfall ra ting system and can be an effective means of evaluating the general scope of rockfall hazard present throughout the system. Individual site assessments are typically more detailed and provide useful information to be used in determining appropriate slope m itigation techniques. 3.2.1 Rockfall Rating Systems When faced with a large number of rockfall sites, t he first step in a successful rockfall mitigation and management plan is identification, inventory, and rating of rockfall slopes. Identification is a logical first step in that organizations must at least be aware of the extent of problem areas. Hazard ratings, which are based on visual inspections and simple calculations as part of a standardized rating system are adopted as a means of providing a relative r anking of potentially hazardous slopes. The slope inventory and rating then provides a framework through which an organization can plan and implement mitigation in a logical and consistent manner. Rockfall rating systems come with a number of benefits incl uding awareness of the extent of potential slope hazards, efficienc y in
18 response to unstable slope activity education of the public or stakeholders, more favorable perception, and reduction of liability (Pierson and Van Vickle 1993). The general Rockfall Hazard Rating System (RHRS) as detailed by Pierson and Van Vickle (1993) can be developed through the following six steps: Slope Inventory Creating a geographic database of rockfall locations using a uniform method Preliminary Hazard Rating Grouping the rockfall sites into three broad categories such as A, B, and C slopes Detailed Hazard Rating Prioritizing the identified rockfall sites from the least to the most hazardous. Preliminary Design and Cost Estimate Adding mitigation information to the rockfall database at least for the highest rated sites. Project Identification and Development Advancing rockfall correction projects toward construction. Routine Review and Update Maintaining the rockfall database on a regular basis. Creation of a sl ope inventory is the first logical step in development of the RHRS. Rockfall problems over broader areas such as national or state transportation and rail networks are often extensive enough that no single individual has a detailed understanding of the ext ent and location of all problem areas. The slope inventory helps
19 to provide a sense of scope and is the foundation for a database management system for slope mitigation. Slope inventories are typically created most effectively with the assistance of person nel who regularly observe or work in an area such as highway maintenance workers. Because rockfall activity is not always immediately apparent, the firsthand information provided by these individuals is valuable in identifying and documenting typical rockf all behavior (Pierson and Van Vickle 1993). The preliminary hazard rating is an optional step that functions to prioritize the detailed rating process. The preliminary rating is often combined with the slope inventory process, during which each slope is g iven a subjective rating by experienced rockfall personnel. The rating categories are broad, such as high, medium, and low or A, B, and C. This process functions to separate the sites into more manageable categories, which improves the efficiency of the de tailed rating process by preventing resources from being directed toward sites with low rockfall potential (Pierson and Van Vickle 1993). The detailed rating is what comes to mind when most individuals envision a rockfall hazard rating system. The purpose of the detailed rating is to quantify the risks posed by rockfall hazards at the sites identified during the inventory so that sites can be scored, ranked, and prioritized for possible mitigat ion work The original RHRS proposed by ( Pierson and Van Vickle 1993) contains 9 categories by which slopes are evaluated and scored: slope height, ditch effectiveness, average vehicle risk (AVR), percentage of decision sight distance, roadway width including paved shoulders, geologic character
20 (including structural c ondition, rock friction, and difference i n erosion rates), block size/ event volume, and climate/presence of water on slope. Each of these categories is evaluated with a score ranging from 1 to 100. The scores are selected based on a combination of professi onal judgment, measurement, and the use of scoring aids such as charts, graphs, and formulas. The original RHRS scoring sheet contains four columns with logical breaks describing the range of conditions that can be found on most rock slopes (F igure 3. 1). E ach column has a set score to help novice raters develop more accurate judgment and scorin g skills, although any score may be used in the range from 1 to 100. As the risk increases from left to right, the scores increase exponentially. This rapid increase in score helps to identify the truly high hazard sites (Pierson and Van Vickle 1993). Various agencies have developed their own modified versions of the RHRS, based on the same principles as the original. Figure 3. 2 shows a similar scoring sheet developed by the Colorado Department of Transportation for their Modified Colorado Rockfall Hazard Rating System. This system contains much of the same data as the RHRS but also accounts for freeze thaw cycles, slope aspect, more specific structural characteristics and vehicle accident history.
21 Figure 3.1: Rockfall Hazard Rating System Scoring Sheet (Pierson and Van Vickle 1993) Figure 3.2: Modified Rockfall Hazard Rating System Scoring Sheet (Image courtesy of R. Group, Colorado Department of Transportation)
22 3.2.2 Individual Site Characterization Once a site has been selected for slope mitigation, a more detailed individual site characterization must be performed. An accurate site characterization coupled with an appropriate mitigation design will ideally reduce the long term rockfall hazard (10 to 20 years) while requiring limited maintenance. Comprehensive site characterization efforts will gather information on terrain characteristics, geological and geotechnical properties of earth materials, rock mass structure, groundwater conditions, and past and present geological processes (Turner and Schuster 2012). This information is then used to evaluate the rockfall potential at the site, including identification of the rockfall source zone, failure mode, block size, tra vel path, and runout zone. As part of the site characterization of rock slopes with respect to rockfall, evaluators should identify the rockfall failure mode. Common rockfall failure modes include planar failure, wedge failure, block failure, key block fa ilure, toppling failure, buckling failure, raveling failure, boulder fall, and differential erosion (FHWA 1994). Figure 3 .3 depicts illustration of several common rockfall failure modes. Mitigation methods corresponding to these failure modes will be discu ssed in a subsequent section. Planar failures occur when rock s slide along existing discontinuities Typically slopes that experience planar failure have been undercut, causing the discontinuities to daylight. Wedge failures occur as planar sliding along t he intersection of two discontinuities that form a wedge shape. Again, failure typically occurs where slopes have been cut steeply, exposing steep discontinuities to daylight. Block failure occurs most often in rock with multiple sets of
23 closely spaced dis continuities, as individual blocks separate along intersecting discontinuities and weak layers then fall from a cut face. Key block failures occur in similar rock types to block failure, except that one or several key rock blocks support numerous rock feat ures above. Failure of the key block or blocks release the entire support ed volume of rock. Toppling failures occur where closely spaced parallel discontinuities dip steeply into the slope, such as bedded sedimentary rocks. As the shear strength of the roc k is exceeded perpendicular to the discontinuities, sections of rock break free from the slope. Buckling failure occurs where closely spaced parallel discontinuities dip steeply away from the slope at such an angle that the friction angle along the bedding planes is exceeded. Again the shear strength of the rock is exceeded perpendicular to the discontinuities, causing the rock to bow out and rupture. Raveling failure occurs in highly fractured rock with multiple sets of closely spaced discontinuities orien ted in different directions. Rocks separate and fall from the slope along the intersecting discontinuities. Raveling failures d iffer from block failures in that raveling is a more regular and continuous process than block failure. Boulder falls occur in co lluvial slopes where larger boulders are supported by a matrix of finer rock and soil material. As the supporting matrix erodes away, the boulders are free to fall or roll out of the slope. Boulder falls are also referred to as block in matrix failures. Di fferentia l erosion occurs in interbedded sedimentary rocks where the different rock units display different weathering rates. As the weaker layers erode away, the more competent layers are left unsupported. When the tensile strength of the unsupported laye rs is exceeded, the rock fractures and falls from the face. Rock faces consisting of interbedded sandstone and
24 shale are one of the most common sources of differential erosional failure. Figure 3.3: Common Rockfall Failure Modes (FHWA 1994) At a minimum site characterization should address the rockfall history of the site; identify the source zone(s), travel path, runout zone, and the geologic properties of each; assess the likely failure mode and associated motion of a rock traveling downslope from the source zone(s); identify expected average and maximum block size; and estimate rollout distance at the base of the slope (Turner and Schuster 2012). Many of these attributes, such as rockfall source zone, failure mode, and block size can be identified by
25 employing preliminary visual assessments followed by a detailed mapping program such as line mapping, cell mapping, or fracture set mapping. Either mapping method is adequate, but the slope investigator should use the preliminary visual assessment to deter mine the type most appropriate for the slope situation (FHWA 1994). The travel path can be identified through visual identification of chutes and pathways or through the use of a three dimensional modeling software such as CRSP 3D, which is available throu gh the FHWA website. Once the rockfall source zone, failure mode, block size, and travel path have been identified and measured, rockfall simulation modeling is employed to estimate the anticipated rockfall trajectory, energy, launch height, and runout distance. Contrary to rockfall stability modeling, rockfall simulation programs assume that a rock has already program s are Color ado Rockfall Simulation Program ( CRSP ) RocFall (produced by RocScience) and CRSP 3D (three dimensional version of Colorado Rockf all Simulation Program). Figure 3. 4 and Figure 3. 5 show screen shots of the CRSP and CRSP 3D programs respectively. Each program allows the user to specify slope geometry and other parameters such as slope hardness and some form of slope friction. When calibrated properly, their results are sufficiently accurate to use as a basis for the specification and design of rockfall mitigation structures (Turner and Schuster 2012). Eac h program also allows the user to specify a rock size and number of simulations, so a broad variety of conditions may be tested. U ser s are able to run simulations until they achieve a
26 reasonably consistent result or until the simulations match a known rock fall site history Rockfall velocity and energy values are typically used to specify design impact energies for rockfall barriers and weight of mesh panels, while launch heights or bounce heights help designers to determine appropriate heights for barriers Runout data is helpful in the design of ditch slope and width. Figure 3.4: CRSP Simulation Result Showing Launch Height and Runout Distance (Image courtesy of R. Group, Colorado Department of Transportation)
27 Figure 3.5: CRSP 3D Simulation Result Showing Rockfall Velocity Distribution (Image courtesy of R. Group, Colorado Department of Transportation) 3.3 Rockfall Mitigation Techniques After a rockfall hazard slope has been identified and characterized, a rockfall mitigation method may be selected, de signed, and constructed. Nearly every engineered rockfall mitigation measure can be classified in one of three broad categories: avoidance, stabilization, or protection (FHWA 1994). Avoidance does not necessarily reduce the rockfall hazard of a particular slope; rather it relocates a road or facility away from the hazard. A common consideration among all avoidance options is that they are typically expensive. Stabilization reduces rockfall hazards by either securing loose rock in place or removing it from t he slope altogether. Protection measures are intended to control loose rock that initiates from a slope before it can cause damage to a facility such as a highway
28 or building. Brief details of common mitigation types within each category are described bel ow. Appendix A provides a comprehensive set of tables listing different mitigation types along with brief descriptions of their purpose. 3.3.1 Avoidance Measures 188.8.131.52 Realignment or Relocation This method involves moving the road or structure horizontally away from the area affected by the hazard. At a minimum, the road or structure must be moved past the impact and runout zone. This method is often used where larger slope instability issues eliminate tunnels or elevated structures as a feasible option. 184.108.40.206 Tunnels Tun nels avoid rockfall by passing underground rather than below long sections of rock slope. Additional rockfall mitigation measures are often required to minimize rockfall hazards at the tunnel portals and within the tunnel itself. 220.127.116.11 Elevated Structures E levated structures avoid rockfall by allowing rocks to pass beneath the structures. Structures are typically elevated through the use of bridges or retaining walls. These structures allow for more flexibility in roadway alignment and can span areas of cont inuous rockfall.
29 3.3.2 Stabilization Measures 18.104.22.168 Rock Scaling Rock scaling is perhaps the most common method of rockfall mitigation and is included to some extent on most rockfall mitigation projects. Scaling consists of removing loose rock and debris from the sl ope using hand tools, inflatable air bags, expanding concrete, or large mechanical equipment. Many scaling efforts require workers to access the slope using ropes or lifting equipment. Figure 3.6 shows workers scaling loose rock using rope access technique s Figure 3.6: Rope Worker Scaling Loose Rock from Slope (Image courtesy of R. Group, Colorado Department of Transportation)
30 22.214.171.124 Trim Blasting Trim blasting involves the use of drilling and explosives detonation to remove loose, potentially unstable, overh anging, or potential launching rock features. Trim blasting relies on careful spacing, layout, and drilling of holes to avoid further destabilization of the slope by blast overbreak. Figure 3.7 depicts the results of blasting at two adjacent cuts. The slop e on the right was blasted using controlled trim blasting techniques. Figure 3.7: Trim Blasting (right) vs. Non Controlled Blasting Techniques (Image Courtesy of T. Ortiz, Colorado Department of Transportation) 126.96.36.199 Rock Dowels Rock dowels consist of steels bars grouted in to holes drilled into a rock block or placed throughout the slope. The dowels are not tensioned at the time of installation. Stabilizing strength is imparted through the additional shear resistance provided by the steel bar. Dowels can also be installed such that when rock blocks begin to move, the bars engage in tension and maintain the normal load on a potential failure zone to resist movement.
31 188.8.131.52 Rock Bolts Rock bolts consist of steel bars grouted in to holes d rilled in to specific rock features or spaced in a pattern throughout the rock face. Unlike dowels, rock bolts are tensioned at the time of installation, providing both additional shear resistance along discontinuities and increased normal force perpendicu lar to the direction of failure. Bolts can be anchored using chemical or mechanical means. Rock bolts provide an advantage over dowels in that no movement must take place before the anchor develops its full desig n tension capacity. Figure 3.8 shows rock bo lts installed to stabilize a large rock feature against sliding and/or toppling Figure 3.8: Rock Reinforcement Bolts (Image courtesy of R. Group, Colorado Department of Transportation) 184.108.40.206 Shotcrete S hotcrete consists of pneumatically applied concrete proj ected on to a rock surface and stabilized with welded wire fabric or reinforcing fibers. The primary application of
32 shotcrete is to prevent weathering and spalling of unstable rock surfaces, such as protecting layers of highly weather susceptible shale in a formation of interbedded shale and sandstone. Shotcrete can also be combined with rock bolts or dowels to create supporting buttresses. Figure 3.9 shows shotcrete installed as weathering and erosion control on a shale layer. Figure 3.9: Shotcrete Erosi on Control on Interbedded Shale and Sandstone Slope (Image courtesy of R. Group, Colorado Department of Transportation) 3.3.3 Protection Measures 220.127.116.11 Catchment Ditches Catchment ditches are the most common and one of the earliest developed forms of rockf all mitigation. (Ritchie 1963) pioneered the development of rockfall catchment ditches through a thorough field testing program using full scale rock drops on steep
33 bedrock slopes and moderate talus slopes. Ritchie ditches are typically deeper and have ste eper sides than most ditches found along highways. Figure 3.10 shows the ditch recommendation chart that Ritchie developed based on his field testing efforts. Federal highway requirements on the slope of ditches (i.e. ditches must allow for the recovery of a vehicle) have prevented most highway departments from adopting Ritchie ditches on a wide scale. To develop ditch recommendations that meet stricter highway standards, the Oregon DOT (Pierson et al. 2001) performed a series of full scale rock drop tests on bedrock slopes and ditches with back slopes of 4H:1V, 6H:1V, and horizontal, resulting in a series of ditch recommendation tables for various slope heights and angles. Figure 3.11 provides an example of an Oregon ditch recommendation chart. In order for ditches to remain effective, they must be cleaned of debris periodically.
34 Figure 3.10: Ditch Catchment C hart Created by ( Ritchie 1963 )
35 Figure 3.11: Example of Oregon DOT D itch R ecommendation C hart (Pierson et al., 2001) 18.104.22.168 Draped Netting Draped steel netting mitigates rockfall risk by either holding rock in place (heavy netting on shallow slopes) or directing rock into a catchment ditch (heavy or light netting on steep slopes). Netting may consist of lightweight wire mesh, woven cable net (heavy or light depending on cable diameter), or high tensile strength wire mesh. Because the majority of draped mesh systems allow rock to travel down the slope between the mesh and the rock face, they require an adequate ditch catchment area. However, th is feature also decreases on slope maintenance needs as the systems are self cleaning. Mesh can be attached directly to the slope brow to limit erosion or suspended by posts to catch additional rocks falling from above. Figure 3.12 shows a sta ndard draped mesh system while F igure 3. 1 3 shows a suspended draped cable net system.
36 Figure 3.12: Draped Cable Net System (Image courtesy of R. Group, Colorado Department of Transportation) Figure 3.13: Suspended Cable Net System (Image courtesy of T. Ortiz, Co lorado Department of Transportation)
37 22.214.171.124 Rigid Barriers Rigid barriers function by being sufficiently rigid or stiff to withstand the kinetic energy of rockfall impacts. Stiffness in rigid barriers is gained principally through material strength and mass (Tur ner and Schuster 2012). As such, they must be much larger than flexible barriers or are rated for far less energy capacity. Common types of rigid barrier s include earthen berms, concrete barrier (Jersey barrier), and structural walls such as MSE walls. Eff ectively, rigid barriers can be treated as an increase in ditch catchment capacity. As such, material must be cleaned from behind the barrier to maintain their effectiveness. Figure 3. 14 shows an MSE wall constructed to mitigate rockfall. Figure 3.14: M SE Rockfall Barrier Wall (Image courtesy of R. Group, Colorado Department of Transportation)
38 126.96.36.199 Rock Sheds Rock sheds provide the most significant risk reduction of any rockfall mitigation protection device. They are used most frequently beneath steep slope s with frequent rockfall (Turner and Schuster 2012). Construction typically consist s of a reinforced concrete roof structure attached to the ground on the uphill side of the highway and supported by columns on the downhill side. Most designs incorporate en ergy dissipating material such as sand or structural foam on the roof to protect it from large impacts. When properly designed, rock sheds may withstand a high number of impacts with little to no maintenance needed. In addition, rockfall trajectory does no t need to be as well understood because the structure covers the entire protected area. Despite these benefits, rock sheds are significantly more expensive to construct compared to other mitigation types and are therefore not commonly implemented. Figure 3 15 shows an image of a high energy capacity rock shed prior to testing in Japan. Figure 3.15: Rock Shed at Test Site in Japan (Image Courtesy of Protec Engineering Inc.)
39 188.8.131.52 Flexible Barriers Flexible barriers, commonly referred to as rockfall fences, consist of steel netting of various construction suspended from bearing cables or wire ropes supported by steel posts anchored to foundations or competent bedrock. When a rock impacts a flexible barrier, the barrier stops the rock through deformation of th e barrier system components. Flexible barrier design and evaluation, and barrier post foundations will be discussed more fully in Section 3.4 and Section 3.5 respectively. 3.4 Flexible Rockfall Barrier Design and Evaluation 3.4.1 Flexible Rockfall Barrier Components Flexible rockfall barriers consist of four main components: the net panel, supporting post assemblies, energy dissipation components, and supporting cables (Turner et al. 2009). Net panels may consist of interlocking ring nets, cable nets, cable ring nets, or woven/twisted wire mesh. Supporting cables consist of upper and lower bearing wire ropes, as well as lateral, uphill, and sometimes downhill tiebacks consisting of wire ropes. Cables are attached to the earth using ground anchors. Net panels are attached to supporting cables suspended on the post assemblies. Flexible rockfall barriers dissipate rockfall impact energy through multiple actions. First, the net panel itself is flexible and will elongate or deform upon impact. Simultaneously, ener gy dissipation devices known as braking elements, which are attached to the supporting cables and tie back elements, will elongate in a controlled manner and dissipate energy either by friction, deformation, or a combination of the two. Figure 3.16 shows t hree types of braking elements ; a friction
40 brake, a deformation brake, and a combination friction/deformation brake. As more deformation capacity is incorporated into the system through more flexible panel materials and additional braking elements, the ene rgy absorption capacity is increased and the fence can s top larger rocks traveling at higher velocities Post assemblies are supported on post foundations anchored to the ground through a variety of different methods discussed in Section 3.5, as well as by upslope and lateral tie back wire ropes connected to ground anchors. The primary purpose of the post assemblies is to maintain the height of the net panel system to intercept falling rocks (Turner et al. 2009). Posts and foundations are typically not view ed as energy dissipating components within the barrier system. Figure 3.17 depicts a generic flexible rockfall barrier system schematic. Figure 3.18 shows a completed flexible rockfall barrier installation on a rock slope. Figure 3.16: Common Braking Element Types: Friction (left), Deformation (center) and Combination Friction/Deformation (right) (Images courtesy of R. Group, Colorado Department of Transportation)
41 Figure 3.17: Generic flexible rock fall barrier schematic (Turner et al. 2009)
42 Figure 3.18: Completed Field Installation of Flexible Rockfall Barrier (Image courtesy of R. Group, Colorado Department of Transportation) 3.4.2 Barrier Performance Assessment 184.108.40.206 Full scale Performance Tests Historically, the performance of flexible rockfall barriers h as been evaluated using full scale testing of prototype barriers (Gentilini et al. 2012). Even barriers that are designed using accurate three dimensional analytical models have their performance verified through rigorous full scale testing. Many manufactu rers operate their own testing facilities (Brunet et al. 2013) in addition to testing facilities operated by governing bodies using published governmental standard procedures (EOTA 2008). These standard
43 procedures are intended to ensure reproducibility of the tests for barriers of varying design strengths produced by multiple manufacturers (Peila and Ronco, 2009). F lexible rockfall barriers are typically rated for a specific range of rock impact energy capacities based on full scale testing programs. In ad dition to impact energy the manufacturer typically provides information on other parameters from test results such as lateral (downhill) deflection, anchor loading energy, and residual barrier height following impact Current testing standards in use thro ughout the world include the NCHRP 2003 Recommended Test Procedures in the United States (Higgins, 2003) and the Guideline for European Technical Approval of Falling Rock Protection Kits or ETAG 027 in Europe (EOTA 2008). Because the ETAG 027 guideline has become the accepted standard among the major barrier manufacturers and the majority of government agencies including those in the U.S., it will be discussed exclusively in this writing with respect to barrier testing, rating and certification practices. 220.127.116.11 ETAG 027 Guideline ETAG 027 was developed in the European Union (E.U.) as a standardized rockfall testing guideline and was put into practice in 2008 (Peila and Ronco, 2009) European and most U.S. manufacturers of rockfall fences are now certifying their products in acco rdance with the E.U. guideline. ETAG 027 tests are conducted by a governing body an advantage in that direct comparisons can be made between many sys tems from many
44 different manufacturers. (Peila and Ronco, 2009) describe the test conditions for ETAG 027 barrier evaluations. The tests are carried out on barrier systems consisting of three net panels and four support posts. The test is conducted by ac celerating a test block into the barrier using either an inclined cable system installed on a slope or a vertical drop system installed on a rock wall. The block impact occurs only in the center of the central net panel, meaning block impacts with posts a nd retaining cables are not evaluated. This is an important consideration, as the center of the central net panel is the strongest portion of the barrier in terms of energy absorbing capacity. Figure 3.19 depicts a profile view and Figure 3.20 depicts a n u phill looking view of the barrier test setup. Measurements obtained during testing include impact energy based on block mass and velocity, barrier elongation, residual barrier height, and various loading data on barrier components. One of the biggest advan tages of ETAG 027 testing is that it requires load measurements at post foundations, lateral anchors, and upslope anchors; loads which can be used by knowledgeable design engineers to guide foundation and anchorage design when data are collected and report ed properly.
45 Figure 3.19: Side View of Typical ETAG 027 Test Setup (EOTA, 2008) Figure 3.20: Uphill looking View of Typical ETAG 027 Test Setup (EOTA, 2008)
46 Based on ETAG 027 criteria, barriers receive ratings for two different impact energy classifications: Maximum Energy Level (MEL) and Service Energy Level (S EL). Table 3.2 indicates the range of SEL and MEL test values for ETAG 027 certifications. To obtain a certification for MEL, the barrier must successfully retain a test block for one t est. Residual barrier height following a MEL test is reported in three ca tegories: A, B, and C. Table 3.3 indicates the residual height categories for MEL tests as a function of nominal (original) barrier height. Figure 3.21 depicts residual barrier height measurement following testing. It is important to note that for a passing MEL test result, there is no minimum residual height requirement; only a reporting requirement for the residual height category. SEL tests are carried out at impact energies equal t o one third of MEL impact energies. To obtain a certification for SEL, the barrier must retain a test block for two tests. The residual barrier height following the first test must be 70% of the nominal barrier height. The residual barrier height following the second test is not specified; that is to say there is no minimum residual height requirement for the second SEL test. Table 3.2 : ETAG 027 SEL and MEL Classifications (EOTA, 2008) Energy level classification 0 1 2 3 4 5 6 7 8 SEL (kJ) 85 170 330 500 660 1,000 1,500 >1,500 100 250 500 1,000 1,500 2,000 3,000 4,500 >4,500
47 Table 3.3 : Residual Height Categories for ETAG 027 MEL Test (EOTA, 2008) Category Residual Height A B Between 30 and 50% of nominal height C Figure 3.21: Residual Barrier Height Following Testing (EOTA, 2008) While ETAG 027 has many advantages, it also has some distinct disadvantages (Arndt et al. 2014). Although foundation loads are acquired during testing, the guidelines do not provide any sort of evaluation or rating of post foundations. These details are left up to the designer who, in the absence of any official guidance, may resort to other sources such as general building codes for foundation s These codes are inadequate for assessing the short term dynamic loads experienced during rockfall events and may lead to the
48 design of overly large foundations (Kane and Shevlin, 2012). Additionally, the loading conditions at the testing site may or may n ot be comparable to what is present at a given construction site. Most barriers are tested in foundations on rock, while many construction sites contain unconsolidated materials. These materials may behave significantly differently under similar loading co nditions. Finally, the ETAG 027 guidelines do not contain any evaluation of anticipated maintenance within a system. 3.4.3 Flexible Barrier Design Flexible rockfall barrier design relies heavily on proper rockfall site characterization coupled with accurate in terpretation of barrier evaluation test results. Designers should perform modeling and field investigations to determine anticipated rockfall pathways, energies, and launch/bounce heights. It is important to match the anticipated rockfall energy and trajec tory determined during modeling or field testing to the layout, hei ght and energy rating of a barrier Increases in energy capacity also typically correspond to increases in the lateral defl ection of the barrier upon impact, so flexible barriers should not be placed directly adjacent to a structure or highway. There are no criteria in the ETAG 027 guidelines limiting lateral deflection at either MEL or SEL impacts. The only requirement is that the rock may not contact the ground prior to maximum elongation during MEL tests. This ensures the barrier withstands the entire rock impact energy without any assistance from ground contact (Peila and Ronco, 2009).
49 As mentioned previously, designers have the benefit of ETAG 027 ratings to guide barrier selection. Ho wever, longevity and desired level of maintenance should also be considered when designing a rockfall barrier. For this analysis, the ETAG 027 ratings must be evaluated in the context of what constitutes a passing test. The guidelines permit a passing cert ification without any residual height requirement for a single impact at a MEL event and for the second impact of SEL events. Barriers with residual heights of less than 70% typically require maintenance to prevent future rockfalls from bouncing over the b arrier. Due to the steep and difficult access terrain in which b arriers are often constructed maintenance can be costly. If long term performance with low maintenance is a high priority, designers should consider installing barriers rated for higher impac t energies than those listed in ETAG 027 results. 3.5 Flexible Barrier Post Foundations 3.5.1 Common Post Foundation Types Rockfall fence post foundations can be categorized into the following types: reinforced or unreinforced cast in place concrete block, drilled shaft, and m icropile (Kane and Shevlin, 2012 ). These foundation types can be installed in the full range of geomaterials from soft colluvium and loose talus to solid, intact bedrock. Cast in place concrete block foundations are typically installed in unconsolidated materials that may be excavated by hand or with light equipment (Turner et al. 2009). When using hand excavation techniques, foundation sizes are typically limited to 1 m 3 and smaller. Drilled shaft
50 foundations have traditionally been ins talled in unconsolidated material as well, but are reserved for higher energy fence designs and/or locations that can easily be accessed by heavy equipment. Micropile foundations consist of any foundation installation that is composed of single or multiple smaller diameter holes (less than 300 mm diameter) drilled and fully grouted with a single anchor reinforcing member per hole (Stelzer and Bichler, 2013). Micropile foundations may be installed in nearly any type of geomaterial. Anchor members can range i n size from 19 mm to 150 mm in diameter with depths from 1 m up to approximately 10 m for high energy installations in loose material. 3.5.2 Post Foundation Loading Per ETAG 027 guidelines, barrier manufacturers are required to measure the loading on foundation anchor components during full scale testing. Table 3. 4 and Table 3. 5 summarize the results of this loading data for multiple barrier manufacturers for barriers both with and without uphill wire rope tiebacks. Data were provided directly by manufacturers, who wish to remain anonymous. All data are from measured impacts during MEL tests. Barriers without uphill tiebacks are constructed with a fixed post base, which imparts a moment to the foundation base plate during impact. Barriers with uphill wire rope ti ebacks utilize a pinned connection between the post and foundation. Theoretically, this allows the post to rotate about its base and eliminates the application of a bending moment to the post foundation (Kane and Shevlin 2012).
51 Table 3.4 : Measured Loads on Components of Barriers with Uphill Tiebacks Energy Rating (kJ) Foundation Compression (kN) Foundation Shear (kN) Uphill Tieback Tension (kN) Lateral Tieback Tension (kN) 500 40 60 70 130 500 110 60 120 165 500 54 84 64 76 1000 100 70 100 250 1000 170 140 170 360 1000 96 158 131 192 1500 226 157 163 253 2000 70 130 240 230 2000 280 205 325 535 2000 278 208 150 237 3000 100 190 300 290 3000 170 315 230 530 3000 276 208 243 300 5000 300 200 290 290 5000 585 479 247 282
52 Table 3. 5 : Measured Load s on Components of Barriers without Uphill Tiebacks Energy Rating (kJ ) Uphill Foundation Tension (kN ) Downhill Foundation Compression (kN ) Foundation Shear (kN) 100 200 210 90 500 305 305 35 500 310 320 130 1000 620 630 160 3.5.3 Current Post Foundation Design Currently there exists no standard design method for flexible rockfall barrier post foundations (Kane and Shevlin, 2012). While foundations constructed in rock have not posed much concern for designers due to the adequate bonding conditions and bearing capacity, th e lack of a standard design is particularly troubling with foundations constructed in unconsolidated materials. Designers lack guidance on how unconsolidated materials respond to the sometimes large dynamic loads experienced during rockfall impact events a nd often tend to design conservatively large foundations (Giacchetti et al. 2014). There also exists no guidance on allowable deflection of post foundations. As such, some designers rely on traditional civil engineering foundation design methods, which att empt to significantly limit strain in order to preserve the integrity of structures. R. Turner et al. (2009) provide an approach to designing post foundations for lateral ultimate passive resistance for laterally loaded piles. The approach is based on a Ra nkine earth pressure approach for a foundation width B. In cohesionless soils, passive resistance
53 develops over three times the footing width, multiplied by the magnitude of the passive earth pressure. For cohesive soils, passive resistance develops over t he width of the footing with a magnitude 9 times the undrained shear strength of the in situ material. cohesive and cohesionless soils. R. Turner et al. (2009) also sugg ested assessing the bearing capacity for concrete block foundations by using presumptive allowable bearing capacities and estimating material properties. The authors did not report on what may constitute an allowable settlement for post foundations. Figu al. 2009) Kane and Shevlin (2012) recommend drilled shaft foundations reinforced per American Concrete Institute (2008) recommendations for all but mid to low energy barrie rs constructed in soil. Foundations are designed using L PILE software with foundation loading data provided by barrier manufacturers and a specified pile displacement of 1 to 1.3 inches. The authors report that drilled shaft foundations are beneficial in that they limit rotation of the post and also limit settlement. However, from a constructability
54 perspective, drilled shafts may not be a realistic option in many cases as barriers are often constructed in remote environments with no access for heavy equip ment. Stelzer and Bichler (2013) provide guidance on the design of micropile post foundations based on recommendations of the Austrian governmental regulation ONR 24810. The authors assert that two design components are require for micropile foundations: the cross section of the steel reinforcement and the bond between the anchor grout and the drill hole wall. The yield strength of the steel is provided by the manufacturer and a safety factor is applied to this value. Anchor pullout forces may be obtained either through direct testing of installed anchors or through values of grout to rock or grout to soil interface strength obtained through literature such as the Post Tensioning Institute (2004) publication on rock and soil anchors. 3.5.4 Direct Post Impacts On e criticism of the ETAG 027 testing guidelines is that the tests only consider impacts to the center of a net panel. Many current practitioners believe that the primary purpose of the supporting posts in a flexible rockfall barrier is to maintain the heigh t of the net panel components. While this is true, posts are not immune to impacts from rockfall events. Arndt et al. (2014) have documented several barrier failures related to rockfall directly impacting a post. Additionally they report that full scale i mpact testing on rockfall barriers and attenuator systems during rockfall events that included multiple rocks revealed that when fence posts are impacted by the first or second rock to reach the
55 barrier, the barrier may lose up to 100% of its effective hei ght, allowing additional rocks to pass by th e barrier. If the support posts can allow the barrier to remain standing for an additional several seconds during such an event, the system i s much more effective at containing secondary rockfall. As such, direct post impacts should at least be considered in rockfall mitigation designs, if not explicitly designed for. W ith the development of more flexible systems, support posts are increasingly viewed as a temporary and replaceable component of the overall barrier system and as such, have not been assessed as a critical element in the successful performance of the system (Kane and Shevlin, 2012).
56 4. Full scale Test Data Collection and Results In this section, detailed descriptions of the experimental test site set ups for both the barrier systems and post base systems are presented, along with an overview of the full scale testing procedures. A description of the various barrier and post base configuration systems that were tested is also presented al ong with results and discussion of the tests. 4.1 Full scale Test Site S etup The test site, located near Empire, Colorado, was designed and developed as a collaboration between the Colorado Department of Transportation and Yeh & Associates Inc. in respons e to a lack of guidelines for post foundation system design and construction, as well as a lack of published case histories on direct post impacts in rockfall barrier systems. The site was constructed between August and November of 2012. The general func tion of the testing setup consisted of creating a pendulum system capable of swinging various concrete test boulders into various posts and post foundation systems. The testing structure consisted of two sets of W10x74 steel support posts and cross members to form frames either 9.1 m or 10.6 m in height and spaced approximately 6 m apart. The frames were anchored to the ground using concrete foundations and braced using 19 mm diameter wire ropes. The 9.1 m frame was used as the axis of rotation of the pendu lum system, while the 10.6 m frame was used for leverage to raise test boulders into position. An overview of the testing site is given in Figure 4.1. Figure
57 4.2 shows a front view of the testing site with a post in place for testing. Test boulders were r aised into position using a forklift attached to a cable running through a pulley on the 10.6 m frame. Concrete test boulder masses ranged from 1,475 kg to 3,628 kg and release heights reached 6.1 m, which corresponded to maximum block impact energies up t o 220 kJ. Test boulders were released and swung into the posts using a pair of pneumatic devices. The first device released the boulder from its hanging position, while the second device released the boulder from the pendulum arm. Due to time constraints a nd to expedite testing procedures, not all test boulders were released from the pendulum arm. Figure 4.3 shows a photo of the pneumatic release devices. Figure 4.1: Side view of the test site setup. Rock is released and swings to right side of ph oto.
58 Figure 4.2: Front view of the test site setup. Rock is directly behind test post prior to raising it into position to be released. Figure 4.3: Pneumatic release devices High spe ed video cameras recording at 24 0 frames per second were installed at four different locations. One camera was installed to provide an overview of the entire test procedure, a second camera was used to provide a full height profile view of the test impact, a third camera provided a plan view of the test impact, and a fourth camera was used to provide a close up view of foundation displacement. Reference lines were painted on the ground and a reference pole was installed to aid in determining displacements.
59 4.2 Post base System Testing C onfigurations A total of twenty nine direct post impacts were conducted with impact energies ranging the ground surface with no anchorage other than wire rope tie back s. Post bases that were fixed to the ground were comprised of either grouted bar foundations (micropiles) or larger diameter deep foundations. Fixed post base systems were tested with various wire rope tie back configurations ranging from no tie backs to b oth upper and lower uphill tie backs as well as low strength downhill tiebacks Floating post base systems were constructed with both upper and lower uphill tie backs as well as low strength downhill tie backs Pinned type post base systems were not evalu ated. The top and bottom uphill wire retaining ropes were connected to a stack of concrete guard rail barriers to approximate an anchor system. The stacked concrete barriers were utilized so that potential replacement of anchor systems if they were pulled out of the ground did not affect the testing schedule. Additional concrete barriers were added as needed to approximate an increase in anchor strength. Each post foundation system configuration was instrumented to measure loading conditions throughout te sting. Circular compression type load cells (referred to throughout as load cells) were placed around the four 25 mm grouted all thread bar foundation elements where applicable. The load cells were placed between the base plate and an upper nut to record c ompression within the load cells, which corresponds to tension in the anchor bars. While the bars themselves were not instrumented directly,
60 axial loading conditions on the bars were inferred from the load cell data. Shear loading of the foundation bar ele ments was not evaluated. The nuts and washers on the bar and load cell system were tightened as much as possible prior to the testing so a load loss could be recorded in the data. It appeared that loads in excess of 60 kN could be pre loaded on the founda tion bars by tightening the nuts prior to testing. Circular link type load cells (link cells) were attached to the two top and two bottom uphill retaining ropes The link cells were used to measure tension in the retaining ropes. Each load cell and link ce ll collected data during testing at a rate of 250 samples per second. Figure 4.4 shows a general plan view of the post foundation system testing and instrumentation configuration. Figure 4.5 depicts placement and tightening of load cells. Figure 4.6 depict Figure 4.4: Plan view of test site setup indicating load and link cell designations (Arndt et al. 2014)
61 Figure 4.5: Placement and tightening of load cells prior to testing. Figure 4.6: Link cell of uphill wire rope anchors attached to the concrete barrier system. This pa per presents a selection of seven specific series of post testing configurations described bel ow, which comprise a total of 20 tests. In A Series tests, post foun dations consisted of a reinforced concrete leveling pad approximately 150 mm thick and 0.9 m in diameter with four 25 mm all thread bar foundation elements. The concrete pad was
62 reinforced with one #4 horizontal rebar stirrup reinforcement. All thread bar elements were installed and grouted approximately 1.5 m in the ground in predrilled holes approximately 50 mm in diameter. Nuts on the all thread bar foundation elements were tensioned with approximately 60 kN of force prior to testing. A Series tests did not include any uphill wire rope support anchors. The post element consisted of a W8x48 steel member 4.5 meters high fully welded to a 508 mm x 508 mm x 25 mm base plate. The base plate was drilled to accommodate the all thread bar foundation elements. Fig ure 4.7 depicts pouring the concrete base pad. Figure 4.8 depicts the typical cross section of the system. Figure 4.7: Pouring the concrete foundation pad around the grouted bar elements.
63 Figure 4.8: Typical section of A Series test setup (Arndt et al. 2014) In B Series and C Series tests, post foundations consisted of reinforced precast circular concrete shafts (deep foundations) embedded in the ground with four all thread bar foundation elements. Circular concrete shafts were approxima tely 0.91 m in diameter with #4 horizontal rebar stirrup reinforcement on 300 mm centers for both testing series. Concrete shafts were embedded to depths of 0.91 m (B Series) and 1.82 meters (C Series). Foundations were backfilled with well graded material that was compacted in 0.3 m lifts. Posts were attached to concrete shaft foundations using four 25 mm all thread bars cast in the shaft and extending the full depth of the foundation. Nuts on the all thread bar foundation elements were tensioned with appr oximately 60 kN of force prior to testing to engage the load cells. B Series and C Series tests did not include any uphill wire rope support anchors. The post elements consisted of W10x60 steel members 4.5 meters high fully welded to 610 mm x 610 mm x 25 m m base plates. The base plates
64 were drilled to accommodate the all thread bar foundation elements. Figure 4.9 depicts construction of the circular concrete shafts. Figure 4.10 depicts the typical cross section of the deep foundation systems. Figure 4 .9: Installation of circular concrete shaft post foundation systems and completed system compacted in place. Figure 4.10: Typical section of B Series and C Series test setup (Arndt et al. 2014)
65 In D Series and E Series tests, post foundations consisted of a reinforced concrete leveling pad with grouted bar foundation elements (micropiles). In each series, posts were set on a concrete leveling pad approximately 150 mm thick and 0.9 m in diameter w ith one #4 horizontal rebar stirrup reinforcement. Posts were anchored using four 25 mm all thread bars installed and grouted approximately 1.5 m into the ground in approximately 50 mm diameter predrilled holes. Nuts on the all thread bar foundation elemen ts were tensioned prior to testing to engage the load cells. Post elements consisted of W8x48 steel members 4.5 meters high fully welded to 508 mm x 508 mm x 25 mm base plates. The base plates were drilled to accommodate the all thread bar foundation eleme nts. The intent of D Series and E Series tests was to c ompare the effects of different wire retaining rope configurations on micropile post foundations. D Series tests included two 19 mm top uphill retaining wire ropes connected to ground anchors. E Series tests included two each of both top and bottom 19 mm uphill wire support ropes connected to ground anchors. Retaining ropes were attached to posts using loops through holes drilled in the web of the steel members. Retaining rope loops were secured with wi re rope clips. Both testing series utilized low strength downhill support ropes attached to the top of the post. Figure 4.11 and Figure 4.12 depict typical cross sections of D Series and E Series tests respectively.
66 Figure 4.11: Typical section of D Series test setup (Arndt et al. 2014) Figure 4.12: Typical section of E Series test setup (Arndt et al. 2014)
67 In F Series and G Series tests, post foundations consisted of only a steel footing plate resting directly on the ground that was free to deflect (float) upon impact. Posts were not fixed to the ground using any additional structural elements but were limited in their lateral deflection by uphill retaining ropes. Overturning in the uphill direction was prevented by installing low strength downhill tiebacks connected to the top of the posts. Ground conditions at the site consisted of well compacted granular material. F Series p ost elements consisted of W8x48 steel members 4.5 meters high fully welded to 508 mm x 508 mm x 25 mm base plates in contact with the ground. F series post bases were resting on level ground. G Series post elements consisted of W8x48 steel members 4.5 meters high fully welded to circular 457 mm diameter x 25 mm thick base plates in contact with ground. G Serie s post bases were set to rest on approximate 20 degree angle ground to simulate placement on a slope. Both F Series and G Series posts were supported with two top and two bottom uphill retaining wire ropes connected to ground anchors. F Series retaining ro pes were 19 mm in diameter and attached to posts using loops through holes drilled in the web of the steel members. G Series retaining ropes were 25 mm in diameter (with the exception of the first G Series test, which used 19 mm retaining ropes) and attach ed to posts by creating a loop around the entire post. G Series tests also included protective sleeves on the bottom retaining ropes. All retaining rope loops were secured with wire rope clips. Figure 4.13 and Figure 4.14 depict typical installations of F Series and G Series tests while Figure 4.15 depicts a typical cross section of F Series and G Series test setups.
68 Figure 4.13: Typical setup for F Series test. Figure 4.14: Typical setup for G Series test Figure 4.15: Typical secti on of F and G series test setup (Arndt et al. 2014)
69 4.3 Test Results Table 4.1 contains the test results from the full scale post foundation testing. A descriptive summary of the results is presented in Section 4.4. The test data are divided into A, B, C, D, E, F, and G systems. Pertinent information in the table includes t he impact energy for each test, peak load cell readings, maximum horizontal force at impact, lateral foundation displacement, and testing comments. The table also includes a column designating whether the post and foundation system remained functional or f ailed. In most cases, if the post retained sufficient height to support a net panel, it was considered functional. This evaluation is consistent with the ETAG 027 first SEL impact, which requires a residual system height that is at least 70% of the nominal height. Exceptions are cases in which structural elements such as the base plate or anchor bars were compromised such that minor additional loads would cause the post to fail. In these cases, the weight of the net panels in the field during an impact woul d likely have resulted in a full failure of the post foundation system and complete loss of post height. Figure 4.4 depicts the load cell orientation, which was maintained consistently throughout testing. The table reports peak load values on the founda tion bar load cells and tieback link cells. Positive results for the load cell data indicate the bars were loaded in tension, while negative results indicate the bars were loaded in compression. Figure 4.16 shows an example of a load vs. time plot for one of the barrier post tests. These plots were used to determine the peak load values, as well as to determine the duration of each impact. Plots for all of the post foundation t ests are presented in Appendix B
70 Figure 4.16: Load vs. time plot for post imp act test High speed camera images from camera position 4 were used to obtain displacement values for each foundation system. Videos results were viewed frame by frame for each test until maximum foundation displacement was achieved. Displacement was deter mined visually by overlaying a grid on the video frames during analysis. The grid was sized using the known dimensions of the posts used in testing. Reference lines placed at 0.3 m intervals at the site during testing were used to verify the displacement r esults obtained by the grid analysis. Unfortunately, the high speed cameras used to determine foundation displacement operated independently of the load cell equipment, making it difficult to correlate the loading and displacement data. -80 -60 -40 -20 0 20 40 60 80 100 0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 0.1600 Load (kN) Time (s) Test D 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8
71 Impact energy was calculated based on the drop height produced by the pendulum system. Measurements of the impact height and raised height of the test blocks were recorded to the 0.1 cm prior to testing. Potential energy of the raised block relates directly to kinetic energ y during impact, assuming no loss of energy in the pendulum system. The test block came to rest during each test, indicating that the full amount of kinetic energy from the block was transferred to the post. The reported horizontal post impact force was c alculated based on the work energy principle (Giordano 2013), which states that the amount of work done on a system equals the change in kinetic energy: (4.1) where W represents work and represents the change in kinetic energy. Because the tra nsfer of kinetic energy to the post was assumed to be complete, the impact force can be calculated using the following equation, along with the measured displacements: (4.2) where F represents impact force, d represents displacement, and is the impact angle. Because the impacts occurred normal to the post, the term reduces to 1. This force analysis is not entirely accurate due to additional material deformation that occurred during some of the impacts, but it provides a means to make re lative comparisons of impact forces between tests.
72 Table 4.1: Results of Post Base Testing (modified from Arndt et al. 2014)
73 Table 4.1: Results of Post Base Testing (continued)
74 Table 4.1: Results of Post Base Testing (continued)
75 Table 4.1: Results of Post Base Testing (continued)
76 4.4 Descriptive Summary of Test Results 4.4.1 A Series At 19.7 kJ of energy (310 kN of horizontal force), the entire post system rotated in a moment failure. The post, concrete base, and foundation elements experienced virtually no damage. The bar elements on the impact side (uphill) pulled out of ground. Both the front and rear foundation bar elements were initially in tension, but the front foundation bar elements transitioned to compression after approximately 0.02 s. The foundation system experienced a maximum lateral deflection of 6.4 cm. Figure 4.17 shows the result of the 19.7 kJ test with no uphill anchors on grouted bar foundation system. Figure 4.17: Result of 19.6 kJ test for A Series test. 4.4.2 B Series At 19.7 kJ of energy (388 kN of horizontal force), the entire post foundation that was embedded 0.9 m in the ground rotated outward in moment failure due to loss of
77 confinement in the surrounding soil. There was no structural damage to the post or concrete. According to the load cell data, both the rear and front foundation bar elements were initially in tension, with only one of the front bar elements transitioning to compression after 0.04 s. The loads were 40% to 90% greater than the loads experienced during the A series test. The overturning failure mechanism indicates the 0.9 m embedment is insufficient to withstan d the post rotation, which is illustrated in the soil failure. Figure 4.18 depicts the results of the failing 19.7 kJ test with no uphill anchors on a 0.9 m drilled shaft type foundation system. Figure 4.18: Result of failing 19.6 kJ test for B Serie s test. 4.4.3 C Series At 19.7 kJ of energy (517 kN of horizontal force), minor damage was observed on the drilled shaft type post foundation that was embedded 1.8 m in the ground. At 67.8 kJ of energy (1483 kN of horizontal force) significant damage occurred t o the post foundation base plate. The steel plate itself sheared, rather than the welded connection between the
78 post and the base plate. No damage to the circular shaft foundation was observed. Load cell data for the 19.7 kJ impact indicate that both the rear and the front foundation bar elements were initially in tension, with the front load cells then experiencing compression after 0.004 to 0.03 s. Load cell data for the 67.8 kJ impact indicate that both the rear and the front foundation bar elements wer e in tension for the duration of the test. Maximum loads were similar between the two highest energy C Series tests but significantly higher than those experienced during A Series and B Series tests. The test results indicate the 1.8 m embedment is suffici ent for the foundation to withstand the post rotation at impacts up to 67.8 kJ. The foundation itself only displaced 4.6 cm laterally. However, the post components are not able to withstand this loading scenario. Because the system is rigid, force is conce ntrated about the weakest link in the system, shearing the post base. Figure 4.19 depicts the result of the 67.8 kJ test with no uphill anchors on a 1.8 m deep rigid drilled shaft type foundation system. Figure 4.19: Result of 67.8 kJ test for C Series test. Note bent and sheared base plate in right image
79 4.4.4 D Series A series of five successive tests was conducted on this system ranging in energy from 8.5 kJ to 116.1 kJ. The post system consisted of a grouted bar foundation with a concrete leveling pad and an upper uphill wire rope anchor attached to the post. Following the 116.1 kJ impact test (514 kN of horizontal force), the post bar and leveling pad experienced significant damage and the post was bent. However, despite these damages the system wa s still functioning overall and the residual height was effective (greater than 70%). At lower impact energies, the foundation bar elements experienced a combination of tension loading in the rear and compression loading in the front. During the 116.1 kJ i mpact, all foundation bar elements experienced tension. Even though impact energy was much high er than Tests B and C, the overall tension in the bar foundation elements was less because the foundation pad broke away. Lateral foundation displacements were m uch higher than A, B, and C Series tests, reaching a maximum of 22.6 cm during the 116.1 kJ impact. Figure 4.20 depicts the result of the 116.1 kJ test with only top uphill retaining rope anchors on a grouted bar foundation system. Figure 4.20: Result of 116.1 kJ test for D Series test.
80 4.4.5 E Series A series of five successive tests was conducted on this system ranging in energy from 8.9 kJ to 112.8 kJ. The post system had both top and bottom uphill wire rope anchors on a grouted bar and concrete pad found ation system. At 112.8 kJ of energy (644 kN of horizontal force), much less damage was observed to the post bar and concrete foundation system as compared to D Series of tests. This is likely due to the bottom wire rope anchors. The post was also bent but the system was still functioning overall and the residual height was acceptable. The overall tension in the bar foundation elements was similar to D Series tests. Lateral foundation displacement was slightly lower than D Series tests, reaching a maximum of 17.8 cm for the 110.8 kJ impact. Figure 4.21 depicts the result of the 112.8 kJ test with both top and bottom uphill anchors on grouted bar foundation system. Figure 4.21: Result of 112.8 kJ test for E Series test.
81 4.4.6 F Series A series of two successive tests was conducted on this system at 95.8 kJ. The post system had both top and bottom uphill wire rope anchors on a floating steel base foundation system (flexible system). One of the tests was considered invalid because the pendulum arm be tween the test block and pendulum frame contacted an upper tieback rope and severed it. For the successful test at 95.8 kJ of energy (341 kN of horizontal force), the post experienced slight damage; being bent but still functioning overall with an acceptab le residual height. The overall loads in the wire rope anchors were higher than other tests, but still well within capacities for reasonable anchor pullout and wire rope strength. Maximum lateral foundation deflection was slightly larger than D Series test s at a value of 28.1 cm. Figure 4.22 depicts the results of the 95.8 kJ test with top and bottom uphill anchors on a floating steel plate foundation. Figure 4.22: Result of 95.8 kJ test for F Series test.
82 4.4.7 G Series A series of two successive tests was conducted on this system at 217.5 kJ. The post system had both top and bottom uphill retaining wire rope anchors on a circular floating steel base foundation system that was inclined approximately 20 degrees. At 217.5 kJ of energy (387 478 kN of hori zontal force), the post was bent but the system was still functioning overall with an acceptable residual height. The overall loads in the wire rope anchors were higher than other tests, but still well within capacities for reasonable anchor pullout and wi re rope strength for 25 mm (1 in) diameter material. Maximum lateral foundation displacements were much higher than other tests, reaching a maximum of 56.1 cm. Figure 4.23 depicts the result of the 217.5 kJ test with top and bottom uphill anchors on a circ ular floating steel plate foundation. Figure 4.23: Result of 217.5 kJ test for G series test.
83 4.4.8 Evaluating Loading and Deflection A broad range of loading, impact forces, deflection, and performance results were obtained based on the full scale post testing effort. Of the 19 tests conducted, 15 were considered functional after impact, three were considered failing, and one was invali d. Figure 4.24 presents a plot of impact force vs. deflection for all of the post tests performed during this effort. As indicated in the plot, all of the failing foundations were limited in deflection to less than 7 cm. Interestingly, the failing tests al l occurred at relatively low impact energies of 68 kJ or less. Flexible systems were able to withstand multiple impacts in excess of 200 kJ without failing. Even though the rigid systems were tested at lower impact energies, the load cell data indicated th at they experienced some of the highest loading values. For example, the highest load reading in the failing C Series test was nearly 200% higher than the next highest load reading of any other test. Based on this load data, it is believed that the rigid s ystems caused the impact forces to be concentrated on the weakest structural elements in the system, which ultimately failed. Figure 4.24: Plot of impact force vs. deflection for all post tests 0 200 400 600 800 1000 1200 1400 1600 0 10 20 30 40 50 60 Impact Force (kN) Deflection (cm) Failed Tests Functional Tests
84 4.4.9 Study Limitations Due to time and budgetary constraints, th e study was limited as to the number of post impact tests that could be performed. As such, the study was stopped short of what could be considered a comprehensive evaluation effort. In order to complete the study, the following post configurations are rec ommended for future testing: Drilled shaft foundations with upper retaining ropes Drilled shaft foundations with upper and lower retaining ropes Pinned post systems with upper retaining ropes Pinned post systems with upper and lower retaining ropes Micropi le foundations installed at varying inclination angles Micropile foundations installed in bedrock In addition to the limitations on the type of post foundations that were tested, an additional limitation was the method in which the posts were loaded. Beca use the posts were impacted with a horizontal force (normal to the post), the post foundations tested cannot be evaluated for significant axial loading along the post into the foundation. Future testing would benefit from dynamic axial loading on posts, as well as oblique rock impacts on posts.
85 5. Design Considerations and Recommendations 5.1 Introduction Based on the results of the full scale post testing experiments, the following design considerations are offered. Design recommendations are proposed bas ed on a discussion of the design considerations. Due to the highly variable terrain in which flexible rockfall barriers are installed, and the range of impact energies available for different barriers, the recommendations stop short of a fully engineered d esign or set of comprehensive guidelines. The goal of the recommendations is to provide design engineers with a basis of making informed decisions on their specific projects with regard to appropriately designed post foundations that contribute to a high l evel of overall system performance. 5.2 Design Considerations 5.2.1 System Flexibility vs. Foundation Flexibility By nature of their design, flexible rockfall barriers are intended to displace down the slope, often on the order of several meters or more. In the context of this significant deflection, designers must consider what constitutes an appropriate degree of de flection within the post foundation system. The desire for limited deflection within foundation systems on more typical civil engineering projects, such as bridges, is driven by the knowledge that the structural systems supported by the foundations cannot tolerate significant deformation and still perform properly. The opposite is true in the case of
86 flexible rockfall barriers; in order to function, the system must deform. R. Turner et al. (2009) state that topping rocks from threatening the safety of the public is the measure scale testing of post foundations has demonstrated that foundations can deflect fairly significantly and still perform adequately. As such, designers should not fear that d eflection within a post foundation system will compromise the performance of the overall system. 5.2.2 System Strength System strength is the most important consideration in selecting an appropriate foundation design in that it has the greatest impact on the o verall performance of a barrier system. There are four potential types of strength to consider when evaluating a post foundation: shear along the base, bearing beneath the base, overturning moment, and pullout of ground anchors. The results of the full sca le post testing demonstrate that stiffness does not necessarily equate to strength within a post foundation system. On the contrary, the more flexible systems performed best at absorbing impact energy while sustaining the least amount of damage. This is fu rther supported by observations of case studies of actual rockfall impacts on lightweight post foundation systems (Giacchetti et al. 2014). A common practice among the current major barrier manufacturers is to construct hinged ween the post and post foundation consisting of a high strength steel fastener in the 19 mm to 32 mm diameter range. The concept behind the
87 pinned connection is that it allows the post to rotate, eliminating the overturning moment applied to the foundation (Kane and Shevlin, 2012). Under ideal conditions, such as those encountered in barrier certification testing, this concept is valid because the rock impact is controlled to occur in the direction of rotation of the pinned connection. In field installation s, the concept is less reliable, as rockfall trajectories are more variable. If a rock impacts the post in a direction that is oblique to the intended direction of post rotation, a moment will be applied to the base and the connection may fail. Additionall y, the pinned connection that allows post rotation comes at the expense of reduced shear strength along the base of the post. A rock impact must only overcome the shear strength of the hinge pin to cause a failure of the system. Posts constructed with fl to displace only have to satisfy two of the four strength requirements outline above: pullout of uphill tieback anchors and bearing beneath the post. Elimination of the traditional foundat ion element eliminates overturning moment and shear along the base of the post. The remaining foundation types tested have to satisfy all four strength requirements. Interestingly, the addition of tieback anchors also increases the strength of rigid found ation types such as piles and grouted bars. This is because the uphill tiebacks act parallel and opposite to shearing forces between the post and foundation elements. Compared to the pinned post system that is commonly installed on most barriers, the fl oating foundation provides a much more robust method of eliminating the overturning
88 moment on the foundation. Because there is no physical element fixing the post to the ground, other than the tiebacks, the post base is not limited to a single direction of rotation. In addition, the shear resistance along the post base is not compromised, as the wire rope tieback size can be adjusted to accommodate a wide range of strength values. Regardless of the direction of rock impact, the wire rope tiebacks utilized i n the floating foundation will engage in a direction that allows utilization of their full strength. 5.2.3 Constructability Because many rockfall barriers are constructed in remote environments that are inaccessible to heavy equipment, constructability consider ations often play a major role in governing designs of barrier systems. Foundation options that can be constructed with hand tools or equipment that can be delivered by crane or helicopter are often preferable from a constructability perspective. For this reason, drilled shaft type foundations may not be a realistic option in remote environments. Flexible foundations require only the installation of anchors and perhaps a small leveling or bearing pad. Portable hand drills can be utilized for the relatively small diameter holes required for the anchors and the holes only require a relatively small volume of grout. Micropile foundations utilize much of the same equipment and materials as flexible foundations, indicating they are also an acceptable option in re mote environments. Ground conditions and properties of on site materials play a role in governing the constructability of barrier systems. In areas where barriers are constructed on solid
89 bedrock, deep foundations are not as constructible as other founda tion types due to the significant cost associated with rock excavation. Conversely, in areas with very soft soils, ground anchors may not be able to develop the strength necessary for proper performance of micropile or flexible foundations. 5.2.4 System Mainten ance When considering system maintenance, a designer should evaluate two criteria; design of components to minimize the need for maintenance and design of components to facilitate maintenance procedures when the need arises. Rockfall barriers are designed to deform under loading from rock impacts and will therefore require maintenance over the life of the barrier. However, if the need for and costs associated with this maintenance can be minimized, the long term value of the barrier can be increased. If a rockfall barrier is an appropriate mitigation type and the barrier is designed and constructed properly, posts and post foundations should only require maintenance in the event of a direct rock impact on a post or foundation element. The performance under these direct post impacts is an important consideration in determining anticipated long term maintenance needs. If the post and foundation incur damage at low impact energies, maintenance will be required more frequently. Additionally, if a large portion o f the post and foundation system is damaged upon impact and must be replaced, the maintenance cost per event will increase.
90 Based on the results of the full scale post testing presented here, rigid post foundation systems without tiebacks are subject to significant damage from direct post impacts, even at impact energies as low as 20 kJ. Replacement of these damaged elements wo uld be costly both in terms of materials and labor. From a maintenance standpoint, rigid post foundations without tiebacks do not appear to be an acceptable foundation type. According to barrier manufacturers, the pinned post system mentioned in Section 5.2.2 provides a benefit in that during direct post impacts, the pinned connection will shear before any other component of the system incurs major damage. Based on the shear strength of 19 mm to 32 mm diameter pins, this failure could potentially occur at relatively low impact energies. While this may provide a maintenance benefit in terms of materials, there is still a labor maintenance cost in re setting the failed pinned connection. In addition, the barrier loses functionality for the time that the post is down. Based on the full scale test results, the flexible post foundation appears to offer a compromise between barrier performance and maintenance during direct post impacts. Posts with flexible foundations survived repeated impacts in excess of 100 k J with no maintenance required between impacts. In the event of a post failure, material replacement costs are somewhat limited in that there is no large foundation to reconstruct and it may be possible to utilize the original anchorages for tieback instal lation.
91 5.3 Design Recommendations Based on the design considerations outlined above and the results of the full scale post testing, the author proposes that a flexible post foundation is the preferred design method for most rockfall barrier applications. Th e flexible foundation should consist of a post base plate installed directly on the ground or on a thin concrete leveling pad. The system should utilize two upper and two lower uphill wire rope tiebacks attached to the post and sized appropriately for the anticipated loading conditions. To prevent overturning of the posts in the uphill direction, a single downhill wire rope tieback per post should be utilized, attached to the top of the posts. The size of the post baseplate itself may be adjusted to provide additional bearing capacity when needed. To aid in constructability, relatively shallow foundation pins may be placed to brace the post during installation of the wire rope tiebacks. The following discussion offers further support for the flexible post fo undation system, as well as discussion of scenarios in which alternatives may be appropriate. The results of the full scale post foundation testing indicate that foundation systems are capable of deflecting and still performing satisfactorily when the only consideration is the ability of the system to absorb rockfall impacts and still maintain a mi nimum level of service. Only when that deflection is related to the compromise of structural systems within the foundation itself is there cause for concern. Direct post impacts are particularly detrimental to barrier systems. If a barrier is installed in an area with frequent rockfalls that increase the likelihood of a post impact, designers should strongly consider a flexible
92 foundation system. In some cases, such as when rockfall barriers are placed adjacent to highways, system deflection is undesirable as high energy events would cause the system to deflect such that the rock and barrier would end up in the highway. In these situations, a stiff foundation should be considered. If possible, designers should consider adding tieback elements to the stiff foundation to decreased damage to the system in the event of multiple impacts. For any proposed barrier and foundation system, a designer should evaluate the various loads the system is expected to encounter. In the case of rockfall barriers, that means d esigners should anticipate loads from impacts to both the panel and post systems. Depending on the foundation type, these loads could involve shear along the base, bearing beneath the base, overturning moment, and pullout of ground anchors. Loads can range anywhere from a fraction of the reported design load from minor impacts, to significantly higher than reported design loads for post impacts. Full scale barrier testing has demonstrated that flexible post foundations are better at absorbing large horizo ntal loads with less damage to the systems as compared to rigid foundation systems. Also, as Giacchetti et al. (2014) reported, axial loads along light systems such as the tested flexible system may not be large enough to result in appreciable barrier heig ht loss. They are also more straightforward to design as the
93 direction and extent of loads are easier to calculate. If axial loading along the post is of significant concern, such as in soft unconsolidated materials, then a rigid deep foundation may be a m ore appropriate choice. If this is the case, tieback elements should be added to reduce the damage that may occur to the system during the concentration of large loads within structural elements.
94 6. Summary A number of objectives are satisfied in this study of full scale post testing. These objectives include reviewing the available literature regarding full scale testing, behavior, and design of rockfall post foundations; providing theoretical background to place the current research in context; analyzing full scale post and foundation testing data based on impact energy and foundation displacement; and finally, providing considerations and recommendations for the design of rockfall barrier post foundations. The literature review reve aled that there is a significantly limited amount of published information regarding current design standards for rockfall barrier post foundations. None of this information was discovered in peer reviewed publications; rather it was available as published papers from professional conference proceedings. Additionally, there was no available literature on full scale dynamic testing of rockfall barrier post foundations aside from the data provided herein. Several publications performed full scale dynamic test ing of barriers themselves or static testing of post foundations. Finally, some of the most valuable information was discovered in case studies of barrier impacts that occurred at actual field installations of barriers. In summarizing the theory behind ro ckfall characterization and mitigation, a number of important factors with respect to barrier post foundation design were discussed. These factors include: Rockfall site characterization is an important component of barrier design
95 in order to select a barr ier of the appropriate size and impact energy rating. Flexible rockfall barriers consist of four main components: the net panel, supporting post assemblies, energy dissipation components, and supporting cables. The primary purpose of the post and foundatio n assemblies is to maintain the height of the net panel system to intercept falling rocks. Due to the complex dynamic interaction of rockfall barrier components, full scale testing of barriers is one of the most valuable tools available for barrier perform ance assessment. ETAG 027 barrier certifications are valuable in that they allow a consistent method of comparing barriers provided by different manufacturers. However, knowledge of the certification process and interpretation of the test results is critic al in selecting an appropriate barrier. ETAG 027 certifications are limited in that they do not address post foundation design nor do they consider direct post impacts during testing. Direct post impacts must be considered in order to evaluate the long ter m performance of a rockfall barrier system. Full scale dynamic impact tests were conducted by the Colorado Department of Transportation to assess the performance of various post foundation barrier configurations. This study examines 19 of those impact tes ts in the context of evaluating
96 impact energy, impact force, foundation displacement, foundation component loading, and overall post and foundation performance. The foundation configurations were separated in to seven categories: A, B, C, D, E, F, and G Se ries. Several general observations can be drawn from the results of the full scale post foundation testing conducted. Overall the most flexible post foundation systems, F Series and G Series, were best at absorbing impact energy while sustaining the least amount of damage. The flexible floating base foundations could absorb more than 3 times the impact energy than fixed base rigid systems. Using both top and bottom uphill wire rope anchors decreased damage to the stiffer foundation systems. The stiffer the post foundation system, the more damage that was concentrated with respect to the structural elements such as steel plates and concrete bases as indicated in C Series tests. Rigid systems experienced component loading of 2 to 3 times the component loading of flexible systems, despite being tested at lower impact energies. Based on the results of the full scale post testing, a series of design considerations are discussed and a general recommendation is made regarding barrier post foundation designs. The design recommendation promotes flexible floating base foundation systems with upper and lower uphill tieback wire rope anchors and one upper downhill wire rope tieback anchor. These foundations appear to be appropriate in most design scenarios, except when deflection must be limited and when ground conditions would not support the installation of ground anchors. In discussing design considerations, for most scenarios there appears to be no drawback to allowing a post foundation to deflect down the slope
97 as long as post height is maintained. Flexible floating foundation designs were chosen based on their higher energy dissipation performance, ease of installation, and potentially low maintenance costs.
98 7. Conclusions Based on the results of the full scale post testing experiments, a number of conclusions can be drawn. From the review of available literature regarding barrier post foundations: Post foundation design for flexible rockfall barriers is in its early stages with regard to devel oping consensus among practicing engineers. There are significant opportunities for research to further advance the field of post foundation design. Post foundation performance is not well understood. More case studies will help to increase the comfort lev el of engineers when assessing appropriate foundations for flexible barrier systems. While the ETAG 027 system is a useful tool for comparing the performance of various systems as a whole under ideal conditions, it lacks in terms of providing useful inform ation for designers looking to evaluate long term performance of barriers under a range of conditions. From the full scale testing of rockfall post foundations: Flexible foundations that are free to deflect within certain reasonable limits offer the best performance in absorbing higher energy rockfall impacts while maintaining an acceptable height Stiff or rigid systems, while giving the appearance of strength, may actually lead to greater incidents of post and foundation failure due higher load values on
99 foundation components resulting in the concentration of stresses into structural elements that are not designed to deflect. While the flexible foundations performed well under horizontal loading conditions, they were not tested with significant axial loads More research is needed to determine the performance of flexible post foundations under axial load. From the discussion of various design considerations: There appears to be no reason to limit deflection of barrier post foundations except in the cases o f barriers installed directly adjacent to protected areas. Flexible floating base foundations provide the desirable characteristics of higher energy dissipation and elimination of moment transfer at the post base. A flexible floating base foundation is rec ommended for foundation installations where deflection is not a concern and where ground materials will support the installation of ground anchors.
100 REFERENCES American Concrete Institute. Building Code and Commentary. (ACI 318 08), American Concrete Institute, 2008. Arndt, B., T. Ortiz, and R. Group. Conference (2014): 148 180. Arndt, B., T. Ortiz, and K. Turner. ull Scale Field Testing of Rockfall Transportation Research Circular E C141, Transportation Research Board of the National Academies, Washington, D.C., 2009. Broms, B. B. Lateral Resistance of Piles in Cohesive Soils. Journal of Soi l Mechanics and Foundations Division, ASCE 90, no. SM2 (1964): 27 63. Broms, B. B. Lateral Resistance of Piles in Cohesionless Soils. Journal of Soil Mechanics and Foundations Division, ASCE 90, no. SM3 (1964): 123 156. Buzzi, O., M. Spadari, A. Giacom ini, S. Fityus, and S. W. Sloan. "Experimental Testing of Rockfall Barriers Designed for the Low Range of Impact Energy." Rock Mechanics and Rock Engineering 46 (2013): 701 712. Brunet, G., G. Giacchetti, and A. Grimod. ultiple th Highway Geology Symposium (2013): 331 348. Cazzani, A., L. Mongiovi, and T. Frenez. "Dynamic Finite Element Analysis of Interceptive Devices for Falling Rocks." International Journal of Rock Mechanics & mining Sciences 39 (2002): 303 321. EOTA, ETAG 27: Guideline for European Technical Approval of Falling Rock Kits : European Organisation for Technical Approvals (EOTA), Brussels, Belgium, 2008. FHWA Federal Highway Administration. Rockfall Hazard M itigation Methods NHI Course No. 13219, Publication No. FHWA SA 93 085, Washington D.C., 1994.
101 Gentilini, C., L Govoni, S. de Miranda, G. Gottardi, and F. Ubertini. "Three dimensional Numerical Modeling of Falling Rock Protection Barriers." Computers and Geotechnics 44, (2012): 58 72. Giacchetti, G., G. Brunet, and A. Grimod. th Highway Geology Symposium (2014): 163 176. Giordano, N. J. College Physics: Reasoning & Relationships. 2 nd ed. Boston, MA: Brooks/Cole, 2013. Higgins, J. D. Recommended Procedures for the Testing of Rock fall Barriers : Prepared as part of NCHRP Project 20 07, Task 138, National Cooperative Research Program, Transportation Resear ch Board of the National Academies, Washington, D.C., 2003. Kane, W. F., and T. Shevlin. rd Highway Geology Symposium (2012): 53 64. McCauley, M. L., B. W. Works, and S. A Naramore. Rockfall Mitigation FHWA/CA/TL 85/12. FHWA, US Department of Transportation, 1985. Muraishi, H., M. Samizo, and T. Sugiyama. "Development of a Flexible Low Energy Rockfall Protection Fence." Quarterly Report of Railway Technical Research Inst itute 46, no. 3 (2005): 161 166. Peila, D. and C. Ronco, Technical Note: Design of Rockfall Net Fences and the New ETAG 027 European Guideline : Natural Hazards and Earth System Sciences 9, (2009): 1291 1298. Pierson, L. A., C. F. Gullixson, and R. G. Chassie. Rockfall Catchment Area Design Guide Final Report SPR 3 (032). Research Group, Oregon Department of Transportation, Salem, 2001.
102 Pierson, L. A. and R. Van Vickle. Manual NHI Course No. 130220 Publication No. FHWA SA 93 057, Washington D.C., 1993. Post Tensioning Institute. Recommendations for Prestressed Rock and Soil Anchors 2004 Ritchie, A. M. Evaluation of Rockfall and Its Control Highway Research Record 17, Highway Research Board, National Research Council, Washington D.C., 1963. Stelzer, G. and A. Bichler. A Comprehensive Guideline for Building th Highway Geology Symposium (2013): 315 330. Turner, R., J. D. Duffy, and J. P. Turner. th Highway Geology Symposium (2009): 125 140. Turner, A. K. and R. L. Schuster. Miscellaneous Report: Rockfall: Characterization and Control Transportation Research Board of the National Academies, Washington D.C., 2012. Wyllie, D. C. and C. W. Mah. Rock Slope Engineering Civil and Mining. 4th ed. New York, N.Y.: Spon Press, 2004.
103 APPENDIX A : Summary of Mitigation Measures Table A.1: Avoidance mitigation options (adapted from Turner and Schuster 2012 ) Mitigation Measure Description or Purpose Limitations/Considerations Realignment Full road realignment or facility relocation to move away from rockfall area Often old road must be maintained for existing access. Commonly there is limited space for this option. Expensive. Tunnels Avoid slope hazards by moving roadway inside rock mass away from external rockfall sources. Hazards associated with traffic in confined sp ace. Long tunnels require lighting and special ventilation. Expensive. Elevated Structures Used to span anticipated rockfall paths, allowing rockfalls to pass beneath. Structure must completely span active area to avoid being damaged by rockfalls. Exp ensive. Table A.2: Stabilization Mitigation Options (adapted from Turner and Schuster 2012 ) Mitigation Measure Description or Purpose Limitations/Considerations Removal Scaling Removal of loose rock from slope by means of hand tools, expansive concrete, or mechanical equipment; commonly used in conjunction with most other design elements. Temporary measure that usually needs to be repeated every 2 10 years as a slope face continues to degrade Rock removal, blast scaling Removal of loose rock or large rock blocks from slope by means of blasting, chemical expanders, or mechanical devices such as hydraulic jacks or air pillows. Damage from fly rock and rockfall and possible undermining or loss of support from removal of key block. Trim blasting Used to remove overhanging faces or protruding knobs that may act as launch features on slope. Difficulties with drilling, debris containment, and safety. Resloping Cutting rock slope at flatter angle to improve slope stability and rockfall trajectories. May have right of way or environmental issues.
104 Table A.2 continued Mitigation Measure Description or Purpose Limitations/Considerations Reinforcement Dowels Untensioned steel bars or bolts installed to increase shear resistance and reinforce a block. Increase normal force friction once block movement occurs. Passive support requires block movement to develop bolt tension. Slope access difficulties. Shear pins Provide shear support at leading edges of dipping rock or slab by using grouted steel bars. Need to provide contact (cast in place concrete) between bars and leading edge of block. Access difficulties. Rock bolts Tensioned steel bars or bolts used to increase normal force friction and shear resistance along potential rock block failure surfaces. Less suitable on slopes made up of small blocks. Difficult to access slope. Shotcrete Pneumatically applied con crete requiring high velocity and proper application to consolidate. Primarily used to halt ongoing loss of support caused by erosion and spalling. Also helps retain small supporting rock blocks. Reduces slope drainage. Can be unsightly unless sculpted or tinted. Wire mesh or fiber reinforcement required. Needs minimum 2 in (5 cm) thickness to resist freeze thaw. Quality and durability dependent on nozzleman skills. Buttresses Provide support to overhanging rock or lateral support to rock face by u sing earth materials, cast concrete, or reinforcing steel. Height limitations, May form roadside hazard and be unsightly. Cable lashing Anchored, tensioned cables used to strap rock block in place. May be used in conjunction with cable nets or wir e mesh. Also used as temporary support during rock bolt and dowel drilling activities. Because of slope or block geometry, typically movement must occur for full cable resistance to develop. Whalers, lagging Anchored beams or steel straps used to hold rock blocks in place between bolt locations. Also used as temporary measure to provide support during rock bolt and dowel drilling activities. Unsightly as permanent application. Movement must occur for full tensioning and resistance to develop. Anchored wire mesh, cable nets, high tensile strength steel mesh Free draining, pinned or anchored in place nets or mesh. Used to apply active retention force to retain rocks on slope. May form pockets of loose rock as rockfall debris accumula tes. Can be difficult to clean out.
105 Table A.3: Protection mitigation options (adapted from Turner and Schuster 2012 ) Mitigation Measure Description or Purpose Limitations/Considerations Protection Mesh or cable nets Draped mesh slope protection Hexagonal wire mesh, cable nets, or high tensile strength steel mesh placed on slope face to slow erosion, control descent of falling rocks, and restrict them to catchment area. Requires debris collection ditch area. Must consider debris and sno w loads on anchors. Depending on mesh type, limited to rocks 4 ft (1.2 m). Hybrid drapery attenuator fences, suspended systems Wire or cable mesh draped by fence posts or suspended across chutes. Fence (impact zone) intercepts and attenuates falling rocks that initiate upslope and directs them beneath mesh and into catchment area. Requires debris collection ditch area. Must consider debris and snow loads on anchors. Depending on mesh type, limited to rocks 4 ft (1.2 m). Catchment areas or sheds Ditches or berms Shaped catchment area at base of slope used to contain rockfall. High slopes require wide fallout areas. May have right of way or environmental issues. Rockfall sheds Covered structures used to intercept or divert rockf alls. Hazards associated with traffic in confined space. Must consider downslope issues. Expensive. Barriers Rigid barriers (with or without fence extension) Rigid barrier walls used to intercept falling rocks and restrict them to prescribed fallout area. Examples include Jersey barriers; guardrails; and other concrete, timber, gabion, or mechanically stabilized earth walls. Rigid systems are more prone to damage by high energy events. Complicate debris cleanout and snowplowing. Flexible barriers Wire ring, high strength wire mesh or cable net panels with high energy absorption capacity supported by steel posts and anchor ropes with braking elements. Typically proprietary systems. Require room for barriers to deflect durin g impacts. Must be cleaned out periodically. Can complicate debris cleanout and snowplowing.
1 06 APPENDIX B: Load Cell Test Data -60 -40 -20 0 20 40 60 80 100 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 Load (kN) Time (s) Test A 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 -200 -150 -100 -50 0 50 100 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 Load (kN) Time (s) Test B 1: 8.9 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4
107 -40 -20 0 20 40 60 80 100 120 140 160 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 Load (kN) Time (s) Test B 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 -100 -50 0 50 100 150 200 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 Load (kN) Time (s) Test C 1: 8.9 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4
108 -50 0 50 100 150 200 250 300 350 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 Load (kN) Time (s) Test C 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 -50 0 50 100 150 200 250 300 350 400 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 Load (kN) Time (s) Test C 3: 67.8 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4
109 -20 0 20 40 60 80 100 120 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 Load (kN) Time (s) Test D 1: 8.5 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8 -80 -60 -40 -20 0 20 40 60 80 100 0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 0.1600 Load (kN) Time (s) Test D 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8
110 -100 -50 0 50 100 150 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 Load (kN) Time (s) Test D 3: 64.8 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8 -100 -50 0 50 100 150 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 Load (kN) Time (s) Test D 4: 113.1 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8
111 -40 -20 0 20 40 60 80 100 120 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8000 Load (kN) Time (s) Test D 5: 116.1 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell2 LinkCell8 -60 -40 -20 0 20 40 60 80 100 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 Load (kN) Time (s) Test E 1: 8.9 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell1 LinkCell2 LinkCell5 LinkCell8
112 -80 -60 -40 -20 0 20 40 60 80 100 120 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 Load (kN) Time (s) Test E 2: 19.7 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell1 LinkCell2 LinkCell5 LinkCell8 -100 -50 0 50 100 150 200 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 Load (kN) Time (s) Test E 3: 70.4 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell1 LinkCell2 LinkCell5 LinkCell8
113 -40 -20 0 20 40 60 80 100 120 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 Load (kN) Time (s) Test E 4: 110.8 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell1 LinkCell2 LinkCell5 LinkCell8 -20 0 20 40 60 80 100 120 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 Load (kN) Time (s) Test E 5: 112.8 kJ Impact Energy LoadCell1 LoadCell2 LoadCell3 LoadCell4 LinkCell1 LinkCell2 LinkCell5 LinkCell8
114 -20 0 20 40 60 80 100 120 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 Load (kN) Time (s) Test F 1: 95.8 kJ Impact Energy LinkCell1 LinkCell2 LinkCell5 LinkCell8 -20 0 20 40 60 80 100 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 Load (kN) Time (s) Test F 2: 95.8 kJ Impact Energy LinkCell1 LinkCell2 LinkCell5 LinkCell8
115 -20 0 20 40 60 80 100 120 140 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 Load (kN) Time (s) Test G 1: 217.5 kJ Impact Energy LinkCell1 LinkCell2 LinkCell5 LinkCell8 -50 0 50 100 150 200 0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500 Load (kN) Time (s) Test G 2: 217.5 kJ Impact Energy LinkCell1 LinkCell2 LinkCell5 LinkCell8