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An investigation of the effectiveness of tensile reinforcement in alleviating approach fill settlement behind bridge abutments

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Title:
An investigation of the effectiveness of tensile reinforcement in alleviating approach fill settlement behind bridge abutments
Creator:
Monley, Gregory John
Publication Date:
Language:
English
Physical Description:
xii, 92 leaves : illustrations ; 29 cm

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Subjects / Keywords:
Tensile architecture ( lcsh )
Bridges -- Abutments ( lcsh )
Bridges -- Abutments ( fast )
Tensile architecture ( fast )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 89-92).
General Note:
Submitted in partial fulfillment of the requirements for the degree of Master of Science, Department of Civil Engineering.
Statement of Responsibility:
by Gregory John Monley.

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|University of Colorado Denver
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|Auraria Library
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Resource Identifier:
17882366 ( OCLC )
ocm17882366
Classification:
LD1190.E53 1988m .M66 ( lcc )

Full Text
AN INVESTIGATION OF THE EFFECTIVENESS OF
TENSILE REINFORCEMENT IN ALLEVIATING
APPROACH FILL SETTLEMENT
BEHIND BRIDGE ABUTMENTS
by
Gregory John Monley
B.A. Geology, University of Colorado at Denver, 1984
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
Department of Civil Engineering
1988


This thesis for the Master of Science degree by
Gregory John Monley
has been approved for the
Department of
Civil Engineering
by
Date:
T)onZM*l&>L R .


Monley, Gregory John (M.S., Civil Engineering)
Investigation of the Effectiveness of Tensile
Reinforcement in Alleviating Approach Fill
Settlement Behind Bridge Abutments
Thesis directed by Associate Professor Tzong H. Wu
It has been reported that the inclusion of
tensile reinforcing in the approach fill behind bridge
abutments increases overall fill stiffness, reduces
approach fill settlement, and decreases or eliminates
lateral earth pressure against abutments. The purpose
of this study was to conduct a preliminary investigation
of the effectiveness of using reinforcing to reduce
settlement of the approach fill.
The finite element method of analysis was
employed in the analytical procedure to simulate the
nonlinear, stress-dependent constitutive behavior of
soils and used linear elastic structural elements to
simulate the reinforcement behavior. A preliminary
analysis was conducted to determine how best to employ
the analytical procedure and how correctly to interpret
the output results in modelling the behavior of
reinforced abutments.
The procedure was verified by comparing results
of the analysis with the measurements of a large-scale
reinforced abutment test. Although the analytical
results indicated substantial local yielding, the


iv
procedure provided a good approximation of measured
settlement. Analytical versus test results showed the
analytical procedure to be correct.
Using the validated procedure, the effectiveness
of using tensile reinforcement to alleviate approach
fill settlement was examined under different foundation,
backfill and reinforcement conditions. The overburden
pressure-dependent and soil-dependent behavior of the
reinforcement and the behavior of reinforced abutments
under surcharge loads were accounted for in this study.
The analytical results indicated that the use of
tensile reinforcement did not significantly alleviate
approach fill settlement behind the abutment. This was
due to the nearly vertical soil movement which caused
the laterally placed reinforcement not to be activated
to any measurable capacity. The apparent success of
reinforcing abutment backfills to alleviate settlement
is more likely due to improved backfill stiffness due to
the inclusion of geotextiles as a compaction aid.
The form and content of this abstract are approved. I
recommend its publicatior
Signed


ACKNOWLEDGEMENTS
I would like to express my sincerest thanks to
Professor Tzong H. Wu for his continual guidance and
encouragement throughout the preparation of this
thesis. Special gratitude is also extended to Mr.
Nelson Chow, Mr. Ahmad Ardani amd Mrs. Deglas Gilbert
of the Colorado Highway Department for their helpful
assistance. I would also like to thank Professor Nien-
Yin Chang and Professor John R. Mays for the
constructive comments they provided during the final
examination.
Finally, I would like to thank my wife, Suzanne
for all the help she gave me in preparing this thesis
and for her love and support.


CONTENTS
CHAPTER
1. INTRODUCTION.................................... 1
Problem Statement............................. 1
Objective..................................... 2
Method of Approach........................... 2
2. CAUSES AND REMEDIES OF
DIFFERENTIAL SETTLEMENT
BETWEEN THE APPROACH
FILL AND BRIDGE DECK.......................... 5
Causes of Differential Settlement...... 5
Conventional Remedial Measures................ 9
Use of Reinforcement to Alleviate
Differential Settlement.................... 12
3. FINITE ELEMENT ANALYSIS PROCEDURE........ 17
SSTIP. and the Duncan-Chang Model........ 17
Method of Analysis........................... 19
Preliminary Test Study....................... 22
4. LARGE-SCALE TEST............................... 32
Introduction............................ 3 2
Method of Experiment......................... 32
Experimental Test Results............... 3 6
Finite Element Results.................. 3 7
Comparison of Results........................ 44
Concluding Remarks........................... 54


vii
5. PARAMETRIC STUDY............................... 55
Introduction................................. 55
Finite Element Discretization................ 55
Pilot Analysis Parameters.................... 62
Boundary and Loading Conditions.............. 67
Parametric Analysis.......................... 75
Concluding Remarks........................... 81
6. SUMMARY, CONCLUSIONS AND
RECOMMENDATIONS FOR
FURTHER STUDY................................ 84
Summary................................. 84
Conclusions................................ 85
Recommendations for Further Study............ 86
REFERENCES............................................ 89


Vlll
TABLES
Table
4-1. Properties of backfill soil and
geogrid reinforcing............................ 35
4- 2. Duncan-Chang hyperbolic stress-strain
parameters and structural bar
element properties............................. 43
5- 1. Properties of initial soil and
reinforcing parameters......................... 61
5-2. Hyperbolic stress-strain
parameters for #30 Ottowa sand................. 65
5-3. Properties of parameters used to
model changing foundation, backfill
and reinforcement conditions........
80


FIGURES
Figure
2-1. Differential settlement caused by
using a deep foundation to support
the abutment.................................. 7
2-2. Wingwall damage resulting from
settlement of approach fill.................... 8
2-3. Methods of alleviating the bump
caused by differential settlement............. 11
2-4. Geotextile-reinforced approach
fill for Douglas Bypass, Wyoming.............. 13
2- 5. Influence of compaction on
pre-strain of soil and pre-stress
of geotextile................................. 15
3- 1. Stress-strain curves for
Polyfilter X (woven fabric)............. 2 0
3-2. Simplified finite element mesh............... 23
3-3. Placement of new layer over a
pre-existing layer during
incremental construction...................... 23
3-4. Accumulation of settlement at
the top of a structure........................ 25
3-5. Structural bar elements placed
as one unit during incremental
loading................................. 2 6
3-6. Collapse of laterally
unsupported structure......................... 28
3-7. Support of laterally unsupported
structure using bar elements.................. 28


X
3- 8. Displacement of structural nodes
and resultant axial straining
in bar elements......................... 3 0
4- 1. Diagrammatic representation of
large-scale test........................ 3 4
4-2. Finite element mesh of
large-scale test........................ 3 9
4-3. Method 1; soft foundation
condition............................... 40
4-4. Method 2; load applied to the
top of a stiff foundation
after construction...................... 40
4-5. Method 1; Comparison of calculated to
measured soil deformation............... 45
4-6. Method 2; Comparison of calculated to
measured soil deformation............... 4 6
4-7. Method 1; minor principal
stress distribution behind
the test abutment....................... 47
4-8. Method 2; minor principal
stress distribution behind
the test abutment............................. 47
4-9. Method 1; stress level
distribution behind the
test abutment............................... 48
4-10. Method 2; stress level
distribution behind the
test abutment................................. 48
4-11. Method 1; comparison of
calculated to measured
strain in the geogrid. ....................... 50
4-12. Method 2; comparison of
calculated to measured strain
in the geogrid................................ 51
4-13. Method 1; comparison of
calculated to measured
vertical stress
52


xi
4- 14. Method 2; comparison of
calculated to measured
vertical stress............................... 53
5-1. Diagrammatic representation
of reinforced bridge abutment................. 56
5-2. Finite element representing
reinforced bridge abutment.................... 58
5-3. Finite element mesh in
which bar elements represent
the geotextile wall facing.................... 60
5-4. Secant modulus at 2% strain
vs. overburden pressure for
nonwoven geotextile, Trevira 1127............. 63
5-5. Calculated and experimental
stress-strain curves for
dry #30 Ottowa sand....................... 66
5-6. Settlement curve for
"no-slip" condition........................... 69
5-7. Settlement curve for
"free-slip" condition......................... 69
5-8. Stress level distribution
for "no-slip" condition....................... 70
5-9. Minor principal stress
distribution for "no-slip"
condition..................................... 70
5- 10. Stress level distribution
for "free-slip" condition..................... 71
5-11. Minor principal stress
distribution for "free-slip"
condition..................................... 71
5-12. Cross-section of a box culvert................ 73
5-13. Group 2; settlement curve..................... 77
5-14. Group 2; soil movement diagram................ 77
5-15. Group 2; strain distribution
in reinforcement.............................. 79


Xll
5-16. Group 3; settlement curve................. 79
5-17. Group 4; settlement curve for
case where backfill parameters
shown in Table 5-3 were used............. 82
5-18. Group 4; settlement curve for
case where foundation parameters
shown in Table 5-3 were used............. 82


CHAPTER 1
INTRODUCTION
Problem Statement
Excessive differential settlement often occurs
between bridge abutments and the approach backfill,
especially where foundation conditions are poor.
Settlement can range from a few inches to a couple of
feet and progress throughout the lifetime of the earth
structure. Because of this, rough and sometimes
hazardous driving conditions are created by the
resultant abrupt step ("bump") at both ends of bridge,
and alleviating this problem often require ongoing and
expensive maintenance such as mudjacking and
resurfacing.
Conventional methods have attempted to reduce
the resultant bump but have not attempted to alleviate
the cause, differential settlement. Two methods are
typically used. A reinforced concrete approach slab
fitted to the abutment and placed on top of the
adjacent backfill is often used to redistribute
differential settlement occurring at the abutment over
the length of the slab. Often in combination with the


2
approach slab, a well compacted material that is not
prone to settlement is constructed as a wedge behind
the abutment and differential settlement is
redistributed along this wedge. Unfortunately, these
methods have enjoyed only limited success and in some
cases have adversely contributed to the problem.
Since 1983, the Wyoming Highway Department has
reported apparent successes in alleviating differential
settlement using multiple layers of geotextile
reinforcing. It is believed that the inclusion of
reinforcing increases the over-all fill stiffness,
reduces settlement of the approach fill and decreases
or eliminates lateral earth pressure against abutments
(Price and Sherman, 1986).
Ob~i ectives
At present the Wyoming Highway Department is
conducting a full-scale test on a reinforced bridge
abutment and similar plans are underway in Colorado.
The objective of this study is to conduct a preliminary
investigation of the effectiveness of using tensile
reinforcement to reduce differential settlement between
the bridge abutment and the approach backfill.
Method of Analysis
To evaluate the effectiveness of tensile
reinforcing and to investigate the behavior of the


3
tensile reinforcement, the finite element method of
analysis was employed. The analytical procedure used
in this investigation incorporated quadrilateral
elements with a nonlinear material model to simulate
the nonlinear, stress-dependent stress-strain behavior
of soils and one-dimensional structural elements with
linear elastic constitutive law to simulate the
geotextile behavior.
A preliminary analysis was first conducted to
gain insight on how best to employ the analytical
procedure and how correctly to interpret the output
results in modelling the complex behavior of reinforced
bridge abutments. The analytical procedure was
verified by comparing results of the analysis with the
measurement data of a large-scale reinforced abutment
test conducted by the Civil Engineering Research
Institute, Japanese Ministry of Construction. Using
the validated procedure, a parametric study was
conducted to evaluate the effectiveness of using
reinforcing to alleviate backfill settlement under
different foundations, backfills and tensile
reinforcements. As part of this study, the results of
triaxial tests conducted on dry Ottawa sand at low
confining pressures combined with tensile strength
tests conducted on a nonwoven geotextile, Trivera 1127,
are incorporated into the analytical procedure to


account for the overburden pressure-dependent and soil
dependent stress-strain behavior of the reinforcing.


CHAPTER 2
CAUSES OF AND REMEDIES OF DIFFERENTIAL SETTLEMENT
BETWEEN THE APPROACH FILL AND BRIDGE DECK
Causes of Differential Settlement
Several factors, often acting together, can
cause differential settlement to occur between the
bridge abutment and the backfill, resulting in an
abrupt step in the road surfacing at the abutment. The
three most important factors are; 1) weak foundation
soils which settle significantly after construction of
the embankment, especially where the bridge abutment is
more strongly supported by a deep foundation; 2)
inadequate compaction effort of the embantanent, due in
part to the inherent difficulty in compacting available
backfill material and in part to the restrictions
imposed by the bridge structure; and 3) poor drainage
of the approach backfill.
As land use becomes more restricted, more and
more bridge abutments are being constructed over poor
or marginal foundation conditions. A number of methods
have been used to improve stability and decrease
settlement of foundation soils. Examples of these
methods are preloading with or without drains, over-


6
excavation and backfill with structural fills,
compaction grouting, vibroflotation, compaction piles,
chemical or thermal treatment, and electro-osmosis.
However, time constraints and high costs may prohibit
the use of these techniques. As shown in Figure 2-1,
using a deep foundation to support the bridge abutments
can also compound this problem. In Colorado, bridges
are typically supported by deep H- or pipe pile
foundations. Bridge designs typically limit settlement
of the bridge abutments to well under 1 inch, and to
abide by the specifications, piles are often driven
deep into a firm stratum. In contrast, approach fill
settlement is primarily controlled by foundation
conditions near the surface. Where these conditions
are poor, the embankment can settle substantially. The
resulting differential settlement can cause structural
distress as well. Figure 2-2 shows the damage to the
wingwalls that have settled along with the backfill
behind the abutment.
Differential settlement also may result from
inadequate compaction of the embankment structural fill
and backfill material. The compaction effort can be
very well controlled by both the engineer and
contractor and yet the resulting soil behavior may be
unsatisfactory. This is due in part to the quality of
the structural fill and backfill material used. Soils


7
b) Settlement of the embankment due to weak
foundation; note that the abutment settlement is
prevented by the deep foundation.
Figure 2-1. Differential settlement caused by using a
deep foundation to support the abutment.


8
Figure 2-2. Wingwall damage resulting from settlement
of approach fill (Ardani, 1987).


9
containing large percentages of silt and/or clay are
difficult to compact and in many cases these are the
only soils available. Also, heavy equipment used to
compact the backfill is not allowed within a given
distance of the abutment and wingwalls in order to
protect these members from structural damage. As an
alternative, smaller hand compactors are used along the
wall parameter which may not adequately compact the
backfill. The result is a nonuniform compactive
effort.
Improper design of the drainage system can
also cause differential settlement to occur by allowing
erosion of fines in the subgrade (Ardani, 1987). To
prevent large hydrostatic pressures from developing
along the abutment wall, a filter layer of crushed
stone and rubble is typically placed down the back of
the abutment wall and connected to the drainage system
placed beneath the constructed backfill (Henry, 1986).
Especially where soils containing large percentages of
fines are used, progressive erosion of the backfill
caused by the percolation and seepage of water can
occur where these filters and drains do not prevent the
migration of fines.
Conventional Remedial Measures
The factors listed above are only among
several which can contribute to differential


10
settlement, making the problem difficult to solve.
Several methods have been attempted to alleviate the
resultant "bump" at the end of the bridge but do not
address the problem of settlement itself (Price et al.,
1986). Two such methods are presented in Figure 2-3.
As shown, a rigid, reinforced concrete approach slab
can be placed behind the bridge abutment to spread out
the differential settlement occurring at the abutment
over a wider area. In general, the slab is attached to
the abutment using dowels or can slide back and forth
along the abutment platform if a construction joint is
provided between the bridge slab and the approach slab
(Yang et al., 1982). Although they have been
beneficial in many cases, where differential settlement
is substantial the use of approach slabs has only been
partially successful and in some cases can aggravate
the problem. As the backfill progressively settles,
the rigid approach slab can crack and may fail if steps
are not taken to support it, usually by mudjacking the
slab from underneath. Mudjacking itself may also
further crack and damage the structural integrity of
this member. The resultant cracks and voids then serve
as a path of least resistance for surface run-off
waters that further erode the subgrade material.
A second method employs a highly granular fill
or pulverized fuel ash that will experience negligible


11
Figure
Reinforced
2-3. Methods of alleviating the bump caused by
differential settlement (Henry, 1986).


12
settlement after placement (Henry, 1986) and is often
used in combination with the approach slab. Much like
the approach slab, a wedge of this material constructed
behind the abutment will also distribute differential
settlement over the length of the wedge. A major
difficulty is presented in finding high-grade material
in the near vicinity that can be economically
transported to the project. The largest costs incurred
in constructing embankments is the excavation and
transport of soils to the construction site. Where
this material is not readily available at a reasonable
cost, it may become prohibitively expensive to use it
as fill material. Also, like the approach slab, the
use of high quality fills has shown only limited
successes.
Use of Reinforcing to Alleviate
Differential Settlement
Since 1983, the Wyoming Highway Department has
built or retrofitted over 38 bridges using geotextiles
to reinforce the backfill. As shown in Figure 2-4,
multiple layers of geotextile material are placed
within the backfill and folded at the sides to form
geotextile facings adjacent to the abutment and
wingwalls. Price et al. (1986) indicated that, by
using a geotextile having a moderately high modulus and
low elongation to break, a stiffer soil mass was


25
Figure 2-4. Geotextile-reinforced approach fill
for Douglas Bypass, Wyoming
(modified from Wyoming Highway
Department, 1986).


14
created which could distribute traffic loads over a
wider area and thus alleviate settlement from occurring
within the reinforced section. It has been further
reported that, since their installment, no repairs have
been made to bridges using reinforcing resulting from
differential settlement.
To alleviate lateral earth pressures from
forming along the abutment backwall and wingwalls
containing the backfill, 3 inch thick soft polystyrene
boards are placed between the geotextile facings of the
reinforced section and the confining abutment and
wingwalls. The "unrestrained geotextile structure may
then act independently of the abutment and exert little
earth pressure on the abutment wall.
Because the geotextile facing can deform
laterally outward as well as downward during
compaction, geotextile at the fabric wall facing can
undergo extension as shown in Figure 2-5. Confined by
the geotextile facing, compaction of the soil will
cause the reinforcement to be prestressed and also
improve bonding at the soil-reinforcement interface.
In turn, this may cause the effective strength and
stiffness of the backfill to improve (Murray, 1982).
Used as a compaction aid, the reinforcing may also
improve the strength thus alleviating differential
settlement.


15
Figure 2
before compaction
after compaction
5. Influence of compaction on pre-strain of
soil and pre-stress of geotextile (Murray,
1982) .


16
Although it has been reported that geotextile
reinforcing reduces differential settlement, very
little research has been conducted to verify its
mechanism. Presently, the Wyoming Highway Department
is conducting a full-scale test to measure the
effectiveness of using geotextile reinforcing and
similar plans are underway in Colorado. The findings
presented by this study should provide useful
information for further research conducted in this
area.


CHAPTER 3
FINITE ELEMENT ANALYSIS PROCEDURE
SSTIP and the Duncan-Chanq Model
In choosing a suitable finite element computer
program to simulate the behavior of soil-reinforced
bridge abutments, certain requirements were needed to
be met. The constitutive model for the soil elements
employed by the program would have to be capable of
simulating the nonlinear stress-strain relationship and
stress level-dependent stress-strain behavior
characteristic of soils. One-dimensional structural
elements were also required to represent the layers of
reinforcement. Finally, the program would have to
include the ability to model the construction sequence
and application of line loads.
To fulfill the above criteria the nonlinear
finite element computer program, SSTIP, was selected
for this study. Originally coded by Y. Ozawa and J.M.
Duncan for soil analyses (Program ISBILD, 1973) using
time-independent nonlinear finite element procedures
(Kulhawy et al., 1969) and later modified by J. Dickens
to include structural elements (1973), SSTIP has been


18
used successfully to predict soil-structure interactive
behavior for a large variety of complex geotechnical
engineering problems (Kulhawy et al., 1960; Duncan and
Chang, 1970; Clough and Duncan, 1971; Bjerrum et al.,
1972; Clough and Duncan, 1972; Duncan, 1972; Duncan,
1973; Jayapalan and Lytton, 1982; Duncan et al., 1985).
The program employs the Duncan-Chang soil model (Duncan
and Chang, 1970). The nonlinear stress-strain
parameters of the soil model are easily obtainable from
conventional triaxial test results and have physical
significance in describing soil behavior. Also, a
large data base of parameters calculated for a wide
variety of soils under both effective and total stress
conditions are readily available (Wong and Duncan,
1974).
Inherent limitations also exist in the soil
and structural models employed by the program which
should be recognized. Only stable soil masses may be
analyzed. This is due in part to the fact that the
stress-strain relationships analyzed in each
construction increment are based on the generalized
Hooke's law and in part because small displacement
formulation is adopted. Simulation of volume change is
also generally poor due to difficulties in simulating
"shear" dilatancy. A more indepth discussion on the
Duncan-Chang model can be found elsewhere (Duncan and


19
Chang, 1970? Wong and Duncan, 1974; Ozawa and Duncan,
1976).
A drawback in using linear elastic structural
elements to represent the layers of soil reinforcement
is that the overburden pressure-dependent stress-strain
behavior of the reinforcing is not accounted for. As
shown by Figure 3-1, the secant modulus for the woven
geotextile decreases in value with decreasing
overburden pressure and experiences nonlinear stress-
strain behavior over a wide range of strain (Et-
Fermaoui and Nowatzki, 1982). SSTIP as well as other
finite element programs known to the writer only
considers linear elastic behavior in using bar and beam
elements to model the structure. Although an
approximate modulus value can be extrapolated from
curves such as those shown in Figure 3-1, future finite
element programs developed to analyze soil-
reinforcement interaction should incorporate this
feature.
Method of Analysis
Two methods of analysis, the composite method
and the discrete method, have been used to simulate the
behavior of reinforced soil. The composite approach
considers both soil and reinforcement to behave as a
homogenous, composite material. In contrast, the
discrete method considers soil and reinforcement to act


EQUIVALENT TENSILE STRESS,
20
Figure 3-1. Stress-strain curves for Polyfilter X
(woven fabric); dry #30 ottowa sand used as
support and cover material (El-Fermaoui et
al., 1982).


21
as individual entities which interact with one another.
The composite method suffers from a number of
disadvantages. It must first be arbitrarily decided
which proportion of a reinforced soil structure behaves
as a composite material and at what distance away from
a layer of reinforcement does the composite "effect"
diminish. If the entire reinforced earth structure
acts compositely then it must be assumed that the
induced strains and deformations equal those of the
soil as the percentage of reinforcement in the soil
typically comprises less than 1% (Collins, 1986). As a
result the method does not supply detailed information
about the behavior of the reinforcement.
To evaluate the soil-reinforcement behavior
the discrete method was employed in this study. One
argument made against using SSTIP to perform a discrete
analysis is that the program does not contain interface
elements able to model slip occurring between the soil
and reinforcing (Collins, 1986). It can be argued,
however, that the procedures used to obtain and
simulate the parameters representing interface
conditions have not advanced to the stage where they
can be used to reliably model actual soil-structure
interactive behavior. Also, no cases were found in
which a measurable amount of slip occurred along the
soil-reinforcement interface. In this study no


22
interface properties are introduced and it was assumed
that no slip occurred between the soil and
reinforcement. Also, where uncertainty existed as to
the true boundary condition, a range of conditions was
examined in order to better evaluate where the true
condition resided.
Preliminary Test Study
To initially test the capability of SSTIP to
be used as a tool for analyzing the performance of
soil-reinforced bridge abutments, a simple problem was
first analyzed. The finite element mesh used in this
analysis is presented in Figure 3-2. The results of
this study are presented below in order to show the
capabilities of SSTIP from an observational point of
view.as well as to demonstrate how the output results
were modified to account for the complex behavior of
the soil-reinforced bridge abutment.
To represent sequential construction, SSTIP
artificially assigns zero displacement to the top of
each newly placed layer to indicate construction of
this layer up to the design grade. As shown by Figure
3-3, complete settlement of the preceding lift due to
placement of the new layer is assumed to occur by the
time the new lift is completed. Due to this simulation
procedure the resulting surface settlement cannot be
directly obtained as the top or last newly placed layer


23
Figure 3-
Figure 3-
Simplified finite element mesh.
mm-t=uT2Tj rm Tim TtmTm****
NEWLY-PLACED LAYER
PRE-EXISTING LAYER
/)//; > A) i i ) ^ i r t v ^ rift
LEGEND
neglected soil displacement
design grade level
Placement of new layer over a pre-existing
layer during incremental construction.
Note that displacement caused by settlement
of the pre-existing layer is neglected in
SSTIP


24
will always show zero displacement. For many practical
engineering problems the surface settlement resulting
from incremental loading is also needed. To obtain
this settlement the summation of deformation incurred
by each consecutively placed layer due to the load
imposed by placement of a new layer above it was used
to represent the resulting surface settlement. Figure
3-4 demonstrates how this settlement was accumulated.
Another problem encountered was caused by the
method of implementing the bar elements representing
multiple layers of soil reinforcement. SSTIP requires
that structural elements be placed as one unit at one
point in the sequence of loading as shown in Figure 3-
5. Therefore, several structural layers were left
indeterminately "hanging in the air" until soil layers
were placed to support the individual structural layers
from underneath. Due to this, nodal points connected
to indeterminate structural elements showed
unrealistically large vertical displacements. However,
the incremental displacements incurred by these
structural layers occurring after they were supported
by soil layers from underneath were reasonable. The
program was therefore modified to nullify all
settlement occurring at the nodes of structural layers
during the period in which they were left unsupported.
To verify that the modification did not affect the


25
LAYER 3
LAYER 2
LAYER 1
LEGEND
^ settlement of layer 1 due to layer 2
settlement of layer 1 due to layer 3
EE settlement of layer 2 due to layer 3
- profile of surface settlement
Figure 3-4. Accumulation of settlement at the top of a
structure. Accumulated settlement equals
settlement of layer 1 due to weight of
layer 2 plus settlement of layer 2 due to
weight of layer 3.


26
LEGEND
y] soil elements of pre-existing layer
soil elements of unplaced layers
_ bar element
Figure 3-5. Structural bar elements placed as one unit
during incremental construction. Three
successively placed soil layers will
eventually support the 3 indeterminate
structural layers shown.


27
soil-structure behavior a simplified model containing
one single layer of structural elements was compared to
a second model containing three structural layers. In
the second model, the middle layer was placed at the
same location and contained the same material
properties as the layer in the first model. Next, the
lower and upper layers of the second model were
assigned very low modulus values to prevent them from
affecting behavior. During sequential construction,
the one structural layer was immediately supported and
did not show large vertical displacement. In contrast,
the unsupported structural layers of the second model
did. However, by nullifying displacements caused to
structural layers while they were unsupported as
described above, the behaviors of the two models were
identical.
Moreover, to demonstrate that the structural
bars could support the soil in tension and alleviate
settlement, the left and right boundary restraints
preventing movement in the horizontal direction were
removed as presented in Figure 3-2. As shown in Figure
3-6, the soil deformed excessively. High strength bar
elements were then placed within the soil layer and the
structural nodes were allowed to move and rotate freely
like the soil. As shown by Figure 3-7, inclusion of
the bar elements greatly alleviated horizontal


28
Figure 3-6. Collapse of laterally unsupported model.
LEGEND
bar element layer
Figure 3-7. Support of laterally unsupported model
using bar elements.


29
deformation and this in turn caused the vertical
surface settlement to also be alleviated.
It was also discovered that in calculating
axial force of structural elements, SSTIP only
considers axial strain resulting from displacement of
structural nodes in the direction that the structural
element was initially placed. For example, in Figure
3-8 the nodes connected to horizontally placed bar
elements displace both horizontally and vertically with
the exception of the two end nodes which are completely
restrained. In calculating axial force, the program
only considers nodal displacement in the horizontal
direction; therefore, length a is used to determine
axial force instead of the true length between the
nodes, length b. When the simple model was loaded as
shown in Figure 3-8, the outer two bar elements showed
axial compression while the two center bars exhibited
tension. The bar element in SSTIP was incorporated to
model horizontally placed tie-backs used to restrain
the lateral outward movement of flexible retaining
structures. Due to the lateral earth pressure on the
wall, deformation of the tie-back occurred
predominately in the horizontal direction and vertical
deformation was neglected. However, in many
reinforced-soil structures significant soil movement
transverse to the initial placement of the reinforcing


30
LINE LOAD
b >
*
y
LEGEND
--- initial position of soil layers
--- final position
initial position of reinforcement
Figure 3-8. Displacement of structural nodes and
resultant axial straining in bar elements.


31
takes place. In this study axial strain in the bar
elements was determined from the deformed position of
the nodal points. The axial force of the bar elements
was in turn calculated from the axial strain.


CHAPTER 4
LARGE-SCALE TEST
Introduction
To gain insight into the behavior of
reinforced bridge abutments and to verify the
analytical procedure, the SSTIP code was used to
analyze the behavior of a large-scale test conducted by
the Civil Engineering Research Institute, Japanese
Ministry of Construction. The purpose of the test was
to determine the ability of high strength geogrid
reinforcement to alleviate differential settlement
between the abutment and backfill (Kutara et al.,
1985). The geogrid layers would be fixed at one end to
the face of the abutment and the abutment itself would
not be allowed to settle, thus representing an abutment
founded on piers.
Method of Experiment
A description of the test, paraphrased from
Japanese, is reported below. To simulate the
prototype, a large bin was constructed 14 m. in length,
2.6m. in height and 1.0 m. in width and the sidings
were built of clear plexiglass to observe the soil-


33
structure interactive behavior throughout the test. A
diagrammatic representation of the test model is shown
in Figure 4-1. The abutment itself was modeled after
one in the field and the backfill stood 1.45 m. in
height and extended 10.0 m. in length behind the
abutment. Five layers of geogrid were placed in the
backfill with one end of each layer securely fixed to
the abutment wall. As shown, the layers of
reinforcement were spaced 20 cm. apart and extended
back 4.0 m. from the abutment. The properties of the
backfill soil and geogrid reinforcement are presented
in Table 4-1. To activate settlement of the backfill,
a 20 cm. thick layer of soluble ammonium sulphate was
placed between the backfill and brick base. After
construction this layer was dissolved, thus simulating
settlement due to a weak foundation. In constructing
the backfill, soil was compacted to an optimum water
content of 20% in 20 cm. lifts. Cut tube samples
indicated a fairly uniform moist density of 1.6
tons/m3.
Instrumentation used in the experiment and
shown in Figure 4-1 included the following:
1. 18 in-ground deformation rods per soil
layer, where there were 7 soil layers in
all and each layer was separated by 20
cm


LEGEND
deformation gauge
< strain gauge
earth pressure gauge
V surface settlement point
** tension gauge
Figure 4-1. Diagrammatic representation of large-scale
test (reproduced from Kutara et al., 1985).


35
Table 4-1. Properties of backfill soil and geogrid
reinforcing (Kutara et al., 1985).
Soil *
*
Geoqrid
Parameter Symbol Value
unit weight (tf/m3) 7 1.6
cohesion (tf/m2) c 2.0
internal friction angle 4> 29.0
Poisson's Ratio V 0.4
Young's Modulus (tf/m2) E 800
Soil classification SM
gravel (%) 0.0
sand (%) 50.1
silt (%) 31.9
clay (%) 18.0
weight (gf/m2) 938
spacing (mm.) 22 x 111
area (m2) A 0.000297
tensile strength (kgm/m) E 8000
rupture strain (%) 12.0
40% of rupture strain (%) 3.0
* Unified soil classification


36
2. Strain gages attached to the upper and
lower face of 2 layers of geogrid, where
the layers were located at 20 cm. and 80
cm. depths, respectively.
3. 9 earth pressure gages placed at 3
different depths in the backfill.
4. 22 surface settlement points.
5. 5 tension gages, one attached to the fixed
end of each of the 5 reinforcement layers.
Two tests were conducted; one in which only
soil occupied the backfill and one in which 5 layers of
geogrid were included in the backfill. After the
completion of each test, observations were made as to
the degree and profile of settlement and the
development of cracks and voids in the backfill. In
the case where geogrid was included in the backfill the
redistribution of stresses in the soil and the amount
of straining developing in the geogrid were also
observed. The test results described by the study are
presented below.
Experimental Test Results
In the test conducted using no reinforcement,
it was reported that friction developing between the
backfill and abutment had very little influence on
overall settlement. Between 17-18 cm. of uneven
settlement measured at the top occurred due to


37
activating 20 cm. of settlement at the base. Also, no
significant cracks developed and it was suggested that
fairly uniform settlement was the cause of this.
In the test in which 5 layers of geogrid were
placed in the backfill and attached on one side to the
abutment wall, it was shown that a negligible amount of
settlement occurred at the backfill-abutment interface
when a foundation settlement of 20 cm. was induced.
The backfill surface profile moving back from the
abutment showed fairly smooth, gradual settlement,
eventually reaching 20 cm. of settlement roughly 1.0 m.
behind the abutment. Large cracks and voids were also
described, especially in the localized region adjacent
to the abutment, reportedly caused by the
redistribution of settlement that was prevented from
occurring at the backfill-abutment interface. After
the completion of this test the resultant settlement
profile, vertical pressure measurements, and profile of
axial straining in the geogrid were presented by the
authors (Kutura et al., 1985).
Finite Element Analysis
To represent the boundary and loading
conditions imposed by the test, the finite element
model must consider the best approach in simulating the
induced settlement at the base and the boundary
conditions imposed on the abutment and the


38
reinforcement layers. The finite element mesh used to
model the large-scale test is presented in Figure 4-2.
Two methods of analysis were used to simulate
the settlement induced at the base. In Method 1, shown
in Figure 4-3, foundation settlement was induced by
assigning material properties to the elements of a
foundation layer to cause it to settle gradually to 20
cm. as soil layers were incrementally loaded above it.
To produce a specified amount of settlement, SSTIP
allows the user to assign displacements occurring at a
given node to successively numbered nodes in the mesh.
The material properties of the foundation elements were
modified until the lowest numbered unrestrained node on
I
top of the foundation layer settled 20 cm., and the
succeeding nodes of that layer were then assigned to
settle the same amount, thus achieving 20 cm. of
uniform settlement. Nodal points attaching the bar
elements to the abutment were restrained both
vertically and horizontally but were allowed to rotate,
and the nodes representing soil in contact with the
wall were also completely restrained to simulate "no-
slip" between the soil and the abutment.
In Method 2, presented in Figure 4-4,
incremental construction of the backfill was completed
over a strong foundation which completely supported the
backfill during construction. After construction, 20


.2.
!
w ^
s 5
Eh ^
D v
PQ
<
.10.0 m..

4.0 m.
1.0
N
^--
& *2
fez.
t . 1.85 m.
- ___
1
~'v-
' 4
/ rV/ftr rVrwYr rkrr A>r>VrAr/*>r/rrr A / r/r Ar / r / /A */ft rhr
LEGEND
bar element
Figure 4-2. Finite element mesh of large-scale test.
U)
KD


40
3- layer 7
layer 6
layer 5
'll layer 4
layer 3
s layer 2
layer 1
so: 1 1 ft foundation
/mm ; / P/ tfirtPt / ^t > H r frr ri) 9/t/ r /} /r/r ftr/ f 1/ A r/r/rJ}
Figure 4-3. Method 1; soft foundation condition.
b-. ! layer 7
N V , i 1 layer 6
' s. _ layer 5
i iayer 4
! iayer 3
> ' layer 2
\ ^ > layer 1
'FJr.Jr Drm Jmt i *4 Oc _LL 4r \ t \ ' > > i f stif: i 4 : foundation
ttW+VtfWWi rVfffrrrfrr/h/iyr/Trrfkrr/tHfmfFSftFrrf)
Figure 4-4.. Method 2; load applied to the_ top of
stiff foundation after construction.


41
cm. of settlement was induced by artificially applying
a large uniform pressure load to the bottom of soil
layer 1 (see Figure 4-4) and by assigning nodal
displacements as was done in Method 1. Also, the nodes
representing soil in contact with the abutments were
allowed to move vertically, simulating a "complete-
slip" condition between the soil and the abutment. It
may be noted that although the final foundation
settlement for the two methods of analyses is the same,
Method 1 simulates the condition in which the
foundation is "drained" during backfilling and
settlement of the foundation occurs predominantly in
response to each construction lift; whereas Method 2
represents an "undrained" foundation during backfilling
and settlement of the foundation occurs after the
entire backfilling is completed.
the. study by Kutara et al. to represent the soil
stiffness. As shown by the Duncan-Chang model, the
value of throughout incremental loading can be
expressed by the following parameters (Duncan et al.,
1976):
A constant Young's modules, E^ was reported in
Rf {1 s i n | cr10^3
2
E
t
2c coset> + 2tfg sin4>
Eg. 4-1


42
where Rf is the failure ratio, og represents the
minor principal stress, { v-\ ^3} is the stress
difference, K is the modulus parameter and n is modulus
exponent, Pa equals atmospheric pressure, and c and are the Mohr-Coulomb strength parameters. To represent
a constant Ef, the value of Rf was made very small and
n was assigned a zero value. The parameters used in
the numerical analysis are presented in Table 4-2.
A constant Poisson's ratio value of 0.4 was
also reported as a property of the backfill. To
incorporate this constant value into the Duncan-Chang
model, the following relationship was employed (Wong
and Duncan, 1974):
G-F|oa(^)
t - d {^13 2
*pa\pg/ L 2c cos+ 20gSin<£J
in which ^ is the tangent value of Poisson's ratio
representing the instantaneous slope of the curve
depicting the variation of axial to radial strain, and
G, F, and d are the Poisson's ratio parameters. To
obtain a constant Poisson's ratio, G was assigned a
value of 0.4 and F and d were assigned zero values. It
should however be recognized that assuming a constant
value of ^ is a simplification. For most soils, with


43
Table 4-2. Duncan-Chang hyperbolic stress-strain
parameters and structural bar element
properties.
Parameter Symbol Value
Soil
unit weight (tf/m3) 7 1.6
cohesion (tf/m2) c 2.0
internal friction angle 29.0
modulus number K 77.44
modulus exponent n 0.0
failure ratio Rf 0.0001
Poisson's ratio parameter G 0.4
Poisson's ratio parameter F 0.0
Poisson's ratio parameter d 0.0
Bar Element
Young's modules (tf/m2) E 400000.0
area (m2) A 0.000297


44
the exception of completely saturated soils tested
under undrained conditions, the value of t>t is stress-
dependent, decreasing with confining pressure (Wong and
Duncan, 1974).
Comparison of Results
Settlement profiles simulated using Method 1
and Method 2 in comparison with the actual reported
settlement are shown in Figures 4-5 and 4-6,
respectively. Both cases provide a good representation
of measured settlement although a slightly better fit
was obtained by Method 2, possibly due in part to
allowing "complete-slip" movement of the soil at the
abutment.
Both methods predicted large zones of shear
and tension yielding as can be seen by the distribution
of og and stress level for both cases presented in
Figures 4-7, 4-8, 4-9, and 4-10. The stress level is
defined herein as
a1
where {a. an { is determined from the Mohr-Coulomb
1 1 3 f
criterion as the deviatoric stress at failure. The
larger zone of yielding experienced by Method 2 is
attributed to the activation of foundation settlement
occurring in one loading increment as compared to


LEGEND
measured displacement
---- calculated displacement
Figure 4-5. Method 1; comparison of calculated to
measured soil deformation.


depth (m.
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
0
distance from abutment (m.)

- ^.
v * 1
.

v
1 4 1 - 1 > t * - ~ i r "i r i i > r i i i 'T" i
.0 1.0 2.0 3.0 4.0 5.0 6.
LEGEND
measured displacement
--- calculated displacement
Figure 4-6. Method 2; comparison of calculated to
measured soil deformation.


47
LEGEND
tension zone
Figure 4-7.
Method 1; minor principal stress
distribution behind the test abutment
(tf/m2).
LEGEND
tension zone
Figure 4-8.
Method 2; minor principal stress
distribution behind the test abutment
(tf/m2).


depth (in.) I?. depth (m.
48
jure 4-9
LEGEND
shear yielding
tension yielding
Method 1; stress level distribution behind
the test abutment.
LEGEND
shear yielding
tension yielding
I
Figure 4-10. Method 2; stress level distribution behind
the test abutment.


49
Method 1 where settlement progressed simultaneously
with successive incremental loading, thereby allowing
redistribution of stresses.
A much better simulation of straining in the
geogrid was also achieved by Method 1 shown in Figures
4-11 as compared to 4-12. As discussed in Chapter 3,
axial straining of the bar elements could not be
obtained directly from the output but instead was
determined by calculating the distance between
structural nodes connecting bar elements. Even with
this modification, some bar elements showed a
comparatively small amount of compression. To
represent the true behavior of the reinforcement which
can not withstand compression, compressional straining
calculated by the program was represented as zero
straining in these figures.
As shown by Figures 4-13 and 4-14, the
calculated vertical stresses for both methods was in
range of the measured values but the simulated curves
appeared to fluctuate. This is attributed to
substantial zones of shear and tension yielding,
especially near the abutment face. When a significant
number of soil elements are at failure, the calculated
stresses are no longer correct as the soil model used
in the analysis is limited to non-failure conditions.


axial strain (%)
LEGEND
A measured strain
-- calculated strain
Figure 4-11. Method 1; comparison to calculated to
measured strain.
depth (m.)


51
10.0-
0.0
-0.2
-0.8
0.0 1.0 2.0 3.0 4.0
distance from abutment (m.)
LEGEND
measured strain
---- calculated strain
Figure 4-12. Method 2; comparison of calculated to
measured strain (note that calculated axial
straining increases by an order of
magnitude over Method 1).
depth (m.)


LEGEND
A measured vertical stress
----- calculated vertical stress
Figure 4-13. Method 1; comparison of calculated to
measured vertical stress.


53

LEGEND
measured vertical stress
----- calculated vertical stress
Figure 4-14. Method 2; comparison of calculated to
measured vertical stress.


54
Concluding Remarks
The results of the large-scale test do
show that geogrid attached to the abutment alleviated
backfill settlement at the abutment. However, no
further study was performed to account for surcharge
loading caused by traffic and other loads or the effect
on performance caused by the development of cracks and
voids resulting from attaching the geogrid to the
abutment wall. Thus, it is difficult to assess the
effectiveness of the geogrid reinforcement under
service conditions. It should also be noted that a
constant modulus value was reported for the backfill
soil used in the test. It was then necessary to assume
that the soil behaved elastically in the study. This
is a gross simplification of the stress-dependent
stress-strain behavior of the soil.


CHAPTER'5
PARAMETRIC STUDY
Introduction
To examine the effectiveness of using
reinforcing in the backfill behind bridge abutments
under a range of varying conditions, a parametric study
was conducted. To provide a rigorous analysis the
study would have to; 1) incorporate a geometry and
boundary conditions representative of full-scale
abutment-embankment structures constructed in the
field; 2) include nonlinear stress-strain parameters
representative of foundation, structural fill and
backfill soils; ,3) consider the overburden pressure-
dependent stress-strain behavior of the reinforcing;
and 4) demonstrate the behavior of reinforced backfills
behind bridge abutments under surcharge loading.
Finite Element Discretization
A typical configuration of a geotextile-
reinforced abutment is presented in Figure 5-1. In
Colorado the abutment is typically founded on H- or
pipe piles which allow negligible settlement of the
abutment to take place caused by placement of the


Figure 5-1. Diagrammatic representation of reinforced
bridge abutment.
ui
cn


57
bridge or due to the resulting traffic loads. Where
the bridge span is greater than 200 ft., the Colorado
Department of Highways also specifies that an approach
slab be included in the design to alleviate the
resultant "bump" caused by differential settlement
between the abutment and approach fill. As shown, the
approach slab is usually connected to the abutment and
is therefore considered an integral part of the
abutment. Multiple layers of geotextile reinforcing
are placed horizontally in the backfill and the
geotextile wall facing is separated from the abutment
by the inclusion of a 3-inch thick polystyrene board.
The backfill is generally composed of a well-compacted,
free-draining granular material. However, similar to
the foundation, backfill conditions vary considerably,
dependent on the burrow site from which the material
was obtained and the compaction effort.
The finite element mesh used in this study is
presented in Figure 5-2. Parameters typical of
foundation soils are used to represent the embankment
which is considered a cut-slope in this study. As
shown, the behavior of the geotextile facing is
simulated by allowing the structural nodes connected to
the boundary adjacent to the abutment vertical freedom
of movement. Also, the 1 ft. thick concrete approach
slab is simply represented by a 150 psf pressure load.



LEGEND
bar element
Figure 5-2. Finite element mesh representing reinforced
bridge abutment.
(ji
00


59
It should be pointed out that due to the inclusion of
the easily-deformable 3-inch thick polystyrene board,
the geotextile wall facing can deform laterally outward
up to about 3 inches, depicting the behavior of an
unrestrained reinforced retaining wall. An attempt was
made to model this behavior by using hinges to connect
the bar elements representing the wall facing as shown
in Figure 5-3. Unfortunately, the bar elements
modelling the facing material did not prevent uneven
lateral deformation from occurring along the height of
the wall facing. This in turn caused nonuniform
settlement at the top of the reinforced backfill.
Further study is needed in this area. The model
presented in Figure 5-2 was used in this study and
should provide an adequate simulation of soil-structure
behavior for comparative purposes.
It was also necessary to develope a set of
soi.l parameters representative of foundation soil and
backfill conditions that were commonly encountered in
the field, and material parameters of a geotextile in
which the overburden pressure-dependent behavior of the
reinforcing had been considered. A set of parameters
modeling the foundation, backfill, and geotextile
material properties are presented in Table 5-1. In the
first part of the study these parameters were used to
examine the boundary and loading conditions. Later,


60

Figure 5-3. Finite element mesh in which hinged bar
elements represent the geotextile wall
facing.


Table 5.1
Properties of initial soil and reinforcing parameters
Backfill
Unified Soil Class. RC* (%) 7 (pcf) 4> (deg) C (pcf) K n Rf G F d
GP 95 125 36 50 300 0.40 0.7 0.30 0.0 0.0
Foundation
CL 120 32 1000 150 0.45 0.7 0.30 0.0 0.0
Geosvnthetic
Type
Geogrid
Secant modules
(psf)
Area Thickness
(ft2) (mil)
112000.0
0.01042 125
* RC = relative compaction (Standard AASHTO)


62
they were substituted by new parameters to examine the
behavior of the reinforced abutments under different
foundations, backfills and reinforcements.
Pilot Analysis Parameters
The structural bar element parameters shown
in Table 5-1 represent properties of the reinforcement
obtained from the results of pull-out tests conducted
on a nonwoven geotextile, Trevira 1127, under a range
of different overburden pressures and using dry #30
Ottowa sand as the confining soil (Siel, 1986). The
density of the Ottowa sand was 107 pcf (70% relative
density). Figure 5-4 shows the variation in secant
modulus at 2% strain mobilization in the geotextile
tested over a range of overburden pressures. Assuming
overburden pressures behind a typical abutment to range
from 0-8 psi, a modulus value was extrapolated from
this curve. The cross-sectional area used for the
reinforcement was determined given the thickness of the
geotextile reported by the manufacturer.
The stress-strain behavior of the geotextile
is also dependent of the properties of the soil under
which it is confined. To account for both the
overburden pressure-dependent and soil-dependent
behavior of the geotextile used in the analysis,
rigorous testing was performed to obtain the Duncan-
Chang model parameters of the Ottowa sand used to


63
secant modulus at 2% strain (psi)
Figure 5-4. Stress-strain relationship for in-
soil geotextile (modified from
Siel, 1986).


64
confine the nonwoven geotextile. A finite element
analysis could then be conducted using the parameters
obtained for both the soil and reinforcement in which a
common soil was used. Three triaxial compression tests
were conducted on dry #30 Ottowa sand prepared at a
density of 107 pcf (70% relative density) under 5 psi,
10 psi, and 15 psi confining pressures, respectively,
and under a rate of loading of 0.5%/min.. The
hyperbolic material parameters were then determined
from the test results. Table 5-2 shows the resulting
stress-strain and volume change parameters obtained
from these tests. To verify that the parameters could
adequately represent the constitutive behavior of the
sand, the simulated stress-strain curves corresponding
to the three tests conducted under different confining
pressures are compared to the actual test curves as
shown in Figure 5-5. These comparisons demonstrate
that the nonlinear, stress-dependent stress-strain
behavior of the soil is accurately modeled by the
Duncan-Chang parameters. The simulated curves were
determined by back-calculation using the parameters
given in table 5-2 and by using the following
hyperbolic relationship:
6
Eq. 5-1


65
Table 5-2.
Hyperbolic stress-strain parameters for
#30 Ottowa sand.
Parameter
unit wt. (pcf)
cohension (psf)
internal friction angle
modulus number
modulus exponent
failure ratio
Poisson's ratio parameter
Poisson's ratio parameter
Poisson's ratio parameter
Symbol Value
7 107
c 0.0
36(2)
K 2700
n 1.05
Rf 0.96
G 0.3
F 0.0
d
0.0


66
LEGEND
----- experimental
----- calculated
Figure 5-5. Calculated and experimental stress-strain
curves for dry #30 Ottowa sand.


67
in which { Oj ct^ } is the deviatoric stress at failure
and E-t is determined for a given strain using the
relation presented by equation 4.1. Therefore, the
stress difference, { _ 03 | for a given amount of
strain, e can be calculated. In light of the fact
that the Poisson's ratio parameters do not generally
provide a reliable simulation of volume change
characteristics, a reasonable value of Poisson's ratio
was assigned for the Ottowa sand as well as other soils
in this study.
Boundary And Loading Conditions
The boundary conditions imposed by the
unreinforced soil resting against an abutment wall are
comparatively different from the conditions established
by the geotextile-reinforced backfill shown in Figure
5-1. It was therefore necessary to investigate these
conditions more carefully prior to conducting the
parametric study. Also, a practical solution was
required to simulate complex traffic loads and other
loads exerted downward onto the earth structure.
The boundary conditions imposed by the
unreinforced backfill were first examined. Two cases
were tested. In one case the backfill soil was
completely restrained from movement along the abutment
boundary, simulating a "no-slip" condition. In the
second case vertical movement was allowed along this


68
boundary, depicting a "free-slip" condition. The
parameters input to represent the foundation and
backfill are those presented in Table 5-1. A
comparison of settlement curves presented in Figures 5-
6 and 5-7 indicated a more gradual settlement profile
depicted by the "no-slip boundary. By completely
fixing the nodal points along the abutment boundary,
however, a narrow zone of shear and tension yielding
resulted as shown by the shear stress level and minor
principal stress distribution diagrams presented in
Figures 5-8 and 5-9, respectively. This is compared to
the more uniform shear stress profile and nearly
horizontal minor principal stress distribution diagrams
shown in Figures 5-10 and 5-11, respectively, which
described the behavior of the "free-slip" boundary
conditions. The actual soil-abutment interface
behavior resides somewhere in between these two
conditions. In the large-scale test presented in
Chapter 4, the backfill behavior was first tested
without the inclusion of geogrid and it was found that
both slip and soil yielding occurred along the wall
when the foundation settled about 14% of the fill
height. It was observed that the zone of slip and
yielding was restricted to a narrow zone adjacent to
the face and did not significantly influence the
profile of settlement resulting at the top of the


depth (in.) ^ depth (in.
69
-12.0*
.gure 5-6,
Settlement curve for "no-slip" condition.
0.0
-4.0
-8.0
*12.0
distance from abutment (ft.)
v > t > t rrrr r t r-7mr r t f r'r t fr-r'r r r ^ ; t
D.O 7.0 14.0
Figure 5-7. Settlement curve for "free-slip" condition.


depth (ft.) £ depth (ft.
70
LEGEND
^ shear yielding
gure 5-8. Stress level distribution for "no-slip"
condition.
Figure 5-9.
Minor principal stress distribution for
"no-slip" condition (in psf).


71
Figure 5-10. Stress level distribution for "free-slip"
condition.
Figure 5-11. Minor principal stress distribution for
"free-slip" condition (in psf).


72
backfill. Because the settlement behavior is of
primary concern in this analysis, the "free-slip"
condition was therefore imposed along the abutment
boundary for the remainder of the study.
In the literature, several studies have
attempted to simulate the behavior of traffic loads by
first representing the dynamic vehicle load as a moving
point load and then determining an equivalent line load
using two-dimensional plain strain finite element
analyses. (Duncan, 1979? Jeyapalan and Lytton, 1983;
Duncan, Seed and Drawsky, 1985). The stress
distribution at a "critical" depth caused by a point
load representing the axle and wheels of a vehicle are
initially determined either by using a Boussinesq
relationship or by measuring actual stress developing
at this depth directly in the field. Then, using two-
dimensional plain strain finite element analysis, an
equivalent line load which produces approximately the
same stresses at this depth is determined. In the case
of the box culvert presented as an example in Figure 5-
12, the equivalent line load is placed at a critical
point directly above the culvert. It is assigned a
load value that produces the approximate distribution
of stresses at the crown that would have been imposed
under the point load of a vehicle axle.
Simulation of traffic loads onto bridge


line load
-------* VMM
critical depth
Figure 5-12. Cross-section of box culvert.


74
abutment earth structures, however, is much more
complex. First, the equivalent line load must simulate
live loads distributed over a range of depth
encompassing the entire earth structure as a "critical"
depth is probably non-existent. Second, the
distribution of stresses through the approach slab
itself needs to be considered. Third, under these
conditions the most critical point along the travelling
surface for the placement of the line load would have
to be assessed. In view of this, a rigorously
performed simulation of live loading was not
incorporated into this study. Instead, a uniform
surcharge pressure was applied to the top of the
backfill at the end of construction to examine the
behavior of the reinforced backfill under surcharge
loading. The value of the surcharge load was assigned
to cause a reasonable amount of settlement to occur at
the surface of the earth structure employing soil
parameters presented in Table 5-1. Although the amount
of settlement behind abutments can vary extremely from
inches to feet in practical construction, a commonly
anticipated amount of settlement is between 1-6 inches.
Therefore, roughly 4 inches of settlement was induced
using a uniform vertical surcharge pressure load of
1000 psf on top of the structure. This approach is
very simplistic; however, it is considered adequate for


75
comparative purposes.
Parametric Analysis
The analyses employed in the parametric study
are grouped as follows:
Group 1. The abutment-embankment system was
first analyzed using the parameters of Table
5-1 with and without the inclusion of
geotextile reinforcing.
Group 2. The same conditions were imposed as
for Group 1 with the exception that the
uniform surcharge load of 1000 psf was imposed
at the end of construction.
Group 3. To examine the effect of backfill
soil on the stress-strain behavior of the
geotextile reinforcing, the parameters used to
model the behavior of Ottowa sand presented in
Table 5-2 were substituted in place of the
backfill parameters provided by Table 5-1.
All other conditions imposed by Group 2 were
kept the same.
Group 4. The behavior of the earth structure
with and without reinforcing was analyzed
under different foundation and backfill
conditions and by improving the strength of
the reinforcing material.
In Group 1, in which all settlement was due to


76
the weight of the earth structure, placing multiple
layers of reinforcing into the backfill was found to
have no measurable effect on alleviating settlement.
The amount of settlement shown in Figure 5-7 is nearly
identical to the amount of settlement occurring when
the geotextile was emplaced. Likewise, the
distribution of stresses and overall stability in the
soil was not noticeably affected.
This was also the case in Group 2 in which a
surcharge load was introduced. The resulting
settlement is shown in Figure 5-13. The reason why is
partly due to the boundary conditions. Figure 5-14
depicts the resulting soil movement that occurred in
Group 2 in which a surcharge load was applied to the
earth structure without the inclusion of the
reinforcement layers. A lateral component of soil
movement can be seen in the foundation in the direction
of the cut-slope. In contrast, the backfill soil shows
nearly exclusive vertical soil movement, caused in part
by the lateral restraints imposed by the abutment.
Tensile reinforcements such as geotextiles add
increased strength to a soil by restraining soil
deformation, through frictional and passive resistance,
parallel to the plane that the reinforcement is
emplaced. Because the reinforcement was placed
laterally whereas a negligible amount of lateral soil


depth (in.
0.0
-4.0
-8.0
-12.0
0.0 7.0 14.0
distance from abutment (ft.)
-
1 '>/' l II 1 r r /-# ITT T" 1 1 fin
1
Figure 5-13. Group 2; settlement curve.
S / / /
/ / 4 4
1 4 4 4 4 4 4 4 4
4 4 4 4 4 + 4 4
4 4 4 4 4 4 4 \ 4
4 4 4 4 4 4 4 i 4
/ ; 4 4 4 4 4 4 4
4 4 4 4 4 4 4 4 4
Figure 5-14. Group 2; soil movement diagram.


78
movement occurred, the analysis indicates that the
tensile resistance of the reinforcing was not activated
in any measurable capacity. This claim is further
substantiated by the distribution of strains resulting
in the bar elements presented in Figure 5-15. It should
be noted that because the amount of deformation is
negligible, the strain distribution shown can easily be
due to round-off errors in the analysis.
In Group 3, the soil parameters developed from
triaxial compression tests on the Ottowa sand shown in
Table 5-2 were used in place of the backfill soil
parameters provided in Table 5-1. The settlement
profile resulting from construction and by imposing the
surcharge load is presented in Figure 5-16. Even with
the fairly different backfill soil, the settlement
profiles resulting with and without the inclusion of
the geotextile reinforcement are practically identical.
This is also true of the resultant stress distributions
calculated for both conditions.
To further substantiate the finding presented
by the study, three additional conditions were analyzed
in Group 4 to examine the behavior of reinforced
backfills under a weaker backfill, a weaker foundation,
and by using a stronger reinforcement, respectively.
Parameters representing each of these conditions are
presented in Table 5-3 and were substituted in place of


depth (in.
79
<#>
c
H
rt
U
-P
W
(C
H
X
rt
0.1-
o.o:
i
0.0
-1.8
o. o:-
0.0 4.0 8.0 12.0 16.0
distance from abutment (ft.)
-7.2
LEGEND
----- calculated strain
Figure 5-15. Group 2; strain distribution in
reinforcement.
0.0
-12.0
0.0
7.0
14.0
distance from abutment (ft.)
Figure 5-16. Group 3; settlement curve.
depth (ft.)


Table 5.3
Properties of parameters used to model changing foundation,
backfill and reinforcement conditions.
Backfill
Unified Soil Class. RC* (%) 7 (pcf)

SP 90 120 32 50 300 0.25 0.7 0.30 0.0 0.0
Foundation
CL 115 30 500 120 0.45 0.7 0.30 0.0 0.0
Geosvnthetic
Type
Geogrid
Secant modules
(psf)
Area Thickness
(ft2) (mil)
79200000.0
0.0125 150
* RC = relative compaction (Standard AASHTO)
00
o


81
their corresponding parameters shown in Table 5-1 for
each condition that was studied. For example, in
examining the settlement behavior of the earth
structure under a weaker foundation, the foundation
parameters of Table 5-3 were substituted in place of
the foundation parameters supplied by Table 5-1. All
other parameters in Table 5-1 remained the same. The
settlement curves corresponding to changing backfill
and foundations conditions is presented in Figures 5-17
and 5-18, respectively, and the settlement curve
resulting from using a stronger reinforcement is the
same as that shown in Figure 5-13. In all three
conditions, the analysis indicated that the inclusion
of the tensile reinforcement did not alleviate
settlement; nor did it influence the distribution of
stresses occurring during incremental loading.
Concluding Remarks
The results of the parametric study tend to
show that, under similar compaction efforts and
boundary conditions, settlement of the backfill is not
alleviated by the inclusion of soil reinforcing. The
findings indicate that, due in part to the boundary
restraints imposed on the abutment wall, the backfill
soil tends to move vertically downward and the amount
of lateral soil deformation needed to mobilize the
tensile resistance of the reinforcing is negligible.


82
c
-4.0- ?>'//// / / / / ? > d t f > ? > /1 i ) i / > j i i /f f / / f / j /
xi CD 0 1
p
ft
a)
T3 -12.0.
0.0 7.0 14.0
distance from abutment (ft.)
Figure 5-17. Group 4; settlement curve for case where
backfill parameters shown in Table 5-3 were
used.
0.0-
Zi -4.0-
£ -8.0-
ft
0)
O -12.0--
0.0 77o 14.0
distance from abutment (ft.)
Figure 5-18. Group 4; settlement curve for case where
foundation parameters shown in Table 5-3
were used.


83
These conclusions are also supported by similar
findings resulting from a recent study conducted at
Purdue University in which the behavior of
reinforcements placed in embankments (which have less
lateral restraint than the backfill placed behind
bridge abutments) of different geometries were
analyzed. The results of this study indicated that
while the use of reinforcements can effectively reduce
shear and lateral strains and increase embankment
stability, they have little effect on alleviating
vertical settlement (Humphrey, 1986).


CHAPTER 6
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
FOR FURTHER STUDY
Summary
Multiple layers of tensile reinforcement are
being emplaced in the approach backfill adjacent to
bridge abutments to reduce differential settlement
between the backfill and bridge abutment. By
restraining lateral deformation of the soil, it is
claimed that a stiffer composite backfill material is
created that can distribute settlement and stresses
caused by traffic and other loads over a greater area,
thus reducing differential settlement. A study was
undertaken to investigate the effectiveness of using
tensile reinforcement to alleviate backfill settlement.
The study was performed using a nonlinear finite
element analysis procedure. A preliminary study was
first performed to verify the analytical procedure for
simulating the complex behavior of soil-reinforced
abutments. In this study, a large-scale reinforced
bridge abutment was analyzed. Two methods of
simulation were used; the first in which foundation
settlement occurred during construction loading of the


85
backfill (Method 1), and the second in which foundation
settlement was imposed after construction was completed
(Method 2) A parametric study was then conducted to
examine the effectiveness of reinforcing to alleviate
backfill settlement under different foundation,
backfill and reinforcement conditions. As a part of
this study, the results of triaxial tests conducted on
#30 Ottowa sand and tensile strength tests conducted on
a nonwoven geotextile, Trevira 1127, were incorporated
to account for the overburden pressure-dependent and
soil-dependent behavior of the geotextile.
Conclusions
Based on the findings of this study, the
following conclusions are advanced:
1. By comparing the analytical results with
the measured data for the large-scale
reinforced bridge abutment test conducted
by the Japanese Ministry of Construction,
the analytical procedure employed in this
study is correct.
2. Although substantial local yielding
resulted in the soil, both methods of
analysis used to simulate the soil-
reinforcement interactive behavior of the
large-scale test provided an excellent
approximation of the measured settlement.


86
Although Method 2 more realistically
simulated the loading sequence employed by
the test, Method 1 provided a better
representation of the range of strains
developing in the geogrid reinforcing and
the distribution of vertical stress
Occurring in the backfill.
3. The analyses indicate that the placement
of multiple layers of geotextile in the
backfill did not reduce differential
settlement between the abutment and the
backfill. This is because there is little
lateral straining in the backfill due in
part to the lateral restraints imposed by
the abutment. The nearly vertical soil
movement that resulted caused the tensile
resistance of the laterally placed
reinforcement layers not to be activated
in any measurable capacity. The apparent
success indicated by the Wyoming Highway
Department more likely results from the
improved backfill stiffness due to
inclusion of geotextile as a compaction
aid.
Recommendations for Further Study
The parametric study presented in Chapter 5


87
represents a preliminary investigation and further
verification is needed. The following recommendations
are made for additional study:
1. A full-scale controlled test should be
conducted to examine the behavior of abutments
with and without the inclusion of tensile
reinforcing in the backfill. Strains induced
in the reinforcing during construction of the
backfill should be separated from those
strains resulting from post-construction
settlement and traffic loads in order to
evaluate the strains occurring in the
reinforcements due to construction loading and
compaction.
2. An experimental testing procedure should be
developed to compare the strengths of
identical soils under the same compactive
effort in which one soil contains tensile
reinforcements. The results would be used to
analyze the benefits of employing
reinforcement as a compaction aid to increase
soil stiffness.
3. A large-scale test modeled after the test
described in Chapter 4 is also recommended.
Foundation settlement may be better controlled
to analyze a variety of foundation conditions


88
using hydraulic jacks placed underneath a
rigid metal plate. Also, surcharge loading
due to traffic and other loads should be
simulated and the development of cracks should
be evaluated to assess the effectiveness of
the reinforcing under service conditions.