Influence of Horizontal Toe Restraint on Reinforced Soil Retaining Wall Behaviour Fawzy Ezzein, Graduate student, GeoEngineering Centre at Queen’s-RMC, Queen’s University, Kingston, Ontario, Canada Richard J. Bathurst, Professor, GeoEngineering Centre at Queen’s-RMC, Department of Civil Engineering, Royal Military College of Canada, Kingston, Ontario, Canada ABSTRACT The research program described in this paper investigates the influence of magnitude of horizontal toe compliance on the performance of 1/6-scale reinforced soil retaining walls at end of construction and during subsequent staged surcharge loading. The walls were 1.2 m high and were constructed with a very stiff horizontal toe support, a free support and with two different spring arrangements resulting in horizontal reaction stiffness values falling between these two limiting conditions. The data showed that the wall performance was sensitive to the range of toe boundary stiffness conditions investigated. As horizontal toe stiffness increased the following observations were made: a) wall deformations decreased but the displacement mode changed from uniform translation to rotation about the toe; b) the toe carried progressively more of the total horizontal earth force acting at the back of the facing column, and; c) strains in the reinforcement layers were attenuated, particularly at the bottom of the wall. RÉSUMÉ Le programme de recherche décrit dans cet article étudie l’influence de l’ampleur de la souplesse horizontale de la base du revêtement sur la performance de murs de soutènement en sol renforcé à une échelle de 1/16, à la fin de la construction et durant des étapes subséquentes de chargement. Les murs avaient 1.2 m de hauteur et furent construit avec un support rigide de la base, avec une base libre et avec deux configurations de ressorts offrant des valeurs intermédiaires de rigidité horizontales. Les données ont montré que la performance du mur était sensible à l’éventail des conditions limites de rigidité de la base à l’étude. Sous l’augmentation de la rigidité horizontale de la base les observations suivantes ont été faites: a) les déformations du mur ont diminué mais le mode de déplacement a changé d’une translation à une rotation autour de la base; b) la base supporte une proportion croissante de la force totale horizontale des terres contre la colonne de revêtement, et; c) les déformations des couches de renforcement se sont atténuées, en particulier à la base du mur. 1. INTRODUCTION The performance of a reinforced soil wall is understood to be influenced by the properties of the foundation on which the structure is seated. Settlement of the structure can be expected to influence the magnitude and distribution of reinforcement loads under serviceability and ultimate (collapse) conditions. Similarly, the lateral stiffness of the foundation soil over the embedded depth of the wall facing can be expected to influence wall behaviour. Nevertheless, current limit-equilibrium based design methods used to compute reinforcement loads ignore the influence of soil compliance below and in front of reinforced soil wall structures. Chou and Wu (1993) carried out a parametric analysis of the influence of foundation rigidity on the behaviour of an idealized geosynthetic reinforced soil wall structure with a timber facing using a finite element model (FEM) approach. Maximum wall deformations were observed to increase by a factor of six between a wall constructed over a rigid foundation and the nominal identical structure built over a soft clay foundation. The mode of wall deformation was also observed to change from predominantly lateral deformation to lateral displacement and rotation about the toe as the rigidity of the foundation

decreased. Reinforcement tensile strains also increased with decreasing foundation stiffness. Rowe and Skinner (2001) carried out a similar investigation using a FEM code to model foundation stiffness effects on a 8 m-high wall constructed using a modular block (segmental) construction. They reported that the soft compressible foundation case resulted in wall deformations that were 150% greater than the same wall built on a rigid foundation. In the same study, they showed that reinforcement strains increased between 80 and 350% for the wall built on a soft foundation. Similar trends in wall response using small-scale physical models have been reported by Palmeira and Monte (1997). A series of full-scale model physical tests on geosynthetic reinforced soil walls has been reported by Bathurst et al. (2000). They measured the horizontal toe force generated at the base of 3.6 m-high modular block walls and the connection loads between the facing and the geosynthetic reinforcement layers. They showed that the vertical and horizontally constrained toe carried about 40% of the horizontal component of earth load acting on the back of the facing. This significant contribution is not considered in current limit-equilibrium based design methods. The practical consequence of this observation

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Displacement potentiometer

Load ring Extensometer Strain Gauge Load cell

Soil pressure cell

1216 mm

133 mm

133 mm

190 mm

190 mm

190 mm

190 mm

190 mm

800 mm

2300 mm

Figure 2 – Cross-section view of model wall and instrumentation

1570 mm

2300

mm

1216

mm

Segmental wall face

Surcharge system

Settlement monitoring

Instrumented footing

Figure 1 – View of test facility strong box and model wall

is that current design methods to estimate reinforcement loads are excessively conservative. In this paper, we report the results of a series of 1/6-scale model tests of geosynthetic reinforced soil walls constructed with different magnitudes of horizontal compliance at the footing. The walls were 1.2 m high and were constructed with a very stiff horizontal toe support, a free support and with two different spring arrangements resulting in horizontal reaction stiffness values falling between these two limiting conditions. 2. EXPERIMENTAL APPROACH 2.1 Test Facility The wall models were constructed in a strong box seated on a rigid base (Figure 1). The strong box is 1.5 m high and 1.57 m wide. The backfill soil extends to a distance of 2.3 m from the front of the wall models. The base of the facility was constructed from four sheets of stiff plywood over a concrete floor. The sidewalls of the test facility are comprised of 18 mm-thick Plexiglas stiffened by an arrangement of steel braces. Sidewall friction was minimized by placing two layers of clear lubricated polypropylene sheets over the Plexiglas. The combination of friction reduction, stiff external bracing and a model width to height ratio of 1.3 resulted in boundary conditions approaching an idealized plane strain condition. A set of hollow structural steel sections spanning the width of the

test facility provides the reaction for air bags used to apply a uniform surcharge to the wall backfill. The reaction beams are anchored to the strong floor of the laboratory using threaded rods. Four layers of plywood are used to transmit the air bag loads to the reaction beams. The airbag arrangement allows pressures up to 50 kPa to be applied to the backfill surface.

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Table 1 – Reinforcement properties

Property Original Modified

Aperture size (mm x mm) 27 x 22 81 x 22

Number of longitudinal members per metre width

36 12

Ultimate strength (kN/m) 17.5 5.6

Strength at 5% strain (kN/m) 4.4 1.3

Stiffness at 5% strain (kN/m) 88 26

Note: strength and stiffness properties from single strand tensile tests carried out at 10% strain per minute.

particle size (mm)

0.010.1110

perc

enta

ge fi

ner

by w

eigh

t (%

)

0

20

40

60

80

100

SandSilt or ClayGravel

Coarse Medium Fine

Figure 3 – Particle size distribution for sand backfill

2.2 General Arrangement of Wall Models Four walls were constructed in this investigation (Walls 8, 9, 10 and 11). The walls were constructed with different horizontal stiffness at the footing which is explained in detail later in the paper. The wall numbering convention was adopted to follow a larger research program currently underway that is focused on the influence of a number of factors on model wall performance including the type of backfill and the influence of vertical foundation compliance. The results of these tests are reserved for future publications. The general arrangement of the model walls and instrumentation is illustrated in Figure 2. The models were built to a scaling factor of 1/6 with respect to prototype scale. The facing of the model walls was constructed from 32 stacked hollow structural steel sections (76 mm by 38 mm with a wall thickness of 4.8 mm). The sections were connected by shear pins. The facing column was seated on an instrumented footing. The footing was supported vertically by six rigid load cells and restrained laterally by a system of load rings and springs (for three of the walls). The vertical and horizontal toe load components were decoupled by mounting the footing on a series of frictionless linear bearings. This arrangement is described in more detail later. Six layers of geogrid reinforcement were attached to the facing and extended 800 mm into the sand backfill. 2.3 Materials 2.3.1 Backfill A uniformly graded washed beach sand with a mean particle size (D50) of 0.55 mm was used as the soil backfill. The particle size distribution for this material is presented in Figure 3. This material was selected because it has a flat compaction curve and repeatable mechanical properties. The plane strain friction angle of the sand interpreted from laboratory plane strain tests is 44 degrees (Hatami and Bathurst 2005). The sand was placed in 190 mm-thick lifts and compacted to a bulk density of 1.68 kg/m3 using a hand plate tamper. 2.3.2 Reinforcement A commercially available knitted and coated polyester geogrid was used for the geosynthetic reinforcement layers. This product was selected because it was the lowest stiffness PET geogrid material available. In addition, the properties of the PET geogrid are sensibly independent of creep effects, which assisted greatly with interpretation of reinforcement strains and loads. In accordance with the scaling laws proposed by Iai (1989), two of every three longitudinal members of the geogrid were removed. Hence, the stiffness of the modified reinforcement was consistent with the 1/6 model scale adopted in this investigation. Reinforcement properties are summarized in Table 1.

2.4 Toe Condition The objective of the current study was to investigate the influence of horizontal toe compliance (stiffness) on wall performance. Wall 11 was constructed with no lateral restraint at the footing. The toe of the other three walls was restrained laterally using three different configurations (Figure 4). The control case (Wall 8) was built with two horizontal load rings. This was the stiffest toe condition. Nevertheless, a small compliance due to the horizontal load rings was present in this test as reported later. Walls 9 and 10 were built with combinations of springs in an attempt to achieve target stiffness values of k = 60 kN/m and 15 kN/m, respectively. Wall 11 with no lateral toe support corresponds to the idealized case of k = 0. The spring stiffness values were selected based on the results of numerical (FLAC) models to ensure detectable

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Facing

Rigid support

Load Ring

Springs

a) Wall 10 (k = 14 kN/m)

Facing

Springs

Rigid support

Load Ring

b) Wall 9 (k = 68 kN/m)

Facing

Plexiglas base

Load Ring

Rigid support

c) Wall 8 (k = 964 kN/m)

Figure 4 – Arrangement of horizontal load rings and compression springs

quantitative differences in wall performance. No attempt was made to relate the toe stiffness values to values that would be expected for a field wall at prototype scale. Each constrained configuration was rigidly connected to a crossbeam anchored to the laboratory strong floor in front of the model wall. To ensure that the alignment of the springs remained perpendicular to the wall face, a series of stiff rods were passed through the springs. Horizontal toe deflections were recorded by displacement potentiometers (Figure 2). 2.5 Instrumentation A total of about 120 instruments, including those described in the previous section, were installed for each wall model (Figure 2). Strain gauges were directly bonded to the longitudinal polyester strands of the reinforcement layers. Wire-line extensometers were attached to the reinforcement and monitored by displacement potentiometers mounted at the back of the test facility. Horizontal connection loads between the reinforcement and the facing column were recorded by three load rings at each reinforcement layer (total of 18 load rings). Six load cells were used to measure the vertical toe loads in all tests and two load rings to measure the horizontal toe loads for restrained footing models. Wall facing displacements in the horizontal direction were recorded by a single column of displacement potentiometers mounted against the centreline of the wall face at the reinforcement elevations. Two displacement potentiometers were used to record the horizontal toe movement. Displacement potentiometers were also arrayed along the top of the facility to record backfill settlements. Finally, three earth pressure cells were embedded in the foundation to record vertical foundation pressures over the course of each test. The measurements from instruments were recorded using a data acquisition system controlled by a PC. 2.6 Wall Construction The wall facing units (hollow steel sections) were placed from the bottom-up in 190 mm vertical sections matching the lifts of compacted sand backfill. The wall facing was braced externally during construction and no horizontal load was carried by the restrained footing models during this stage. Following construction the air bag surcharging system was installed. Initial readings were taken and the external props removed (end of construction). Next, a series of constant surcharge pressure increments of 5 kPa was applied to the backfill surface using the air bag arrangement. Each surcharge load increment was maintained for 24 hours. Surcharging was continued to a maximum value of 50 kPa and then the wall unloaded in 10 kPa increments held for a duration of 24 hours. Finally, the wall was carefully excavated in horizontal lifts to examine the reinforcement and to survey vertical settlements at each reinforcement layer.

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toe horizontal displacement (m)

0.000 0.005 0.010 0.015 0.020

horiz

onta

l toe

forc

e (k

N)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

k = 14 kN/m

k = 68 kN/m

k = 964 kN/mWall 8

Wall 9

Wall 10

Figure 5 – Back-calculated horizontal footing stiffness

14

68

k (kN/m)

964

0

surcharge pressure (kPa)

0 10 20 30 40 50 60

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onta

l toe

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e (k

N)

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Wall 8Wall 9Wall 10Wall 11

Figure 6 – Horizontal toe forces

surcharge pressure (kPa)

0 10 20 30 40 50 60

vert

ical

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forc

e (k

N)

2

4

6

8

10

12

Wall 8 Wall 9Wall 10 Wall 11

14

68

k (kN/m)

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k (kN/m)

964

Figure 7 – Vertical toe forces

3. TEST RESULTS Due to page constraints only selected test results are presented. Figure 5 shows the measured horizontal toe stiffness for the three walls with a horizontally constrained toe condition. The data points were calculated from measured footing loads and displacements after the props were removed. The measured values for Walls 9 and 10 are close to the target values reported in Section 2.4. Wall 8 has a horizontal toe stiffness that is 14 and 69 times greater than that of Walls 9 and 10, respectively. The total horizontal force recorded at the toe of each wall is plotted against surcharge pressure in Figure 6. With the exception of the laterally unrestrained test (Wall 11), the horizontal toe force can be seen to increase with increasing surcharge pressure. However, for the same surcharge pressure, the magnitude of horizontal toe force can be seen to increase with increasing lateral footing stiffness. At the end of construction, the stiffest toe condition (Wall 8) generated toe loads that were 16 and 33 times greater than for Walls 9 and 10, respectively. At a surcharge pressure of 50 kPa, the toe load for Wall 8 was about 3 and 10 times greater than for Walls 9 and 10, respectively. The influence of toe stiffness on vertical footing loads is presented in Figure 7. As surcharge pressures increase the magnitude of vertical toe increases. This is likely due to friction between the back of the facing column and the soil and down-drag forces due to the soil hanging up on the reinforcement at the connections with the wall. The data shows that differences in vertical loads are not as great as those recorded for the horizontal toe forces and the vertical toe response is similar for all walls with unrestrained or soft spring toe conditions. For example, at the end of construction, Wall 8 recorded a vertical load that was about 1.4 times that of the other walls. At the maximum surcharge pressure increment of 50 kPa the vertical toe loads for Walls 9, 10 and 11 were within 9% of the value recorded by the stiffest wall (Wall 8). Wall displacement profiles recorded at the end of the 50 kPa surcharge load increment are plotted in Figure 8. The datum for these measurements is the vertical orientation of the facing column just prior to prop release. The data shows that as the horizontal stiffness of the toe support decreases the magnitude of lateral deformation at the base of the facing column increases. Furthermore, as the stiffness of the toe support increases the rotation of the wall with respect to the toe increases. For Wall 11 that was laterally unrestrained at the toe, the top and bottom of the wall displaced laterally by about the same amount. Figure 9 shows connection and horizontal toe loads measured at the end of the 50 kPa surcharge load increment. The figure shows that for the stiffest toe condition, the restrained toe carries a load that is much larger than any individual connection (reinforcement) load. The lowest reinforcement layer for this wall carries less load than the bottom layer of any of the other more

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facing horizontal displacement (mm)

0 10 20 30

elev

atio

n (m

m)

0

200

400

600

800

1000

1200

1400

1468k = 964 0 (kN/m)

14 68 964 (kN/m) k = 0Wall 8Wall 9Wall 10Wall 11

Figure 8 – Horizontal facing displacements at end of 50 kPa surcharge

68k = 14 0

14 68 k = 964 (kN/m) 0

964 (kN/m)

horizontal load (kN)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

elev

atio

n (m

m)

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800

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Wall 8Wall 9Wall 10Wall 11

connections

toe

Figure 9 – Horizontal loads at connections and wall toe at end of 50 kPa surcharge

horizontal toe stiffness, k (kN/m)

10 100 1000

ratio

of h

oriz

onta

l toe

load

to

sum

of c

onne

ctio

n an

d to

e lo

ads

(%)

0

10

20

30

40

50

60

68

964

14

ratio = 27.2 log(k) -27.2

Figure 10 – Fraction of total horizontal earth force carried by wall toe versus stiffness of wall toe

compliant toe models. It is interesting to note that the stiffness of the toe for Wall 8 is a factor of 37 times stiffer than the reinforcement layers (i.e. k = 964 kN/m for the toe and k = 26 kN/m for the reinforcement at a strain of 5% in Table 1). For Wall 10, the toe reaction stiffness is k = 14 kN/m which is about one half of the stiffness of the reinforcement. Not surprisingly, the footing load is lower than the connection loads for this wall. The contribution of the restrained horizontal toe to the total horizontal earth load acting against the facing column can be appreciated from Figure 10. The fraction of the total load carried by the toe increases with increasing horizontal toe reaction stiffness. For the stiffest toe case (Wall 8), the toe carries about 50% of the total earth load. For the wall models in this study, there is an approximate log-linear relationship between the relative contribution of the restrained toe to load capacity and toe stiffness. Figure 11 shows the magnitude and distribution of strains recorded at the end of the 50 kPa surcharge load for the walls with the stiffest and least stiff (free) toe conditions (Walls 8 and 11, respectively). The strains were calculated from measurements recorded by pairs of extensometer points attached to the geogrid layers. Consistent with the data reported for wall deformations in close proximity to the toe, the strains in the lower reinforcement layers for the wall with the free toe condition are greater than for the stiffer toe condition. The differences in strain values diminish as the elevation above the toe increases. 4. CONCLUSIONS An experimental program was carried out using four 1/6-scale reinforced soil models 1.2 m in height. The walls were constructed with six layers of PET geogrid layers, a high quality sand backfill and a structural facing. The walls were heavily instrumented and were stage surcharge loaded following end of construction. The difference between tests was the horizontal stiffness at the footing supporting the wall facing. The data showed that the wall performance was sensitive to the range of toe boundary stiffness conditions investigated. As horizontal toe stiffness increased the following observations were made: a) wall deformations decreased but the displacement mode changed from uniform translation to rotation about the toe; b) the toe carried progressively more of the total horizontal earth force acting at the back of the facing column, and; c) strains in the reinforcement layers were attenuated, particularly at the bottom of the wall. 5. ACKNOWLEDGEMENTS The writers would like to acknowledge the contribution of Simon Leung who assisted with the experimental work reported here.

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References Bathurst, R. J., Walters, D., Vlachopoulos, N., Burgess,

P. and Allen, T. M. (2000) Full Scale Testing of Geosynthetic Reinforced Walls, Keynote paper, ASCE

Special Publication No. 103, Advances in

Transportation and Geoenvironmental Systems using

Geosynthetics, Proceedings of Geo-Denver 2000, 5-8 August 2000, Denver, Colorado, pp. 201-217.

Chou, N. N. S. and Wu, J. T. H. (1993) Effects of Foundation on the Performance of Geosynthetic-Reinforced Soil Walls, Proceedings of Geosynthetics

1993, Vancouver, Vol. 1, pp. 189–202. Hatami, K. and Bathurst, R. J. (2005) Development and

Verification of a Numerical Model for the Analysis of Geosynthetic Reinforced Soil Segmental Walls Under Working Stress Conditions, Canadian Geotechnical

Journal, Vol. 42, No. 4, pp. 1066-1085. Iai, S. (1989) Similitude for Shaking Table Tests on Soil-

Structure-Fluid Model in 1g Gravitational Field, Soils

and Foundations, Vol. 29, No. 1, pp. 105-118. Palmeira, E. M. and Monte, L. M. (1997) The Behaviour of

Model Reinforced Walls on Soft Soils. Proceedings of

Geosynthetics ’97, Long Beach, California, pp. 73–84. Rowe, R. K. and Skinner, G. D. (2001) Numerical

Analysis of Geosynthetic Reinforced Retaining Wall Constructed on a Layered Soil Foundation, Geotextiles and Geomembranes, Vol. 19, No. 7, pp. 387-412.

distance from back of facing (mm)

0 200 400 600 80002468

Wall 8Wall 11

02468

02468

02468

02468

02468

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

stra

in (

%)

Figure 11 – Strains in reinforcement layers at end of 50 kPa surcharge load increment for Wall 8 (k = 964 kN/m) and Wall 11 (k = 0)

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