Effects of backfill on the performance of GRS retaining walls

Geotextiles and Geomembranes 17 (1999) 1—16

Effects of backfill on the performance of GRS retaining walls S.M.B. Helwany *, G. Reardon
, J.T.H. Wu Department of Civil Engineering, University of Wisconsin at Milwaukee, EMS, P.O. Box 784, Milwaukee, WI 53201, USA
Division of Engineering, University of Texas at San Antonio, 6900 N. 1604 Loop W., San Antonio, TX 78249, USA Department of Civil Engineering, Reinforced Soil Research Center, University of Colorado at Denver, Denver, CO 80217, USA Received 30 September 1997; received in revised form 9 April 1998; accepted 13 September 1998

Abstract In this study, a finite element program was validated by comparing its analytical results with the results of a well-instrumented large-scale laboratory test conducted on a geosynthetic reinforced soil (GRS) retaining wall under well-controlled test conditions. The validated computer program was then used to investigate the effects of backfill type on the behavior of GRS retaining walls. Three different geosynthetic reinforcements and sixteen different backfills were implemented in the analysis of three different wall configurations to produce 144 analysis combinations. It was shown that the type of backfill had the most profound effect on the behavior of the GRS retaining wall. It was also shown that the stiffness of the geosynthetic reinforcement had a considerable effect on the behavior of the GRS retaining wall when the backfill was of lower stiffness and shear strength. A parametric study was performed on GRS retaining walls based on the finite element analyses to assist the design engineer in choosing the appropriate backfill and the appropriate geosynthetic reinforcement for GRS retaining walls in order to satisfy the prescribed requirements of maximum lateral displacement, maximum axial strain in the reinforcements, and/or average safety factors.  1999 Elsevier Science Ltd. Keywords: Geosynthetics; Design; GRS retaining wall; Soil—structure interaction; Finite element analysis

*Corresponding author. Fax: 001 414 229 6958. 0266—1144/99/$ — see front matter  1999 Elsevier Science Ltd. PII: S 02 6 6— 1 14 4 ( 98 ) 0 00 2 1— 1

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1. Introduction Geosynthetic-reinforced soil (GRS) retaining walls, that consist of soil and facing units together with geosynthetic as reinforcement, have gained vast popularity due to their cost effectiveness and ease of construction. In this paper, the finite element method is utilized to investigate the effects of various backfill materials on the performance of GRS retaining walls, and to perform a parametric study on GRS retaining walls with different configurations, different backfill materials, and different reinforcement stiffnesses. The finite element program used herein was first verified by comparing the analytical results with measured values of a well-instrumented fullscale GRS retaining wall laboratory test conducted under well-controlled test conditions, referred to as the Denver Test Wall (Wu, 1992a, b). The finite element method has been utilized by numerous researchers to analyze geosynthetic-reinforced soil structures. For example, ten of the thirteen researchers participated in the prediction of the behavior of the Denver Test Wall, in the International Symposium on Geosynthetic-Reinforced Soil Retaining Walls (Wu, 1992), have utilized the finite element method in their analyses. Other examples include Rowe and Ho (1997); Ho and Rowe (1997); Cai and Bathurst (1995); Karpurapu and Bathurst (1995); Ka-Ching et al. (1994); Ho and Rowe (1994); Bathurst and Simac (1993); Chou (1992); Rowe and Ho (1992); and Rowe and Ho (1987). The finite element computer program used in this study was a modified version of ‘DACSAR’ (Deformation Analysis Considering Stress Anisotropy and Reorientation) originally coded by Ohta and Iizuka (1986). An elaborate finite element input generator for GRS retaining walls was implemented in DACSAR to facilitate data input for various GRS retaining wall configurations. The nonlinear soil model by Duncan et al. (1980) was also implemented in DACSAR to simulate the nonlinear behavior of backfill. The accuracy of this finite element program was verified by comparing the analytical results with the measured results of the Denver Test Wall. A series of numerical analyses was then conducted to investigate the behavior of GRS retaining walls of different configurations with various types of backfills and different degrees of compaction. The parametric analyses resulted in ‘‘parametric charts’’ for GRS retaining walls. These charts are useful to the design engineer in choosing the appropriate backfill and geosynthetic reinforcement for GRS retaining walls to satisfy prescribed requirements of maximum lateral wall movement, maximum strain in the geosynthetic reinforcement, and/or safety factors.

2. The Denver Test Wall The plane strain testing facility within which the Denver Test Wall was constructed has been described in detail by Wu (1992a). The Denver Test Wall, as shown in Fig. 1, was 3 m high, 1.2 m wide and 2.0 m deep. An Ottawa sand was used as backfill in the test and placed by the air pluviation method in an air-dried condition. A geosynthetic reinforcement was placed at approximately 28 cm vertical spacing. The wall facing

S.M.B. Helwany et al./Geotextiles and Geomembranes 17 (1999) 1—16

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comprised inter-connected timber logs. Uniform pressure increments of 35 kPa were applied to the top surface of the Denver Test Wall utilizing an air bag. The reinforcement used in the test was a fairly light weight nonwoven heat-bonded polypropylene geotextile. Some of its index properties provided by the manufacturer are presented in Table 1. The uniaxial load-deformation behavior of the geotextile tested at a constant strain rate of 1 percent per minute is shown in Fig. 2. The results shown in the figure were obtained by testing a geotextile specimen of 30 cm in width and 2.5 cm in gage length to achieve a near plane strain condition. The test was performed without pressure confinement on the geotextile specimen. Previous tests of this geotextile have indicated that the load-extension behavior of the geotextile was not affected by confining pressure up to about 300 kPa (Wu, 1991). The backfill used in the Denver Test Wall was a sub-rounded silica Ottawa sand. The specific gravity of the sand was 2.65 and the maximum and minimum unit weights

Fig. 1. Schematic diagram of ‘Denver Wall’ test.

Table 1 Some properties of the geosynthetic reinforcement Unit weight (ASTM D-3776) Grab tensile (ASTM D-4632) Elongation at break (ASTM D-4632) Modulus at 10% elongation (ASTM D-4632) AOS (ASTM D-4751) Permittivity (ASTM D-4491) Coefficient of permeability

1.93 N/m 890 N 60% 4.45 kN 0.101 mm 0.1/sec 1.99;10\ cm/sec

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Fig. 2. Load—deformation behavior of the geotextile.

Fig. 3. Triaxial compression test results for Ottawa sand.

per ASTM D 854 were 17.7 kN/m and 15.3 kN/m, respectively. The placement unit weight was approximately 16.8 kN/m. Triaxial compression tests results for this sand at confining pressures of 70, 205, and 350 kPa are shown in Fig. 3.

3. Finite element analysis of the Denver Test Wall The finite element mesh used in the analysis is shown in Fig. 4. The timber facing was represented by beam elements, the geotextile reinforcement was represented by truss elements, and the soil was represented by plane strain 4-node quadrilateral

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Fig. 4. Finite element discretization of ‘Denver Wall’ test.

elements. A total of 1050 elements and 961 nodal points were used in the finite element analyses. The construction sequence was simulated by 10 construction lifts. The fineness of the finite element mesh was checked by comparing its results with the results of a coarser finite element mesh with 600 nodes — the difference in the maximum lateral deformation of the facing, both in location and magnitude, was negligible. The nonlinear, stress-dependent stress—strain—strength behavior of the soil was simulated by the modified Duncan soil model. The model parameters for the granular soil used in the Denver Test Wall were determined from the results of the triaxial compression tests. These parameters, shown in Table 2, were used to back calculate the triaxial compression tests results. A good agreement between the model simulation and the triaxial test results was obtained, as indicated in Fig. 3. The stress—strain

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behavior of the geotextile reinforcement and the timber facing was simulated as linear elastic. This was considered sufficient as the stress level in the service condition was rather low. The measured long-term deformations of the Denver Test Wall were deemed negligible. Therefore, creep was not considered in the current analysis. The calculated and measured responses of the Denver Test Wall under 105 kPa surcharge pressure were compared. Fig. 5 indicates the close agreement between the calculated and measured lateral displacements at 105 kPa surcharge pressure. The measured axial strain distributions along the geotextiles at three different elevations of

Table 2 Modified Duncan soil parameters for the Ottawa sand in the Denver Test Wall Parameters

 deg

*

deg

C N/m

K

n

R 

K


m

Values

38.4

2

0

1116

0.66

0.87

907

0

Notes: C Friction angle

K n R  K
m

Cohesion intercept (N/m)

and * (deg) 

"friction angle for p "1 atmosphere   * "reduction in friction angle for a 10-fold increase in p  Modulus number Modulus exponent Failure ratio Bulk modulus number Bulk modulus exponent

Fig. 5. Comparison between the calculated and measured lateral displacements in ‘Denver Wall’ test.

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the Denver Test Wall were compared with the analytical results as shown in Fig. 6. It is noted that the calculated strains represent the average measured values. The good agreement in the displacements and strains indicated that DACSAR code was capable of simulating the behavior of GRS retaining walls. It is to be noted that the Denver Test Wall, shown in Fig. 1, does not represent a typical cross-section of a GRS retaining wall because it has very little backfill behind the reinforced soil zone. Hence, the effects of retained soil are nearly excluded. The Denver Test Wall also restricted the horizontal movement of the top of the facing of the wall resulting in an unusual deflection profile noted in Fig. 5. Nevertheless, in the above analysis, the Denver Test Wall was merely considered as a ‘boundary value’ problem which was utilized to validate the usefulness of the finite element code DACSAR. Consequently, the validated code can be utilized, with some confidence, to analyze more realistic GRS retaining wall configurations.

4. Effects of backfill on the performance of GRS retaining walls In this study, three GRS retaining walls of different heights were analyzed. Sixteen different backfills, with material parameters listed in Table 3, and geosynthetic reinforcements of three different values of stiffness were used in each retaining wall. A total of 144 cases were investigated. The material parameters of the aforementioned sixteen backfill materials were deduced from the triaxial tests results conducted on 135 different backfill materials (Duncan et al., 1980). Table 4 shows the reinforcement configuration of each wall height. The thickness of each construction layer was 30 cm. Each wall was subjected to a uniform surcharge pressure of 35 kPa, a value representing typical traffic load, at the end of construction. Special attention was given to the initial stress conditions in newly added soil layers during the construction of the GRS retaining walls. Compaction was accounted for in

Fig. 6. Comparison between the calculated and measured strains in the geotextile reinforcement in ‘Denver Wall’ test.

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Table 3 Representative soil parameters (after Duncan et al., 1980) United soil classification

Backfill RC (%) designation standard number proctor

c

 kN/m deg

*

deg

C K kN/m

n

R 

K


m

GW, GP SW, SP

1 2 3 4

105 100 95 90

23.6 22.8 22.1 21.3

42 39 36 33

9 7 5 3

0 0 0 0

600 450 300 200

0.4 0.4 0.4 0.4

0.7 0.7 0.7 0.7

175 125 75 50

0.2 0.2 0.2 0.2

SM

5 6 7 8

100 95 90 85

21.3 20.5 19.7 18.9

36 34 32 30

8 6 4 2

0 0 0 0

600 450 330 150

0.25 0.25 0.25 0.25

0.7 0.7 0.7 0.7

450 350 250 150

0.0 0.0 0.0 0.0

SM-SC

9 10 11 12

100 95 90 85

21.3 20.5 19.7 18.9

33 33 33 33

0 0 0 0

24 19 14 10

400 200 150 100

0.6 0.6 0.6 0.6

0.7 0.6 0.7 0.7

200 100 75 50

0.5 0.5 0.5 0.5

CL

13 14 15 16

100 95 90 85

21.3 20.5 19.7 18.9

30 30 30 30

0 0 0 0

19 14 10 5

150 120 90 60

0.45 0.45 0.45 0.45

0.7 0.7 0.7 0.7

140 110 80 50

0.2 0.2 0.2 0.2

Table 4 GRS retaining wall dimensions Wall height (m)

Depth of backfill (m)

Length of geotextile (m)

Number of reinforcement layers

3 4.5 6

3.7 5.5 7.3

1.8 2.7 3.7

10 15 20

a simplified manner utilizing the compaction model by Seed and Duncan (1983), in which the initial lateral stress in each soil element of the newly added soil layer was increased by a ‘residual’ lateral stress due to compaction. The residual lateral stress is a function of soil properties and the pressure exerted by the compaction machine to the top surface of the soil layer. Only the first pass of the compaction machine, with an average pressure of approximately 800 kPa, was considered in the analysis. It is to be noted that the simulation of multiple passes of the compaction machine is possible as described by Seed and Duncan (1984). The effect of facing rigidity on the performance of GRS retaining walls has not been fully understood and is usually ignored in the design. Most of the current design procedures assume that the reinforcing layers, connected to the facing and extended beyond the potential failure plane — the plane passing through the bottom of the

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retaining wall face and making an angle of 45°# °/2 with the horizontal, will resist the lateral earth pressure acting on the reinforced soil zone. Schlosser and Juran (1983) have noted that the rigidity of the metal facing element in a retaining wall model test can influence the model height at failure. The effect of facing rigidity on the performance of GRS retaining walls has been noted in laboratory and field tests and in finite element analyses as well (Tatsuoka et al., 1992; Tatsuoka, 1993; Helwany et al., 1996). In this study, a continuous facing with ‘global’ stiffness and minor bending resistance was assumed. This continuous facing had the following properties: elastic modulus (E)"140 MPa, moment of inertia (I)"40 cm/cm, and cross-section area (A)"25 cm/cm. Three respo nse parameters were chosen for this parametric study, namely, the maximum lateral displacements of the wall facing, the maximum axial strain in the reinforcement, and the ‘average’ safety factor for the GRS retaining wall. The average safety factor is calculated by averaging the ‘apparent’ safety factors of the soil elements located on or near the potential failure plane which passes through the bottom of the retaining wall face and makes an angle of 45°# °/2 with the horizontal. The apparent safety factor is defined as the ratio of the deviatoric stress of a soil element to the deviatoric stress at failure using the Mohr—Coulomb failure criterion. For most of the GRS wall configurations analyzed herein, the average safety factor was found to be minimum (most critical) along the potential failure plane. Fig. 7 shows the maximum lateral displacements, the maximum axial strains in the reinforcements, and

Fig. 7(a). Parametric analysis of a 3 m high GRS retaining wall — maximum lateral displacement versus geosynthetic stiffness.

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Fig. 7(b). Parametric analysis of a 3 m high GRS retaining wall — maximum strain in reinforcement versus geosynthetic stiffness.

Fig. 7(c). Parametric analysis of a 3 m high GRS retaining wall — average safety factor versus geosynthetic stiffness.

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the average safety factors, respectively, as functions of geosynthetic stiffness for a 3 m high GRS retaining wall. Fig. 8 gives the results of the analyses conducted on a 4.5 m high GRS retaining wall, and Fig. 9 gives the results of the analyses for a 6 m high GRS retaining wall. Figs. 7—9 can be utilized for the selection of backfill materials and geosynthetic reinforcements for various GRS retaining walls, with heights ranging from 3 to 6 meters, to satisfy prescribed conditions such as the maximum lateral movement of the wall facing. For example, the design engineer can utilize Fig. 8 to determine the geosynthetic stiffness required to limit the maximum lateral displacement of the facing to 2.5 cm for a 4.5 m high GRS retaining wall with a silty sand backfill having a relative compaction of 85% of the Standard Proctor (Backfill C12 in Table 3). The required geosynthetic stiffness in this example is approximately 220 kN/m as indicated in Fig. 10 (taken from Fig. 8). The corresponding maximum strain in the reinforcement is approximately 0.5% and the corresponding average safety factor is 6.4 as shown in the same figure. It is noted from Figures 7—9 that the type of backfill has the most profound effect on the behavior of GRS retaining walls. It is also noted that the stiffness of the geosynthetic reinforcement has a more pronounced effect on the behavior of GRS retaining walls when the backfill is of lower stiffness and shear strength. For example, the 3 m high GRS retaining walls with backfills C15 and C16 (lower stiffness and shear strength) exhibited significant improvement when a stiffer geosynthetic was utilized as indicated in Fig. 7. On the other hand, the 3-m high GRS retaining walls

Fig. 8(a). Parametric analysis of a 4.5 m high GRS retaining wall — maximum lateral displacement versus geosynthetic stiffness.

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Fig. 8(b). Parametric analysis of a 4.5 m high GRS retaining wall — maximum strain in the reinforcement versus geosynthetic stiffness.

Fig. 8(c). Parametric analysis of a 4.5 m high GRS retaining wall — average safety factor versus geosynthetic stiffness.

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Fig. 9(a). Parametric analysis of a 6 m high GRS retaining wall — maximum lateral displacement versus geosynthetic stiffness.

Fig. 9(b). Parametric analysis of a 6 m high GRS retaining wall — maximum strain in the reinforcement versus geosynthetic stiffness.

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Fig. 9(c). Parametric analysis of a 6 m high GRS retaining wall — average safety factor versus geosynthetic stiffness.

with backfills C13 and C14 (higher stiffness and shear strength) exhibited relatively small improvement when the geosynthetic stiffness was increased.

5. Summary and conclusions In this paper, the finite element program DACSAR was validated by comparing its results with the results of a well-controlled well-instrumented large-scale laboratory test conducted on a GRS retaining wall. The validated computer program was then used to investigate the effects of backfill type on the behavior of GRS retaining walls. Three different geosynthetic reinforcements and sixteen different backfills were implemented in the analysis of three different wall configurations to produce 144 analysis combinations. It was shown that the type of backfill had the most profound effect on the behavior of the GRS retaining wall. It was also shown that the stiffness of the geosynthetic reinforcement had a considerable effect on the behavior of the GRS retaining wall when the backfill was of lower stiffness and shear strength. Parametric charts were established for GRS retaining walls based on the finite element analyses. These charts are useful to the design engineer in choosing the appropriate backfill and the appropriate geosynthetic reinforcement for GRS retaining walls in order to satisfy the prescribed requirements of maximum lateral displacement, maximum axial strain in the reinforcements, and/or average safety factors.

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Fig. 10. Performance example of a 4.5 m high GRS retaining wall.

6. References Bathurst, R.J., Simac, M.R., 1993. Two computer programs for the design and analysis of geosyntheticreinforced soil retaining walls. Geotextiles and Geomembranes 12 (5) 381—396. Cai, Z., Bathurst, R.J., 1995. Seismic response analysis of geosynthetic reinforced soil segmental retaining walls by finite element method. Computers and Geotechnics 17 (4) 523. Chou, N.S., 1992. Performance of geosynthetic reinforced soil walls. Ph.D. Dissertation, University of Colorado at Boulder. Duncan, J.M., Byrne, P.M., Wong, K.S., Mabry, P., 1980. Strength, stress—strain and bulk modulus parameters for finite element analyses of stresses and movements in soil masses.’ Report No. UCB/GT/80-01. Helwany, S.M.B., Tatsuoka, F., Tateyama, M., Kojima, K., 1996. Effects of facing rigidity on the performance of geosynthetic-reinforced soil retaining walls. Soils and Foundations, 36 (1) 27—38. Ho, S.K., Rowe, R.K., 1994. Predicted behavior of two centrifugally modelled soil walls. ASCE Journal of Geotechnical Engineering 120 (10) 1845—1873.

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Ho, S.K., Rowe, R.K., 1997. Effect of wall geometry on the behavior of reinforced soil walls. International Journal for Geotextiles and Geomembranes 14 (10) 521—541. Ka-Ching, S., Leshchinsky, D., Matsui, T., 1994. Geosynthetic reinforced slopes; limit equilibrium and finite element analyses. Soils and Foundations 34 (2) 79—85. Karpurapu, R., Bathurst, R.J., 1995. Behavior of geosynthetic reinforced soil retaining walls using the finite element method. Computers and Geotechnics 17 (3) 279. Ohta, H., Iizuka, A., 1986. DACSAR, deformation analysis considering stress anisotropy and reorientation. Report, Soil Mechanics and Foundation Engineering Laboratory, Department of Civil Engineering, Kanazawa University, Japan. Rowe, R.K., Ho, S.K., 1997. Continuous panel reinforced soil wall on rigid foundations. ASCE Journal of Geotechnical and Geoenvironmental Engineering 123 (10) 912—920. Rowe, R.K., Ho, S.K., 1992. A review of the behavior of reinforced soil walls. Keynote lecture, International Symposium on Soil Reinforcement, Kyushu, November, pp. 47—76. Rowe, R.K., Ho, S.K., 1987. Application of finite element technique to the analysis of reinforced soil walls. Proceedings of the NATO Advanced Research Workshop on Applications for Polymeric Reinforcement in Soil Retaining Structures, Kingston, pp. 541—554. Schlosser, F., Juran, I., 1983. Behavior of reinforced earth retaining walls from model studies. Developments in SMFE-1, Applied Science Publishers, pp. 197—229. Seed, R.B., Duncan, J.M., 1983. Soil-structure interaction effects of compaction-induced stresses and deflections. Geotechnical Engineering Research Report No. UCB/GT/83-06, University of California, Berkeley. Seed, R.B., Duncan, J.M., 1984. SSCOMP: a finite element analysis program for evaluation of soil-structure interaction and compaction effects. Geotechnical Engineering Research Report No. UCB/GT/84-02, University of California, Berkeley. Tatsuoka, F., Murata, O., Tateyama, M., 1992. Permanent geosynthetic-reinforced soil retaining walls used for railway embankments in Japan. Proceedings of the International Symposium on GeosyntheticReinforced Soil Retaining Walls, Rotterdam, Balkema. Tatsuoka, F., 1993. Roles of facing rigidity in soil reinforcing. Proceedings of International Symposium on Earth Reinforcement Practice, Fukuoka, Kyushu, Japan, Balkema Publisher, 11—13 November 1992, Vol. 2, pp. 831—870. Wu, J. T. H., 1992a. Predicting performance of the Denver walls: general report. Geosynthetic-Reinforced Soil Retaining Walls, Wu (Ed.), Balkema Publisher, pp. 3—20. Wu, J. T. H., 1992b. Construction and instrumentation of the Denver walls. Geosynthetic-Reinforced Soil Retaining Walls, Wu (Ed.), Balkema Publisher, pp. 21—30. Wu, J. T. H., 1991. Measuring inherent load-extension properties of geotextiles for design of geosyntheticreinforced structures. ASTM Geotechnical Testing Journal 14 (2) (June 1991) 157—165. Wu, J. T. H., 1992. Geosynthetic-reinforced soil retaining walls. International Symposium on Geosynthetic-Reinforced Soil Retaining Walls, Wu (Ed.), Balkema Publisher.