Simplified Procedure to Evaluate Earthquake-Induced Residual Displacement of Geosynthetic Reinforced Soil Retaining Walls

SOILS AND FOUNDATIONS Vol. 50, No. 5, 659–677, Oct. 2010 Japanese Geotechnical Society SIMPLIFIED PROCEDURE TO EVALUATE EARTHQUAKE-INDUCED RESIDUAL DISPLACEMENT OF GEOSYNTHETIC REINFORCED SOIL RETAINING WALLS SUSUMU NAKAJIMAi),JUNICHI KOSEKIii),KENJI WATANABEiii) and MASARU TATEYAMAiii) ABSTRACT Based on a series of shaking table model tests, it was found that the eŠects of 1) subsoil and backˆll deformation, 2) failure plane formation in backˆll, and 3) pullout resistance mobilized by the reinforcements on the seismic behaviors of the geosynthetic reinforced soil retaining walls (GRS walls) were signiˆcant. These eŠects cannot be taken into account in the conventional pseudo-static based limit equilibrium analyses or Newmark's rigid sliding block analogy, which are usually adopted as the seismic design procedure. Therefore, this study attempts to develop a simpliêd procedure to evaluate earthquake-induced residual displacement of GRS walls by re‰ecting the knowledge on the seismic behaviors of GRS walls obtained from the shaking table model tests. In the proposed method, 1) the deformation characteristics of subsoil and backˆll are modeled based on the model test results and 2) the eŠect of failure plane formation is considered by using residual soil strength after the failure plane formation while the peak soil strength is used before the failure plane formation, and 3) the eŠect of the pullout resistance mobilized by the reinforcement is also introduced by evaluating the pullout resistance based on the results from the pullout tests of the reinforcements. By using the proposed method, simulations were performed on the shaking table model test results conducted under a wide variety of testing conditions and good agreements between the cal- culated and measured displacements were observed. Key words: failure plane, geosynthetic retaining wall, Newmark's method, residual displacement, shaking table test (IGC: E12/E14/E8) equilibrium design approaches, which are not directly INTRODUCTION related to the residual state of the retaining walls after the Case history of recent large earthquake revealed good earthquake. seismic performance of the geosynthetic-reinforced soil In the performance-based design, the seismic perfor- retaining walls (GRS walls) with full-height rigid facing mance of the retaining walls can be typically evaluated by (e.g., Tatsuoka et al., 1998; Koseki et al., 2006). For ex- comparing the residual displacements with allowable ample, after the 1995 Hyogo ken Nanbu-Earthquake, the values. Therefore, the performance-based design ap- GRS wall constructed to support the railway embank- proach is more suitable for considering the diŠerent seis- ment at Tanata site could be used after minor retroˆtting mic performances between the GRS walls and conven- works although many conventional concrete retaining tional retaining walls, while more detailed investigations walls (e.g., gravity, cantilever and leaning type retaining on the seismic behavior of the GRS walls are required so walls) needed reconstruction works due to their severe as to evaluate their seismic performance accurately. damage. However, these diŠerent seismic performances In relation to the above, many experimental and ana- cannot be properly evaluated based on the conventional lytical studies on seismic behavior of the GRS walls have pseudo-static limit-equilibrium approaches. been carried out so as to investigate into the mechanisms On the other hand, there is a trend in Japan to shift the of their ductile seismic behaviors. design procedure to performance-based design from con- Watanabe et al. (2003) have revealed through a series ventional pseudo-static limit-equilibrium approaches of shaking table model tests that mobilized tensile (Koseki et al., 2007) because it is di‹cult to evaluate duc- resistance of the reinforcement which increased with the tile seismic performance of the GRS walls based on the accumulation of the wall displacements could resist eŠec- factor of safety obtained from the pseudo-static limit tively against the seismic thrusts (i.e., the inertia force i) Public Works Research Institute, Japan (s-nakaji55＠pwri.go.jp). ii) Professor, Institute of Industrial Science, University of Tokyo, Japan. iii) Railway Technical Research Institute, Japan. The manuscript for this paper was received for review on April 21, 2009; approved on May 19, 2010. Written discussions on this paper should be submitted before May 1, 2011 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 659 660 NAKAJIMA ET AL. and dynamic earth pressure). They also found that the to residual strength. rapid increase in the wall displacement of the convention- The other approach which has also been attempted is to al type retaining wall was triggered by local bearing apply the ˆnite element (FE) method to the displacement capacity failure of the subsoil beneath the toe of the prediction for the GRS walls (e.g., Fujii et al., 2006). retaining wall. It should be noted that increase of the wall This approach will be more suitable for simulating the displacements following the bearing capacity failure was seismic behavior of the GRS walls especially at relatively not observed with the GRS walls. small displacement levels, while the reinforced soil Experimental studies on the GRS walls with segmental behaves as a continuum although the modeling of the wall facing panels have also been carried out (e.g., geosynthetic-reinforcements will become complicated. Bathurst et al., 2002; Matsuo et al., 1998; Ling et al., However, there seems to be a limitation of the FE ap- 2005). The eŠects of the facing inclination and shear proaches for the displacement prediction of the GRS resistance of the facing interface (i.e., strong connection walls because the FE analyses considering the eŠects of each segmental panel) on the seismic performance of strain localization in the backˆll layer which would result GRS walls were highlighted through their experiments. into the failure plane formation and strain softening be- The eŠects of the material properties (e.g., pullout havior have not yet been fully developed although seismic resistance and rupture strength of the reinforcements) of behavior of the GRS walls are aŠected by these phenome- the geosynthetics reinforcements on the seismic perfor- na. As far as the authors know, there does not exist any mance and failure mode of the GRS walls have also been procedure to evaluate earthquake-induced displacement investigated (e.g., Izawa et al., 2002; Nakajima et al., which considers the deformation characteristics of the 2008a). Izawa et al. (2002) found out through the series of GRS walls. the centrifuge model tests on the GRS walls subjected to In view of the above, Koseki et al. (2004) proposed a the sinusoidal excitation that the failure plane would not simpliêd displacement evaluation method for the GRS form in the backˆll in which the reinforcements were in- walls by combining the NM method with considering the stalled (reinforced backˆll) in the case of the GRS walls eŠect of the shear deformation of reinforced backˆll and with rigid reinforcements while the failure plane could be subsoil layer while the eŠect of the strain softening behav- formed within the reinforced backˆll in the case of soft ior with the failure plane formation in the backˆll layer reinforcements. In addition to the deformation mode, that is under dense condition in general was also taken they pointed out that the pullout resistance of the reinfor- into account. However, the shear deformation character- cements should be appropriately taken into account in istics of the reinforced backˆll have not yet been investi- developing the displacement evaluation method. Similar gated in detail. considerations were also discussed by Nakajima et al. This study, therefore, attempts to develop a simpliêd (2008a) based on the series of shaking table model tests displacement evaluation method for the GRS walls by which were conducted under the gravitational êld using considering their seismic behaviors observed in the shak- the irregular excitation. ing table model tests and examine its applicability. Based on the knowledge obtained from the above experimental observations, studies to develop a displacement evaluation procedure for GRS walls were also car- PROCEDURE AND GENERAL RESULTS OF ried out. SHAKING TABLE MODEL TESTS Horii et al. (1998) proposed a displacement evaluation Model Test Procedure method based on the Newmark's sliding block approach The model tests were conducted using a shaking table (Newmark, 1965), while the eŠect of the shear deforma- at Railway Technical Research Institute in Japan. The tion of the reinforced backˆll was also taken into ac- size of the soil container was 260 cm long, 60 cm wide and count. This approach has been adopted as a procedure to 140 cm high. The cross sections of the GRS wall models evaluate the earthquake-induced residual displacement with three diŠerent reinforcement arrangements used in under large seismic load in the current railway design the series of shaking table model tests are shown in Fig. guideline in Japan (RTRI, 2007). 1. Because the concept of Newmark's method (NM Two types of geogrid reinforcement models were used method) is simple, there have been many studies to adopt in the model tests. As shown in Fig. 2, a geogrid model the NM method for the evaluation of the earthquake-in- made of lattice-shaped phosphor bronze strips with a duced residual displacement of GRS walls (Matsuo et al., thickness of 0.1 mm and a width of 3 mm for each strip 1998; Cai and Barthurst, 1996; Huang and Wang, 2005). was used. Particles of Toyoura sand were glued on the However, it should be noted that the sliding or rotating surface of the reinforcements so as to mobilize the fric- mass and its subsoil are assumed to be rigid in the NM tional resistances eŠectively. For the other type of geo- method.

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