Stability analyses of a reinforced soil wall on soft soils using strength reduction method
Introduction
Reinforced soil retaining walls (RSWs) have been widely used throughout the world since their first emergence in the early 1980s as the structures have many advantages including esthetics, short construction period, good wall stability, cost effectiveness, good seismic response, strong adaptability on highly compressible soft foundation soil, and the ability to tolerate large differential settlement (Tatsuoka et al., 1997, Bloomfield et al., 2001, Yoo and Jung, 2004).
The limit equilibrium method (LEM) has been used for decades to design major geotechnical structures (e.g. slopes, embankments, pits) and is favored by engineers due to its simplicity. LEM has been also recommended to evaluate the stability of RSWs (British Standards Institute, 1995, American Association of State Highway and Transportation Officials (AASHTO), 1996, American Association of State Highway and Transportation Officials (AASHTO), 2002, Federal Highway Administration (FHWA), 1996, Federal Highway Administration (FHWA), 2009, National Concrete Masonry Association (NCMA), 1997, National Concrete Masonry Association (NCMA), 2010). The RSWs on soft clay may suffer large deformation and incur compound or global failure if the shear strength in the subsoil is not large enough. Usually, staged-loading method is employed for the construction of these structures to speed up the consolidation and thus improve shear strength in soft subsoil. However, the increase of soil strength due to consolidation during the loading process cannot be readily considered in LEM, thus it is mainly used to assess the short-term (undrained) stability of the structures on soft clay (Rowe and Li, 2005). Yet the soil hardening during consolidation can be considered and manually increased as an input in LEM (Leroueil et al., 2001, Suzuki and Yasuhar, 2007). Chen et al. (2014) analyzed a geogrid reinforced wall on soft clay with LEM by manually increasing the soil strength in the drained zone, and the obtained factor of safety (FS) is quite comparable with the result from FEM using hardening soil model. The strength reduction method (SRM) was first used for slope stability analysis by Zienkiewicz et al. (1975). Later, it has been applied widely for analyzing slope stability including the computation of factor of safety, especially after the technique was adopted in several well-known commercial geotechnical finite element (FE) or finite difference (FD) programs, e.g. PLAXIS or Fast Lagrangian Analysis of Continua (FLAC) (Cheng et al., 2007, Abusharar and Han, 2011). Some researchers have used the SRM incorporated in FLAC to analyze the stability of reinforced soil slopes and walls. Han et al. (2002) analyzed the stability of unreinforced and geosynthetic-reinforced slopes by using FLAC and obtained FS values similar to those computed by simplified Bishop's method (Bishop, 1955). Leshchinsky and Han (2004) used FLAC to characterize the stability of multi-tiered reinforced walls by quantifying the effects of offset distance, fill quality, foundation soil, reinforcement length and stiffness, water, surcharge, and number of tiers. Abdelouhab et al. (2011) analyzed the stability of RSWs with different types of strips using FLAC. Liu et al. (2012) used FLAC to examine the mechanisms and causes contributing to the failure of a high steep geogrid-reinforced slope. As compared with the LEM, the SRM incorporated in FE and FD programs has the following advantages (Dawson et al., 1999, Cala and Flisiak, 2001, Cheng et al., 2007): (1) the critical failure surface is found automatically from the reduction of shear strength; (2) there are no slices, so SRM requires no assumption on the inter-slice shear force distribution; (3) multiple failure surfaces can be obtained; (4) structures (e.g. reinforcements, beams, cables) and interfaces can be included without concerning about compatibility; (5) it is applicable to many complex conditions and can give information such as stress, movements, and pore pressure variation during construction that are not possible in the LEM.
In this study, a 7.6 m high RSW with wrapped-around facing was constructed in stages on very thick soft Shanghai clay (see Fig. 1). A deep-seated global failure was observed during the construction of the RSW (Chen et al., 2014, Xue et al., 2014). The RSW was analyzed by using two-dimensional (2D) coupled mechanical and hydraulic FE package, PLAXIS Version 8.2 (Brinkgreve et al., 2004). The stability of the RSW was examined with the SRM incorporated in the PLAXIS program. The effects of extending, strengthening, and stiffening bottom reinforcement layers on the stability of the RSW were further investigated by using the FE modeling.
Section snippets
Site description
The subsoil profile of the construction site consists of 5 compressible soil layers within the depth of 31 m from the ground level. The 5 soil layers are (from top to bottom): silty clay (2.6 m thick), mucky silty clay (4.4 m thick), clay (3.6 m thick), silty clay (7.4 m thick) and silty clay (13.0 m thick). The soil underneath is a 6.3 m thick stiff clayey silt with average SPT blow count of 31, underlain by stiff silty sand and clay. The ground water table is shallow and fluctuates within 0.5 m below
Conversion of axisymmetric drainage to plane drainage
The actual field consolidation of soil around each drain is close to an axisymmetric condition. To simulate an axisymmetric drainage in plane strain conditions, such as in the 2D FE program, Hird et al. (1995) proposed a geometry and permeability matching equation to convert from axisymmetric to plane strain conditions as follows:where kh = the horizontal permeability of the subsoil; khp = the equivalent horizontal permeability of the drainage system in plane strain condition;
Numerical results
Fig. 7 shows the computed and measured ground surface settlements. It can be seen that the computed curve agrees well with the measurement within 200 days. The computed curve also exhibits a sudden settlement from the 190th day as the RSW experienced in the field.
The computed and measured displacement curves of the RSW toe are compared in Fig. 8. It can be seen that, simulated displacements of the toe are quite comparable with the measured values. Both computed and measured displacements at the
Effects of extending, strengthening and stiffening bottom reinforcement layers
A minimum reinforcement length of 0.7H (H denotes wall height) is recommended for RSWs. Longer reinforcement lengths or foundation improvements are required for structures where foundation conditions affect global stability (American Association of State Highway and Transportation Officials (AASHTO), 2002, Federal Highway Administration (FHWA), 2009). Foundation improvements can fundamentally meet the global stability of RSWs but typically are cost and time-consuming. Extending reinforcement
Summary and conclusions
A two-dimensional coupled mechanical and hydraulic finite element (FE) modeling was performed on a 7.6 m high built-to-failure RSW on very thick soft clay. The stability of the RSW was examined by the SRM incorporated in the FE program. The observed ground settlement, toe displacements, and the deep-seated global failure of the RSW were well captured with the FE modeling. The effects of extending, strengthening and stiffening bottom layers of the reinforcement on the stability of the RSW were
Acknowledgments
The support from the Natural Science Foundation of China under grant no. 41072200 and from the Fundamental Research Funds for the Central Universities is gratefully acknowledged. The third author is on sabbatical leave in the University College Dublin, Ireland when the paper was written. The author would like to acknowledge the funding received from Monash University and EU FP7 SMART RAIL project for his visit. The authors appreciate the valuable comments and suggestions from the anonymous
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