تجزیه و تحلیل حساسیت عددی و شبیه سازی از آزمون خاکریزی در مقیاس کامل با موادجغرافیایی تقویت شده بسیار سبک وزن و در خاک رس نرم بانکوک

A full-scale test embankment was constructed on soft Bangkok clay using rubber tire chip–sand mixture as a lightweight geomaterial reinforced with geogrid under working stress conditions. The facing of the embankment was made of segmental concrete blocks with rock filled gabion boxes as the facing to the sloping sides. This paper attempts to simulate the behavior of the full-scale test embankment using PLAXIS finite element 2D program by means of undrained analysis in the construction stage and thereafter consolidation analysis was performed during the service stage. The settlement predictions of the soft clay foundation mostly depended on the assumed thickness of the weathered crust and the OCR values of the soft clay layer. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. The lateral wall movements were overpredicted by the analysis due to the partially drained consolidation process at the early stage of the construction. The FEM computed geogrid movements were smaller than the observed field data due to the use of lightweight tire chips-sand backfill. The maximum tension line agreed reasonably well with the coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying the weathered crust thickness, the OCR values of soft clay, the permeability values of the soft clay and the interface coefficient of the geogrid. The settlements and the excess pore water pressures changed significantly when the OCR and the permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements and tensions in the geogrids. The higher interface coefficient yielded less wall/geogrid movement and resulted in higher tensions in geogrids as expected. The results of analyses show that the FEM analysis using 2D plane strain conditions provided satisfactory predictions for the field performance.

Geosynthetic-reinforced segmental retaining walls/embankments have been well accepted in practice as alternatives to conventional retaining structures; their benefits include sound performance, aesthetic appearance, cost effectiveness and expediency of construction. This is especially true in soft ground area such as Bangkok, Thailand. Although many geosynthetic-reinforced soil walls have been safely constructed and are still performing well, there are many areas such as alluvial clay or soft clay area that need in-depth studies in order to better understand the mechanical behavior of this system under more aggressive and harsh environments (Yoo and Song, 2006). Issues related to the design and factors affecting the performance of reinforced soil have been addressed by many researches in recent times (e.g. Bathurst et al., 2005, Park and Tan, 2005, Skinner and Rowe, 2005b, Al Hattamleh and Muhunthan, 2006, Hufenus et al., 2006, Nouri et al., 2006 and Chen and Chiu, 2008). Also, the behavior of reinforced earth structures has been comprehensively studied through field observations of full-scale physical model, laboratory model testing, and numerical simulation (Bergado et al., 1995, Bergado et al., 2000 and Bergado et al., 2003; Frankowska, 2005, Varuso et al., 2005, Bergado and Teerawattanasuk, 2007, Hatami and Bathurst, 2005, Hatami and Bathurst, 2006, Ling and Leshchinsky, 2003 and Won and Kim, 2007). The use of geosynthetic-reinforced subgrade, railway and pile support embankment is studied by many researchers such as Frankowska, 2007 and Brown et al., 2007 and Min et al. (2007). Chen et al. (2007) conducted a series of centrifuge modeling test of a geotextile-reinforced wall to study its behavior in wet state due to poor drainage conditions. The study of Sarsby (2007) concerns the use of ‘Limited Life Geotextiles’ (LLGs) which are designed on the basis of having a limited working life as basal reinforcement for an embankment built on soft clay. An alternative method such as numerical or simulation by means of appropriate methods such as finite element (FE) (e.g. Basudhar et al., 2008) or finite-difference (FD) techniques (e.g. Ho and Rowe, 1994) is essentially required for a better understanding of the mechanical response of reinforced soil walls subjected to different loading conditions to develop more advanced design methodologies compared to the current limit equilibrium-based approaches. A strategy to meet this goal is to investigate the performance of reinforced soil walls using numerical models validated against physical data gathered from field or laboratory model (Hatami and Bathurst, 2006). Most researchers have assumed plane strain condition for numerical simulations of reinforced earth structures (Chai, 1992, Chai and Bergado, 1993a, Chai and Bergado, 1993b, Bergado et al., 2000, Bergado et al., 2003, Karpurapu and Bathurst, 1995, Alfaro et al., 1997, Rowe and Ho, 1998, Rowe and Li, 2002 and Hinchberger and Rowe, 2003). Recently, Skinner and Rowe (2005a) have presented the results of a numerical investigation into the bearing capacity stability of geosynthetic-reinforced retaining walls constructed on yielding foundations. Skinner and Rowe (2005b) also performed a numerical investigation into the long-term effect of foundation yielding on a geosynthetic-reinforced retaining wall and bridge abutment, to examine both the internal and the external stability of the wall. Hatami and Bathurst (2005) reported a survey of published work on numerical simulation of reinforced soil walls and categorized this work according to: (1) whether numerical models were verified against experimental/field evidence or were simply idealized model; (2) size of the experimental models used for the verification of the numerical models; (3) quality and extent of the measured data reported for each experimental/field case; (4) assessment of the accuracy of the physical data; (5) simulation of the construction sequence and the compaction effects; (6) constitutive models for the soil backfill and the availability of laboratory data from which model parameters can be selected; and (7) consideration of load–strain–time effects on the mechanical behavior of polymeric reinforcement layers. Hatami and Bathurst (2005) concluded that validation of numerical models was deficient with respect to many of the issues identified previously. Ideally, a numerical model should be robust enough to capture qualitatively and quantitatively the isolated influence of each wall component as it is varied between otherwise identical structures. Previous simulations of reinforced walls/embankments on soft Bangkok clay were steel grid reinforced wall with poor quality backfill and wrapped up facing (Bergado et al., 1995), hexagonal wire reinforced wall with sand backfill and gabion facing (Bergado et al., 2000) and hexagonal wire reinforced wall with sand backfill and precast concrete facing panels (Lai et al., 2006). The conclusions of these 2D plane strain studies were that the response of reinforced embankments was principally controlled by the interaction between the reinforcement and the backfill and therefore appropriate model and interface properties were needed; the permeability of the foundation soil influenced the finite element analysis; the embankment loading during the construction stage needed to be modeled appropriately and numerical analysis yielded better predictions of performance than the analytical models. Chai and Bergado (1993a) used finite element method to analyze geogrid reinforced embankments on soft clay and conclude that the stage construction of embankment on soft ground can be simulated by finite element analysis considering the large deformation phenomenon and the variation of the foundation permeability during consolidation. This paper deals with the predictions, numerical simulations and sensitivity analyses of the performance of a fully instrumented full-scale test embankment constructed on soft Bangkok clay using lightweight rubber tire chips-sand backfill with geogrid reinforcements. A commercially available finite element program was used to predict the performance of the test embankment during construction and post construction stages.

The numerical simulation based on finite element analyses assuming plane strain condition using PLAXIS was carried out to study the behavior of a lightweight embankment reinforced with geogrid on soft ground foundation. The numerical simulation techniques adopted in this paper captured well the overall behavior of the reinforced soil wall/embankment system on a soft soil. The important simulation considerations in the FEM analysis consisted of the method of applying the embankment loading during the construction process, the selection of an appropriate soil and reinforcement models, the estimation of soil permeabilities of the soft foundation, and the selection of appropriate parameters at the interface between the lightweight tire chips–sand backfill and the geogrid reinforcement corresponding to the interaction mechanism. The predicted results were shown to be generally in good agreement with the measured settlements, excess pore water pressures and lateral wall movements. The numerical simulation overpredicted the movements and tensions in the geogrid reinforcement. A number of important conclusions from this study are summarized below: 1. The settlements' predictions of the soft clay foundation mostly depended on the assumed thickness of the uppermost weather crust layer and the overconsolidation ratios of the soft clay layer. 2. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. 3. The computed lateral wall movements overpredicted the measurements since the partially drained consolidation process at the early stage of construction was not modeled well by the stage construction finite element analysis. Moreover, the predicted lateral wall movement can be overpredicted due to the influence of the inclinometer casing. 4. The simulated geogrid movements underpredicted the observed field data due to the use of lightweight tire chips–sand backfill. 5. The predicted tension forces along the geogrid reinforcement overpredicted the observed field data due to the limitations of the simulation during the consolidation process. The maximum tension line generally agreed well with coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying weathered crust thickness, OCR values of soft clay, permeability values of soft clay and interface coefficient of geogrid. The settlements and the excess pore water pressures were much affected when the OCR and permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements, and tensions in the geogrids. The higher interface coefficient yielded lower wall/geogrid movement and resulted in higher tensions in geogrids as expected.