Laboratory tests on geosynthetic-encapsulated sand columns
Introduction
Although construction on soft soils is most commonly undertaken using basal reinforcement, with or without prefabricated vertical drains (Rowe and Li, 2005, Briançon and Villard, 2008, Ghazavi and Lavasan, 2008, Li and Rowe, 2008, Rowe and Taechakumthorn, 2008, Chen et al., 2008, Bergado and Teerawattanasuk, 2008, Basudhar et al., 2008, Kazimierowicz-Frankowska, 2007) there is growing interest in the use of granular columns constructed by filling a cylindrical column with granular material. This technique has been introduced into engineering practice to improve the bearing capacity and reduce settlement of sand column foundations resting on the weak soil (Bergado et al., 1991, Bergado et al., 1992, Raithel et al., 2002). The improvements on bearing capacity via granular columns are achieved through the inclusion of a stronger granular material. In response to a vertical load, an expanded granular column will squeeze the native soil, and result in an additional confining pressure onto the column. That leads to an increase in the stiffness and strength of granular column. However, insufficient lateral support at shallow column depth (top portion) frequently causes bulging failure at the top portion of the column (Hughes and Withers, 1974, Madhav and Miura, 1994). Therefore, reinforcement on granular columns, especially over the top few meters, is needed to provide lateral support to enhance the lateral confinement of column. The reinforcement can be achieved by enveloping a granular column with a flexible fabric or by placing horizontally laminated reinforcing sheets on the granular column either in full or partial height (Rao and Bhandari, 1977, Alamgir, 1989, Ayadat and Hanna, 1991, Cai and Li, 1994, Madhav et al., 1994, Broms, 1995, Nods, 2002, Raithel et al., 2002, Kempfert, 2003, Sharma et al., 2004). Geosynthetic-encased sand columns were successfully used to found a dike in very soft soil for land reclamation (Raithel et al., 2002).
The reinforcing effects on sand columns have been verified by laboratory triaxial compression tests performed on sand columns reinforced with horizontal disks or external encapsulation (Broms, 1977, Gray and Al-Refai, 1986, Chandrasekaran et al., 1989, Al-Joulani, 1995, Ashmawy and Bourrdeau, 1998, Haeri et al., 2000, Ayadat and Hanna, 2005, Sivakumar et al., 2004). Test results show that the reinforcement on sand columns increases the peak strength and the axial strain at failure, and reduces the loss of post-peak column strength. However, there is a dearth of literature on the analytical study of reinforced granular columns. Murugesan and Rajagopal (2006) presented numerical analysis results based on finite element technique that investigated the effect of external encapsulation on the confining pressure on a sand column embedded in soft clay. Raithel and Kempfert (2000) proposed analytical and numerical methods to study the performance of geosynthetic-encased sand columns. Kempfert (2003) studied the effectiveness of the encased columns for several projects. He reported on the improvement of column strength for encased granular column over stone column. He also showed a significant improvement on the column strength that increases with the geotextile stiffness.
Wu and Hong (2008) and Wu et al. (in press) reported an analytical method that investigated the stress–strain relation of granular columns reinforced with horizontal disks or external encapsulation. In this paper, we will report the results obtained from a series of laboratory triaxial compression tests conducted on sand columns encapsulated by geotextiles. The effect of encapsulating sleeve on the deviatoric stress increase and the volumetric reduction were investigated. The contributions of the sleeve toward the confining pressure were analyzed. The mobilized pseudo-cohesion and friction angle corresponding to various axial strains were studied to interpret the reinforcing effect. Finally, the experimental results are compared with analytical results reported in the literature (Wu et al., in press).
Section snippets
Materials used for testing
The experimental program consisted of performing triaxial compression tests on 140 mm high × 70 mm diameter dry sand samples encapsulated in geotextile sleeves. The soil used was uniformly graded angular quartz sand with a specific gravity of 2.65, effective size of 0.7 mm, uniformity coefficient of 1.23 and coefficient of gradation of 0.84. According to the Unified Soil Classification System this soil can be classified as SP. The maximum and minimum dry unit weights of the soil
Experimental results
The experimental results for test materials (geotextiles and soil), unreinforced and reinforced sand columns with two relative densities are presented in this section.
Analysis of strength parameters
The peak cohesion and friction angle of soil are usually evaluated using ultimate strength under various chamber pressures. However, ultimate strengths of the reinforced granular columns in the current experiments were not reached (Fig. 5). Therefore, cohesion and friction angle corresponding to different strains are evaluated and referred as mobilized pseudo-cohesion and mobilized friction angle . The mobilized pseudo-cohesion and mobilized friction angle of reinforced soil specimens were
Prediction of the experimental results using analytical approach
An experimental study was conducted by performing tests under specific conditions as dictated by a set of control parameters including reinforcement, soil and confining pressure. It is desirable to generalize the experimental results into a dimensionless form that can be easily adopted in design practice for field applications. However, many factors govern the behavior of the encapsulated granular column that include the material properties (in situ soil, filled granular material and
Conclusions
A series of laboratory experimental tests were conducted to investigate the effectiveness of external encapsulation on the strength of sand column. Three types of geotextile were sewed into sleeves to encapsulate sand specimens with two relative densities. For triaxial compression tests on reinforced specimens, the deviatoric stress and volumetric strain were measured and used to evaluate the inspired confining pressure, radial strain and volumetric strain reductions. The cohesion and friction
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