Keywords: granular column, numerical analysis, parametric study, encased, geosynthetic.

Weak bearing capacity and consolidation problems are frequently encountered during foundation construction on soft soil. The base reinforcement and granular columns are the most commonly techniques used to solve these problems. The inclusion of granular material in soft soil reduces the drain path and improves the bearing capacity. However, insufficient lateral support at shallow column depth frequently causes bulging failure in the top portion of the column. There is a practice to enhance the bearing capacity of a granular column, especially within the top portion, by reinforcing the column with tensile resistant material.

Many elements affect the bearing capacity of a reinforced column embedded in soft soil, e.g. granular and reinforcing materials, surrounding soft soil, interfacial characteristics of different materials and the geometric and mechanical boundary conditions. The numerical method has advantages over the theoretical method in analyzing structures with complicated geometry and load conditions such as columns embedded in limited radial spacing and non-uniform pressure distribution along the column length. Because confining pressure significantly affects the mechanical properties of a granular material, variations in the confining pressure during axial column loading should be explicitly expressed. Volumetric strain induces column expansion and radial strain, which in turn increases the circumferential stress in the encapsulating reinforcement and cavity pressure offered by the surrounding soft soil.

This paper provides a numerical method to analyze the mechanical performance of a reinforced granular column. The proposed method employs the elastic-plastic constitutive law with a non-associated flow rule for filled material, and the elastic-perfectly plastic law for the reinforcement. The mechanical parameters of the materials are extracted from simple tests, e.g. a triaxial compression test for pure sand and a load-elongation test for the reinforcement. The results obtained from the proposed method were compared with those obtained from laboratory experimental triaxial tests. A series of parametric studies regarding the effects of column diameter, and encasement stiffness and strength on encased column behaviour is performed. The encasement stiffness and column diameter have significant effect on the inspired confining pressure of an encased column. The numerical results reveal that using low stiffness encasement to reinforce a large diameter column produces limited reinforcement effect. The sleeve hoop stress exerts an additional confining pressure on the encased column, which mobilizes its compressive strength and resistance to further deformation in an interactive manner. Therefore, no clear sign of strength yield can be found in the encased columns prior to when the encasement reaches its yield strength.

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