Keywords: sand column, numerical analysis, encapsulated, triaxial compression test, geotextile.

Foundation construction on soft soil usually encounters weak bearing capacity and consolidation problems. The basal reinforcement and granular columns are the most common techniques used to solve these problems [1,2,3,4,5,6]. 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 (top portion) frequently causes bulging failure in the top portion of the column [7,8,9,10].

Encapsulating cylindrical granular columns using tensile resistant material provides a great improvement in the column strength. For an encapsulated granular column subject to axial load, the axial deformation and volumetric expansion causes lateral expansion, which stimulates circumferential strain and stress in the encapsulating reinforcement and provides additional confining pressure to the column. This in turn strengthens the sand column in an interactive manner. This study numerically analyzes the behaviour of geosynthetic-encased sand columns. Because of the volumetric strain, the filled material has a decisive effect on the expansive behaviour of the column. Numerical functions are developed to formulate the volumetric strain in the sand.

An elastic-plastic constitutive model with non-associated flow rule is used to characterize the prominent expansive behaviour of medium to dense sands. The mechanical properties of the materials involved in the reinforced column are extracted from simple experimental tests. Results from the present numerical analysis are compared with experimental results for encapsulated sand columns subjected to triaxial compression. The tested sand columns are made of two different sands, encapsulated by sleeves fabricated from two different geotextiles. The numerical analysis results are also compared with data obtained from a theoretical method reported in the literature. The numerical results exhibited good agreement with those obtained from laboratory tests.

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