Behavior of dual layered encased stone columns in loose sand under shear load

Abstract

Rapid urbanization and civilization have led to a higher demand for land, requiring effective ground improvement techniques to enhance the load-bearing capacity and stability of weak soils. This thesis specifically examines stone columns as a commonly employed ground improvement method. Stone columns involve the placement of crushed stones into weak soils to enhance their engineering characteristics. The focus was on bulging failure to improve the vertical load-bearing capacity of stone columns. Additionally, there were other failure modes identified that are associated with column groups and underlying issues. One of the observed failures in the stone column is shear failure, which has been observed beneath the embankments at the corner line of the column or might be found where the stone columns are susceptible to the lateral load. Additionally, the use of encasement has become one of the most popular concepts to enhance the efficiency of the stone columns. This study explores the possibility of enhancing the shear resistance of stone columns by implementing a new concept of duallayered encasement. The use of two layers of geosynthetics to encase the stone columns is anticipated to enhance the overall stiffness of the soi-stone column combined body, resulting in enhanced shear strength and overall performance. The study aims to examine the behavior of dual-layered encased (DLE) stone columns under different normal pressures. The findings suggest that the inclusion of stone columns in loose sand enhances their capacity to bear lateral loads. The preliminary observation suggested that the soilstone column system demonstrates greater rigidity in comparison to the loose sand bed, and unencased stone columns rupture along the predetermined shear plane, whereas encased columns undergo bending without rupture. Encasement enhances the lateral load capacity of the stone column by mobilizing tensile forces. This results in significantly higher lateral resistance forces, up to 2.5 times greater. Dual-layered encased (DLE) stone columns exhibit a significant increase in strength mobilization when compared to other stone columns. This enhancement is particularly noticeable for smaller column diameters, while it becomes less prominent for higher area replacement ratios. Additionally, it has been observed that stone columns enhance the shear strength characteristics of the underlying soil bed. The largest increase in apparent friction angle is observed with the use of larger-diameter stone columns. Moreover, the most significant change in apparent cohesiveness is observed at lower levels of area replacement. Also, both the second moduli of elasticity, i.e., E10 and E20, were higher for the dual-layered-encased stone column than the single-layered-encased stone column, indicating the enhanced stiffness of the combined soil-stone column body. The study demonstrates that the dual-layer encasement concept is superior to the conventional stone column. The dual-layered stone column approach can be recommended for areas where the foundation's underlying column is at high risk of failure due to lateral loads, as well as at the corner line of stone columns beneath structures such as embankments.