Experimental and numerical studies on geosynthetic encased stone columns in saturated and unsaturated soils

Abstract

Geosynthetic encased stone columns are conventionally designed based on the principles of saturated soil mechanics ignoring in-situ unsaturated conditions. Such an approach results in over conservative and, in certain scenarios, unrealistic designs. A more rigorous approach is required for design of stone columns considering the influence of saturated and unsaturated soil conditions taking account of area replacement ratio of stone columns and the stiffness of geosynthetic encasement considering site specific conditions. However, such an approach is presently not available in the literature. For this reason, in this study, 42 physical model tests were conducted that suggest an increase in carrying capacity of stone columns by 14 times understand unsaturated conditions in comparison to saturated conditions for the soil tested. A non-linear increase in load-bearing capacity was observed in boundary effect and primary transition zones while a decline was noted in secondary transition and residual zones due to reduced matric suction contribution. Additionally, comparisons were drawn using 3D finite element analyses, which were extended to prototype-scale studies to understand variations in confining pressure and soil deformation around stone columns in unsaturated conditions. The numerical results validated the experimental results, showing a 50% reduction in settlements due to the contribution of matric suction. Finally, a simple framework is proposed for predicting the load-carrying capacity of encased stone columns extending the mechanics of saturated and unsaturated soils. The rational design methodology summarized in this study for extending it into geotechnical engineering practice can contribute to potential cost savings by optimizing geosynthetic stiffness, stone column diameter, and area replacement ratio.