Deformation and consolidation around encased stone columns

Abstract

A new analytical solution is presented to study soft soil improvement, both reduction of settlement and consolidation time, by means of encased stone columns. The proposed solution pretends to be a simple and useful tool for design. Therefore, only a unit cell, i.e. an end-bearing column and its surrounding soil, is modelled in axial symmetry under a rigid and constant load. Soil is assumed as elastic but plastic strains are considered in the column using the Mohr-Coulomb yield criterion and a non-associated flow rule, with a constant dilatancy angle. An elasto-plastic behaviour is also considered for the encasement by means of a limit tensile strength. The solution is presented in a closed form and is directly usable in a spreadsheet. Parametric studies of the settlement reduction, stress concentration and consolidation time show the efficiency of encasing the columns, which is mainly ruled by the encasement stiffness compared to that of the soil. Column encasement is equally useful for common area replacement ratios but columns of smaller diameters are better confined. Besides, the applied load should be limited to prevent the encasement from reaching its tensile strength. A simplified formulation of the solution is developed assuming drained condition. The results agree well with numerical analyses.