Consolidation Analysis of Soft Ground Improved by Stone Columns Incorporating Foundation Stiffness

The consolidation of soft ground improved by stone columns is normally analyzed under equal strain or free strain conditions. In this study a new consolidation model for stone columns is proposed to capture actual field conditions that lie between these two hypotheses, where a permeable foundation layer on top of the unit cell is introduced. By considering the stiffness of this layer, a closed-form solution can be derived, which indicates a considerable difference between the equal strain and free strain conditions. The influence of foundations with varying values of stiffness is examined, and the results demonstrate that as the foundation layer becomes stiffer, the time needed to achieve a 90% degree of consolidation decreases and so does the differential settlement, but the steady stress concentration ratio increases. This is also confirmed by a parametric study carried out under varying dimensionless ratios with respect to soil modulus, column spacing, and permeability. A computational example is provided to show the implications of these results on actual design. Finally, a case study is presented to illustrate that the proposed model is able to provide more realistic predictions of settlement and stress concentrations on top of the unit cell.