Numerical modeling of stone columns with installation effects

Stone column installation modifies the state of the surrounding soil, and therefore, influences the ground improvement achieved with the stone column treatment. The paper presents numerical simulations investigating the settlement reduction caused by stone columns in a natural soft clay, accounting for the modification of soil state caused by stone column installation. These installation effects have been previously studied by modelling the installation as an undrained expansion of a cylindrical cavity. A "unit cell", i.e. only one endbearing stone column and its corresponding surrounding soil, is modelled in axial symmetry using the finite element code Plaxis. The properties of the soft clay correspond to Bothkennar clay, a soft (Carse) clay from Scotland (UK). The complexity of this material is simulated via an advanced constitutive formulation able to account for the soil anisotropy, namely S-CLAY1. The results show that the changes in the stress field, such as the increase of radial and mean stresses, and the loss of overconsolidation have a positive influence on the settlement reduction, excepting for low loads. Furthermore, column installation and subsequent loading cause several changes in the soil fabric. Those changes have a positive effect in reducing the settlement because energy is dissipated in the evolution of anisotropy. Parametric studies of the influence of the coefficient of earth pressure at rest after column installation are also presented