Predicting Stiffness and Stress Variation of Saturated Clay Improved with Vibro Stone Columns and Evaluating its Effect on Improving Reinforced Foundations

Vibro stone column techniques create an improved composite foundation in fine grained soils because of: (1) the installed load bearing columns of well-compacted, coarse-grained material and (2) the improvements to the surrounding soil due to the construction of the stone columns consolidating the surrounding soil. Extensive research work has been carried out over the last 20 years to understand the improvement in the composite foundation performance due to the consolidated soil. Few of these studies have quantified the changes in the stiffness and stress state of the treated soil, or have considered the impact that these changes have upon the performance of the composite foundation. Consequently, empirical and conservative design methods are still being used by ground improvement companies leading to a significant range of results in engineering practice. Based on cylindrical cavity expansion theory, two-dimensional finite element study to develop an axisymmetric model of a single stone column reinforced foundation was performed using PLAXIS 2D to quantify the effect of the vibro installation of this column in soft saturated clay by producing the load settlement response of the foundations. An updated mesh was used to cope with the large deformation of the soft clay around the installed column caused by the lateral expansion due to the Vibro technique. Different amounts of lateral expansion were simulated to determine the change in the stress state, stiffness and load settlement response. It was found that the radial expansion increases the pore pressure in the clay that starts to dissipate immediately after finishing the column installation leading to a permanent improvement of the stiffness of the soil which decreases with distance from the column. The radial stress acting on the column also changes creating a new coefficient of lateral earth pressure K, a key design parameter. The effect of these altered soil characteristics were assessed by applying a load to the composite foundation and calculating the resulting settlement.
The previous model results have been validated and applied for a well-documented field case of stone column groups using Plaxis 3D after adopting a conceptual model for accumulating the installation effect of two adjacent stone columns. A very good agreement between the recorded and simulated load-settlement curves was achieved after performing few calculation cycles of different degrees of expansion cavity. A simplified design framework base on numerical analysis in how to account for the stone column installation and the recommended degree of applied radial cavity during stone column installation was the main output of this research to achieve more efficient composite foundations.