Liquefaction mitigation in silty soils using stone columns supplemented with wick drains
Vibro replacement stone columns are in use to mitigate liquefaction hazards in sandy soils for almost three decades. There are three mechanisms that help reduce liquefaction potential of a sandy soil improved using stone columns. During stone column installation sandy soils densify due to installation vibration. Further, the stiffness of the composite improved soil increases leading to a reduction in cyclic shear stress induced on the soil surrounding the stone columns during earthquakes. In addition, pore pressures generated in the soil during earthquakes are quickly dissipated through the highly permeable stone columns. These combined mechanisms reduce the liquefaction potential of the improved soil. Sandy soil sites improved using stone columns have performed well during earthquakes. However, its effectiveness in silty soils is limited. Recent case histories show stone columns supplemented with wick drains work well in such soils. This study focuses on three aspects: (i) examining the reasons for the sub-performance of stone columns in silty soils, identifying key soil parameters that hinder the effectiveness of stone columns, and developing means to improve the effectiveness of this method in silty soils including provision of supplementary wick drains, (ii) developing a numerical model to simulate stone column installation with and without wick drains, and qualitatively evaluate the degree of ground improvement, and (iii) verifying the numerical simulation results using case histories and field experimental studies, and developing modified design charts and guidelines for designing stone columns with and without wick drains to improve sands and silty soils. Pore pressure generation, post-liquefaction dissipation, and densification characteristics of an artificial silty soil and three natural silty soils were experimentally studied and compared with sand. A careful analysis of such data indicates that liquefaction characteristics of silty soils and sands are not very different when compared using grain contact density indices as the basis for comparison. However, post-liquefaction dissipation characteristics are very much dependent on grain size characteristics. Low coefficient of consolidation associated with silty soils precludes faster pore pressure dissipation during stone column installation and therefore hinders densification around the stone columns during installation. It also hinders drainage during earthquakes. This appears to be the primary reason for the lack of effectiveness of stone columns in silty soils. Numerical studies of pore pressure behavior of silty and sandy soils support this view. Based on the experimental results, a numerical model was developed to simulate the stone column installation process. During installation, pore pressure generated due to the vibratory energy imparted into the surrounding ground was estimated, and the ground densification associated with pore pressure dissipation was calculated. Several simulations were done for sands and silty soils with varying initial conditions improved using stone columns with and without wick drains. The model was fine tuned and tested using case studies and field measurements. Design charts and design guidelines that were developed based on the extensive experimental and numerical study are presented. Recommendations for improving the stone column design methodology, and for further research in this subject are presented as well.