Physico-mechanical aspects of liquefaction risk reduction in sand using geotextile-encased granular columns

Liquefaction-induced flow failures, excessive settlements, lateral spreading, and loss of shear strength in granular soils can become massive hazards during earthquakes. Among the various mitigation techniques available, soil reinforcement using dense granular columns can be considered as a very effective technique, and its effectiveness gets further improved by encasing the columns in a geotextile to maintain the integrity of the columns during earthquakes. This paper presents findings from a first-of-its-kind study of simple shear tests on sand reinforced with geotextile-encased granular columns (EGC) to understand the fundamental mechanisms leading to its improved liquefaction resistance and shearing response. The effects of area replacement ratio and grouping action of columns on the overall response are established by performing a series of multi-stage constant volume simple shear tests on unreinforced and EGC-reinforced sands. The area replacement ratio was varied between 4 and 16% in different tests, and the tests with 16% area replacement ratio were conducted on sand with a single column and a group of columns. The particle sizes, encasement tensile strength, and column configurations are carefully chosen to avoid scaling and boundary effects on the test results. The performance of EGCs against liquefaction was evaluated considering all fundamental mechanisms, including the progression of pore pressures, nonlinear hysteretic behaviour, strain energy accumulation, and shear modulus degradation. The potential of EGCs for mitigating the liquefaction and improving the post-liquefaction shear strength of sand was found to improve with the increase in the area replacement ratio. For a specific area replacement ratio, the beneficial effects were more significant when the EGCs were spread into a group of symmetrically placed columns instead of a single column at the centre. The stress concentration on the EGCs due to modulus contrast and additional confinement offered by the EGCs to intervening soil have collectively benefited the shearing response of sand before, during, and after liquefaction.