NUMERICAL STUDY ON EMBANKMENTS RESTING ON LIQUEFIABLE SOIL WITH MITIGATION MEASURES

Abstract

Interaction of pore fluid with soil solids during ground excitation results in reduction of load sharing capability of soil. This phenomenon causes increase in excess pore pressure and further causes reduction in shear strength of the soil. A stage may come when there will be complete loss in the strength. This stage is termed as ‘Liquefaction’ of soil. Seed and Lee (1966), Seed and Idriss (1971), Seed (1979), and Seed et al. (1985) provided significant experimental framework related to the liquefaction phenomena and cyclic mobility. Physical understanding is well established, whereas analytical and numerical approaches are still an area of challenge. Highway infrastructures (embankments, slopes, retaining wall etc.) when built on liquefiable soil layer can be a cause of disruption when underlying foundation soil liquefies during earthquake event. Based on detailed literature study on embankment-liquefiable foundation soil systems, a numerical study has been planned to examine the effect of liquefaction on the embankment. Based on the preliminary study, different mitigation methods are considered further. Numerical models have been developed in PLAXIS 2D and PLAXIS 3D. The constitutive behavior of foundations soils has been represented by UBC3D-PLM model. The same has been defined by Mohr-Coulomb model for embankment soil. Due to the increase in sideways shear stresses below the embankment toe, embankment undergoes excessive settlement. The soil beneath the embankment never attains liquefaction condition, however the region near the toe and free-field liquefies. In order to mitigate the liquefaction of foundation soil below the toe regions, stone column mitigation has been adopted in the numerical model. In case of plane strain idealization, the equivalent plane strip approach has been considered to derive the equivalent parameters (bulk modulus, permeability). Based on a detailed parametric study considering the number, diameter and spacing of stone columns, optimum spacing to diameter ratio for effective mitigation of liquefaction has been found out. Further, a seismic study is carried out considering 10 different ground motions with a wide range of Arias Intensity. With increasing Arias intensity of ground motions, the embankment settlement is observed to be increasing almost linearly. Stone column mitigation is less effective in case of higher ground accelerations.