EFFECT OF SUBSURFACE LIQUEFACTION ON THE STRENGTH OF SURFACE SOIL

After a subsurface layer of soil is liquefied completely, the excess pore water pressure therein is dissipated through the overlying layer of soil. The Terzaghi consolidation theory is applied to the problem to determine the excess pore water pressure, and numerical solutions are obtained using the finite element method for a variety of initial and boundary conditions. If the compressibility of the surface soil is one order of magnitude smaller than that of the initially liquefied soil, the maximum pore pressure in the surface soil is not significantly affected by the initial excess pore pressure in the surface soil, the relative thickness of the surface soil with respect to the initially liquefied soil, the drainage condition at the lower boundary of the initially liquefied soil, or by the depth of the water table. The smaller the permeability and compressibility of the surface soil, the smaller becomes the shear strength of the soil due to the upward seepage. The surface soil can even be liquefied if its permeability and compressibility are sufficiently low as compared to those of the initially liquefied soil.