Bearing Capacity of Liquefiable Sand under Seismic Loads

Abstract

The liquefaction phenomenon is considered one of the most challenging problems in geotechnical research. Saturated sand is the most hazardous type of soil with respect to liquefaction. Seed and Idriss (1971) proposed the “Simplified Procedure” to estimate the factor of safety against liquefaction. If the factor of safety against liquefaction is less than unity (FSliq< 1), the sand will liquefy, and hence mitigation measures should be applied. If FSliq ≥ 2, the excess pore pressure is relatively small, and the hazard of liquefaction is diminished. For soils where the factor of safety against liquefaction ranges between 1.0 and 2.0, the literature suggests a conservative procedure to estimate the seismic bearing capacity of shallow foundations,where the failure is assumed to occur by punching shear (i.e. the foundation depth has no effect on the seismic bearing capacity). This leads to a significant reduction in the seismic bearing capacity and conservative foundation design. In this thesis, a nonlinear numerical model based on the Finite Element Method (FEM) is implemented in order to estimate the seismic bearing capacity of partially-liquefiable sands (1 <FSliq< 2) from the stress-settlement curve. Different stresses are applied, seismic analyses are conducted, and hence the post-earthquake settlement that takes place due to redistribution of the excess pore pressure after the end of the earthquake episode is estimated. The nonlinear dynamic model is advantageous over other constitutive models for its ability to capture the excess pore pressure generation at each time step during the earthquake. Studies are carried out considering partially liquefiable sand (SPT blow count of 10 to 30). An extensive parametric study is conducted, where the effects of footing width, sand relative density, foundation depth, partially liquefiable layer thickness and earthquake intensity are investigated. The stress-settlement curve for each case is plotted.