The effects of local geology and liquefaction on seismic ground motion

摘要

Observations from major earthquakes show that local geology affects the seismic ground motion and the liquefaction potential of soil layers. Case studies also indicate that less damage happened to structures located on the sites where liquefaction was detected. However, numerical analyses to these phenomena have been seldom reported.;In this thesis, a stress-strain-liquefaction model is proposed based on the energy approach. The model involves the use of the non-linear constitutive relationship of soil and the modulus degradation curve determined from laboratory and field tests. A new boundary element formulation for visco-elastodynamic analysis in time domain is presented. The velocity and the acceleration terms in the dynamic equilibrium differential equation are approximated by an appropriate temporal discretization scheme. The differential equation is solved by the direct boundary element method in which the fundamental solution for elastostatic problems has been adopted. The domain integrals generated by the velocity and acceleration terms are transformed to boundary integrals by means of the dual reciprocity method. The new liquefaction model has been introduced into the boundary element formulation, to conduct the one-dimensional and two-dimensional liquefaction analysis.;One-dimensional liquefaction analysis has been conducted for the Port Island case. The computed motion agrees with the record of seismic motion. The analysis shows that the ground motion with the period from 0.2 second to 2.5 second has been significantly reduced by the liquefaction of soil. This result explains the fact that less damage to most of the engineering structures occurs in liquefied zone.;A parametric study has been carried out on the effects of the two-dimensional valley on ground motion and liquefaction. It is found that for low-frequency motion, the maximum amplification occurs at the center of the valley. The amplification is significantly affected by two factors: the shear wave velocity of the fill material and the base material in the valley, and the depth and width of the valley. The liquefaction potential in an alluvial valley is generally higher than that in an evenly layered site. Liquefaction starts at the edges of the valley.;Based on the analyses, it is suggested that in a site with high liquefaction potential, pile foundation and structures with high resonant frequency will help to reduce the seismic damage to the structures. It should be noted that in earthquake zone, the existence of alluvial valleys may increase the seismic damage and the liquefaction potential.