Two dimensional nonlinear seismic ground response studies
The work in this thesis investigates the nature of the two dimensional nonlinear seismic ground response of alluvial basins. Computer programs are developed to analyse both the out-of-plane (SH) and in-plane (PSV) two dimensional solution spaces. A finite difference approach is utilised. The seismic input motion may originate from either below or from within the two dimensional mesh modelled. The analyses are performed in terms of total stresses and strains, and pore water pressures are not taken into account. A transmitting boundary allows energy to radiate from the solution space. The computations are performed with a nonlinear soil model which incorporates hysteretic material damping. The shape of the initial loading curve of the soil model may be arbitrary, but the majority of the analyses presented utilise a hyperbolic initial loading curve.
Alluvial basins of constant L/H ratio are analysed subject to both vertically propagating seismic waves and seismic waves inclined at angles to the vertical. Both simple displacement pulse and complex transient earthquake acceleration input forms are utilised. Varying levels of excitation are employed to study the effect of strain level on the basin response. Two dimensional basins of varying L/H ratios are analysed subject to vertically propagating waves, and the results compared to a one dimensional nonlinear formulation. The results obtained using a hyperbolic initial loading curve are compared to those produced using initial loading curves derived from empirical relationships for dynamic soil properties. Results from the two dimensional nonlinear analysis are compared to those obtained from a two dimensional linear visco-elastic solution. A detailed case study is investigated with the two dimensional nonlinear analysis forming part of the Ashigara Blind Prediction Test. Elastic closed form solutions for simple two dimensional configurations are calculated, and time domain results compared to the nonlinear analysis. In-plane Rayleigh wave and out-of-plane Love wave characteristics in the nonlinear medium are investigated. A study of soil-structure interaction in the nonlinear medium is made. The feasibility of including discrete fault source models in the two dimensional analysis is investigated, with both dislocation and stress drop methods of input.
The two dimensional nonlinear site response analysis method presented in this thesis is found to be a very flexible tool in calculating the ground response of alluvial basins to seismic waves. The method is relatively inexpensive computationally in the light of present computer capabilities. The method is therefore preferred to one dimensional and linear visco-elastic analyses in calculating the response of alluvial basins, given sufficient site data.