Numerical prediction of seepage and seismic behavior of unsaturated fill slope

This paper presents a case study on a numerical prediction of the seepage and seismic behaviors of an unsaturated fill slope. In this study, the numerical prediction is performed with a dynamic three-phase (soil, water and air) coupled analysis based on porous media theory and constitutive models. The weak forms of three governing equations, momentum balance equations of the overall three-phase material and mass and the momentum balance equations of the pore fluids (water and air), are implemented in a finite element model. The discretized equations are solved by the fully implicit method and the skeleton stress is also implicitly integrated. The in-situ observation at the target fill slope of the case study has been ongoing since 2006. First, distributions of water saturation in the fill slope are simulated by performing seepage analyses to reproduce the in-situ ground water level in the fill slope. Second, seismic responses of the fill slope during two past earthquakes are simulated, and the numerical method is validated by comparing observed acceleration records and numerical one. Finally, seismic responses of the fill slope during a future scenario earthquake are predicted under different moisture conditions of unsaturated fill. As a result, the following findings were obtained. 1) The shape of the ground water level was partially reproduced from the quite dry fill slope element by using virtual constant precipitation. However, the observation results of moisture distribution above the ground water level were not reproduced. 2) In the validation analyses after two earthquakes, the numerical method reproduced the positions of peak frequency of acceleration Fourier spectra. 3) The numerical results clearly showed that frequency characteristics of the seismic response and the residual displacement of the fill slope were affected by the initial distribution of the degree of saturation in the fill ground.