Rainfall Induced Seepage and Slope Stability Analyses

Abstract:

This study investigates the rainfall induced seepage behaviors and slope stability of an unsaturated natural slope of colluviums along the A-A' profile of Lu-Shan landslide using two-dimensional finite element method. At first, a steady/transient seepage analysis was carried out using 42 days rainfall records from Mat-Sa typhoon in 2005. Through the inspection of the coincidence of the groundwater variation between simulation and measurement, a set of best fit unsaturated hydraulic conductivity function k_r(ψ)~(ψ) and horizontal and vertical saturated conductivities k_x and k_y for colluviums can be determined. Where, the variable ψ denotes the matrix suction of soil stratum. The function, k_r(ψ)~(ψ), considers the seepage behaviors of unsaturated colluviums gradual transition from unsaturated to saturated state.

For a 48-hrs design rainfall with different return periods 5, 25 and 50 years, the range of the transient saturated zone formed in the slope during rainfall will expand with the increase of rainfall intensity. The self-weight of soil mass increases due to the rainwater absorption and which alternately introduces a higher down sliding force to the slope and leads to a large extent reduction of factor safety FS of the unsaturated natural slope (A-A'profile).

When the matrix suction, ψ, in the function k_r(ψ)~(ψ) was adjusted to a higher value (ψ→10ψ), physically it represents a soil stratum with finer particle, the infiltration and pore-water pressure variation becomes not observable in the rainfall induced seepage analysis. Conclusively, an unsaturated natural slope with higher matrix suction (ψ→10ψ) always possesses a higher FS value than that with lower matrix suction (ψ→0.10ψ). For the slope with anisotropic hydraulic conductivity ratio (k_y/k_x =0.01), due to the downward infiltration rate of rainwater is lower than that with isotropic hydraulic conductivity (k_x/k_y =1), the occurrence time for a FS value starting to downgrade may lag behind during rainfall.