Unsolved effects of soil depth and its non-uniform distribution on rain-induced landslides remain although soil depth is known to affect both the infiltration and the equilibrium forces in the critical shear zone. A series of laboratory experiments by a sloping flume packed a sandy soil placed under a rainfall simulator were conducted to elucidate the effects of soil thickness. Five kinds of soil depths of uniform distribution and two types of non-uniform layers were tested. The rainfall

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... ty was set at constant (110 mm/h) in every experiment to eliminate the effects of rainfall intensity on landslide initiation. Initial soil wetness was also controlled at a constant level. Pore water pressures and the motion of sliding masses were observed. Numerical simulations, which were composed of a slope stability and a seepage model, were also tried. Initiation times of failures resulted in a liner relationship with soil depths in both the laboratory and numerical experiments of uniform soil layers. The penetration velocities of the wetting fronts were observed to be constant in every experiment of uniform soil layers. The saturated area simultaneously appeared over all the bottom boundary of the uniform depth slope. Contrary to this phenomenon, the wetting front reached shallower part at first, and the saturated area also sequentially appeared in the same place of heterogenous soil distributions. Because the velocities of wetting front were always constant as the same as the uniform soil layers, soil depth became essential to the time when the wetting fronts reached the bottom ends. Failures were initiated at the shallower parts of the slope where the saturated areas first appeared in the slope of uniform soil layers. The simulations' results by a limit equilibrium analysis combined with finite seepage model corresponded well to those of the laboratory experiments for the critical times of failure and locations of slip circles. These results show that soil depth and its non-uniformity strongly regulate both initiation times and locations of slope failures.