Hysteresis Effect of Water Retention and Permeability of Unsaturated Soils on Instability of Slope Due to Rainfall

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The test for hysteresis of water retention curve and that for unsaturated permeability were carried out on Masa soils produced in Okayama. The effect of the unsaturated properties of slope soils on the reduction behavior of the safety factor is examined with the numerical program of the slope stability analysis considering the reduction of the shear strength of unsaturated soils due to the rain infiltration. The numerical program is also improved for considering both the effect of the rain infiltration and that of an earthquake on the slope stability. The numerical results give the following conclusions.(1) The hysteresis of the relationship between the volumetric water contents and the relative permeability can be neglected.(2) The safety factor of a slope, whose soil has the water retention curve showing a small magnitude near saturation, can show the sudden drop due to rain infiltration.(3) The test obtaining the water retention curve should be carried out under absorbing condition, when the results are supposed to be applied to the instability problem due to rain infiltration.(4) The slope suffering from the rain infiltration becomes more unstable when the earthquake strikes the slope.