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

在冈山地区的马萨土上进行了保水曲线滞回试验和非饱和渗透率试验。采用考虑雨水入渗导致非饱和土抗剪强度降低的边坡稳定性分析数值程序，研究了非饱和土的非饱和特性对边坡安全系数折减行为的影响。同时考虑了降雨入渗和地震对边坡稳定性的影响，对计算程序进行了改进。数值结果表明：(1)体积含水率与相对渗透率关系的滞后性可以忽略不计。(2)土体的保水性曲线在饱和附近呈小幅度变化时，边坡的安全系数可以表现为雨水入渗引起的陡然下降。(3)获得保水性曲线的试验应在吸水条件下进行，其结果应适用于雨水入渗失稳问题。(4)受雨水入渗影响的边坡在地震作用下变得更加不稳定。

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.