Study Of Water Retention Characteristics And Strength Degradation Of Unsaturated Red Clay Slope Under Extreme Rainfall

Rainfall infiltration is the important inducing factor of slope instability, and landslides induced by rainfall get its severity, especially in rainy areas. Rainfall infiltration is a complex process that saturated and unsaturated seepage of rainwater infiltration happens concurrently. This process varies the distributions of seepage field and stress field of the original slope, and stability is affected ultimately. Necessary physical and mechanical parameters of saturated and unsaturated red clay were obtained before numerical simulation. Characteristics of water retention、displacements、stability and strength were observed under continuous and unimodal-type rainfall conditions by coupling seepage field and stress field, combining with monitoring points(six in total) located in palces closed to slope crest、hillslope、slope toe respectively.Water retention property of unsaturated red clay slope was observed through distribution of pore water pressure and its variation on monitoring points in rainfall stages. We can draw conclusion that rainfall intensity and the duration of time are significant factors on the basis that isotropic hypothesis and the given initial conditions. Rainfall infiltration made negative pore water pressure increased; negative pore water pressure of unsaturated zone in shallow depth increased obviously when the rainfall intensity was small; drainage phenomenon occurred in the saturated layers located in bottom when the rainfall intensity was greater and formed closed negative pore water pressure zones.In the saturated-unsaturated seepage process, the void ratio of soil shifted with variation of saturability or water content, according to the basic relationship of quality & volume of soil. And this physical phenomenon performs variaton of displacements of clay slope in two-dimensional analysis. Analysis of displacements manifested that subsidence of slope top happened in early rainfall stage but it would restore in subsequent periods, that slope surface、slope toe would uplift gradually at the same time and this change would become more and more apparent during rainfall.Slope stability would reduce gradually during rainfall, immediate cause is a decrease of shear strength in unsaturated soil, but from the formative root, is the result of the increase of negative pore water pressure in unsaturated zone. Soil’s stress state 、stress path and other factors take significant effects on shear strength. Selecting absorbing stress as indicator of strength parameter can not only avoid the difficulties above, but also reflect the condition of the matrix suction and retain the linear feature of Mohr – coulomb criterion. Calculation and analysis of absorbing stresses were performed by extracting the effective stresses and total stresses on monitoring points in the spectrum of rainfall duration. The results show that absorbing stress, which is a key index to control the shear strength, dominates in the effective stress in duration. This conclusion is expected to provide theoretical basis for the analysis of shallow landslide.