Research On Permeability Mechanism And Slope Stability Of Unsaturated Loess Under Dry-wet Cycle Conditions

As the loess in the northwest region is in a cyclical change of rainfall-evaporation and dry-wetting,the structure and hydraulic characteristics of unsaturated loess always change dynamically with the change of external environment,and the dynamic change of external environment will affect the stability of unsaturated loess slope,so it’s of certain significance to study the rain infiltration of unsaturated loess under precipitation-evaporation conditions to remain slope stability.The unsaturated loess used in this test was taken from Tongwei,Gansu Province.Firstly,the infiltration mechanism of the unsaturated loess slope was obtained through discussion;and the relationship between the change of soil-water characteristic curve and the change of micropore structure of unsaturated loess is discussed through laboratory soil-water characteristic curve test and nuclear magnetic resonance test.Finally,established a unsaturated loess slope model based on the soil-water characteristic curve test,and performed the finite element numerical simulation.This article mainly has the following research consequences:(1)Developed a model for infiltration of unsaturated loess slopes based on hydraulics theory and the porous media theory,and the formula for calculating the transient moisture content of unsaturated loess slope under different rainfall intensity and duration is deduced by means of mathematics and physics,and the infiltration front depth and saturation layer thickness are obtained after the formula is changed according to different initial conditions;(2)Through the indoor soil-water characteristic curve test,the relationship curves between the suction of in-situ loess and remolded loess matrix and the volume moisture content under different cycles of wetting and drying are obtained.By comparing the differences the soil-water characteristic curves between the as-built and remoulded soils under different working conditions,analyze the effect of the wet-dry cycle and initial dry density on the soil-water characteristic curve,the soil-water characteristic curves corresponding to an arbitrary number of wet-dry cycles can be effectively predicted,laying the foundation for deriving the unsaturated permeability coefficient of the loess.(3)Based on the soil-water characteristic curve,the matrix suction needed for predicting the unsaturated permeability coefficient is obtained,and the corresponding permeability coefficients of different matrix suction are obtained by combining with the Childs &Collis-Geroge prediction model;and the relationship between dry-wet cycle times,initial dry density and unsaturated permeability coefficient is fitted according to the prediction results,and confirmed the applicability of the predictive model,so as to provide data support for the stability analysis of unsaturated loess slopes.(4)Using the Macro MR12-150H-I multi-functional microstructure analysis and imaging system to conduct nuclear magnetic resonance tests,the variation patterns of T2 profiles corresponding to unsaturated loesses with different numbers of wet and dry cycles,different initial dry densities,different drying temperatures and different soil properties were analyzed.It explains the reason why the permeability of unsaturated loess depends on the change of internal pore structure under wet-dry cycle conditions,and provides a microscopic mechanism for the stability analysis of unsaturated loess slopes;(5)Based on the results of soil-water characteristic curve test,the finite element analysis model of unsaturated loess slope is established,and the seepage analysis and stability analysis of the unsaturated loess slope under different design conditions are carried out.The results show that the stability coefficient of slope is inversely proportional to slope gradient;the greater the rainfall intensity,the faster the volumetric water content infiltration rate.As the rainfall progresses,the soil gradually becomes saturated,the rate of water infiltration decreases,the range of change of volume water content decreases,and the stability of slope decreases continuously in the whole process of rainfall.In addition,the rainfall-evaporation process has a greater impact on the soil infiltration characteristics,which will reduce the stability of the slope.