Research On Strength Evolution Mechanism Of Remolded Unsaturated Loess And Slope Stability Under Drying-wetting Cycles

Loess is widely distributed in the northwest of China,because of its arid climate conditions,the groundwater level is relatively low,therefore,the soil is often in the unsaturated state.The mechanical properties of unsaturated soil is generally more complex than that of saturated soil due to the existence of pore gas,and the shallow soil of a large number of natural slopes and high slopes exposed by engineering excavation are exposed to the atmosphere,which are often affected by climatic conditions.Among them,the repeated alternating cycles of rainfall and evaporation make the water and gas state in shallow soil of slope in a dynamic change,the change of water vapor causes the change of suction in soil,which affects the mechanical and permeability characteristics of soil,and even endangers the stability of slope.Therefore,in order to study this kind of engineering problem,it is necessary to have a comprehensive and correct understanding of unsaturated soil in the process of drying-wetting cycles,and to analyse the stress state of unsaturated soil reasonably.Neverthless,in most cases,the slope is not destroyed during the peak rainfall in the process of rainfall,but by the non-peak rainfall after repeated rainfall-evaporation cycles.It seems oversimplification to study the stability of the slopes under rainfall infiltration process only.Therefore,the strength evolution mechanism of remolded unsaturated loess under drying-wetting cycles is studied by SWCC test and triaxial test of unsaturated soil.And the experimental results are applied to the stability analysis of loess slope under rainfall-evaporation action.The main contents are as follows:(1)The water retention characteristics of remolded unsaturated loess under different times of drying-wetting cycles(0,1,2,5)were studied through SWCC tests of unsaturated loess,and the change rules of SWCC of remolded unsaturated loess during dehumidification-hygroscopicity cycles were obtained.It is found that the water holding capacity of remolded unsaturated loess decreases with the increase of drying-wetting cycles,that is,the matrix suction decreases at the same moisture content.With the increase of drying-wetting cycles,the hysteretic effect decreases gradually.Three different SWCC models are used to fit the results,and a more suitable model for the soil-water characteristic curve of remolded unsaturated loess is obtained.Considering the influence of drying-wetting cycles,the prediction model of soil-water characteristic curve of remolded unsaturated loess under drying-wetting cycles is established.The permeability coefficient of remolded unsaturated loess is predicted by Childs&Collis-Geroge permeability coefficient model,and the variation of unsaturated permeability coefficient with matrix suction and volume water content under different times of drying-wetting cycles is obtained.Then the formulas for calculating the permeability coefficient of remolded unsaturated loess with different volumetric water content under any times of drying-wetting cycles are established.(2)By using the WF unsaturated soil triaxial apparatus,through controlling different net confining pressures(50kPa,100kPa,200kPa,400kPa),different matrix suction(50kPa,100kPa,200kPa),the consolidation drainage tests of remolded unsaturated loess specimens with different times of drying-wetting cycles(0,1,2,5)were carried out,and the deformation characteristics and strength evolution after different times of drying-wetting cycles were obtained.It is found that the soil sample after drying-wetting cycle is more prone to shear failure,and the greater the suction,the more obvious the phenomenon is;the greater the confining pressure and the matrix suction,the greater strength of unsaturated loess,and the contribution of matrix suction to the strength is mainly realized by cohesion c,while the internal friction angle??varies little with the increase of the matrix suction;accordingly,the unsaturated strength parameters c?,??and?~b are obtained,and the change rule of the parameters with the drying-wetting times,the results show that all the unsaturated strength parameters decrease with the increase of wet and dry cycles under the same test condition.And decrease most in the first wet and dry cycle,and c'is most affected by the cycles,followed by?~b,and has the least influence on?'.The experimental results provide a parameter basis for the subsequent study of the stability of loess slope under rainfall-evaporation.(3)Seepage and stability of loess slope under rainfall-evaporation conditions are studied by using finite element software Geo-studio.Through its built-in seepage evaporation module(VADOSE/W),the temporal and spatial distribution characteristics of seepage field in the process of rainfall-evaporation are calculated by setting reasonable climatic boundary conditions.Based on seepage calculation results,the The evolution law of slope stability during rainfall-evaporation process is obtained by stability calculation module SLOPE/W.And the relationships between slope stability and slope gradient and rainfall intensity are studied by changing slope gradient and rainfall intensity,which provides reference for practical engineering.It is found that with the increase of dry-wet cycle times,the overall safety factor of slope decreases gradually,and the larger the slope gradient,the smaller the safety factor.