Seepage And Stability Analysis Of Sandy Overburden Landslide Under Rainfall Conditions

Sandy overburden is a loose deposit or sediment covering the bedrock.The main component is sand.Rainfall has adverse influence on slope stability,which is the major factor causing sandy overburden landslide.Sandy overburden landslide has the characteristics of wide distribution,frequent occurrence and great damage.Based on the basic theory of rainfall infiltration and the evaluation methods of landslide stability,this paper has studied the law of seepage change,failure mode and stability of sandy overburden landslide under rainfall conditions by means of indoor rainfall model test,sand hydraulic characteristic experiment and numerical simulation analysis,and has discussed the influence of slope,rainfall intensity and geological structure on the developing process of sandy overburden landslide.The major achievements are as follows:1.The indoor model test about rainfall-induced sandy overburden landslide was carried out.With the help of the developed comprehensive geological environment simulation experimental device(consist of model trough and artificial rainfall system),the measurement system(including various sensors and monitoring recorders,high-definition industrial cameras),image processing technology,the rainfall infiltration process,and the changes of slope deformation,volume water content,pore water pressure and matrix suction of sandy overburden under rainfall conditions had been studied.Before the test,the uniformity of rainfall intensity and the accuracy of sensors were calibrated.The results met the requirements of test,and the basic physical properties of the selected materials were tested.2.Through the observation records and comparative analysis of indoor rainfall model tests,the seepage development,the deformation and failure law of sandy overburden landslide under rainfall conditions are obtained.The influence of slope,rainfall intensity and geological structure on the development and change of sandy overburden landslide has been discussed,and the failure mode of sandy overburden landslide under rainfall conditions is summarized.(1)In the early stage of rainfall,the wetting front moved downward,the infiltration rate at the bottom of sandy overburden was the largest,and the pore water pressure,volume water content and matrix suction change frequently,while the change at themiddle and top was relatively slow,and the deformation of slope was not obvious.Due to the effect of rainfall scouring,the deformation was relatively concentrated in the shallow areas of sandy overburden;with the wetting front reaching the bedrock surface,the entire overburden became full gradually.The volume water content and pore water pressure reached a larger value,the matrix suction decreased,the seepage effect gradually strengthened,and the slope deformation began to increase continuously;Before the failure occurred,the pore water pressure fluctuated greatly,the seepage changed frequently and violently,and the slope deformation was obvious;In the middle and later stage of rainfall,the damage had developed from the bottom to the top of the overburden,and the pore water pressure reduced to a steady state gradually,the matrix suction increased little,the slope seepage weakened,and the sandy overburden reached a state of overall instability and failure.(2)Slope,rainfall intensity and geological structure play important roles in the development and change of sandy overburden landslide.With the increase of slope,the infiltration rate of rainfall at the bottom of sandy overburden increases,and the seepage effect inside the overburden becomes more intense when sandy overburden has reached the saturation state as a whole,the development of deformation and destruction becomes more obvious,and the overall stability decreases.With the increase of rainfall intensity,the infiltration rate of rainfall in the sandy overburden increases,the overburden reaches the saturation state faster as a whole,and the time related to development of seepage change,deformation and destruction advanced.The overall stability decreases.The setting of underlying bedrock under sandy overburden is cancelled,and the model is changed from binary geological structure to single geological structure.The integrality increases greatly,the seepage change and slope deformation develop slowly,and the overall stability is improved largely.(3)The failure mode of sandy overburden landslide under rainfall is progressive retrogressive failure.Its failure developing process can be divided into three stages.The first stage is that the shallow layer of sandy overburden is subjected to intense erosion;The second stage is that the bottom of sandy overburden is damaged by tension crack;And the third stage is that the crack moves upward to form progressive failure,which ultimately leads to the overall failure of sandy overburden.3.The hydraulic characteristics of sandy soil samples were tested under the conditions of dehydration and wetting by using the transient dehydration and wetting experimental device(TRIM).Based on Richards equation under one-dimensional flow,the parameters related to hydraulic action in the sample of sand inversely calculated by van Genuchten-Mualem model,and then the data obtained from the experiment are fitted by the HYDRUS-1D software.Finally,the hydraulic characteristics' parameters of the sand,Soil-Water Characteristic Curve(SWCC)and Hydraulic Coefficient Curve(HCF)will be obtained correctly.4.Based on the basic theory of rainfall infiltration and the evaluation methods of slope stability,with the help of SEEP/W ? SIGMA/W and SLOPE/W modules of GeoStudio software,according to the designing scheme and relevant data of indoor rainfall model test and the hydraulic characteristic parameters of sand obtained from TRIM test,the numerical simulation analysis of sandy overburden landslide under rainfall is completed.The numerical simulation agrees well with the model test results,but there are some differences in the variation of pore water pressure in the late stage of rainfall ? the regularity of slope strain and the failure mode of sandy overburden landslide,which are mainly related to the boundary effect of the model,the difference of compactness and homogeneity of sand,and the uneven distribution of initial matrix suction.