Study On Mechanism Of Rainfall Instability Of Loess-bedrock Fill Slope With Weak Zone

With the rapid development of urbanization in the Loess Plateau,the utilization rate of urban land resources has reached an unprecedented height.The urban-rural construction is densely distributed,and the population is crowded and the traffic is inconvenient,which seriously affects the healthy development of urban economy.In order to solve these problems,Yan'an city has taken the lead in carrying out the construction strategy of large-scale gully control and land reclamation projects.However,these projects have formed a large number of cut and fill loess slopes.Due to the special engineering geological properties of loess,the problems of slope erosion,collapse,landslides and soil erosion are more prominent under rainfall,Among them,the stability of loess fill slope is more complex,and lack of scientific guidance and theoretical basis.Therefore,it is of great theoretical and practical significance to study the instability mechanism of loess filled slope under rainfall.This paper takes the engineering construction of cutting mountain to build cities and gully land reclamation projects as the research background,through field data survey and drilling sampling,indoor soil mechanical parameter test,and indoor rainfall model experiment combined with finite element numerical analysis,the instability mechanism of the fill slope under the action of rainfall in the loess-bedrock containing the weak zone are studied,and the change law of internal seepage,soil pressure increase,pore pressure increase,water content,slope displacement and deformation failure of the slope are revealed.Meanwhile the influence of rainfall patterns,rainfall soil parameters,and rainfall slope type changes on slope stability is discussed.The research results can provide a scientific guidance for the engineering design and geological disaster prevention and control of the weak zone loess-bedrock fill slope produced in the construction of cutting cutting mountain to build cities and gully land reclamation projects in the Loess Plateau.The main research results are as follows:(1)Through on-site investigation of engineering geological background in Yan'an area,drilling sampling and indoor geotechnical tests,the basic physical parameters of the soil in the study area are obtained,which provides parameter basis for subsequent model tests and finite element numerical analysis.(2)Based on the indoor rainfall model test,the instability mechanism of the slope under the action of rainfall on the slope filled with loess-bedrock in the weak zone is revealed:(1)The weak zone and the slope surface are the"two-sided"dominant channel for rainwater infiltration;(2)The overall change trend of the slope soil pressure in the early rainfall period is that the soil pressure rises under the influence of rainwater infiltration,after that,the soil pressure decreases due to the softening of the soil inside the slope.In the later stage of rainfall and the process of stopping rainfall,the soil pressure recovers and rises due to the dissipation of rainfall.The variation law of the soil pressure in three days of rainfall under the condition of intermittent rainfall shows a circular fluctuation on the whole;(3)In intermittent rainfall,the trend of the pore water pressure of the downhill slope is in three stages:the steady stage before the rainfall-the rise of the pore water pressure in the middle of the rainfall and the steady stage-the pore water pressure decreases at the end of the rainfall;(4)The change trend of slope water content shows three stages:stable in the early stage of rainfall,rising to peak in the middle stage of rainfall and slightly decreasing at the end of rainfall;(5)The development process of slope damage is mainly slope toe erosion and slippage,trace erosion,erosion cutting,side erosion,and finally shallow damage on the slope;(6)The boundary between the weak zone on the top of the slope and the filling loess is affected by rainfall and rainwater infiltration,which leads to shear-shrinkage failure in the upper area of the slope body,and the maximum settlement is at the slope shoulder;(7)The toe of the filling loess slope suffers from"two sides"advantages Under the influence of seepage,local area of shallow slippage occurs and an empty surface is generated.Under the action of the empty surface,the traceable erosion of the shallow landslide eventually constitutes a multi-level shallow sliding failure.(3)Based on Geo-studio finite element numerical simulation,the influence law of slope stability coefficient under different working conditions is revealedthat:(1)The slope stability coefficient decreases exponentially with the increase of rainfall intensity and rainfall duration,and the rainfall infiltration increases with the rainfall depth;(2)The stability factor linearly increases with the cohesion and internal friction angle before and after rainfall,and the increase of permeability coefficient leads to the rapid decrease of slope stability coefficient;(3)With the linear increase of the slope,slope height,and thickness of the weak zone of the fill slope,factor of stability decreases exponentially and the combined formula isy=e^(a+bx+cx2).