Rainwater Movement Characteristics And Failure Mechanism Of Zhonglin Village Residual Soil Slope In Feiyun River Basin

The Feiyun River Basin is located in the southeast coastal area of Zhejiang Province.During the typhoon and rainstorms,a large number of landslides are unstable,causing serious casualties and property losses.Revealing rainwater characteristics of residual soil slope and instability mechanism under Typhoon rainstorm has important theoretical and practical significance for the prediction and prevention of rainfall-induced landslides in the study area.Based on the background of the geological environment and the basic characteristics of the landslide in the Feiyun River Basin,taking Zhonglin Villiage landslide as an example,we investigate the rainwater infiltration characteristics of tuff and granite residual soil under different rainfall conditions,then the SEEP/W mode of Geo-studio software was then used to replicate the above soil column experiments to back-analyze unsaturated hydraulic parameters of the two soils,which were later used to simulate seepage fields of the slope under rainfall infiltration.Finally,we used the Geo-studio software to analyze the slope stability the seepage field,deformation,and stability of Zhonglin Village landslide under different rainfall conditions.On the basis of the above research,the instability mechanism of the Zhonglin Village landslide was summarized and analyzed.Main conclusions obtained through the above research of which are as follows:(1)Landslides in Feiyun River Basin mainly occur in tuff,granite,basalt and sandstone strata.88% of the landslides in Feiyun River Basin are about 2-3 m and 1000 m2 in depth and area.About 5% of the landslides are very small in scale,with the instability depth less than 1 m and the sliding area about 500 m2.About 5% of the landslides are large in scale,with an average depth of 6.5 m and a maximum depth of 12 m,with an average sliding area of about 4500 m2,the back edge of which are often intruded by dikes(2)The infiltration velocity of rainwater in Zhonglin Village landslide residual soil during typhoon rainstorm is much greater than that during plum rains.The permeability of granite soils of the slope is much higher than that of tuff soils,during typhoon rainstorm and plum rains,wetting front velocities in granite residual soil are 2-3 times and 1.2-3 times than that in tuff soil,respectively.The infiltration velocity of rainwater decreases with the increase of depth,during typhoon rainstorm and plum rains,wetting front velocities of soils at 100 cm depth is only 0.2-0.3 times and 0.3-0.5 times than that at shallow soils,respectively.(3)Under the typhoon rainstorm and the short-term heavy rainfall,rainwater infiltrates only into the shallow surface of the tuff soil where a temporary saturated region was formed,while at the same time,rainwater is able to infiltrate through the granite residual soil deeply into its contact with the strongly weathered rock due to the differences of hydraulic properties of the two soils,then the ground water flows laterally along the interface between the strongly weathered layer and the residual soil,the water table in the slope increase with time,subsequently leading to an increase in pore-water pressure in the slope.Under the plum rains,the pore water pressure in the shallow residual soil layer of slope increases rapidly,but it does not reach saturation,and rainfall has little effect on the groundwater table.(4)There are two failure surfaces in the shallow and deep layers of the Zhonglin Village landslide.Under the rainfall,the porewater pressure of the superficial tuff residual soil increases,the shear strength decreases and the shear strain increases significantly,the shallow failure surface is formed at the depth of 4-5 meters under the control of topography and structural plane.Rainwater infiltrates rapidly through the granite residual soils,the porewater pressure of the soils near bedrock surface increases,the shear strength decreases and the shear strain increases significantly,under the control of topography and structural plane,a deep sliding surface is formed along the bedrock surface,and the failure depth can reach 11 meters.(5)During the typhoon rainstorm and the short-term heavy rainfall,the stability coefficient of the shallow failure slope is rapidly reduced to the minimum,and shallow slope instability is potentially the most dangerous during rainfall periods.After the rainfall,the stability coefficient of the deep failure surface continues to decrease,and the deep slope instability is potentially the most dangerous after rainfall.(6)The granite dyke and its residual soil have played a key role in the deep failure of Zhonglin Village landslide,changing both the water pressure distribution and failure mode of the slope.Rainfall infiltrates into the shallow layers of the residual soil of the tuff and only triggers shallow landslides.However,due to the slope-structure change caused by the granite dyke and its residual soil,which has much greater permeability,rainfall could infiltrate deeply into the interface between the strongly weathered layers and the residual soils,leading to a rising water table and consequently the deep failure in the slope.