Soil Water Dynamic Characteristics On Slope Near Benggang Walls And Stability Of Benggang Walls

Benggang erosion is one of the most serious soil erosion in the red soil hilly area of southern China,which seriously threatens the life safety and production activities of local people.The research on Benggang erosion is mostly carried out from the aspects of surface runoff erosion sediment and gully development process,and the influence of soil infiltration and lateral flow on Benggang erosion process is less studied.The soil properties,soil water seepage and lateral flow affect the water distribution and stability of Benggang.This paper study the dynamic characteristics of soil water and how soil properties affect Benggang’s stability,in an effort to provide theoretical support for revealing the Benggang’s development mechanism.In this paper,the soil moisture of different soil depths(20,40,60,80 cm)and different slope positions(upslope,midslope and downhill)on the slope near the Benggang wall was measured for a long time in a typical active period of collapse in Tongcheng,Hubei Province.The collapse time was observed,and the soil water infiltration,leakage,net water storage and lateral flow at each slope position were calculated.According to the soil properties of the Benggang wall,five rainfall scenarios(50,100,150,200 and 250 mm/d)and the ratios of the thickness of the sandy soil layer and the red soil layer to the saturated hydraulic conductivity(1:1,3:1,5:1,7:1,9:1,20:1,40:1)were designed.The slope stability software was used to simulate the safety and stability coefficient of the Benggang wall under different conditions.The following results were obtained:(1)In the slope direction and profile direction,soil properties showed obvious spatial variation.Along the slope direction,the closer to the Benggang wall,the less the soil clay content,the greater the bulk density,the worse the hydraulic conductivity.Along the profile direction,as the soil depth increases,the soil clay content gradually decreases and the sand content gradually increases.The texture also changes from sandy clay loam to sand,and the deep soil sand content reaches 91%.With the change of soil texture profile,the saturated hydraulic conductivity in the vertical direction of Benggang wall decreases first and then increases.The shear strength of the soil on the Benggang slope also exhibits a certain degree of variability.The shear strength of surface soil is greater than that of deep soil,and the shear strength of surface soil decreases significantly with the change of water content,from 256 k Pa to73 k Pa.The red soil layer has large shear strength,strong water holding capacity and weak hydraulic conductivity.The shear strength of the sand layer is small,the water holding capacity is weak,and the hydraulic conductivity is strong.(2)The soil moisture distribution on the slope near the Benggang wall also showed spatial variability in space and time.During the monitoring period of2016-2021,there are differences of soil water distribution regularity in different soil layer,and downward slope,the soil moisture content gradually decreased,while with the increase of soil depth,the soil moisture content on the slope up and down increased while the soil moisture content in the slope down gradually decreased.The water infiltration is greater on the slope than in the middle and bottom of the slope,and the water content is greater on the slope than in the middle and bottom of the slope.The spatial variation of precipitation,infiltration and soil water holding capacity determines the spatial and temporal dynamic distribution of soil water on the slope near the Benggang wall.(3)The lateral flow in the soil within 24 h after rainfall is related to the infiltration capacity and layer variation of the soil.The overall performance is that the upper slope is smaller than the middle slope and the lower slope.The average annual lateral flow in the upper slope,middle slope and lower slope is 1233.7 mm,1522.4mm and 1785.8 mm,respectively.The saturated lateral flow was detected when the soil moisture content was 0.4 cm~3/cm~3and the precipitation was 58.5 mm.Higher initial moisture content and larger rainfall were two necessary conditions for the occurrence of saturated lateral flow,and the wall collapse was accompanied by saturated lateral flow.Saturated lateral flow in near Benggang wall soil is a necessary but insufficient condition for Benggang wall collapse.(4)Scenario simulation results show that the stability of Benggang wall is affected by rainfall characteristics and soil properties.When the rainfall intensity is less than 2 mm/h,the safety and stability coefficient of the Benggang soil is always greater than 1.When the rainfall intensity is greater than 4 mm/h,continue to increase the rainfall intensity,the safety and stability coefficient of soil does not change much,and the stability of Benggang wall is mainly affected by the red soil layer’s thickness and soil saturated hydraulic conductivity.When the red soil layer is thicker(the ratio of thickness of red soil layer to sandy soil layer is 1),the water is not easy to reach the deep sandy soil layer,which is conducive to the stability of the Benggang wall.When the sandy soil layer becomes thicker and the red soil layer becomes thinner(the thickness ratio of sandy soil layer to red soil layer is 40),the soil moisture penetrates more into the sandy soil layer,so that the overall moisture content of the sandy layer increases,the shear strength decreases,and the Benggang wall begins to destabilize.In summary,the dynamic state of soil moisture near the Benggang wall indicates the stability of the Benggang.Precipitation infiltration increases the soil moisture content,resulting in a decrease in the shear strength and stability of the soil.High intensity rainfall contributes to the occurrence of lateral soil flow,increases soil moisture and reduces shear strength in the sandy soil layer near the Benggang wall,and promotes the instability of the Benggang wall.