Laboratory Model And Numerical Simulation Of Instability On Collapsing Wall Induced By Rainfall Infiltration

Benggang,a the fragmented erosional landform,mainly distributes in granite weathering mantles.A huge sediment yields of Benggang generate environmental detoration,including siltation of rivers and reservoirs,burying farmland,destroying water conservancy facilities.Benggang has been highlighted as a national concern due to its significant threat to ecosystem and social services in tropical and subtropical China.Collapsing wall as the most activity part of Benggang system is critical to formation and development of the Benggang due to its high sensitivity to slope failure.To realize an effective control of the Benggang,it is essential to investigate slope failure mechanisms of collapsing wall.Therefore,the mechanism of rainfall-induced slope failure of the collapsing wall was investigate by laboratory modelling and numerical simulation.The obtained results would facilitate a better understanding of formation and evolution of Benggang erosion.Four typical granitic Benggang areas were selected from Tongcheng（TC）of Hubei,Ganxian（GX）of Jiangxi,Changting（CT）of Fujian and Wuhua（WH）of Guangdong provinces as the study objects.The soil properties,including basic physical and chemical properties,hydraulic characteristics and shear strength characteristic,of the collapsing wall were measured and analyzed.In order to understand the mechanism of rainfall-induced instability of collapsing wall,the soil volumetric water content,pore water pressure,displacement and shear stress distribution of the model slope under rainfall conditions were analyzed by laboratory test and numerical analysis method.Meanwhile,to explore the primary and secondary factors affecting the seepage stability collapsing wall,the sensitivity analysis of the factors was carried out by using parameters study and grey correlation analysis.The main conclusions are as follows:（1）The soil properties showed obvious profile heterogeneity,and the basic physical properties,hydraulic and mechanical properties between residual soils and completely weathered soils were significantly different.The soil texture in the profile of collapsing wall had a changing trend from clay to loam sandy soil from the top soil layer to the bottom,and the clay content（28.92⁴8.38%）of the residual soils was significantly higher than that of the completely weathered soils（5.54³6.65%）,while the sand content showed an opposite trend.The liquid and plastic limit decreased with the increased soil depth,and the liquid（48.28⁵4.27%）and plastic limit（32.12³8.54%）of residual soils were relatively higher.（2）the water holding capacity of completely weathered soils was worse than that of residual soils.The air entry values of RS（0.208⁰.795 k Pa）were relatively smaller than that of completely weathered soils（0.491⁴.198 k Pa）.S1 and S2 showed an increased trend with the increased soil depth.The residual water contents(0.101⁰.179 cm³ cm^-3)of completely weathered soils were lower than that of residual soils(0.201⁰.289 cm³ cm^-3).The unsaturated hydraulic conductivity of residual soils was higher than that of the completely weathered soils,and the decreased rate of the permeability coefficient of completely weathered soils was faster than that of residual soils,which is related to the stronger water holding capacity of the residual soils.The shear strength of completely weathered soils were more sensitive to the water variation.The effective cohesion,c′,and effective friction angle,φ′,of residual soils（15.65⁴2.82 k Pa,12.74³0.60 °）and the completely weathered soils（0¹5.23 k P,11.59³4.91 °）were significantly different.The effective cohesion of residual soils were greater than that of completely weathered soils,while the effective friction angle showed the opposite trend.The values of φ^b of residual soils were smaller than that of completely weathered soils,and the changing values of φ^b with soil suction of residual soils were also smaller than that of completely weathered soils.With the increase of soil saturation,the soil suction decreased,and the shear strength of completely weathered soils decreased faster than that of residual soils.（3）The maximum displacement of collapsing wall was located near the foot of the slope under the natural gravity stree field,while the maximum displacement of collapsing wall was located at the shoulder under the rainfall condition.The shear failure area of collapsing wall was mainly concentrated at the foot of the slope,and the maximum shear strain increased with the rainfall intensity.The displacement varied exponentially with time.The rainfall water gradually infiltrated into the soil along the slope surface,and the soil at the shoulder of slope formed seepage area firstly resulted from the water infiltration on the catchment and collapsing wall.Then,the foot of the slope also formed a certain seepage area because of rainfall,runoff and ponded water.The point H1 monitored the variation of water firstly,and the movement rate of the wetting front at the monitoring point H1 was o.161 cm min-1.Then followed by H2,H4 and H3,and the movement rate of the wetting front at the monitoring points were 0.153 cm min^-1、0.133 cm min^-1 and 0.116 cm min^-1,respectively.At the end of the test,the maximum value of XY-displacement（0.54⁰.66 mm）was mainly concentrated at the slope shoulder,and gradually decreased towards the interior of the slope along the slope shoulder.Meanwhile,the maximum shear stress（14¹6 k Pa）was concentrated at the foot of the slope and gradually decreased in the upward direction of syncline.With the extension of rainfall time,the seepage area at the shoulder and foot of the slope gradually developed,and the stress was concentrated in the soil at the foot of slope which was subjected to gravity of the upper soil and seepage force.The stress at the toe of the slope was transferred to the top soil of toe that was still in the unsaturated area.Finally,the soil of the unsaturated area at the upper of the toe also reached the point of yielding.The results of numerical analysis were in good agreement with the results of laboratory tests in terms of the volumetric water content,the cross section of the failure plane and the time of soil mass failure.（4）Rainfall infiltration changed the stress distribution of the collapsing wall,and the maximum displacement was concentrated on the shoulder.The shoulder of collapsing wall had an obvious trend of lateral displacement,and vertical crack occurred under the influence of tensile force.The collapse of the collapsing wall was controlled by the crack development.The equation f（λ,X,Z,c,ψ,a） was proposed to judge the slope failure of collapsing wall.The stability of collapsing gully was most sensitive to the initial soil suction,the fitting parameters "m" and "n",the effective friction angle and rainfall intensity,and the grey correlation degrees were all greater than 0.84.The larger the value of parameter "α",the stronger the water-holding capacity,the higher the unsaturated permeability coefficient,the lower the factor of safety of collapsing wall under rainfall conditions,and the faster the decreasing rate of the factor of safety,while the parameter "m" showed the opposite trend.The influence of the parameter "n" on the factor of safety of collapsing wall under rainfall conditions depends on the relationship between the initial soil suction and the air entry value.The saturated hydraulic conductivity,K_s,affects the unsaturated permeability coefficient,k_w,thus affecting the rate of water entering the soil.The smaller the value of K_s,the smaller the soil unsaturated permeability coefficient,and the greater the factor of safety of collapsing wall under rainfall conditions.The smaller the value of "ψ₀",the lower the factor of safety of collapsing wall,while the parameter "I" showed the oppsite trend.When the effective cohesion and effective friction angle were bigger,the soil unsaturated shear strength was also larger,and the factor of safety of collapsing wall was greater.Meanwhile,the sensitivity of the stability of collapsing wall to the effective friction angle was greater than effective cohesion.