| Shacaotou old landslide is located at the left bank of the Nangou gully in Weizhou Town,Wenchuan County,Sichuan Province.It is an old insequent rock landslide developed in the phyllite slope.The total volume of the accumulation mass is about 550,000 m~3.In the rainy season of 2017,the middle and lower parts of the old landslide accumulation mass began to show signs of revival.On the revived mass,a large number of cracks were formed on the ground and houses,and the road were broken,which directly threatened the safety of the lives and properties of the residents as well as the traffic in the landslide area.We ascertained the characteristics of Shacaotou old landslide and the revived mass,such as distribution range,deformation characteristics and slope structure,etc.in the field investigation,and divided the revived mass into three zones.On this basis,we studied the revival mechanism of Shacaotou old landslide through geological analysis combined with numerical simulations.And the stability and the development tendency of the revived mass were calculated and analyzed.The following main results were obtained:(1)The revived mass is in the middle-lower part of the old landslide accumulation.The revived mass is about 150m wide at the front edge,75m wide at the back edge,about300m long,with a thickness of 7.0m?18.0m,a total volume of about 35×10~4m~3,the main sliding direction is about 55°.According to the intensity of deformation and the deformation mode,the revived mass can be divided into three zones,which are strongly deformed zone?and slightly deformed zone?in the upper part of the revived mass and strongly deformed zone?in the lower part of the revived mass.(2)The revival of Shacaotou old landslides was mainly influenced by the following factors:topography,physical and mechanical properties of the accumulation,groundwater,erosion at slope foot by Nangou gully and rainfall infiltration during the rainy season.The continuous rainfall in the 2020 rainy season is the direct cause of the increased deformation of the revived mass.According to the deformation characteristics of the revived mass and the results of 3D numerical simulation,the deformation mode of the revived mass is considered as follows:the upper part of the strongly deformed zone?and slightly deformed zone?shear damaged first,the soil mass slid downward,pushing the soil mass below to deform.Compound deformation mainly composed of thrust load-caused deformation occurred in the strongly deformed zone?:the soil mass in the steep area crept downward,and caused the soil mass at the back edge to tensile fracture.At the same time,the soil mass at the foot of the slope collapsed affected by the erosion of the Nangou gully,causing the deformation of the shallow soil mass behind.(3)We numerically simulated the changes of the seepage field in the slope during the rainfall and 10 days after the rainfall under the once-in-50-years five-day continuous rainfall conditions by the SEEP/W program.The simulation results show that during the rainfall,especially at the early stage,rainwater is enriched in the shallow soil,the strength of the shallow soil is greatly reduced,and the soil self-weight increases,so that the shallow soil slip is likely to occur.During the late stage of rainfall and a period of time after the rainfall,the pore water in the shallow soil seep downward,causing the pore water pressure in the saturated zone at the bottom of the accumulation to rise,generating an uplift pressure on the upper soil mass,resulting in the reduction of the stability of the potential sliding surface in the deep part of the accumulation.(4)We calculated the stability coefficients of the different zones of the revived mass of Shacaotou landslide by Morgenstern-Price method.The results show that in the natural condition,the strongly deformed zone I and the strongly deformed zone II are understable,the slightly deformed zone I are basically stable.In the rainstorm condition,the strongly deformed zone I is unstable,the slightly deformed zone I and the strongly deformed zone II are understable.All the zones are unstable under the earthquake condition.(5)The effect of rainfall on the stability of the revived mass is influenced by the thickness of the accumulation layer and the groundwater depth.There is a significant lag in the weakening of the stability of the revived mass by rainfall in strongly deformed zone?and slightly deformed zone?.The weakening of the revived mass stability at the early stage of rainfall is mainly due to the increase of the soil self-weight because of the rainfall infiltration,the stability coefficient decreases slowly.During the late stage of rainfall and a period of time after the rainfall,the stability coefficient decreases rapidly as the rainwater infiltrates deeper into the accumulation,and reaches the minimum value 10 days after the rainfall.The stability of the strongly deformed zone II decreases linearly during the rainfall without obvious hysteresis phenomenon. |
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