| At 8:30 pm of August 27, 2014, a large landslide occurred in Fuquan, Guizhou Province. The huge mountain slid at the speed of 50m/s, rock and soil block rush to the pit with water in the right front edge of slope.The landslide surge occurred as a result of the impact action of high speed landslide on water in the pit, the high velocity flow and air caused huge great destruction to Xiaoba and Xinwan.It caused23 deaths and 22 injured, 77 house damaged of 67 households, and caused a lot of life and property losses.The conformation of Xiaoba landslide is extremely rare in the country and the destruction of which is severe.Relying on scientific research project“study on geological mechanic behavior and hazard-formation during long-term evolution of large-scale landslide”,which is the sub project of 973 scientific research project “study on indentification of disaster-inducing factor,acquisition of predictive information,early waring method of large scale landslides in the west of China,I have participated the fieldword of Xiaoba landslide,and then carefully studied the characteristics and the formation mechanism of it.The main research contents and results show that:(1)Xiaoba slope is a kind of high slopoe with two-layer structure of upper hard and lower soft, which is moderate-dip cataclinal slope with upper hard and lower soft.Based on the rock occurrednce of left and right slide wall of landslide, we found that the space shape of Qingshuijiang rock formation bedding plane is wedge-shaped,the dip direction of intersecton of which is 140°and the dip angle of which is 30°.There is a reverse fault with fault attitude of N36°E/NW/62° in the anterior part of slope. The dolomite rock mass of Sinian Denying Formation(Z2dy1) in the front edge of slope is the locked segment which controls the stability of slope.(2)Xiaoba landslide is a large-scale and high-speed bedding rock landslide.The maximum moving speed of sliding mass is 50m/s, the total volume of sliding masss is1,410,000 cubic meters, the volume of residual sliding mass in sliding source area is560,000 cubic meters, the volume of accumulation of sliding mass in accumulationarea is 810,000 cubic meters and the thickness of slidebody is about 20~50m.Sliding surface developes along lithologic interface of the underlying soft rock as well as bedding surface. The main sliding mass is a wedge-shaped block, the landslide slided along double sliding surface. The plane shape of the landslide is generally fan shaped.The elevation of the trailing edge of the landslide is 1430m~1450m, the elevation of the front edge of which is 1250m~1265m,the level difference between the two is about 200 m.The maximum horizontal longitudinal is 780 m between trailing and front edge,the maximum horizontal width is 470 m. The main sliding direction of Xiaoba landslide is 140°~145°.(3)Signs of deformation and failure for a detailed site survey, the slope is devided into sliding source region A and accumulation area B according to the configuration and the characteristics of material source of landslide.Depending on the characteristics of nature of sliding, sliding source region A is devided into main sliding mass area A1 and sliding trough A2.Depending on the characteristics of the order of accumulation, accumulation area B is devided into?accumulation left accumulation area B1,?accumulation right accumulation area B2, ? accumulation shallow surface material accumulation area B3, ?accumulaiton silicalite rock block concentrated accumulation area B4.(4)Based on field investigation,it is concluded that unfavorable characteristics of slope structure of bad geological basis for landslide formation,excavation in the front edge of slope promoted the formation of landslide, excavation and continuous rainfall role for the formation of landslide by inducing factors.The unfavorable geological conditions is the basis of landslide, the comprehensive effect of excavation and continuous rainfall induced landslide.Landslide instability of the basic failure modes are devided into three stages:(1)slope occurred bedding slip, then tension cracking of trailing edge of the slope,(2)the locked section appeared at the toe of slope,(3)the locked section was cut through, then slide mass started and slid down.The geomechanical modes for deformation and failure of Xiaoba slope is:sliding-tension-shear break.(5) Based on mechanical experiments,We found the internal friction angle and cohesion along the bedding surface of clayey siltstone of underlying Qingshuijiang Formation(Qbq) clayey siltstone are 31°and 3.2MPa respectively, the shear capacity of which is low. The internal friction angle and cohesion of the dolomite rock mass of Sinian Denying Formation(Z2dy1) are 50.4 ° and 16.7MPa respectively, the shearcapacity of which is high, and the uniaxial compressive strength of which can be more than 70 MPa.The mechanical properties of both are basic reason for Sliding surface developing along the bedding surface of clayey siltstone of underlying Qingshuijiang Formation and the dolomite rock mass of Sinian Denying Formation(Z2dy1)developing into the locked section. The internal friction angle and cohesion of Nantuo formation moraine rock mass are 45°and 21.3MPa respectively, but it is easy to disintegrate after absorbing water and the strength of which decline quickly.(6)Midas/GTS software using the slope were three-dimensional numerical simulation, the results show:(1) It is concluded that under the condition of excavation,the effect of underground excavation in the left front edge of slope on slope stability and the deformation of slope caused by it is greater than that of open excavation in the right front edge of slope. Compared with rock mass in the right front edge of slope, the degree of stress concentration and deformation of rock mass in the left front edge of slope is larger.(2)Under the condition of rainfall,stess concentration of locked section in the toe of slope is more obvious;compared with the open excavated slope, the deformation of the underground excavated slope is greater, the value and distribution range of the relatively high value area in the Qingshuijiang Formation(Qbq) palimpsest clayey siltstone is greater, the deformation and the degree of stress concentration of the locked section in the left front edge is greater. It is inferred that the first failure of the locked section in the left toe of slope is the reason for instability of Xiaoba slope.(7)Geo-slope software using the slope-dimensional numerical simulation, the results show:(1)First, without considering groundwater situation, the underground excavation and open excavation process simulation.Calculations shows that excavation led to maxium shear strain concentration belt appearing along interface of clayey siltstone and silty slate in the middle part of slope, and which comes into being the potential sliding surface. Compared with the rock mass in the right toe of slope, degree of stress concentration and distribution of plastic zone of the locked section consisting of Sinian Denying Formation(Z2dy1) dolomite rock mass in the left toe of slope is larger.Then initial seepage field and transient seepage field under the condition of rainfall of two sections simulation, the results indicates that rainfall has certain effect on groundwater level of Xioaba slope.(2)Finally coupled seepage and stress calculation simulated the effect of rainfall on Xiaoba slope.It is conculuted that rainfall led to the distribution of maxium shear strain concentration belt increasing, the potential sliding surface evolves further.The degree of stress concentration and distribution of rock mass in the toe of slope is larger compared with condition without considering groundwater situation. Compared with the rock mass in the right toe of slope, degree of stress concentration and distribution of plastic zone of the locked section consisting of Sinian Denying Formation(Z2dy1) dolomite rock mass in the left toe of slope is significantly larger,and the corresponding slope of which comes into critical sliding condition. We inferred that the loced section of left toe of slope first becomes instability, which leads to final instability of Xiaoba slope.(8) Based on the analysis of the three hazard factors of the slope geological condition, artificial excavation and continuous rainfall, we inferred that the key hazard factors of Xiaoba landslide are excavation and continous rainfall.we can study on the idenfication of Xiaoba landslide as following four aspects: unfavorable characteristics of slope structure,artificial excavation,deformation characteristics,rainfall duration and density. |
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