Analysis On The Topography And Rainfall Starting Factors Of Shallow Soil Landslide

"6·06" debris flows in Wangmo County in Guizhou province have caused heavy casualties and property loss, there are 66 valley in this region, only 19 missed mudslide. Thought investigation, there occurred 230 soil landslide in this event of debris flow, the thickness of landslide is generally 0.5-1m,it′s typical shallow soil landslide, landslides became the main source of the debris flow.Shallow soil landslide is one of the most widely distributed, high frequency of outbreaks and most harmful geological hazards.such landslide are vulnerable to rainfall and reservoir level cyclical effect because of its loose sliding body, so it has poor stability. Once the landslide occurs, it will not only bring great danger to local people’s life and property safety, but also cause widespread soil erosion. In alpine areas when the landslides down through the slope, It will most likely convert to debris flow, if not, the landslide slide into the valley channel, if rains continue to supply, the landslide deposit will also be started to form debris flow. So strengthening the formation, movement and mechanism study of such soil landslide is of great theoretical significance to correctly evaluate the stability of soil slope, reduce or avoid the occurrence of landslide hazard, protect people’s life and property safety and reduce economic losses.The formation of this type of landslide is closely related to topography, soil condition and rainfall factors, rainfall is trigger condition, topography and soil conditions are potential conditions for landslides. Predecessors ′ study on topography, soil texture and rainfall condition were all lack of more in-depth and more detailed feature analysis on individual landslide, comprehensive study on several conditions was also not considered. This paper is mainly in the case of Wangmo, Guizhou "6.6" debris flow disaster, we selected 230 surface soil landslide in the study area as the research object. As the study area is small, geological structure and the lithology in this area is simple, the residual soil covered on the hill is basically the same, the soil can be considered to be the same type, so this paper focused on the influence of topography and rainfall factors to the shallow soil landslide. First of all, the study area is divided into 4 areas(dayi,dabian,nabao,xintun) by rainfall condition, each rainfall in the region is broadly the same, we separately study the effects of topography on shallow soil landslide in each district through which we obtained the terrain discriminant model, and then we explore the effect of rainfall through which we got the rainfall discriminant model. Through comprehensive analysis of these two model, we obtained a composite factor discriminant model of shallow soil landslide. Eventually we apply the topographic discriminant model to Tennessee, Oregon, Split in America and Hofu in Japan to verify, very good results are achieved.(1)In the event of ―6.6‖ debris flow in Wangmo,Guizhou, the main source comes from shallow soil landslide on the side slope in the valley basin, it is closely related to topographic features(hillslide slope,etc) and rainfall conditions. When the landslide downhill in case of enough water and sliding surfaces, it is likely to evolve into hill-slope debris flow sliding into the main channel to expand the scale of debris flow.(2)Hill slope has an important influence on the shallow soil landslide, through the survey, we found that the sensitive slope of shallow landslides in Wangmo,Guizhou is ranging from 18 °-45 °,and the minimum sensitive slope decrease with increasing in rainfall intensity and amount. The minimum sensitive slope of shallow soil landslide in the four rainfall area is 18.5 °, 20 °, 21.5 °, 22.5 °.(3)In the paper, we selected landslide slopes(α) and length, width(L,W)、slope(β) and length(Lu) above the trailing edge of landslide、fluctuate slope(θ1、θ2) and width(W1、W2) on either side of landslide、free surface conditions(t),and then make the slope factor S=tan(α),upper gentle slope topographic factor U=tan(α-β) *(Lu/L), topography factor on either side of the landslide C=tan(θ1)*(W1/W)+tan(θ2)*(W2/W). Through the comprehensive analysis on slope factor(S),upper factor(U),side factor(C) and the free surface factor(t),we get a comprehensive topographic model: T = S+1.8U+0.8C+t,it can well represent the shallow surficial soil landslide terrain conditions.(4)Through the above studies of shallow soil landslide occurred in Wangmo County of Guizhou province, we conclude that the critical terrain conditions is: T≥Cr, S≥Cr0.That is to say when in case of same rainfall and geological condition, not only topographic T-factor must be greater than a certain value, and the slope S-factor must be greater than the minimum sensitive gradient, the Dayi area Cr=0.73,Cr0=0.33;Dabian area Cr=0.78, Cr0=0.36; Nabao area Cr=0.83, Cr0=0.39; Xintun area Cr=0.87, Cr0=0.41.(5)On the base of formation mechanism of surface soil landslide and in combination with the characteristics of the rainfall and time of disaster happened, the paper chooses the I-P critical rainfall mode including cumulative rainfall and hourly rainfall:0RKIBR+=, it confine K-value is 5.5 according to the formation mechanism of shallow soil landslide and characteristics of the rainfall, average annual rainfall is used to normalize the rainfall.(6)By comparing the correlation on the topographic T-factor and the rainfall R-factor,we get a comprehensive P-factor of surface soil landslide that its expression is 9.009.0)8.08.1)(5.5( CUSRIBRTP ￠+++==,Composite P-factor can judge the possibility of the slope development into surface soil landslide on the base of same geological condition.(7)Using the T-factor model in the surface soil landslide disaster of Tennessee, Oregon, Split in the United States and Hofu area in Japan, The differences in soil conditions and rainfall in different regions does not take into account temporarily, we get a good verification result. So the studies on surface soil landslide model has universal applicability.(8)According to integrated terrain T-factor value of 5 places(including study area and 4 verify area),we divided it into four hazard grades: higher risk when T>0.9;medium risk when 0.7<T≦0.9;low risk when 0.5<T≦0.7;very low risk when T≦0.5.The division of the risk grade is suitable for different regions.The division of risk grade is rough, In the absence of detailed information on rainfall, geological data,this method allows you quickly assess their risk initially.