| Lots of slow-inclination shallow soil landslides were triggered byrainfall on September 16 th in Nanjiang. The sliding body of slow-inclination shallow soil landslideswas thin and mainly concentrated from 1 to 5m, the slope was slow and mainly distributed between 10~30°,the strata were obviously divided into two layers and the overlying soil layeralwaysslided along the bedrock cover interface which was a smooth surface.The traditional mechanical balance views shows that the overlying soil thin, the strata gently inclined and gravity component along slip direction is small,so the slope stability is better in the natural state, but Lots of slow-inclination soil landslides were triggered byrainfall on September 16 th in Nanjiang, the formation mechanismwas special.Single slow-inclination shallow soil landslide is little harm, but the group, large-scale landslide is in the human living area, it is worth attention.At present, there were some studies about development environment, distribution law, influence factors and formation mechanism of slow-inclination shallow soil landslides.But there was no clear to reveal the reason why the thickness of slip masswas mainly in the 1~5m and the overlying soil alwaysbedding slided along the bedrock cover interface.This paper relies on the Chinese Geological Bureau geological survey projectthat “Southwest important geological disaster investigation and warning division” and the Sichuan Province Science and technology support project that “Study on the Mechanics of Rainfall-induced Landslide and Monitoring Forecast Demonstration Project of Low-Angle Dip Bedded Slope in East Sichuan”.Based on the slow-inclination shallow soil landslides induced byrainfall on September 16 th,through the rainfall infiltration test, the G-A infiltration model and the calculation of atmosphere influence depth considering the effect of atmosphere on the slopestudies the rainfall infiltration depth, and then analyzingthe distribution characteristicsof slide massthickness; Thereasonwhy overlaying soilalways slide along the bedrock cover interfacewas analysed through the unsaturated soil strength theory when infiltration peakis located in soil layer and the base cover interface under the natural and rainfall conditions;ApplingGeo-studio toevaluate the slope stabilityconsidering the pore water pressure and the softening effect;I-D model was built under different environmental factors using the finite element, andthe time warning modelwasestablishedusing the Green-Ampt infiltration model combining unsaturated soil shear strength theory and limit equilibrium method.Landslides susceptibility zoningwasresearched based onlogitic regression model and Arcgis;Finally, establishing a nonlinear model between influence factors and the slope stability using BP neural network, choosing the most accurate and reasonable evaluation parameters,calculating the weight of each influence factor,predicting slope instability state, and making susceptibility zoning of landslide hazard,the main conclusions and understanding as follows:1. The rainfall infiltration test andthe G-A infiltration model show that the permeability of red clay in Nanjiang is extremely low,the rain infiltration depthwill be very limitedinshort and heavy rainfall,it is generally only about 50 cm.2. The atmosphere influence depth is 5.37 m in study area,the rain easy permeateinatmosphere influence depth, but with increase ofsoil layerdepth, the weathering degree of the soil gradually weakened, the pore ratio and permeability gradually decreased, infiltration becamemore difficult,the rainfall infiltration depth is the main cause ofthe thickness of slip masswas mainly in the 1~5m.3. Under the condition of nature, The overlying soil thin and the strata gently inclined and the gravity component along slip direction is small, the slope stability is better;Under the condition of rainfall, when the depth of infiltration peak is less than the thickness of soil layer and the potential sliding surface is in the infiltration peak surface, the slope stability. When the depth of infiltration peak is equal to the thickness of soil layer and the potential slip surface is located on the base cover interface, stagnant watergeneratedon interface, water level rise, and interface produce softening and lubrication,the interface effect is the main cause that slow-inclination soil landslidesalways bedding slided along bedrock cover interface which is a smooth surface.4. Gravity and pore water pressure make slopestability with thicker soil be lower, but softening effect make slope stabilitywith thinner soil be lower, slope stability is mainly controlled by pore pressure in earlier rainfall, but slope stability is mainly controlled by softening effect in late rainfall.The coupling of pore water pressure and softening effect makeslow-inclination shallow soil landslides happened.5. Change of soil layerthickness, slope angle, shear strength and saturated permeabilitycoefficient influence the I-D model to some extent; With increase in soil layerthickness, slope angle andsaturated permeabilitycoefficient, and decrease in shear strength of soil, the rainfall duration causing slope failure shows a decrease trend.These I-D modelscoincide with the real rainfall intensity and duration induced slow-inclination shallow soil landslides in Nanjiang, the slope was often failure under heavy rains of two days.6. Establishing two types of time early warning model based on Green-Ampt infiltration modelcombining the unsaturated shear strength theory and limit equilibrium method.①the time early warning modelof red clay slope withfracture;②the time early warning modelof red clay slope withoutfracture.7. Logisitic regression establish space early warning model with high prediction, thefitting degree of model is more 82.1%.The contribution rate of landslidefrom large to small is slope, antecedent cumulativerainfall, slope elevation and the "9.16" cumulative rainfall. Susceptibility zoning and the actual distribution of landslide is consistent8. BP neural network models establishthree space warningmodel(different evaluation parameters) having highly reliable, The correlation coefficient(R) between predictive value andactual value is respectively0.862、0.88、and0.839.Reasonable choice of evaluation parameters has an important influence on accurate prediction of landslide stability. Landslide susceptibility is accuracy.9. Formation lithology(argillaceous soft rock in red bed), topography(the monoclinal structuredevelops long and slowconsequent slope) and adequate cumulativerainfall cause the Large area and mass slow-inclination shallow soil landslides. |
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