Study On Formation Mechanism Of Gentle Inclined And Shallow Accumulative Landslide Induced By Heavy Rainfall

A large number of landslides are caused by rainfall.In recent years,landslides occurred in southwest mountainous areas,mostly caused by heavy rainfall,for example,2004 "9.3" floods caused Xuanhan Tiantai Township landslide and in 2007 Dazhou "7.5" heavy rainfall caused multiple mass landslides and 2011 "9.16" heavy rainstorms caused the large-scale mass landslide disaster in Nanjiang County.Nanjiang County in 2011 "9.16" caused a lot of accumulative landslides,the thick of this type of slope is 3 to 5m,sliding along the bedrock cover interface,the dip of sliding bed is 10~20°.It is generally believed that the possibility of landslides in this kind of slope is very small,but it triggers large-scale mass landslide under strong rainfall conditions.This kind of landslide has its unique formation mechanism.Therefore,it is of great significance to study the formation mechanism of landslides in shallow layer landslides.Based on the typical landslide survey in the red soil area of Nanjiang County,Sichuan Province,this paper summarizes distribution law,deformation and failure mode and movement of the landslide.And the installation of on-site penetration test on typical landslide body,installation monitoring instruments of volume moisture content,pore water pressure and rainfall,field test of high density resistivity method and numerical simulation of rainfall infiltration for rainfall infiltration process to explore the slope of the rainwater infiltration law;The shear test of sliding soil and bedrock cover interface by using the improved direct shear apparatus and ring shear apparatus,the study on the mechanical properties of the bedrock cover interface.The deformation characteristics of the landslide hazard were studied by centrifugal modeling tests and physical simulation tests.And the stability of the landslide is analyzed by numerical simulation.Through the research of this paper,the following main achievements are achieved:(1)The law of development of gentle and shallow accumulative landslides in the red layer of Nanjiang County was identified and the deformation and failure mode was proposed.The results show that the landslides in the gentle and shallow layers of the red layer in Nanjiang County are dominated by small and medium-sized ones,and the thickness of the slopes is mostly 3 to 5m.The ?-type landslide(side of the bedrock,side of the cliff,leading edge of the free faces)And type I(no gully or small gully bounded on both sides of the landslide,and the leading edge of the free faces)as the main boundary condition.From topography and geomorphology,the dominant elevation of accumulative landslides are 500 ~ 1000 m,and the landslides are concentrated on the slope of 10~20°.The geomorphic units are mainly located in the areas of single-sided mountain,table-shaped mountain and Single-sided mountain slope on the surface.Accumulative landslides are developed in located in Jurassic and Cretaceous strata.The failure mode is slip-crack along the bedrock cover interface.(2)In order to study the law of rainfall infiltration of gentle inclined and shallow accumulative landslide in Nanjiang,groundwater monitoring and high-density electric test were carried out on typical landslides.The results show that the permeability coefficients vary greatly between different parts of the slope.The response of groundwater level to the rainfall is basically the same,but the response of groundwater level is slightly lower than that of rainfall.The groundwater level of slope in flood season stays above the bedrock cover interface.after the flood season,the pressure of pore water rapidly declines,and the groundwater level gradually descends below the bedrock cover interface.By analyzing the distribution of moisture content of accumulation layer at different time before and after rainfall,it is found that the surface soil is sensitive to rainfall and its water content is highly fluctuant.The main source of water is vertical infiltration of rainfall.The main source of soil moisture in deep soil is vertical and horizontal infiltration.The soil is saturated at the interface of the front of the slope,and the transient saturated seepage field is formedunder the action of heavy rainfall.As the rainfall continues,the saturated area is continuously raised by the slope along the base interface to the rear of the slope of the slope,the water level continues to rise,the pore water pressure increases gradually.(3)In order to find out why the accumulative landslide in the red layer mainly slides along the 3~5m deep bedrock cover interface,Shear test of the interfaces between the sliding body,the slip zone soil and the bedrock,the saturated slip zone soil and the bedrock.The results show that the shear strength of the sliding body,slip zone soil and bedrock interface were decreased with the increase of water content.Under the same water content(unsaturated)condition,the shear strength of slip zone soil and rough bedrock is the highest,the shearing strength of the slippery soil is next to that of the slippery soil and the smooth bedrock.The shearing strength of the saturated slippery soil of the landslide with 3~5m thickness is the highest,the shear strength of bedrock is second,and the shear strength of saturated slip zone soil and smooth bedrock face is the smallest.(4)Geotechnical centrifuge model test was conducted on the formation process for the gently inclined and shallow accumulative landslide along bedrock cover interface in Nanjiang County.The deformation and failure characteristics,process and genesis mechanism of the landslide were analyzed.The experimental results show that the slope instability is controlled by the weak layer near the bedrock cover interface and the strong rainfall.Under strong rainfall,the water enters into the bedrock cover interface and cladding along the dominant infiltration channel and causes the slope to slip.The failure mode of the accumulation slope is slip-pull.(5)In order to study the influence of rainfall intensity,slope thickness and sliding bed slope on the deformation and failure of slope,the series of physical simulation tests of the formation process of gentle inclined and shallow accumulative landslide were carried out.The experimental results show that when other conditions are the same,the rainfall intensity is larger and the rainwater tends to converge to the bedrock cover interface,which is more prone to destabilization damage.The shorter the time required for landslide sliding,the smaller the accumulated rainfall.Under other conditions,the slope thickness of 3~5m,the greater the thickness of the slope is not easy to slip,the longer the landslide slide,the cumulative amount of rainfall is more,the sliding time is longer,the pore water pressure required at the bedrock cover interface is greater.When other conditions are the same,the slope gradient of the sliding bed will be slow and the landslide will occur longer time and more accumulated rainfall.The greater the pore water pressure at bedrock cover interface for sliding.On the contrary,the opposite.The deformation and failure mode of the accumulation slope is slip-crack that slides along the bedrock cover interface.(6)In order to find out why the accumulative landslides in the red layer occurred mainly in 3~5m slopes,the critical pore water pressure at the start of slope was studied.The results show that the slop will instability when the pore water pressure is greater than the critical pore water pressure when the slope is 3 to 5m.The thickness of the slope body is more than 5m,and it is difficult for rainwater to infiltrate into the bedrock cover interface.The pore water pressure at the bedrock cover interface are 0,which is much lower than the critical pore water pressure.accumulative landslides occur mainly in the genesis mechanism that the slope body thickness is 3 to 5m.