Study On Source Characteristics And Start-up Mechanism Of Shallow Landslide In Reneyong Post-fire Debris Flow

Post-fire debris flow is a kind of special geological disaster.It refers to the debris flow which occurs near the burned area and is closely related to the forest fire.On June 1^st,2014,a forest fire occurred in Yuangen Mountain,the upper reache of the Reneyong Gully in Zhengdou Township,Xiangcheng County,Sichuan Province.The fire lasted four days and was put out on June 5^th.In the following years,multiple debris flows occurred in Reneyong Gully successively due to the impact of local heavy rainfall,which had a significant impact on the personal and property safety of local residents.Taking six branch ditches on the left bank of Reneyong Gully affected by forest fires as examples,a detailed investigation of the study area was made on the basis of fully collecting relevant geological data,and the distribution characteristics of fire intensity and material sources in the basin were mastered by combining remote sensing interpretation.Subsequently,a lot of indoor and outdoor experiments on soil properties were carried out in order to explore the influence of fire behavior on starting landslide material source.Meanwhile,Geo-Studio simulation software was used to analyze the stability of a typical shallow landslide,so as to obtain the influence rule of forest fire on slope stability.The main findings of this paper are as follows:?1?Remote sensing interpretation combined with on-site survey results show that the total area of the forest fire occurred in Yuangen Mountain on June 1^st,2014,was approximately6.9km².According to the fire intensity standard,the burned area can be divided into four levels:severe,moderate,mild and unfired.Post-fire debris flow all occurred in the basin of the six branch ditches that the fire was excessive and the vegetation was burned seriously.The main active material source in the basin is landslide,and the distribution and development of landslide are highly correlated with the fire intensity.?2?The slope surface soil in the unburned area is gravel-sand soil,and the content of fine particle from the top to the foot of the slope has little change,the slope surface has strong resistance to rain erosion.The slope surface soil in the severe burned area is gravelly soil,and the content of fine particle from the top to the foot of the slope increases obviously,which indicates that the burned soil is susceptible to rain erosion and has a certain sediment transport capacity.Meanwhile,forest fire also affects soil permeability,which reflected by the reduction of permeability and saturation permeability coefficient,vegetation recovery could improve soil permeability to a certain extent.?3?After the forest fire,the number of roots,root diameter,root mass density and root length density in the superficial soil of severe burned area reduced significantly compared with those in the unburned area,and under the condition of the same root diameter,the soil resistance to tension and tensile strength decreased after fire.At different soil depths of 0 cm,5 cm and 10 cm,the shear strength of the soil-root composite in the burned area is lower than that in the unburned area,which decreased by 27%,25%and 16%respectively.Specifically,the cohesion of soil decreased obviously after forest fire,while internal friction angle changed little.?4?The formation of shallow landslide in burned area is mainly influenced by landform,fire behavior and rainfall.The steep terrain provides the gravitational potential energy difference for the slope,the fire behavior makes soil lose the protection of vegetation,makes the slope more vulnerable to rain erosion and reduces soil shear strength,rainfall can increase the gravity of the slope,and increase the pore water pressure,hydrostatic pressure and hydrodynamic pressure of slopes,further reduce the mechanical properties of soil,resulting in increased deformation and failure of slopes and accelerating the instability of landslides.?5?Stability analysis of a typical shallow landslide in severe burning area,the results show that under natural conditions,the stability coefficient of the slope before and after the fire is1.504,1.134,which is stable and basically stable and under rainfall conditions,continuous rainfall for 1h,2h and 3h,the stability coefficients of the slope before fire are 1.398,1.242 and1.120,which are in a stable state to a basic stable state,the stability coefficients of the slope after fire are 0.907,0.855 and 0.807,which are in a unstable state.