Study On Controlling Factors Of Rainfall Induced Shallow Landslide In Well Vegetated Area

China is a large mountainous country.The mountainous area accounts for about two-thirds of the countrys land area.The geological structure is complex,the topography is undulating,and the precipitation is concentrated.There are a lot of shallow landslides prone areas sensitive to rainfall in the mountainous region.The vegetation effectively prevents and controls shallow landslides,debris flow disasters and soil erosion and other mountain disasters.Although vegetation has a good ecological disaster reduction effect,in recent years,under extreme rain conditions,a large number of shallow landslides and debris flows have occurred in some areas with high vegetation coverage in China and abroad.The function of vegetation to prevent and control shallow landslides has limited conditions,and may play different roles under different geological environmental conditions.The function of vegetation to prevent and control shallow landslides has limited conditions,and may play different roles under different geological environmental conditions.In order to further reveal the control factors of shallow landslides induced by rainfall in areas with high vegetation coverage under special geological environmental conditions.This study taked the group-occurring shallow landslides in the well-vegetated area of Liziyuan Village,Qinzhou District,Gansu Province,induced by heavy rainfall events from June to July 2013 as the research case.Interpret the shallow landslide and extract the spatial distribution and topographical conditions of the landslide.Field investigation of vegetation parameters and shallow underground structure of the hillslope,field sampling and indoor tests to determine the rock and soil characteristics,and in-situ monitoring of the hydrological process of the hillslope,and finally analysis of the slope stability.The results are as follows:(1)Shallow landslides induced by rainfall in Liziyuan Village of Qinzhou District were mostly distributed in the sunny slopes of Larix kaempferi stands.The terrain conditions of landslides were mostly steep slopes,and most of them were concave slopes.The upslope contributing area of the landslides and the gradient showed a power-law decreasing relationship.The depth of the landslide was generally shallow,and the area of the landslide and the depth of the slide also showed a power-law decreasing relationship.(2)Vegetation mainly exerted ecological disaster mitigation effects through hydrological and mechanical effects.In the hydrological process,the canopy interception effect showed the performance of Larix kaempferi stand on shady slope > Larix kaempferi stand on sunny slope>Quercus aliena stand on sunny slope,but its effect on the prevention and control of shallow landslides induced by rainfall was very limited.The hydrological effect of the root system to improve the water conductivity of the soil was smaller for the shady slope soil with high water conductivity.On the sunny slope,the root biomass of the Quercus aliena stand was much greater than that of the Larix kaempferi stand,which could better improve the drainage performance of the soil.The mechanical effect of root reinforcement was that the Quercus aliena(deep-rooted tree species)stand was much better than the other two Larix kaempferi(shallow-rooted tree species)stand.(3)The geophysical results of the geological radar indicated that the weathering degree of the granite residual soil on the sunny slope was higher than that of the shady slope.The profile investigation of the slope also found that the weathering degree of the sunny slope and the shaded slope soil layer was obviously different,which was manifested in the sunny slope soil layer 1 and soil layer 2 with a higher content of fine particles in the weathered soil layer,and a higher sand content in the soil layer 3,While the shady slope had a higher sand content in the entire soil profile.Affected by the differential weathering of the granite residual soil on the sunny slope and the shaded slope,the shear strength characteristics showed that the soil layer 1 and soil layer 2 on the sunny slope had higher effective cohesion.The effective cohesion of soil layer 3 on the sunny slope and all soil layers on the shaded slope was relatively small,but the effective internal friction angle was relatively high.The hydraulic characteristics showed that both the soil-water characteristic curve(SWCC)and the permeability coefficient curve(HCF)of the sunny slope and the shady slope soil layers had obvious hysteresis effects,and the degree of hysteresis was as follows: the soil layer 2 of the sunny slope > the soil layer 1 of the sunny slope >the soil layer 1 of shady slope>the soil layer2 of shady slope>the soil layer 3 of shady slope>the soil layer 3 of sunny slope.At the same time,the excellent performance of water retention performance was: the soil layer 2 of sunny slope > the soil layer 1 of sunny slope > the soil layer 1 of shady slope > the soil layer 2 of shady slope> the soil layer 3 of shady slope>the soil layer 3 of sunny slope.The permeability performance was as follows: the soil layer 3 of shady slope > the soil layer 2 of shady slope >the soil layer 3 of sunny slope >the soil layer 1 of shady slope> the soil layer 1 of sunny slope > the soil layer 2 of sunny slope.(4)The hydrological process monitoring results of the three forest land slopes showed that the soil temperature of the Larix kaempferi stand and the Quercus aliena stand on the sunny slope were relatively close,and was significantly higher than that of the Larix kaempferi stand on the shady slope.The peak of soil water content was synchronized with the peak of rainfall,and most of the peaks of saturation of the Larix kaempferi stand on sunny slopes were greater than those of the other two stands.The water storage performance of the slope was as follows: the Larix kaempferi stand on sunny slopes> the Quercus aliena stand on sunny slopes> the Larix kaempferi stand on shady slopes,and the drainage performance was reversed.The suction stress of soil layer 1 and soil layer 2 of the two types of stands on the sunny slope was greatly affected by the rainfall events,but the suction stress of the shade slope did not respond significantly to the rainfall,and the value of the suction stress was small.(5)The resistance of the slope was mainly composed of the pulling force of the back wall of the landslide,the anti-sliding force of the side wall and the anti-sliding force of the bottom of the landslide.The dynamic results of the force analysis of the landslide showed that the sliding resistance of the sidewall of the landslide was as follows: the sunny slope of the Quercus aliena stand > the sunny slope of the Larix kaempferi stand > the shady slope of the Larix kaempferi stand.The anti-sliding force at the bottom of the landslide showed as follows: the shady slope of the Larix kaempferi stand> the sunny slope of the Quercus aliena stand> the sunny slope of the Larix kaempferi stand.The peak of the driving force of the downturn appeared synchronously with the rainfall event with heavy rainfall.Affected by the difference of the hydrological process of the slope,the size manifested as: the sunny slope of the Larix kaempferi stand> the sunny slope of the Quercus aliena stand> the shady slope of the Larix kaempferi stand.In the events of heavy rainfall or heavy accumulated rainfall,it was controlled by the difference in lithological weathering and vegetation on the shady and sunny slopes.The vegetation slopes of the Larix kaempferi stand on sunny slopes were at risk of instability,and the sunny slopes of the Quercus aliena stand and shady slopes of the Larix kaempferi stand had better stability.