Stability Assessment Of Lakani Paleo Landslide Accumulation Body

Landslides are one of the main types of geological disasters in mountainous areas.The accumulation residue of Lakani paleo landslide in Shigu Town,the northwestern mountainous area of Yunnan Province is the main research objective in the present research.The Lakani Paleo Landslide accumulation body is deposited on the steep bank slope of the west tributary of Jinsha River near Shigu Town,about 8.67 km away from Shigu Town.The study area is located on the southeastern edge of the Qinghai-Tibet Plateau.This area has complex topography,special climate,and frequent geological disasters such as landslides and debris flows,posing a serious threat to infrastructure and human life.Affected by the southeast and southwest monsoon,the study area receives heavy rainfall.In study area there are present obvious geological structures such as folds,faults,and joints.According to the seismic map of China our study area is seismically active and having average horizontal seismic load of a=0.2 g.The Lakani paleo landslide deposit is mainly formed by the mixed accumulation and consolidation of rubble and fine-grained soil,and the bedrock is mainly quartz schist.Due to the large scale of accumulated residues,the landslide accumulation body is divided into blocks according to topography and geological characteristics.The landslide accumulation area can be divided into two blocks,namely area(1)and area(2).The estimated area of area(1)is about 7.62×10~5 m~2,volume is about 29.74×10~6 m~3,average inclination is 39°and thickness is 40m.The estimated area of area(2)is about 1.3×10~5 m~2,volume is about 3.98×10~6 m~3,average inclination is 29°and thickness is 30 m.In general,the geological characteristics and material composition of landslide accumulation area(1)and area(2)are similar and basically the same.Landslides are caused by internal factors and external factors.Internal factors include lithology,rock mass structure,weathering degree of rock mass,etc.;external factors mainly include vibration,rainfall,excavation or unloading the slope toe,loading of slope crest etc.Thermoluminescence Dating results are showing the age of the landslide accumulation body of our study area Lakani is about 43.9±2.4 Ka.It means that this ancient accumulation landslide occurred 43,000 years ago.In this research,limit equilibrium and numerical analysis methods are used to evaluate the stability of ancient landslide deposits.(1)Firstly,the remote sensing image data was used to interpret the paleo landslide,and then prototype investigation(or archetypal investigation)of the ancient landslide was carried out in the field.The first-hand information of the Lakani ancient landslide was obtained,and the genesis mechanism of the landslide was established.Based on the geological understanding,DEM data was used to build a three-dimensional geological model on the Arc GIS software platform,and three longitudinal profiles of the ancient landslide were drawn.Through the basic understanding of the field investigation and consulting a lot of related literature,the shear strength parameters of the ancient landslide body and the sliding surface are determined by the back analysis method.(2)For limit equilibrium analysis,we used three methods:Morgenstern-Price method,Spencer method and Bishop Method.We performed limit equilibrium analysis by using Slope/W software.In the present research under limit equilibrium analysis,we have determined the factor of safety(FOS)of 3 longitudinal profiles(upper,middle and lower longitudinal profiles)of the Lakani deposits in the study area under 8 working conditions:(1)when the Groundwater is at a depth of 50m;(2)when the groundwater level is at a depths of 50 m,40 m and 35 m and 30 m and with a horizontal seismic load a=0.2 g;(3)when the groundwater level is at a depths of40 m,35 m and 30 m.The study found that the middle longitudinal profile and the lower longitudinal profile are stable under above conditions,but upper longitudinal profile is relatively less stable.Sensitivity analysis was also carried out and the results showed that the friction angle and the water table were the most sensitive factors.When the friction angle decreases and the groundwater level rises,the possibility of slope failure is greater.(3)In this thesis,numerical analysis method is used to further evaluate the stability of Lakani landslide body.The calculation of stability is achieved by reduction of physical and mechanical parameters.This research uses FLAC software.FOS calculated by numerical analysis shows that Upper,middle and lower longitudinal profiles are stable when slope is dry or in its natural condition and when slope is wet,but upper longitudinal profile is not stable when slope is wet and seismic load of a=0.2 g is applied while other two(middle and lower)profiles are stable under this condition.(4)The stability calculation results of various limit equilibrium methods and numerical analysis methods show that the Lakani accumulation body is currently stable,that is,no landslide will occur along the original slip surface.In addition,the accumulation body is also stable in the scale of the next few decades,and the buildings on the accumulation body,such as houses,ponds,farmland,schools,playgrounds,etc.,will not be damaged due to the instability of the accumulation body.No tensile fissures and signs of large-scale deformation and failure were found during the on-site investigation,which can also indicate that the accumulation body is in a stable state.