Study On The Development Characteristics And Stability Of The Xiongba Giant Ancient Landslide In The Jinshajiang Fault Zone

There are many landslides within 1km along the Jinsha River fault zone in the eastern margin of the Qinghai-Tibet Plateau,among which giant ancient landslides account for an important proportion.Giant ancient landslides are mainly developed along the Jinsha River valley with obvious high-level characteristics,and are easy to form landslide-blocking-dam break disaster chain after instability,such as Baige landslide,Temi landslide,Woda landslide,Sela landslide,etc.A giant ancient landslide was discovered at Shadong Township,Gongjue County,Changdu,Tibet Autonomous Region,about 6km east of the Luona-Buxu fault,a branch fault of the Jinsha River fault zone,on the right bank of the upper reaches of the Jinsha River.Once the landslide loses stability and slides,it will block the Jinsha River and form a dammed lake.After the dam breaks,it will cause serious harm to the surrounding and upstream and downstream areas.Under the action of the alpine climate and geological structure,the landslide occurrence area causes the internal stress field to be readjusted,the degree of rock mass fragmentation increases,the mechanical properties of the rock mass deteriorate,and weak structural planes are formed.This area had already experienced instability sliding before the Holocene.Therefore,the current Xiongba ancient landslide was formed.Based on the collected data,this paper uses various methods such as field ground survey,geophysical prospecting,drilling and numerical simulation to carry out a refined study on the Xiongba ancient landslide.This study concludes that:1.The Xiongba ancient landslide has an irregular plane shape,with an average length of 2130m and a width of 2680m.The overall sliding direction is NE70°.On the plane,it is divided into two large areas,the sliding source area and the accumulation area.The accumulation area is divided into a relatively stable area and a strong deformation area.The main material of the landslide deposit is broken stone soil(Q₄^del),and the bedrock is mainly composed of mica quartz schist.There are mainly two-level sliding zones in the deep.The shallow sliding zone(S₁)is mainly composed of gray-yellow breccia,and the deep sliding zone(S₂)is gravel breccia,and the breccia is weathered from mica quartz schist.2.The potential sliding zone(S₁)of the Xiongba ancient landslide corresponds to a landslide volume of 2.16×10⁸m³,and the deep sliding zone(S₂)corresponds to a landslide volume of 5.98×10⁸m³.At present,landslides are mainly affected by rainfall,faulted structures,river erosion,and‘locked sections'developed in the middle of the landslide.Especially,the deep‘locked sections'developed in the slope play a key role in controlling the stability of the landslide.3.The strength of sliding soil is mainly determined by the particle framework.The increase in water content will increase the deformation rate and pore pressure of the soil,and the mechanical strength will decrease rapidly.The content of the sliding zone with a particle diameter of less than 10mm is higher than other depths,which is due to the long-term creep of Xiongba ancient landslide along the sliding belt resulted in the formation of particles broken.Its strength is related to the content of coarse particles.At the same time,with the increase of water content,the effect of water content on the internal friction angle is not obvious,but it will lead to a decrease in cohesion.Fresh mica quartz schist is relatively dense,with low water content,and the failure surface is basically along the schistal direction.Under long-term creep conditions,the internal friction angle of mica quartz schist is often greatly reduced.4.Through FLAC^3Dsimulation analysis,it is believed that the landslide under its own weight is basically in a stable state as a whole,but the plastic zone of the shallow sliding zone is basically connected except for a small part of the front edge.After inputting a typical Ms8.0 earthquake acceleration,the plastic zone of the shallow slip zone is completely connected and the plastic zone of the deep slip zone expands,and the landslide is prone to instability at the shallow surface.Analogous to the Baige landslide,the Xiongba ancient landslide blocked the Jinsha River with a volume of about 1400×10⁴m³ after instability,forming a dam about 2400m long and 45m high.