Formation Mechanism Of Large-scale High-position And Long-distance Landslide In Emeishan Basalt

The Emeishan basalt group,widely distributed in southwestern China,is often considered as an ideal place for building large hydropower project dams due to its deep canyon landforms.Historically,the large-scale high-position and long-distance landslides in Emeishan basalt have caused a large number of casualties,property damage and far-reaching environmental effects.Currently,there is no deep and systematic research for the development process of such landslides over the world,and it is still difficult to objectively access the risk of large-scale high-position landslides in southwestern China.Therefore,the study on the formation mechanism of large-scale high-position and long-distance landslides in Emeishan basalt has significant scientific and practical sense.In this study,we consider the large-scale high-position and long-distance landslides in Emeishan basalt(LHLLEB)as the research object,and through the use of remote sensing interpretation,field large-scale investigation,laboratory tests and numerical simulation,the distribution characteristics,the development characteristics,the geological types,the formation conditions and the motion evolution processes of landslides are studied in depth.On this basis,combined with the unique geological environment conditions in the southwest,the engineering geological characteristics of Emeishan basalt,and the research results of landslide kinematics,the formation mechanism of the LHLLEB is systematically analyzed,and the main understandings and progress are listed in blow:(1)The LHLLEB are most widely developed in the high-intensity and high mountain gorge areas.The landslides are mainly distributed along the main stream of large rivers and its tributaries in a strip-like dense cluster.Four distribution areas are formed in the study area: the upper reaches of the Jinsha River and its tributary distribution areas(landslides account for 35%),the middle and lower reaches of the Jinsha River and its tributary distribution areas(landslides account for 51%),the middle reaches of the Dadu River and its tributary distribution areas(landslides account for 9%),the lower reaches of the Dadu River and its tributary distribution areas(landslides account for 5%).The LHLLEB are most widely developed in the slope with mid-dip and mid-gentle-dip bedding structure.(2)The Emeishan basalt group in southwestern China is composed of multiple overflow cycles.For example,14 overflow layers are developed in Xiluodu area,and the Emeishan basalt has the characteristics of huge thick layer structure,strong rock strength,and soft and hard alternating layered rock mass.Strong tectonic reworking resulted in polyphase fold deformation remade of Emeishan basalts,development of shear joints and interlayer shear dislocation.The coupling of strong internal and external dynamics during the neotectonic period resulted in the formation of canyon landforms with great topographic contrast in the basalt distribution area.The rock mass of the valley slope was unloaded strongly,the interaction between tuff and water was strong in the valley area,and the interlayer bonding force along the inclined slope was greatly reduced.(3)The LHLLEB are are mainly divided into three geological types: the bedding landslide developed on the wing of anticline in ejective fold,the high-position bedding landslide developed in mid-gentle-dip monoclinal structure,and the bedding landslide developed on the upper wall of fault.The bedding landslide are often developed on the anticline flank of ejective fold,due to the Emeishan basalt is belong to the brittle-hard rock,folding is accomplished in brittle and ductile environments buried thousands of meters deep,and fracture zones are formed in the transition zone between anticline and syncline due to abrupt change of strata occurrence.X-long joints are developed in plane and section,and basalt is cut into plate structure.The rock mass at the fracture zone is fractured,and streams and valleys develop along the zone,the foot of the basalt bedding valley slope is empty,and rock mass slides along bed surface due to creep at the foot of the slope,weakening the interlayer bonding force,and strong earthquake eventually causes rock mass tension fracture and instability.If the high-position bedding landslide developed in mid-gentle-dip monoclinal structure,because the dip angle of the structural plane is smaller than that of the valley slope,the height difference between the exposed position of tuff on high-position slope(potential shear outlet)and the foot of slope reaches hundreds of meters.Under the action of gravity,the upper slope slides along the tuff to the free surface,and the trailing edge is pulled apart,coupled with other unfavorable factors such as unloading weathering and flow erosion.Finally,the large-scale high-position bedding landslides are triggered by strong earthquakes.If the bedding landslide developed on the upper wall of fault,the landslide is affected by the fault activity,the integrity of the layered slope is poor.When the foot of the slope is empty,the fault zone is compressed and plastic extrusion,which leads to bedding slip of the slope rock mass and greatly weakens the interlayer bonding force.When coupled with long structural planes on both sides to form side crack planes,a large-scale inclined plate structure is formed.Under the action of strong earthquake and other external forces,the rock mass near the fault can be damaged by tension,and a large-scale high-position landslides can be formed by the mode of compression-slip-tension fracture.(4)The formation mechanism of large-scale high-position and long-distance landslides in Emeishan basalt: the development of faults,joints and interlayer dislocations in rock mass caused by strong tectonic reconstruction.Active neotectonic movement causes the deformed and fractured Emeishan basalt form canyon landform;Under the background of valley stress field,rock mass is strongly unloaded and water-rock interaction is repeated;Slope rock mass slides along layers and shears along lateral fissure surface,interlayer bonding force and integrality of slope rock mass are thoroughly destroyed,and the large-scale high-position and long-distance landslides are formed by the sudden instability of the partitioned inclined slab structure under the action of seismic inertia force.Therefore,the formation of landslide has undergone two stages of long-term "deformation accumulation" and "trigger instability".Deformation and failure modes are mainly break-slip-tension fracture,slip-tension fracture and compression-slip-tension fracture,and the typical representative landslides are Mahu landslide,Aizigou landslide and Jiaopenba landslide.The occurs of remotely slide of the basalt landslides need four factors: The landslide body is at a high position and has a high potential energy;Rock mass in sliding source area is divided by primary structural plane and structural plane,and the rock mass has a high degree of fragmentation;The particles after disintegration are nearly equiaxed(sphericity is good)and lack of fine particulate matter;The friction energy dissipation between particles is weak after the landslide body starts to move sharply,and it can keep high-speed movement for a long time.(5)The kinematic characteristics of high-position landslide debris flow are studied by indoor chute model test in this study: The basalt debris particles with higher degree of fracture have better particle sphericity(the proportion of debris particles with sphericity above 0.6 in the study area is about 60%.),and particles with good sphericity are prone to bounce and roll during movement.Under this movement mode,the effective friction coefficient between particles and sliding surface is lower,and has a momentum transfer effect during the movement,so that the basalt debris particles exhibit stronger mobility and thus can slide further distances,and then the scope of the landslide will be even greater.(6)The kinematic behavior of the landslide was simulated using a 3-D discrete element model(3DEC),with four stages: the start-up activity stage,the short-range activity stage,the high-speed and long-distance debris flow stage,and the accumulation and river blocking stage.The results show that with the increase of slope elevation in the sliding source area,the horizontal and vertical acceleration of the slope have significant amplification effect and the seismic acceleration near the structural plane has a multiplier effect(amplification is about 6-7 times).The significant amplification of seismic acceleration is the main reason for the earthquake-induced high-position landslide.