Research On Development Characteristics And Formation Mechanism Of Large-scale Landslides In Bailongjiang River Basin

The Bailongjiang river basin is located in the transition zone between the Qinghai-Tibet Plateau and the Loess plateau,which is a steep deformation zone.The large-scale landslides in the Bailongjiang river basin has the characteristics of wide distribution,high density,intensive activity,large scale,serious damage and so on.Based on the engineering geological environment,this paper selectively analyze the development characteristics of large-scale landslides and simply classify the types of large-scale landslides and discuss the key factors,evolution history and formation mechanism of various types and summarizes the concept model of geomechanics of various types of large-scale landslides,which is expected to obtain the important theory and the practical significance of the development characteristics and formation mechanism of large-scale landslides in the Bailongjiang river basin.This paper,based on the previous research and field investigation,utilize statistical analysis,qualitative analysis,semi-quantitative analysis and numerical simulation to systematically analyze and study on the engineering geological conditions,distribution rules,development characteristics,evolution process,formation mechanism of the large-scale landslides and the distribution,influencing factors,revival mechanism of dammed landslides in the Bailongjiang river basin.we can draw the following conclusions:(1)The large-scale landslides in the Bailongjiang river basin are characterized by linear distribution along the river and zonal distribution along the fault zone and the distribution has no obvious selectivity to the river bank.More than 90% of the large-scale landslides are distributed within the range of 5.0km from the fault,and the farther distance is the lower number of large-scale landslides is from the fault.At the same time,the large-scale landslides are mainly distributed in the zones closely related with the crossing,staggering and transforming section of the fault.The number of large-scale landslides(N)and the recently vertical distance(Dr)statisfy following relationship: N= 113.9e-0.4Dr(R2 = 0.9553).(2)The mainly sliding directions are the NE and W,but it shows the same characteristics as the strike of the fault,namely SE and NW oriented in Liangshui,Wudu to Lazikou,Zhouqu.As the landslide volume increases,the friction coefficient of the large-scale landslides decreases exponentially,and some large-scale landslides show strong debris flow characteristics.The apparent friction angle and the volume of landslide in the Bailongjiang river basin statisfy following relationship: f=11.422e-0.472lg(V/m3)(R2 = 0.5148).(3)The large-scale landslides in the Bailongjiang river basin are mainly affected by topography,lithology,slope structure,fault structure,neotectonic movement and earthquake.1)The large-scale landslides have corresponding relationship with the elevation and micro-landform;for most large-scale landslides occur in river valley and canyon section below the elevation of 1200 to 2600 m(upstream is 2200 to 2600 m,midstream is1600 to 2200 m,downstream is 1200 to 1600m),particularly in the upper segment of canyon section.2)The soft and hard stratified rock group in the basin is most favorable for the development of large-scale landslides,and more than 93% of the large-scale landslides are associated with phyllite and slate metamorphic rocks.3)The consequent slope is the most favorable to the development of large-scale landslides;and the steeper the dip angle is,slope stability is worse.The consequent slope and reverse slope of the structural plane with the same slope direction are also unstable.(4)According to the types of rock mass and the location of sliding plane,the large-scale landslides in the Bailongjiang river basin can be divided into four categories: the homogeneous loess landslide,the loess interface landslide,the loess-bedrock landslide and the bedrock landslide.At the same time,combined with the specific slope structure,formation mechanism of the homogeneous loess landslide is creep-sliding and fracturing;formation mechanism of the loess interface landslide are fracturing and ejectting,seismic liquefying and plastic flowing,fracturing and shattering-sliding;formation mechanism of the loess-bedrock landslide and the bedrock landslide are fracturing and ejectting,fracturing and shattering-sliding,sliding and fracturing,bending and fracturing,sliding and bending-fracturing,the floding wing sliding,faulted and shattered zone sliding.(5)The ratio of incompletely dammed landslides and completely dammed landslides in study area is 2:1.Landslides damming are influenced by topography,geological structure,hydrodynamic condition,earthquake and so on.According to field investigation,at least 8 areas remain lacustrine sediments at Caopo landslide to the Jugan country,Wudu and elevation of top sediment to river is generally between 55~70m.The sediment are mainly composed of silt,fine sand.Due to the lack of years of experiment,it is not clear that the 8 were all formed by the Caopo landslide.The FLAC-3D numerical model shows: under natural condition,there is obvious shear strain concentration zone in the middle and posterior part of the landslide accumulation,and the shear strain increment concentration zone also appears in the back of the landslide but not through the whole landslide accumulation;under seismic condition,the maximum value of the total displacement and horizontal displacement of the accumulation body appears in the middle and the back of the landslide and shear strain concentration zone and the shear strain increment concentration zone obviously appears in whole landslide accumulation,which indicates that the earthquake is the main factor of the landslide resurrection.