The Genetic Mechanism And Motion Characteristics Research On Dangjiacha Landslide Induced By The Haiyuan Earthquake

China is located between the world's two seismic belts which are the Circum-Pacific seismic belt and the Eurasian seismic belt.Due to the Pacific Plate,the Philippine Sea Plate and the Indian Plate extrusion,earthquake fault belt is very developed,the earthquake occurs frequently.Haiyuan earthquake occurred in the south of Ningxia province which was famous as its high magnitude,great intensity,strong damage,wide affection and so on in the world in 1920.The earthquake directly induce a large number of geological disasters such as landslides and collapses,it was so classical that caused morphological diversity and large numbers of typical loess landslides,Dangjiacha landslide is one of these which we will study in this paper.The landslide is located in the Dangjiacha Village of Xiji county in the Ningxia province.It belongs to the ? band intensity of the earthquake.The landslide get much geologist attention because of the special genetic mechanism and motion characteristics.This article used field survey,mapping,drilling,trenching and other survey methods to identified the topography,stratum,geological structure,hydrogeological conditions,engineering geological conditions and landslide scope,scale,shape,deformation and failure mode of the landslide area.Then we taked the soil samples of field and the core of bole hole to do physics,mechanics experiments,we Looked forward to exploring the failure mechanical modes in shock loads.Finally,we used the discrete element numerical method such as PFC to analog the deformation and destruction of the whole process of the landslide.What's more,monitored the changes in the process of destruction of the relevant physical and mechanical parameters,And then make a study on the genetic mechanism and the motion characteristics of landslides and got some results as follows;(1)Dangjiacha landslide located in the middle reaches of the Lanni river which is the second tributaries of Yellow River.The terrain is ups and downs and the elevation in South is higher than north and west is higher than east.There are many loess plateaus,beams,hills and valleys which are very typical.The topography was cut strongly and the gully developed very much.So the geological disasters grown strongly.The main portion of the landslide area is covered by the middle Pleistocene and later Pleistocene loess of Quaternary.It underlain by red Neogene mudstone.The regional groundwater types is the class of Quaternary loess soil diving which mainly rely on the local rainfall to supplies throw the loess joints and voids channels.The new district intense tectonic acting strongly,One of them caused Haiyuan earthquake was a major thrusts with large displacement of left rotation.It released enormous energy and induced Dangjiacha landslide directly.(2)According to the accumulation characteristic and sliding movement of Dangjiacha landslide,which divided into three sub landslides with main sliding direction of 295 °,300 ° and 80 °.the boundary of Landslide is clear and the sliding walls is high and steep.Their shape looks like chairs and their profile looks like a fish.The accumulation are ups and downs,The total volume of the three landslides is nearly 20 millions cubic meters.According to the movement of the slide into the source region,circulation area and accumulation area.The terrain of source regions is relief and alternate,the trailing edge is sag.The circulation area with "U" shaped of cross section which can accelerate and bound the flowing material.The Accumulation area blocked river and cut off the water which formed the second famous berry lake in Asia.(3)The main reason of the Dangjiacha landslide includes topography conditions,Stratum,groundwater and earthquake.It experienced four stages include the free face formed,the potential slip surface formed,the slip surface colluded due to earthquake and started with high speed.The mechanism of deformation is mainly reflected in two aspects.On the one hand,the loess dynamic strength reduced by the seismic load.Especially saturated loess structure occurs shear failure,Several shear failure surface formed in loess and its underlying mudstone or nearby area.On the other hand,Partial saturated loess liquefied.The loess at the foot of the slopes with shallow groundwater liquefied.The shear strength of loess dropped significantly even lost.Under the combined effect of these two cases,Eventually leading to the slippery surfaces collude and the slope became unstable.The upper loess started to slip and think of Neogene mudstone as a sliding bed.(4)We divided the slid movement into five stages according to the state of motion and the speed direction of Dangjiacha landslides.(1)Instability and the starting stage,It means the sliding surface colluded,Three sub landslides begin started.(2)Collision and change sliding direction stage,It means the landslide slipped for a smaller distance after left the bed,then they collided together with a big angel in their common free surface during the Xiaochazi ditch,And continued to run with a changed direction.(3)Disintegration and flow-slide stage.It means the front section of landslide almost completely disintegrated because of the maximum disturbance.The dry loess blocks and debris flows afloat on the wet loess with high speed.(4)After blocking and anti-alice stage,It means the bodies of the three landslides buried even crossed the Baoyu ditch,after that,the running substance hit the high and steep mountain opposite,then it was strongly anti-alice and formed super covering accumulation,so it already complete the first sliding way.(5)Ateral migration stage,It means the followed substance with rapid migration which composed by the loess and mud still with high energy,so they slid along the upstream and downstream of Baoyu ditch with a nearly vertical direction,so it started the second sliding way.By the relevant measurements and calculated,we got a conclusion that the landslide running with a super long distance and super high speed,and it slid with a phenomenon of loess liquefaction and ultra distance.(5)We used the particle flow discrete element numerical simulation PFC2 D software to establish a two-dimensional simulation model,then execute the seismic dynamic response analysis which can reappear the process of Dangjiacha landslides.At First,the shock loads in vertical and horizontal seismic waves made a extended stress concentration in the sloping shoulder position and overwhelming stress concentration at the sloping foot,the slope body became more dense,then there appeared series of pull cracks and formed a steep edge,And it appeared to applaud in the export position at the sloping foot.After that,it finally formed a "slipping-pushing induced extended crack" failure mode.throw monitoring the speed of sliding,we discovery that the Landslide experienced the first acceleration and deceleration process.By monitoring the porosity of the model during the sliding process,we discovery that the bonding relationship between each materials have been destroyed,the landslide change from disinter grating to collapsing step by step,then it became a motive status of debris flow in the end.