Dynamic Response And Deformation Mechanism Under The Earthquake Of A Large Ancient Landslide In Front Of The Zipingpu Dam

Among the lots of landslides frequently occurred in China,earthquake-triggerred landslide is more destructive due to its high degree of concentration,high density and complex dynamic conditions of slopes.Many scholars have conducted the research on the seismic landslide,but most of them are aimed at some hot issues,such as high-speed and long runout landslides or high-speed landslide-avalanches that have super dynamic characteristics,but the failure mechanism of ancient landslides under the earthquake is lack of systematic research.On May 12 th,2008,a strong earthquakes(Ms=8.0)occurred in Yingxiu Town,Wenchuan County,Sichuan Province,and triggerred a large number of geological disasters such as landslides and avalanches.Field investigation shows that almost all the large rapid landslides induced by this seismic event were rock slides whose materials had not previously failed,yet none of the large ancient landslides had been reactivated,only the steep/gentle part of the slope collapsed and cracked.This seems a major discrepancy from the conventional occurrences.Is it a coincidence or a certain event? To sove this problem,an ancient landslide on the left bank of the Zipingpu dam has been taken as the research object.Combined with field investigation,numerical simulation and model test,the dynamic response in the deformation process of the ancient landslide under earthquake is systematically studied.Based on this,the overall and local stability of the ancient landslide are analyzed,the stress distribution and failure mechanism of slope under strong earthquake are emphatically stated and the influencing factors on deformation and failure of the slope under strong earthquake are also discussed.The main results of the manuscript are as follows:(1)Based on the landforms and topography of the ancient landslide and deformation monitoring data in this scismic event,the deformation and failure characteristics and their forming conditions are analyzed.The results show that: The displacement-time curve of the surface and the deep part of the slope suddenly increased when the earthquake occurred.The accumulation layer slid along the rock-soil interface.And the surface failure mainly occurred in the front area of Dengzhanping platform.While after the principal earthquake,the slope has no obvious deformation.The flat terrain,firm structure,gentle sliding surface and attenuation of seismic wave are the main factors that made the ancient landslide maintain stable during the earthquake.(2)By using the conventional limit equilibrium method and GeoStudio for calculating the stability of each potential sliding surface of the ancient landslide on different conditions and different water level,it could be concluded that: the whole ancient landslide with original terrain was stable under ? degree of seismic intensity,the stability coefficient was 1.134.The sliding surface underlying the Hudouping platform in the front of the ancient landslide is steep with a inclinaton of 22°,which leads to a stability coefficient of 0.797 there.At the same time,the front crest of Dengzhanping platform could be 35°,and the stability there under earthquake is poor,the stability coefficient is 0.794.After ballasted,the stability coefficient of the whole ancient landslide has been raised up to 1.198 ~ 1.273.Because the original Hudouping platform is now covered,the stability coefficient there was 1.289~1.741 now,but the steep crest of the Dengzhanping platform was unstable with a stability coefficient of 0.975.(3)Based on the actual terrain of the Zipingpu ancient landslide,the deformation and failure process of the slope have been reproduced by the model tests,the dynamic response and failure process of Zipingpu ancient landslide under the earthquake have been analyzed by monitoring the acceleration and failure phenomenon on different parts of the slope.The results show that the acceleration on different positions of the slope shows obvious “height amplification effects” and “landform amplification effect”,different landform not only has different acceleration,but also has different failure forms and different damage degree.(4)By using the finite difference software FLAC3 D to simulate the deformation of Zipingpu ancient landslide under ? degree seismic intensity,it could be concluded that the seismic power has a great influence on shear stress and shear strain of the slope,the slope crest and the front part of Dengzhanping platform has been plastic yielded.Shear strain concentration occured on the sliding surface,but the shear strain did not penetrate the whole slope,and the ancient landslide will not be reactived to slide along the sliding surface.(5)A shaking table test was conducted to simulate the failure process and dynamic response of the accumulation slope when the gradients of slope surface and rock-soil interface are different.The results show that the initial acceleration on the rock-soil interface is the smallest,while that on the middle of slope surface is secondary and on the slope shoulder is the biggest.The steeper the slope is,the more obvious the acceleration amplification effects is.The failure modes of the slopes can be summarized into three types:(1)Local collapse,when the the dip angle of the rock-soil interface is less than 15°,there is only collapse on local parts of slope,while the entire slope would not slide along the rock-soil interface.(2)Local collapse with entire creep,when the dip angle of the rock-soil interface is between 15°~25°,local collapse is still dominant,but a slight dislocation along the rock-soil interface has been occurred.(3)Entire slide,when the rock-soil interface is steeper than 25°,the slope will slide along the rock-soil interface with inferior local collapse in the sliding body.(6)The failure process of the slope with the different geological elements has been simulated by discrete element software PFC2 D.The outcomes show that whatever the failure mode is,the rear scarp is always generated in the tension region on the slope shoulder,and it is always a steep tension crack.The stress concentration area influenced by the slope angle is an important factor on slope collapse,and the collapse extends when the slope angle increases.