Study On The Stability Of Complex Ancient Deposits In The Zangqu River Basin,East Tibet

Because of the active tectonic activities,a set of large ancient deposit developed in the eastern Tibetan plateau.With the development of human engineering activities,the large ancient deposits are harnessed more and more widely,and the study on stability of them become the core problem.Zangqu river basin this paper selected the research area is located in the Tongpu town,Jiangda city,a area of about 672.7 sq km.Based on the field investigation and the engineering geological conditions research,this paper study on the engineering geological characteristics,Stability influence factors,mechanismof deformation and failure,evaluation of the stability systematically by means of a large number of laboratory tests.the limit equilibrium analysis,and three-dimensional numerical simulation.In the process,we got the following understandings:?Proceed from the sedimentary properties and structural characteristics of typical ancient deposits,we have found that the main geotechnical type are gravel soil,brecciated soil,and they have a large difference in the lithology of soil,size grading,the weathering degree,the roundness and the cementation degree with each other.As for the structure of ancient deposits,the characteristics mainly include that the good occlusion between coarse particle and fine particle,the interstitial of fine particle,the duality of structure and the lamellar reunion in micro-structure.?By large number of laboratory tests we found that: most ancient deposits are moderate~dense compactness and have the calcareous or argillaceous cementation of different degree.The mineral composition of every ancient deposits show little changes,and the main mineral are feldspar,quartz,kaolin,mica,chlorite and so on,and it is bad for the stability of huge deposits that the structure of kaolin,chlorite,mica is shape-layer.By the constant head hydraulic conductivity test,we found that the velocity of seepage of ancient deposits areabout 10-2~10-3,belong to weak~moderate permeable layer.?According to direct shear test,the cohesion of ancient deposits changes a lot,which has basically reflected they are of uneven.At the same time,the change of moisture content also shows on the cohesion.By the dynamic strength pressure test,we found that dynamic load makes the cohesion and friction angle decrease compared with static load,what is more,the change of failure vibration has great influence on the dynamic bond,but the dynamic friction angle changes a little.Through dynamic modulus of damping ratio test,the maximum dynamic modulus,the maximum damping ratio and the maximum dynamic shear modulus have no significant difference under the same consolidation pressure.?The Main factors which affects the stability of the ancient deposits are structure characteristics,physical properties,water influence and human engineering activities.By analyzing Denongnongba ancient landslide deposit,the mechanism of deformation and failure of most deposits are collapse sliding deformation under gravity stress.Using the limit equilibrium analysis,we get the stability coefficient is 1.27 on the light rain condition and the condition of heavy rain is 1.153 through MATLAB,which shows that deposits are in permanent stable statein l under light rain conditions,and in a state of temporary stability for the heavy rain condition.?Analyzing the stability of the ancient deposits by FLAC3 Dthree dimensional numerical modeling,the results show:(1)compared with the heavy rain condition,the range and values of stress concentration,displacement deformation and plastic zone are smaller under the light rain,and it shows that ancient deposit are more stable under the light rain condition;(2)Shear stress concentration belt has not been formed transfixion plane under the two conditions,which indicates that deposits are in the overall stable state;(3)Near the surface of the deposits,the y direction displacement are pointing to the Zangqu valley,which indicates the deposit have the outward displacement,and the zdirection displacement is vertical downward,which shows that gravity stress is a important factor of deformation.The y and z direction displacement deformation concentration area indicates that the deformation and failure form of deposit are related to the front traction of the back,and it remains the same with the deformation mechanism analysis and limit equilibrium calculation results.Comprehensive view,the ancient deposits have local instability phenomenon,but in the good overall stability in the study area.