Formation And Evolution Of A Deep-seated Toppling Failure Rock Mass Under The Unloading In Double Directions

Yagong study area is located on the right bank of the upstream of Lancang River near the dam of the ancient water hydropower station,and in the alpine valley area on the eastern edge of the Qinghai Tibet Plateau.There are large-scale deep toppling deformations in the section near the dam of Gushui hydropower station.At the same time,Meilishi landslide group,Zhenggang landslide and Gendakan landslide in this section are all formed by the cumulative deformation of the toppling rock mass.The geological structure of the study area is relatively complex,especially in the early stage of the bank slope affected by the F₁₀₅ fault cutting,and the occurrence of the strata in the slope body changes.At the same time,the slope is not only affected by the Lancang River undercutting,but also by the Mushui River rapid undercutting.In particular,the rock mass unloaded and toppled intensively in double directions.In order to study the formation mechanism and evolution process of the dump deformation,this paper takes the Yagong dump deformation in the upper reaches of Lancang River as the research object,and makes a detailed analysis on the geological conditions of the studied slope,the evolution history of the river valley,the weathering unloading characteristics of the rock mass,the development characteristics of the dump deformation,the relationship between the dump deformation and the unloading of the bank slope in the process of valley cutting,the change characteristics and laws of the stress field in the bank slope,the deformation characteristics of the slope body,especially the formation mechanism and evolution process of the toppling deformation are studied in depth by means of physical simulation test.The main research results are as follows:?1?According to the change of the attitude of the slope rock stratum in the field investigation,it is found that the strike of the stratum on the left bank of the mushui river is deflected due to the left-hand strike slip movement of the strike slip fault F₁₀₅,which intersects with the mushui river at a small angle,forming a layered rock mass with a steep reverse dip on the left bank of the Mushui River,which provides a precondition for the bank slope to dump to the Mushui River.According to the geological characteristics of the adit and the opening statistics of the unloading fracture,the unloading zone in the study area can be divided into strong unloading zone,weak unloading zone and non unloading zone.According to the structural characteristics of the rock mass in the adit and the development characteristics of the fracture zone,the strength classification and division of the dump rock mass are carried out,which can be divided into ighly toppled,strongly toppled,weakly toppled and normal rock mass.?2?The numerical simulation results of valley undercutting show that the stress field of the slope is redistributed with the undercutting of the valley and the unloading of the slope rock mass.The displacement of the bank slope on the Lancang River side is larger than that on the Mushui River side,which is mainly affected by the undercutting of the Lancang River Valley.The displacement and deformation of Mushui River is mainly caused by the undercutting of Lancang River Valley in the early stage.After the formation of Mushui River,it is mainly controlled by the undercutting of Mushui River Valley.It can be concluded that the toppling deformation body is the result of the continuous cumulative deformation evolution under the condition of two-way unloading.?3?The toppling deformation is the result of internal and external factors.The slope itself is mainly anti dip and steep thin-layer rock mass,and the bank slope is steep with large relative height difference.As the valley continues to cut,the bank slope increases and becomes steep,and the increase of the free face promotes the further weathering unloading of the rock mass,thus aggravating the process of rock mass collapse deformation.?4?The results of physical simulation test show that the evolution process of dumping in the study area can be divided into three stages:unloading deformation,bending deformation and dumping fracture.In the process of evolution and formation,the failure mechanism is mainly tension fracture in the layer,shear dislocation in the layer and between layers,namely tension shear fracture and compression shear fracture.The bank slope on the side of mushui river has a relatively strong dumping degree and a relatively deep development depth,which is closely related to the strong unloading of Lancang River cutting rock mass in the early stage and the rapid cutting of Mushui River in the later stage.?5?Based on the results of physical simulation test and numerical analysis,the formation mechanism of dumping in the study area can be summarized as follows:the original ground stress is high,with the continuous cutting of river valley,the rock mass has strong unloading rebound,intensively unloading destroyed the structure of rock mass and weaken the mechanical properties of rock mass with forming many unloading cracks in rock mass,and caused the stress field of slope to be redistributed furtherly and formed a stress relaxation zone in the strongly unloading zone,then the direction dominant principal stress in the stress relaxation zone rotated to that of gravity,and the deep-seated toppling failure formed in the steep layered rock mass.The local deformation of these slopes will cause the huge cumulative deformation of the whole slope,and then evolve into the deep dumping failure.