Formation And Evolution Of A Deep-seated Toppling Failure Rock Mass During The Rapid Downcutting Of The Lancang River

Deep-seated toppling failure is a common manifesation of the steeply dipping layered rock slope in Southwest China.Because of its large scale,there is the possibility of further evolution into a large landslide,which often poses a threat to the production and construction activities in the adjacent areas.However,its formation mechanism,evolution process and its relationship with the evolution process of large rivers in this area still need further exploration and research.Based on the detailed investigation and study of the toppling and failure characteristics of the deep-seated toppling rock slope in front of the dam of the Gushui hydropower station in the upper reaches of Lancang River,the formation mechanism and evolution process of the deep-seated toppling rock slope were deeply explored and studied by means of physical simulation and numerical simulation,and the following main results are obtained:(1)The investigation results show that the deep-seated toppling rock mass is mainly developed in the left bank of Longitudinal Valley in the upstream reach of the dam front of the Gushui hydropower station of the upper Lancang River,with an elevation of 2100-2600m.The maximum depth of the deep-seated toppling rock is more than 150.3m,the length along the river is more than 1km,and the total volume is about 6×10~6m~3.With the change of elevation,the degree of rock mass toppling shows the following characteristics:the rock mass below the elevation of 2100m has almost no toppling;within the elevation of 2100-2300m,the degree of rock mass toppling increases with the increase of elevation;above the elevation of 2300m,the degree of rock mass toppling decreases with the increase of elevation.(2)The development density of the structural plane in the deep-seated toppling rock slope is relatively high,mostly along the layer or along the original weak structural plane;the dip angle of the strata varies with the elevation(the original dip angle in the area is about 75-85°),and the dip angle of the strata is about 60-85°below the elevation of 2100m and above the river bed;the dip angle of the strata is about 20-50°within the elevation of 2100-2300m in the middle of the bank slope;In the upper part of the bank slope with an elevation of 2500-2600m,the dip angle of the strata is about 52-62°;there are many collapse fracture zones in the deep-seated toppling rock slope,and the dip angle of the strata in the adit fluctuates sharply near the collapse fracture zone.(3)Based on the four factors of dip angle,development of toppling fractural zone,unloading degree of rock mass and P-wave velocity,the deep-seated toppling rock mass is divided into three grades:highly toppling rock mass,strongly toppling rock mass and weakly toppling rock mass.(4)Since the Quaternary,the Lancang River in the study area has been cut down for more than 600m,especially from the elevation of 2300m to the present river valley,which has undergone a rapid downcutting process,forming a steep river valley landform on both sides.This kind of landform condition provides a good external condition for the toppling and failure of the steeply dipping layered rock slope in the area.The strata in the area is in the form of reverse dip and steepness,and the layer trend is basically the same as that of the slope.The thickness of the strata is between2-20cm.During the structural evolution,the strong compression makes the rock mass metamorphism more obvious,which makes the rock mass in the area show a typical thin medium-thick layer structure,and the mechanical properties of the strata show obvious anisotropic characteristics.This is the deep-seated toppling failure developed in the area Internal factors of destruction.(5)The rapid downcutting of river results in a strong redistribution of the stress field of the slope on both sides.The unloading causes by the downcutting of river further destroys the rock structure and forms a large number of unloading and fracture phenomena in the rock mass.Therefore,a large depth of stress relaxation zone is formed on both sides of the valley,which makes the valley stress can be divided into three zones in the horizontal direction,i.e.stress relaxation zone,stress concentration zone and original rock stress zone.In the relaxation zone,the stress of rock mass is mainly controlled by gravity field.This is one of the important reasons for the deep-seated toppling failure of the steeply dipping layered rock slope in the area.(6)The deep-seated toppling failure in the area is the result of the long-term geological process of river downcutting,and the product of the long-term gravity effect of the rock mass accompanied by river downcutting.The evolution process can be summarized as follows:in the early development stage of the valley,unloading and stress redistribution form a relatively shallow stress relaxation zone on both sides of the valley,which makes the steeply dipping layered rock mass in the relaxation zone toppling to a certain extent towards the free direction under the action of gravity;with the further downcutting of the river,the stress relaxation zone on both sides of the valley further to the depth and The low elevation extension results in the formation of a greater range and depth of stress relaxation zone in the valley,which makes the toppling failure of steeply dipping layered rock mass further expand to the depth and the lower part of the slope.