| The proposed Banda hydropower station is located in the territory of Chai county and Zuo Gong county in Changdu area of Tibet.The hydropower station is the fourth level of the seven level development scheme recommended for the hydropower planning of the upper reaches of the Lancang River(Tibet river section in Tibet).In this paper,the left bank slope of the middle dam of Banda hydropower station in Lancangjiang River as the research object,its deformation and failure characteristics and the possible instability failure mode are systematically studied,after consulting the relevant previous research results and carrying out the field geological survey of the dam site,the engineering geological conditions,the rock structure characteristics.According to the deformation and failure characteristics of slopes and the development characteristics of controlled structural planes,the possible deformation and failure modes of slopes are analyzed.The geological analysis method,rigid limit equilibrium method and numerical simulation analysis method are used to analyze the deformation response characteristics of the left bank natural slope and the slope after excavation,and the stability evaluation is given.Through systematic analysis and research,this article has obtained several basic understandings and conclusions:(1)The river banks in the study area are narrow and the terrain gradient is steep.The valley along the dam axis is generally symmetrical "V" shape deep cut valley,and the rock mass weathering and unloading are intense.The horizontal thickness of the slope weathered zone and the unloading zone has a tendency to increase obviously with the elevation,and on the similar elevation,the unloading zone and the weathering zone of the ridge are obviously larger than the gully,the gentle slope,the platform or the flat slope.(2)The geological survey data are consulted and the geological conditions are investigated.The detailed investigation and rechecking of each adit are carried out,and the rock mass structure characteristics of the left bank slope of the middle dam are studied.The rock mass structure of the slope is poor,and there is no structural plane of I and II grade.The Ⅲand Ⅲgrade structural plane are more developed in the abutment slope,which are mostly filled with gravel or secondary mud,and the depth of the weathering and unloading is larger.The fault type III structural plane within and outside the steep slope has great influence on the stability of the slope,most of which is the trailing edge of the slope instability.The long drawn fracture outside the gentle dip slope is more developed and may become the sliding interface of the slope instability.(3)There is mostly hard Angan rock in the left bank slope of the middle dam.It is found that the main rock mass structure of the study area is the following kinds: the sub block-block structure,the mosaic – sub block structure,the fractured structure and the bulk structure;From the tunnel mouth to the bottom of the tunnel,most of the adits are fragmented,sub block and massive rock mass structure.The bulk structure rock mass is mostly distributed in high elevation shallow table or cavern.The high elevation PDS05 adit passes through the SL7 fractured loose rock mass and is on the ridge,which is greatly influenced by the strong unloading,which causes the rock mass to be loose and fragmentized.(4)Based on the analysis of the deformation and failure characteristics of the left bank of the central dam and the development characteristics of the controlled structural plane,the possible slip combination mode of the slope are analyzed.The slope development has a certain degree of toppling,shallow surface slipping,block slipping and sliding fracturing.Most of the deformation and failure takes the cataclastic loose rock mass or the strong unloading bottom interface as buttom,and the failure outside the slope is the trailing edge of the failure.The possible deformation and failure modes of the left bank of the middle dam are as follows:(1)plane slip pull induced fracturing;(2)plane slip press induced fracturing;(3)Partial toppling deformation.In addition,the development of plane slip pull induced fracturing is more common,especially affected by the NW moderate dip slope fault of slope and the NE long tensile fracture outside gently inclined slope,this deformation and failure mode is particularly common.(5)The slope stability of the left bank of the middle dam is analyzed through geological analysis method,rigid limit equilibrium method and FLAC-3D numerical simulation method.The stability of the potential sliding surface passing the unloading boundary is mostly not lower than the safety index.The results show that the stability of slope is better under the natural condition,but in the case of rainfall,the stability of the abutment is poor,the broken and loose rock mass may occur local or integral instability,and the strong unloading part is also in the understable state,which may occur local collapse or overall instability,under the long-term effect of adverse weathering camp force or extreme adverse condition.(6)By comparing and analyzing the stress situation,deformation(displacement)and plastic zone distribution before and after the slope excavation,it is found that the overall stability of the intake slope is better under the natural condition,whose type of failure is mainly local deformation and failure,which is similar to the abutment.But after the excavation of the slope,the displacement and deformation of the rock mass near the new free surface and excavation platform are large.After the second stage excavation,the shear failure zone is penetrated near the second stage excavation platform.At this time,the slope may occur shear failure near the second stage excavation platform.After the fourth stage excavation,a new shear failure zone was formed near the original shear failure zone,and the possibility of shear failure increased.It is suggested that the rock mass of each excavation should be strengthened before the next excavation,after the second stage excavation. |
PDF Full Text Request