Study On Surrounding Rock Stability Of Busbar Tunnels Of Yangfanggou Hydropower Station

Yangfanggou Hydropower Station on Yalong River is located in Muli County,Liangshan Prefecture,Sichuan Province.Its total storage capacity is 514.8 billion cubic metres and installed capacity is 1500MW.It is currently in the construction stage and is expected to be completed in 2022.The underground powerhouse adopts the first development plan on the left bank.The ground elevation of the underground powerhouse is 2240-2370m,the thickness of overlying rock mass is 197-328m,the thickness of horizontal rock mass is 125-320m,and the lithology is granodiorite.The rock mass is relatively complete and the rock is hard.The main and auxiliary powerhouse caverns,main transformer caverns and tail tunnels are arranged in parallel with each other.The longitudinal axis direction is N5°E.Four busbar caverns are arranged in parallel with each other in the rock pillars between the powerhouse caverns and the main transformer caverns.The caverns are orthogonal to the axis of the powerhouse.The excavation dimension is 45m×9m?11m?×7.5m?length×width×height?,and the net distance between each busbar cavern is 24m.According to the rock mass properties revealed by excavation,the surrounding rock mass structure of busbar tunnels is mainly block-to-sub-block,and the structural plane is mainly small faults and joints,mainly in NNE,NEE and NWW directions.The dip angle is mainly medium-steep dip angle,and the surrounding rock types are mainly type II and III₁.As the main and auxiliary powerhouse tunnels continue to lie down,the busbar tunnels gradually locate in the middle and upper part of the high side wall,and the stress is constantly adjusted.The stress state of the busbar tunnels will become very complex due to the cutting effect of NNE on the unfavorable structural plane.Through field investigation,it is found that at different stages of excavation,there are many different types of deformation and failure of busbar tunnels.In the early stage of excavation,there are blocks falling and rib spalling.In the middle and later stages of excavation,the rock mass,concrete shotcrete,concrete cushion and secondary lining crack in varying degrees,some horizontal distribution and some circumferential distribution.The stability of the surrounding rock directly affects the stability of the downstream side wall of the powerhouse,so it is necessary to study its stability.This study relies on the mentor project to carry out the geological survey during the construction period of Yangfanggou Hydropower Station,participate in the field geological sketch work,make a detailed catalogue of the excavation face of each section of the cavern,collate the first-hand geological data in time,evaluate the rock mass structure and surrounding rock quality,and investigate and analyze the deformation and destruction phenomena in each excavation stage.Based on the study of the deformation and failure mechanism of the surrounding rock of the busbar tunnels of Yangfanggou Hydropower Station and aimed at the stability of the surrounding rock,the overall route and method are studied.Through the above research,the main achievements are as follows:?1?Before the support of busbar tunnels,the deformation and failure of surrounding rock are mainly the block type failure of side-crown arch,rib spalling and cracks in floor foundation.After support,the main deformation and failure phenomena are spray layer cracks of side arch,floor cushion cracks and secondary lining concrete cracks.According to the spatial development characteristics of cracks,cracks can be divided into horizontal cracks parallel to the axis of busbar tunnels and circular cracks perpendicular to the axis of busbar tunnels.Horizontal cracks mostly occur at the right arch shoulder of the busbar tunnels after the initial lining,while circumferential cracks distribute more,including floor foundation cracks,spray layer circumferential cracks,floor cushion cracks,which develop more near the factory building and less near the main transformer.?2?The deformation and failure mechanisms of surrounding rock of busbar tunnels are mainly structural plane-controlled,stress-controlled and stress-structural plane combination.Structural plane control type is mainly caused by block slipping and collapse and eventually block falling.Stress-controlled failure is mainly caused by the concentration of tangential compressive stress at the arch shoulder and arch foot,resulting in rib spalling and horizontal cracks in the spray layer.Stress-structural plane combination type is caused by surrounding rock fracturing caused by adjusting initial three-dimensional compressive stress state to vertical loading and lateral unloading state.The dominant structural plane with steep dip angle further deforms the surrounding rock to the inside of the powerhouse,resulting in tension-type annular cracks.?3?The local stability of busbar tunnels is affected by the characteristics of eccentric stress and secondary stress in valley.Under the"jumping tunnel excavation"mode,the displacement of adjacent busbar tunnelss changes little,the excavation disturbance is weak,and the surrounding rock stability of busbar tunnelss group is good.By using 3DEC numerical simulation software to simulate the response characteristics of surrounding rock of busbar tunnels under layered excavation,it can be concluded that under the action of high side wall effect,lateral unloading and vertical loading of surrounding rock of busbar tunnels,compressive tension deformation and failure occur,resulting in tension-type annular cracks in surrounding rock and lining of busbar tunnels.The maximum axial displacement of the busbar tunnels is 9.64cm.If the rock pillars are not supported in time,the stability of the surrounding rock of the busbar tunnels can not be ensured.?4?After systematic support of rock pillars,it is known that after the implementation of support measures,the maximum principal stress of surrounding rock near busbar tunnels increases from -2.5?-5.0MPa to-5.0?-7.5MPa,and the minimum principal stress changes from positive value to negative value,and the original tensile stress zone has basically disappeared.The axial displacement of busbar tunnels is reduced from 9.64cm before support to 4.08cm.The plastic zone only distributes in the shallow surface layer.The deep part has disappeared.The deformation of busbar tunnels has been effectively controlled and the stability of surrounding rock has been greatly improved.