| Modern large-scale underground engineering is affected by factors such as high regional in-situ stress level,complex geological conditions,large excavation space scale and long construction period.The stability of surrounding rock becomes very prominent in the whole life cycle of underground space.In this paper,according to the actual topographic and geological conditions of the underground powerhouse area of Shuangjiangkou Hydropower Station and the measured initial in-situ stress results in the plant area,the regression and inversion analysis of the initial in-situ stress field is carried out by using the three-dimensional finite difference method.The excavation of the cavern group is carried out according to the graded excavation and graded support.The stress-strain response of the cavern excavation under the two working conditions of no support and support is analyzed,and the most unfavorable situation of the parallel construction of the three caverns is considered.The most important outcomes include:(1)According to the objective situation of the plant area in this area and according to the measured initial in-situ stress results in the plant area,the three-dimensional finite difference method is used for regression.The results show that the initial in-situ stress field of the plant on the right bank shows the characteristics of zoning and zoning.Near the upper shallow layer,there is a low stress area,the middle area is a stress transition area,and the rock mass stress in the deep area is relatively uniform,which is a stress stable area.The crossing of fault and dislocation zone has a great iMPact on the stress state of surrounding rock in the plant area,and there is a certain phenomenon of stress concentration or reduction.(2)The deformation field distribution of the typical section of the underground powerhouse of Shuangjiangkou Hydropower Station is analyzed under the two working conditions of support and no support.The results show that the natural in-situ stress in the plant site area is high,In the process of excavating surrounding rock,the unloading effect is very significant.In addition,during the specific construction process,the deformation of the surrounding rock of the cavern group presents the characteristics of developing to the free face as a whole,which is specifically manifested in the downward deformation of the surrounding rock of the vault of the cavern,the deformation of the surrounding rock of the upstream and downstream side walls and end walls into the tunnel,the upward rebound deformation of the bottom plate of the cavern at all levels,With the continuous implementation of the project,the deformation of surrounding rock becomes more prominent,The deformation of the surrounding rock at the arch crown converges and stabilizes gradually after the previous excavation.Under the supporting condition,the maximum displacement of the main power house still occurs at the intersection of the upstream side wall and fault F1 after the excavation of the cavern.The maximum displacement is 33.8mm,which is 14.6mm less than that under the unsupported condition.The maximum deformation of the side end wall of the auxiliary power house is 28.3mm,which is 7.5mm less than that under the unsupported condition.The maximum deformation at the arch crown of the main power house is 15.7mm,which is 6.5mm less than that under the unsupported condition;The excavation scale of the main transformer tunnel and the tail dispatching room is relatively small,and the deformation response is relatively small.The displacement of the left end wall of the main transformer tunnel and the tail dispatching room is relatively large.After the excavation,the maximum displacement of the end wall of the main transformer tunnel is 14.6mm,which is 3.9mm less than that of the unsupported working condition.The maximum displacement of the end wall of the tail dispatching room is 20.0mm,which is 6.6mm less than that of the unsupported working condition.(3)The stress field distribution of the typical section of the underground powerhouse of Shuangjiangkou Hydropower Station is analyzed under the two working conditions of support and no support.The results show that the natural in-situ stress in the plant site area is high,In the process of excavating surrounding rock,the unloading effect is very significant.The excavation will cause a certain disturbance to the initial in-situ stress in the plant area and cause stress redistribution.During the construction of the project,the stress distribution in this area is largely reflected in large deformation stress release of the surface layer around the tunnel and the slab rock at the bottom of the tunnel,the significant reduction of the principal stress,the stress concentration and the increase of the principal stress at the corners such as the bottom corner of the side wall and the arch foot.The influence range of surrounding rock stress redistribution increases step by step with the excavation,and its main influence range is within 2～3 tunnel diameters.Under the support condition,the stress law of surrounding rock is similar to that under the non support condition.The excavation of cavern causes the unloading of surrounding rock,and with the potential plastic damage of surrounding rock,the stress level of surrounding rock is greatly reduced.However,the influence range of surrounding rock stress redistribution under the support condition is reduced.The small principal stress of the surface rock mass of cavern is 0～2mpa,the large principal stress is 5～10mpa,and the stress concentration will occur at the bottom corner of side wall and arch crown,and the maximum principal stress can reach 30～33mpa.The excavation scale of bus tunnel and tailrace tunnel is small,and the stress concentration effect is more obvious.There will be obvious stress concentration zones above and below them,and the stress value is more than 30MPa.There will still be large stress concentration at the bottom and top corners of the tunnel end wall,with the stress value above 32Mpa.The stress level in the area where the fault is located is relatively small,causing a certain dislocation to the stress distribution of the surrounding rock in the plant area.(4)For the underground cavern,it has a very representative plastic zone powerhouse of Shuangjiangkou Hydropower Station is analyzed under two working conditions of support and no support.The results show that the natural in-situ stress in the plant site area is high,In the process of excavating surrounding rock,the unloading effect is very significant.The rock around the powerhouse will appear plastic areas in varying degrees,mainly manifested as shear plastic areas and tensile plastic areas,and the shear plastic areas are mainly accoMPanied by sporadic tensile plastic areas in some areas.The distribution areas are mainly at the side wall,vault and bottom plate of the cavern.Under the support condition,the distribution of plastic zone in the surrounding rock of underground powerhouse is greatly reduced.With the gradual excavation of the cavern,the three caverns will also have sporadic plastic zones.After the excavation,the plastic zone basically only appears on the surface of the side wall of the cavern.The development depth of plastic shape is greatly reduced compared with the non support condition,and the maximum development depth is less than 4m.The development depth and range of plastic zone are greatly improved compared with the non support condition.The plastic zone mainly occurs in the powerhouse bottom plate and the end walls on both sides.The plastic shape at the bottom plate is mainly represented by the shear plastic zone,and the plastic area at the end walls on both sides is represented by the tension plastic zone.After the excavation is completed,the plastic zone is basically scattered only on the surface of the tunnel end wall and the bottom plate.The development depth of the plastic shape is greatly reduced compared with the unsupported condition,and the maximum development depth is less than 3m.It shows that the supporting measures can have a good effect. |
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