| Due to the complex structure of the tunnel,the deformation is large and the rheological and creep characteristics are often caused by the high ground stress.The damage of the supporting structure is often caused,the initial support is cracked,the uplift is raised,which has restricted the normal construction of the tunnel and affected the long-term stability of the supporting structure.At present,the research on the deformation and stress of the weak surrounding rock at home and abroad has not yet been relatively mature.The tunnel industry is a worldwide problem.It is necessary to carry out the research on the characteristics of the tunnel structure and the deformation control technology under the high ground stress environment.Based on the example of the Muzhailing tunnel project in Lanzhou-Chongqing railway,this paper adopts the methods of on-site monitoring,indoor and outdoor test,and numerical simulation analysis and so on.In view of the existing research results on the deformation and stress of carbonaceous slate under high ground stress,the deformation of the support and its characteristics,the stress characteristics and deformation control technology of the large-deformation carbonaceous slate tunnel under high ground stress are studied.The main contents and conclusions are as follows:Through the field point test and the shear test,the strength range and the related parameter values of the carbonaceous slate in the Muzhailing tunnel are obtained.The average strength is 1.9MPa.The internal friction angle is 27.5°.The cohesion 0.21 MPa and deformation modulus is 0.502 GPa.The strength of carbonaceous slate in Muzhailing tunnel is low,and the layer strength is lower.The softening property is remarkable.The stress distribution and structure,the principal stress and the stress state of the cross-section of the tunnel in the Muzhailing tunnel are analyzed by the hydraulic fracturing method.The maximum horizontal principal stress is between 26.22 and 38.38MPa,and the minimum horizontal principal stress is between 15.73 and 21.52 MPa,and the maximum stress principal stress is between 25.23 and 21.52 MPa.The side pressure coefficient of the tunnel cross section of the vertical tunnel axis in the northern section of the tunnel area is?_?/?_v=2.92.The side pressure coefficient of the tunnel cross section in the southern section of the tunnel area is?horizontal?_?/?_v=2.14.The ground stress structure is favorable to the development and activity of the reverse fault.According to the results of ground stress test of Muzhailing tunnel,the stress release and loosening of the surrounding rock of the tunnel are measured.It is concluded that the influence of loosening circle is more than 8m in the surrounding area of the tunnel.Carbonaceous slate surrounding rock in Muzhailing Tunnel takes four forms of support,including the initial support for the three which followed by anchor spray,net spray and rigid frame,and the fourth layer of support for the two lining.The finite difference software is used to establish the calculation model of the deformation of the surrounding rock supporting structure of the two sections with simulated horseshoe and circular sections.The results show that the cumulative convergence value of the vault position has been about 450mm before the fourth layer of support facilities in horseshoe shape,which has exceeded the design of the reservation value of about 1times.Cumulative convergence value is 1400mmin the side wall position which is beyond the design of the reservation value of about 4 times.After the second lining,the deformation rate of surrounding rock has slowed down.However,the deformation is still growing and the four-story support cannot reach the control of the deformation of the surrounding rock.The cumulative convergence value of the vault position has been about250mm in circular excavation section,which is lower than the design reserve value.The cumulative convergence value is about 800mm in the side wall position,which exceeds design reservation 15%.Within the controllable range,the deformation of the surrounding rock is obviously convergent,and the final settlement of the vault is below300mm,which is lower than the designed deformation.The four-layer support can effectively control the deformation of the surrounding rock.According to the monitoring of the deformation of the two sections of the surrounding rock,it is found that the deformation of the brackets in the carbonaceous slate increases steadily and then increases sharply,and the deformation rate is obviously slowed down after the third layer support.The deformation of the horseshoe section and the surrounding rock in the circular excavation section is converging.Based on the established numerical analysis model and the field monitoring data,the mechanical behavior of the surrounding rock support structure of the carbonaceous slate tunnel under high ground stress is studied.The result shows that the stress values of the supporting structures are higher at the side wall and arch foot.The value reach to800kPa of the horseshoe-shaped section at the arch.The surface stress of the concrete is the largest at the side wall.It is found that concrete is cracked and broken firstly.Circular expansion section of the two lining contact pressure at the arch is 500kPa.The value of concrete surface stress is reached the maximum in the arch,which is about 15MPa.After the tunnel laid,the weak support structure of the tunnel appear in the side wall and the invert arch.The horizontal compressive stress of the dome is about 1284.58KN,and the vertical stress of the side wall is about 5699.75 KN.According to the results of the force and deformation analysis of the surrounding rock supporting structure,the principle of the large deformation treatment of the carbonaceous slate section is put forward.In the construction process,flexible support is used firstly,then the rigid support is used.The support structure is step by step to strengthen and the overall performance of the supporting system performanceis improved.The program of“circular cross-section+guide hole extension+four-tier support system”is adapt to high stress tunnel structure deformation,andthe traditional method is made up for the shortcomings.In which the guide hole adopts the two step method,and the opening section of the arch with an inverted arch or arch control deformation.Three step method is used in circular excavation section construction.According to the tunnel monitoring experiment,the total displacement of the initial branch of the tunnel is obtained.The first layer is released by about 60%.The second layer is released by about 25%~35%.The third layer is about 10%~15%after the completion of the three-tier initial support.The vault settlement rate is reduced by 80.6%compared with that before the construction.The convergence rate of the upper side wall is reduced by 82.4%.The deformation has been effectively controlled and the deformation is basically converged.The results show that the circular cross section is uniformly compressed,and the bending moment and shear force are significantly reduced.The minimum principal stress(compressive stress)is reduced by more than 80%.The circular section shows radial uniform shrinkage.From the structural force and deformation point of view,the circular section is more advantageous than the existing optimized soft rock section.The results show that the treatment scheme has better control effect on the deformation of the rock mass rock.The safety reserves of the tunnel structure are large.Surrounding rock deformation conforms to specification requirements,and the desired results are reached under the treatment measures.The controllable and the technology has been improved,which can provide reference for similar projects in the future. |
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