| In recent years,with the rapid development of urban construction,land resources on the ground are becoming increasingly scarce.A large number of high voltage transmission lines in cities not only affect the beautiful environment,but also greatly reduce the utilization efficiency of surrounding soil.The best solution to this kind of problem is that the transmission line "enters the ground".By building the power tunnel in the urban central area,the high-voltage lines which were originally affected by the space and external environment are arranged in the underground tunnel as a whole,so as to reduce the potential safety hazards caused by the adverse weather conditions.Tunnel construction will inevitably disturb the rock and soil mass,causing surface subsidence.Although there are many studies on the ground settlement caused by the excavation of straight-headed tunnels,there are few studies on the ground settlement caused by the excavation of corners,and the relevant experience is seriously insufficient.In this paper,the gradual large cross-section 90 expansion turning tunnel of Dalian Nanyan Fourth Circuit High Voltage Transmission Project is taken as the engineering background.According to the design requirements,Midas GTS NX finite element software is used to study the surface settlement law during the construction of corner tunnel,and the comparison with the field measured values verifies the rationality of the numerical model,analyses the stress and displacement of surrounding rock of tunnel,and provides engineering design experience for similar projects,and draws the following conclusions:(1)The simulation results of corner tunnels show that the transverse surface settlement curve is approximately symmetrical along the central line of the cross-section,and the farther away from the central line,the smaller the settlement.The maximum simulated surface settlement is 3.94 mm,and the monitoring side value is 4.0 mm,which is in good agreement with the field monitoring value.Longitudinal surface settlement increases with the increase of monitoring days,and the settlement value is larger in the variable section section section of corner tunnel.(2)The stratum above the tunnel vault produces settlement deformation.The settlement of the same section increases gradually from the surface to the tunnel vault,and reaches the maximum settlement deformation around the tunnel vault.The law of vault settlement of tunnel section is divided into three stages according to the relative position with excavation face: the surrounding rock deformation is slow in the early stage,the settlement rate increasesrapidly when excavation face passes,and the settlement tends to be stable when excavation face is far away;the formation uplift deformation occurs below the tunnel inverted arch,the closer the stratum uplift is to the inverted arch,the larger the deformation,and the maximum uplift value near the inverted arch at the bottom of the tunnel.(3)When excavating with the method of middle partition,stress concentration occurs at the joint between the middle partition and the vault.The stress and deformation of the vault and inverted arch are obviously larger than those of the side walls on both sides.The lateral steel support has a good effect on the stability of the side walls.(4)When the tunnel is excavated,the initial support(lining)should be applied as early as possible.The lining and the surrounding rock of the tunnel should be deformed in coordination.After the steel support is applied,the surrounding rock-lining-steel support forms a strong structure.The maximum surface settlement value simulated changes from11.23 mm without support to 3.94 mm when support is applied in time.It shows that the supporting structure system composed of lining and steel support can be very good.Good restraint of surface deformation. |
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