Analysis Of Face Stability During Large Cross-section Rectangular Tunnel Jacking

Compared with circular tunnels,box tunnels can increase the space utilization ratio by approximately 20 percent.They have been increasingly used in urban utility tunnels,underground passages,highway and subway tunnels.Due to their poor arching effect,box tunnels are usually excavated by open-cut.With the increasingly stringent constraints from environment protection and the advancement of pipe jacking technique,more and more rectangular tunnels have been installed through jacking.In the meantime,the cross-sectional area and alignment length of the tunnels are continuously increasing.Due to the shape characteristics of large cross-section rectangular tunnel jacking,the stability of working face is worse than that of circular tunnel.If the support pressure is not set properly,it is easy to cause the instability of the working face.In this paper,the face stability of large cross-section rectangular tunnel jacking is fully investigated.Theoretical analysis and numerical simulation are used to solve this problem.The research results have a certain reference value for the stability evaluation of tunnel face of rectangular tunnel jacking.The main research contents and conclusions of this paper are as follows:(1)By dividing the two-dimensional rotational mechanism into discrete elements,the dissipation rate associated with the mechanism is accurately calculated.Results of numerical verification show that the active rotational mechanism can predict the spiral shear zone at the bottom of tunnel well,and has higher adaptability in large cross-section rectangular tunnel jacking.The coincidence degree between the passive rotational mechanism and numerical failure mode is less than that of active rotational mechanism.The prediction effect of failure mode of deep tunnels is better than that of shallow tunnels.The active and passive critical support based on the rotational mechanism are between the existing two-dimensional and three-dimensional analytical solutions,and agree well with the three dimensional numerical simulation results(2)The composite mechanism is proposed by combining rotational and translational blocks,and the compatibility is satisfied by constructing an interface at the mechanisms.The results of parametric analysis show that with the increase of shear strength of soil,the effect of translational mechanism decreases gradually with the increase of frictional angle.Numerical verification shows that the prediction effect of active composite mechanism on the spiral shear zone at the bottom of tunnel is better than that of single rotational mechanism.With the increase of the ratio of width to height of rectangular tunnels,the numerical critical support pressure is gradually close to the active composite mechanism.The variation of passive composite failure mode is opposite to that of active composite failure mode.The effect of rotational mechanism in high-friction soil is almost lost.The coincidence degree between the numerical solution and the composite mechanism is greater than that of single rotational mechanism.With the increase of the ratio of buried depth to height,the gap between the numerical solution and the passive composite mechanism gradually increases.(3)The validation of the current solutions is conducted by investigating the face stability of rectangular tunnel jacking in the project of Shilianjie Station of No.5 metro line in Suzhou.Compared with the monitoring value of the support pressure,it is found that the value calculated by the composite mechanism is closer to than that calculated by the rotational mechanism.The support pressure drop value caused by the suspension of rectangular tunnel jacking is close to the active limit support pressure predicted by the composite mechanism,which indicates that there is a risk of instability in the working face.The failure mechanism proposed in this paper can provide a theoretical basis for the design of rectangular tunnel jacking.