Numerical Study On Construction Control Of Shallow Buried Tunnel With Unsymmetrical Loading And Weak Surrounding Rock Mass

Construction of shallow buried tunnel in the mountain area,will inevitably face unfavorable geological conditions of bias pressure and weak surrounding rock mass.To ensure the construction safety of shallow-buried and biased tunnels in weak surrounding rocks,there is urgent and necessary to reveal the stress characteristics and deformation laws of the surrounding rocks and supporting structures by numerical simulation.According to the field survey data and the actual conditions of construction and environment of the Hongchuan tunnel project of the Wuhu to Huangshan highway,numerical analysis models were set up to carry out the analysis of excavation and support construction process from Wuhu to Huangshan by the finite difference method.The mechanical properties of the surrounding rock and supporting structure and the stability of the opening,as well as the influence of the discontinuous stratum on the construction of shallow tunnels were further investigated.The results obtained from the simulation were also verified with the field monitoring data.Some conclusions are obtained as following.(1)Results from numerical simulations of the shallow buried bias section indicate that the values of horizontal and vertical maximum displacement of the surrounding rock in the left tunnel are 4.81 mm and 3.03 mm,respectively,while 3.33 mm and 8.58 mm for the right tunnel.They gradually decrease from the center to both sides;the pressure and displacement between the surrounding rock and the initial support gradually increase with the excavation,and gradually converge at a position about 15 m to 20 m away from the palm surface of the upper step.The axial stresses of anchors in the left and right tunnels are tensile stress with the maximum values of 9.5MPa and 41.5MPa respectively.This suggests that the inside tensile stress is larger than the outside one,and t are,and the inside anchor has a good restraining effect on the deformation of the surrounding rock.(2)The stress and deformation of the surrounding rock in the tunnel section are symmetrically distributed on the left and right sides in spite of axially discontinuous strata,while the deformation of the surrounding rock in the left and right tunnels is different.The maximum values of horizontal and vertical displacements are 1.98 mm and 6.84 mm in the left tunnel,respectively,and 3.81 mm and 11.65 mm in the right tunnel.The settlement value of the vault at the location of discontinuous strata in the right tunnel is greater than that of the left tunnel.This may be due to the poorer integrity of the connection between the supporting structure and the surrounding rock,and suggests that the tunnel construction should be strengthened at the location of discontinuities stratum.(3)The maximum settlement at the entrance of the biased tunnel is at the measuringpoint above the vault,and the settlement on the left side of the tunnel axis is greater than that of the right side.With the completion of the supporting structure,the safety factor of slope stability increased from 1.27 before excavation to 1.32,indicating that the supporting structure constrained the development of the sliding surface;the semi-bright and semi-dark supporting structure's stress form changed in the from of "push type" to "push combination type" with the excavation progresses,and "push type".Among the three typical sections,the minimum principal stress of 6.65 MPa is the largest at the foot of the wall of the dark tunnel in the semi-bright and semi-dark supporting structure,while the maximum stress all appeared at the left wall and the left arch waist.(4)Under different geological conditions and different working conditions,the simulated settlement of the vault,the convergence of the side wall and the arch waist,the surface settlement,the size and change of the pressure between the surrounding rock and the initial support at the position of the vault and the side wall,basically coincides with actual monitoring data.This may indicate that the numerical simulation model proposed here is reasonable and reliable for the analysis of the stress field and deformation law in tunnel construction under different sections and working conditions,and also provides a basis for the control of actual engineering construction.