To study the construction mechanical characteristics of shallow buried large span subway station tunnel, the Shangxinjie subway station in Chongqing is selected to establish three-dimensional numerical model using Midas-GTS. The deformation and stress distribution of surrounding rock mass are highlighted. Besides, the monitoring date of the preliminary lining deformation was analyzed and compared to the numerical results to validate the correctness of numerical simulation. The simulation results show that the maximum settlement located at the crown of tunnel with the value of 5.37 mm, and the maximum lateral displacement located at arc springing of tunnel with the value of 4.39 mm. The pre-settlement takes up to 25% of the final settlement, and has great implication when evaluating monitored data to gain reliable information about ground behavior. Tunnel crown settlement develops rapidly when the core soil (part 7, 8, and 9) are excavated. Totally, displacements caused by the excavation of core soil takes 55 percent of the final displacement, thus the soil core is the key to control the tunnel deformation. The maximum principle stress concentrates at the arch foot at both sides with the value of 1.87 MPa. The maximum tensile stress is 0.05 MPa which is approximate to tensile strength of rock and mainly distribute in the upper left region of tunnel. In practice, tensile cracks of ground appeared in this region, and underground water pipe of this region was damaged leading to water gush. The curves of monitoring date with construction steps are similar with that of numerical results, but the monitored settlement is 2.02 mm that is less than numerical results, for the reason that only the residual displacement after completing the preliminary lining can be monitored based on the conventional measurements.

1. Introduction

In response to rapid growth in urban development there has been a pressing need for construction of new tunnels for transportation systems and underground utilities. Subway is construction in more and more cities in China to solve the increasingly serious traffic congestion problem. Based on existing research results, when a tunnel or underground space is excavated, it inevitably disturbs the in situ stress field which causes ground movements leading to surface settlement. Generally, the section of subway tunnel is quite large to meets the request for utilization. The sectional area of the subway station is approximate to 400 m2 that is much larger than that of highway tunnel. In addition, the buried depth of subway station always ranges from 10m to 30m which is nearly one times of tunnel diameter. Thus, many subway station tunnels have the characteristics of shallow buried and large span. The construction mechanical characteristic is more complex than highway tunnel. Thus construction mechanical characteristics of shallow buried large span tunnel should be studied to acknowledge the stress and deformation distribution during tunneling to ensure the stability of tunnel and to reduce project cost.

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