Deformation Characteristics and Countermeasures of shallow and Large-span Tunnel Under-crossing the Existing Highway in Soft Soil: a Case Study

Hongmian Road goes under Jihe highway by the way of tunnel in Shenzhen City, Guangdong Province, China. The tunnel has some special characteristics such as shallow depth, large span, soft soil, busy traffic, and long distance under-crossing the operating highway, thus there is great difficulty and risk during its construction. A series of in-situ tests were conducted to reveal deformation characteristics during tunneling beneath the existing highway by the double-side-drift method (DSDM). The results show that initial construction method and measures can’t meet the safety requirement of tunnel construction, and the level of risk is extremely high, so the construction method need to be timely adjusted. By comprehensive analysis, the double-side-drift optimum method (DSDOM) is put forward, on the basis of which a series of optimum measures also have been put forward, such as reasonable construction methods, the suitable length of bench and excavation, soil reinforcement measures on tunnel face, optimizing construction process, closing primary support in time and speeding up the construction of secondary lining. Then the feasibility of this optimized construction method and measures are verified by the numerical simulations. Finally, from feedbacks of monitoring results, the optimum method and measures have been proved to be reasonable and feasible, and they can guarantee a successful pass beneath the operating highway.