Deformation Characteristics of Large Section Tunnel Surrounding Rock and Optimization of Tunnel Construction Scheme for Qingdao Metro Line

Objective The large section tunnel, due to flat shape and large span, usually adopts multiple steps and staged excavation. In the process of construction, the surrounding rock and the temporary support are repeatedly disturbed, affecting the stability of surrounding rock and supporting structure. Therefore, the deformation characteristics of the surrounding rock should be studied and the construction scheme should be optimized. Method Taking the mined tunnel of Qingdao Metro Line 4 in turn-back section as the example, based on the on-site monitoring data and using finite element simulation, the deformation characteristics of the surrounding rock in the excavation process of the large-section metro tunnel with three-step and nine-stage method are analyzed, and then compared with the optimized three-step and five-stage method. Result & Conclusion The simulation results show that the ground settlement is relatively large caused by the current construction scheme within the horizontal distance of 26m from the tunnel center line, accounting for 89.77% of the total settlement. The arch and the spandrel deform are large after excavation, and the maximum settlement reaches 102 mm. The stress of the surrounding rock of the arch increases rapidly with excavation. As the excavation advances, the local principal stress concentration appears in the upper part of the haunch and the corner. The optimized three-step and five-stage excavation method can realize rapid excavation and support, and simplify the construction process by 40%, while reducing disturbance to the surrounding rock and controlling its deformation within a reasonable range of 10 mm. Thus, the stress redistribution of the arch and the stress concentration in the corner are improved.