Study on Excavation Sequence of Pilot Tunnels for a Rectangular Tunnel Using Numerical Simulation and Field Monitoring Method

Cross diaphragm (CRD) method divides the tunnel into multiple pilot tunnels to reduce the span of a single excavation, which is beneficial to reduce the disturbance of excavation construction to surrounding rocks. In this method, the excavation sequence of pilot tunnels is a key factor affecting the excavation disturbance. In this study, two schemes of excavation sequence of pilot tunnels were simulated by numerical method, based on a newly built shallow buried rectangular tunnel using CRD method. By comparing and analyzing the vertical displacements of tunnel surrounding rocks, ground surface settlements, crown settlements and bending moment of the primary support structure, the surrounding rock deformation characteristics and trends of the tunnel were revealed, which determined the more reasonable excavation sequence scheme for the construction of the rectangular tunnel. By comparing with the field monitoring data, the numerical simulation results were verified to a certain extent. In addition, the comparison of calculation results obtained from the Mohr–Coulomb (MC) and the hardening soil small (HSS) constitutive models were discussed. The investigation results show that: (1) the displacement of surrounding rock and bending moment of the primary support structure are closely related to the excavation sequence of the pilot tunnels; (2) the deformation trends revealed by the numerical results are similar to those from the field data; (3) according to the field construction process, the asymmetric excavation sequence of "left upper pilot tunnel—left lower pilot tunnel—right-upper pilot tunnel—right lower pilot tunnel" is safe and reliable for the rectangular tunnel construction.