Study on optimization of excavation scheme for pilot tunnels of pile–beam–arch method in upper soft and lower hard stratum

Abstract

Based on the Labor Park Metro Station Project in Dalian area located at the upper soft and lower hard composite stratum, in order to determine the reasonable excavation sequence and excavation footage of pilot tunnels of PBA method, the yield approach index (YAI) is introduced. The YAI of surrounding rock, surface settlement, stress of surrounding rock and LDP curves under different construction sequence and excavation footage of pilot tunnels are analyzed, and the optimal construction scheme of pilot tunnel is determined. The results show that the multi-cavity effect of the pilot tunnels gradually appears with the progressive excavation of pilot tunnels. After the completed excavation of six pilot tunnels, the shape of the surface settlement groove is basically in the form of groove. The peak value of the settlement groove is related to the excavation order of the pilot tunnels. The superposition effect to the surface subsidence is obviously observed from upper and lower pilot tunnels. The excavating sequence from top to bottom and from side pilot tunnel to central pilot tunnel is the best to control the surface settlement. Under this geological condition, the circular excavation footage of pilot tunnel should be selected as 1.5 m. The numerical simulation results1 of pilot tunnels excavation are consistent with the field-measured results, verifying the feasibility of the simulation process. The scheme can provide the basis for the subsequent projects, as well as the construction of the subway station with similar geological conditions using pile-beam-arch method (PBA) method.