A Study On The Multi-tunnel Effect Caused By Pilot Tunnel Excavation For Subway Station Constructed In PBA Method

ABSTRACT:With the rapid economic development and accelerating urbanization, urban rail transit construction is entering the most prosperous periods in history. Various innovative construction methods which are adapted to urban underground engineering excavation emerged. The pile-beam-arch (PBA) tunneling method of underground excavation in shallow-burial tunnel combines the technical achievements of traditional sallow buried-tunneling method with the advantages of cap and excavation method. With the organic combination of small section pilot tunnel, supporting arch and drilled pile, pile, beam and arch share the load during the construction period. The multi-cavern effect will inevitable generate at the excavation stage because of the numerous and close-spaced pilot tunnels. Meanwhile, the excavation orders of the pilot tunnel are still in dispute.In this thesis, which is based on the ChaoYangMen subway station, the numerical simulation to the construction orders of the pilot tunnel is carried on by FLAC3D software. In order to compare the various construction schemes and determine the optimal excavation order, the final settlement of the ground surface, the surface sedimentation rate and the distribution of the soil plastic zone are taken as control essentials. It gets the impact of different construction orders on the structure and stratum through the analysis of the ground and arch crown settlement, the uplift at the bottom of tunnel, the distribution of soil plastic zone, stress distribution of the surrounding soil and the moment and displacement of primary support. Research results show that:(1) Comparing with the different schemes, the differences in the final settlement of the ground surface and the horizontal affected area are very little, but there are significant differences in the surface sedimentation rate.(2) Through the analysis of displacement, stress and plastic zone, it is found that the superimposition effect in the area between the central pilot tunnels is very strong, which indicates that the multi-tunnel effect in the central part of the multi-tunnels is significant. Therefore advancing support in the soil between the central pilot tunnels should be strengthened during the excavation of the multi-tunnels.(3) After multi-factor comparison and scoring, the "upper-lower, edge-central" excavation order is recommended. The research conclusion of this thesis can be taken as a reference for determining excavation order and excavation scheme of pilot tunnels.