Abstract: In order to compare and evaluate the most safe and stable cross section type in the excavation of the multi-arch tunnel without a pilot tunnel excavated first as well as to improve the safety and stability of the tunnel lining structure during the tunnel excavation, this paper presents a case history of the excavation and evaluation results of a multi-arched tunnel in Fujian Province. Three different tunnel excavation sequences with the amount of backfill at the top of mid-wall area were numerically evaluated for the factors of safety for the lining structure at final stage, temporary supporting structures and midwall stability. The stability of the tunnel structures and its variation during three different tunnel excavation sequences at various stage of excavation are compared in detail. The results indicate that that the maximum stress changes are found in the lining structure at upper step of the later excavated tunnel. In all three excavation sequences, the immediate filling the gap between the lining structure and the mid-wall is beneficial to the stability of the lining structure. It is also concluded that the method three can be used as the crosssection type of the tunnel excavation without a pilot tunnel excavated. The stability of the lining structure in the multiarched tunnel excavation can be improved by the immediate backfilling of the top of mid-wall.

Key words: Tunnel Engineering, Tunnel Excavation without Pilot Tunnel Excavated, Numerical Simulations, Multiarch Tunnel, Factor of Safety