Abstract: Expressway tunnel of three driveways is often large-span tunnel with oblate cross-section because excavation span is large and ratio of height to span is small. This structure shape could make a bad influence on stability of surrounding rock and stress distribution of support system, so it is necessary to determine a reasonable tunneling scheme by academic analysis. The east Mao Mountain tunnel on the Ning-Chang Expressway in Jiangsu province is designed as a separate tunnel with uplink and downlink both having three driveways respectively. It is a typical shallow, large-span and oblate tunnel. The three dimensional numerical simulation using the FLAC3D software is conducted in this paper to model different excavation schemes, including CRD method proposed in pre-designing and partial bench cut method adopted in practice. The support parameters used in these two numerical models are both accordant with the data in design. The results show that partial bench cut method is feasible. A comparison between numerical results and observation data in practice is also conducted in the paper, which is more approved that the actual tunneling scheme is reasonable. The results of this study are expected to provide academic reference to design and construction for the similar engineering.

Key words: large-span tunnel, shallow tunnel, numerical simulation, FLAC3D, tunneling scheme, stability analysis