Investigating the Mechanical Responses and Construction Optimization for Shallow Super-Large Span Tunnels in Weathered Tuff Stratum Based on Field Monitoring and Flac3D Modeling

To investigate the mechanical responses and construction optimization of shallow super-large span (SLS) tunnels in weathered tuff stratum, the field monitoring scheme and the refined three-dimensional numerical model for Xiabeishan No.2 tunnel were established, and a sequence of field and laboratory tests was conducted to acquire the calculation parameters. Then, the surrounding rock pressure and shotcrete stress in grades III-V rock mass were comprehensively monitored and analyzed. Finally, according to the problems occurred in site construction, some optimized construction methods were proposed and then, systematically discussed by numerical simulation. The results show that: (1) it was discovered that the construction disturbance of the shallow SLS tunnel is mainly concentrated in the excavation of the upper benches; (2) the excavation of the upper bench of the middle heading, in which the super-large span structure is formed, is the most critical constructio n step; (3) the blasting vibration created by the excavation of middle heading caused serious damage to the temporary support, and the stress adjustment between the rock mass and supporting structures was completed before the demolition temporary support; (4) both the "middle first and side later" double side heading method in grades IV-V rock mass and the three-step method in grade III rock mass are feasible under certain conditions.