Objective As a rod-shaped component embedded into strata, the advance small pipe (hereinafter abbreviated as ASP) possesses certain bearing capacity when the form of primary support structure changes or the external loads change abruptly. For tunnel support removal in fractured rock strata, it is necessary to investigate the influence pattern of ASP design parameters on construction stability.

Method Based on a large-span running tunnel in Qingdao Metro, the risks of support removal during multi-heading construction by the primary support arch cover method in soft weak strata are analyzed. The bearing mechanism of ASP in improving primary support during the support removal stage is studied. A three-dimensional numerical calculation model is established using FLAC3D software. Under self-weight loads, the influence pattern of ASP design parameters on stratum settlement is systematically analyzed. Design parameters of ASP need prioritized are determined, and theoretical analysis and numerical results are verified through field monitoring data.

Result & Conclusion  Because ASP can significantly strengthen the spatial shed protection effect of the residual bearing system after the support removal, it is helpful to restrain surrounding rock deformation and reduce construction risks. The influence degree of each design parameter on optimizing the mechanical performance of primary support and controlling stratum deformation is ranked in descending order as circumferential spacing, pipe diameter, wall thickness. The improvement effect becomes insignificant when the circumferential spacing exceeds 450 mm. It is recommended for ASP to prioritize using design parameters of small pipe diameter, small circumferential spacing and moderate wall thickness. The ratio of the vault settlement control effects by ASP in the excavation stage to that in the support removal stage is approximately 2:1. Field practice and monitoring data verify the remarkable comprehensive bearing capacity of ASP.