Experimental Study on the Mechanical Characteristics of Shield Tunnels under Vertical Jacking
In vertical jacking construction,the effect of jacking reaction forces applied to the bottom of shield tunnels is a critical issue concerning the safety and stability of the surrounding soil structure.To explore the impact of vertical jacking construction on the mechanical behavior of shield tunnels,a model test was conducted using a test apparatus developed for a typical project.The test aimed to reveal the development pattern of the reaction force distribution during the jacking process and to study the mechanical and deformation characteristics of the tunnel lining and joints during different jacking stages,comparing them with theoretical calculations.The results indicate that the dis-tribution of jacking reaction forces exhibits a typical nonlinear pattern,characterized by a significant increase before soil failure,a notable decrease after soil failure,and a gradual stabilization phase.The convergence deformation of tunnel segments undergoes four stages:a sharp increase,rapid decrease,slow decline,and eventual stabilization,af-fecting the opening ring and adjacent two rings.Deformation control during the jacking process should prioritize ver-tical reinforcement based on displacement characteristics.Longitudinal bending deformation occurs in the tunnel during the jacking stage,with the maximum gap observed between the opening rings.Due to sudden longitudinal load shifts,the largest misalignment occurs between the opening ring and adjacent rings.The segment ring joint opening and misalignment decrease significantly along the longitudinal axis,with displacement mainly resulting from segment body deformation and minimal joint deformation contribution.Under jacking forces,the tunnel crown and inner base are subject to tension and compression,respectively,with stress initially increasing and then de-creasing.The upper part of the tunnel waist experiences initial compression followed by tension,while the lower part is under tension.The jacking reaction force induces a redistribution of segment stress disturbances,primarily affecting the opening ring and adjacent rings.The segment stress levels remain low,and no tensile or compressive damage is observed in the structure.
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