Longitudinal Mechanical Properties of Shield Tunnels Crossing Soft and Hard Mutation Strata

Abstract

Shield tunnels that cross soft and hard mutation strata will experience severe longitudinal uneven settlement, which results in local damage to the shield tunnel. Previous studies on the longitudinal mechanical properties of shield tunnels generally assumed that the strata were homogeneous or level-layered and that the lining segment ring was uniform and ignored the shield tunnel’s discontinuity and the inhomogeneity in the strata adjacent to the shield tunnel. Therefore, a three-dimensional (3D) longitudinal refined numerical calculation model of a shield tunnel is developed under complex geological conditions, which realistically considers the effects of a staggered assembly of a shield tunnel, longitudinal and circumferential bolts between segments, and the interface between the strata and tunnel structure. An improved Timoshenko beam theory method that has been validated by a model test in the literature is adopted to verify the numerical calculation model’s reliability and applicability and to highlight its advantages. Then, a study is conducted to analyze the longitudinal mechanical properties of shield tunnels that cross soft and hard mutation strata through this numerical calculation model. The influence law for strata on the longitudinal mechanical properties of the shield tunnel is highlighted by comparisons with another numerical calculation model with simplified strata, which includes the longitudinal uneven settlement and local deformation of the shield tunnel, the opening and dislocation of segment rings, deformation, and yield state of the longitudinal and circumferential bolts. Furthermore, a parametric study is further presented to analyze the influence of the elastic modulus difference between soft and hard strata on the longitudinal mechanical properties of the shield tunnel. An empirical formula is introduced to calculate the maximum opening and dislocation of the segment rings. The longitudinal uneven settlement of the shield tunnel becomes more significant and concentrated as the elastic modulus difference increases, which increases the local deformation and longitudinal internal force in the shield tunnel. Similarly, the deformation and internal force of bolts and the interface increase in the longitudinal direction. Significant differences in the elastic modulus between the soft and hard strata will likely induce severe deformation damage and bolt failure into the shield tunnel.