Quantitative determination method for bearing state of shallow buried shield tunnel under loading conditions

In coastal areas, the soil strata often fluctuate greatly and are uneven in softness and hardness. Due to the difference in the resistance of the soil strata, the bearing state of shield tunnel structure varies greatly. The 1∶5 fine model tests and numerical simulations are carried out to study the difference of structural bearing state of a staggered jointed shield tunnel under different stratum conditions, and a quantitative determination method for structural bearing state is proposed. The results show that: (1) The obvious cracking of the arch and the lateral opening of the longitudinal seam are obvious at the arch waist position, and the extrusion cracking of the joint outside the arch waist and inside the arch and the yield of the bolt at the bottom of the arch occur successively during the test loading process. (2) There are six critical bearing states in the segment bearing evolution process of silt layer, clay layer and sand-filled layer, but there are differences in the segment cracks, joint openings, reinforcement forces and bolt forces. (3) According to the damage degree of segment, the bearing state is divided into four grades, and the corresponding control values of vertical convergence and residual bearing capacity of each grade are determined. Taking the silty clay layer as an example, the control values of vertical convergence of each grade are 2.0‰D, 3.9‰D, 15.3‰D, and the proportion control values of residual bearing capacity are 88.0%, 76.9%, and 19.7%. The conclusion of this study may provide reference for the evaluation of the service status of metros in coastal cities.