Back Analysis of Water and Earth Loads on Shield Tunnel and Structure Ultimate Limit State Assessment: A Case Study

A method based on the least-squares method was proposed for the back analysis of water and earth loads using in situ monitoring data of a limited amount of structural strain values for a shallow shield tunnel buried in the soil stratum. Theoretical formulas were used to calculate the internal forces, conduct an assessment of the ultimate limit state, and determine the safety factors of compression and shear. Based upon all of the above, an assessment system was established, and its procedures were illustrated. The highlights of this assessment system are that only a few strain monitoring points are required for the back analysis of the external loads, the internal forces in every position of the segment are calculated by the uniform rigidity ring model, and the safety assessment is conducted by the proposed ultimate limit state formulas. According to the in situ monitoring project in Nanjing subway line 1, the monitoring data of internal forces were close to the back analysis data, which proves the reliability of this method.