Evaluation of earth pressure around a deeply buried metro shield tunnel with a large cross-section under high water pressure conditions

A field monitoring program is performed to measure the earth pressures acting on the linings and the internal forces in segments of a slurry shield tunnel of Yangtze River tunnel project in Nanjing. The magnitude and distribution of the earth pressure are analyzed around the tunnel deeply buried in a silty sand stratum with high pore water pressure. The internal forces in the segments of concern are calculated under three different vertical loading combinations(i.e. effective overlying soil pressure plus water pressure, Terzaghi's soil pressure plus water pressure, and water pressure only), and the results are compared to the ones measured in the field. The results show that: 1) the water pressure acting on the shield tunnel lining is almost equal to the theoretical hydrostatic pressure; 2) the observed vertical earth pressure on the crown of shield tunnel accounts for about 80% of that yield by the Terzaghi's soil pressure plus water pressure formulation, showing soil arching effect exists above the tunnel; 3) the observed bending moments are smaller than theoretical results of the three combination methods, and the measured axial forces are approximately twice as much as the theoretical results. In addition, the influence of the ground reaction on the internal force in the segments is also discussed. The insights provided from this study can contribute to the improvement of large-section shield lining design.