Discrete element simulation on soil arch effect in curved shield tunnel with super large diameter

Objective The asymmetric stratigraphic loss will induce complex stress transfer during the excavation in curved shield tunnels with super large diameter. This study investigates the formation rule, internal and external boundary characteristics, and asymmetry of soil arch effect above curved tunnels; as well as quantify its influence on the stress and displacement distribution of surrounding soil. Method Based on discrete element method, the simulations of different asymmetric stratigraphic loss rates were achieved by controlling shield tail gap and over-excavation gap on curve inner side, so as to simulate the soil disturbance caused by excavation. The reliability of model was verified by comparing with the field-measured surface settlement. The soil arch effect around tunnel and the determination of its internal and external boundaries during shield tunneling along curved section were investigated by using this validated model. It was proven that the soil arch induced by excavation exhibited asymmetry, and its influence on tunnel was analyzed. Result With the increase of stratigraphic loss rate, the scope of loosened zone above tunnel expands. The disturbance presents a typical inverted triangular distribution, with a soil arch zone where stress redistribution occurs above the loosened zone. The locations of two points, at which the tangential stress and the tangential stress recovery to 90% of the initial tangential stress, are taken as the internal and external boundaries of soil arch. The soil above tunnel could be divided into three zones from bottom to top, i.e., loosened zone, soil arch zone, and stable zone. The over-excavation of curved tunnel leads to increased asymmetric stratigraphic loss and more significant changes in the direction of the maximum principal stress, which is different from that induced by straight tunnels, thereby forming an asymmetric arched stress transfer trajectory, i.e., an asymmetric soil arch. Moreover, the asymmetric soil arch results in a greater reduction in radial stress. Conclusion The study findings provide reliable numerical basis and theoretical reference for the stress evaluation on lining structures, support design, and construction control of curved shield tunnels.