Effect of the sand relative density on the tunneling-induced ground settlement in three-dimensional space

Abstract

Relative density (D_r) is a critical factor that determines the physical and mechanical properties of cohesionless soil. The existing literature investigated its effect on the transverse ground settlement trough induced by tunnel excavation, but a comprehensive understanding about its effect in three-dimensional space has not been provided. In this work, a series of shield excavation model tests was carried out in sandy ground. Two filling methods were used to control the relative density of the sand in the strongbox, and a scaled shield machine whose cutterhead can advance and rotate was employed to simulate the shield construction procedure. The results show that when D_r = 35 %, ground surface settlement is produced before the cutterhead reaches the monitoring section, whereas when D_r = 55 % or 80 %, settlement does not appear until the cutterhead has already passed. As D_r increases, the surface settlement trough in the transverse section becomes shallower and narrower, with its shape evolving from a normal distribution curve to a triangle. The ground volume loss increases with decreasing depth when D_r = 35 %, indicating contraction of the sand above the tunnel. In contrast, dense sand experiences shear dilation, which restricts the spread of volume loss away from the tunnel excavation section. Additionally, the soil arching effect is introduced to explain the hysteresis observed in the surface settlement relative to the position of the cutterhead. According to the test results, different measures should be taken for tunnel construction in the ground with different compactness degree.