Effects of Pillar Depth and Shielding on the Interaction of Crossing Multitunnels

Abstract

Any new tunnel excavation may damage adjacent existing tunnels in congested cities. To evaluate the impact of new tunnel construction on nearby existing tunnels, a series of three-dimensional centrifuge model tests were conducted with numerical backanalyses using an advanced hypoplasticity constitutive model. The influences of the pillar depth-to-diameter ratio (P/D) on two-tunnel interaction and the effects of shielding on three-tunnel interaction were investigated. The maximum measured settlement of an existing tunnel caused by a new tunnel excavation at P/D of 0.5 underneath was approximately 50% larger than that when P/D was 2.0. This is attributed to a smaller mobilized shear modulus, resulting from a larger reduction in confining stress of soil acting on the invert of the existing tunnel in the former than in the latter. When the new tunnel was excavated underneath two perpendicularly crossing tunnels, the lower existing tunnel “shielded” the upper one from the influence of tunnel excavation. As a result, the settlement of the upper existing tunnel was 25% smaller than in the case without the shielding tunnel.