Effects of construction sequence and cover depth on crossing-tunnel interaction

An appropriate construction sequence for crossing tunnels can help minimize the adverse impact on the tunnel that is constructed first (considered as the existing tunnel). However, the influence of construction sequence on crossing-tunnel interaction is complex. Two pairs of three-dimensional centrifuge tests were carried out to investigate the effects of construction sequence on crossing-tunnel interaction. In the first pair of tests, the new tunnel was excavated beneath the existing tunnel in a reference test, while in the other test the new tunnel advanced above the existing tunnel. To study the effects of cover depth on the construction sequence, the depths of the existing and new tunnels were increased in the second pair of tests. An advanced hypoplasticity constitutive model with small-strain stiffness was adopted to back-analyze the tests. The existing tunnel was found to be vertically compressed when the new tunnel was excavated underneath, but vertically elongated when the new tunnel advanced above. This is because the reduction of stress acting on the existing tunnel in the horizontal direction was larger than in the vertical direction when the new tunnel was constructed beneath. On the other hand, the decrease in vertical stress on the existing tunnel was larger than the horizontal stress reduction when the new tunnel was excavated above. This behavior was observed in both pairs of tests, irrespective of the cover depths of the tunnels. As the cover depths of the existing and new tunnels increased, settlement of the existing tunnel due to the new tunnel construction beneath decreased. This is because with the larger cover depths of the tunnels, the increase in mobilized shear stiffness of the soil dominated the increase in stress relief caused by the tunnel excavation.