Sheltering effect of vertical twin tunnel excavation for different horizontal distances on the ground and pipelines

Subway tunnels are often designed as twin or multiline overlapping tunnels. The sheltering effect from the upper tunnel on the excavation of the lower tunnel is obvious. However, the mechanism of this sheltering effect has not been examined via detailed systematic research. Therefore, in this paper, centrifugal model tests on the excavation of vertical twin tunnels under typical sheltering conditions were carried out. Then, relevant numerical simulations were conducted, focusing on the change law of the ground and pipeline settlements and the principal stress and shear strain of soil under different sheltering conditions. Combining the results of the centrifugal model tests and relevant numerical simulations, for twin tunnels in an overlapping arrangement, the maximum ground settlement is caused by the excavation in the sequence of the upper tunnel first. However, for twin tunnels in a shouldering arrangement, the opposite sequence is caused by the sheltering effect. Sheltering effect is most prominent when the twin tunnels are in an overlapping arrangement. When the horizontal distance between the twin tunnels is 1DT, the ground settlement caused by excavation in the different sequences is the same. It is a critical state in which the sheltering effect is balanced with a disturbance effect. When the horizontal distance is from 1DT to 1.8DT, the sheltering effect gradually decreases, and the disturbance effect dominates as the distance increases. When the horizontal distance is more than 1.8DT, the sheltering effect basically disappears, and only the disturbance effect exists. Finally, the sheltering coefficient C is proposed, which is related to the horizontal distance between the twin tunnels. By introducing the stochastic medium theory, an equation is proposed, which can quantitatively predict the ground settlement by excavation under sheltering effects at different horizontal distances.