Prediction of Ground Settlement Induced by The Construction of Twin Piggy Back Tunnels

The construction of subway tunnels represents an effective solution to address transportation demands in large cities. During the planning, design, and construction stages, subway tunnels in urban areas are often arranged as two closely spaced parallel tunnels to facilitate efficient operation. However, the close spacing of twin tunnels induces mechanical interactions between them, which alter the displacement of the surrounding soil and affect both the magnitude and distribution of ground settlement. In this study, the finite element method was employed to predict and analyze the settlement trough induced by the construction of piggyback twin tunnels. The results demonstrate that piggyback twin tunnel excavation produces a larger settlement trough compared with single - tunnel excavation. In particular, the findings reveal that the construction sequence of the two tunnels significantly influences the displacement profile of the settlement trough. Specifically, the second tunnel is excavated within a soil mass that has already been disturbed and weakened by the excavation of the first tunnel, resulting in a volume loss that differs from that of the first tunnel. This study also investigates the key parameters affecting the ground settlement trough, including construction sequence, volume loss, tunnel spacing, and tunnel depth. These findings provide valuable insights for the planning, design, and construction of subway tunnels in urban environments.