Construction of Large-Span Twin Tunnels Below a High-Rise Transmission Tower: A Case Study

The structure on the ground bears impact from the tunneling-induced settlement, especially when the tunnel is shallow and its span is large. This paper presents the construction of Lijiachong twin tunnels below a high-rise transmission tower and the prevention techniques to avoid the failure of the tower. The span of each tunnel is about 18 m, and the tunnel embedded depth below the tower foundations is only 15.5 m. There was a high risk of influencing the performance of the tower during the tunneling. To reduce the risk of collapse of the transmission tower, the available methods in field to reinforce the tower foundations were proposed. For engineering cost and construction convenience, the separated pile foundations were connected with framed girders, resulting in that the separated foundations worked as a whole. The effectiveness of the reinforcement was evaluated by monitoring deformation and stress in the tower, indicating that the measured values were lower than their respective allowable ones. With reinforcing the tower foundations using the framed girders, the Lijiachong twin tunnels safely passed beneath the transmission tower, and the continuous performance of the transmission tower was ensured.