Field Monitoring of an Existing Cut-and-Cover Tunnel between Two Large-Scale Deep Excavations

Abstract

The safety of operational tunnels impacted by nearby deep excavations has led to growing interest in the field of practical civil engineering. Many researchers have studied the performance of shield tunnels during adjacent excavation processes, but few discussed the behavior of a cut-and-cover tunnel bounded by a new excavation. Based on the field-monitoring results of a case in Shanghai, this paper investigates the performance of an existing cut-and-cover tunnel between two large-scale deep excavations. In this case, the diaphragm walls of the cut-and-cover tunnel are also used as retaining walls of the new deep excavations. Vertical displacements of the tunnel and different diaphragm walls, variation of underground water level, and the stratified settlement of subsoil close to the tunnel are monitored and analyzed. The results show that the cut-and-cover tunnel uplifted due to the nearby deep excavations while the shield tunnels settled. The vertical displacement distribution curves for the cut-and-cover tunnel were bell-shaped. By installing the partition walls, the direct influence of the large pits was avoided so that the impact of the large-scale excavation on the tunnel was reduced. Because all diaphragm walls (partition walls, cross walls, and retaining walls) and the cut-and-cover tunnel were connected, responses of the tunnel were mainly affected by the diaphragm wall system. The effect of the soil heave inside the excavation on the partition walls can be transferred to the tunnel by cross walls. To control the displacement of the cut-and-cover tunnel, the cross walls should be constructed after the excavation of large pits, and dewatering in the confined aquifer also has an active effect to reduce the heave.