Performances of Shield-Driven Tunnels Subjected to Excavation of a Large-Scale Basement in Soft Soils

This paper reports a field case of the excavation of a large-scale basement parallel to twin existing shield-driven tunnels in soft soils. A comprehensive field monitoring program was conducted to measure the performances of the shield tunnels during excavation. The trapezoid-shaped basement with an excavation area of 26,982 m2 was excavated at the side of twin existing shield tunnels using a bottom-up excavation sequence. The tunnel horizontal and vertical displacements, and the tunnel convergence, were extensively measured. Based on field observations, the existing shield tunnels developed horizontal movements toward the excavation side, and settlements in the shield tunnels were observed due to the stress relief of the excavation of the large basement. The tunnel rings within the excavation width generally experienced horizontal elongation as a result of the reduction in the lateral earth pressure because of excavation. During the excavation of the elevator shaft, soil flow occurred, which resulted in a sudden and continuous increase in the tunnel horizontal displacements, tunnel settlements, and tunnel convergences. The down track suffered more serious adverse impacts than the up track, with eight leakages and 61 cracks observed in the down track. The newly induced cracks mainly occurred in a range within the excavation width, and were generally consistent with the distributions of the tunnel horizontal displacement and tunnel convergence. The measured data for the normalized tunnel horizontal and vertical displacements mainly fell between delta h/|delta hm| = delta v/|delta vm| - 0.5 and delta h/|delta hm| = delta v/|delta vm| + 0.4, and it was difficult to establish a direct linear relationship between them. However, a fairly good relationship between the tunnel convergence and horizontal displacement can be observed, the measured data mainly falling along delta c/|delta cm| = delta h/|delta hm|. An approximately linear relationship can be roughly found between the normalized tunnel vertical displacement and the tunnel convergence. The data mainly fell between delta c/|delta cm| = delta v/|delta vm| + 0.2 and delta c/|delta cm| = delta v/|delta vm| - 0.3.