Performance of a Deep Excavation and Its Effect on Adjacent Tunnels in Shanghai Soft Clay

A 267-m-long, 54-m-wide, and 14.9-16.4-m-deep multipropped deep excavation with three cross walls in Shanghai soft clay was constructed at a side of an existing metro station. To investigate the performance of this deep excavation and the effect of this deep excavation on adjacent metro station and shield tunnels, the ground deformation and structure responses were extensively monitored. Based on the field observations, the deflections and vertical movements of diaphragm walls, surface settlements, vertical and horizontal displacements of the up track and the down track within the station and nearby shield tunnels, as well as tunnel convergence were extensively analyzed. By adopting cross walls, jet grouting and large dimension concrete struts, the measured maximum lateral wall deflection and ground deformation were much smaller than those at other sites without installing cross walls in Shanghai soft clay. The maximum excavation-induced lateral wall movements and ground settlement behind the retaining wall were 0.11% He (final excavation depth) and 0.12% He, respectively. Because of stress relief resulting from the deep excavation, the metro station experienced heave with a maximum value of 9.6mm. Due to horizontal stress relief resulting from the adjacent basement excavation, the existing tunnel moved as much as 5.5mm toward the basement. Affected by the horizontal stress reduction around the tunnel lining, the existing tunnel became horizontally elongated by up to 4.9mm.