Protective effect of partition excavations of a large-deep foundation pit on adjacent tunnels in soft soils: a case study

The safety of metro tunnels is seriously threatened when protective measures are not well organized during adjacent foundation pit excavations in soft ground. To date, the protective effects obtained from adopting partition excavations and a deep diaphragm wall in large-deep excavations have not been fully understood. In this study, 3D numerical analyses incorporating the hypoplastic constitutive model, which can take into account the small strain stiffness of soil, were performed to reproduce tunnel responses subjected to lateral excavations. The mechanism of crack development in tunnel lining, the protective effects of partition excavations, and diaphragm walls are investigated in sequence. The obtained results indicate that crack development in tunnel lining is mainly dominated by lining deformation in the longitudinal direction, instead of deformations in the radial and circumferential directions. Through firstly excavating the nearest excavation to the tunnel that is small in size, the tunnel displacement and bending moment are reduced, since the system rigidity of the nearest excavation increases significantly after applying supporting structures. By contrast, such reduction is found to be detrimental when the nearest excavation is large in size and is performed at the end. This indicates that a comprehensive consideration of both the order and the size of partition excavations is important. The inhibiting effect on the tunnel displacement caused by increasing the depth of the diaphragm wall is found to function nonlinearly, depending highly on its referred depth. Additionally, this effect is more effective than increasing the width of the diaphragm wall under constant volume conditions.