More and more subway tunnels have been planned and constructed underneath the ground of urban cities to relieve the congested traffic and to reduce environmental impacts. The clear distance between tunnels becomes closer and closer. Based on the previous studies, potential damage may occur in existing tunnels if new tunnels were constructed too close. So far, previous studies mainly focused on the tunnel-tunnel interactions with circular tunnels. However, the deformation mechanism of non-circular shaped such as horseshoe-shaped tunnel can be totally different with that of a circular tunnel. Currently, no systematic study has been carried out to investigate the interaction between existing horseshoe-shaped tunnel and new tunnels. The main objective of this research is to study th...[ Read more ]

More and more subway tunnels have been planned and constructed underneath the ground of urban cities to relieve the congested traffic and to reduce environmental impacts. The clear distance between tunnels becomes closer and closer. Based on the previous studies, potential damage may occur in existing tunnels if new tunnels were constructed too close. So far, previous studies mainly focused on the tunnel-tunnel interactions with circular tunnels. However, the deformation mechanism of non-circular shaped such as horseshoe-shaped tunnel can be totally different with that of a circular tunnel. Currently, no systematic study has been carried out to investigate the interaction between existing horseshoe-shaped tunnel and new tunnels. The main objective of this research is to study the response of an existing horseshoe shaped tunnel due to new twin-tunnel excavation

Based on the geometry of a real project, two series of three-dimensional centrifuge model tests were designed and carried out to simulate the advancement of two new circular tunnels perpendicularly undercrossing an existing horseshoe shaped tunnel in dry sand. The effects of settlement joint and pillar depth on crossing tunnel interactions were investigated. Moreover, three-dimensional numerical back-analyses were conducted using an advanced hypoplasticity soil model to assist in the interpretation of the test data. Furthermore, a thorough numerical parametric study was carried out to explore the shape effect on various aspects of multiple tunnel interaction.

More significant deformations and bending strains both in the longitudinal and transverse directions were observed in an existing horseshoe shaped tunnel than those of an existing circular tunnel. The vertical elongation and horizontal compression of existing horseshoe shaped tunnel are 81% and 50% larger than those in existing circular tunnel, respectively. This is attributed to the larger transverse bending strain distributed along the circumference of horseshoe shaped tunnel. The hoop stress cannot transfer continuously at two corners in horseshoe shaped tunnel. As a result, the shear stress needs to be mobilized around the corners to equalize the hoop stress. The bending strain along the bottom plate of horseshoe shaped tunnel is 40% larger than that in the lower part of circular existing tunnel, especially around the corners.

For existing jointed tunnel simulated, the longitudinal flexural rigidity of existing tunnel was substantially reduced. As a result, the settlement of existing tunnel increased by more than 100% while the longitudinal bending strain and deduced shear stress dropped by more than 60%.