Centrifuge And Numerical Studies Of The Influence Of Basement Excavation On The Underlying Tunnel

Rapid urban development in cities often requires basement excavation close to or directly above existing tunnels. Some reports of correlative projects have indicated that an excavation may induce upward displacement and deflection on tunnel lining and affect the serviceability and safety of the tunnel. In soft soil, more attention should be paid on this complex interaction between a basement excavation and an existing tunnel. The research in this theis aims to study the mechanism of this complex interaction and investigate the influence factors by means of the finite element and centrifuge model tests. The research can futher the understanding of this complex interaction and provide reasonable advice to protect the tunnel lining.Based on the research, following conclusions have been obtained:1.A series of two-dimensional finite element analyses were performed to study the influence of basement excavation on underlying tunnel in soft soil. The following conclusions are drawn: (1) Excavation-induced displacements of tunnels are consistent with the soil movement around excavation; (2) Excavation-induced tunnel distortions lie on the soil stress changes around tunnel lining during excavation; (3) For the tunnels located in the centerline of excavation, maximum diameter change does not necessarily occur in the tunnel with smallest vertical distance to the formation level of excavation. For the excavation with smaller excavation width(2B_e/h_e≤10/3),the magnitude of relative deformation of the tunnel located in the centerline of excavation is approximate to that located close to the diaphragm wall with the same tunnel cover depth. For the excavation with larger excavation width(2B_e/h_e≤16/3),the relative deformation of the tunnel close to diaphragm wall is larger than that located in the centerline of the excavation with the same tunnel cover depth; (4)The bending moment changes of the cross sections of the tunnels are consistent with the tunnel relative deformation, and the mangnitude of the bending moment changes are linearly proportional to the relative deformation.(5)The consolidation effect of excavation on the vertical displacement of underlying tunnel is significant due to the dissipation of excess negative pore water pressure aroud tunnel lining. The influence of consolidation effect of excavation on tunnel distortions is trivial. (6)Excavation-induced tunnel distortion will decrease as the increase of the elastic modulus of the tunnel lining. After certain magnitude of the elastic modulus of the tunnel lining ia achieved, the tunnel distortion during excavation will decrease insignificantly as the increase of the elastic modulus of the tunnel lining.2. A series of three-dimesional finite element analyses were carried out to study the tunnel responses during excavation in soft soil. Based on the research, it can be obtained that excavation-induced displacement and distortion of the cross section of the tunnel close to the center face of the excavation( hty=0) is approximate to the result of two-dimensional analysis. The excavation-induced displacement and distortion of the cross section of the tunnel change slightly when 18m and decrease with the increase ofhty when hty≤18m. When exceeds 54m (the location of diaphragm wall), the excavation-induced displacement and distortion of the cross section of the tunnel will decrease significantly with the increase ofhty . The tunnel cross section located outside the excavation with hty≥81m is insignficantly influenced by basement excavation.3.The result of the two centrifuge tests in sand depicts that the bending moment and vertical diameter changes of underlying tunnels with diffirent tunnel location. The numerical simulation of the centrifuge tests were also performed and the following conclusions are obtained: (1)The result of numerical simulation is consistent with the variation tendency of the bending moment of the tunnel cross section in centrifuge tests(;2)For an basement excavation in sand with excavation width of 300mm(24m in prototype) and excavation depth of 150mm(12m in prototype), excavation-induced distortion of the tunnel cross section will decrease as the increase of the tunnel cover depth when the tunnel cover depth is larger than the depth of the bottom of the diaphragm wall. (3) Excavation-induced distortion of the tunnel cross section will decrease remarkly with the increase of the soil elastic modulus around tunnel.