Long-term performance of cast-iron tunnel cross passage in London clay

A series of 3D soil-fluid coupled finite element analyses was conducted to examine the long-term tunnel behaviour of an old cast-iron cross passage in stiff London clay. In the proposed geotechnical FE model, an advanced critical state constitutive model was employed to simulate complex soil behaviour, whereas the details of the tunnel linings was simplified using shell elements. The computed time-dependent soil load derived from the geotechnical model was then applied to a structural finite element model where the details of the cross passage structure such as bolted-joints and tunnel segments were explicitly modelled. This proposed semi-coupled soil-structure model shows agreement against field observations and it was found in this particular case that the structural condition evaluated from the FE model is more critical than that of the conventional bedded ring method. Results show that the surface ground settlement is governed mainly by the twin tunnel construction and the presence of the cross passage does not increase the settlement. On the other hand, the construction of a cross passage affects the structural performance of the adjacent running tunnel in both short term (undrained) and long term (soil consolidation). In particular, the critical location is identified to be the tunnel segment adjacent to the opening at the axis level and its stresses and deformation increase with consolidation time.