Structural Health of a concrete tunnel lining under complex in situ loading

Abstract: Supporting structures were designed for a utility tunnel in hard rock which was to be intersectedby some road tunnels. With the intersections involving partial overlapping between the idealcross-sections, a concrete lining and post-tensioned steel cables were added to support theutility tunnel prior to the excavation of the road tunnels. The objectives in this work wereto identify the structural behaviour of the system, assess the present state of the supportingstructures and suggest an effective monitoring strategy. Preliminary information was collectedfrom the original drawings, technical specifications as well as from site visits. Using on thefinite elements software Comsol Multiphysics, suitable models were built to represent thetunnel intersections and to test hypotheses. The significant uncertainties in the problem wereaddressed by studying limit cases and discussing their results. A measure of validation wasgained from comparing cracks in concrete on site with cracking hotspots predicted by themodel. It was concluded that the behaviour of the lining is not compatible with that of cablesuspended structures and that concrete is subject to relatively high state of stress. Monitoringcable forces was found to be an ineffective strategy for identifying changes in the structuralsystem. Monitoring the evolution of cracks in concrete was found to be the most feasiblestrategy.