ABSTRACT
The construction of new tunnels in urban areas can present significant challenges when excavations occur near existing underground structures. It usually necessitates adherence to stringent impact criteria imposed by asset owners. While tunnelling technology can deliver works to small tolerances, it was often impossible to fulfil a no-movement mandate, which may lead to disruptions and delays due to impractical trigger levels in monitoring regimes. This paper presents a case study from the Sydney Metro Central Tunnelling Package, where limits for a highway tunnel ventilation structure located beneath twin TBM (tunnel boring machine) tunnels were redefined in collaboration with the asset owner. Initial analyses indicated that the expected displacements from tunneling would exceed the owner’s original limits but would not cause structural damage to the existing structure. The study concentrated on critical structural aspects that met the performance targets. The findings demonstrated that structural performance was more influenced by relative displacements than the total magnitude of movement. By prioritizing the actual structural effects of movements rather than the perceived impact, revised and more practical trigger levels were developed and accepted. This collaborative and performance-based approach allowed for the continuation of the project while ensuring the integrity of the existing structure.
