Jet grouting campaign to minimise tunnelling risks in Coode Island silt

Melbourne Water’s Hobsons Bay Main Sewer Project will divert around 30 percent of Melbourne’s wastewater and enable remediation of the existing sewer that was built in the 1960s. This upgrade includes a connection tunnel between the upstream connection utility hole on the existing main sewer and the siphon inlet vortex, which connects to the on-grade conduit under the Yarra River. This upstream connection will enable isolation and flow diversion between the existing conduit and new siphon. Site investigations indicated very soft to soft Coode Island Silt in the tunnelling zone on the Yarra River East bank, necessitating measures to mitigate substantial tunnelling and ground settlement risks. The short drive length for the connection tunnel made tunnel boring machines (TBMs) and pipe jacking cost-prohibitive and impractical, and although Victoria’s history includes weak soil tunnelling with cast-iron shields and hand excavation, safety awareness has made that approach effectively obsolete. Moreover, Victoria’s recent expertise in larger diameter tunnelling is generally through siltstone and occasionally in more competent soils. Given these constraints, jet grout columns were installed to create a grouted mass in the tunnelling zone prior to sequential excavation method (SEM) tunnelling. The tunnel temporary support design accounted for two scenarios: (1) fully treated and (2) major zones of untreated ground underneath an abandoned sewer above the tunnelling zone. Inspections were undertaken after every excavation advance to observe ground improvement quality and predict support type selection. This paper summarises this portion of the project from initial design through design development and construction.