Groundwater control around a large basement

Melbourne’s Crown Casino was constructed on a site bordering the Yarra River and underlain by problem soils of the Coode Island Silt Formation. The development needed to provide two levels of basement car park over the entire site. An innovative approach to groundwater control around the excavation was required to avoid depressurisation of adjoining soils, leading to settlements. Analysis showed a conventional bentonite cut-off wall would still allow depressurisation by lateral flow through the Coode Island Silt during the construction period. The high cost and construction difficulty of a very low permeability wall mitigated against it. An hydraulic wall was proposed in conjunction with a conventional cut-off wall. This comprised a curtain of wick drains surrounding the cut-off wall and charged with water. Control of seepage through an underlying aquifer by a cut-off wall was considered, but a more cost-effective method using recharge by wells was adopted when shown necessary. Monitoring of groundwater pressures around the site showed that the maximum change in water pressure was less than 1 m head, the design criterion. Part way through construction, recharge was initiated when monitoring of the deep aquifer showed pressure reduction attributed to vertical leakage through a basalt tongue.