Geotechnical challenges of a deep basement excavation: a case study

A 59 storey tower underlain by a 30m deep basement is currently under construction within the expanding central business district of Parramatta, NSW. The tower is surrounded by and abuts two multi-storey buildings with basement levels, a heritage listed church, a 100 year old water main, and is set back approximately 50m from the Parramatta River. A comprehensive site investigation program was carried out to characterise the complex geotechnical conditions of the site, including deep cored boreholes, insitu permeability testing and borehole imaging. Due to the challenging ground conditions encountered, proximity of the neighbouring structures and shallow groundwater, the shoring system consists of secant pile walls terminated above bulk excavation level in competent bedrock, below which a grout curtain extends approximately 50m below surface level to limit seepage inflows. An Instrumentation and Monitoring (I&M) program was implemented to assess actual basement wall movements against those predicted from the numerical analyses.