Increased traffic delays during peak commuter periods, combined with forecast population growth has resulted in the NSW Government upgrading Sydney’s road network. Motorway tunnels are currently being constructed beneath the city to link major arterial roads to create an orbital road system and ease traffic congestion. Although the tunnels are to be constructed predominately as drained or un-tanked tunnels, grouting will be undertaken during construction to reduce groundwater inflows to maintain the operational inflows below the design criteria of 1L/sec/km. Ongoing passive groundwater inflow to the tunnels during the construction and operations phases will cause impacts to the local hydrogeological regime, including groundwater losses, groundwater drawdown, impacts to groundwater users and groundwater dependent ecosystems and potentially saltwater intrusion. The palaeochannel aquifer, underlying some of the modern creeks are composed of unconsolidated sediments are hydraulically linked to the tunnels, and are also likely to be impacted.
Proposed tunneling depths for the WestConnex projects including the M4 -East, New M5 and M4-M5 Link extend up to 80 metres below ground surface. To support the groundwater impact assessments geotechnical and hydrogeological field investigations included extensive drilling programs along the alignment, packer testing, construction of monitoring wells, groundwater level and quality monitoring and laboratory testing of core to measure permeability and porosity. Fully cored boreholes extend up to 100 metres depth. Baseline data was used to characterize groundwater and develop a hydrogeological conceptual model. This conceptual model formed the platform for a numerical groundwater model that simulated the hydrogeological conditions along the alignment and was used to predict potential groundwater impacts. At St Peters Interchange a cut-off wall was designed to reduce leachate generation by restricting groundwater inflow from the Botany Sands aquifer to the former Alexandria Landfill. The outcomes of the hydrogeological investigations were used to minimize groundwater impacts and develop mitigation measures.