Groundwater flow through rocks is mainly governed by the fracture permeability, which is related to the nature, intensity and interconnectivity of the rock fractures. Excavations and underground openings in rock are frequently affected by the groundwater inflows that incurs heavy maintenance costs, delays in construction, threats to the natural groundwater resources and groundwater drawdown related risks.
A proposed multi-storey mixed use commercial and residential building at Parramatta NSW encountered significant groundwater inflow (approximately 12 L/s) during the excavation of the 24 m deep basement. The large inflow was due to sheared and crushed zones which is associated with faults and folds. Treatment and disposal of the encountered groundwater was not economically feasible. To address this issue, a grouting programme was employed to seal the rock fractures below the excavated depth. The grouting programme aimed to create a “grout curtain” to permanently reduce groundwater inflow into the basement excavation. The initial grouting stage targeted known sheared/crushed zones based on the assessment of available site investigations and mapping of the exposed rock faces in the excavation. This was followed by review of grouting records to identify deeper rock fractures and to inform the subsequent target grouting to help achieve an adequate grout curtain. This paper also discusses how the development of a new graphical tool help in identifying areas offering high potential for reduction in inflow. The method successfully achieved reduction of groundwater inflow by 85%. Post-construction back analysis of the grouting records also provided useful insight into the hydraulic properties of the fractured rock aquifer in Parramatta.