Central Interceptor – How does groundwater modelling and instrumentation stack up in the real world
A new wastewater tunnel referred to as the Central Interceptor (CI) will collect stormwater and wastewater overflows from the Auckland isthmus area and transfer them to the Māngere Wastewater Treatment Plant. This will increase the wastewater network capacity by reducing the overflows and make the system more resilient. Numerical and analytical groundwater flow models representing the spatially varying geological settings and different construction methodologies were developed to assess the potential groundwater inflows and drawdown from the construction of the shafts and tunnels. Potential adverse effects arising from groundwater drawdown were assessed to demonstrate that the construction can meet the consented envelope of effects. The modelling results were also used to inform the construction planning in terms of groundwater inflow control and the setting of suitable trigger levels for groundwater drawdown and ground settlement that is part of a project wide monitoring network. Installed instrumentation includes real time data reporting and visualisation, that facilitates monitoring of the construction and consent compliance, and also provides feedback to the groundwater modelling should model updates be required. This paper highlights the lessons learnt during the construction of the CI through the eyes of installed instrumentation and how groundwater monitoring and modelling could complement each other, including:
- Groundwater response during excavation of the Māngere Pump Station (MPS) in high permeability aquifers, and
- Groundwater behaviour during launch of the tunnel boring machine in a confined, controlled environment.