ESI used a groundwater model of the basal Sands and Chalk underlying central London to assess the dewatering impacts of tunnelling as part of the construction of the Channel Tunnel Rail Link (CTRL).
A number of boreholes were to be installed for dewatering purposes whilst the tunnel was being driven. Due to access constraints, the selection of borehole locations was difficult. The model was used to optimise borehole locations to ensure that adequate dewatering of the tunnel was achieved.
The groundwater model was based on MODFLOW, but a new well package was used which converted boundary cells from constant rate to constant head when the water level in the cell falls to the elevation of the pump intake.
This prevents the model from over estimating the amount of drawdown that can be induced by the pumping wells.
Actual time series data for groundwater levels prior to dewatering and during dewatering and groundwater levels and pumping rates for the dewatering system were incorporated into the model and used as a basis for calibration.
The model simulated actual and predicted future drawdown and recovery over a period of 6 years. The results were used to predict recovery and groundwater levels at various points along the trajectory of the tunnel where subterranean engineering works were continuing.