ESI, now part of Stantec, provided groundwater modelling of the Basal Sands and Chalk underlying central London to assess the dewatering impacts of tunnelling construction for the Channel Tunnel Rail Link (CTRL).
At the driving stage of tunnel development a number of boreholes were required for dewatering but due to access constraints selecting locations proved difficult for the client. ESI were asked to provide a groundwater model to optimise borehole locations and ensure adequate dewatering of the tunnel was achieved.
A groundwater model was developed based on MODFLOW, with a new well package for converting boundary cells from constant rate to constant head when water level in the cell falls to the elevation of the pump intake. This subsequently prevented the model from over estimating the amount of drawdown induced by the pumping wells.
Incorporated into the model and used as a basis for calibration were actual time series data for groundwater levels prior to and during dewatering and also groundwater levels and pumping rates for the dewatering system.
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 taking place.
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