Tunnelling in Chalk: Groundwater monitoring and risk assessment

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Courtesy of ESI Ltd - Environment Specialists

Morgan Est is constructing a tunnel to carry high voltage electricity cables between two electrical substations.  The tunnel will mostly be excavated in Chalk, with some excavation occurring in the Thanet Sands.

ESI was appointed as Morgan Est’s environmental consultant for the project and has managed liaison between Morgan Est, regulators and other stakeholders over the last four years.

A central demand of the project was the need to minimise environmental impacts on groundwater.  To this end, a large amount of effort was put into ‘engineering out’ any potential for an impact at the design stage.  ESI played a key role in this process liaising with engineers and regulators.
The tunnel will be constructed close to a number of sensitive public water supply wells.  Purpose drilled monitoring wells were equipped with sensors to monitor groundwater level, electrical conductivity and turbidity.  For a subset of the wells, the data collected is transmitted to ESI via a telemetry system, on a daily basis. 

This allows almost real time monitoring of rapid events such as changes in turbidity following rainfall events.  The wells are also installed with dedicated ‘bladder pumps’ allowing high quality groundwater samples to be taken and subject to chemical analysis. 

The data are being used to provide an early warning in the unlikely event that the tunnelling causes problems.  Based on the data collected, ESI has undertaken a number of risk assessments to ensure that shaft and tunnel construction will not have a detrimental effect on groundwater quality or quantity.

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