Tunnelling in Chalk: Groundwater Monitoring and Risk Assessment

Case Study

A construction company wanted to construct a tunnel close to a number of public water supply wells in the London region. A central demand of the project was the need to minimise environmental impacts on groundwater. ESI played a key role in the engineering design process allowing the construction to cause no significant impacts on the public water supply boreholes. Rapid events (such as those following rainfall) were monitored to provide early warning of problems caused by the tunnelling.

Tunnelling in ChalkThe construction company Morgan Est was constructing a tunnel to carry high voltage electricity cables between two electrical substations. The tunnel was mostly excavated in Chalk, with some excavation occurring in the Thanet Sands.

 

ESI was appointed as Morgan Est’s environmental consultant for the project and managed liaison between Morgan Est, regulators and other stakeholders over a four-year period.

 

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 was 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 was transmitted to ESI via a telemetry system on a daily basis.

 

This allowed almost real time monitoring of rapid events such as changes in turbidity following rainfall. Wells were installed with dedicated ‘bladder pumps’ allowing high quality groundwater samples to be taken and subjected to chemical analysis.

 

The data was used to provide early warning of the unlikely event that the tunnelling caused problems. Based on the data collected, ESI undertook a number of risk assessments to ensure that shaft and tunnel construction did not have a detrimental effect on groundwater quality or quantity.

 

The tunnel was constructed with no significant impacts on the sensitive public water supply boreholes.

 

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