A regional scale groundwater monitoring programme for the Crossrail project: strategy and implementation

Summary

Crossrail is a 21km tunnelling project under London from Paddington in the west and Pudding Mill Lane and Plumstead in the east. The entire sequence of London Basin geology is traversed along the route including the deep aquifer. Ground investigations began in the first phase of Crossrail during the 1990’s and continued through the 2000’s finally finishing in 2011. A total of 1150 groundwater monitoring installations were constructed during some 40 phases of ground investigation. The installations were monitored frequently during individual phases of ground investigations but less so afterwards. A project wide, co-ordinated strategy was set up in order to maintain the existing network, carry out remedial repairs, replace critical installations that had been lost, damaged or become unreliable and ensure that the monitoring continued in a co-ordinated manner to provide a robust groundwater baseline prior to construction and any dewatering/depressurisation that was required. The monitoring was carried out over several years to demonstrate seasonal and other temporal effects and allow a project wide comparison of results that could indicate the effects and influence of faulting or other geological features, for example, the presence of sand channels.

The quality of the data was reviewed centrally for quality control and for both regional and local scale interpretation, culminating in the production of a single, periodically updated ground water monitoring report for the project. A single company was given the task of carrying out the monitoring to a specification which included all the different kinds of installations that had been constructed over the years. A comparison of the different forms of monitoring installations, in terms of performance, relative cost, ease of installation, and use is discussed.

The strategy adopted by the project included the setting up of a series of sentinel wells in order to monitor third party contamination sources. The project took no responsibility for the third party sources but if there was a risk of mobilisation through interaction with construction dewatering, the sentinel system would provide advance warning. This would provide sufficient time to allow mitigation measures to be put into place without cessation of construction dewatering thereby minimising risk to the project.