Extensive monitoring of groundwater levels and surface settlement is being carried out during construction of the Thames Tunnel and its approaches across low-lying marsh areas at Swanscombe and West Thurrock.
Settlement could be induced by either the tunnelling or the temporary dewatering being carried out as part of the work. Ten metres of soft alluvial clay and peat lie over permeable gravels and chalk at the site. Water levels are tidal and close to ground surface in the gravels and chalk.
Numerous piezometers and surface settlement stations have been installed in the surrounding area up to 1km from the works to monitor changes in ground levels and in the groundwater regime within each soil type, and recharge wells have been installed on the Thurrock side to mitigate against settlement.
Ground settlement caused by the tunnelling operations is also being monitored by several arrays of surface survey stations.