Ground Investigations – Piezometers below the Thames

Hydrogeological data from the tidal river zone was required to fully characterise ground risk and accurately inform the tunnel design. A broad grid of 125 geotechnical boreholes was installed across the site, including 12 installed below the River Thames (directly drilled and deployed from Jack-up), utilising Geosense Piezometers to determine the effects of water table drawdown in the ground layers.

A weighted armoured marine cable running along the riverbed linked the Vibrating Wire Piezometers (VWPs) to a Geosense data box on the river wall. The box enabled sensor data to be linked in near real-time to the monitoring Contractors portal, which was available 24/7 to key stakeholders.

Site build – Geosense supply

Geosense supplied sensors for baseline monitoring 12 months before the start of site works and on the main build in both Greenwich and Silvertown, monitoring the construction of the cut and cover, the open cut, and the tunnel portal worksites. Geosense provided over 600 in-place Inclinometers (IPIs), effectively managing stock availability despite simultaneous high demand from the HS2 project. The IPIs allowed automatic and continuous measurement of deformation profiles within the inclinometer casings in vertical, inclined, or horizontal orientations.

Push-in Pressure Cells were installed between the TBM and flood defence walls. Geosense MEMS Tilt Beams were also welded to steel props for some of the structural elements of the build. These provided a differential displacement profile of the structure between anchor points.

Monitoring third-party existing infrastructure

Geosense sensors were deployed as part of a wider package of monitoring existing infrastructure across the site. This included the Eastbound and Westbound Jubilee Line tunnels beneath the site, the A1011 Silvertown Way viaduct, and Thames Wharf River Wall in Silvertown, where rope access teams installed prisms and Geosense tilt sensors on the north face.

Accessible data

Over 150 wireless dataloggers/communications nodes and three communications gateways from Geosense and other wireless providers were used on the project, linking the data from the IPIs and tilt sensors to a central monitoring system. Here, the client and third-party stakeholders had access to all the monitoring data in one place, along with automated monitoring reports and monitoring alarms in near-real-time.