The Lower Thames Crossing is a new highway project 40km from central London that is seen as an essential component in the UK’s future transport infrastructure. Located east of the existing Dartford Crossing, the new crossing over the River Thames will benefit the Lower Thames area around Kent, Thurrock, and Essex and will increase capacity from four lanes in each direction (at Dartford) to seven lanes each way.
At the centre of the ambitious project, are two 4km tunnels crossing below the river. The scheme will add 23km of new highway to connect the tunnels to the existing road network.
Murphy Geospatial was asked to complete a comprehensive ground investigation monitoring campaign at the planning stage of the project. The programme included the creation of 400 boreholes, several pump tests, and geophysical surveys at more than 100 locations.
A baseline monitoring campaign was required to be commissioned in the area where the future tunnel portals would be, taking place over several months to enable a better understanding of the ground movement during dewatering pump tests.
Our task was to create a stable control network in an area influenced by tidal movements, from the nearby River Thames, and from the ground dewatering works to monitor the surrounding area which included a live railway line. This was achieved by installing a system with GNSS monitoring stations; a network of robotic total stations installed on concrete monuments and ground prisms throughout the monitoring site.
To achieve a stable project co-ordinate datum, we installed four GNSS stations, equipped with TOPCON MR2 receivers and PG-S1 antennas. Two of the GNSS points were placed approximately 400m from the tracks, where the influence of the tides was supposed to be reduced. The other two GNSS points were placed at the beginning and end of the observed embankment section, to create a link between the monitoring and the remote areas. The GNSS antennas were placed on top of 360-degree prisms on a stable pillar mounted on a concrete cube. Solar panels were installed to power the receivers.
Working in challenging conditions, we were able to set up a reference epoch, three solutions of each baseline, and six two-faced cycles of each station. In total, 24 GNSS baselines and 12 total station cycles were selected. The network adjustments of the GNSS and total station observation resulted in a mean accuracy of:
- ±1.3 mm for the plan point positions
- ±2.1 mm for the heights of the points
For the observation of the prisms along the tracks and the 360-degree prisms of the GNSS points, three TOPCON MS1AXII total stations were installed along the embankment. A fourth total station was placed remotely, giving a “terrestrial link” between the remote stable points and the monitoring points along the tracks.
Our highly-skilled monitoring engineering team installed an automated deformation monitoring system. This gave us detailed information on daily variations and the tidal effects of the railway along the River Thames embankment, ensuring that we provided vital support in these areas during the planning phases of the Lower Thames Crossing project.
We are renowned for our knowledge and experience in providing a comprehensive range of monitoring services to clients and delivering verified geospatial data recorded over time.
The observation data from the total stations and GNSS receivers was captured and transmitted fully automated, making use of the TOPCON Delta monitoring system. The processing was running fully automated by the Delta Watch software applying rigorous geodetic network adjustments – combing terrestrial and GNSS-based observation in a parametric model.
The success of this project underlines our reputation in delivering services to equip our clients with the data and information they need to make decisions at every stage. We draw on the experience and skills of our people to deliver the certainty that our clients can trust.
For more information on this project, please contact Andrew Masters at email@example.com or +44 203 598 3775