Tunnelling of the new cable tunnel below the Thames begins
National Grid, the transmission owner in England and Wales, is expanding its high-voltage grid as part of The Great Grid Upgrade. A key project is a 2,200- meter-long cable tunnel under the Thames from Gravesend to Tilbury. Herrenknecht supplied a tunnel boring machine for the project, which began tunnelling at the start of May. The launch and reception shafts for the cable tunnel are being constructed using Europe’s largest shaft sinking system, also supplied by Herrenknecht.
Demand for electricity is expected to increase significantly and is forecast to double by 2050 as homes, businesses, public services, transport, and industry increasingly switch to electric power. National Grid’s Great Grid Upgrade is the largest overhaul of the electricity network in generations, helping to strengthen energy security by connecting more home-grown energy from more affordable sources.
Herrenknecht supplied the machinery for Grain to Tilbury, a key project in the grid upgrade. After a Herrenknecht Vertical Shaft Sinking Machine had excavated the launch shaft, tunnelling began at the start of May on the 2,200-meter-long cable tunnel. On behalf of National Grid, the Ferrovial Bemo Joint Venture, consisting of Ferrovial Construction and BEMO Tunnelling UK Ltd., is constructing the tunnel for
the 400 kV high-voltage cable.
Tunnel boring machine specifically tailored for the project
Herrenknecht AG has received an order from the Ferrovial Bemo JV for a tunnel boring machine (TBM) specifically tailored to the project’s requirements. The TBM of the Mixshield type has a diameter of 4,730 millimeters, is 108 meters long, and weighs 464 tonnes in total. Beneath the Thames, the TBM is predicted to encounter a mixed ground of chalk and flintstones up to a compressive strength of 1000 MPa. Additionally, the high water pressure beneath the Thames poses a challenge. The construction site is located at the river’s mouth where it flows into the North Sea.
Accordingly, the TBM is designed to withstand a water pressure of 4.5 bar, partly due to the tidal range. While water and earth pressure prevail at the cutting face, the rest of the machine is maintained at atmospheric pressure. The highly sophisticated Mixshield, a technology developed by Herrenknecht, offers maximum tunnelling safety thanks to precise tunnel face support. It is also sealed against water pressure 41
meters below the Thames with a multiple sealing system. For work at the tunnel face, such as changing the cutting tools on the cutting wheel, the Mixshield features a personnel airlock that allows workers to gradually acclimate to the different pressure conditions – similar to a scuba dive.
Custom solutions and a comprehensive delivery package
The TBM not only excavates the tunnel but also lines it with precast concrete segments, known as lining segments. The tunnel will have an internal diameter of 4 meters and an external diameter of 4.50 meters. Using a hydraulic overcutter, the excavation diameter can be increased. This enables the machine to navigate curves and make course corrections more easily. The tunnel’s curve radius is 350 meters.
Due to the complex geology, the Mixshield is designed including a Mini gripper and Anti-roll fins. This allows the machine to be secured so that the cutting wheel can be retracted if necessary. Additionally, the Mixshield is equipped with a drilling rig for preliminary exploration and a telescopic camera to inspect the tunnel face without hyperbaric interventions needed by the personnel. This allows obstacles in the ground
to be detected in a timely manner and the necessary precautions to be taken.
The delivery package for the tunnel construction includes a separation plant, navigation technology from Herrenknecht’s subsidiary VMT and multi-service vehicles that transport the segments from the launch shaft to the TBM inside the tunnel.
Europe’s largest Vertical Shaft Sinking Machine excavates launch and target shaft
Europe’s largest Vertical Shaft Sinking Machine (VSM), with a diameter of 15,900 millimeters, is being used to construct the launch and reception shafts. The Herrenknecht VSM excavated the 45-meter-deep launch shaft in Tilbury on the north bank of the Thames in just four weeks, from mid-November to mid-December 2025. VSM technology is particularly well-suited for soft soils with high groundwater levels,
as it does not require costly groundwater dewatering. By simultaneously driving the shaft wall and constructing the rings, driving speeds of up to 2.7 meters per day have been achieved. This method shortens construction time, minimizes environmental impact, and increases safety, as no personnel are required in the shaft and all work is controlled entirely from the surface. Herrenknecht is supplying a comprehensive equipment package for the shaft construction: in addition to the VSM, a high-performance separation plant for processing the excavated material and segment molds for ring construction. This is complemented by extensive services ranging from logistics to on-site support.
After the construction of the launch shaft in Tilbury, the VSM was dismantled and transported to the construction site of the receiving shaft in Gravesend. There, it has been constructing the receiving shaft – 48 meters deep – from mid-March to the beginning of May. The tunnel boring machine will break through the shaft wall at a predefined, unreinforced location.
PROJECT DATA THAMES CABLE TUNNEL
› Client: National Grid
› Contractors: Ferrovial Bemo JV (Ferrovial Construction und BEMO Tunnelling UK Ltd.)
› Application: Tunnel for high-voltage underground cables
› Tunnelling length: 2.2 km
› Geology: Chalk, flint
MACHINE DATA
› Machine type: Mixshield
› Diameter: 4,730 mm
› Cutterhead drive power: 720 kW
› Torque: 2,369 kNm