Construction of New Cable Tunnel Under the Thames
National Grid, the transmission system operator in England and Wales, is expanding its high-voltage grid as part of The Great Grid Upgrade. A key project is a 2200 m long cable tunnel under the Thames from Gravesend to Tilbury.
Thames Cable tunnel – the Mixshield is lifted into the launch shaft in Tilbury
Credit/Quelle: Herrenknecht
Herrenknecht supplied a tunnel boring machine for the project, which began tunnelling at the start of May 2026. The launch and reception shafts for the cable tunnel were constructed using Europe’s largest shaft sinking system, also supplied by Herrenknecht. National Grid awarded the contract for the construction of the cable tunnel for the 400 kV high-voltage cable to the joint venture Ferrovial Bemo (Ferrovial Construction and Bemo Tunnelling UK Ltd.).
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.
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 Mixshield TBM has a diameter of 4730 mm, is 108 m long, and weighs 464 t 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. It is also sealed against the high water pressure 41 m 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.
Due to the complex geology, the Mixshield is also designed as a ”Mini Gripper” and includes 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 (VSM), with a diameter of 15 900 mm, was used to construct the launch and reception shafts. Herrenknecht supplied 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 was 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 built the 48 m deep receiving shaft from mid-March until early May 2026. The tunnel boring machine will break through the shaft wall at a predefined, unreinforced location.
