Advanced mixshield TBM tackles chalk and flint beneath the Thames

Construction has begun on a major sub-river power tunnel beneath the River Thames as part of National Grid’s Great Grid Upgrade, with a Herrenknecht Mixshield tunnel boring machine now driving a 2.2 km cable tunnel between Tilbury and Gravesend. The project involves a technically challenging utility tunnel drive due to the combination of mixed chalk–flint geology, high groundwater pressures and a tight alignment geometry.

The tunnel is being constructed by the Ferrovial Bemo Joint Venture for a new 400 kV electricity connection designed to strengthen long-term energy infrastructure capacity as electricity demand continues to increase across the UK. The tunnel boring machine, specifically configured for the project, has an excavation diameter of 4.73 m, a total length of 108 m and a weight of approximately 464 tonnes. Engineers expect the machine to encounter mixed chalk and flintstone ground, with flint inclusions reported to reach compressive strengths of up to 1,000 MPa in localized zones.

One of the project’s primary engineering challenges is the high water pressure beneath the Thames estuary, where tidal influences contribute to design pressures of up to 4.5 bar at depths of around 41 m below the Thames. To address this, the Mixshield TBM incorporates multiple sealing systems and maintains controlled pressure at the excavation face while the remainder of the machine operates under atmospheric conditions. The system also includes a personnel airlock to support hyperbaric interventions during cutterhead inspections or tool replacement operations.

The tunnel will be lined using precast concrete segments, producing a finished internal diameter of 4.0 m and an external diameter of 4.5 m. A hydraulic overcutter allows the TBM to slightly enlarge the excavation profile when required, improving steering capability along the project’s 350 m radius alignment. Additional stabilization features, including anti-roll fins and a mini gripper system, have been integrated to manage risks associated with mixed-ground tunnelling and abrasive flint conditions.

To reduce the need for manned interventions under pressure, the TBM has been equipped with a telescopic inspection camera and a forward probing drilling rig capable of identifying obstructions and adverse ground conditions ahead of excavation. Navigation is supported by VMT guidance systems, while dedicated multi-service vehicles transport segments and materials between the launch shaft and the TBM.

Before tunnelling operations began, Europe’s largest Vertical Shaft Sinking Machine, measuring 15.9 m in diameter, was used to construct the launch shaft in Tilbury and the reception shaft in Gravesend. The 45 m deep launch shaft was completed in approximately four weeks, while the reception shaft was constructed to a depth of 48 m. The method avoids the need for costly groundwater dewatering and allows shaft work to be controlled from the surface.

The project highlights the increasing technical requirements of utility tunnelling in the UK, where mixed geology, groundwater pressures and constrained river-crossing alignments continue to drive the use of advanced shielded TBM systems, mechanized shaft sinking and integrated navigation technologies.