National Grid TBM under the Thames: tunnelling design and risk notes for engineers
Reviewed by Tom Sullivan

First reported on Tunnelling Journal – News
30 Second Briefing
A 271.5‑tonne Herrenknecht Mixshield TBM, Caroline, has started driving a 2.2km electricity cable tunnel with a 4m internal diameter beneath the River Thames in Essex for National Grid’s Grain to Tilbury project, delivered by the Ferrovial BEMO joint venture. The drive will pass through variable Thames estuary ground conditions between 35m‑deep launch and reception shafts of 15m and 12m diameter, with tunnelling continuing into 2026 and overall scheme completion targeted for 2029. The new tunnel will replace the 1969 Thames Cable Tunnel and carry new high‑voltage circuits between Grain and Tilbury substations.
Technical Brief
- Herrenknecht’s 271.5‑tonne Mixshield is a tailor‑made machine configured specifically for this cable tunnel.
- Mixshield technology indicates pressurised slurry support at the face, suited to variable, water‑bearing estuarine deposits.
- Ferrovial BEMO JV is responsible not only for tunnelling but also headhouses and CSE compounds at each shaft.
- Cable Sealing End compounds will form the transition interface between underground circuits and overhead/substation equipment.
- Shaft depths of 35m imply tunnelling well below riverbed level, reducing hydraulic connection and scour risk.
- Differing shaft diameters (15m and 12m) suggest asymmetric construction logistics and equipment layout between Grain and Tilbury.
- Replacement of the 1969 Thames Cable Tunnel indicates end‑of‑life asset renewal and upgraded transmission capacity across the estuary.
Our Take
Herrenknecht’s role on National Grid’s Grain to Tilbury scheme mirrors its presence on other complex undercrossings in our database, such as the Panama Canal and HS2 Euston Tunnel, signalling that National Grid is leaning on a contractor with a recent track record in high‑consequence, water‑saturated drives.
Within our 721 Infrastructure stories, only a subset involve UK power transmission upgrades, so the Thames Cable Tunnel positions the Great Grid Upgrade alongside rail megaprojects like HS2 in terms of underground construction complexity rather than conventional cable laying.
The 2.2 km, 4 m‑diameter Thames Cable Tunnel is modest in length compared with long rail tunnels like the Brenner Base Tunnel section in our coverage, but the 2026 tunnelling and 2029 project completion dates suggest National Grid is pacing works to align with broader grid reinforcement milestones rather than pushing for a rapid, stand‑alone delivery.
Prepared by collating external sources, AI-assisted tools, and Geomechanics.io’s proprietary mining database, then reviewed for technical accuracy & edited by our geotechnical team.
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