Sizewell C tunnel redesign vs Hinkley Point C: geotechnical lessons for engineers
Reviewed by Tom Sullivan

First reported on New Civil Engineer
30 Second Briefing
Ground conditions at Sizewell C have forced the Civil Works Alliance to depart from the Hinkley Point C reference design for the offshore intake and outfall tunnels, driving new solutions for lining, support and construction sequencing. Contractors have had to re-optimise tunnel geometry and TBM drive strategy for the North Sea sediments and local stratigraphy, rather than the harder rock and different stress regime at Hinkley. The changes affect segment design, joint detailing and groundwater control, with direct implications for durability, settlement behaviour and marine interface works.
Technical Brief
- Hinkley Point C experience is being reused only where compatible, with design teams explicitly re-validating each assumption.
- Design reviews have focused on TBM launch and reception structures, given different overburden and seabed morphology at Sizewell.
- Alliance governance uses integrated digital models so geotechnical, structural and marine disciplines iterate tunnel options in near‑real time.
- Construction sequencing is being co-developed with marine package contractors to manage weather windows and North Sea access constraints.
- Risk allocation in the alliance contract incentivises early identification of ground-related design changes rather than late construction claims.
- Lessons from this Hinkley-to-Sizewell transfer are expected to inform future UK nuclear tunnelling schemes using reference designs.
Our Take
Within the 446 project-tagged pieces in our database, Sizewell C and Hinkley Point C are among the few UK nuclear megaprojects where geotechnical design changes between ‘reference’ and ‘follower’ plants are being documented in detail, giving practitioners rare visibility on how lessons learned are actually translated into new tunnel schemes.
For civil contractors in the Civil Works Alliance, the move from Hinkley Point C to Sizewell C tunnel designs likely affects risk allocation on ground conditions and constructability, as UK nuclear work packages increasingly price in experience from earlier packages rather than treating each site as a clean-sheet design.
Across several geotechnical stories, nuclear projects like Sizewell C stand out for their long programme horizons, meaning early tunnel design choices can lock in methodology and plant fleets for a decade or more, with limited scope to retrofit innovations seen later on other major infrastructure schemes.
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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