Sizewell C puts geotechnical works at the centre of nuclear construction

The next major phase of Sizewell C will depend heavily on specialist ground engineering, as the project prepares the construction platform for one of the UK’s largest new energy infrastructure schemes.

The planned 3.2GW nuclear power station on the Suffolk coast requires a stable and watertight working platform before major above-ground construction can progress. The geotechnical scope includes piling, soil mixing, diaphragm walls and plastic cut-off walls. These systems will support excavation, groundwater control, temporary works, logistics routes, heavy lifting equipment and the permanent nuclear structures that follow.

This is not simply enabling work. On a project of this scale, ground engineering becomes one of the main risk controls for the entire construction sequence. Before major concrete, structural and mechanical packages can progress, the ground treatment and retaining systems must be installed, tested and accepted. Disruption at that stage can spread rapidly into excavation, crane deployment, lifting plans, concrete sequencing and the mobilisation of downstream contractors.

The groundworks must address two fundamental requirements: bearing capacity and water control. Sizewell C has said the works will create a platform capable of supporting the immense loads associated with construction of the new power station, including one of the world’s largest land-based cranes, alongside construction plant, logistics areas, equipment compounds and the permanent nuclear structures.

Groundwater is one of the site-specific factors that still requires dedicated engineering attention, alongside local geology, site access and enabling infrastructure. In broad geotechnical practice, diaphragm walls and plastic cut-off walls of this type are used to limit seepage, control excavation conditions and reduce the risk of instability during deep groundworks, while soil mixing can improve weak or variable ground and reduce permeability. The project has not published which technique will be applied to which function.

Piling typically provides load transfer where shallow ground conditions are not sufficient, while retaining and cut-off systems help create dry and controlled work zones. These elements need to be designed as an integrated system rather than as separate packages, because each one affects excavation sequence, groundwater drawdown, ground movements, temporary works and long-term performance.

A milestone diagram in Sizewell C’s annual report and accounts for FY25/26, published on 6 July and reported by New Civil Engineer, shows main earthworks — covering site preparation, excavation and groundworks — scheduled to take approximately three years. The report does not specify when those main earthworks will begin. That duration reflects the complexity of preparing a nuclear construction site before visible superstructure works can dominate the programme.

The technical assurance requirements are also higher than on many conventional civil engineering projects. Ground conditions, workmanship, materials, testing, inspection records and as-built information must be robust enough to support the safety case for the structures above, and design changes require disciplined review and traceability.

The key issue is therefore not only how many specialist firms are involved, but how well the geotechnical design, temporary works, monitoring, quality control and construction sequencing are integrated. Nuclear projects leave little room for fragmented decisions at ground level.