AECOM provided detailed geotechnical design on the new nuclear power station at Hinkley Point C, currently the largest geotechnical design contract for nuclear new build in the UK.

The enabling works for the new power station include 5.5 million cubic metres of excavations and the geotechnical design work of 240,000 square metres of rock slope stabilisation, including the 35-metre-deep excavation slopes for the heat sink areas, overall site earthworks design, safe construction platforms, and a system of temporary drainage and utility services.

The rock stability design was undertaken using state-of-the art 2D and 3D numerical modelling. This challenging programme for the delivery of the geotechnical design packages was carefully managed to mirror the construction progress with the deliverables delivered in four packages to ensure the site preparation construction works could be completed without interruption.

Onshore Tunnels and Shafts

The AECOM team have used their experience to provide detailed design of the Primary SCL Ground Support for the Onshore Tunnels and Shafts. These works comprise three large shafts and a network of galleries and tunnels which form part of the liaison and cooling water facilities at the power station.

This package of works also includes launch adits for the three Tunnel Boring Machines used to construct the outfall tunnel and two intake tunnels which transfer water to and from the Bristol Channel to the power station. In addition, the geotechnical team have analysed and assessed other temporary works, including heavy crane lifts at the crest of the excavation slopes, for three TBMs and other permanent works, into the deep excavations and shafts.

Challenges included designing shafts and tunnels within, below and around the deep excavations. The ground support included sprayed concrete linings and rock-bolt support systems within the interbedded mudstone and limestone strata at the tunnelling excavation horizon with shafts ranging from 10.0m to 17.5m diameter and up to 45m deep. Two of the shafts are situated at the toe of reinforced slopes 20m in height and the design team have worked to ensure that there are no conflicts between shaft and slope support. The deepest shaft also includes three large openings connecting the two outfall galleries with the main outfall tunnel.

Design includes a network of eight tunnels with lengths up to 200m and cross-sections ranging up to 8.75m internal diameter. Each of the tunnels features a range of profile variations to suit construction methodology, including; enlargements to facilitate TBM launch, horse-shoe profiles, circular and ovoid profiles and enlargements to provide passing areas for plant. The tunnels include some challenging geometries necessary to facilitate the permanent works, including; tunnel portals at the corners of reinforced excavations and tight bends in close proximity to slope surfaces.

AECOM have been responsible for developing suitable construction sequences to allow efficient and safe excavation and support. Alongside this, AECOM have fulfilled the SCL Design Representative role in the RESS meetings, with responsibility for the design and inputs to the daily monitoring and works review process.