Horizon Nuclear Power developed plans to construct the Wylfa Newydd nuclear power station, Wales, which was expected to produce enough energy to power about 620,000 homes. Before any construction began, ground investigations were needed because of proposed structure layout changes. More critically, investigations needed to verify, develop and refine the seismic (shear and compression wave) profiles of the rock and superficial soils to withstand natural disasters, such as winds generated by a hurricane or even an earthquake.
Horizon commissioned Structural Soils, an RSK company, to carry out the UK’s largest ground investigation in collaboration with Atkins and the British Geological Survey. Phase 1 included drilling 282 vertical and 32 inclined rotary cored boreholes, many using cable percussion drilling techniques. More than 4 km of drilling produced 400 m of soil and 4 km of rock cores (low-grade metamorphic bedrock), which required meticulous logging by a team of experienced geologists. There were more than 10 drilling rigs and 50 personnel on-site and 9 staff at the core logging facility.
The amount of seismic testing at Wylfa was the most ever performed on a UK hard-rock site, while the data interpretation was the most extensive carried out on a hard rock site in the world. To adhere to Horizon’s specifications and British Standards requirements, Structural Soils implemented mobile data capture via touchpads, which made it simple for engineers to log borehole and trial pit data uniformly. Summary and detailed logs for all exploratory hole positions were logged and stored securely. Automated reporting scripts were programmed to generate sections of the report, thereby saving days of administrative time.
Ensuring health and safety
Structural Soils, as always, ran the site with a strict health and safety code. All personnel committed a whole day to behavioural safety training. A health and safety manager took charge of inductions, passed on health and safety information, randomly tested staff for drugs and alcohol and produced risk assessments. Daily health and safety briefings were mandatory for all personnel, and weekly toolbox talks discussed environmental, archaeological, ecological and health and safety issues.
From the outset of the project, Structural Soils aimed to source labour and materials locally. About one-third of the workforce was from Anglesey and North Wales. Staff who would otherwise have commuted stayed in local accommodation and supported local businesses. Moreover, carpooling reduced expenditure, fuel consumption and the site’s carbon footprint.
The site comprises agricultural land, divided by drystone walls and included sites of special scientific interest. Consequently, the investigations were conducted with extreme caution to avoid damage. A network of gravel haul roads was used to access the exploratory hole locations. Where exploratory holes were located away from these tracks, plastic matting protected temporary access routes, preserved the integrity of the ground surface and minimised silt runoff. Each exploratory hole was surveyed by an ecologist, an environmental engineer and an archaeologist before Structural Soils carried out intrusive work. Rotary boreholes close to watercourses were given particular attention. Engineers used recirculation drilling techniques, and no water was discharged to ground to avoid silting of adjacent watercourses. Drilling flush fluid was contained and transported to a secure holding tank for appropriate disposal after chemical testing and analysis.
Structural Soils, Horizon and Atkins were the joint winners of the Ground Investigation Project of the Year award for the Wylfa Newydd project in 2017. The judges said how impressed they were with the deft handling of sensitive data for a contract of its size and how the project excelled in innovation, sustainability, health and safety, and value engineering.