Uskmouth Power Station

Friday 30 April 2010

Uskmouth Power Station - Development, demolition and redevelopment.

The Challenge
Severn Power Ltd needed to build a new 800 MW power station at Uskmouth, Near Newport, South Wales on a demolition site previously occupied by the old coal fired ‘Station A’ power station built in the 1950s. The geology of the site comprised up to 30m of often soft alluvial soils overlying Mercia Mudstone bedrock. However an 8m to 9m deep piled back filled basement that occupied roughly half the available site had been left after the demolition. The back fill was made up of demolition rubble, including steel work, and also pulverised fuel ash (PFA) from the adjacent still operating ‘Station B’ power station built in the 1960s. The PFA had been formed into a heap 6m to 8m above ground level and covered the whole of the basement area. Although at the time of the investigation, details of the development had not yet been formulated, it was understood that two gas fired turbines were to be constructed on the footprint of the original power station, spanning the old backfilled basement. Utilising this site would therefore require removing and levelling the spoil heap to ground level with all the costs of disposal of spoil that would entail before construction could commence. It would also be necessary to formulate a suitable foundation design to cope with the restraints due to the old basement and its foundations.

The Solution
Soil Mechanics is experienced in the preparation of geotechnical and geoenvironmental investigations for construction purposes. From their own historical records, which for this site involved some seventeen previous investigations dating back to the 1940s, a comprehensive and cost effective proposal was prepared, which following approval by the client, was subsequently carried out. From the investigation and historical records Soil Mechanics were able to advise the client of the constraints posed by the existing piles that supported the basement etc, this information being vital to the redevelopment of the site. From the records existing piles were found to be pre-cast concrete that were driven into particularly difficult ground conditions, notably interbedded alluvial soils at depth. Piling had involved driving through a 3m thick sand layer then a soft cohesive layer to found in gravel at a depth of 15 to 20m. It required particular high energies for the time (3 to 4 tonne hammers falling through 1.5m) with up to 2000 blows often being required to achieve only 300mm of penetration through the sand. In total 9180 piles had been installed. With relevant information from the investigation, the client and his advisors could formulate plans for the existing piles and ensure that the project economics remained realistic. Options for new foundations included:

• Spanning or partially spanning the basement.
• Founding on the existing piles that supported the basement.
• Constructing new foundations altogether. This would however entail locating the existing foundations and removing obstructions from within the basement.

As part of the investigation a comprehensive geoenvironmental assessment was carried out using in house laboratories to undertake chemical testing. Of particular concern were the significant quantities of PFA (150 000m3) that would have to be disposed of unless it could be demonstrated that it could be left elsewhere on the site. Following this assessment it was possible to persuade the EA to allow the material to be retained with all the cost savings that would result.

Results
The flexible, intuitive thinking by Soil Mechanics geotechnical and geoenvironmental engineers resulted in significant cost savings. Ground investigation of this large site cost less than £157,000, shaved months of the construction schedule and saved:-

• £12 - £15million (compared to removal of pulverised fuel ash)
• Costs due to early identification of risk, much of which could be made in advance of the intrusive investigation due to the significant historical records held by Soil Mechanics.

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