Groundforce Shorco has employed its strongest ground-support equipment to assist with the construction of a major waste-to-energy project near Bristol.
Main contractor Clugston Construction is building the £252 million facility for waste specialist Viridor. When completed next year, the plant will use non-recyclable waste as the fuel to generate around 34MW of power – enough electricity to supply 44,000 homes, equivalent to a population larger than the city of Bath.
Central to the project is the waste bunker in which the incoming MSW (‘municipal solid waste’, better known as ‘black bin bags’) is stored prior to incineration.
This massive bunker measures approximately 50m by 17m and has a total depth of 32m – 12m of which is below ground level.
Engineering consultants Byrne Looby and TSP Projects, Byrne Looby headquartered in the Irish Republic, and TSP in York, collaborated on the design of the bunker, with TSP providing the design for the secant-piled retaining wall within the excavation.
After installation of the concrete secant piled wall, a reinforced capping beam was cast at ground level and propped using three of Groundforce Shorco’s massive MP500 modular props spanning the excavation. Each MP500 is capable of supporting a load of 500 tonnes.
Clugston then commenced construction of the above-ground insitu concrete structure while simultaneously excavating below ground.
“Once we had cast the capping beam we propped it and when we’d excavated to about 3m we put the second level of props in,” says Clugston’s senior project manager, Gary Parkinson.
The lower level props comprised seven MP250 props fitted with Groundforce’s 1,220mm diameter Supertube extensions for maximum rigidity. These were braced against a Super Mega Brace waling beam – the highest-capacity proprietary waling beam on the market.
Poor ground conditions guaranteed high lateral loadings but construction of the above-ground structure directly on top of the capping beam considerably increased the forces involved – hence the use of the strongest available support equipment.
“It’s a rectangular excavation, so not particularly complicated in design terms,” observes Groundforce Shorco’s senior engineer, Adam Fletcher. “But it is a deep excavation and highly loaded – with deflection to be kept to a minimum, that’s why we had to use our strongest kit”, he explains.
Dewatering was employed to bring down temporary propping loads and minimise deflection, but even so the loadings came in at 325kN/m ULS for the upper level and between 475kN/m and 525kN/m ULS on the lower level.
According to Gary Parkinson it is unusual, though not unheard-of, to carry out deep excavations while at the same time building upwards above the excavation itself. “We did a similar job with Groundforce a couple of years ago on the Wilton EFW on Teesside,” he says.
“The benefit of using this type of bracing equipment is that we can work quickly, progressing both up and down at the same time”, he adds.
In December, once the massive 3m-thick base slab had been cast, the temporary propping equipment was removed. The total value of Groundforce’s hire contract was around £150,000.
With both the excavation and the above-ground structural work complete, Clugston is now casting the insitu concrete liner walls within the bunker. Completion of this part of the project is expected in February.