Main contractor Clugston Construction is building a £252m waste-fuelled power station for waste specialist Viridor. When completed next year, the plant will use non-recyclable waste to generate around 34MW of power – enough electricity to supply 44,000 homes.
Central to the project is the waste bunker in which the incoming black-bag municipal solid waste is stored prior to incineration.
This massive measures approximately 50 metres by 17 metres and has a total depth of 32 metres – 12 metres of which is below ground level.
Consulting engineers Byrne Looby and TSP Projects 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 500-tonne rated MP500 modular props spanning the excavation.
Clugston then started construction of the above-ground in situ concrete structure while simultaneously excavating below ground.
“Once we had cast the capping beam we propped it and when we’d excavated to about three metres we put the second level of props in,” said Clugston senior project manager Gary Parkinson.
The lower level props comprised seven MP250 props fitted with Groundforce’s 1220mm 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,” said Groundforce Shorco 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.”
Dewatering was employed to bring down temporary propping loads and minimise deflection, but the loadings still came in at 325kN/m ULS for the upper level and between 475kN/m and 525kN/m ULS on the lower level.
According to Clugston, 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 [energy from waste] on Teesside,” Gary Parkinson said. “The benefit of using this type of bracing equipment is that we can work quickly, progressing both up and down at the same time.”
The temporary propping equipment was removed in December, once the base slab had been cast.
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.
The total value of Groundforce’s hire contract was around £150,000.