Energy from Waste, Derby

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Mabey UK Hire Head Office

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Derby Waste is a Mechanical Biological Treatment (MBT) facility with an on-site gasification plant. The new facility is expected to divert up to 98% of residents’ residual waste, while also generating enough electricity to power 14,000 homes. Mabey was called in to support the reception pit - an excavation to facilitate the tipping of waste prior to it being sorted for processing within the plant.

Mabey’s engineers design specified a number of frames, using both Super Shaftbrace and Supershaft Plus to support the deep excavation. At 39m long, 19.2m wide and 12.3m deep, it is one of the largest excavation support projects ever undertaken by Mabey, with subcontractor Ivor King Piling installing the 600mm wide sheet piles to a depth of 19m. The external measurements of the concrete bunker are 32.7m x 12.7m, with a wall thickness of 1200mm lower and a step to 800mm upper and a base thickness of 1800mm. Five levels of frames were required to support this significantly deep excavation. The top frame used Super Shaftbrace with Super Bracing Strut 400 props, with the four remaining frames using Supershaft Plus with Super Bracing Strut 600 props. An overall total of 450t of equipment was used to support the excavation.

There was geotechnical variability across the excavation as it is partly located in a backfilled Tannery settlement pond. Consequently there was variability between boreholes and differences in groundworks levels. In addition, construction sequencing was critical as wall lifts had to be poured in two 6m high lifts.

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Mabey’s design allowed the three lower frames to be removed after the base was cast against sheets acting as a strut. This allowed the first wall lift to be poured without any framing obstructing the shutters. The excavation could then be backfilled to this level, allowing the upper two levels of frames to be removed, and the sheets to stand in cantilever, again with no obstruction. To aid the planning of the construction sequencing, Interserve requested Mabey’s Digital Engineering team to provide a Revit model of the excavation, with project phases created. This allowed the site teams to visualise the complex installation and removal sequence of the support frames around the excavation, concrete construction and finally the backfill operations.

Shaun May, Temporary Works Design Engineer at Interserve commented: “We utilised the BIM design in our Q&A procedures in terms of complex design and Mabey is Level 2 BIM compliant so fell in line with that.  It is not currently a mandatory requirement for any temporary works but the industry and Mabey’s procedures are heading that way – so this was the first step. There were huge benefits in the visualisation from the sub-contractor in getting a feel on how the excavation would look and in particular, the sequencing of the scheme in line with our hold points. Mabey’s sequencing drawing was printed out large, laminated and various copies cable tied to the edge of the excavation. This in turn, aided in the Health and Safety of the scheme as the site operatives had prior knowledge and understanding of the purpose of the scheme, thus allowing them to plan accordingly. It also highlighted possible construction constraints we could encounter on site through the process. It brought upon questions from all levels of the site team that ordinarily may not have been raised.”

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