The challenge
We were 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. Our 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 we have ever undertaken. Subcontractor Ivor King Piling provided the installation of 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.
Our solution
Five levels of frames were required to support this significantly deep excavation. The top frame used Super Shaftbrace and Super Bracing Struts with 400mm extensions. The four remaining frames used Supershaft Plus and Super Bracing Struts with 600mm extensions. Overall, a total of 450t of groundworks equipment was used to support the excavation.
There was geotechnical variability across the excavation, as it was 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. Our 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, our Digital Engineering team provided Interserve with 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.
