Introduction
Proprietary shoring equipment from Groundforce Shorco supported a major excavation at a £8.2m upgrade to a wastewater treatment plant near Rainham in Kent.
Project
The main contractor is building a massive 5,500m3 underground storage tank for the client, Southern Water, to prevent heavy rainwater surcharges overwhelming the plant and causing untreated sewage to be discharged into the River Medway.
The plant serves more than 100,000 households and this will more than double the site’s stormwater storage capacity from 4,489m3 to 10,000m3.
Work began on site in June 2025 to dig out an 8.2m-deep excavation measuring 37m x 33m to accommodate the tank, which will be built using in-situ concrete construction.
Challenges
Due to the soft alluvial soil at the site, a robust support structure was required to reinforce the sides of the excavation during construction of the tank. For this, the contractor turned to Groundforce Shorco.
Southern Water had originally wanted the whole thing built above ground but that would have required extensive pumping equipment and a complex computer system. A simpler and more cost-effective solution was to build it below ground.
Solution
The perimeter of the excavation was lined with 12m-long GU 21N interlocking steel sheet piles, installed by piling specialist Berryrange, to create a cofferdam. Toe-in is 4.3m below formation level (8.2m).
Within the cofferdam, three levels of frame were used to brace the sides of the excavation. The topmost frame, at 1m depth, comprises Groundforce Shorco’s Mega Brace beams with MP150 modular hydraulic struts installed across the four corners as knee-braces.
The second frame was installed at 3.5m below ground and used the heavier Super Mega Brace beams with MP250 struts as knee-braces.
The bottom frame was the same configuration as the second frame and was installed at 5.8m depth. Each frame comprised eight hydraulic struts – two across each corner – with the longest span being 22.2m.
To support the weight of the equipment, the top level of frame was supported by hanging chains and the two lower frames were supported by steel gallows brackets welded to the sheet piles.
Once the excavation reached formation level at 8.2m, 100mm of backfill was poured, followed by a 1.2m-thick concrete slab. Once this slab achieved a strength of 10N/mm2, the bottom frame was then removed.
A further 350mm concrete was then poured to bring the slab thickness to 1.55m; when this cured, the second frame was removed.
At this point, the contractor installed the formwork and cast the 750mm-thick tank walls up to 1.5m below ground level. When the walls reached the required strength, the top frame was removed, the remaining excavation back-filled and the tank completed.
The design of the scheme changed significantly from the original plan,
says Groundforce Shorco area manager Arpad Nagy:
Even the underground design was changed. “They had intended to use precast caisson rings but that would have meant digging deeper – at least 16m. Instead they made it wider and shallower,”
says Arpad Nagy.
The contractor battered the sides of the site to reduce the surrounding ground level by 0.5m. So while formation level was 8.2m, the excavation itself was only 7.7m.
The revised design and methodology not only simplified the construction process but also accelerated it.