FOUNDATION CONTRACTOR Aarsleff Piling has constructed its first steel sheet piled cofferdam using the Halt Lock pile clutch sealing system for a £1M wastewater project in Greater Manchester.
The Halt Lock system was developed by Dutch groundworks contractor Hei & Grondwerken and is licensed in the UK by British Steel. It uses a rolled steel angle section, factory-welded onto the side of a standard sheet pile, to create a void around the clutch which is filled with a bitumen sealant.
As the adjacent pile is driven in, it bends the angle section back to bite the incoming clutch, displacing and forcing the sealant out through the vertical joint to create a watertight connection.
The 57m long, 12.5m wide watertight cofferdam will form the permanent walls of a large capacity on-line storage tank for a combined foul and storm water sewer overflow pollution management scheme next to the River Medlock at Tamesside on the eastern outskirts of Manchester.
Main contractor Norwest Holst Construction and its engineer R Richter & Associates proposed a permanent watertight cofferdam and top down construction instead of building the tank inside a temporary cofferdam. This, it is claimed, saved 15% in costs and cut the programme by 10 weeks.
Aarsleff, working for NHC, installed the 120 pairs of Larssen LX32 sheet piles between 7.3m and 9m long. The middle joint of each pair was welded in the factory and the pair fitted with one Halt Lock clutch.
The first 40 pairs of piles were driven using an ICE 815 vibrator. This installed the critical first pair through stiff clay and 300mm into the underlying sandstone.
The remaining piles were driven using a Junttan HHK6a hydraulically accelerated impact hammer. This proved twice as productive in installing the sheet piles. Original production rates were predicted at 10 to 15 pairs per day but in fact, up to 28 per day were achieved.
Once the cofferdam was finished, Norwest started top down construction which involved casting the 400mm roof slab, then excavating down to cast the 600mm thick floor. Work is due to finish next month.
The project is one of several North West Water schemes along the Medlock to upgrade combined foul and stormwater unscreened effluent overflow discharges and improve river quality.