KVAENER CEMENTATION FOUNDATIONS has installed its Cemloc plunge columns to support the roof beams during top down construction of a massive underground treatment works in Weymouth, Dorset.
The £20M primary and secondary treatment works at Wyke Regis near Chesil Bank are being built to comply with the Urban Waste Water Treatment Directive. The cramped site, alongside a headworks and long sea outfall, has houses on two sides and a holiday camp on the third. On the seaward side lies a meadow and Fleet Lagoon, both sites of special scientific interest.
KCF's work is worth £2M and the contract is being run under a partnering agreement with client Wessex Water and M&E contractor OTV Birwelco.
Work began in June 1998 and the first job was to build acoustic bunds next to the houses. A sewer had to be rerouted around the site before piling for the perimeter wall of the 12m deep, 108m long and 72m wide excavation could begin.
The site is underlain by the Kimmeridge Clay Formation over the mudstones of the Ringstead Waxy Clays and finally the sandstones of the Sandsfoot Grits.
The perimeter wall was formed using a watertight secant piled wall designed by KCF with finite element analysis by Geotechnical Consulting Group. The wall comprises 356, 900mm diameter, 15.2m long hard concrete CFA piles and 349, 900mm diameter, 13.2m long soft bentonite cement CFA piles.
Some of the piles are fitted with inclinometers to monitor wall deflection, which has been 10-15mm. This wall is now being lined with cast insitu concrete.
Piling across the excavation area consisted of 54 bored piles - 1500mm diameter over the top 8m (to accommodate the plunge column frame) and 1200mm diameter below to founding depth between 18m and 22m.
The 12m long steel Cemloc plunge columns were then sunk into the top 3m of these piles. They are accurately positioned in the centre of the piles to 1:600 verticality. These support precast concrete beams which are supporting the perimeter walls of the box during excavation, preventing twisting and slipping due to the artesian groundwater below the site.
A fleet of hydraulic excavators and lorries removed about 83,000m3 of clay spoil. A blinding layer was placed on the floor of the excavation and the floor slab cast.
Some 540, 225mm diameter tension minipiles anchor the floor slab to counteract the uplift of 120kN/m2 caused by groundwater pressure and clay heave. These are between 10m and 15m deep and each has a central 63.5mm diameter Dywidag bar with working loads up to 1200kN.
Another important part of the scheme is the odour control plant, which had not been finalised before work started. This sits next to the existing works and was built with two contiguous piled walls, one of 41, 900mm and 20m long piles and the other of 26, 600mm diameter and 9m long piles.
Work is due to finish in December 2000.