Top down construction is being used to create a new treatment works on a difficult and exposed site. Judith Cruickshank reports.
On a sunny day in June the site for Wessex Water's new treatment works is idyllic, looking out on to the blue waters of Chesil Bank. In wet winter weather it is a different matter. With no kind of wind break between the site and the sea, the rain is virtually horizontal, and the clay soils which make up the site become heavy and slippery.
It is not just weather which makes the location of the new primary and secondary treatment works at Wyke Regis near Weymouth problematic. Site confines leave little space for manoeuvre; the grassy meadow and the lagoon which lie between it and the sea are both designated SSSI; there is housing on two of the remaining sides and a holiday camp on the third. And the headworks already located on the site has to remain operational at all times.
Despite this the team on site appears cheerful. Time lost in the bad winter weather has been more than made up and the partnering contract between Kvaerner Construction, M&E contractor OTVB and Wessex Water seems to have promoted a thoroughly harmonious atmosphere.
Gaining planning permission was a drawn out business as the local authority was concerned about the effect of the plant on the residents of adjoining properties.
Permission was finally granted in April 1998, shortly after the partnership had been formed, and work began on site in June that year for final completion by December 2000.
Top down construction had always formed part of the plan, but the first action of the partners on gaining possession was to fence off the site, with acoustic barriers where it backed onto housing. Topsoil was stripped and stored on site for later replacement. It wasn't quite so simple, though. Provision had to be made for nesting birds, archaeological remains and the almost inevitable badgers.
Kvaerner Cementation Foundations then set about the construction of a secant pile box, with openings on one side. Ground conditions consist of Kimmeridge clay overlaying Ringstead Waxy clays (mudstones) and then Sandsfoot grits. There is significant groundwater pressure.
The box consists of 349 female piles 900mm in diameter, 13.2m long and 356 male piles, 15.2m long and 900mm diameter. Some have been fitted with inclinometers to monitor deflection since a primary concern has been to prevent twisting and slippage due to pressure of groundwater. Maximum recorded deflection has been in the range 10mm to 15mm. Actual dimensions of the box are 72m by 108m on the sea-facing side.
A diaphragm slab was cast around the interior edge of the piled box. A rebated edge supports a grillage of cast insitu beams which assure rigidity and will eventually support the precast roof planks. The beams are supported on 54 load bearing plunge piles, installed using Kvaerner Cementation's Cemloc process. This allows accurate placing, vital around a live sewer taking waste to the headworks.
This sewer was however eventually re-routed around the edge of the box, the kind of decision and operation which could only have come about with a partnering contract. The move was agreed and implemented in just two weeks.
Once the Cemloc columns were in place excavation could begin. A ramp was constructed leading down to the open side of the box and some 83,000m3 of heavy, sticky clay dug out by a fleet of 360degrees excavators and tracked loaders. The process was carefully balanced to control pressure on the box.
Just as tightly controlled was the fleet of trucks which took away the muck. Although part of the haul route took advantage of a disused MoD road, all site access ran through residential roads. Almost the first act of the partnership was to install a wheel washing plant and at the height of the muckshift two dedicated traffic marshalls were employed in addition to the gate man.
The excavation complete, a blinding layer was applied at base level and lining of the piled walls began. But problems did not stop here. The concrete, a Class 3 mix of Portland cement and 70% ground granulated blast furnace slag, is well able to cope with the harsh conditions but takes considerably longer to cure. This adds as much as two days before the climbing wall shutters can be stripped.
As part of the lining process, the anchor which connects to the pile wall has an end fitting adapted to take a temporary works connection for the shutter ready for the next lift.
Currently under way is the anchoring process for the base slab, which has to resist combined uplift pressures from clay heave and the artesian water table of 120kN/m2.
Some 540 tension mini piles are being installed at depth of 10m to 15m. Each contains a central 63.5mm Dywidag bar for working loads up to 1,200kN and is fitted with an extension on to which is screwed a 50mm thick square steel plate. This takes the load and transfers it to the slab.
One essential element of the project is the odour control plant, located alongside the existing works. Not even the outline of this was decided at the time work started on site. The fact that it has been dovetailed into the programme and is now taking shape is a tribute to the co-operative attitude engendered by the partnering contract.
This spirit has extended to the point where two Wessex engineers have been working on site as part of the team of Kvaerner's construction project manager Brian McColm.
This is Wessex' first experience of partnering for a capital works project but says Kim Hall, project manager for Wessex Water, 'without partnering we wouldn't be as far on as we are in the programme'. And in a final endorsement he adds: 'It has allowed us to get what we want'.