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Process of elimination

URBAN FOUNDATIONS

A 1km long,31m deep basement will make sure that a new waste management facility close to the heart of Paris has minimal visual impact.

Alittle stretch of urban industrial activity on Roosevelt Quay on Paris' otherwise leafy left bank is for the moment overshadowed by construction of a huge new waste-to-energy plant that will handle the detritus from more than a million of Paris' inhabitants.

Across the road from a wellestablished cement works, client Syctom's Isseane facility will have the capacity to treat 460,000t of household waste and 55,000t of industrial waste a year, when it is fully open at the end of 2007.

However the emphasis at Isseane is very much on minimising environmental impact - both visually and operationally. The new plant replaces a facility whose tall chimney dominates the skyline of Paris' western suburbs. In contrast the new facility, when finished, will be just three storeys high and all that will be visible above ground level is a modern architect-designed office and administrative block.

There will be no chimney and no release of water-cooling vapour. In fact liberal planting of trees and clever landscaping, coupled with the light and airy lowrise glass-clad superstructure, should result - at least according to the architect's impressions - in a finished project that looks more like a park than a waste processing facility. Syctom has dubbed its development a 'green factory'.

The site is nevertheless enormous, measuring about 300m by 1000m and with a design that requires a 31m deep basement to house the processing plant, the project must rank as one of the largest foundation contracts under way in the world.

Unsurprisingly then, the foundation work is being carried by a joint venture made up of some of Europe's leading geotechnical contractors. France's Soletanche Bachy and Germany's Bilfinger Berger both make up 35% of the 'special foundations' joint venture contractor; while French companies Spie Foundations and SEFI each input another 15%.

Thies Helbig, project manager with Soletanche Bachy - which acts as the joint venture's 'technical pilot' - says they are working as a fully integrated joint venture, sharing both personnel and equipment.

'It's not like we are doing this section and Bilfinger Berger is tackling that, ' he says. 'We are totally mixed.' This, he maintains, is an interesting challenge but one that has provided clear advantages once the initial hurdle of cultural differences was resolved: 'We have found that we complement each other very well - the philosophy on this site is we are acting as one family.'

The scale of the contract means geotechnical work will continue right through until January 2005. First task on mobilising to site in April 2003 was the perimeter diaphragm wall needed to support the up to 31m deep basement and keep out water from the adjacent River Seine.

The diaphragm wall extends down to 50m and the cut-off is extended a further 20m into low permeability chalk, with grout injection around the entire perimeter.

The JV contractors have assembled an impressive kit list for tackling the walls including two hydrofraises, three mechanical diaphragm wall grabs, two hydraulic grabs and seven service cranes.

Despite the depth of the cut-off there is still a possibility that water will enter the basement through isolated fissures in the chalk, explains Soletanche Bachy's newly graduated engineering geologist Anne Gandilhon. For this reason pumping tests will be carried out after the wall has been completed, and it is possible that the team will still need to inject a deep basal cut-off slab to provide the required degree of sealing.

The basement is divided into three main areas and the perimeter wall varies in thickness from 0.6m to 1.5m, depending upon the depth-induced earth pressures and structurally imposed lateral loading.

Much of the diaphragm wall has been designed to be auto-stable, meaning it will not require propping and is not tied back with anchors. This is achieved using Tshaped wall panels.

Other foundation works include around 400 load-bearing piles, some of which are up to 2m in diameter. More typically piles are between 0.7m and 1.4m in diameter.

Some of the columns incorporate steel H-section plunge columns, which have to be installed to a very tight vertical tolerance - quite a tall order when the pile top level is 31m below the ground surface.

No wonder then the project is being carried out by the very top end of Europe's contracting fraternity.

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