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Ground conditions blamed for £900M overrun on Boston Central Artery project

UNFORESEEN GROUND conditions are being blamed as one of the main causes of the £900M cost overrun on the Boston Central Artery project.

Costs have risen to £7.6bn and could go even higher, according to leaks from a Federal Highway Administration report.

The FHA is the main funder for the project and is threatening to freeze payments unless a refinancing plan is put in place.

This has yet to be finalised but is expected to be a combination of state surpluses and cash from the client, the Massachusetts Turnpike Authority, and the Massachusetts Port Authority.

A report on the overrun, due as GE went to press, is expected to criticise management of the project and will detail results of an investigation into the overruns as well as identifying cost saving measures.

The Central Artery project is being managed by a joint venture of Bechtel and Parsons Brinckerhof. It is one of the most significant urban transport schemes in the world, aiming to relieve road traffic congestion through the heart of the city estimated to cost motorists more than £300M every year.

Cut and cover tunnels are replacing huge sections of elevated highway, tunnels are being jacked beneath railway lines and immersed tube tunnels are being built to cross the harbour area, completely reconfiguring the road layout (GE July 1999). In all, some 10M. m 3of material is being excavated.

There are some formidable geotechnical challenges. Much of the natural harbour, originally a series of inlets, was backfilled to allow Boston to grow, leading to highly variable mix of fill, sand, silt and peat overlying the Boston Blue Clay, which softens with depth.

Some of the fill contains a variety of construction materials including concrete, steel, timber and granite blocks.

There is also a high water table, with groundwater 2m to 3m below the surface.

Despite extensive site investigations, some of the obstructions in the fill did not come to light until construction had begun. MTA director Joe Allegro claimed comprehensive geotechnical surveys on an excavation of this size were not possible.

'There is no industry standard for checking the ground conditions on longitudinal tunnels. The ground was tested every 100ft [30m]. If we had tested it every 10ft [3m], the process would have taken 10 times as long and been 10 times as expensive.'

Pressure on the project management team is likely to grow if the FHA report confirms that project costs could rise by a further £875M to £8.5bn.

But with 70% of the excavation finished, 90% of the slurry wall for the main tunnels installed, 70% of the tunnel jacking completed, the first immersed tube tunnels in place and the cable stayed bridge 40% complete, some argue that the worst technical challenges to hit the budget are over.

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