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Walking on water

Main construction challenge on the southernmost section was not the motorway itself but a large earth dam.

Concern for a flock of Canada geese roosting on the warm asphalt of a newly completed carriageway plus the possibility of stray golf balls landing on the adjacent motorway were at the forefront of site engineers' minds on the new M60's most difficult contract. This is the southerly section running 6km from the River Medlock to link into the existing route at Denton.

Pushed to the backs of their minds at the opening celebrations will be the problems of last minute redesigns for 28km of cable ducting plus new foundations for a large rail bridge and the major hassles of building the UK's largest earth dam in decades.

These suppressed thoughts summarise the main reasons why contracting joint venture Amec/ Alfred McAlpine ended up with a 50% overrun on its planned three year, £101M contract and with a claims bill likely to be settled at around £40M.

That most of the delays have already been awarded by the client as legitimate extensions of time proves, claims the joint venture's chief engineer Frank Platt, that the problems were not generally of the contractor's making.

'This has really been three jobs in one, ' says Platt. 'All the structures were built by ourselves, rather than the client packaging major crossings into an advance works contract. We then formed a complex section of urban motorway and built a 1km long dam.'

Most of the 15 bridges, over or under roads, rail and a canal, had to be completed within a year of the contract's April 1996 start to allow an unobstructed haul road, for a second year muckshift, right through the job. And most were ready in time. The exception was a 130m long skewed tunnel-like rail underbridge which needed its piled foundations redesigned when Railtrack banned the piling rigs.

Reason for the speed was the need to haul much of the contract's 2.5Mm 3earthworks from the north of the project to the south where the contractor faced one of the more unusual tasks to befall a motorway contractor: the construction of a 10m high, fully engineered, earth dam.

The motorway route cuts through the 2,680 megalitre Audenshaw reservoir. Divided into three separate sections, the reservoir is temporarily losing about a third of its volume.

One section was drained allowing the motorway to cross it just in front of the new dam. With both complete, the reshaped area behind the dam is being refilled.

Both the dam's 60,000m 3clay core, and its 340,000m 3sloping shoulders formed of weaker clay, were to be won from a deep cutting near the other end of the contract. All went well with the core and about a fifth of the shoulder material until clay quality in the cut deteriorated dramatically.

'The remaining clay was too wet and weak to use so we faced a shortfall of 270,000m 3needed for the shoulders, ' says Highways Agency project engineer John Dutson. 'The extent and interpretation of original site investigation boreholes missed what proved to be an extensive volume of unsuitable material.'

An alternative clay source was eventually found, ironically right alongside the dam, in the bed of the now drained section of reservoir. But, only a 1.25m depth of shoulder could be laid per month, and none at all during the seven month closed earthworks season, to allow dissipation of pore water pressures. Knock on setbacks accounted for a year of the contract's 18 month delay.

'Given the unpredictability and high risks on this contract, it would have been grossly unfair to the contractor had it been awarded as design and build, ' says Rees Evans, deputy resident engineer for HA consultant Mouchel.

'Comparison with the adjacent contract is similarly unfair because the challenges, especially the dam, were very different.'

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