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Shouldering blame

Construction of the latest section of Manchester's M60 outer ring road is running a year late and at least £20M over budget. But the problem has little to do with the road and a lot to do with creation of Britain's largest earth dam in a decade.

Road contractors are well used to earthworks difficulties. But here the shortage of suitable fill material - and submission of 'unexpected ground conditions' claims - relate not to motorway embankments but to the shoulders of a 1km long clay dam lying physically and chronologically on the contract's critical path.

'Our timetable is controlled not by earthworks seasons but by the much more strictly dictated dam construction programme,' explains contracting joint venture Amec/Alfred McAlpine's project manager Phil Girling. 'We are forbidden from working for seven months over each winter to allow controlled dissipation of pore water pressures.'

Girling's design meetings are as much with Panel One dam engineers and the Reservoirs Panel as they are with his client the Highways Agency. The 500,000m3 dam is equally crucial to motorway progress.

The £102M contract will provide a key 6km section of the planned motorway box around Manchester. The overall 56km ring road, made up of several existing motorway sections and now already renamed the M60 - should be open in summer 2000.

Most is complete, bar a 17km gap to the east of the city now being plugged by four separate contracts. Two are finished, one was awarded to Balfour Beatty a month ago and the ongoing Amec/Alfred McAlpine section between Denton and the river Medlock started in April 1996.

Dominating the alignment of the joint venture's route is the need to cut across a corner of a storage reservoir used by North West Water as an emergency supply for Manchester. The 7,000M litre Audenshaw reservoir is conveniently divided into three separate sections, each surrounded by a 10m high earth bund. So it was only necessary to dewater the westerly 2,700M litre section and to route the eight lane motorway across the middle.

Eventually one half of this section will be refilled, with the area outside the motorway route backfilled and landscaped.

The curved clay bund being formed adjacent to the inner edge of the motorway trace, to link with the two severed ends of the original perimeter wall, is regarded as a 'new' 10m high earth dam, complete with a cupboard full of dam regulations.

Fortunately the £25M earthmoving programme included excavation of a deep cutting through a moss bog and removal of 300,000m3 of peat plus underlying clay (NCE Roads supplement, 18 June).

This clay, 3km along the route from the reservoir, was to provide the dam core and stiffer shoulder material. There are no problems winning the 60,000m3 core clay, but the 340,000m3 stronger clay needed for the shoulders was simply not there.

'We were quite satisfied with the original borehole survey by our consultant Mouchel which suggested there would be enough clay for both areas of the dam,' concedes Highways Agency project manager Steve Edwards. 'But though there is no shortage of clay, its quality is not good enough for the shoulders. With hindsight, we should have sunk more boreholes.'

Some 190,000m3 of shoulder clay has already been won from beneath the bog, but the frantic search to source and then extract the remaining 150,000m3 fell foul of last year's dam closed season between October and April. As a result, the up to 100m wide dam, which should have been finished this autumn, is currently just a third complete.

Ironically, the new clay source turns out to be under the noses of engineers. The clay bed of the now redundant reservoir section alongside the dam is proving ideal and has added bonuses. Not only is there now negligible haul distance to the site, but the clay's removal is creating a convenient hole into which to bulldoze the redundant old reservoir wall.

Hassle at the dam represents most of the contract's 12 month delay and cost overrun. There was, however another setback, which nearly added an embarrassing second year's delay.

Most important of the project's 15 bridges is the crossing over the main trans-Pennine Manchester to Leeds rail line. This tunnel-like, 130m long skewed concrete bridge consists of precast decking over insitu solid walls and bored concrete piles.

Located roughly midway along the route, the bridge's early completion was considered vital to allow access through the job, especially for muckshifting plant. And, within days of setting up site cabins, Girling was preparing to start piling for the bridge's long side walls.

But between tender discussions and site start, Railtrack had tightened up its trackside safety rules, effectively banning the contractor's planned 20m high piling rigs in case they toppled across the live railway. Instead the JV had to resort to sturdier 4m high tripod rigs and to use more - but shorter and narrower - bored piles.

The consequent bridge redesign by Mouchel introduced soft-toe piles to share vertical loading with the spread footings above. It also specified reinforced earth behind the side walls to reduce lateral loadings. The build programme was ex- tended from nine months to 16.

Luckily this part of the programme has been kept intact as a result of an innovative yet simple temporary works solution.

A nearby small stone arch bridge over the railway had early on been ruled out as too weak for an alternative haul route.

But lateral thinking engineers from the JV and Mouchel came up with the idea of overlying the old bridge with a £250,000 temporary Bailey bridge. Load transfer was shared between old bridge deck and concrete ground slabs formed at either end allowing safe passage for dumptrucks as heavy as 40t.

Fortunately for the contractor, Mouchel and the Highways Agency have already accepted in principle that, under the standard ICE 5th Edition contract, bridge redesign and clay shortages were not of the JV's making. Contract extensions totalling 48 weeks have already been awarded with £10M claims and additional works payments settled 'on account'.

No-one is willing to discuss the end bill, but it is reliably understood that it could top £20M.

Edwards stresses that the 12 month hold-up on this contract has not in itself delayed overall completion of the ring road. Unconnected setbacks before last May's award of the final £50M contract to the north - which included government funding hold-ups, extra public inquiries and the ongoing roads review - means the overall ring road cannot open before summer 2000. That is three months after the Amec/Alfred McAlpine job is now expected to finish.

With extra finances and contract extensions more or less agreed, there seems a welcome dearth of contractual rowing.

The only argument evidenced between Girling and Mouchel deputy resident engineer Rees Evans during NCE's visit, was the local pub's correct name.

Evans' claim that it was 'Ye Olde Blue Pig' and Girling's 'Hungry Horse', both proved correct. The presence of both names on the hostelry wall remains as much a mystery as, to some, does the recent improvement in the performance of golfers at the fourth hole on nearby Fairfield golf course.

Here Evans and Girling do know the answer. They had to realign the hole, effectively halving its distance, to create the motorway route past it. Rumour has it that the regulars remain resolutely ignorant of the engineers' help as they boast impressive scorecards.

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