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Difficult approach

M9 extension - Keeping the trafc running is key for Morrison Construction as it widens the southern approach to Scotland's Forth Road Bridge. Adrian Greeman reports.

Few £30M construction projects can attract liquidated damages running to hundreds of millions of pounds. But 4.5km of new road approaches under construction for the south side of the Forth Road Bridge might have done.

Massive penalties are not strictly part of the contract being done for City of Edinburgh Council by Galliford Try subsidiary Morrison. But part of the project involves crossing a major BP oil pipeline and rupturing it would have been extremely costly.

'It's mega and carries about 25% of all oil exports from Scotland, ' says Morrison project manager Alan Smith. Rupturing would be unthinkable in terms of lost revenue and environmental damage. 'Informally I was told it could cost as much as £1bn overall, ' he says.

To make matters worse, the oil line crosses the project where is runs over ancient coal mines close to the junction of the new road and the A90 which feeds the Forth crossing.

A £9M preparation contract let to Forker dealt with those.

It drilled and grouted the voids found in the area before Morrison's work began in May last year.

To Smith and Morrison's relief, subsequent bridging of the line has gone relatively well.

A membrane was laid over the 600mm diameter pipe after it was exposed and a simple concrete box bridge structure has been built around it.

BP representatives were on site continuously while the work was done along with representatives from the client's consultant Scott Wilson.

'Having the client engineer here was unusual, ' says Smith.

The £28M contract is design and build, but for this element the client supplied the design in detail. Consultant Carl Bro, recently taken over by Dutch rm Grontmij, has done the remaining design work as subcontractor.

No plant was allowed to work across the pipeline which meant that a ramp was built across it to allow access from one side to the other.

Backlling around the pipe, which crosses the new two lane dual carriageway motorway section at right angles, is virtually complete and the bridge has all but vanished.

Despite the client's attention, the pipeline has not been Smith's major concern. He has been more worried about keeping traffic running on two busy trunk roads while work is underway.

The point of the project is to improve links to the southern side of the Forth road bridge from the central Scotland motorway network.

Currently the M9 links to the bridge via a dual two lane spur road that peters out into the single lane A8000 which carries trafc north to the bridge. It is a notorious bottleneck, says Smith.

The project will therefore extend the spur some 3km to join into the larger dual carriageway A90 which has always been the main route to the road bridge from Edinburgh and the east.

The A90 is being upgraded to dual three lane road for 2.5km and will virtually merge via a sweeping curve into the extended M90 two lane section.

The existing A90 will pass under a new bridge section westbound, while eastbound traffic peels off on a new slip road. The underpass, which incorporates 100 precast beams 26m long, is the biggest structure on the project.

Morrison must keep two lanes of the A90 open each way during the day, although single lane each way working is allowed at night.

Daytime work involves widening the road next to the busy carriageways using just the 3.65m single lane width of the grass verge to work in.

'That has been difficult in places because the road will have a much changed vertical alignment, ' says Smith. Two existing overbridges have to be dealt with, and for these the road needs to drop down to achieve the headroom needed.

Though the changes are only up to 1m, excavation for road base and structure has meant differentials of upto 2.5m between the existing live carriageways and excavations for the new. In a few places some temporary sheet piling has helped matters.

As the new third carriageway is built, traffic can be moved over onto completed sections.

Altogether a seven phase traffic scheme is to be used for replacing some of the new outer carriageway. 'We have to build part of that as temporary road and then remake the new levels when we have more access, ' explains Smith.

Meanwhile, the M90 extension has had its complications too as it has the pipleline and two other crossings to contend with.

One of these is a road with a busy industrial depot on it. Building the new bridge involved moving the existing road sideways onto a temporary bund, leaving space to create an overpass.

'It is quite awkward because much of the new motorway section is in cutting up to 7m deep and this is the deepest part, ' says Smith.

The temporary bund has had another function as it acts as a dam along the alignment. 'And there is only one drainage point on the project, a burn further eastwards, ' he adds.

Luckily last summer was especially warm and dry and the only problems occurred in November which, conversely, was one of the wettest on record. Working out a good sequence of pumps and lines has been necessary.

The warm summer helped local subcontractor MacGregor Plant Hire push ahead with the muck shifting. The contract design eliminated off site disposal for the glacial Blue Clay which makes up the area.

'But we do need fill imported, ' says Smith. Some 650,000m 3 of extra material is needed for embankments.

It comes from one of the spent oil shale, or 'blaes' tips in the area.

The material is highly visible on a new embankment for a crossing at the existing M9 end where it connects with the new.

Traffic meanwhile has been diverted onto a 800m long temporary road alongside the old M9 spur end.

The project is on schedule to finish by autumn.

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