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Mission: impeccable

PILING & DEEP FOUNDATIONS

A change of foundation design after pile testing has saved time and money on a replacement railway viaduct in south west Scotland. Max Soudain reports.

Traffic on the Kilmarnock to Dumfries rail line in south west Scotland has been dominated by freight trains since the mid-1990s. Apart from when Hollywood paid a visit that is.

In 1995 the makers of Mission: Impossible wanted to film a helicopter chasing a Eurostar passenger train into the Channel Tunnel for the movie's final action sequence. They did not seem to mind that the tunnel portal in the Upper Nithside valley looked completely different from the real one in Kent, 650km to the south.

After its 15 minutes of fame, the Kilmarnock to Dumfries to Carlisle line reverted to its main job - forming an important part of the Third Anglo-Scottish Freight Route, a key rail link between London and Glasgow.

This is now being upgraded as part of plans by rail network manager Railtrack Scotland to transfer more freight traffic from road to rail over the next decade.

The improvements will allow more flexible train movements and heavier and longer trains to use the line.

Near Auldgirth, 15km north of Dumfries, the line crosses the River Nith on the Portrack Viaduct. Some 125M. t of freight passes over the structure each year and it is beginning to show signs of wear.

Carillion Rail site manager Ian MacDonald says: 'One of the piers is sinking and tilting and the bearings have gone, so Railtrack Scotland was worried about movement.' Carillion Rail is the main contractor on the design and build scheme for a replacement viaduct alongside the old one, that will allow heavier trains to pass safely.

Work also includes building 800m long clay fill embankments up to 5m high, north and south of the viaduct. Design was carried out by Carillion Rail's consultant Scott Wilson.

Ground conditions beneath the viaduct piers and abutments are boulder clay and sands down to 6m with very dense gravel and cobbles beneath. Groundwater is at about 2m. To the south, ground steadily worsens, with up to 10m of soft and loose alluvial material and peat deposits.

The poor ground meant the abutments and three piers for the 90m long steel truss girder viaduct would have to be piled.

Bury St Edmunds-based contractor Piling Solutions won the contract, but not before it had proposed changes to the foundation design.

'Due to the ground conditions the early scheme for the abutments and piers was to use Odex piles down to bedrock, ' explains piling manager Gary Brindle.

'This would have meant installing a large number of piles up to 20m long, which would have been expensive. Instead we proposed shorter, more heavily reinforced injection piles founded in the gravels.' Shorter piles means faster installation and less materials - with obvious savings on the original scheme, he adds.

Similar to CFA piles, injection piles are formed using small piling rigs with shorter masts which means hollow stem sectional augers rather than a single auger have to be used to form the pile.

To prove its proposal would work, Piling Solutions installed two 600mm diameter test piles, 11m and 13m long, on both the north and south abutments in March this year. Testing was carried out using digital logging to allow full analysis of the results.

'The results were as predicted, ' Brindle says. 'When subjected to double the working load of 1000kN, settlements were between 12mm and 15mm, which was a good performance.' This convinced Scott Wilson and Carillion Rail that the alternative design would work.

Construction of an access road to the remote site began in April.

Piling Solutions arrived in May to start the first phase of its £450,000 contract, which involved installation of the piles for the north abutment and the three piers, plus the south abutment.

Brazilian-made Clo Zironi rigs were used on this phase, two CR18s and a larger CA40. All three are designed for difficult access work, Brindle says. While the CA40 has a 18m high mast and could install the piles using a single auger, it is still smaller than a normal CFA rig, he adds.

Conventional equipment would have been too heavy for the poor ground, Brindle says, and height restrictions, based on fall over distance, meant that even the CA40 could not be used within 18m of the railway. This meant using the two CR18s on the piles closest to the viaduct, with 1.2m long sectional augers used to form the up to 9m long piles.

Ground level was lowered by 1m to create a working platform, further reducing pile lengths and the amount of cut-off needed.

Work then began on the 70 piles for the 23m by 11m pier which, along with the south abutment, will support the largest span of the new viaduct where it crosses the River Nith.

The other two piers are 16m by 8m and have 32 and 28 piles respectively, while the 22m by 13m north abutment has 72 piles.

Typical spacing is 2m and all the piles are reinforced with 8.5m long cages of T32 bars, weighing between 300kg and 400kg - 'fairly meaty', says Brindle. Pile caps are 1.5m thick.

The first six week phase finished in mid-July. Before leaving, Piling Solutions carried out another pile test for a smaller bridge 400m south of the south abutment.

Despite the poor ground conditions in this area, test results were very positive, reports Brindle, with only 6mm of settlement under a 1,500kN test load.

The pile also held over 3,000kN and a 200kN lateral load test was also successfully carried out.

Brindle expects work on the small bridge to start in the autumn. Piling for the south abutment is likely to be carried out before the end of the year, with about 70 piles installed to depths of 12m as the gravels here are deeper, overlain by a thick lens of alluvium. Additional work may include piling for settlement control of the section of embankment running over the poor ground between the two bridges, to prevent any differential settlement.

Once the new viaduct is in place next year, rail traffic will switch to the new line, allowing the ageing Portrack Viaduct to be demolished. Although it may not be glamorous enough to feature in a Hollywood production, the new viaduct will certainly be an example of well thought out geotechnics overcoming a difficult (if not impossible) mission.

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