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SOFT TOUCH

GEOTECHNICS OF TRANSPORT

A soft engineering solution is preventing railway embankment scour on the banks of Britain's oldest canal. Dave Parker reports.

Network Rail engineers spotted something seriously wrong during a 2003 inspection of the mid-19th century embankment that carries the Spalding to Doncaster line alongside the Foss Dyke canal west of Lincoln.

Where a sheet pile wall ended, close to a recently opened pleasure boat marina, the canal bank was suffering significant scour.

'Erosion was getting close to our safety limits for embankments, ' says Network Rail project manager Nigel Lea.

'We began regular monitoring and programmed remedial works for the 2007-08 maintenance period. But the monitoring revealed the bank had been scoured back up to 2.5m in a year, over more than 2km, so we had to take action much faster.'

Dug by the Romans nearly 2,000 years ago (see box), the 18km long Foss Dyke was one of the first UK canals to see a boom in pleasure boating after the disappearance of commercial traffic in the 1950s.

Being wider than most canals - 18m on average - it can take larger boats, which often exceed the 6kph (4mph) speed limit. The wakes are much more destructive than anything the Roman engineers dreamed of.

Protecting this stretch of the canal was not easy. Logistical and environmental concerns meant that the hard option - extending the sheet piling along the area under threat - was far from attractive.

Lea explains: 'There were kingfi shers and water voles to consider.

Access for piling rigs from land would only be possible during railway possessions, and the cost estimates were high.'

Relatively shallow water near the banks ruled out heavy piling barges.

A second feasibility study, however, uncovered a soft option already being considered for another part of the canal network.

'The idea was to install treated softwood stakes in the canal close to the bank to support a protective 'wall' made up of dead willow faggots topped with coir rolls planted up with water plants in advance, ' says Craig O'Brien, rail environment manager for main design and build contractor May Gurney.

'Behind the wall would be dredged material or imported washed stone backfill supporting pre-planted coir mattresses.'

May Gurney won the contract with a £650,000 tender. But when it teamed up with specialist subcontractor Salix River & Wetland Services, the basic scheme underwent some subtle modifications.

Salix sales manager Richard Evans says: 'We did a value engineering exercise and decided we didn't really need stone or gravel simply as a void filler. The coir mattresses needed very little support in practice.

'The sides of the embankment and the bank of the canal were being cleared of bush and scrub as part of the contract, and the original plan was to chip the brushwood and leave it on site. Instead we decided to recycle it into the void behind the faggots.'

Although the coir will rot away in four or five years, the faggots and brushwood will last indefinitely in their anaerobic environment.

Sedges, rushes and reeds will root down from the coir before it disintegrates, while the treated softwood stakes should last at least 50 years, Evans says.

Salix made up the rolls and mattresses from free trade Sri Lankan coir last year and planted them up in its nursery in Wales. By the time they arrived on site in February the water plants were well established and breaking into new growth.

Out on the water, May Gurney's mini-armada is led by a bank clearing barge, which also retrieves and recycles the brushwood.

Behind it comes the stake driving barge bearing a Hitachi long reach excavator fitted with a pneumatic hammer.

Stakes, up to 1.8m long, are driven to refusal then trimmed.

Salix operatives wade in the canal to install the faggots and other components - in the deeper areas up to five layers of faggots are needed.

'Since we started on 6 February we've had temperatures down to - 5degreesC, ' says May Gurney site agent Mark Anderson. 'Nevertheless we expect to complete the job well inside the original 22-week programme - maybe in as little as 14 weeks.'

Concerns about nesting birds ruled out 'devegetation' of the banks between April and the end of August. Several mature oaks will be left in position, as they attract so much wildlife.

Lea reports that checks on the embankment, made up of well compacted sand overlying a sandy clay layer, have confi rmed that no track settlement has occurred so far, and that loss to scour has slowed recently.

He adds: 'This type of scour protection is likely to be used again in similar locations on the network.

We're already looking very closely at using it on the South Forty Foot Drain near Boston, where the Sleaford to Boston line runs close to the water.'

Roman legacy

Believed to be the first artificial navigation channel in Britain, the Foss Dyke was probably dug by the Romans in about 120AD to link the Rivers Witham and Trent. Nearby Lincoln (Lindum Colonia) was one of the four most important Roman cities in England, built on one of the few hills in the fertile East Anglian plains.

After straightening and deepening the Witham to connect Lincoln to the sea, the next project for Roman engineers was the Foss Dyke, giving access to central England via the navigable Trent.

Unlike most more recently built canals, the Foss Dyke also acted as a major drainage channel. It had a long period of decline after the Romans left, but in 1121 Henry I is recorded as ordering the 'scouring of the channel'. It was restored again in the late 17th century, but by 1735 it was impassable to boats and perilously close to disappearing. Fortunately, it was saved by its importance as a drainage channel, and in 1741 it was leased to a certain Richard Ellison. The Ellison family operated it for more than a century, once retaining Marc Isambard Brunel to carry out a survey.

But in 1848, like many other UK canals, the Foss Dyke fell into the hands of its new competitor, the railways. It was owned by the Great Northern Railway until nationalisation in 1950, although commercial traffic virtually disappeared after the Second World War, and no doubt the railway's directors would have been happy to let it silt up.

Once again, the Foss Dyke's vital drainage function proved its salvation, saving it from the fate of so many other railway-owned canals.

It now forms part of a complex drainage system, the Lincoln Flood Alleviation Scheme, which gives a 1 in 100 year standard of protection against flooding from the River Witham, Sincil Dyke and River Till.

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