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Nailing down the problem

Britain's first use of carbon fibre soil nails is solving separate problems faced by Railtrack and its contractor extending a North East metro. David Hayward reports.

The extension of Newcastle's Tyne & Wear metro westward to Sunderland is a UK first for both rail and construction engineers.

A novel partnership between Railtrack and public sector operator Nexus, it will see light and heavy rail traffic not only sharing the same route but, for the first time, the same tracks. And less visible though equally record setting for construction is what lies hidden in the slopes of new rail cuttings - over 7,000 lightweight Italian soil nails.

Railtrack's unusual £40M stake in the total £98M expansion of the metro is regarded as a new market opportunity in both sharing tracks and the future economic growth of the rail network. To extend the existing Tyne & Wear metro network 11km to Sunderland, Railtrack is using its own framework contractors to convert and electrify an existing twin track rail link already operating between Sunderland and Newcastle. An average of six metro trains a day will slot into this route alongside a less frequent diesel commuter service.

But extending the metro beyond Sunderland, and 5km through western suburbs to South Hylton, involves the resurrection and reconstruction of a long abandoned and infilled 1960s rail line from Durham. All that remains of the original cutting is its route, preserved as a narrow cycle track.

Here, a key role of main contractor Christiani & Nielsen, with its partnered subcontractor Bachy Soletanche, is to re-excavate the old rail cutting and create, in the tight confines of this largely urban route, a 10m wide track bed in a 6m deep cutting.

At tender stage, some three quarters of Bachy Soletanche's £4M subcontract cost was earmarked for concrete retaining walls needing 1,800 large diameter bored piles. But a decision to import the carbon fibre soil nails from Italy triggered a major value engineering exercise along the entire route. At Sunderland, the nails' installer, foundations specialist Bachy Soletanche, claims the technique is proving twice as quick and half as expensive as concrete piled retaining walls. Steeper slopes can be formed compared to an engineered or earth reinforced embankment and the lack of clutter at the bottom of the slope allows the railway trace to double instantly as a haul road through the site.

But something even more unusual is being achieved at Sunderland. The UK's largest soil nailing operation to date is under way in slopes just a few metres from the planned route of electrified metro trains. What about the stray current?

Railtrack has always had a problem with steel soil nails. If installed too close to an electrified track stray currents can earth through the steel rods - which does no harm to the track but can lead to severe corrosion of the metal nails themselves.

Carbon fibre, although a good conductor of electricity, suffers no such problems.

One senior rail engineer who was instantly impressed is Railtrack's project manager Kieran Dunkin. 'Soil nailing offers a clean, quick and cheap way to stabilise rail embankments but is generally ruled out by the likelihood of steel rods attracting stray currents, ' he says. 'Carbon fibre nails seem the ideal solution and I see their use expanding considerably in future rail contracts.'

On this contract piling has been reduced by 40%, with soil nailing - able to support a range of slopes from 25degrees-55degrees - now stabilising 1.7km of embankment.

The average 45degrees slopes need up to seven rows of nails inserted as the cutting is excavated downward in two 3m deep benches. Positioning an upper row of conventional steel nails would need not only lifting machinery but also a man rider and a total team of three people.

The carbon fibre version, five times lighter, requires one man.

Now three quarters through the six month contract, Bachy Soletanche project manager Julian Gatward claims total insertion rates for his fleet of six soil nailing machines of 160 nails a day - double a good average had they been made of steel.

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