Soil nails provide a wealth of advantages over other slope stabilisation methods. Half the cost and twice as quick to install as concrete piled retaining walls, they can be used to form steeper slopes than an engineered or earth reinforced embankment.
A third important benefit peculiar to carbon fibre nails has led to their use on Britain's largest soil nailing operation, the ú98M ($141M) extension of the Tyne and Wear metro for the Railtrack-led consortium Sunderland Direct.
Foundation contractor Bachy Soletanche was asked by main design and build contractor Christiani & Nielsen to value engineer the best way of stabilising the 6m high steep-sided slopes for the 5km of cutting through suburban Sunderland.
Bachy Soletanche knew that conventional steel nails would be unacceptable, as they could act as an earth for currents induced in the metro tracks, leading to severe corrosion of the nails.
Carbon fibre nails made in Italy by manufacturer and supplier Sireg, using raw material from Japan, not only solved this engineering problem but are also cheaper than steel, do not require corrosion protection and are light and easy to handle.
For the client, the fact that they do not act as an earth was the deciding factor when it sanctioned this first British use of carbon fibre soil nails.
From the contractor's point of view, their light weight is a major advantage - 'about the weight of two bags of sugar, ' estimates Bachy Soletanche project manager Julian Gatward as he balances a 7m long, 16mm diameter carbon fibre nail on one hand.
'That's five times lighter than an 11kg steel equivalent which would need a crane or excavator to raise it to a hole 3m up the embankment.'
The high 45degrees slope needs up to seven rows of nails inserted as the cutting is excavated in two 3m deep benches. To position the uppermost row of steel nails from each bench floor would not only need lifting machinery but also a man rider and a threestrong team. The carbon fibre version requires one man with a good javelin-style aim.
Gatward claims a daily rate of 160 nails for the six machines Bachy has on site - double a good average rate for steel.
Progress is helped by the fact that most of the firm's ú1M fleet of soil nail rigs - Casagrande C6, Mustang, Klem and Soilmec - are fitted with a carousel attachment for easy extension of drill rods.
This machine gun style framework holds two additional drill sections, allowing them to be automatically added or removed from the drill string as it bores up to 12m long holes for each nail.
Operating 3m above the working platform avoids the delay of lowering the machine each time the drill needs extending.
At tender stage, Bachy Soletanche had planned on using 1800 nails. The value engineering exercise increased that total fourfold and the operation, twothirds complete, now accounts for half the ú4M subcontract.
'We exhausted our Japanese source of carbon fibre and later supplies have come from the US, ' says Gatward. 'Bulk supply of nails totalling 45km means they work out 20% cheaper than steel and I reckon future use can only grow.'