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CALLED TO THE BAR

Flat strip carbon fibre soil nails are making their UK debut on CTRL contract 103.

One of the most intricate of the Channel Tunnel Rail Link contracts is unquestionably no 103, immediately to the north of King's Cross Station.

Within a few hundred metres, joint venture contractor Kier/ Edmund Nuttall has to deal with the East Coast Main Line, Midland Main Line, North London Line, Regents Canal and York Way.

And as the high-speed line nears its destination of St Pancras, there is very little room to play with in terms of vertical alignment.

Set against the scale of the overall project and almost heroic nature of much of the engineering on contract 103, you might expect geotechnical contractor Ritchies' novel use of carbon fibre soil nails to be on an epic scale too.

But as is often the case with geotechnical innovations, its groundbreaking subcontract is rather low-key. Tucked away at the northern end of the section, in a nailing scheme designed by CL Associates, Ritchies is using the carbon fibre nails to reshape the abutment wingwalls of a Victorian bridge that carries the North London Line railway over York Way.

The work is needed because Kier/Nuttall jv is realigning a section of York Way as part of the elaborate CTRL enabling works.

The realigned York Way rejoins the existing road immediately south of the North London Line; but, as Kier/Nuttall sub-agent Fiachra Page explains, the carriageway needs to be lowered by 2m as it passes through the bridge, known on the contract as Bridge 126.

Not only does the bridge have to be underpinned, but the square brick abutments on its south side are being cut back to improve sight lines, in particular for traffic exiting the adjacent Camden Depot.

The two resulting 9m high vertical wingwalls are constructed top-down. Soil nails are covered by mesh and the whole face covered in sprayed concrete, fully encapsulating the nail heads. A cosmetic brick facing should, over time, visually merge with the original brickwork.

Nothing too dramatic in that, but Ritchies' business development manager David Gibson believes it is only the second use of carbon fibre soil nails in the UK, and certainly the first use of a flat strip 'nail' Nail is a somewhat misleading term, as the 30mm wide, 4mm thick flat bars are similar in profile to a plastic ruler. The nails, supplied by Italian geotechnical equipment manufacturer Sireg, are made up of longitudinal carbon fibre strands within a vinyl ester matrix and have tensile strength of 250kN. The outer surface of the bar is coated with a coarse sand to ensure a good mechanical bond with the surrounding grout.

Installation is similar to conventional soil nailing, albeit much easier in most respects. The 9m long nails are manually placed in 100mm diameter rotary augered holes and grouted into place using a pure Ordinary Portland Cement grout. Nominal tension - about 10kN - is applied to the nails before they are locked off using a standard endplate with Sireg's special slotted wedges that clasp the bar.

Ritchies will complete working down from the top in seven lifts, installing nails at 1.2m centres horizontally and 750mm centres vertically. The 250mm high-strength structural sprayed concrete is built up in two layers and incorporates two layers of mesh.

The bar is very flexible and lightweight, which is the main reason project engineer RLE specified it. With a confined working space, installing conventional nails would have meant using many couplers to join short sections of nail together, explains Gibson; particularly on the west wingwall where the new face is tucked in tight beside structures on the Camden Depot site. The carbon fibre's flexibility meant all the nails could be inserted as single lengths.

For the same reason, carbon fibre nails have big advantages when long nails have to be installed from scaffold. Because there is no additional corrosion protection required, the nails are easy and very quick to assemble and install.

The carbon fibre is delivered to site in 100m lengths, coiled into an approximately 2m diameter hoop that can be safely carried by a single person. Ritchies simply cuts the strip to length on site using a hacksaw - getting 10 nails out of each coil. Spacers are placed over the bar to ensure they sit centrally within the augered hole.

The only downside, says Gibson, is the care is needed when cutting the strip to length, as the bar can otherwise fray and lose its integrity - and if the bar is knocked about it can chip.

Although carbon fibre is more expensive than steel per metre, the ease and speed of construction can make it cost effective, particularly on congested sites such as this.

'As always the biggest difficulty is getting people to accept a new technology, ' says Gibson, 'but I expect to see more use of carbon fibre. It is after all more established on the Continent.'

Ritchies is now installing the eastern wing wall, which includes 140 nails.The western wing wall, with 245 nails, was completed last year.

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