Download Flat strip carbon fibre soil nails are making their UK debut on CTRL

ne 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 lowkey. 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 topdown. 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
O
14
Flat strip carbon fibre
soil nails are making
their UK debut on CTRL
contract 103.
I nstallation of the flat strip carbon fibre nails is similar to conventional
soil nailing, albeit easier. Left: end plate detail.
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 I00mm
diameter rotary angered 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
t wo 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.
GROUND ENGINEERING MAY
2004