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Strengthening secures tunnels’
future
Ore Tunnel remedial works
BAM Ritchies working with BAM Nuttall carried out remedial
stabilisation in Ore Tunnel, East Sussex to address lining
deformation and water ingress problems. Where bulging of the
side wall had occurred, affecting the train kinematic clearance
envelope, the solution consisted of 6.0m long glass-fibre
anchors. Drilling was completed by a road-rail excavator-
mounted drill mast. Grout-socks were used to limit grout loss
into the voids behind the tunnel and open rock joints.
The ground conditions surrounding the tunnel consist of weakly
cemented sandstone with bands of silt, as well as mudstone
with persistent, strong ironstone bands. Owing to access restrictions,
the ground investigation information was limited so the root
cause of the tunnel's problems was difficult to diagnose.
BAM Ritchies worked collaboratively with BAM Nuttall, White
Young Green and Network Rail to develop the design as information
became available through the project duration.
To deal with water ingress at various locations, holes were
drilled to 5m depth and 112mm perforated pipes installed with
geotextile socks to conduct water to pipework leading to an
existing drain.
Sizeable voiding between the tunnel lining and the surrounding
ground - which occurred at various locations - posed a risk
to the lining as sandstone blocks could fall and cause damage.
The solution was to drill holes and inject a polymer void-filling
grout which also sealed the tunnel, reducing water ingress.
Folkestone tunnels
A series of stabilisation measures were carried out by BAM
Ritchies at Abbotscliffe and Shakespeare tunnels, forming
part of major ongoing maintenance on the 160 year-old Folkestone
tunnels. Abbotscliffe is a 1,800 metre long twin-track tunnel.
With increasing frequency, the locally famous Lyddon Spout
erupts, suddenly releasing a large build-up of water from
within the surrounding chalk. An adit - used to drain the
groundwater that enters the tunnel, transferring it out to
the sea - was showing signs of distress as a result of rock
fall failures. Stabilisation was required to ensure the safety
of the maintenance crew working on the drainage channel so
108 resin-fixed glass- fibre rock bolts were installed and
tested. Rock fall mesh was placed along the bolted section
and a tell-tale monitoring to record future movements was
set up.
Shakespeare is a twin-bore tunnel, approximately 1,200 metres
long, which exhibit a Gothic influence as both bores consist
of high arches. During previous works, when tie bars were
installed through the pier between the two tunnels, a series
of voids were found. Network Rail required these voids to
be filled to ensure the tie-bar scheme would perform as designed.
100 holes were drilled from one bore towards the other and
perforated pipes installed. Neat cement grout was then injected
through these to fill the voids.
Rodney Street tunnel strengthening
In Edinburgh, BAM Ritchies Concrete Techniques section worked
on strengthening an old railway tunnel as part of the Sustrans
National Cycle Network.
Sustrans is constantly reviewing the status of the National
Cycle Network and is working with local authorities and other
partners to bring all sections of the network up to standard.
The Rodney Street project at Canonmills was part of this programme.
The tunnel was originally built to carry trains to the Granton
Ferry before the Forth Rail Bridge was constructed. The rail
line was closed in 1968 and the area beneath the tunnel was
partially infilled. As part of the development of the National
Cycle Network, Sustrans strengthened and reopened the tunnel
to make it safe for use by cyclists.
The future aspiration, in partnership with the council, is
to use the tunnel as part of Route 75, enabling cyclists and
pedestrians to avoid the busy road junction that sits above
it.
Prior to any works being undertaken, traffic management reduced
live traffic above the section of tunnel to be lined to a
single carriageway as cover to the tunnel crown was only 300mm.
It was necessary to survey the road levels to monitor any
potential road settlement.
The strengthening works were undertaken in two phases to
coincide with the traffic management regime. Each phase consisted
of daily monitoring of road levels, excavation of backfill,
application of initial steel fibre reinforced sprayed concrete,
fixing of two layers of steel reinforcement and application
of 400mm sprayed concrete. A final work layer of polymer fibre-reinforced
sprayed concrete was applied to the whole area to provide
fire resistance.
Elsenham footbridge
In 2005, two children were killed crossing the railway at
a level crossing at Elsenham Station. As a result of this
tragedy, a footbridge was designed to take the public safely
over the line.
BAM Ritchies installed 34 mini piles - each 7m deep and with
a nominal diameter of 350mm - using auger drilling and casings
to support the new bridge. The piles were designed to take
the weight of the constructed footbridge and also withstand
a collision load if a derailed train hit the structure.
Piles were reinforced with a full- length reinforcement cage
in three short coupled sections owing to adjacent overhead
power cables and then grouted with a neat cement grout. Although
the piles were close to the track, work was completed without
disruption to traffic or inconvenience to the public.
Article courtesy of The Rail Engineer - may 2009
Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
peter.bishop@bamnuttall.co.uk
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