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The refurbishment of Conisbrough tunnel is
part of Railtrack’s ongoing programme of improving the rail network.
The tunnel is 235 metres long on the main line between Doncaster
and Sheffield. The 6.1m wide tunnel was causing speed restrictions
on the line due to inadequate clearances and repairs were required
in two areas. The repairs involved taking out the existing
brick lined steel arch reinforcement from previous repairs carried
out in the late 1800’s.
Before this work could be carried out the areas to be repaired
had to be reinforced. A system of stainless steel rockbolts was
used for this, however, the strata behind the brick lining was
very wet and the lower 12m long rockbolt holes, which dipped at
5 degrees to the horizontal, penetrated the water table. This
made grouting the steel bolts very difficult. |
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To improve the strata reinforcement
in the sides of the tunnel, Weldgrip Gripforce
® fibreglass rockbolts were used.
The Gripforce ®
Rockbolts were 31/15 OD/ID tubes
with a tensile strength of 480kN. The tubes, 12 metres long, were
installed in 60mm diameter holes 1 metre above track height and
dipped at 5 degrees to the horizontal.
Four stainless steel centralisers were used to hold the rockbolts
central to the hole, these were positioned equidistant along each
bolt. A 200 kN rated high load nut assembly secured each end plate. |
A Gripforce ®
tube was installed immediately into each hole after
drilling and grouted with Sulphate resistant cementitious grout.
This was to eliminate any build up of water and sludge and prevent
degradation of the holes.
The grout was pumped through the centre of the tube filling
the hole from the bottom. It was noted during grouting operations,
that water was displaced from the bottom of the hole before
grout flowed out.
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A total of 100 Weldgrip Gripforce
® Rockbolts were installed at 1
metre above rail height 50 on either side of the tunnel.
Pull out tests on the Gripforce ®
Rockbolts gave significantly better results than the equivalent
stainless steel bolts. |
An additional 10 Gripforce
®, 31/15 rockbolts 10 metre's long, were used at 3 metre's above
rail height inclined into the roof, to reinforce a fractured section
of the existing brickwork. These were installed using the internal
bore of the Gripforce ®
Rockbolt as a breather tube.
The Gripforce ® Rockbolt and a plastic
grout tube was installed into each inclined hole and expanding
foam used to form a seal. Cementitious grout was pumped through
the grout tube filling the hole from the sealed entrance to the
top of the hole, until grout started to flow from the Gripforce
® tube.
The orifice of the tube was then plugged with a stainless steel
peg. |
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| The advantages of using Gripforce
® fibreglass rockbolts in Conisbrough tunnel were: |
Download the Printable PDF Brochure
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4.2 meg |
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Bar
& 'AT' Rockbolts
Ease of installing 12 metre lengths by being able to bend the
bolts in the confined space of the 6.1m Wide tunnel.
