Update on Gotherington Slip
article by: Darren Fairley
posted on: 10 June 2010
updated on: 21 August 2010
The slip - the facts
Although there seems to be little new external movement around
the area of the running line the slip is a major one.
Monitoring has already recorded significant movement deep in the
core of the embankment, far greater than happened on a number of
the Severn Valley Railway's slips prior to their catastrophic
failures following heavy rain in 2007.
Bore holes are being inserted to assess the below ground
movement and to decide how best to arrest or repair the damage
dependant on what is found. The area affected is much longer and
deeper than the slip at Cheltenham two years ago and there are in
fact at least four more separate areas of movement along the
entire length of the loop embankment, which is about
500 yards long.
There is no doubt that we made the right decision to stop trains
when we did. The ground has sheered between the running lines in an
arc round previous BR repairs. The main line support would have
deflected with continued train movements and the additional damage
that would have resulted would have made the reinstatement works
much more difficult and expensive. The area which was repaired by
BR when the railway was still operational has not stood the test of
time. The bore holes have established that ash fill was used and is
much deeper in the loop section than on the main line - showing the
repair was a patch and make-do.
The present slip is clearly related to British Railways repairs
and has been affected by poor ground drainage in the area.
Water on the east side drains diagonally, following the furrows of
the mediaeval 'ridge and furrow' farmland over which the line was
built and through the area that has slipped. The very cold winter
and snow loading has also been a factor. Whilst badgers are in the
area and there is some rabbit damage, we are confident that animals
are not responsible for the slip.
The repair will be a major engineering work. It is not just a
matter of digging out and replacing the damaged embankment. The
existing embankment under the running line is now fragile and the
track sits on the edge of a sheer face fracture in the
embankment.
What happens next?
The ground level surveys are complete and they are some of the
most detailed I have seen. They show a number of reasons why
the water sits were it does and what we will need to do to rectify
the situation. For example, a pond on the east side is 1.5m higher
than the point at which water is percolating out on the opposite
side; the water is travelling directly along an ancient furrow with
no active drainage to intercept it and the clay is therefore
starting to slump again.
Soil samples are being taken and analysed to best decide what
system(s) will be employed to arrest further movement and just how
far down the excavation works will need to extend. Each of the
slips could have a different mechanism / method applied to them and
we will be able to choose the most cost effective option when the
report is completed and conclusions presented. The site works will
be concluded at the end of May and a report issued with options and
assessment mid June.
We will now start removing the signalling cables and fittings as
they are now becoming stretched and likely to be damaged with
further movement.
Once the final bore holes have been completed, we will be able
carefully to remove the loop track work. This will have to be
undertaken in a manner which prevents the bore holes from being
damaged, probably by unclipping and dropping the sleepers and then
pulling the rails out from either end. We will not be able to use a
road rail machine from the running line directly at the slip site
due to risk of topple as there is no support to the shoulder of the
track.
Repairing the embankment
The insurance loss adjuster has visited site and has agreed to
fund a major part of the repair claim. The repair options have not
yet been concluded but it will nevertheless be a major civil
engineering undertaking and as such will be subject to optioneering
and tendered to give us the best repairs, at the best price. The
works will be broken down into suitable packages, but the main slip
and drainage repairs are clearly not within the scope of our
volunteer workforce.
It has already been established that early work will be required
to establish new drainage and it is likely this will have to be
undertaken before the main repair starts. We have also identified
other areas where drainage improvements are an absolute priority
and these could be undertaken within existing budgets if
efficiencies are identified from doing them at the same time.
Conclusion
I can not at present give an indication how long the repairs
will take, as it will depend on a number of factors including what
repair options we choose. However do we have one of the best
engineering contractors dealing with the slip and as such I have
full confidence in the monitoring works and assessment that they
are undertaking, based on my own experience of dealing with similar
works on the main line.
The accompanying photographs underline the severity of the slip
and have been annotated to show how and where the damage has
happened.
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