Contractors World
 

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Contractors World - 2015 Vol 5 No 2

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Tunnel Engineers & Poor Ground Conditions

 

A complex tunnelling project to enable electric trains to run between Bolton and Manchester has been delayed, possibly until December, by exceptionally poor ground conditions.

Network Rail engineers enlarging the Farnworth Tunnel, built in 1834, have run into large swathes of sand. Rather than a firm material to bore through, sand pours down into the excavated area halting progress.

It has proved impossible to safely excavate while concurrently installing and grouting sections of tunnel wall as was originally planned.

Engineers are now pumping resin into the ground to firm it up before the 9 m wide TBM - Fillie, the UK's largest tunnelling machine, bores through it.

Martin Frobisher, route managing director for Network Rail, said:

This delay is intensely frustrating for our engineers and, more importantly, for passengers.
    We first hit an area of running sand on August 14 when our engineers saw it suddenly pouring from the working face. This has slowed progress and created big voids, the largest of which needed filling with around 35 tonnes of grout.
    Again on August 27, sand poured into the excavated area and our engineers had to remove 100 tonnes of material by hand.
    The nature of civil engineering, especially deep below ground, is that you never fully know the exact ground conditions until you start tunnelling or excavating. Taking soil samples acts as a guide but is never 100% accurate because conditions vary greatly with the amount of water present.
    The rate of progress is very dependent upon the conditions and we are working around the clock to complete this as quickly as possible.
    Our top consideration is safety. Our engineers face a huge challenge. We must allow them the time they need to tackle it safely. We will of course provide further updates over the coming weeks when we know more.

 

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