The Farnworth tunnelling project to enable electric trains to run between Bolton and Manchester has been delayed, due to exceptionally poor ground conditions.
Tunnelling was originally scheduled to be complete by 5 October, but is now likely to be delayed until December.
Network Rail engineers enlarging the Farnworth tunnel, which was originally built in 1834, have run into large swathes of sand.
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 9m-wide tunnel boring machine (TBM) bores through it.
The TBM, named Fillie, was expected to be able to tunnel 5.6 to 7m every 24 hours. Due to the poor ground conditions, the machine has only been able to bore at a rate of 2.8m every 24 hours.
Martin Frobisher, route managing director for Network Rail, said: “We first hit an area of running sand on 14 August 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 35t of grout.
“Again on 27 August, sand poured into the excavated area and our engineers had to remove 100t of material by hand.”
Frobisher added: “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.”