Tunnel Engineers & Poor Ground Conditions - page 2 of 2

Major Upgrade

The original 276 m long Farnworth Tunnel was built to accommodate a single track for steam-powered trains. It is being enlarged by a 9 m-wide TBM so it can house two tracks with overhead electric equipment.

Creating a pit above the tunnel to safely remove material

Tunnelling started on August 1 and was originally planned to be completed by October 5. Prior to this more than 1,500 soil samples were taken, each one eight metres in depth, to try and get as accurate an idea as possible of the ground conditions to be encountered, but such testing can only ever act as a guide.

The schedule was for the TBM, named Fillie, built in Oldham, to tunnel 5.6 m to 7 m every 24 hrs, but because of the very poor ground conditions, she has only been able to bore at a rate of 2.8 m / 24 hrs.

Looking into the Farnworth Tunnel where engineers are racing against time to overcome unexpected poor ground conditions.

No Simple Engineering Task

Another particular engineering challenge, are the twin tunnels which carry the railway under the main A666 Manchester to Bolton road. These are not big enough to accommodate the overhead lines which will power a new fleet of electric trains in future.

The solution is to keep one of the 270 m-long tunnels open to trains, while the other – just 1.5 m away – is pumped full of 7,500 m³ of foam concrete before a completely new tunnel, big enough for two tracks and their overhead power lines, is re-bored through the new concrete by the TBM and her team of expert tunnelling engineers.

Around 120 people are working round-the-clock on the project, which has already seen:

• the complete demolition of Farnworth and Moses Gate train stations, with work started to rebuild them on their new alignments
• over 1,600 m of tracks lowered
• 7,500 m³ of foam concrete pumped into the tunnel
• the construction of a 300 m motorway standard access road, removing 40-120 lorries from residential roads each day
• a new footbridge at Kearsley
• the installation of a concrete lining and permanent rib supports to the operational tunnel.

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