PROJECT OUTLINE

Scheme to enlarge Farnworth Tunnel near Bolton to allow the electrification of the railway line between Manchester and Preston.

Two original 270m-long brick tunnels constructed between 1838 and 1880 at Farnworth were showing their age with water causing the lining of one of the tunnels to deform with open joints and cracking. The electrification project was also an opportunity to ensure that the structure would be secured and safeguarded for the future by Murphy carrying out strengthening works.

Murphy had to fill the up-tunnel with 7,500m3 of foam concrete before re-boring it using the UK’s biggest tunnel boring machine (TBM), Fillie, to create a bigger tunnel housing two tracks with room for overhead electric cables.

KEY CHALLENGES

• Network Rail continued to keep the tunnel open for passenger services throughout the project meaning that Murphy had to work in limited time periods.
• The tight timeframe of only 11 weekends meant only being able to take possession of the tunnel for 54 hours at a time.
• Work was carried out just metres underneath the A666 – a main road linking the M61 and Bolton – and 1.5m from a live railway in the adjacent tunnel.
• The team came up against difficult ground conditions that meant before re-boring the up-tunnel, Murphy had to strengthen it by spraying a 200mm reinforced shotcrete lining through the tunnel.
• Difficult ground conditions meant the solutions had to be created as the project work went along, including further reinforcing the tunnel before the TBM could be put to work. The 7,500m3 of special foam concrete had to be soft enough to bore through but sturdy enough to prevent a collapse of the structure, so a special mix had to be created.
• To remove 500,000 tonnes of material from the tunnel while not compromising the structure.

PROJECT DELIVERY & INNOVATIONS

The original 270m-long tunnel was lined with a mixture of brick and interlocked solid stone, running through a mixture of ground – laminated clay, sand, glacial fill and silt – which posed challenges for Murphy in undertaking the re-bore. The up-tunnel which previously took trains north east on a single line was enlarged by 3m to accommodate two new electrified rails.

The single track of the down-tunnel remained in use during the up-tunnel works but required strengthening for the TBM drive and to safeguard the structure. Over 1,500 ground investigation boreholes were drilled by Murphy to establish the ground types which would need to be handled by the tunnelling machine. Up to 500,000 tonnes of material was removed using a feeding conveyor belt and Moxy trucks.

Almost 2,000 concrete lining segments which make up the tunnel were put in place by an erector arm within the rear tail-skin section of the TBM. This process drives the TBM through the tunnel – as each ring is completed the whole TBM is jacked forward against the previous set of rings.

The special design of the TBM also allowed interchangeable devices to be mounted onto the shield to address the varying ground conditions. These included fore-poling boards to provide initial ground support at the face with centrally mounted twin mining booms, which could carry a roadheader drum cutter, hydraulic breaker or bucket excavator, depending on the ground type encountered.

The project - from start to finish - took a year, including design, build, testing, delivery, commissioning, construction of the TBM, tunnelling and dismantling of the TBM. 

AWARDS

• Highly commended at the RICS NW Awards 2016 in the category of best infrastructure project. 
• Murphy won Project of the Year and was selected as Health & Safety Winner, at the North West Regional Construction Awards 2016.