Workers Protected During Emergency Works on Farnworth Tunnel

Project Overview

Farnworth’s first two-track tunnel was engineered by John Hawkshaw, who was well-known for his work on the Severn Tunnel, a significant engineering feat. The Farnworth Tunnel was constructed for the Manchester Bolton & Bury Railway between 1835 and 1838. At the centre of the 270m tunnel, light shone down a shaft sunk from the surface, which was covered by a glass dome.

With the introduction of Pullman coaches on the Midland Railway route between Manchester and Carlisle, the track gauge proved too small. The tunnel was also in need of extensive repairs. Instead of enlarging it, the solution was to bore a second tunnel for the ‘Down line’. This took seven months and it opened in December1880. The lining of the 1838 tunnel was repaired and thickened, and the two tracks were reduced to a single track in the centre. This tunnel was then used for the Manchester-bound ‘Up line’.

The older tunnel was from the ‘pioneering first phase’ of railway building and was accorded a Grade II listing by English Heritage. However, with the next stage in its evolution in 2015 – the driving of a bigger bore on a different alignment – Hawkshaw’s structure, along with its shaft and 13 passages, ceased to exist

Challenge

This £20.8 million project was completed as part of the North West Electrification Programme, with the conversion of the diesel line between Manchester and Preston to an electrified line. As neither of the tunnels could accommodate two tracks and overhead line equipment, a larger one needed to be re-bored in the older tunnel.

Firstly the 1880 tunnel, which had been affected by water damage, needed to be strengthened in order for a reduced train service to operate, while the new tunnel was being mined. Three groups of workers installed a mesh frame to the internal walls, sprayed 200mm shotcrete, and replaced the deformed lining along a 20m stretch. A diesel plant was operated within the tunnel.

The 1838 tunnel was to be filled with 7,500m3 of specially formulated foam concrete and then re-bored using the UK’s largest tunnel boring machine, ‘Fillie’, which took 12 weeks. Despite initial ground investigations, sand unexpectedly poured into the excavated area on two occasions, and 100,000 tonnes of material had to be removed by hand. As a result, the TBM broke through on 25 October 2015, 21 days later than planned. However, the new tunnel still opened on schedule, on 14 December 2015.

Solution

As the UK specialists in air quality control, we were pleased to assist with this unique project. For the first phase, J Murphy required a temporary ventilation system to ensure a constant flow of fresh air, for a safe working environment. Using an 800CF ventilation kit with an adjacent generator, we forced fresh air through the tunnel via 30m of 800mm duct, creating an entrainment of air. This kit was in operation for three months.

The second phase posed challenging conditions at times for the crew. We specified and provided a VENTEX 800CF fan to extract and filter the dust created by the boring works. The intake of the fan was connected to a 300mm duct and a filtration bank on the fan outlet. This system extracted continuously from the workface, via 270m of 300mm duct, at a rate of 21m3 per second. In this way, the effect of the dust on the workers’ environment was minimised.