The scheme to widen Farnworth Tunnel near Bolton includes enabling works which saw their fair share of innovation with a sensitively applied shotcrete strengthening solution.
The project to enlarge Farnworth Tunnel near Bolton will eventually see a much more efficient rail service due to electrification of the railway between Manchester and Preston, with faster, more environmentally-friendly trains with more space for passengers, benefitting individuals and the local economy.
The project recently achieved a major milestone for main contractor Murphy and client Network Rail with the 9m diameter, 293 tonne tunnel boring machine breaking through the up-tunnel, which Murphy had filled with 7,500m3 of foam concrete. This was re-bored to provide an enlarged tunnel to house two tracks running north-west and south-west, and offering the necessary overhead clearance for electrification.
The two original 270m-long brick tunnels were constructed between 1838 and 1880 and were showing their age – a survey to the down-tunnel showed water ingress had caused the tunnel lining to deform with open joints and cracking. The electrification project was also an opportunity to ensure that this key railway infrastructure would be secured for the future by addressing the whole structure. 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.
Murphy sprayed a new 200mm reinforced shotcrete lining through the tunnel but this was far from a straightforward job due to the fact that Network Rail would continue to use the tunnel for regular passenger services throughout the project. This meant that the work would have to be completed in 11 weekends, with Murphy only able to take possession of the tunnel for 54 hours at a time working between midnight on Friday and 6am Monday morning.
This required careful planning by Murphy as the team needed to protect the track and hand back the tunnel to Network Rail at the end of each possession. Every Tuesday morning during the project Murphy held a session to discuss the lessons learned from the previous possession and how to improve any areas so that work would progress as efficiently as possible over the next weekend.
The steel reinforcement for the down-tunnel lining consisted of A252 mesh pre-cut into 1.2m by 2.4m lengths to enable a gang of three or four Murphy staff to install it easily and safely. Murphy demonstrated innovative use of road rail vehicles (RRVs) by designing a bespoke scaffold which was mounted on an RRV trailer to safely install the steel reinforcement.
Fineturret Limited carried out the precision spraying of the shotcrete onto the mesh, using a Meyco Oruga robot mounted on another RRV trailer specially adapted by Murphy to accommodate its range of movement. The shotcrete was a CPI EuroMix product located in four 30 tonne silos sited on the surface above the tunnel’s central ventilation shaft. This enabled the mix to be conveniently pumped down the shaft and directly into the tunnel. A total of approximately 1,800 tonnes of the CPI shotcrete was used for the down-tunnel reinforcement, with 100 tonnes a day being pumped in during some of the possessions.
The CPI silo approach provides the material in a dry state, meaning that it was flexible to the sensitive schedule of this project unlike ready-mixed alternatives that have a limited shelf-life in comparison. This novel method, first seen on Crossrail, allowed Murphy to switch on the electrically powered silos to blend the mix with water at the nozzle as and when required, meaning that waste was virtually zero. CPI made deliveries as needed to replenish the silos so that Murphy was able to use the shotcrete in high volumes at its own convenience.
The second major constraint, which presented an engineering challenge for Murphy, was a range of tolerances within the tunnel with some sections being narrower than others. Here only 170mm of shotcrete could be used to maintain enough room for train clearance.
The crown of the existing brick lining was also deformed in one 12m section of the tunnel, which meant that spraying was not an option. Murphy’s design engineers decided that the section had to be removed and that ten steel rib arches should be installed to strengthen the structure, before installing the mesh and shotcrete lining. Murphy designed a special swivel arm-lifting device that was hydraulically attached to an RRV so that staff could install the upper sections of the rib arches safely, without any manual handling being required. This alone saved approximately 60 hours over the whole project.
The down tunnel reinforcement project has been carried out to Eurocode standards, meaning that Murphy has helped protect this important structure for Network Rail for another 100 years. A challenging project, Farnworth tunnel saw Murphy display its expertise in employing an innovative yet careful approach to working with shotcrete at low tolerance.
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