Now fit for purpose in the 22nd century
Railway passengers planning journeys earlier this year to and from the south west between Monday 18 February and Friday 8 March 2019 were being urged to check before travelling, owing to essential maintenance work that took place at the 1,000-metre long Whiteball tunnel near Tiverton Parkway in Somerset.
Rail Engineer readers with good memories will recall that the work in Whiteball Tunnel is linked to a project which started in 2011, when preparatory work was carried out in the tunnel. The preparatory work caused little disruption to trains but was the launch of a multi-staged plan to significantly improve the state of the old brick lining throughout the tunnel.
The tunnel, named after the nearby village of White Ball, was built by Brunel. The 1,000-metre-long Victorian brick-arch structure, opened in 1844, straddles the Somerset/Devon border. It provides a path for trains to travel under the white sandstone of the Blackdown Hills located between Taunton and Exeter.
Sulphurous steam trains
The clay used for the millions of bricks used for the tunnel lining came from local pits. The engineering bricks produced were of a good quality but, over the years, weathering, chemical reaction from the sulphurous steam trains, voiding behind the brick lining and the degradation of the mortar joints, has meant that there has been a rolling programme of repairs to the lining throughout most of the 20th century, right up to the present day.
Following on from the work carried out in 2011, a similar three-week closure of this route took place in 2014. This enabled this stage of the programme of work in Whiteball Tunnel to take place.
The structure gauge within Whiteball tunnel is very generous, due to Brunel’s ‘broad’ track gauge of 2,140mm. As a consequence, past generations of engineers have been able to carry out substantial repairs, turning new brick arch rings within the existing lining using shields to support the brickwork and workers during construction.
These shields, usually made out of old bullhead rail, were designed to match the profile of the tunnel and they could be clamped together to accommodate the varying lengths of the tunnel that needed repairing. The arch ring structure also included a staging platform at a level that allows trains to run underneath. Over time, these rings were left in situ, forming a permanent feature within the tunnel.
Unhealthy and hazardous working environment
The shields were placed over concrete strip foundations designed to match the length of arch required to be turned. Once in place, skilled bricklaying teams constructed a two-brick arch inside the existing lining. The process was very effective, but also very time consuming and expensive. It required a highly skilled workforce, which had to endure difficult circumstances, working at height in an often very uncomfortable and unhealthy environment.
This hostile environment, coupled with the accelerating deterioration of the tunnel lining, is the reason why Network Rail decided to change its strategy and adopt the Ram Arch System which is in use in the tunnels on London Underground.
A different approach
The Ram Arch System consists of easy to handle, two-metre lengths of galvanised steel mesh. These are bolted together on site to form arch rings one metre wide that are then interlocked and supported on slotted angle brackets fixed into the brick lining about two metres from the cess level. Each ring is secured to the brick lining using two 200mm long, temporary dowels, fixed with a five-second “resin hit”.
Once everything is in place and secure, a more permanent fixing is introduced. The temporary dowels are replaced with five permanent rock anchors that are fixed with a 10-second “resin hit” for each individual arch ring. Finally, spine wires are threaded along the profile of the arch to link the individual arch rings together and provide additional stiffness. These spine wires are then fixed to the now continuous arch structure using a pneumatic Hog Ring Gun.
Once the rock anchors are in place and the spine wires fixed, the nuts on the rock anchors are loosened to create a 2 to 3mm gap between the new galvanised arches and the existing brick lining. This is the work Network Rail completed in 2011 and 2014, which formed Stages 1 and 2 of the project.
In situ or precast?
The work undertaken this year, Stage 3, includes the installation of 1,500 “Butterfly” dowels, spaced 50cm apart, fixed into the side walls of the tunnel following six areas within the tunnel where the Ram Arch system has been installed.
The original plan was to fix precast concrete slabs to the sidewalls of the tunnel to underpin the Ram Arches. However, this was considered too complex, given the irregularities within the tunnel and the length of possessions required to install the units. The dowels were fixed by a two-man team working with a trolley. It was a simple and effective approach and, most importantly, it was carried out without impacting on the train service.
Having installed the dowels, the work carried out during the three-week closure was now ready to start. The principal contractor for Stage 3 was Murphy, with BEDI Consulting undertaking the design work. The work involved spraying ‘shotcrete’ concrete to the profile of the tunnel throughout the repaired areas.
Scott Pillinger, Network Rail’s programme manager for this work, outlined the logistics involved in completing this task and they were challenging, to say the least.
The logistical challenge
The first challenge was the location of the site. The closest access to the tunnel is one mile away from the Taunton end portal and access to it was down narrow, winding country lanes. Throughout the three weeks, two twelve-hour shifts, with 50 or more operatives on each shift, would be working and lodging in the vicinity. Appropriate facilities were required both on the site and within the tunnel throughout the work period.
In addition, as in any tunnel environment, everything has to work in a linear fashion so that what goes in must not hinder what is coming out, and vice versa. But that’s just the easy bit, which most good contractors will deal with almost as a matter of course.
The real challenge was to shotcrete (spray concrete) the tunnel lining throughout the 355-metre length of repair which was, in fact, made up of six different locations in the tunnel. The plan was to spray the concrete in two-metre strips working from the cess to the crown and back down to the other cess. Scott explained that the first week was not as successful as they had hoped but that the second and third week were very productive.
Two robots, ‘Ivy’ and ‘Holly’, carried out the spraying but, to ensure that safety was not compromised, only one could be used in the tunnel at any one time. Each robot was rail mounted with outriggers and was operated by a ‘nozzle man’ standing close by, using a remote control.
Monitoring the sprayed lining
To ensure the quality of spray thickness was maintained, a surveyor was located about 10 metres away. He was in constant communication with the nozzle man and, using a point cloud laser survey, he was able to confirm the depth of the concrete being sprayed. The aim was for a constant 150mm depth throughout the tunnel profile.
While this operation was underway, concrete was being continually transported to site using a rail-mounted concrete mixer to feed the robot. Because of the special design of the shotcrete, with its carefully designed fibre content, the nearest plant capable of providing the quality and quantity required was based in Derby. To keep the site productive, each shift needed up to 60 cubic metres of shotcrete concrete. Murphy had erected eight silos at the main site with a total storage capacity of only 40 cubic metres. Therefore, deliveries from Derby to site had to be reliable and very regular.
A concrete mixer holds about five cubic metres, so it is not necessary to go into any more detail to emphasise how critical it was that every aspect of this project had to work effectively and efficiently. That includes allowing for the vagaries of the somewhat-unpredictable motorway system. The concrete was transferred to smaller wagons on reaching Bristol and, to ensure that there was a continuous flow, wagons were held at strategic locations en route. The narrow lanes close to the site were, according to Scott, “the cleanest in the county”, helping to minimise disruption to the local community.
Review of the ‘bounce back’
All went well and the work was completed on time. However, the team identified that the ‘bounce back’ of sprayed concrete, calculated to be about 20 per cent maximum at design stage, was actually closer to 30 per cent. The track had been protected throughout using boarding and plastic sheeting, so there was no risk of the track ballast being contaminated, but this was obviously a concern and detailed analysis of the whole process is now underway to better understand the reasons why this happened.
Outstanding safety record
The result of this work is that Whiteball tunnel is now a safer space. A significant length of spalling brickwork is now protected. The ingress of water is managed and channelled into the tunnel drainage system and the drainage system itself has subsequently been overhauled and renewed.
Scott was eager to point out that Murphy performed to a very high standard ensuring, that the nail biting ‘just in time’ logistics worked effectively.
As always, a good indicator of a well-run project is its safety record. Throughout the work, over 400 ‘close call’ issues were raised, highlighting the level of care and ownership that existed on this site, and, with well over 20,000 hours worked over the 19 days of the blockade, only one incident was recorded, when someone slipped on some steps but did not incur any injury.
What is most exciting is that this work is testing and pushing the boundaries of engineering maintenance in tunnels to ensure that Network Rail’s significant stock of operational tunnels in the UK will be fit for purpose well into the 22nd century.