writer Collin Carr
In the heart of Devon, situated on the main line between Taunton and Exeter, is Whiteball Tunnel. Brick lined, it was designed by Brunel and built in 1842. The great man was clearly not thinking in metric as it is a tantalising 999m long, the equivalent of 1,092 yards. Whiteball Tunnel is also where the first ever speed of 100 mph was achieved by a means of transport – this momentous event happening in 1904 with the ‘City of Truro’ being the locomotive involved.
The clay used for the millions of engineering bricks required for the tunnel lining came from local pits. Whilst they were of good quality, over the years they have suffered through weathering, chemical reaction from the sulphurous steam trains, voiding behind the lining and the degradation of mortar joints.
Permanent features
Rectifying all this has necessitated a rolling programme of repairs throughout most of the 20th century, right up to the present day. Fortunately, as the route was built for Brunel’s broad gauge, the tunnel’s structure gauge is very generous. Consequently, it has been possible for engineers to turn new brick arch rings within the existing lining. Over the years, special shields – usually constructed from old bullhead rail – have been designed to match the tunnel’s profile. These could be clamped together to accommodate the varying lengths to be repaired and included a staging platform at a height that would allow trains to run underneath. Over time, they became a permanent fixture within the tunnel.
In preparation for the repair, ballast was removed from the cess and a concrete foundation strip constructed. The shield was placed over this to match the length of arch to be turned; skilled and experienced bricklaying teams would then usually turn a two-brick arch inside the existing lining. The process was effective but very time consuming and expensive. Working at height, it also presented considerable difficulties for those involved and often a very uncomfortable environment.
New approach needed
Network Rail has persevered with that repair programme. However, as the condition of the brickwork had started to deteriorate more rapidly, it was clear that the tunnel could become less safe unless something was done. In addition, the anticipated possessions needed to carry out the work could inhibit the running of an effective train service. Consequently, in consultation with First Great Western, Network Rail’s tunnel sponsor Colin Sims agreed that a new approach would have to be found and adopted.
During the investigations, Network Rail met Andy Sorley and Keith John, respectively the Contracts Director and Senior Contracts Manager for Amco – a Barnsley-based company established in 1970. Andy and Keith discussed the requirements with Innovative Support Systems (ISS), a firm with a wealth of tunnel expertise and also from Barnsley. ISS was instrumental in developing a tunnel lining technique first used by London Underground. In discussion with Network Rail, it was realised that an adaptation of this system should provide the solution that Colin Sims was looking for.
Steel mesh rings
Network Rail appointed Amco in conjunction with their designer Donaldsons. They identified an existing product, created by ISS called the RamArch® System. This resulted in a two-stage approach: the first would treat a 355m length of the lining – that’s one third of the tunnel – and entailed starting from the Taunton portal, working in a six-week series of 48-hour weekend possessions. This has recently been completed at a cost of £1.8 million.
Deborah Elliott was Network Rail’s Scheme Project Manager and she explained what was involved. The first stage was designed to stop bricks from spalling and falling out, thus removing any potential risks to passing trains.
The RamArch® System consists of 2.2m lengths of easy-to-handle galvanised steel mesh. These are bolted together on site to form arch rings 1.3m wide and interlocked using a 300mm overlap to create a 1m advance for every cycle installed. These arches are then supported on slotted angle brackets that have been fixed into the brickwork about 2m above cess level. Each ring is secured to the lining using two 200mm threaded pins, fixed with a five-second ‘resin hit’.
Spine wires are threaded along the profile of the arch and lines of continuity installed to the back of the arch during the assembly process, prior to it being lifted into position. These link the individual rings together to provide additional stiffness and are fixed to the arch segments using a CL50A pneumatic hog ring gun – a more efficient method than using traditional tie wires.
Each galvanised cycle weighs 138kg in its bare form, but the spine wires and lines of continuity bring the overall weight to 154kg. However, pull out tests have revealed that each threaded pin is capable of supporting significantly more load than this – 4 tonnes in some cases – although this result is very much dependant upon the condition of the existing brickwork and the presence of voiding.
Once everything is in place and secure, a more permanent fixing is introduced. The threaded pins are supplemented with five permanent rock anchors that are fixed with a ten-second resin hit for each individual arch ring. It is then the intention to loosen the nuts on the rock anchors to create a 40mm gap between the new galvanised arches and the existing brick lining. The purpose of this is to ensure that cementitious cover can be achieved. This will take place during the second stage.
Production line
At any one time during the 48-hour possessions, there were around 30 workers in the tunnel. As principal contractor, Amco was responsible for the effective development and safe execution of the process. Deborah explained that the system, as well as the organisational skills and tunnelling experience of Amco’s Construction Managers Russ Hardwicke and Steve Blackledge, ensured that all the work was carried out at or from ground level, apart from the use of a MEWP to provide fixings at the crown of the arch. ISS developed a construction and lifting attachment to be use alongside a road-rail vehicle, enabling each section of RamArch® ring to be assembled at track level and positioned without scaffolding or any form of staging.
This meant that typical risks associated with tunnel working were engineered out of the process. It also allowed a production line approach to be adopted, meaning that everyone knew what they were doing and what was going on around them. As a consequence, the work was completed in five possessions, with any snagging issues addressed during the sixth. A galvanised steel cover is now in place to protect trains from any loose brickwork. There were no recorded accidents or incidents and the work was completed on time and within budget.
The second stage
Everything is now ready for the second stage which will involve the tunnel being closed for between one and two weeks. Discussions have been taking place with First Great Western to determine how and when Network Rail can gain the necessary access. The reassuring aspect is that the work completed to date will be adequate for at least the next five years – probably a lot longer.
In terms of the engineering, stage two will comprise three separate elements. The first is to grout behind the lining at ground level where the brickwork is still exposed below the new arch rings. Previous surveys have identified the need for this work, ensuring that the stratum behind the lining is consolidated. This will be followed by the installation of precast concrete units that will be fixed to and through the lining using rake anchors. Lastly, shotcrete will be applied behind and over the newly-fixed, galvanised steel archway, providing a solution to the problem for the next 125 years – the specified design life.
This is an exciting project that appears to be progressing really well. Hopefully, detailed plans for the next stage of this important work will be finalised soon, allowing Network Rail to evaluate the benefits of the completed scheme and consider what needs to be done to the rest of the tunnel. This process also designs out many of the significant risks that we associate with traditional tunnel repair methods that are still used on parts of the network. It will enable an assessment to be made of the true financial benefits offered by this approach.
Finally, we must not forget that this 170-year-old structure provides the only pathway for trains to venture into the south-west of England. As such, Whiteball Tunnel is a valuable asset and deserves the very worthwhile, skilled engineering attention that it is currently receiving.
