While one might be tempted to think of the United Kingdom as a country with few tunnels – in comparison to Norway or an Alpine country, for example, in fact there are more than 300km of railway tunnel in the UK. Much of them date from the mid to late 19th century, the heyday of railway expansion.

The Springs Tunnel is a typical example of one of these old brick-lined railway tunnels. Located on the Wharfedale Line from Apperley Junction, near Leeds, to Ilkley, the tunnel was first opened by the Midland Railway in approximately 1865. After more than 150 years old, the tunnel is showing its age. The operator, Network Rail, decided to undertake a refurbishment of the tunnel. This article describes the problems encountered, which are typical of tunnels of this age, and some innovative solutions used to solve them. Finally, the progress of the work is reviewed.

The Existing Springs Tunnel

In common with many tunnels from this period, the Springs tunnel consists of a brick lining to support the rock around the tunnel which had been excavated originally using blasting.

Condition of the tunnel

As one would expect from a structure which is well past the design life envisaged by the original engineers, there are a range of structural and serviceability problems with the tunnel. That said, it is worth recognising that, despite the limitations of the original tunnellers, this tunnel has performed well over the previous one and a half centuries.

Water ingress

Like similar tunnels, there were a number of places were water was coming in. Besides being inconvenient, uncontrolled water ingress brings with it a host of durability risks both for the lining and equipment inside tunnels. In exposed locations, the water can freeze into icicles which can damage the lining or even trains. In this case the worst affected area was the portal where freeze-thaw had damaged the brickwork. Water ingress dripping onto the overhead catenary can also cause short-circuits due to electrical arcing.

Brick lining

In the main, the tunnel is in fair condition, the exception being a 12m length from the north portal which was showing significant signs of deterioration, including bulging, cracking and spalling.

Behind the tunnel, the rock surface is quite irregular due to the crude blasting techniques which were used at the time of its construction. Hence the load and the bedding of the arch is far from even.

Usually there is not much space inside the tunnels, between the lining intrados and the kinematic envelope, since the trains are larger than envisaged in the original design. Therefore the challenge is to find solutions which can strengthen the existing lining or which take up very little space.

Design & Considerations

General
Besides the normal design challenges, the age of these old tunnels and the live operations present distinct challenges for the designers. Springs Tunnel, like many of its vintage, is so old that the chances of finding any reliable information on its design and construction is slim. This is coupled with the difficulty in gaining access to investigate the tunnel lining and the geology surrounding it.

In structural terms, the arch of a tunnel lining works best when it is evenly loaded and the arch had an even shape and thickness. Clearly, this was not the case here. However, the first problem to address was the water ingress. The designer, Donaldson Associates (now Cowi UK), produced a series of measures to deal with all of these issues.

Water ingress
Given the lack of space inside the tunnel, the design envisaged stopping the water ingress by grouting rather than adding a waterproof lining inside the tunnel. Initially cementitious grout was foreseen on the grounds of cost and simplicity.

However, this raises some issues in the case of a brick arch with voids behind it. Cement grout is slow to hydrate and so the quantities that are pumped behind the lining need to be controlled to avoid a large fluid pressure developing and overloading the arch. In the permanent case, there will be some weep holes (drainage holes) to direct any residual water ingress away from the overhead catenary.

Ground consolidation

As mentioned earlier, uneven loading on an arch creates the risk of overloading it locally. Over time the rock mass can deteriorate and collapse into the voids around an old brick arch lining. To stabilize this and avoid further deterioration, the designer specified that these voids should be filled with grout, initially intended to be cementitious grout.

Brick lining

The first two steps handle the watertightness of the lining and the overall stability. This leaves the question of what to do with the sections of the brickwork which had deteriorated to the point that they had lost the strength needed to carry the loads in the lining. As mentioned earlier, there was no space to put another lining inside. There was no option but to remove the brickwork and replace it with a new structural arch.

This was the case for the first 12m of the crown and haunches of the tunnel at the Ilkley portal. Removing parts of a lining which is under load is a delicate exercise. As Figure 2 and Figure 3 show the solution which first involves removing the first three courses of brick and installing RAM Arch preformed mesh. RAM Arch, produced by Foulstone Forge and originally designed by David Hindle, Phil Richardson and Chris Scott, has enough rigidity, not only to support its own weight in the short term but also to provide support to the rest of the brickwork. The spring off angle brackets for each RAM Arch panel are fixed to the tunnel lining.

The RAM Arch has a clever “jacking system” which allows it to maintain intimate contact with the brickwork. Additional fixings are installed through the RAM Arch in order to profile any mesh that cannot contour itself to the lining. All the fixings are 400mm x 12mm and held in position by Minova 14mm Fast Lokset Resin. To transform the temporary RAM Arch structure into a permanent solution, 250mm of sprayed concrete is applied to encase the RAM Arch.

Refurbishment Works

The Principle Contractor was Amalgamated Construction (AMCO) who were awarded the Design and Build Contract under NR9 Conditions. AMCO procured Cowi and Minova to help with the project.

Possessions and working time

One of the greatest challenges of working on live railways is the limited time available for work. This emphasizes the importance of simple, clean and fast-acting solutions. To compound this constraint, for reasons beyond AMCO’s control, the original plan of possessions had to be changed. This led to a re-think of the design. In the original design Minova’s Carbofill grout would have been used to create bulkheads along the line of the tunnel behind the lining. The voids in between would then have been filled with a cementitious grout. However, this would need 28 days to gain sufficient strength to permit the damaged part of the lining to be milled out. This would not fit within the available possessions.

Chemical grouting

Having worked successfully together on other similarly challenging projects, AMCO sought the advice of Minova, a specialist supplier of construction materials and chemicals. Despite a lead time of only 10 days, Minova was able to design a new programme of injections, based on their expanding polyurethane and non-foaming silicate resin grouts, and to mobilise a team to perform the injections during the six night track possessions that were available. Wilkitfoam ‘T’, Carbostop ‘E’ and Geoflex were used in the injections which were performed by Minova’s specialist staff.

Phase one:

Injection of Wilkit Foam T, into the side walls to fill voids behind the brickwork. Wilkit Foam T has a high expansion ratio, and thus was ideal for filling the extensive voids behind the side walls. The injection work also helped to prevent water ingress at this level, and acted as a plug to prevent further water migration that might occur as a result of the subsequent injection of the crown with Carbostop E.

Phase two:

Injection of Carbostop E into the crown to stop water ingress. 10mm holes were then drilled into the crown of the tunnel and Carbostop E, polyurethane foam, injected to stop water ingress. Carbostop E expansion is generally limited to that required to fill the voids, and as such it does not generate high pressures on the existing strata/structures. This avoided adding any excessive load onto the existing lining.

Phases three:

Injection of Geoflex to consolidate the structure prior to the milling operations to remove the weak brickwork. Holes of 30mm diameter were drilled to a 400mm depth into the crown section of the arch that was to be milled.

The silicate resin, Geoflex, was chosen because it does not expand so no additional load would be imposed on the weakened lining. Equally importantly, Geoflex provides a strong bond, effectively gluing the lining to the ground. This provided the immediate support to the lining while the damaged bricks were milled out.

These sophisticated chemical grouts have been developed in the toughest of environments in the mining industry and are ideal for the more challenging situations in civil engineering. Minova’s chemicals have helped complete some of the most challenging tunnelling projects such as the Hallandsas tunnel in Sweden and Valdaheidi tunnel in Iceland as well as being used regularly on tunnel repair projects in the UK

Relining

The next step was to mill out the brickwork, historical information and site investigation suggested a minimum of seven rings of brickwork, the design took a conservative view of six rings but in reality there turned out to be four rings. Obviously removing three rings of a four ring tunnel would be asking for trouble and this led to a hasty design change in order to deliver the works as planned.

Once the old brickwork had been removed, the Ramarch panels could be lifted into place and fixed using Minova supplied the 14 mm diameter, fast-acting Lokset resin capsules and 400mm masonry fixings

Finally the Ramarch steel structure was covered in sprayed concrete to form the permanent support, replacing the first three of courses of the original brickwork.

Conclusions and acknowledgements

The Springs tunnel is a good example of the challenges facing us with ageing infrastructure, often more than 100 years old. The problems seen in this tunnel are typical of the old brick-lined tunnels often found on the British rail network. Refurbishing live railway assets demands fast-acting, simple solutions which can be applied in the minimum amount of space. The original constraints on this project were further tightened, leading to a review of the design. The introduction of innovative solutions like Minova’s chemical grouts and the RAM Arch system enabled the work to be completed with the limited time available.

The authors would like to thank AMCO and Network Rail for their kind permission to publish this article. The success of the work is a result of the hard work of all those on site. In particular Minova would like to acknowledge Ian Rea and his team for their efforts on the scheme development and injection works.