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Polyurethane slab track

Old bridges. What can you do with them? They are expensive to replace, costly to maintain, often difficult to access and can cause problems such as speed and gauge restrictions. Masonry arches are worst. They suffer from spalling, bulging and general dilapidation. To cure their ills very often takes stitching, bracing, sheathing, anchoring or sleeving. All have their benefits, and their problems.

And some bridges are listed, so you can’t do any of that – at least not cheaply.

So masonry arch bridges are a problem. Network Rail has 25,000 of them. Many are over 100 years old, one of the oldest being the Whiley Hill bridge on the Stockton to Darlington railway.

It’s not a very exciting bridge to look at, but it was built in 1824 by George Stephenson, making it one of the oldest railway bridges in the world.

Venerable structure

While George Stephenson was a forward thinker, even he didn’t predict freight trains running at 80mph with 25 tonne axle loads over his bridge. So he hadn’t calculated for those stresses. The bridge has done a good job of coping with them for 190 years, but it was getting a bit tired.

Fortunately, those clever people at Balfour Beatty had been working with more clever people at Heriot Watt University on a solution to just such a problem. The answer was not to uprate the bridge at all, but to make sure that the loads it was experiencing were no higher than those it was designed to withstand, all those years ago.

Most railway track in the UK sits on ballast – which is basically a bed of stones. As each stone can move in relation to its neighbours, although that movement is restricted to an extent by the interlocking of the sharp edges of the individual pieces of ballast, the force of a passing train goes almost straight down into the trackbed, and then into the structure under the track – the bridge.

There is a bit of spreading of that load through the ballast, so the loading is actually in the shape of a pyramid, but not much.

What is needed is a way to spread that load, to make the base of the pyramid larger, so that the point loading on the bridge is reduced. If it can be spread enough, then the bridge will be able to withstand it in its current condition, removing the need for all that expensive remedial work.

And it’s not just the expense. A major bridge reconstruction involves closing the railway, and the road underneath, and inconveniencing a lot of people.

Unifying ballast

So what did all of those clever people come up with?

They developed a way of turning the loose ballast into a more unified structure. That doesn’t mean sticking it together, that’s been tried before. Glue is usually hard, and somewhat brittle. Sticking pieces of ballast together means that there is a lot of point contact, which causes high stresses, which breaks the glue, and you are back to square one.

The answer is to use a two-part polyurethane. This fills up the voids between the stones, turning the whole thing into a homogeneous mass.

Now back off the amount of polyurethane until it only fills around 20% of the voids. That is enough to stiffen the ballast, but it means that the track still drains normally through the remaining 80%. It also makes the whole system more flexible – still able to spread loads but also resilient enough to withstand the shock loadings of a passing train.

The trick is to apply just the right amount of the right grade of polyurethane, in just the right place.

After a lot of testing in the laboratory at Heriot Watt, the new product, now named XiSPAN as it was a derivation of the XiTRACK ballast reinforcement that has been used to bind damaged track together for some time, was ready for its first practical application.

Dermot Kelly, Balfour Beatty Rail’s senior project manager in charge of the development, explained the process. First of all, the quality of the existing ballast was checked, and found to be poor. So a large vacuum excavator was brought along to the roadway under the bridge, pipes run up, and the whole lot sucked away leaving the track propped up on jacks.

Fresh ballast was installed, up to 200mm below sleeper base. The polyurethane compound, mixed in the nozzles, was then pumped onto the ballast. It soaked away and then gelled about 15 seconds later.

More ballast was added, up to sleeper bottom, and the track aligned and tamped. Story Contracting provided the plant, and 1stinrail the track team.

And that was it. Job done. George Stephenson’s Whiley Hill bridge is good to go, if not for another 190 years, at least for some considerable time.

Troublesome tunnel

So what next for this interesting new technique?

Well, there was Toadmoor tunnel. It’s another George Stephenson design, but is a little bit newer – it was only built in 1840. Just 128 yards long, it runs through an unstable hillside near Ambergate north of Derby.

With an elliptical bore, the tunnel is twin track. There is an invert under the track, tapering down to a low point in the middle of the six foot.

Due to tight clearances, the depth of ballast under the track is kept to a minimum. This means that, although there is an adequate amount in the six foot, there is almost nothing close to the tunnel walls. This arrangement causes its own problems – settlement in the middle and crushed ballast at the edges.

A model of the tunnel was built at Heriot Watt and tested at a simulated 80mph with 25 tonne axle loads. Once the correct grade of polyurethane had been formulated, the system was ready to go into the tunnel.

Once again the current ballast was in poor condition, so that was all removed. The existing rails and timber sleepers were retained – there are some interesting sleeper lengths in the tunnel due to its particular geometry.

Fresh ballast was brought in, and the track relayed in the usual way. Once all of the gauging checks had been completed – it is impossible to use a tamper – then the polyurethane could be applied. First of all the shoulders were treated, creating an edge beam which would retain the track. Then the ballast was excavated between the sleepers down to the level of the sleeper bottom and the polyurethane poured in, effectively consolidating the ballast from sleeper bottom downwards.

Finally, the top level of ballast was replaced

All of this took just four Saturdays, and Toadmoor tunnel has now effectively been slab tracked. What is more, it is a resilient slab that drains as well as conventional ballast and which dampens vibration – which makes it superior to a concrete slab in some applications.

So what next for XiSPAN?

XiSPAN won Balfour Beatty Rail the Heritage Award at the 2014 Network Rail Partnership Awards.

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