Timber sleepers bedded in insitu concrete have been the norm on the London Underground ever since the first lines were opened in the 19th century. And they have served well - some still in use were installed in the 1930s. But lately it has been increasingly difficult to source the high quality timbers needed, especially the exotic hardwoods like Australian Jarra.
'Even if you can get them, they are invariably kiln dried these days, ' explains operator Tube Lines standards and production innovation manager Brian Hall. 'This means there is still a central core which is green, so the sleepers shrink in a relatively short time and break the bond with the concrete bedding.' Tube Lines took over responsibility for the maintenance and upgrading of the Jubilee, Northern and Piccadilly Lines in 2003. By this time many of the timber sleepers were in urgent need of replacement. Ride quality was deteriorating, especially on tightly curved sections. Faced with a tight timetable to get all the lines up to acceptable standards, Tube Lines had to consider the concrete option, already in use on the Jubilee Line Extension.
'Concrete sleepers started to come in above ground in the late 1980s, ' project manager Floris Klinck reports. 'Used with resilient pads between the rails and the sleeper, concrete is no noisier than timber in use, yet it should last 50 years easily. Steel is out of the question, due to all the electric currents down there.' But replacing timber with concrete when working deep underground is a much more complex undertaking than it is out in the open air. Most upgrading work has to take place during the brief four 'engineering hours' between 1am and 5am (NCE 6 October 2005). Even with 30 weekend possessions a year, progress is painstakingly slow.
Klinck says: 'Our target is to replace five sleepers per night. A 200m length of track will take 12 months to upgrade completely at this rate - but getting five sleepers in doesn't leave us much spare time.' The pinch point is the bedding of the sleeper ends (see diagram). Existing insitu concrete is checked for compressive strength well in advance - it has to exceed 15MPa. Old bullhead rails are removed; more modern rails are jacked up. Jackhammers are used to break out every third or fourth of the old sleepers, which have to be lugged away.
Substandard concrete is broken back, surfaces are scabbled, then the replacement sleepers, which weigh up to 110kg, are moved into position by four men.
Next, if rail replacement is taking place, new flat bottomed rails are installed; otherwise the existing rail is fixed back into place on the untouched sleepers.
Then, the new sleepers are raised into position and clipped onto the rails.
'Now comes the really tricky bit, ' says assurance manager Lionel Abbey. 'We have to fill the approximately 15mm gaps between the sleeper ends and the insitu concrete with something that's very fluid and virtually self-compacting - but which gains strength very quickly.
'It has to achieve a minimum compressive strength of 15MPa before we can hand it back. We have used a special grout, but it's very expensive.' Specialist materials company Tecroc Products came up with the answer. Its BD LUL 15/15 is basically a fast setting rapid hardening highly fluid concrete with a nominal maximum aggregate size of 6mm. In the high temperatures and humidity of an Underground tunnel it remains workable for at least 20 minutes, yet hits the crucial 15Mpa mark in one hour. At 28 days it can top 50MPa.
Abbey says that for such a high performance product BD LUL 15/15 is relatively insensitive to water addition.
'We've done tests at 10% too much water and it's still cohesive and free from bleeding and plastic cracking.
'And if you add less than the specified water you get a useful mortar for haunching and so on.' Although the product comesas a manhandleable 25kg bag of dry powder, the risks of excess water being added on site are very low. Tecroc trains all operatives using BD LUL 15/15 supplying calibrated measuring buckets as well as hand held mixers.
Minimal shuttering is needed before the mix can be poured around the sleeper ends, and little in the way of compaction is required, reducing the need for plant to be hauled in and out of the work site each night.
Until recently timber still had to be used on tight curves. 'There have to be internal check rails there to keep the wheels on the track, so standard sleepers couldn't be used, ' Klinck says.
'Now we've just started getting concrete alternatives from Tarmac, along with concrete replacements for the timber pit blocks we use in stations.' In all, more than 60km of track is awaiting an upgrade at the moment. Tube Lines has 350 men in action on 12 different sites. The hope is that in the not too distant future an effective mechanical sleeper handling technique will be developed to speed the overall process.
Tecroc's magic concrete will still be needed, of course.
And passengers on the Tube Lines tracks will notice a significant improvement in ride quality as a result.