CTRL: Track Complete to within a Mile of St Pancras
Slab track has been completed two months early, and a test train has run from Paris to St Pancras. Who would have guessed in August 2002, when the joint venture signed the contract for CTRL2 Track and OCS (Overhead Catenary System), with a completion date of December 2006, that they would have all the track laid in open route and in tunnels, sufficient to run the Mauzin track measuring train from Paris to within one mile of St. Pancras by the middle of August 2005?
This is actually 2 months ahead of the programme they set themselves in April 2004! CTRL1-is the completed section from the tunnel portal to Fawkham Junction just south of Ebbsfleet and the Thames; CTRL2 is the section currently being constructed to connect right through to St. Pancras.
As we went to press, Eurailtest’s track measuring train, Mauzin, arrived on CTRL2 having travelled from Paris, through the Channel Tunnel, up CTRL1, across the new connection onto CTRL2, through the Thames and London Tunnels to within a mile of St. Pancras. By then, the slab track through the tunnels, and the ballasted track across the structures of the open route were both complete and accurately aligned, leaving only the additional platform tracks at Stratford remaining to be done.
40 km of railway, half in tunnels!
On CTRL2 the ACT-JV (the Alstom, Carillion, Travaux Sud-Ouest (TSO), Joint Venture) has been laying a total length of 40km of twin-track high-speed railway. Half is on open route, where there is one 6-track station at Ebbsfleet. Half is in tunnels, 2.5km under the Thames, and 17.5km under London, where Stratford 6-track station is in a 1.1km box, 7.5 km from the west London portal.
Although ACT-JV began planning in 2002, it wasn’t until April 2004 that they started in earnest on site, laying ballasted track at Ebbsfleet to give access to the south end of the Thames tunnels, which they started concreting in October 2004 and finished by Christmas. Five kilometres of plain-line slab track were concreted using ready-mix concrete, handled by two shuttles, a concrete pump and a multi-purpose gantry (MPG); albeit not without some difficulties with the machinery!
Shuttles - problems and solutions
The shuttles suffered mainly traction problems, which were overcome by hauling with a small locomotive until the problem was resolved. The MPG was used for installing sleepers, pulling rails, and moving the props and jigs used for rail position adjustment.
In the Thames tunnel, one shuttle remained linked to the concrete pump, around 140 metres behind the area being concreted, to incrementally move it and to act as its reservoir of concrete. The second shuttle was used to transport concrete from the tunnel portal, where it was delivered as readymix, to the first shuttle. This system of working ensured that there were no interruptions to the supply of concrete for pumping and hence the process was continuous, with the first shuttle and pumping system moving forward at the speed of concreting.
Concrete with polypropylene fibres, and additives
In January of this year, they started concreting the slab track in the London tunnels using the full scale concrete train; a 450 metre length of tightly packed machinery, attended all the time by a couple of fitters. This machinery proved quite reliable, after the first few weeks of difficulty with the concrete pump. By the 12th August they had finished concreting the 35 km of plain line slab track in the four London tunnels, and were within one mile of St. Pancras.
The concrete mix was designed for a 200mm slump; a minimum four hours workability, and a 28-day strength of 37 Mpa, following a 24-hour strength of 8.5 Mpa. In practice, strengths at 28-days exceeded 50 Mpa. 400 kg/cm of cement were used including 28% PFA (pulverised fly ash) to provide a suitable mix for pumping with early strength.
The pre-mixed aggregates also included polypropylene fibres, not as reinforcement, but to provide the necessary freeze/thaw resistance at the tunnel portals and resist early age cracking. Additionally two additives were used; Degussa Sky 545-a superplasticizer, and Degussa Stream 5, as a pumping aid.
Pumping 350 metres uphill
With a maximum gradient of 2.5%, pumping uphill was sometimes necessary. The concrete was pumped to 350 metres in front of the train, spread manually, compacted using vibrating pokers and finished to level by steel trowel. The first stage concrete and adjacent completed walkway, meant that no shuttering was needed.
The concrete train is made up of the following wagons:
- 12 of pre-mixed aggregates, including reinforcing fibres.
- 1 of water.
- 1 of cement, in four silos.
- 1 carrying two generators.
- 1 with mixing plant, weigh hoppers and a control cabin.
- 1 dump wagon, in case of a mixing error.
- 1 carrying a re-mixing hopper and concrete pump.
- 4 welfare and workshop wagons, back and front.
During concreting, all 450 metres and 22 wagons were inched forward by Tractorail, but a Freightliner Class 66 locomotive was used when positioning, and Class 14 shunter, when stabling.
The challenge of concreting in tunnels
The challenge of concreting 40km of track in tunnels was seen as very high risk; hence the employment of a ready mix and shuttle system in the Thames tunnel, which was key to the logistics of the bulk of the contract, and a separate full scale concrete train in the London tunnels. In the event, Thames went very well, and London simply followed; both were on time. Why was this successful?
There were a number of contributory factors:
- A long time spent in detailed planning.
- Careful choice and design of equipment, and inspection of its manufacture.
- Thereafter great care in its maintenance and repair.
- Very skilled staff on the management of the equipment and the processes.
- Comprehensive training of the operatives to use the equipment efficiently.
- Careful control using sophisticated electronics to produce high strength easily placed concrete, pumpable over a distance of 350 metres uphill.
- High strength jacks and jigs, robust yet easily adjustable, to ensure the rails were precisely positioned and held secure to the nearest millimetre, as the operatives clambered all over them placing the concrete.
UIC 60 rail in 216 metre lengths
Key to the continuity of all long-welded rail and turnout installation, was our logistical interface with Corus and their haulier EWS (English Welsh and Scottish Railways). This was especially true of track in tunnels, because the long welded rail trains had to be propelled all the way up the tunnels, and past Stratford Box on recently constructed railway. Each 32-bar train had to be pushed up the tunnel four times so that the rail could be unloaded 8 rails at a time, into the side channels of the 1st stage concrete.
The rail section used was 60E1 (UIC60) Grade 260 supplied by Corus in 216 metre lengths, which had each been flash butt welded from six 36 metre long rolled rails. Railtech’s PLA process of aluminothermic welding was used for the site welds.
Time to become ‘slick’
After each unloading of long welded rails, trains loaded with sleepers were propelled up the tunnel and the MPG would straddle the train, lift a panel of sleepers, roll to position and place them accurately to line. The rails were then lifted by a threader from the channels, onto the sleepers and bolted down loosely. They were then welded together into long lengths and bolted up, leaving them assembled, but lying on the first stage bed.
TSO provided an RND 92, the front of which ran on the assembled track, while the back end lifted sleeper and rails off the bed, and left them supported by jacks and jigs in almost the final position. The surveying team precisely aligned the rails by adjusting the jacks and jigs, then TSO’s CM10 rolls on the rails, checking the vertical and horizontal curves. It is worth remembering that they have up to 160mm of cant on the curves (none of which will be fitted with lubricators), combined with some steep gradients. This is not easy; a big team of people were needed, and they took quite some time to become ‘slick’ at their jobs!
Vossloh not Pandrol preferred after technical evaluation
Design of the slab track was driven by the desire to reduce to an absolute minimum, the effects of noise and vibration. The precast twin block sleepers have each block encased in a plastic boot, lined with stiff neoprene pads restraining horizontal movement, and a single softer neoprene pad supporting the full plan area of the sleeper. This allows some vertical movement under the weight of a train.
This design has been tested endlessly in SNCF’s Paris laboratory, and judging by the effect under the construction trains so far, it looks as though it should be successful when the Eurostars run. The only places where twin block sleepers were not used was in turnouts, and where interfacing with Network Rail’s 3rd rail system would have presented difficulties!
Both mono-block and twin block were considered initially, but twin block was preferred due to their proven high speed performance and cost. Their better lateral resistance on curves was also a factor with high-speed curves down to 1750 metres and even 400 metres at 80 kph, at the West London tunnel portal! Following a technical evaluation, Vossloh W14 fastenings were chosen in preference to the Pandrol alternative.
So here is a success story in regard to track construction on CTRL2; hopefully before the year is out they will have a similarly upbeat report on the OCS (Overhead Catenary System - known elsewhere as the overhead line equipment!), installation. This part of the work was a slow starter, but is coming up fast on the outside rail even as I write!
My thanks to Mike Casebourne, ACT JV Project Director, for all his help with this article; and we look forward to bringing you an update when the next stage of the work is complete.