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A trick of the rail

Special gadgets and ingenious techniques, plus careful planning and diplomacy were crucial for the JLE track work, says Adrian Greeman.

The logical way to lay a railway is to start from one end and finish at the other, using the newly laid track to bring in materials and equipment. This is how Tarmac, now Carillion, and its French partner Montocol planned to tackle the Jubilee Line Extension, but programme pressures meant the joint venture was obliged to jump about a little.

'It sometimes meant walking through other contractors' sites or negotiating specific times to bring in our materials,' says Carillion senior planning engineer Roy Clark.

'For each site we had to have the safety training appropriate for that contract - some of my lads had seven or eight safety tickets.'

Not surprisingly, the trackwork was all under one contract. As project manager Jim Boaden says: 'It is all down to logistics once you get the basic technical stuff done.'

But Boaden's 'basic' was an understatement. The JLE tunnel track uses two advanced designs for noise and vibration suppression: a padded connector on a precast sleeper embedded in concrete for most of the tunnel, and a boat-shaped floating slab for particularly sensitive stretches beneath Westminster and Pall Mall.

It all required a high level of skill and experience, along with inventiveness. Several machines were devised or modified.

Work began on site at Stratford Market Depot late in 1995. As with a subsequent 3.5km of above ground double track to the portals, the depot has relatively conventional concrete sleeper and ballast system, 'though it was complex enough forming some 60 or so turnouts', says Boaden.

Out on the line the JV shaped grades, compacted and rolled ballast and installed trackwork and a drainage layer.

Materials arrived by road through four access points but once into tunnel everything came by train through the portals.

The idea was to take possession of the whole line 'chunk by chunk', station to station, to move in and store materials. In the yard Tarmac welded French Material de Voie-manufactured steel rails to give 72m lengths which were loaded on to wagons.

The final concrete track bed is formed around a rebar mat woven through five rows of steel fixing hoops left projecting from the concreted tunnel invert. This meant rails could not be pulled in over the mat.

'So we fixed a system of rollers against the hoops and hauled with a reinforced dumper,' says Boaden.

With rails on wooden blocks, the rebar could be placed and then the sleepers, nicknamed 'hedgehogs' because of rebar sticking out. For all their untidy appearance they were very precisely made, with exactly positioned fixings. The JV and supplier Tarmac Precast invested a couple of months to get the moulds right, says Boaden.

Carillion brought the sleepers packed over two steel channels on the bottom of three rail wagons. A trolley using a Hiab lifting arm ran in the channels, ferrying one sleeper at a time to the unloading point where they were picked up by a special cantilever gantry.

The front cantilever could place 12 sleepers on to the unformed track before the whole worktrain ran forwards on the new rail.

To fit sleepers the rail was jacked up by a special frame further ahead to leave space for connection. Bolting is a precise operation because a special 30mm thick resilient composite pad sits beneath the spheroidal graphite iron (SGI) base plate; bolts were tightened precisely.

Special template jacks in the gap between every third sleeper clamp and hold the rails. 'Quite a clever thing that,' says Boaden. Fine adjustments for height and position to 3mm tolerance were made within the tunnel.

'But the trick with rails is not only the absolute survey but the relative tolerance of the rail inner face which should not alter more than about 1mm,' explains Boaden. Two measuring machines checked the versine, the distance between the chord and the curve on bends, and then the crossfall and twist.

Only after a tunnel section was complete with track was the concreting started. Big static Putzmeister units were used to pump concrete forwards up to 700m with generally 150m3 being placed at a time.

In the floating slab section the tunnel invert was left curved. Carillion fitted steel plates to the walls which held a 200mm long diameter bearing pad supporting the centre point of a 2.5m curved concrete trough.

Load transfer between troughs was achieved by 40mm diameter, 10m long steel reinforcing bars. Two holes formed in the trough sides were extended by sleeve through the insitu concrete to the surface to provide jacking points should the bearing pads have to be changed at a later date.

Track lengths fitted with baseplates and 'resilient pads' were then hung on template supports and precisely adjusted before concrete was poured to fill the trough. With a pad and a bearing, as well as the high mass of the concrete, much of the train vibration is absorbed.

One of the most complex tasks was at Green Park where the JLE joins on to the existing live Jubilee Line. An earlier contract had already created a step plate junction around the running tunnel to curve the new line southwards, explains Clark. The track installation could only be done in 'engineering hours' from 1am to 4.30am using Green Park for surface access.

'Just outside the station the lower third of the old cast iron segment tunnel wall was left to support the track and we had to break through with plasma flame cutters,' says Clark. Concrete blocks with the resilient pads were installed in between the existing sleepers before old timber elements were removed.

To maintain the existing train service the bullhead rail was first substituted with flat bottom rail. The existing tunnel track concrete between the timber sleepers was broken out and specially designed 'Barlow' blocks with resilient pads installed before the old timber sleepers were removed.

Using variable width temporary baseplates and blocks allowed the support system of the turnouts to be in place before changing the plain rail to switch and crossing.

'We had three weekend possessions for the permanent concreting,' says Clark. Blocks were positioned precisely using interlinking steel angle irons. After checks and repositioning of existing running rail, a finished section was temporarily grouted in time for trains.

Trackwork was completed in the tunnels in November 1997 with depot sidings being completed following the resiting of S&E contractors around Easter 1998.

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