A pair of 180 year old rail tunnels should soon be safe and waterproof, thanks to some innovative technology. Dave Parker reports.
Dogged by chalk falls ever since they opened 1824, the Strood and Higham tunnels on the North Kent line have had an eventful history (see box).
In December 1999 four carriages of a train passing through a tunnel were derailed by an unusually large fall. Six months later one of the original access shafts collapsed, and the tunnel was closed for four weeks, re-opening only after a 30km/hr speed limit was applied (NCE 29 June 2000).
This was clearly not a viable long-term solution. In January this year a 12 month blockade began to allow the UK's longest railway tunnel refurbishment in living memory to get under way. Target cost for the civil works is £18M, but in total Network Rail will be investing some £35M.
'We're taking the opportunity to relay all the track and install a new drainage system as well, ' explains Network Rail project manager John Kingsnorth.
'But the key to the project is the lining work. The technique we adopted after extensive off site trials is more akin to mining technology than conventional railway practice, ' he adds.
'To make everything simpler we've isolated the entire site from the rail network by removing track and erecting barriers at each end. So what we have is a conventional construction site that just happens to have tracks running through it.'
This means that 176 'nonrailway' staff can work without having to undergo railway safety training courses but the site can still make use of the tracks.
This is convenient, as the water table in the tunnels is just about at sleeper level and the ground far from rock solid.
The first major operation was to move all services - including a 33kV cable - from the cess to a temporary position between the tracks. 'This gave the piling rigs access', explains John Russell, project manager with lining contractor Costain.
Costain has just passed the halfway mark installing 300mm diameter bored raking piles along each side of the tunnels, generally 7m into virgin chalk, rather than the canal fill (see box).
These are linked by an insitu capping beam which supports steel arches (see diagram).
Reinforcement for the insitu concrete piles comes from the steel tremie pipes used to place the concrete, which are left in place. A total of 1,500 of these piles is due to be installed.
In all there were originally 16 unlined tunnel sections up to 250m long, totalling more than 1.4km. Kingsnorth reports that the original 19th century masonry is still in good condition, as are the shotcreted linings installed four years ago, but almost all the previous emergency support work may have to be replaced.
The instability of the chalk was emphasised back in March when there was a small fall during piling operations. Additional protective canopies had to be installed.
Carving the tunnels' gnarled, smoke-blackened walls into a standard cross section is still being undertaken with some care. Rockbolting is used from track level to tunnel crown.
Two Costain teams are working towards each other from the outer portals of both tunnels.
Access is also possible through the 'bombhole', the local nickname for the vast crater dug out of the middle of the tunnel to form a passing bay (see box).
Once the arches are in position, typically at 2m centres, grout bags or timber props temporarily stabilise the chalk.
A 600mm wide ABG Piledrain drainage blanket is fixed to the wall, leading down to a polypipe fin drain.
Then Costain follows up with the Italian made track mounted lining rigs, which feature a complex arch shutter with multiple access points to encourage complete and even concrete distribution.
Only a very special concrete could be guaranteed to penetrate every crevice, encapsulate the grout bags and give a good finish, however. The answer was the first large scale use of the latest technology self compacting concrete (SCC) on the rail network - some 8,000m 3in all.
'And it's no ordinary SCC, ' Kingsnorth points out. 'This contains plastic fibres as well.
It's supplied ready mixed by Hanson.'
There was a time when SCC was rarely seen outside a precast concrete factory. The water-reducing admixtures used then to produce concrete that was at the same time extremely fluid and exceptionally cohesive had only a short effective life.
Getting concrete into the lining shutters on the Strood and Higham tunnels involves a long and complex journey, a real test for the mix formulators.
From Hanson's truckmixers the C40 concrete is transferred to a pair of rail mounted mixer/ agitators running on adjacent tracks. These shuttle backwards and forwards to the lining rig, discharging into a concrete pump which is connected in sequence to the access points on the shutter above.
The two lining teams are about 2.5km apart. Costain aims to complete the lining programme by the end of August, giving Network Rail plenty of time to relay the track and complete all the other works before the tunnels re-open on 17 January next year.