During the Napoleonic Wars, a safe inland route between the royal dockyards at Woolwich and Chatham seemed an eminently sensible idea.
Work on what eventually became the Thames and Medway Canal began in 1800. A year later the length from Gravesend on the River Thames in north Kent had reached Higham, where it stalled for 18 years.
A chalk ridge blocked the shortest route to the River Medway. Various diversions were examined before it was eventually decided to excavate what was, at 3.5km long and 10m wide, the largest canal tunnel in the UK.
Five years of excavation followed, with the tunnellers digging 12 shafts from above and working several faces at the same time. Chalk falls were common, with the worst lengths lined with masonry.
Shortly after it opened in 1824, it became obvious the canal could never be economic if only one vessel could travel through the tunnel at a time. In 1830 a massive pit was dug near the halfway point and an open-air passing bay was built, splitting the tunnel into two separate structures, 60m apart.
But competition from the new railway companies continued, and in 1845 the tunnel was sold to the South Eastern Railway Company. The canal was filled in and two tracks laid.
Over the years chalk falls continued, and more sections were lined with everything from traditional masonry to Armco culverting.
Eventually 60% of the tunnel was protected in this way.
In December 1999 four carriages 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 30kmh speed limit was applied.
This was clearly not a viable long-term solution. In January this year a 12-month blockade began to allow railway tunnel refurbishment 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.
'To make everything simpler we've isolated the site by removing track and erecting barriers at each end. What we have is a conventional construction site that just happens to have tracks running through it.'
This means 176 'non-railway' staff can work without having to undergo railway safety training courses but still make use of the tracks. The tracks are 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 the services - including a 33kV cable - from the cess to between the tracks.'This gave the piling rigs access, ' explains John Russell, project manager for lining contractor Costain.
Costain installed 300mm diameter bored raking piles along each side of the tunnels, generally 7m into the underlying chalk rather than the canal fill above.
The 1,500 piles are reinforced with the steel tremie pipes left in place after concreting. Piling was completed in early August.
Piles are linked by a cast insitu capping beam which supports the steel arches forming a key part of the new lining. Horizontal ground anchors were installed through the capping beam to ensure loads are tranferred into the raking piles effectively. There were originally 16 unlined tunnel sections up to 250m long, totalling more than 1.4km. Kingsnorth says the 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 in March when there was a small fall during piling.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. Rock bolting has been used to reduce the risk of rock falls, with 3m and 4m long bolts installed where needed.
Once the arches are in position, typically at 2m centres, grout bags or timber props stabilise the chalk temporarily. A 600mm wide ABG Piledrain drainage blanket is fixed to the wall, leading down to a polypipe fin drain.
Then Costain follows with the Italian trackmounted lining rigs, which feature a complex arch shutter with multiple access points to encourage complete and even concrete distribution (GE April 2004).
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 - 8,000m 3inall.
'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 SCCs were 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.
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 reopen on 17 January next year.