Typical of BW's approach to historic structures is the recently-completed restoration of Lock 10 on the short Tame Valley Canal near Perry Barr. Originally a layer of puddled clay sealed the sides of the masonry chamber, but below was only sand. Over the years, canal water percolating beneath the lock had eroded the sand away, with inevitable results.
Subsidence was well advanced, Bligh reports, but demolition and reconstruction was an unattractive option, not least because of the close proximity of a high pressure gas main. 'This really ruled out the large sheetpile cofferdam that would have been needed if we were to demolish and rebuild - or even underpin the structure conventionally.'
Underpinning from the inside of the dewatered lock chamber was a much more attractive option. No cofferdam would be needed and the gas main, and other adjacent services, would not be disturbed. The problem was how to transfer loads from piles driven through the bottom of the lock to the lock chamber itself, which had well and truly broken its back, without seriously reducing the lock's effective cross-section.
'Luckily, this is quite a complex design of lock,' Bligh says. 'Water entered it through a series of culverts that discharged at intervals along its length.
'This feature was intended to minimise turbulence in the lock when the paddles were raised, but it meant there was a continuous circular void running below the main walls which we could exploit.'
Bligh's team decided to transform these culverts into structural members by inserting reinforcement cages and filling with concrete. 'These are now effectively internal longitudinal ground beams, supported off the new pilecap,' he explains.
Water now enters the lock through two conventional culverts each side of the upstream gates. Overall, the refurbished lock with its oak gates and stone copings looks no different to other locks on the same stretch of canal, which is exactly how Bligh likes it.