News feature: major repairs progress at Marple Locks
PUBLISHED: 11:23 04 April 2018 | UPDATED: 11:23 04 April 2018
As the emergency closure to rebuild a lock wall at Marple extends to over seven months, we take an in-depth look at what happened, and what's being done to repair it and prevent it happening again
At a first glance, you could be forgiven for thinking that Marple Locks look pretty indestructible. These aren’t the typical small brick-built narrow locks typical of the Midlands; they’re monumental structures, each with a 13ft rise, put together from huge blocks of stone and looking like they’ll last for ever.
But that’s the problem with first impressions. Yes, the size of the stonework may mean it takes a while for any problems to reach the stage where they need to be dealt with – many boaters will recall using the locks when there must have been some sizeable voids in the back of the walls, from the amount of water draining out of them as the locks empty. But these voids do need filling (and today that’s usually done by pressure-grouting through holes drilled in the locksides), otherwise eventually something more serious will happen. Even then, a problem can develop unexpectedly quickly and catch the engineers out. And that’s what happened in September 2017, when in a very short space of time, the Lock 15 suddenly went from developing some subsidence on the lockside (indicating that all wasn’t well underground) to the main chamber wall moving inwards by 80mm in three days.
After two unsuccessful attempts at stabilising the wall by grouting, engineers realised it had reached the point where a rebuild was needed. And there were two extra issues to further complicate (and delay) the work. Firstly, as a Grade II listed structure, it would all have to be agreed with English Heritage. And secondly, it’s a particularly awkward site to have to do this kind of work on. The main issue is that it’s right next to a public road, which couldn’t be closed as it’s needed for possible emergency vehicle access.
So before work on dismantling the wall could start, a deep and wide trench had to be dug alongside the road and filled with mass concrete, to provide a retaining wall to support the road and stop it collapsing into the worksite. And in the meantime, the lock was filled up with gravel to stop the wall from moving any further.
Only then could the wall be gradually taken down, one course (layer of stones) at a time, with all stones carefully numbered ready for reassembly. By the time we visited the site in March, where the lockside had been, there was a large hole (occupied by a large excavator). Project manager Rob Jowitt explained that there was still another 2m of wall to take out, right down to the invert (the curved bottom of the chamber) at a depth of some 7m below lockside height, before reassembly could begin – but pointed out where already a series of reinforced concrete ‘downstands’ (pillars going down into the ground) had been installed behind the wall. These will form the strengthening at the back of the new wall, replacing the stone buttresses packed around with clay which have been doing the job for the last 200-odd years.
As the wall is gradually rebuilt, the reinforced concrete behind it will also be gradually extended upwards, stopping just below coping stone level so that the traditional grass lockside can be reinstated.
That was the plan, and when the main work got under way in January, although the original planned reopening date slipped by a couple of weeks owing to the complications of working in a constricted site, CRT was hopeful of getting the canal open in time for Easter. But then another problem arose.
The plan had been to rebuild the wall from the original stones – but unfortunately they weren’t in as good condition as had been hoped. A number of them had suffered from cracks which couldn’t be seen until the wall was taken down, others split or crumbled during the dismantling. Some can be pinned back together with dowels; however out of the total of over 400 blocks taken out, something like one in five will need replacing. And that has meant finding a suitable quarry to source matching stone, then cutting the stones to the right size, all with EH’s approval – which has added another month’s delay.
But it’s an ill wind… CRT has taken advantage of the enforced closure, and of £1.7m coming in from its supporter the People’s Postcode Lottery, to carry out a number of jobs all down the flight. These include:
. New gates at two locks (one of them would have been scheduled this winter anyway; the other has been brought forward).
. Repairs to canal walls (using stone recycled from the back of the Lock 15 wall) in two places.
. Grouting the walls of ten locks.
. Re-pointing lower gate quadrants.
. Re-surfacing towpath ramps alongside locks.
. Replacing a rickety flagstone towpath surface above Lock 8.
So does that mean that there won’t be a repeat of the Lock 15 problem at another lock? Hopefully, yes – but Rob admits that some of the other ten locks grouted were “quite bad”, implying that some vigilance is needed. And it’s a measure of just how much variation occurs just within a single flight that the off-side chamber wall at Lock 15 (the partner to the one being rebuilt) is in perfect condition, and appears to have been built from completely different sized blocks.
But all in all, the flight should be in decent condition when it reopens, and (fingers crossed) boaters and CRT can look forward to some trouble-free years to come at this most impressive lock flight.