The hardest part of rebuilding a new lock wall was dismantling the Grade-II listed original structure, as Paul Kirkwood reports from Cheshire.
“It’s like piecing together a jigsaw but not knowing how many pieces you should have or where they fit in,” is how site agent Dave Jones sums up a £2M job that Morrison Construction is currently undertaking for British Waterways at Vale Royal Locks in Cheshire.
The pieces are huge blocks of masonry and the jigsaw is a rounded bullnose wall that forms an island between a sluice gate and one of two locks on a stretch of the River Weaver Navigation, which is under the client’s stewardship.
“It was difficult to make head or tail of what was there. The masonry was covered in lichen and water-born life.”
Dave Jones, Morrison Construction
Seven years ago the bullnose partially collapsed into the water. Opening and closing the lock and sluice gate had, over time, scoured the canal bed either side of the bullnose eventually undermining the structure and causing it to collapse.
The first stage of rebuilding and restoring the bullnose to its 19th century glory involved dewatering a section of the lock system using a sheet piled cofferdam and lock gate stop planks so that a structural assessment could take place. Engineers were confronted with a mess.
“The wall had separated into two pieces,” says Jones. “One piece had completely fallen into the scoured canal bed and the other piece had semi-collapsed, half-leaning into the water but not fully over.”
The original wall construction at the base was 2m to 2.5m-wide with facing stone at the front.
Since part of the wall had fallen flat on its face, the team first had to dig through concrete, brickwork and rubble and everything that had been thrown in behind when it was first built to find the facing masonry stones right at the bottom.
This had fallen into the canal bed scour hole so was lower than natural canal bed level. “It was difficult to make head or tail of what was there,” says Jones. “A lot of the masonry was covered in lichen and other water-born life.”
Recovering the masonry
Recovering the masonry for the section of wall that was hanging over was just as challenging but for different reasons.
“The stability of the structure was unknown,” says Jones. “Any collapse would be in a forward direction so we placed straw bales in front to cushion any falling masonry.”
“The stability of the structure was unknown. We placed straw bales to cushion any falling masonry. ”
Dave Jones, Morrison Construction
The contractor built an access ramp with a recovery pad formed from the existing soft fill material behind the wall. This allowed an excavator to stand side-on to the wall and push the top masonry stones down on to the recovery pad one at a time.
This method ensured that operatives were kept safely away from the wall as it was dismantled.
Rubber cushions tied to the excavator’s bucket gave extra protection. Inevitably, some masonry stones sheared but these were repaired with dowels.
Putting the bullnose back together again
- Sheet piles temporarily squared off the bullnose
- 12.5m long anti-scour piles formed the original alignment of the nose
- Anti-scour piles are backfilled with 6F2 recycled fill and topped with a 600mm-thick reinforced concrete slab
- Recovered stone from the collapsed wall will sit on top of the concrete slab
Heritage masonry subcontractor Bullen Conservation recovered around 220, 3t facing stones in all, along with additional backing stones and larger stones that formed the original foundation courses.
These foundation stones have not been used in the new structure but British Waterways plans to reuse them elsewhere.
Each facing and through stone was to be reused and so each was numbered with spray paint for immediate reference and then the rebuild numbering system was chiselled in before the stones were laid out in course for cleaning.
Repiecing the puzzle
“No drawings of the original bullnose exist so, to work out which stones went where, their dimensions, shapes and finish were inputted to create an AutoCad drawing,” says Jones.
“This was then overlaid onto the foundation drawing provided by designer Arup as a first estimate of how the wall would have looked. From that we could make an accurate assessment of the exact alignment of the wall.” In plan the bullnose wall is rugby ball-shaped with a slight point and is flatter on one side.
An added quirk is that the sluice channel side is 6m-high and the lock side 8m − having been built up in the mid-19th century to accommodate a greater depth of water when the canal’s capacity was increased.
“No drawings of the original bullnose exist so, to work out which stones went where, their dimensions, shapes and finish were inputted to create an AutoCad drawing. From that we could make an accurate assessment.”
Dave Jones, Morrison Construction
To begin the reconstruction, the bull nose was truncated and Morrison drove in a 10m long line of sheet piles across it to temporarily square it off and hold back the material behind.
Two internal lines of LX32 bearing sheet piles were then driven into the stiff mudstone with a third, 23m-long curved line of 12.5m-deep anti-scour piles forming the original alignment of the bull nose.
Site workers then excavated the area behind the anti-scour piles and backfilled it with 6F2 recycled fill before topping with a 600mm-thick reinforced concrete slab which overhangs the piles by 150mm. This provides a foundation for the wall in place of the lowest three courses of original stonework and will be out of sight under the waterline after the canal is rewatered.
The slab’s alignment is critical since it has to match precisely the curvature that can be achieved by the reclaimed facing stones while providing a seamless join with existing masonry to ensure boats are not caught on the lock wall where it starts to taper towards the bull nose.
To ensure this happened, timber shutters faced with 3m lengths of 3mm steel were positioned to form the curve just outside the sheet piled edge.
Challenges and difficulties
Facing stones and backing stones are currently being relaid using a type of lime mortar that tests have shown most closely matches the original. After every three courses, the wall is backed by concrete with 6F2 fill behind.
Work is also being carried out at the southernmost “large” lock. There, during the initial assessment phase of the project, a second scour hole was identified, as a potential cause of instability for the main lock. As part of the contract, Morrison is infilling it with 400mm to 700mm of rip-rap topped with smaller stones while maintaining a clear draft of 3.5m to ensure safe passage of boats. The scour hole beside the bull nose was infilled with similar gritstone in the early stages of the project.
An otherwise simple operation has been hampered by access difficulties. The only vehicular access is down a 1.6km-long wooded track. A bridge at the end of it has been strengthened to take 5t loads rather than 3t, which means it can be used for concrete deliveries. However, most larger deliveries including one of the two site cranes, excavators, some materials, and even the site cabins − have been floated up on the canal.
The last phase of the job will be the design and construction of what is likely to be a weir-type structure of sheet piles to prevent future scouring next to the new bull nose. Final completion is set for October.
The semi-collapse of the bull nose at Vale Royal was a problem that has been over a century in the making.
Built in 1791, today’s sluice gate started life as the navigation’s first lock.
A second lock was added alongside in 1861 with the bull nose in between as part of a scheme to increase the depth, capacity and efficiency.
A third big lock was added to the south of the small lock in 1889. The bullnose’s original foundations and canal bed were never designed to cope with what grew into such a substantial and busy lock system.
The Weaver Navigation runs from Winsford to Weston Docks in Runcorn and in its heyday provided transport for the Northwich salt trade.