Project Denham Culvert Repair
ClientBritish Waterways London Region
Project value £450,000
Main contractor Morrison Construction
Denham, south Buckinghamshire, is the archetypal Home Counties commuter village. It has pretty much everything a stressed-out City worker might want in his country retreat – wisteria-clad cottages, village hall, school, pub, river winding lazily through it – it ticks all the boxes.
Problems would seem to have no place in this rural idyll but Morrison Construction has had no easy time of it on its contract to repair an essential waterway in the village. Morrison found itself battling with access difficulties, environmental constraints and Mother Nature herself in its bid to prolong the life of the Denham Culvert, a 200-year-old watercourse passing underneath the Grand Union Canal’s deepest lock.
“It is more of an aqueduct than a culvert, really,” says Tim Walker, Morrison’s project manager on the job. “We were brought in after our client, British Waterways, carried out an inspection and found the structure to be in need of repair.”
The masonry culvert enables the River Frays to pass underneath the Grand Union Canal and no major repair work has ever been carried out on the ageing structure.
“That initial survey showed that some structural repair work needed doing but it didn’t really highlight the full problem,” continues Mr Walker.
“Basically the survey was done by touch without dewatering so it wasn’t until we managed to drain the structure completely that we found out exactly what sort of a state it was in.”
British Waterways has a sliding scale of structural soundness, with Grade A denoting brand new and stepping down in levels of dilapidation to Grade E. Denham culvert was listed at Grade D.
Without a fully dewatered inspection, even this poor grading was little more than a guesstimate, but it was all Morrison
had to go on when pricing the repair project.
The initial pricings came in at around £350,000, based on a contract period of 15 weeks – two weeks’ work to revamp each of the five masonry ‘barrels’ that make up the culvert and a further five weeks timetabled for access works, mobilisation and any additional work that might arise.
But subsequent inspections revealed the problems were much more severe than initially thought.
Scouring (the eroding action of the river’s flow around structures) underneath the foundations of the barrels at the upstream end, cracks in the walls between the barrels, loose masonry in the barrel soffits and severe damage to the inverts were all evident.
Immediately the scope of the works changed. Inevitably the project cost rose – to £450,000 – and the contract period stretched out to 22 weeks. One of the major practical challenges for the Morrison site team was posed by the poor access; how to get plant and equipment as well as materials to the site.
Several options were considered, including bringing a temporary access road across fields the half-mile or so from the busy A40, before the team hit on the eventual access arrangement.
British Waterways project manager Antonia Zotali explains: “The temporary road was not ideal. Access to the river would have been from canal level and the towpath narrows considerably around the lock-keeper’s cottage, so it would have been difficult to get plant and machinery in from there.
“Also that access would have brought slow-moving delivery lorries and site vehicles out onto the A40, so it was too dangerous.”
Other options, including bringing in water-borne plant, were explored before eventually the team decided to use a wooden bridge used by green keepers at the nearby Buckinghamshire Golf Club.
But, with a weight limit of 5 tonnes set on the bridge, it was unable to take all the loading from the large plant Morrison’s
needed for the work.
“We used a 3-tonne tracked dumper to ferry material in and out. All the steel sheet piles for the cofferdam, all the concrete
and all the waste was ferried across the bridge,” says Mr Walker.
A large 13-tonne excavator was brought across a ford further upstream to carry out pile driving, excavating and concrete placing work.
A temporary ramp has been constructed down the northern bank of the river to allow the machine access to the site while further upstream where the river flow is being temporarily stemmed by a barrage of 1 tonne sandbags, a temporary haul road has been built using timber railway sleepers and geotextile matting.
“We are right on the flood plain of the river and the haul roads have been flooded,” says British Waterways asset and programme manager, Peter Walker. “But overall we have not been badly affected by the weather.”
The two northernmost barrels of the culvert were the first to be worked on. The site team drove a series of 4.5 m-long LX16 steel sheet piles 2.5 m into the riverbed to form a coffer dam around them.
This allowed repair work to be carried out in dry conditions despite Tim Walker’s fears about the proximity of the driving operations to the ageing structure.
“It’s just a bit close and you never know what could happen. In the end we plugged the last 200 mm between the steel sheet and the culvert with sandbags,” he says.
Indeed the sheet piles have proved such a hit that a moorhen has built its nest at the right angled joint of the coffer dam.
Now the project team can’t pull them out until it has incubated its eggs – which could take around three weeks. It is a timetable extension that everyone is relaxed about.
Sound for another 200 years
Full inspection of the culverts revealed the level of damage that had been done to the original structure. The existing timber invert was missing from most of the culvert barrels and the foundations of the headwall had been significantly
weakened by the river’s flow scouring material from underneath them.
When first built the engineers used wooden cross-beams with a packed clay insert and timber ‘floorboards’ to run longitudinally down the barrels.
“It is 200 years old so in fairness it had lasted quite well,” says Morrison’s project manager, Tim Walker. “But the old timber inverts had been ripped out in all but one of the barrels and so it was decided that it should be replaced with a
cast in situ slab.”
This involved the excavation of the barrels by hand with workers using small skips on a bogey and rail system to transfer the arisings out of the 18 m-long, 1.9 m-wide and 1.25 m-high culvert barrels.
The site team used A393 steel mesh coupled with C40 concrete to underpin the scoured foundations and cast a slab back
to original invert levels along the barrels. This slab was cast a further 1 m upstream of the culvert headwall to form a scour apron and a row of 1.5 m-long cut-off steel sheet piles driven in front of the apron to help prevent scouring beneath it.
Further structural repairs included the rebuilding and repointing of the cracks in the masonry walls and barrel arches.
The cracks were stitched with helical bar. Where this was not possible, external steel straps were bolted to the face of the
masonry over the crack.
“The structure is not listed but it is in a conservation area. We used lime mortar on the headwalls but Portland cement-based mortar inside,” says Mr Walker.
Working within Environment Agency Guildlines
The Denham culvert site is surrounded by areas of environmental note. Sitting in the middle of the Denham Country Park with the area immediately downstream of the culvert denoted a Site of Special Scientific Interest, British Waterways project manager Antonia Zotali has been working hard to ensure the work does not breach Environment Agency guidelines.
One of its key requirements was that the flow of the River Frays, from the crest of the weir where it splits from the River Colne downstream to the culvert, could only be reduced by 40 per cent or 300 mm below normal water level. This control was put in place to protect fish spawning downstream.
To control the flow through the river, 1 tonne sandbags were placed at the crest of the weir and river level monitoring points set up above and below the weir and culvert. Sandbags were added or removed to control the flow.
A further agreement as part of the agency’s Temporary Works Consent was that just two of the culverts could be dewatered at a time. But the level of scour between the culvert diaphragm walls made it difficult to work on just two barrels and Ms Zotali had to go back to the Environment Agency to ask to be allowed to block three of the barrels.
“Having just two closed at any one time didn’t work when we were underpinning. It meant we were working against a barrel with a full bore of water running through it,” says Tim Walker.To download a PDF of a typical section of the Denham Culvert Repair click on the resource box on the right hand side of the page