Contractors had to devise a way of ensuring only clean water made it to the Manchester Ship Canal – without flooding neighbouring Media City.
Project: SAL172 UID
Client: United Utilities
Main contractor/engineer: GCA JV – Galliford Try, Costain and Atkins
Mechanical subcontractor: Franklyn Yates
Electrical subcontractor: Picow Engineering
Piling subcontractor: Skanska Cementation
Start date: September 2013
Completion date: February 2016
“Once it’s finished, you won’t even know it was there,” says GCA senior agent Gary Thompson.
Unlike some arguably more glamorous projects, the one he refers to will end up almost completely invisible and under what will most likely be a car park once construction has finished.
But while the end product may not turn out to be eye-catching, the technical challenges of delivering it certainly are.
United Utilities’ SAL172 UID (unsatisfactory intermittent discharge) project will sit beneath a small parcel of land on the edge of Media City in Manchester.
It is being taken on by main contractor GCA – a joint venture between Galliford Try, Costain and Atkins.
Part of the AMP5 asset management programme, the project involves the installation of a tank that will filter and screen stormwater, as well as providing stormwater storage.
Previously, water was directed from the sewer, via a brick culvert, straight into the Manchester Ship Canal.
The UID, which is already operational, will screen any excessive stormwater flows before they enter the sewer, and store any additional water that will then be directed to treatment works once the storm flows have abated.
The main addition is a detention tank 20 m in diameter and 20 m deep, into which the redirected culvert will flow.
The construction of the diversion chamber, which adjoins to the detention tank, has proved incredibly challenging for the GCA team, not least because the tank itself has been constructed just off the original 120-year-old Victorian brick culvert.
When Construction News first visited the project in March 2015 as part of the Open Doors initiative, this part of the works was just about to begin and the brick culvert itself had been fully excavated.
This was the most challenging stage of the project, Mr Thompson says. “There are no records, and probably no-one alive had ever seen it before we dug down.
“We didn’t even know what the shape at the bottom was, so there was very little known about it.”
The main danger for the team was that, in order to work on the culvert and redirect the flow of water into the tank, they would have to stop water flowing through the culvert.
But this came with its own set of risks.
In the event of a storm surge, which could be up to 9,000 litres a second, blocking up the culvert would be problematic to say the least.
“If we did flood, there’s no other way of that water getting to the canal – it would’ve backed up and hit the BBC in Media City,” Mr Thompson says.
The team looked at a number of different ways of working on the culvert while stopping the water and removing the risk of a flood.
“We saw-cut through the existing culvert, and first we tried to use inflatable stoppers to stop the flow of the water,” Mr Thompson explains.
“But they didn’t seal it properly and it would have been too slow getting them out in the event of a surge of water.”
Eventually, the team decided a bespoke solution would be necessary.
“If we did flood, it would have backed up and hit the BBC in Media City”
Gary Thompson, GCA
Once the culvert was cut through, divers were employed to discover the shape and size of the culvert itself.
They also took measurements so the team could make two plates to stop the flow of water on either side of what would be the working area.
“When the divers went in, that was the first time we knew what the shape of the culvert was,” Mr Thompson says.
Once the divers had measured the culvert – which turned out to be more pentagonal on the bottom, rather than curved like its roof – a timber template was made to test stopping the water.
After this, a final metal version was made and welded in place. Once the plates were in situ, the water was pumped out and work on diverting the culvert into the main chamber could begin.
But even though the plates were installed, there was still a significant risk of flooding, meaning the team had to work round the clock and put in place a number of precautions.
In the event of a surge, the plates would have to be removed within minutes to prevent the culvert backing up and flooding Media City and the surrounding area.
Working round the clock
Having installed the plates in late June, the team switched their attention to diverting the culvert into the chamber, putting in a week-long, 24-hour-a-day blitz to make sure work wouldn’t be interrupted by a storm.
As luck would have it, the weather held.
“We put the contingency in the programme for pulling the plates in the evening and working during the day, but it was a lot better and more efficient to get it done in one go,” Mr Thompson says.
On top of this, an early warning system placed spill alarms on the chambers, which would tell the team when the water was at 75 per cent capacity.
The site is around a kilometre from the Ship Canal itself – and the remainder of the water still flows down through the 120-year-old culvert to its eventual destination.
“We had a site to make the project work, but we needed a very innovative solution to make it work”
Aled Edwards, GCA
The 24-hour-a-day week was part of an overall effort by the team to maximise productivity.
This approach was complicated by the site being not only extremely compact but also hemmed in by commercial buildings just 5 m away on one side, while the other looked out over Weaste Cemetery, which has more the 300,000 burial plots.
The site was “basically a postage stamp”, says GCA construction framework manager Aled Edwards.
“There were a lot of solutions that we looked at in the area, but this solution was the preferred one – albeit on a very small site.
“So we had a site, but we needed a very innovative solution to make it work.”
The site’s location adjacent to the cemetery added some degree of uncertainty for the team, given that there are very few records of exact burial locations.
The majority of records were destroyed in the Second World War, while the cemetery itself was hit by bombing over the same period.
According to Mr Thompson, the team found a bone during the excavation of the culvert, meaning works had to be stopped – but luckily, it turned out to be from a horse.
The team also uncovered a wartime air-raid shelter while excavating near where the site is accessed.
Mr Edwards says the client’s schedule could have seen them extend their 24-hour-a-day working pattern beyond just one week, but consideration for their neighbours led to an alternative solution.
“Considering there’s hotels very nearby, we worked 16-hour shifts – two eight-hour shifts back to back – to achieve the same output.”
Once the culvert was completed, the works on the screening and diversion chamber could get under way.
Because of the site’s size and location, the cylindrical detention tank was constructed using 1.2 m-diameter secant piled columns.
These columns went down around 25 m, meaning the team were working on “the limits of secant piling capability”, according to Mr Edwards.
The detention tank, which was one of the first parts of the project to be constructed, will boast a unique, energy-saving flushing system (see box).
No head for heights
Once complete, the tank will also require the addition of a roof, which will need to have access from above via 18 different lids.
This roof was designed in such a way that all the gaps that would eventually be covered by lids would remain covered by steel plates during construction, eliminating any need for the team to work at height.
“All the screens and walkways were prefabricated off site,” Mr Edwards explains.
“Essentially the lid is prefabbed to fit to perfection off site, and then you’ve got an onsite assembly and installation in a single crane lift. We’ve driven out the need for working at height wherever possible.”
“We installed the screens in sections of four rather than individually, so rather than 20 lifts, you’re doing it in five”
Gary Thompson, GCA
Mr Thompson adds that part of the prefabrication process meant the installation was much more efficient on site, particularly for the screens inside the detention tank.
“We installed the screens in sections of four rather than individually, which is what has traditionally been done, so rather than 20 lifts, you’re doing it in five,” he says.
All the efficiency measures used throughout the construction process have helped reduce the overall capex on the project, which is slated to come in below its stated £9m budget.
With the tank itself having been operational since the end of June, the team is well on course to hit the project’s completion date of February 2016 – by which time there will be few signs there was ever a construction project here in the first place.
Although the project will no longer be visible to the general public, the team is proud of what has been a successful and innovative job.
“We can look back at the scheme and say it’s been hugely successful,” Mr Edwards says. “It’s brought out a lot of really good engineering and put everybody’s skills to the test.”
The tank flushing system
The bell flushing system installed on the detention tank is a first for client United Utilities. The system, which automatically cleans the tank in the event of a storm, has been designed to save energy and increase efficiency.
When water enters the tank, it rises up a bell positioned just above the base of the tank and up a vertical pipe running from the bell up the side of the tank.
When water within the tank rises to a certain point, it passes through a screen. These screened flows are then channelled into the canal.
Any water that remains in the tank is pumped via a rising main to wastewater treatment works in Salford.
Once the tank is all but empty, the seal on the bottom of the bell and pipe is broken, meaning the hydraulic pressure is released and the water flushes any deposits from the screening process into a gutter and away to the wastewater treatment works – a process Mr Thompson describes as “like holding on to a top of a straw full of water”.
“Once you release it, all the water drains away, taking all the waste with it,” he says.