The aqueduct was an impressive bit of engineering when it was built – but it was built more than 70 years ago. Regular inspections by United Utilities in the 2010s found the aqueduct was still performing well, but it was unlikely to last another 70 years without substantial work. A £3bn plan was put in place to replace six tunnel sections, and to keep the water flowing for the 2.5 million people who depend on it.
The aqueduct alternates between tunnels that carry water through higher ground and sections United Utilities calls syphons: sets of four large pipes which descend into valleys, cross underneath the rivers that sit at the bottom of them then climb out the other side. The whole aqueduct was designed to take advantage of a hydraulic gradient that keeps the water moving.
Those syphons have allowed the remedial work to take place on the aqueduct. One pipe can be switched off for inspection while the remaining three carry the water.
Each tunnel section is different, however. “You have to turn the whole thing off to actually go in and see it,” says John McNeill, who leads the Haweswater Aqueduct Resilience Programme (Harp) project at United Utilities. “They’re single-conduit sections.”
To overcome that problem, six replacement tunnels will be dug alongside the existing ones before being tested and connected into the aqueduct at either end, allowing water to continue to move as the old section is decommissioned.
The project comes with some unique engineering challenges. The most difficult section is a 16km tunnel in the Trough of Bowland, which sits 300-400 metres below ground in parts. The techniques used will be a world away from those used by the engineers who built the original aqueduct between 1933 and 1955, who drilled into the rock, blasting their way through hillsides. “It was a really arduous, hard way of doing a tunnel,” says McNeill. “But do you know what? It’s lasted 70 years.”
This time, tunnel boring machines will cut through the rock while precast concrete segments are installed behind them to prop up the tunnel. The aim is to create what McNeill calls a “one-pass tunnel”: the machine goes through once and leaves a largely complete tunnel behind it, rather than requiring engineers to return and install a second lining.
That lining also comes with its own engineering requirements. The water arriving at the aqueduct has already been treated to be potable, and everything inside the finished tunnel is therefore transporting drinking water. “You can’t just build it and then give it a really good scrub at the end,” McNeill says. The concrete segments need to be designed, manufactured, transported, stored and installed with water quality in mind. Some oils used in hydraulic equipment need to be food grade, while some commonly used solvents cannot be deployed.
Materials need to comply with Regulation 31 of the Water Supply (Water Quality) Regulations, which governs products and substances used by water companies that come into contact with drinking water. “You can’t just rock up with your standard concrete mix and throw it in and cross your fingers,” says McNeill. “It’s a totally different game.”
Keeping things out of the water matters too. The new tunnels need to be watertight, meaning the team behind it have paid extra attention on gasket design, segment thickness and particularly how those segments are installed.
Rather than the 70 years that the tunnels lasted, United Utilities wants their replacements to last 120 years. To do so, engineers have had to consider the slow erosion caused by water passing across their surfaces, how compounds in the water might react with the concrete and whether the aqueduct could be required to carry more water as demand and the climate changes.
Small tweaks can make big changes on a project as big as this. Increasing the length of individual tunnel segments from one metre to 1.2 metres meant, across roughly 50km of new tunnel, fewer segments need to be manufactured and transported. United Utilities reckons it removed about 6,000 HGV journeys from the roads.
The programme could eventually employ 1,200-1,500 people, with the utility company aiming for apprentices to account for up to 5% of the workforce. For the engineers who get to work on it, opportunities on this scale do not turn up often.
“It’s a once-in-a-generation project to work on,” McNeill says, and “a multi-generational project that will last for years to come.”
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