Stitching together an aqueduct

Martin Lewes
South Lakeland reporter, BBC Radio Cumbria

The tunnellers emerging at Nab Scar in the 1890s

United Utilities engineers have drained the huge underground tunnel that takes water from the Lake District to Manchester.

It has been drained for maintenance checks, one of which, a year or two ago, showed there was a risk of a problem near Rydal.

Now engineers are repairing the crack in the Thirlmere aqueduct at Nab Scar.

By using steel and specialised grout the engineers are effectively stitching it back together.

A black deposit

Twenty feet or so down two ladders and you're standing where five feet of water would normally be flowing at a stately four miles an hour in the general direction of Manchester.

The concrete walls and ceiling look as if they were built only a few years ago - only the occasional small stalactite of calcium hanging from a joint betrays the 100 years this aqueduct has lain beneath the hills.

It is about the size of a stripped-out railway carriage and although there is only an inch of water on the floor, the reflections in the arc lights of the ceiling give an illusion of great depth when you look down the tunnel into the pitch darkness beyond. John Butcher of United Utilities says that when you walk long distances inspecting the walls, you can begin to see and hear things - "It gets spookier the longer you stay down here."

Where the water has flowed, there is a black deposit - the mineral manganese, a natural and harmless trace element. You will occasionally find it in your kettle as well.

In a few places the rock was solid enough when the Victorian navvies bored through in the 1880s and 1890s to leave it alone. Otherwise the tunnel was lined with up to two feet of concrete. For much of its length it's actually cut and cover - they dug a big trench, built a canal, roofed it over, and re-laid the soil.

The gentle slope of the aqueduct carries the water along

Biggest danger

Occasionally it breaks cover to cross a small valley or ravine. Often, it is well-camouflaged - looking like a large retaining wall. In other places it looks like a railway bridge.

For larger valleys, the water's sent through four pipelines, down and back up again. It forms a siphon - once the pipes are full, the pressure of the flow from one end and the suction from the other pulls the liquid through.

Every year, three quarters of a billion litres of water make their way down the 96 miles to the Heaton Park reservoir near Manchester without the use of any pumps. The journey takes about 24 hours.

The engineers working in the tunnel and around it come from United Utilities, who are making an annual inspection, and from a specialist firm, Bachy Soletanche. The biggest danger to the aqueduct comes from earth movements. A crack in that concrete lining is often the first sign of a developing problem.

For much of the last year, navvies in one tunnel could hear their colleagues hammering from the other side, 24 hours a day. When the two tunnels met, they were only eight inches out.

Amazed by the exploits

Currently they are dealing with just such a problem, at Nab Scar, near Rydal. The aqueduct here crosses a small steep ravine between two rocky outcrops. In the 1880s the navvies tunnelled through the rock, filling the ravine with the loose spoil. They they cut the aqueduct into the loose spoil.

Several years ago inspections inside the tunnel picked up an unusual crack, spiralling around the concrete. Now Bachy Solentanche engineers are drilling piles into the fill above and below the aqueduct, effectively stitching it onto the rock beneath to stop it sliding downhill and taking Manchester's water supply with it.

It took four years to dig the aqueduct during the 1880s

They are using technology that would have amazed the Victorian engineers who made the tunnel. Ground penetrating radar established the problem. Steel and specialised grouting are being used for the repairs. While the drills bore the holes into the rock, an electronic system keeps watch on the loose scree above. Any sign of movement and it flashes a message - to computers, and even by text to mobile phones - to the supervising engineers.

But those engineers remain just as amazed by the exploits of their Victorian forbears. John Butcher of United Utilities says he is in awe of the tunnelers who with little more than experience spent four years tunnelling three miles under Dunmail Raise from both sides. For much of the last year, navvies in one tunnel could hear their colleagues hammering from the other side, 24 hours a day. When the two tunnels met, they were only eight inches out.