Discover north London's underground reservoir holding 67 million litres of water

Tunnels at Maiden Lane Reservoir in Dartmouth Park

Tunnels at Maiden Lane Reservoir - Credit: Thames Water

A £2.5 million construction project to renew and reinforce an 1850s covered reservoir on Dartmouth Park Hill is due to be completed by the end of May, according to Thames Water.

The brick-built Maiden Lane Reservoir, which can hold 67 million litres of water or the equivalent of 27 Olympic-size swimming pools, supplies 200,000 north Londoners.

Thames Water engineers say that after almost a year’s work on the site, at its peak involving as many as 30 engineers and construction staff, the reservoir should be good for at least another 60 years, and all being well for another century or longer.

The Victorian-era reservoirs, treatment works and sewer network which kept a rapidly growing city supplied with water and able to deal with sewage and waste are regarded as an extraordinary civil engineering achievement.

Construction work at Maiden Lane Reservoir in Dartmouth Park

Construction work at Maiden Lane Reservoir in Dartmouth Park - Credit: Thames Water

The Maiden Lane Reservoir  – Maiden Lane was the old name for Dartmouth Park Hill – was completed in 1855. At first it stood among green fields. The Smallpox and Vaccination Hospital now at the heart of the Whittington Hospital estate had just been built, but housing didn’t spring up along Dartmouth Park Hill and Junction Road until the following decade.

The structural work on the reservoir was triggered by signs of slippage, evident in large cracks on the path leading through the park surrounding the reservoir. A Thames Water engineer commented that the eastern embankment had slipped by up to 40cm – quite a lot, given that such movement is measured in millimetres.

To shore up the structure, two rows of concrete piles have been put in place beneath ground level – 135 in total, at a depth of up to 11 metres. They are positioned every two metres to stop the soil shifting and inclinometers are in place to measure any slight movement.

Most Read

Thames Water says that noise and vibration monitors have been in use throughout, and drilling has been done in corkscrew style both to limit the noise and to keep to a minimum any vibrations which might affect the structure.

The Dartmouth Park area in the mid-1860s

The Dartmouth Park area in the mid-1860s - Credit: Ordnance Survey

The reservoir itself is said to be "one of the better ones" of its era in London. The water stored is already treated, most coming from the Copper Mill works on the River Lea, so there’s little sediment or erosion. The reservoir consists of two separate cavernous compartments, with water normally about six feet deep. Water is pumped up mainly at night, when the power is cheaper, and then gets depleted during the day. 

While some older reservoirs have fairly flimsy coverings, the Maiden Lane Reservoir has a sturdy arched brick roof.

"You could play football on top of it," one of the construction team said. But the crown of the reservoir is to remain fenced off, a decision that has disappointed some local residents.

Although the park is a no-go area for the public during the construction work, the engineers on site say that no curious passer-by is turned away. They are happy to explain what they are up to and why.

Tunnels at Maiden Lane Reservoir in Dartmouth Park

Tunnels at Maiden Lane Reservoir - Credit: Thames Water

Once the construction team is finished, Islington Council will decide when the northern section of the surrounding park, including a refurbished play area, will reopen. It should be up and running for the summer. The steep-sloping, east-facing section of the park is being seeded as a wildflower meadow and will only be accessible to the public later in the year.

Dartmouth Park, to give the green space surrounding the reservoir its formal name, is not the most widely used of local public spaces. It doesn’t have the charm of Waterlow Park or the expanse of Hampstead Heath. But it is a well-regarded local amenity and offers spectacular views to the east and south-east. 

While most in the community will be happy to reclaim the space from the "high vis" teams, some local residents will miss the construction workers. At the start of day, foxes often congregate at the site office. There are three cubs and three adults. Engineers insist they don’t feed the foxes, but their denial is not entirely convincing.

Andrew Whitehead is the author of the Curious series of books about different localities of north London.

Maiden Lane Reservoir in Dartmouth Park

Maiden Lane Reservoir in Dartmouth Park - Credit: Brian Kelly