Water company sheds light on invisible tunnel: The pounds 250m Thames Ring Main is the capital's best-kept secret, reports Martin Whitfield

THE invisibility of Britain's longest tunnel is both the secret of its success and the reason that at least 90 per cent of the public does not know it exists.

Lying deep below the streets of London, the 50-mile long Thames Water Ring Main is one of the capital's biggest engineering feats, taking eight years and pounds 250m to complete.

But above ground there is nothing. Not even a hint of the huge pumping stations, built 40 metres underneath Park Lane, the Holland Park roundabout and Barrow Hill, next to Regent's Park.

People who live near the 21 access shafts may have been aware of the construction but most others had little idea of what was going on beneath their feet.

To rectify the ignorance, Thames Water is spending some of the savings the Ring Main will deliver ( pounds 2m a year off the pumping electricity bill alone) on an advertising campaign to sing the project's praises to customers. The Queen will officially open the main on 11 November.

'Most people have no idea of the work that goes into delivering water in the taps,' Mike Dick, principal construction engineer, said. Many of the capital's residents were only too aware of one of the main reasons for the Ring Main's construction - burst water pipes that caused serious traffic congestion.

The system of surface water mains, buried a few feet under arterial roads, remains largely a Victorian network placed under ever increasing pressure from higher demand and rising traffic.

About three-quarters of London's water comes from the Thames but pollution in the 19th century led to legislation saying that no drinking water could be extracted downstream from Teddington, well to the west of the city.

Water is drawn from the river to the large reservoirs at Hampton, Ashford, Stanwell and Wraysbury before being cleaned and then pumped to customers in central London.

It was the expense and failure of this pumping system that necessitated the building of the Ring Main, a circular tunnel wide enough to take the average family saloon with plenty of room to spare. It carries half of the capital's drinking water.

Built at a depth of 40m from where the water enters the system in west London, the flow is by gravity to central London pumping stations, so cutting the need for expensive pumping.

The water is raised at 11 pumping stations to join the distributor pipes at the surface.

Because it is a circular tunnel, water can flow in either direction. A section can be closed for maintenance and water sent in different directions to either side of the break in order to guarantee continual supply.

Most of the main, built in phases since 1986, is now working, with just a small section to be completed near the Ashford Common treatment works.

'It means there is now less pressure on the surface mains, many of which are 100 years old. We will be able to look at some for the first time because they were too important to be taken out of service,' Mr Dick said.

Later phases of the project followed the construction of Channel tunnel and workers and equipment came from Folkestone. Burrowing through London clay, the work is straightforward in tunnelling terms although the ground at Tooting Bec Common, which is Thanet sand, had to be frozen first.

The tunnelling crews have not had far to go for the next job - London Underground's Jubilee Line extension.

(Photograph omitted)

Start your day with The Independent, sign up for daily news emails
News
Young Winstone: His ‘tough-guy’ image is a misconception
people
Sport
Adnan Januzaj and Gareth Bale
footballManchester United set to loan out Januzaj to make room for Bale - if a move for the Welshman firms up
Arts and Entertainment
Ellie Levenson’s The Election book demystifies politics for children
bookNew children's book primes the next generation for politics
News
Outspoken: Alexander Fury, John Rentoul, Ellen E Jones and Katy Guest
newsFrom the Scottish referendum to the Ice Bucket Challenge, our writers voice their opinions
PROMOTED VIDEO
Have you tried new the Independent Digital Edition apps?
ebooks
ebooksA year of political gossip, levity and intrigue from the sharpest pen in Westminster
News
i100
Sport
Yaya Sanogo, Mats Hummels, Troy Deeney and Adnan Januzaj
footballMost Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
Arts and Entertainment
L to R: Hawkeye (Jeremy Renner), Captain America (Chris Evans) & Black Widow (Scarlett Johansson) in Avengers Assemble
film
News
Nigel Farage celebrates with a pint after early local election results in the Hoy and Helmet pub in South Benfleet in Essex
peopleHe has shaped British politics 'for good or ill'
Arts and Entertainment
Pharrell Williams' “Happy” was the most searched-for song lyric of 2014
musicThe power of song never greater, according to our internet searches
Sport
Tim Sherwood raises his hand after the 1-0 victory over Stoke
footballFormer Tottenham boss leads list of candidates to replace Neil Warnock
Arts and Entertainment
Sink the Pink's 2013 New Year's Eve party
musicFour of Britain's top DJs give their verdict on how to party into 2015
Voices
Strictly Come Dancing was watched by 6.9m viewers
voicesIt has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Arts and Entertainment
Roffey says: 'All of us carry shame and taboo around about our sexuality. But I was determined not to let shame stop me writing my memoir.'
books
News
i100
News
Caplan says of Jacobs: 'She is a very collaborative director, and gives actors a lot of freedom. She makes things happen.'
people
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Selby Jennings: VP/SVP Credit Quant- NY- Investment Bank

Not specified: Selby Jennings: VP/SVP Credit Quant Top tier investment bank i...

Ashdown Group: Senior Marketing Executive- City of London, Old Street

£40000 - £43000 per annum + benefits: Ashdown Group: Senior Marketing Executiv...

Ashdown Group: Marketing Manager

£40000 - £43000 per annum + benefits: Ashdown Group: An international organisa...

Ashdown Group: Internal Recruiter -Rugby, Warwickshire

£25000 - £30000 per annum: Ashdown Group: Internal Recruiter -Rugby, Warwicksh...

Day In a Page

War with Isis: The West needs more than a White Knight

The West needs more than a White Knight

Despite billions spent on weapons, the US has not been able to counter Isis's gruesome tactics, says Patrick Cockburn
Return to Helmand: Private Davey Graham recalls the day he was shot by the Taliban

'The day I was shot by the Taliban'

Private Davey Graham was shot five times during an ambush in 2007 - it was the first, controversial photograph to show the dangers our soldiers faced in Helmand province
Revealed: the best and worst airlines for delays

Revealed: the best and worst airlines for delays

Many flyers are failing to claim compensation to which they are entitled, a new survey has found
The stories that defined 2014: From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions

The stories that defined 2014

From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions
Stoke-on-Trent becomes first British city to be classified as 'disaster resilient' by the United Nations

Disaster looming? Now you know where to head...

Which British city has become the first to be awarded special 'resilience' status by the UN?
Finally, a diet that works: Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced

Finally, a diet that works

Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced
Say it with... lyrics: The power of song was never greater, according to our internet searches

Say it with... lyrics

The power of song was never greater, according to our internet searches
Professor Danielle George: On a mission to bring back the art of 'thinkering'

The joys of 'thinkering'

Professor Danielle George on why we have to nurture tomorrow's scientists today
Monique Roffey: The author on father figures, the nation's narcissism and New Year reflections

Monique Roffey interview

The author on father figures, the nation's narcissism and New Year reflections
Introducing my anti-heroes of 2014

Introducing my anti-heroes of 2014

Their outrageousness and originality makes the world a bit more interesting, says Ellen E Jones
DJ Taylor: Good taste? It's all a matter of timing...

Good taste? It's all a matter of timing...

It has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Olivia Jacobs & Ben Caplan: 'Ben thought the play was called 'Christian Love'. It was 'Christie in Love' - about a necrophiliac serial killer'

How we met

Olivia Jacobs and Ben Caplan
Bill Granger recipes: Our chef's breakfasts will revitalise you in time for the New Year

Bill Granger's healthy breakfasts

Our chef's healthy recipes are perfect if you've overindulged during the festive season
Transfer guide: From Arsenal to West Ham - what does your club need in the January transfer window?

Who does your club need in the transfer window?

Most Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
The Last Word: From aliens at FA to yak’s milk in the Tour, here’s to 2015

Michael Calvin's Last Word

From aliens at FA to yak’s milk in the Tour, here’s to 2015