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)