London’s Water—A New Dimension

By Awake! correspondent in Britain

LONDON, England’s capital city, now has one of the most advanced water-supply systems in the world. It was completed two years ahead of schedule at a cost of some $375 million. The expertise gained in its construction is already being traded to other countries.

Why was such an expensive project necessary, and what has it achieved?

New for Old

London’s oldest trunk water main was built in 1838. Forty years later water was still being carried in buckets from communal street standpipes in the poorer areas of the city. “The turning on of the tap in the early morning by a man with a key was an event of importance, . . . for once the authority with the key was gone not a drop of water could be drawn until the following morning,” relates one writer.

Victorian engineers did a masterful job when they extended this water supply to individual homes, laying iron water mains and building conduits at varying depths under road surfaces. Since then, however, the increasing volume, weight, and vibration of motor traffic, along with the greater pumping pressure needed to ensure an adequate flow of water over long distances—up to 18 miles [30 km] in some cases—have taken their toll in ruptured mains. This results in traffic chaos when roads have to be shut off for water-main repair. It is estimated that 25 percent of all water drawn from reservoirs in England is lost through defects in delivery piping.

Additionally, London’s demand for water has escalated over the past 150 years—from 88 million gallons [330 million L.] to over 600 million gallons [2 billion L.] each day. Washing machines, dishwashers, car washing, and the watering of gardens during dry summers have all helped to heighten demand. The need to improve the supply of water to the metropolis became urgent. But what could be done?

Thinking Big

To replace the old pipes by laying stronger ones under the same road system was out of the question. Costs were as prohibitive as the inconvenience to Londoners was unacceptable. Thus, ten years ago the Thames Water Ring Main project was conceived. It would greatly increase London’s water supply. The project is a 50-mile [80 km], 8-foot [2.5 m]-wide water main, or tunnel, buried at an average depth of 130 feet [40 m] under the city and capable of carrying 285 million gallons [over one billion L.] of water a day. Such a ring main would allow for the flow to be controlled in either direction, making it possible for any section to be taken out of service for maintenance at any time. Water would be gravity-fed into the tunnel from water treatment plants and then pumped directly into existing local mains supplies, or holding reservoirs.

Why did the tunnel, the longest in Britain, have to be so deep? Because underground London is honeycombed with 12 railway systems as well as the usual mass of public service utilities, and the tunnel obviously had to clear all of them. When engineers unexpectedly encountered deep pile foundations of one building, which had been missed in the initial survey, work was delayed for over ten months.

Construction was scheduled in stages. No great problems were expected digging through London clay, but the tunneling had to be abandoned for over a year at the initial site, south of the Thames at Tooting Bec. There the tunnelers entered a stratum of sand containing water under high pressure, which eventually engulfed the boring machine. To resolve this difficulty, contractors decided to freeze the ground by circulating a brine solution at minus 18 degrees Fahrenheit [−28° C.] through the boreholes. Sinking another shaft nearby, they were able to dig through the ice block to retrieve the entombed machine and continue drilling.

Because of this experience, engineers saw the need to devise a new system of lining the tunnel with concrete. It also became evident that a different type of tunneling machine was needed to cope with such unstable ground. A Canadian earth pressure balance machine was the answer. Three were purchased, and as a result, the tunneling speed doubled to a mile [1.5 km] a month.

Computer Construction

Traditional theodolite land surveys were made from rooftops to give line-of-sight measurements for shaft locations, and the results were then checked electronically. This method was adequate initially, but once tunneling began, how could exact alignment be ensured underground?

Here, modern technology took over by means of the Global Positioning System (GPS). This survey equipment consists of a satellite receiver tuned into a GPS spacecraft orbiting the earth. The equipment could compare signals from a number of orbiting satellites. Once these measurements had been coordinated by computer, the positions of all 21 shafts and 580 boreholes were pinpointed along the route on Ordnance Survey maps. Armed with this data, tunnelers were guided with precision.

Computer Control

To meet the needs of six million customers is no easy task. Demand can fluctuate not just season by season but day by day. This calls for around-the-clock monitoring to ensure that correct water pressure and quality are maintained at all times. How is this vital coordination possible? By means of a computer control system that cost $5 million.

Each shaft pump is controlled by its own computer, and cost is kept to a minimum by using inexpensive, off-peak electricity. Master computers at Hampton, in the west of London, regulate the whole network. The computers draw data from fiber-optic cables fixed to ducts in the tunnel walls and relay it via closed-circuit television monitors.

Water quality is checked at daily, weekly, and monthly intervals. “There are 60 mandatory tests for 120 substances in testing water quality. They include analyses for substances like nitrates, trace elements, pesticides and other chemical solvents,” The Times newspaper explains. These measurements are now made automatically and are relayed to the computer headquarters for interpretation and action as the need may be. Water tasters also make periodic assessments of quality.

Thinking Ahead

This marvel of modern engineering is already providing 154 million gallons [583 million L.] of drinking water daily to a population spread over 580 square miles [1,500 sq km] of Greater London. When it is fully operational, it will meet some 50 percent of present demand, taking the strain off other sources of supply.

Even this will not be enough. Therefore, plans are now being made to extend the ring main by another 40 miles [60 km] early next century. Truly, an ingenious solution to a difficult problem!

[Diagram on page 15]

Cross section beneath London, showing water main underneath other tunnel services

S

New water main and shafts

River Thames

Underground railway tunnels

N

[Credit Line]

Based on photograph: Thames Water

[Picture on page 16]

Water-main tunneling machine

[Credit Line]

Photograph: Thames Water

[Picture on page 17]

Water-main construction work

[Credit Line]

Photograph: Thames Water