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Water: Main switchers

This month marks completion of the main phase of arguably the world’s largest and most complex urban water mains renewal programme. David Hayward reports from London’s West End.

London water mains replacement

Squeezed space: Roadworks along central routes need months of detailed planning

Beneath the streets of central London an antiquated maze of Victorian cast iron distribution pipes is being replaced by an engineered 21st century network, guaranteeing an ongoing safe and reliable drinking water supply for the capital.

Over the last 12 years the water demands of 600,000 of Thames Water’s London customers have been reassessed using the latest hydraulic modelling techniques. And the result is over 2,300km of new pipe being laid beneath a thousand streets to reduce water leakage by 30%.

“It has been a massive and challenging operation, which at times proved a logistical nightmare, but we have now provided central London with a water distribution network fit for at least another century,” reflects John Chambers, construction manager for the project’s lead contractor Murphy.

On route, a team of consultants and contractors, led by client Thames Water Utilities, has had to match the need to impose on London major, potentially disrupting, roadworks - or complete closures of several hundred streets - while at the same time minimising inconvenience to the capital’s already road network.
Excavations have been opened up in the midst of central London’s several million pedestrians; while the concerns of major customers - ranging from hospitals and high court judges to theatre owners and Olympic Games organisers - have been a constant priority.

“It has been the most difficult renewal programme we have ever undertaken, and was much more complex than any of us anticipated”

John Chambers, Murphy

The skills of Britain’s Victorian civil engineers have seldom been more appreciated than beneath London’s streets. Here, complex, but once ordered, networks of brick tunnels and cast iron pipes have provided the capital with world-renowned clean and dirty water distribution systems for over 150 years.

Now past their original design life, these networks still fulfil their aim. But water supply leakage, increasing collapse and duplication by more recent network additions, have signalled the need for a more cost effective engineered water mains replacement.

Launched in 2002, Thames Water’s Victorian mains replacement programme boasted all the credentials to be one of the most technically and logistically challenging water projects yet attempted beneath any major city. When the current phase is completed this month, at a cost of over £650M, the project will have proved to be that and more.

“It has been the most difficult renewal programme we have ever undertaken, and was much more complex than any of us anticipated,” says Chambers.

Over 20,000ha of north and central London which, until the programme started, received virtually all its main water supply through Victorian pipework, has been divided into 100 zones called district metered areas (DMAs). Each DMA contains, on average, 2,000 customers. When work was underway, supplies could only be cut off for a couple of hours while
connecting the new network and without affecting a neighbouring area.

With more recent, often uncharted, pipe runs creating an up to five fold duplication of distribution mains, each DMA has been re-engineered. The cast iron network has been completely abandoned but left insitu, while rationalisation results sometimes in new pipes which have only half the capacity of their predecessors.

And the renewal programme has produced a 30% reduction in the pipework needed.

West End Challenge

London water mains replacement

Tight constraint: Ensuring the safety of central London’s 250,000 pedestrians proved a constant challenge to mains replacement contractors

Ask any member of Murphy’s team which area has been the most challenging to upgrade, and most will reply London’s West End.

Over three years, a 100 strong dedicated workforce replaced 90km of cast iron pipes beneath the most congested streets of one of the world’s busiest capital cities.

A total of 270 streets have been dug up to upgrade 11,000 customers. They were streets crowded with high value business traffic plus a high percentage of London’s 8M daily pedestrians.

And over much of the 600ha West End, including Oxford Street, Piccadilly and Leicester Square, minimising inconvenience to commuters and visitors has been of equal importance as reducing traffic disruption. Along just one short upgraded pedestrian route from Leicester Square to Covent Garden, the safe control of over 250,000 pedestrians every day occupied considerably more planning and focus than the construction work.

Murphy had continually to satisfy the often conflicting needs of unusually high profile customers. London’s theatreland would not tolerate roadworks noise or disturbance during evening or matinee performances.

Equally sensitive was the Strand’s Royal Courts of Justice, and the possibility of a major leakage forcing the closure of an underground Tube station.

Large public events proved a constant challenge. Traffic planning for the 2012 London Olympics meant pipe laying beneath 28 major traffic routes had to be completed early; installed to their own separate schedule and yet still integrated into the overall programme.

And the construction team learned to know well the dates of prestigious Leicester Square film premieres, marathon road races and central London demonstration marches.

 

But before even the first metre of new polyethylene pipe could be laid, the contractor’s logistics team held centre stage. Roadworks have been needed simultaneously in up to 30 DMAs scattered across London, and every street to be covered needed a pre-agreed construction programme.

A dozen local authorities - all needing a say in street works permits, parking bay suspensions or temporary traffic lights - had to sit down alongside the emergency services plus representatives of every property or business being upgraded. Key construction personnel could easily be expected to attend three external meetings a day, and at peak more than 2,500 letters a week were being sent to customers. Relaying in the busiest streets could take six months to plan.

Murphy has been involved in the programme continuously since a pilot study in 2002. Initially the company replaced pipes through individual contracts under its framework agreements with Thames Water.

But, from 2010, the work has been undertaken as part of the Optimise joint venture between Murphy, contractors Clancy Docwra and Barhale Construction plus consultant MWH.

The joint venture’s 400 strong workforce was split into around 80, two to five person teams, which were employed initially in laying either 450mm down to 125mm diameter distribution mains or 90mm to 25mm lateral service pipes to individual properties. A third team would arrive later to complete carriageway reinstatement.

Murphy has, however, over the years repeatedly value engineered the pipe laying process, developing an innovative but simple “one hit” system, resulting in a 12% saving in time and cost. Instead of using two separate crews laying mains or follow-on smaller service pipes, there is now only one team completing both operations at the same time.

This has resulted in one, rather than two, sets of potentially disrupting roadworks or carriageway closures.

An additional, much smaller, second pipe team is still required later to install individual house connections, but as this team can operate within footpath boundaries, there is minimal disruption.

Scrape away the surface roadway and the ground around much of the distribution network being upgraded contains a maze of different utility services. Some are mapped, some remain a total surprise until unearthed, and most form a complex intertwined jungle of pipes and cables. For this reason, the most successful of the three main construction techniques for new pipe installation have been trenchless techniques - pipe insertion or bursting.

Both demand that the new pipe runs follow exactly the alignment of the old cast iron routes. These now abandoned networks can then act as host pipes or exact guide routes for their replacements.

Best for the smaller diameter distribution pipes, and for most of the lateral property supply network, is pipe insertion. This involves a narrow fibreglass nose cone leading the new polyethylene pipe run fixed behind it. Cone and pipe run are pulled through the old, slightly larger, redundant cast iron pipes, which remain in place.

For larger mains distribution routes, pipe bursting has proved the value engineered solution. Relatively new to water main construction at the start of the programme, the technique has been fine tuned by the contractor into a cost effective installation method, especially for replacement pipes larger than their originals.

Here, a more robust steel nose cone is used, again with the new pipe run strung out behind it. Fitted with splitter blades, the cone is also pulled through the old cast iron pipes, but with more powerful jacks and linked steel rods replacing the insertion technique’s rope haulage mechanism.

The brittle cast iron is cracked and burst apart, opening up a wider diameter bore into which the larger plastic pipe run is positioned.

In utility-congested central areas, which also demand larger distribution pipes, trenchless techniques are impractical. Here, conventional open cut, or narrow, trenching could not be avoided, demanding larger land take, roadworks and higher cost.

As the last few metres of new pipe are being laid, Thames Water can start assessing the success of the whole programme’s primary driver - reduced leakage.

Back in 2002 the company was losing over 900Ml per day throughout its region, due largely to its cast iron pipework.

Today that leakage figure is, 270Ml per day less - a 30% reduction. And further water savings are possible from next year as Thames Water has
earmarked another 881km of similar pipework improvements.

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