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Costain finds there's brass among Heathrow 's muck

When Britain's largest sludge dewatering plant got in the way of the Heathrow Terminal 5 construction project Costain was brought in to work up a £106 million solution.

Alasdair Reisner visits the firm's Iver South site to see how it is getting on SLUDGE. Even the word is unattractive.The half-liquid, half-solid sediment squelching through the nation's sewer network is hardly the stuff to raise much excitement.

Unless, that is, your name is Jon Clackett.The sludge Mr Clackett deals with stands in the way of the £4.5 billion Heathrow Terminal 5 airport works.All of a sudden this sludge, lying just inside the M25 at Perry Oaks in Berkshire, looks slightly more interesting.

Despite its country-cute name, Perry Oaks is home to the largest sludge dewatering plant in Britain. Every week it treats 20,000 cu m of the slimy stuff, recycling the waste back into the water network, leaving dried sludge cake that can be used as fertiliser.

Once every few minutes a plane roars through the skies above Perry Oaks making its way to or from London's Heathrow Airport. Increasing demand for the airport led to the requirement for construction of the gargantuan fifth terminal at the airport.The only problem was the small matter of the sewage plant lying slap-bang in the middle of the area earmarked for the new runways.

All of a sudden the problem of Perry Oaks became a big-money issue.

The solution was to move all of the functions of Perry Oaks to nearby Iver South, the site of an existing sewage treatment works.

Enter Mr Clackett as Costain's project manager for the scheme.The firm initially only held the contract to build a pipeline linking up the Iver South unit to the main sewage treatment plant at Mogden in Twickenham. But late in 2001 the firm was told it had also scooped the contract to develop the Iver South facility itself. Client Thames Water agreed to wrap up the two deals under a single management structure to reduce administration costs.

But if the management structure was simplified, the nature of the job was anything but. Putting aside the new plant at Iver South and the pipeline upgrade from Mogden, the team also had to upgrade the plant at Mogden itself, build new bridges over the A4 to create a site entrance at Iver South and build a pipeline from Perry Oaks.All together the scheme will be worth £106 million to Costain.

Once produced by the Mogden plant the sludge used to be transferred to Perry Oaks in pipes within the main 2.9 m sewer.As some of these were installed nearly 70 years ago it was decided the pipes should be updated as part of the project.

But, as the sewers needed to remain in use at all times, this task was by no means easy.

'It was all night work, because we could only get access between 11.30 at night and 4.30 in the morning.We had a penstock (gate used to control the flow of water) that was lowered and the sewer behind it acted as a catchment area, ' says Mr Clackett.

The penstock had around 400 mm clearance at its base to allow a small amount of flow through the tunnel, preventing the preceding sewer from backing up before the team had finished work each evening.Yet, even with this system in place it was important to keep conditions monitored both above and below ground.

'The smell is one thing, but the real issue is heat. It can get very hot underground so we have forced ventilation to keep the air cool and fresh.

We have to keep that monitored - we are a bit more advanced than having a canary in a cage now, ' says Mr Clackett.

Despite the penstock it was also important to ensure that rainwater was not entering the tunnel through drains downstream.

'Obviously a dry day makes for the best conditions so the wet summer didn't really help matters.We had a system of two warnings.The first call would be to come and work closer to the tunnel entrance.The second would be to get out of there, which we had to do a reasonable number of times, ' says Mr Clackett The limited space offered inside the tunnel didn't leave much room for the team so they had to think laterally when it came to the plant they used.

'We came up with the tracked tunnel units. It started out as a small dump truck and was converted to carry three 5.5 m lengths of pipe on a carousel.The first thing we do is take down the old pipes then replace them with new. But at the same time we are replacing brackets that allow movement in the joints between the sections.We found that, despite having been down there for 70 years, the pipes were in pretty good condition but the brackets definitely needed replacement, ' says Mr Clackett.

But the team found this long-winded.With speed of work vital, it was frustrating that the tracked units could only reach one bracket at a time.

The simple innovation of a platform at one end of the unit allowed access to two brackets.

The TTUs worked in pairs so that if one machine broke down it could be piggy-backed out using the hydraulic power of its sister machine.This was vital, as losing one of the machines down the tunnel would not only mean that it would need to be replaced, it could also block the tunnel and cause flooding.

The specialist nature of the TTUs means that once Costain has finished the work on the tunnels it is unlikely to find another use for them. Instead they will be handed over to Thames Water, which will use them for sewer maintenance.

Costain contracts n B1: Pipeline from Perry Oaks to Iver South - £8 million n B2: New junction on A4 leading to Iver South- £2.5 million n B3: Upgrade of existing junction of A4 to Iver South - £3.5 million n B4: Upgrade pipeline between Mogden and Iver South - £14 million n B5: Modifications at Mogden - £34 million n B6: Design and management - £6 million n B10: Dewatering plant at Iver South - £38 million Total value: £106 million Project details Client: Thames Water Main contractor: Costain Designer: Black & Veatch Centrifuges: Alfa Lavel Bentonite Wall: Keller Odour stripping: Aquabio Air mixing: Monsal Shafts: Joseph Gallagher Cofferdams: Sheetpile UK Cakestore concrete: MJ Gallagher Steel (dewatering building): James Brothers Steel (cake stores): Royden

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