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FLOOD PAIN

TUNELLING - Efuent being pumped from Borehamwood includes Big Brother, EastEnders and sewage from a network that is being upgraded.Damon Schünmann reports.

Borehamwood in south Hertfordshire, perhaps best known as home of the BBC's Elstree film studios, does not just suffer from chronic diarrhoea by producing the long-running soap about deranged and angry folk from east London. In recent times it has also been afflicted with flooding when high volumes of rainwater has become too much for the high street's central sewage pipe to handle.

'We are working here because of the internal ooding to properties on Shenley Road [the high street] as recently as last year, ' says Arthur Gugan, project construction engineer at client Thames Water.

Main contractor Barhale Construction site agent Andy Crawford adds: 'At the moment the water comes down to Brook Road [where work has begun] and outfalls via a gravity sewer ending up at Maple Lodge sewage treatment works. But because this system can't cope, it's backing up into people's basements.' Because of this inability of the sewers to cope, 29 properties in Shenley Road and the surrounding area have been ooded.

The system comprises large clay and concrete pipes up to 600mm diameter and the catchment sewers take both surface and foul water.

'It's an old system that surcharges and over ws causing ooding and people have contacted Thames Water when gardens and properties are affected. So we put the current system through a model that showed it couldn't cope with a one in 15year storm event, ' says Gugan.

Barhale won the £2.8M NEC (new engineering contract) work from Thames Water under an AMP 4 (asset management plan) framework agreement, alongside Anglian Water and Severn Trent Water. It is installing pipework in open cut sections, generally built in 50m lengths, as well as in microtunnelled deeper sections that will also minimise disruption.

Consultant Faber Maunsell principal engineer Neil Gibbon says: 'The new sewer will provide over ow for the existing one and a pumping station [in Eldon Road] will provide [additional] overflow for when the new system fills. This station will empty after the storm event. But both sewers will always have a small amount of water running down them as it makes sense to have constant ow in a foul water pipe to stop it becoming stagnant.' For the open cut work, site workers will lay about 600m of pipes ranging from 375mm to 900mm internal diameter up to 4.5m deep, with the pipes bedded within a shingle surround.

For the microtunnelling work, an Iseki Unclemole TCC780 will bore about 640m to install 600mm inside diameter pipe up to 8m deep. To control face stability for the laser-guided machine, earth pressure balance is achieved through mechanical support from the cutter head, while a slurry system counterbalances and helps stabilise hydrostatic pressures.

The tunnelling machine will travel at up to 12m a day and can tunnel up to about 110m per drive, the length being dependent on friction on the outside of the jacked pipe and the pumping limits of the ushing system.

The strata at the site - just inside the north curve of the M25 orbital motorway - is made ground overlying firm to stiff clay.

'We are doing something innovative here', says Crawford, '[We are] hoping to reuse most sitewon material as backfill if we get Highway Authority approval.' Before being sent offsite for processing, spoil from the Unclemole's return line is fed into Barhale's bespoke onsite three-tier separation plant. Here, a closed system recycles the slurry and separates oversize material (see ow chart below left).

'Clay is generally not used as a backfill material but we hope it will meet the Highway Authority speci cation, ' says Crawford. 'The benefits of reuse are not taking it to landfill which means reduced lorry movements and reduced costs with landfill tax and aggregate tax.

'There is also the advantage of not bringing in virgin material from a non-sustainable source. The material taken out to make way for the pipe's shingle surround will still have to be removed either to landfill or other Barhale projects if possible.' The company is also trying to reprocess the tarmac from the roads covering the open cut sections, but this depends on its makeup as some may contain coal tars that are unacceptable for reuse.

Work began on 1 June and is due to complete by the beginning of February next year.

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