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Geotech special: Inner strength

Vibro stone columns have been used to strengthen an embankment which is being used to form part of a stormwater dam.

For nearly a century White Cart Water has inflicted serious flooding on homes and other properties on the south side of Glasgow. This shallow, fast flowing river is prone to flash flooding and only 12 hours of rain can raise water levels by 6m.

More than 20 significant floods have taken place since 1908 and in 1984 over 500 homes were inundated.

Historically, flood defences along the White Cart Water corridor have been piecemeal and isolated. Major investment was required to protect properties from current flood risks and from more frequent inundations expected as a result of global climate change.

The threat of repeated flooding also presents local householders and businesses with major insurance problems. At the end of 2002 the insurance industry withdrew its guarantee of affordable flood insurance in high-risk areas, saying it would have to consider charging higher insurance premiums or even refuse flood cover altogether. This could result in property values being greatly reduced.

Holding back water

In response, Glasgow City Council decided to build a dam upstream of the flood risk area to hold back water during periods of intense rain. The main contract went to Carillion, which in turn awarded specialist contracts to Pennine and Nattrass Construction. Pennine has installed 2,500 top feed vibro stone columns under a three and a half week, £124,000 contract into the hillside above Newton Mearns where the River Earn flows through the valley. It was working in mainly sandy clays and soft silts.

The vibro stone columns made from locally sourced stone combined with sheet piling, being carried out by Nattrass Construction, will strengthen a natural embankment and together form a dam. Carillion has also installed a large concrete culvert fitted with a hydrobreak flow control device. This will restrict storm water flows, with the dam retaining excess water.

Pennine has installed 2,500 top feed vibro stone columns into the hillside, under a three and a half week contract worth £124,000

Pennine claims that using stone columns is more economical and cost effective than alternative solutions which would have included cutting the hillside down and rebuilding it in engineered layers. Doing this would have taken longer and damaged local wildlife habitats, says the contractor.

Carillion constructed three working platforms on the hillside for Pennine to enable it to install the vibro stone columns.

Pennine had five operatives on the site and two rigs − a Stratacaster MarkII and a Telescopic All Purpose Rig for Displacement Implements (Tardis).

The smaller Tardis rig was used because it is only 6m high, enabling the contractor to work closer to overhead power cables which cross part of the site. Now that piling is complete, the hillside is to be re-graded and the platforms removed before the site is returned to local farmers.

Glasgow City Council will retain control of the dam for maintenance.

Battling floods

The dam project is one of three being undertaken for Glasgow City Council to the south of the city. All are flood water retention schemes which form part of a £29.5M flood prevention programme.

The water retention dams are not considered enough to wipe out the risk of flooding as the river is expected to rise significantly after heavy rain, even when flow is held back by the dams. As a result the council is funding flood defence improvement work along the water courses downstream of the dams.

This involves a mixture of strengthening existing defence walls and building new flood walls and embankments. Work also involves building pumping stations to transfer surface water to water courses, and installing surface water drainage systems to areas prone to surface water flooding.

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