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Integrated approach pays off

KELLER GROUND ENGINEERING is integrating site remediation with ground improvement on a contract in North Wa les .

The firm says that running the two operations in parallel saves time and money.

'By being in control of all aspects of the site preparation we can accelerate the programme and also design the site clean-up to complement the ground improvement operation, ensuring the soils remain suitable for ground treatment, ' said Keller contracts manager Steve Baxter.

Work is being carried out for Turkish manufacturer Toprak Ceramics'new factory on the Deeside Industrial Park. The 33ha site was formerly part of Shotton Steelworks. This was demolished and the site reclaimed, with ground levels raised using up to 6m ofhydraulically placed silty sand.

The sand is loose to medium dense, underlain by a thin layer ofsoft organic soil, a residual of the salt marsh that once covered the low-lying area. Local deposits ofdemolition material are also present.

Although the site was remediated during reclamation, investigations showed contamination was still present and that landfill gas was migrating on to the site. As work had to be carried out within a 12-week programme, it was decided to consider dealing with the site conditions as one 'integrated problem'.

Other large buildings on the industrial estate have been piled but after discussions with Keller, consultant Nickson Davis opted for dynamic deep compaction, which proved to be more economical. Ground improvement had to densify the soils to provide a uniform ground-bearing pressure of100kN/m 2across the entire site.

'A high level of settlement control was an essential requirement for Toprak's manufacturing process, ' said John Davis of Nickson Davis. 'Dynamic compaction was able to provide the most cost-effective way of achieving this and providing the flexibility for any future redevelopment work. '

Two dynamic compaction rigs are carrying out 67,700m 2ofground treatment, which involves high energy tamping by dropping large weights on a regular grid from controlled heights to compact the underlying soils.

Testing is used to prove the level ofcompaction achieved and the level of differential across the vast site area. 'We are using pre- and post-treatment static cone tests on a 25m grid to measure the level of improvement along with full-scale load tests to monitor settlement performance, ' said Keller engineering manager Barry Slocombe.

In all,11 long-term load tests are being carried out, using kentiledge to load 2m by 2m bases to 250Kn/m 2. 'Settlement after treatment, at two-and-half-times the working load of 100kN/m 2, has generally been around 7mm, ' Slocombe said.

While compaction is under way, Keller is dealing with contamination on the site. This is concentrated in hot-spots with elevated levels ofVOCs. These are being bio-treated using 'landfarming techniques'.

To counter gas migration and potential spread of contamination from the surrounding area, Keller is installing an 8m deep cement/bentonite slurry cut-offwall and gas venting system across the northern boundary.

And, despite an 'atrociously'wet autumn, Keller looks set to complete the £500K contract two weeks ahead ofschedule.

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