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MAKE SOIL NOT SPOIL

FOUNDATIONS

The Landfill Directive, partly introduced in July, could provide the impetus for a more creative approach to foundation engineering.

Leaving aside the fourfold increase in cost of disposing of hazardous wastes as a direct result of the Landfill Directive in July, the general trend over the last few years has been a steep rise in all spoil disposal costs.

Add to this the government's drive to redevelop and regenerate brownfield sites where they are more likely to encounter hazardous waste, and developers face a double-whammy.

Common sense says that in this context, foundation methods that displace the soil sideways, rather than bringing it to the surface, have an automatic advantage. Expect therefore to see an increase in driven cast insitu and auger displacement piling.

The spoil disposal issue could be good news for ground improvement methods too. As Martyn Singleton, business development director with Keller Ground Engineering sees it, the pressure to minimise spoil is encouraging people to 'use soil more creatively than bypassing the problem with piles' According to Singleton 'most problem soils are simply competent materials in a loose state' With the exception of degradable soil, organic clays and peat, he says: 'If you can make them denser you improve their engineering characteristics.'

The obvious application of this concept is the well-proven method of vibro-compaction, originally developed by Keller in Germany in the 1930s and established in the UK for more than 40 years.

In recent times Keller has helped push reliability and quality control of vibro-replacement to the point that the method is starting to be used as an alternative to piles for buildings up to five or six storeys and is certainly making inroads into the buoyant housing sector. The basic approach is to improve the ground so that the structures can be founded on shallow strip footings or trench fill foundations.

Vibro concrete columns, a close cousin to vibro-compaction, also has good potential as a displacement foundation method on contaminated sites - particularly as it is possible to create bulb-ended piles that can take very high working loads.

Quieter than driven piling, another advantage of this method, says Singleton, is that it is possible to provide foundations for sites where contaminated materials overlie, say, highly permeable gravels. The enlarged base means a vibro concrete column can obtain adequate capacity without having to penetrate the gravel, negating the risk of creating a migration path from the contaminated ground.

Another displacement foundation technique, dynamic compaction, is also frequently used to deal with the problems of redeveloping old refuse tips. Keller recently undertook one such contract for Pro Logis at Thornton Heath, where dynamic compaction was used to reduce site levels by 1m. This approach not only enabled planning requirements on building heights to be met, but the developer also saved close to £1M by not having to remove excess soil from site.

In this case, Keller installed driven cast insitu piles with enlarged heads to support the structures and floor slabs. Although this method has been around a long time, it is enjoying a new lease of life - helped by the development of enlarged heads - but primarily because it is a displacement method.

Singleton says it is significant that when Keller restructured its piling activities last summer, it discarded the supposedly more sophisticated large diameter bored piling, but kept its displacement methods.

And while many contractors are failing to make a major impact with rotary displacement auger piling methods, which produce very little or no spoil, Singleton considers a good compromise to be the move towards high displacement CFA piles.

These reduce arisings by around 50% compared to conventional CFA, but are not limited by the high torque requirements of full rotary displacement methods, or for that matter, the issue of controlling heave in the ground surrounding the pile.

Singleton believes clients are becoming more open to ground improvement as an alternative to a conventional piling solution - and when there is no alternative to generating spoil you can use it more creatively and for instance turn unsuitable fills into engineering-grade materials.

Even earth retaining structures, which can be built from site-won material, are in effect a ground improvement method, says Singleton. It is possible to reinforce the material and form slopes at up to 65infinity.

'Reinforced soil systems such as Textomur have been developed specifically so that cohesive as well as granular soils can be used to form retaining structures, ' he says.

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