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INTO THE LIMELIGHT

The outlook is good for the lime stabilisation industry despite recent well publicised setbacks.

Paul Wheeler reports.

Just when everything seemed to be going right for the soil stabilisation industry, a high profile job for the Highways Agency goes dramatically wrong, and a leading contractor is thrown off the Heathrow Terminal 5 project for 'financial irregularities' But the future of the technique remains good insists Chris Harnan, who chairs the soil stabilisation task group of the British In-situ Concrete Paving Association (Britpave) and is managing director of contractor O'Keefe's soil remediation division.

One reason is impending implementation of the first part of the EU Landfill Directive. It is estimated that from July the number of UK sites eligible to accept contaminated materials will drop from about 200 to a dozen.

While it is difficult to predict the precise effects of this legislation, it is a safe bet the cost of disposal will rocket and haulage distances to landfill sites will increase. Some predict that the dig and dump method of site remediation will disappear.

Soil stabilisation, which allows weak and soft materials to be modified into engineering materials on-site, offers an increasingly attractive alternative.

Opponents of the technique point out that production of the lime and cement that it requires involves a lot of embodied energy.

Harnan argues that, on balance, soil stabilisation is 'a very suitable process from a sustainability viewpoint' This is because there is no need to transport soft clays off site, dump and then replace them with virgin granular aggregates. It is claimed that one tanker-load of quicklime eliminates 100 truck journeys.

Harnan believes there has been an overreaction to the well publicised heave problems discovered on the lime-stabilised A10 Wadesmill Bypass in Hertfordshire earlier this year.

'It's a little like saying that the use of all concrete should have been banned after the problem of thaumasite attack was recognised in Somerset a few years ago, ' he says.

Understanding of the chemical processes during soil stabilisation has improved greatly in the last few years as a result of research into thaumasite. Experiments have shown that given a source of calcium, alumina and sulphate, the expansive mineral ettringite can form in the presence of water - essentially the same process that leads to thaumasite formation in concrete.

Like thaumasite attack in concrete, heave in lime-stabilised soil is a rare occurrence.

Problems do occur from time to time, but, says Harnan, this is 'on no more than a handful of jobs every year, as far as we are aware.'

The problem is nevertheless a potentially serious one, given the basic 'ingredients' involved in soil stabilisation. Lime and cement provide a source of calcium while alumina is a common clay mineral and sulphates are often present in the ground.

Although these compounds, together with water, need to be present under specific thermodynamic conditions for ettringite to form, says Harnan, 'one has to assume that the potential for heave is always there' One of the industry's biggest problems is that a number of contractors have entered the sector in recent years who, putting it bluntly, do not know what they are doing.

'To be successful at shallow soil stabilisation requires technical input and engineering, not just equipment and powder, ' Harnan says.

An expensive piece of kit does not guarantee success. Harnan outlines the vital elements of any soil stabilisation project: a good investigation of the near-surface soils, the right technical knowledge, rigorous testing before the job, proper design, use of appropriate equipment and products and - 'of course' - quality control testing during the work.

With this approach, he says, the technique can be used successfully in quite extreme conditions such as high-sulphate soils.

Although case studies reveal situations where heave has occurred in lime-stabilised soil with sulphate levels as low as 0.25%, Harnan cites a project in Texas where the technique was used on a major road project with sulphate levels in excess of 5%.'It worked fine because they were very aware of the problem and engineered around it, ' he says.

Britpave's soil stabilisation task group will publish a technical note on treating cohesive soils later this summer. See www. britpave. org. uk

Much of the sector's technical expertise lies with material suppliers - visit www. britishlime. org for an example.

All (lime stabilised) roads lead to- Roman road builders pioneered lime stabilisation. Lime burning was a major industry, producing mainly slaked lime for mortar and cement.

But what first comes out of the kiln is quicklime: there was no shortage of material for causeway construction. Stretches of the Appian Way in Rome still sit on lime-stabilised soils two millennia old.

In modern times the UK has lagged behind other countries in use of the technique. Latest figures indicate the UK stabilises about 2Mm3 of soil a year using either lime, cement or a combination of both. The latest US figure is 40Mm3.Long-term experience in the US confirms the Romans knew what they were doing. A Texan study of more than 40 lime stabilised clays up to 25 years old showed they were up to 25 times stronger than untreated equivalents. In the 1990s lime stabilisation was used extensively at both Denver and Houston airports, a solution that has also been adopted at Heathrow Terminal 5.

The sheer convenience of the technique, coupled with significant technological improvements, have given it real credibility. Lime stabilisation is now seen as a 'win-win' solution, saving time and money as well as being more environmentally friendly.

Despite the recent problems, lime stabilisation has a long track record and its environmental benefits are so marked that it is likely to be used on an expanding scale.

Road to ruin The UK's last major problem with sulphate attack on lime stabilised capping layers occurred on the M40 in 1990.

Experts agree the subsequent and continuing upgrading of the Highways Agency's specification for lime stabilisation had produced effective guidelines, and there was genuine surprise at the problems on the A10 bypass in Hertfordshire, particularly as the Agency says correct procedures were followed.

It may be cruel timing that the Highways Agency specification for lime stabilisation was superseded in November last year by TRL report 447, which marks a dramatic change in the approach to testing.

In the meantime the Highways Agency says it is investigating the A10 problem and will not be drawn on its long-term position on the use of lime stabilisation until this testing is concluded.

Some in the industry believe that blaming sulphates for the problems on the M40 is an oversimplification. Other factors come into the equation, including the fact that the M40 was constructed during a very dry summer - just as the A10 was.

Lime stabilisation only works over a fairly narrow spectrum of moisture content, and it is easy to get it wrong. If lime is added when the ground is too dry, it can subsequently heave if the groundwater regime changes and the ground wets up.

One solution is to allow the chemical reaction to take place at an early stage so the ettringite forms and any volume increase occurs before compaction.

That's magic!

The basic process of lime stabilisation involves adding about 3% by weight of quicklime to soft, sticky clay, mixing well, then watching an apparently magical transformation. Almost immediately the clay becomes dry, crumbly and easy to handle.

Not only does the lime dry out the wet clay, it also breaks down its structure. This means cement, and other strengthening additives such as pulverised fuel ash or blast furnace slag, work more effectively.

On a site, this means clay soils too soft to support construction or construction traffic can be transformed into strong, dry working platforms, ready to carry roads, runways, car parks or base slabs with the minimum of effort.

In granular materials addition of dry cement without lime can produce a material similar to a lean mix concrete.

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