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Learning to love lime stabilisation

Encapsulation of contaminated soil by lime and cement stabilisation resulted in considerable cost and time savings during the design and pre-construction stages of a new school in east London.

Cement lime stabilisation has been around for decades.Ground otherwise considered unsuitable for construction can be improved substantially simply by adding cement and/or lime.

Always an attractive idea, current re-evaluation of what is meant by sustainable development has given it fresh appeal.

Using marginal materials on site has many advantages, and they are not confined to the direct cost of materials used on a project.

However the reality of soil stabilisation work, especially where lime is used as well as cement, is less attractive. In urban areas, where the technique has the greatest potential, stabilisation work is often seen as environmentally problematic.

The normal method of working is to spread the lime and cement out and then run over the ground with a rotavator, mixing the soil and additives insitu. This can be a messy process, especially in windy weather. Lime and cement can be blown off site, and it is difficult to convince locals that you are being 'environmental' when surrounding trees are covered in lime and cement dust.

Nor do clients like the idea of paying for lime and cement that ends up on trees . . . and then there is the quality control issue. If product is being blown around site, what percentage is actually going into the ground and at what concentration - how consistent is the finished stabilised ground?

O'Keefe Soil Remediation, based in south-east London, was the first soil stabilisation contractor to address these shortcomings in an otherwise attractive technique. In 1997 the company was responsible for developing a new type of mixing machine with German construction equipment manufacturer Wirtgen.

Essentially, Wirtgen chopped one of its rotavating machines in half and put a four tonne hopper in the middle to produce the WR 2500K, the world's first fully integrated soil stabilisation machine. Lime and cement are delivered directly to the milling drum so mixing takes place immediately, within the skirted confines of the slowly moving mixing unit.

The mixing process is computer controlled, with release of the mixing product varied to match the speed of the machine to produce the predetermined concentration in the ground.

After the mixing process come the laser-controlled bulldozers that can achieve a tolerance in level within plus or minus 10mm. The result is a much cleaner and better controlled construction process which results in a much more reliable product.

O'Keefe Soil Remediation envisaged using the process to improve the engineering characteristics of marginal materials for roads and car parks (one of the company's early projects was car parking areas at the Millennium Dome).However, given the consistency and reliability of the process, plus the high permeabilities achieved (typically k values of 1 x 10 m/s), O'Keefe realised the process was capable of encapsulating pollutants on contaminated sites.

Last year a project came up on which to test the concept. The results were hugely successful with benefits beyond those originally conceived. The new PFI-procured Leighton primary school, built on a brownfield site in east London, was the first school to be built in the area for 100 years. Construction partners included WS Atkins Environmental and Wates Construction.

The site was contaminated with about 6000m 3oftypical heavy metals such as zinc, lead, copper and mercury.

Traditional off-site disposal was prohibitively expensive, so Atkins and O'Keefe started investigating options for encapsulating the contaminants.

O'Keefe Soil Remediation was confident it could easily deliver the 1x10 m/s required, and Atkins was sufficiently interested in the concept to enable talks to develop. The resulting scheme involved ground improvement with very closely space stone columns - installed by Keller Ground Engineering to 3m depth - and O'Keefe Soil Remediation forming a 500mm cement stabilised capping layer over the top.

The mix design, with 5% added cement, was developed and tested by the materials testing section of Weeks Consulting. Because the ground was essentially granular, there was no need to incorporate lime into the mix. The resulting material achieved a permeability in laboratory tests of better than 1 x 10 m/s and CBR values of at least 90%.

On site, the 500mm slab was formed in two 250mm layers, to ensure the Bomag roller achieved full compaction throughout. Using laser Controlled caterpillar bulldozers, the encapsulated sealing slab was placed to within plus or minus 4mm of design, and the whole process was completed within four weeks.

With these properties and the precision with which the slab could be formed, it was realised the ground-floor foundation slab could be cast directly on the encapsulation slab. 'O'Keefe's early involvement at the pre-tender design stage lead to phenomenal savings in both time and money for the project, ' says O'Keefe Soil Remediation's operations manager Mark Jones.

For the follow-on groundworks, O'Keefe Construction simply used a rock trenching machine to cut shallow foundation and service trenches within the stabilised layer, removing the need for formwork and blinding and considerably speeding up construction.

O'Keefe Soil Remediation's biggest hurdle was not proving the technical capability, but getting the process accepted by the Environment Agency. During the course of the contract the company, aided by Weeks, applied for a mobile plant licence which it achieved in early autumn.

This required the company to adopt a generic working plan which sets out the construction process, health and safety matters, and quality control and monitoring regimes for remediating heavy metals and PAHs. This is a significant achievement and should improve acceptance of soil stabilisation as a sustainable remediation tool.

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