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AuthorJonathan Cook

DateAug 12 2019

Cement and chemical stabilisation is not the only green and cost-effective alternative to dig and replace for delivering a robust and long-lasting roads network.

Two of the central pillars of the UK government’s Road Investment Strategy are to reduce maintenance costs by 30% to 50%, compared with current levels, as well as to reduce the environmental impact of the road network.

The green roads agenda will, of course, be achieved by reducing carbon emissions of vehicles and also by building-in environmental measures to all new schemes – and rectifying environmentally-poor designs when repairing, maintaining and upgrading existing roads.

Geotechnics has a lot to offer in this regard, particularly when building new roads over weak or waterlogged ground.

Thankfully, today’s engineers are less likely to resort to the traditional approach of dig and replace and will consider a number of different approaches to increase bearing capacity. Use of chemical stabilisation and stabilisation geogrids can, not only be faster and more cost-effective, but that are greener too.

Chemical stabilisation – using lime, cement and other hydraulic binders – can reduce environmental impact by reducing the amount of material having to be disposed of and the amount of imported, as well as the associated reduction in lorry movements. However, the method does have its limitations.

For a start, not all soils are suitable for chemical stabilisation – the method is most effective on high plasticity clays, silts and fine sands – and soils containing sulphates can react negatively to the binder. Testing is needed to confirm suitability prior to treatment and treated ground needs to be left to cure for a few days afterward, before construction can continue. There is also a risk of treated soil deteriorating if there are frequent wet-dry cycles, which can lead to cracking.

Additionally, stabilisation plant and equipment is heavy, making it difficult to reach the areas with the softest soils, and treatment cannot be carried out in heavy rain.

Installing stabilisation geogrids in a capping layer or in the unbound layers of a road pavement, on the other hand, is often faster, simpler and more cost-effective and delivers significant improvements in bearing capacity.

Geogrids can be used with every soil type, there are no requirements to assess suitability and they can be installed by hand, without specialist skills, in any weather. Plus, there is no “curing time” so construction of the road pavement can proceed immediately.

This is a tried and tested approach, accepted by many highways authorities around the world, provides stable foundations for new roads and to help repair roads damaged through settlement of underlying ground or the pavement structure itself.

From an environmental point of view, like chemical stabilisation, geogrids can reduce the amount of unsuitable material that has to be excavated and, in some cases, remove the need entirely. They can also be used with recycled, site-won marginal fills and help reduce the thickness of aggregate layers – and hence use fewer materials, while improving overall pavement performance.

Of course, there is no getting away from the fact that geogrids are made from plastic but it is important to note that the polypropylene used in their manufacture is chemically-inert, does not degrade and can be recycled at the end of a road’s operational life.

Full-scale field testing has demonstrated that mechanically stabilised aggregate layers deliver more robust and longer-lasting road pavements, increasing traffic capacity by up to six times that of traditional designs. This will obviously help reduce the maintenance and repair burden on owners and operators, further strengthening the argument that geogrids should be at the heart of helping to meet the government’s drive for a roads network that is greener, sustainable and delivers value for money.

Jonathan Cook is senior product manager at Tensar International

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