Roads built on clay have a familiar weakness. When the soil is wet, it softens and a soft subgrade cannot carry the loads that the layers above it pass down. The usual remedies are to add binders such as lime or cement, or to bring in gravel and sand and each of these has a cost, especially where good stone is scarce and transport makes up a large share of the bill. A possible source of help is biomass plants and wood-processing factories. Wood combustion results in ash, which is generally dumped in a landfill. The ash contains calcium that reacts and hardens over time, which raises a question for road builders whether a waste product can replace a purchased binder or imported aggregate.
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