Soil Evaluation for Engineering Purposes

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Soil as an engineering material
Soil has a role as an engineering medium in sustainable land use activities. If we wish to, build erosion control structures, build farm dams, construct track and roads, or develop urban settlements; the engineering characteristics of soil need to be understood.

The following are the key soil properties which need to be considered when wishing to use soil in such a way. The questions listed under each property are to focus assessment.

Particle size distribution

What is the soil to be used for?
What is the range of particle sizes in the material?
How much clay is in the soil?
(Too much clay can make materials intractable; too little – unsuitable)
What is the Unified soil classification?

Organic matter

Is there any significant content of organic matter?
(Organic matter plays havoc with consistency and longevity of any construction)

Clay stability

Is the clay dispersive?
How dispersive?

Shrinkage potential

What is the linear shrinkage?

Moisture content at time of construction

What is the optimum moisture content for compaction?
Is/was the soil anywhere near this at time of construction?

Evaluation of soil for engineering use employs the same tools as for biological robustness above (with perhaps one significant addition: assessment of linear shrinkage) but requires a quite different interpretation system.

Engineering use of soil is about building things with it. Usually this requires minimising all those requirements for a vigorous biological environment. Water entry rate, porosity, conductivity, and rate of biological activity have to be kept as low as possible.

Compaction is important to minimise porosity and permeability. High levels of clay are undesirable for their negative influence on tractability, compactability and stability. Moderate to high levels of linear shrinkage will often lead to failure if earthworks dry and rewet. High levels of dispersion aggravate any small problems arising with shrinkage. Very low levels of dispersion are often undesirable because of its association with friability (try making a mud brick from such material) and high rates of conductivity. Organic material will oxidise and change over time, leading to instability.

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Further information about Soil Biology.

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