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Plastic fantastic: soil plasticity and contamination

Soil is a very complicated substance. Neither dead, nor alive, nor organic, nor inorganic, even describing what a soil is can be difficult. Scientists and engineers therefore characterise a soil based on its physical and chemical properties but, due to the complexity of soils, there can be a misunderstanding of how a soil’s properties relate to its behaviour. Soil plasticity is often described in contaminated lands studies, for example, because it is assumed a soil’s plasticity relates to its ability to transmit or impede contaminants. What is meant by soil plasticity and the impacts of soil plasticity on contaminant transport is the topic of this blog.

Plastic soil

Soil plasticity is the ability of a soil to change shape and rearrange its molecular structure without changing its volume. Plasticity is most accurately measured in laboratories through the Atterberg test, which determines: the point at which a soil goes from being a liquid to a semi-solid (its liquid limit); from a mouldable semi-solid to an unmouldable solid (its plastic limit); and from an unmouldable solid to a shrinking solid (its shrinkage limit).

A soil cannot be plastic if it is not cohesive. Soil particles smaller than approximately 2 μm (the exact number depends on the field of study) are called clays and are often both very cohesive and highly plastic. Soil particles greater than 63 μm are termed sands. Sandy soils are generally not cohesive and the soil particles will not stick together. Sandy soils therefore cannot be moulded and hence have neglible plasticity. The distribution of particle sizes in a soil (soil gradation) is therefore one of the major factors determining its plasticity.

While all plastic soils contain clays, one cannot correlate a soil’s degree of plasticity to its clay content, as not all clays will have the same plastic properties. An organic clay called dy, found at the bottom of lakes and made from the left over remains of life, has low plasticity. My favourite type of clay, and those highly pertinent to the topic of this blog, are the fabulous black and grey cracking clays of the Liverpool Plains in Central West New South Wales. The bulk of the black and grey cracking clay matrix is made up of a clay mineral called smectite. This mineral, formed by water reacting with basaltic rocks, is both highly plastic and highly productive for agricultural purposes.

Smectite’s plasticity is due to its chemical structure. It has long sheets of molecules, with these sheets aligning face to face, allowing water to be held in between in a thin film. This, along with a few other things which affect the way clay molecules are attracted to each other, such as the soil’s salt, water, and organic matter concentrations, is what gives soil its plasticity.  

Plastic soils are prone to swelling: that is they increase in volume as they adsorb water. They also shrink and crack as they lose water. The smectitic soils of the Liverpool Plains are plastic soils which shrink and swell. Little hillocks are created at the surface of the grey cracking clays when the soil swells into itself and has nowhere else to go but up. As these soils dry they then shrink, with cracks forming in between clods of dirt. The clods fall back into the cracks, over a metre deep in some cases, making these soils self-mulching.

Smectitic soils can allow chemicals and water to rapidly move through cracks and fissures to the groundwater or neighbouring areas without ever entering the soil matrix, in what is termed bypass flow. This occurs when the soil is rapidly wetted (such as in a thunderstorm) and the soil’s cracks do not have time to swell shut before water infiltrates to the deeper layers of the soil. It should also be noted that how a clay reacts to water is very dependent on what is dissolved in the water, and in the context of contamination it should not be assumed that a cracked clay will behave the same with all dissolved constituents.

Bentonite is another clay with highly plastic properties. The bentonite clays are close relatives of smectite and these clays have many similar physical and chemical properties. Bentonite, like smectite, is highly plastic and swells more than any other clay. Bentonite differs from smectite, however, because even if it dries and shrinks, any cracks formed will seal tight as it again becomes moist, preventing water flow as if it had never been cracked. This is why bentonite clay is often used as a sealant in dams and groundwater wells.  

Both bentonite and smecite are clay minerals which make soils highly plastic. Smectitic soils can allow water to flow through, while soils high in bentonite do not. So why then are environmental scientists often required to describe a soil’s plasticity if it can have contrasting effects on contaminant mobility? It seems this is related to Lambe and Whitman’s seminal geotechnical textbook “Soil Mechanics”. The Unified Soil Classification System described in this textbook (complete with a table relating soil plasticity to compacted soil permeability) is commonly used by environmental scientists in the contaminated lands sector to classify soils. Although Lambe and Whitman note that the degree to which water will move through a soil is directly related to particle size, void ratio, composition, fabric, and degree of saturation as well as plasticity, these additional factors are not often measured in contaminated lands studies.

The testing of a soil’s plastic properties is not a simple task and, while it can be roughly estimated in the field after appropriate training, for the results of any soil plasticity test to be accurate it must be done in the laboratory and according to the standard methods. A rough estimate may be all that is required for engineering purposes, however, results are quite likely to be inaccurate without undertaking the necessary steps to test a soil’s liquid limit and its plastic limit. Moreover, if soil plasticity is truly a value which needs to be known for engineering purposes, further tests such as a soil’s shrinkage may also be needed and cannot be done in the field.

Finally these clays have been talked about as if it is common to find a “bentonite” soil. In reality, pure clay deposits such as bentonite or kaolin are commercially important and relatively rare. The soils we see on a day- to-day basis are instead a mixture of all sorts of clays with all sorts of histories and properties. Knowing what type of clay occurs in the soil again requires laboratory analyses by specialist scientists. 

In conclusion, soil plasticity is an interesting property of soils which has important ramifications for engineering purposes, especially when measured in association with other measurements such as compactability and shrinkage. In contaminated lands projects, measurement of soil plasticity without thorough and systematic following of the standard method tells one very little about how a plastic soil will behave in a contamination transport context; highly plastic soils may have very high hydraulic conductivity and/or very low hydraulic conductivity, depending on the soil’s other properties and the conditions in which it occurs. Furthermore, without adequate training as to the methods for estimating plasticity in the field, the recording of plasticity is an essentially guessed and arbitrary measurement, at best wasting both valuable time and resources and possibly even exposing consultants to unnecessary liability.

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