Sofia Martinez
Non-plastic soil is a type of soil that does not contain clay. The presence of clay in a soil sample is determined by its behaviour, specifically its Atterberg limits, which refer to the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic, and liquid. The plasticity of soil is important to understand when designing structures, as it determines the soil's shear strength and change in volume as it expands and contracts with different moisture contents.
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What You'll Learn
Non-plastic soils are free of clay
The concept of non-plastic soils is related to the Atterberg limits, which are a basic measure of the critical water content of a fine-grained soil. Depending on its water content, soil may exist in one of four states: solid, semi-solid, plastic, and liquid. The transition between these states is gradual, and each state exhibits distinct consistency and behaviour, resulting in varying engineering properties.
Non-plastic soils refer to soils that do not possess clay-like characteristics. In engineering, the presence or absence of clay is determined by the behaviour of the soil sample, specifically its Atterberg limits. Non-plastic soils are defined as those where a thread of soil cannot be rolled out to a diameter of 3.2 mm or less at any moisture content. This indicates that the soil lacks the cohesive properties typically associated with clay particles.
The plasticity index and liquid limit ratio, known as the clay factor (CF), have been used to classify soils according to their texture and plasticity. However, existing classification systems, such as the USDA classification, have limitations and may not accurately reflect the plastic behaviour of certain soils. For example, some non-plastic soils classified as clay may actually exhibit plastic behaviour.
To address these limitations, additional group symbols have been proposed to classify non-plastic soils more comprehensively. These include GMN (non-plastic silty gravels) and SMN (non-plastic silty sands) for coarse-grained non-plastic soils, as well as MLN (non-plastic, inorganic coarse silt-sized fractions), MIN (non-plastic, inorganic medium silt-sized fractions), and MHN (non-plastic, inorganic fine silt + clay-sized fractions) for fine-grained, non-plastic silts.
In summary, non-plastic soils are free of clay in the sense that they lack the plastic behaviour and cohesive properties typically associated with clay particles. However, the presence or absence of clay can be determined through various methods, including engineering, agronomy, and mineralogy, each with its own unique perspective on the definition of "clay".
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Atterberg limits define soil states
The Atterberg limits are a basic measure of the critical water content of fine-grained soils, such as silt and clay, as they transition from a solid to a liquid state. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic, and liquid. In each state, the consistency and behaviour of the soil are different, and consequently, so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behaviour.
The Atterberg limits were created by Albert Atterberg, a Swedish chemist and agronomist, in 1911. They were later refined by Arthur Casagrande, an Austrian geotechnical engineer and a close collaborator of Karl Terzaghi (both pioneers of soil mechanics). The limits are named after Atterberg.
The Atterberg limits are: the shrinkage limit, plastic limit, and liquid limit. The shrinkage limit (SL) is the water content at which the soil changes from a semi-solid to a solid state. At this moisture content, the volume of the soil mass ceases to change with further drying of the material. The plastic limit (PL) is the minimum water content at which a soil is considered to behave in a 'plastic' manner, i.e. it is capable of being moulded. The liquid limit (LL) is the maximum water content a silt or clay can have before becoming a liquid.
The transition from plastic to liquid behaviour is gradual over a range of water contents, and the shear strength of the soil is not actually zero at the liquid limit. The liquid limit is measured by either the Casagrande cup method or a cone penetrometer. The Casagrande cup method involves placing a soil sample in a cup of standard size, cutting a groove through the middle, and then repeatedly dropping the cup and measuring the number of blows required for the groove to close. The cone penetrometer method involves measuring the penetration of a standardised stainless steel cone into the soil.
The plasticity index (PI) is a measure of the plasticity of soil. It is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid and plastic limits. Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay.
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Plastic limit testing
Non-plastic soil refers to soil that does not contain clay or silt particles. Clay and silt particles are cohesive and interact with water, causing the soil to change sizes and vary in shear strength. The presence of clay in a soil sample can be determined by its behaviour (i.e. Atterberg limits), the diameter of the soil grain, or by x-ray diffraction.
Atterberg limits are a basic measure of the critical water content of a fine-grained soil and were created by Swedish chemist Albert Atterberg in 1911. The limits were later refined by Austrian geotechnical engineer Arthur Casagrande. The Atterberg limits are:
- Shrinkage limit (SL)
- Plastic limit (PL)
- Liquid limit (LL)
The plastic limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. This procedure is defined in ASTM Standard D 4318 and AASHTO T90. If the soil is at a moisture content where its behaviour is plastic, the thread will retain its shape down to a very narrow diameter. The sample can then be remoulded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is defined as the gravimetric moisture content where the thread breaks apart at a diameter of 3.2 mm (approximately 1/8 inch). A soil is considered non-plastic if a thread of 3.2 mm cannot be rolled out at any moisture content.
The liquid limit (LL) is the water content at which the behaviour of a clayey soil changes from a plastic state to a liquid state. The transition from plastic to liquid behaviour is gradual over a range of water contents, and the shear strength of the soil is not zero at the liquid limit. Atterberg's original liquid limit test involved mixing a pat of clay in a round-bottomed porcelain bowl of 10–12 cm diameter. A groove was then cut through the clay with a spatula, and the bowl was struck many times against the palm of one hand. Casagrande later standardised the apparatus and procedures to make the measurement more repeatable.
The plasticity index (PI) is the size of the range of water contents where the soil exhibits plastic properties. The PI is calculated by subtracting the plastic limit from the liquid limit (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little to no silt or clay.
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Soil classification systems
Soil classification deals with the systematic categorisation of soils based on distinguishing characteristics and criteria that dictate choices in use. Soil classification systems can be approached from the perspective of soil as a material and soil as a resource. For instance, the French Soil Reference System is based on the presumed genesis of the soil.
Ancient Greek scholars produced a number of classifications based on several different qualities of the soil. Geotechnical engineers classify soils according to their engineering properties as they relate to use for foundation support or building material. Modern engineering classification systems are designed to allow an easy transition from field observations to basic predictions of soil engineering properties and behaviours.
The European Union uses the World Reference Base for Soil Resources (WRB), which is currently in its fourth edition. The European soil classification system (ISO 14688) is very similar, differing primarily in coding and in adding an "intermediate-plasticity" classification for silts and clays, and in minor details.
Other engineering soil classification systems in the United States include the AASHTO Soil Classification System, which classifies soils and aggregates relative to their suitability for pavement construction, and the Modified Burmister system, which works similarly to the USCS but includes more coding for various soil properties. The AASHTO system was developed by the American Association of State Highway and Transportation Officials and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes. It roughly divides soils into two groups: granular and silt-clay materials, based on sieve analysis. Granular materials are considered good as a subgrade, while silt-clay materials are less satisfactory.
The USCS and additional engineering descriptions are standardized in ASTM D 2487. The Unified Soil Classification System (USCS) is used in engineering and geology to describe the texture and grain size of a soil. The USDA classification system, on the other hand, determines soil texture in terms of particle size, but not how this texture affects actual soil properties.
The Atterberg limits are a basic measure of the critical water content of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic, and liquid. In each state, the consistency and behaviour of the soil are different, and consequently, so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behaviour. The liquid limit is the water content at which the behaviour of a clayey soil changes from the plastic state to the liquid state. The plastic limit is determined by rolling out a thread of the fine portion of a soil on a flat, non-porous surface. A soil is considered non-plastic if a thread of 3mm in diameter cannot be rolled out at any moisture content.
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Clay factor (CF)
Clay soil is a compact soil type with a particle size of less than 2 micrometres. Clay soils are often sticky and plasticine-like, and they can hold more water than most other soil types. Clay soils are also characterised by their ability to swell when wet and shrink when dry.
The Atterberg limits are a basic measure of the critical water content of fine-grained soils, including clayey soils. The limits are defined by the soil's shrinkage limit, plastic limit, and liquid limit, which correspond to the various states of the soil: solid, semi-solid, plastic, and liquid. The plastic limit (PL) is determined by rolling out a thread of fine soil on a flat, non-porous surface. If the thread can be rolled out to a diameter of 3.2 mm or less, the soil is considered plastic. Non-plastic soils, on the other hand, cannot be rolled into a thread at any moisture content.
The clay factor (CF) is a parameter used in research to classify soils according to a texture-plasticity system. It is calculated using the plasticity index and liquid limit ratio. The clay factor helps to address the limitations of the USDA soil classification system, which only considers particle size and not the resulting properties of the soil. By evaluating the clay factor, researchers can better understand the plastic behaviour of soils that may be mislabelled as non-plastic under the USDA system.
The presence of clay in a soil sample can be determined in several ways, depending on the field. In engineering, the Atterberg limits are used, whereas agronomy or soil science relies on the diameter of the soil grain, and mineralogy may use x-ray diffraction.
Overall, the clay factor is a useful parameter for understanding the plastic behaviour of soils and improving soil classification systems.
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Frequently asked questions
Non-plastic soil refers to soil that does not contain clay. Depending on its water content, soil can appear in one of four states: solid, semi-solid, plastic, and liquid. The transition between these states depends on the soil's composition and the presence of clay, silt, or sand. Non-plastic soils are typically classified as coarse-grained or fine-grained and can include silty gravels, silty sands, and inorganic silt.
One method to determine if soil is non-plastic is by performing the Atterberg liquid limit test. This test involves mixing clay with water in a round-bottomed bowl, striking the bowl against the palm, and observing the behavior of the soil. Non-plastic soils will not exhibit plastic behavior and will not form a thread when rolled out on a flat, non-porous surface, regardless of moisture content.
Soil classification is important in geotechnical applications to ensure the soil is suitable for a particular use. For example, when designing structures, it is crucial to understand the shear strength and volume change characteristics of the soil. Classifying soils as non-plastic helps engineers and agronomists make informed decisions about the behavior and properties of the soil.
