Chloe Zhang
Soil classification is a complex and varied process, with many types of soils existing in nature, all with different compositions, characteristics, and sizes. The classification of non-plastic soils is a particularly challenging area, with existing systems such as the USDA texture triangle and Casagrande Unified Soil Classification failing to adequately classify inorganic silts, silty sands, and silty gravels that are non-plastic. The plasticity of a soil is determined by its water content, and a soil is considered non-plastic if a thread cannot be rolled out of it at any moisture level. To address the limitations of current systems, new group symbols have been proposed to classify non-plastic soils, including GMN (non-plastic silty gravels) and SMN (non-plastic silty sands) for coarse-grained non-plastic soils, and MLN (non-plastic inorganic coarse silt-sized fractions), MIN (non-plastic inorganic medium silt-sized fractions), and MHN (non-plastic inorganic fine silt and clay-sized fractions) for fine-grained non-plastic soils.
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What You'll Learn
The limitations of the USDA classification system
The USDA soil classification system has several limitations. Firstly, it only determines soil texture based on particle size, without considering how this texture relates to actual soil properties. This is a significant limitation as it can lead to inaccurate classifications; for instance, certain soils classified as "non-plastic" using the USDA system may still exhibit plastic behaviour.
Moreover, the USDA system does not adequately address the classification of inorganic silts, silty sands, and silty gravels that are non-plastic. This is because the system lacks specific group symbols to represent these types of soils. As a result, there is a lack of clarity in classifying non-plastic soils, which can impact their suitability for geotechnical applications.
The USDA system also does not account for the clay mineralogical composition of fine-grained soils and its effect on their liquid limit behaviour. This is important because the undrained shear strengths at liquid and plastic limit water contents can vary, and an understanding of this variation is crucial for certain applications.
Additionally, the USDA classification system may not be as comprehensive as other systems, such as the World Reference Base (WRB) for soil resources, which is the international standard endorsed by the International Union of Soil Sciences. The WRB system incorporates a wider range of criteria and parameters for soil classification, potentially making it more versatile and applicable worldwide.
Lastly, the USDA system may not be as widely used or recognised in certain regions or applications as other classification systems, such as the Unified Soil Classification System (USCS) or the AASHTO Soil Classification System. These alternative systems are commonly employed in engineering, geology, and transportation projects, where factors like soil strength and uniformity are critical for structural design.
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The importance of clay mineralogical composition
Firstly, the clay mineralogical composition determines the plasticity of soils. Plasticity refers to the ability of clay to be moulded into different forms when it retains water. The chemical composition of the material present in the clay greatly affects its plasticity. For example, some species of chlorite and mica are found to be non-plastic, even when ground into macroscopic flakes with more than 70% of the material less than 2 micrometres in size. On the other hand, certain species of chlorites and micas become plastic when ground, even if only 3% of the material is below the 2-micrometre threshold. This highlights the complex relationship between clay composition and plasticity.
Secondly, the clay mineralogical composition influences the swelling capacity of soils. Swelling clay minerals are geological materials containing more than 50% of mineral particles with a size of less than 2 micrometres. The swelling properties of clay minerals are crucial in designing lightweight buildings. However, the mobilisation of swelling capacity can also lead to stability concerns and foundation threats for structures such as tunnels and slopes. Therefore, understanding the clay composition is essential for ensuring the safety of such structures.
Moreover, the clay mineralogical composition impacts the classification of non-plastic soils. Existing soil classification systems often rely on the plasticity chart to categorise fine-grained soils. However, these systems struggle to classify non-plastic granular materials unambiguously. To address this issue, researchers have proposed additional group symbols to classify coarse-grained non-plastic soils (GMN and SMN) and fine-grained, non-plastic silts (MLN, MIN, and MHN). These symbols provide a more comprehensive description of non-plastic soils, filling the gaps in traditional classification systems.
Lastly, the clay mineralogical composition is relevant in soil remediation and environmental protection. Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. Clay minerals are among the soil amendments used to immobilise PTEs and reduce their bioavailability. By understanding the mineralogical composition of clay soils, scientists can develop effective remediation techniques to mitigate the negative consequences of soil contamination. In conclusion, the clay mineralogical composition is of utmost importance in classifying non-plastic soils, predicting their behaviour, and applying that knowledge in various geotechnical and environmental contexts.
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The cone penetration method
One such experiment, conducted by Tao-Wei Feng in 2004, utilised a cone with specific dimensions and weight (a 30° angle, 20 mm diameter, 20 mm depth, and 80g weight). Four soil samples with varying Plasticity Index (PI) values were tested using this method, and the results were compared with those obtained from the standard thread-rolling method. The cone penetration method proved to be feasible for performing the plastic limit test, with a good correlation to the standard method.
The British Standards Institute (BSI) has recognised the cone penetration method in its BS 1377 (1975) standard, referring to it as the cone penetrometer test. This test determines the plastic limit of soils based on the relationship between water content and undrained shear strength. The BSI standard highlights the importance of calibration when using the cone penetrometer, as soil texture can impact the calibration factor.
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The liquidity index
For example, a soil with a liquid limit of 60%, a shrinkage limit of 20%, a plastic limit of 30%, and a natural moisture content of 40% would have a liquidity index of 0.5.
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The plasticity index
Soils with a plasticity index of zero are considered non-plastic. Organic soils, which have high liquid and plastic limits, tend to have very low plasticity indexes. The plasticity index is also affected by the amount of clay present in the soil, with the index generally decreasing as the clay content increases. The addition of fly ash can also reduce the plasticity index of a soil due to the presence of calcium, which increases clay flocculation and reduces plasticity.
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Frequently asked questions
Non-plastic soils are those that have little to no silt or clay content.
You can use the Atterberg limits to identify the soil's classification. The plasticity index (PI) is a measure of the plasticity of the soil, with soils that have a PI of 0 being non-plastic.
The Atterberg limits are a basic measure of the critical water content of fine-grained soils. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic, and liquid.
There are five proposed symbols for classifying non-plastic soils: GMN (non-plastic silty gravels), SMN (non-plastic silty sands), MLN (non-plastic, inorganic coarse silt-sized fractions), MIN (non-plastic, inorganic medium silt-sized fractions), and MHN (non-plastic, inorganic fine silt and clay-sized fractions).
Plastic soils are cohesive, meaning they stick together. Non-plastic soils lack this cohesive property and will not remain together.
