What is a silty clay and what is a clayey silt? John Atkinson and David Norbury look at the issue of field description of soils and the challenges of describing fine grained materials
An engineering description of a soil should be made from simple observation and manipulation of samples in the field or in the laboratory before testing. The description should reflect the engineering behaviour of the soil and the most fundamental engineering description of soil starts with the particle size.
Most standards and codes relate descriptive words such as clay, silt, sand and gravel to a particular range of sizes, usually on a logarithmic scale. However, when the grains are too small to be observed, the description calls for another approach.
Most soils contain a range of sizes and these are designated by a main term describing the principal fraction and by one or more qualifying terms describing the secondary fractions. For example sandy gravel describes a soil with a range of sizes from 0.063mm to 63mm but containing a greater proportion by weight of gravel sized grains in the range 2mm to 63mm.
“The words silt or clay as a main term or as a qualifying term such as silty clay or clayey silt describe a behaviour and not a grading”
Soil grains that are sand-sized and larger are visible to the naked eye so it is relatively easy to describe the grading and draw an approximate grading curve of these soils from visual inspection. Soil grains that are silt and clay-sized are too small to be seen with the naked eye so descriptions of fine grained soils and the fine fractions of well graded soils cannot be made from direct visual observation of their sizes. Instead descriptions of these soils have to be made from observations of their behaviour in simple hand manipulations such as dilatancy, toughness, plasticity and dry strength (Norbury, 2010). As a result, the words silt or clay as a main term or as a qualifying term such as silty clay or clayey silt describe a behaviour and not a grading.
The evolution of the different ways with which fine grained soils have been described following British codes since before 1981 was summarised by Norbury (2010). Codes within the Eurocode family are inconsistent; in some places soils are described in terms of grading and in other places in terms of behaviour. BS EN ISO14688- 1:2002 includes “identification of soil calls for naming and description of a soil on the basis of its grading…and plasticity”; it declares that “particle size is the fundamental basis for designating mineral soils using particle fractions to distinguish the soil mechanical behaviour”; and that “fine soils… shall be termed either ‘clay’ or ‘silt’, depending on the plasticity of the fines fraction and not on the grading”.
So in the space of a few pages the code requires soils to be described on the basis of both grading and plasticity, then on grading only and then on plasticity only.
The words clay, silt, sand and gravel describe grain size but if the grains are platy, flaky or elongated it is not clear what grain size is. The grading of a coarse grained soil is normally determined by sieving and so grain size is determined by what can and cannot pass a given square sieve aperture.
The grading of a fine grained soil is normally determined by sedimentation making use of Stokes Law for a sedimenting sphere. Consequently a soil grain that settles at the same speed as a sphere with diameter 0.0063mm is said to be 0.0063mm irrespective of its actual size and shape.
A typical soil of engineering interest contains grains that are broken rock and grains that belong to one of the clay mineral families such as kaolinite, illite or smectite. There is not a direct relationship between clay as a size and clay as a mineralogy. For example, rock flour may be clay sized quartz grains while clay mineral grains may be in silt sized aggregations.
“The description of a fine grained soil should not use sizerelated words such as silt and clay unless a grading has been determined by direct measurement”
Since grading and clay mineralogy are independent both cannot be captured in one measurement or description.
For example a soil with a plasticity index 30 would probably be described in the field as silty clay but it could have 60% kaolinite (activity = 0.5) or 30% illite (activity = 1) or 15% smectite (activity = 2). Plasticity on its own does not therefore provide sufficient information to estimate grading or mineralogy.
Aspects of soil behaviour that influence ground engineering are permeability and shrinking and swelling. Permeability is closely related to (the square of) the nominal grain size and shrinking and swelling is related to the nature of the clay minerals. Fine grained soils with little clay have a higher permeability and drain faster than those with larger clay fractions. Fine grained soils with active clay minerals, such as smectites, swell and shrink more than those with less active clay mineral, such as kaolinites. Soil description should capture both grading and mineralogy, at least qualitatively.
The grading of a coarse soil, or the coarse fraction of a well graded soil (in an engineering sense), can be estimated and a grading curve drawn from visual observation in the field. In addition the shape, texture and mineralogy of the grains can be determined. These descriptions are often sufficient to estimate basic engineering behaviour. In this case, the words sand and gravel as main or sandy and gravelly as qualifying terms correctly describe grading.
However, the words silt and clay as main or qualifying terms do not clearly describe the grading of a fine grained soil or the fine fraction of a well graded soil. Fine grained soils are assessed in the field on the basis of several of the touch and feel field hand tests (Norbury 2010). The description of a fine grained soil should not use size-related words such as silt and clay unless a grading has been determined by direct measurement. Instead the description of a fine grained soil should include the outcomes of the simple manipulations described by Norbury.
It is most unlikely that ground engineers will suddenly stop using words such as silty clay and clayey silt as descriptions. However, when they see them on borehole logs and laboratory test sheets they should remember that they do not necessarily describe either grading or mineralogy and they should call for further testing. It would help if field description sheets included space to record the outcomes of the field hand tests.
BS EN ISO 14688-1:2002 Geotechnical investigation and testing — Identification and classification of soil — Part 1: Identification and description. Norbury D. R. (2010). Soil and Rock Description in Engineering Practice. Whittles Publishing.