DHARMIDENTIFICATION AND CLASSIFICATION OF SOILS 95
(i) It should be acceptable to all engineers.
These are rather very ambitious requirements for a soil classification system and can-
not be expected to be met cent per cent by any system, primarily because soil is a complex
material in nature and does not lend itself to a simple classification. Therefore, an engineering
soil classification system is probably satisfactory only for the specific kind of geotechnical en-
gineering project for which it is hopefully developed.
4.5 CLASSIFICATION SYSTEMS—MORE COMMON ONES
A number of systems of classification of soils have been evolved for engineering purposes.
Certain of these have been developed specifically in connection with ascertaining the suitabil-
ity of soil for use in particular soil engineering projects. Some are rather preliminary in char-
acter while a few are relatively more exhausitve, although some degree of arbitrariness is
necessarily inherent in each of the systems.
The more common classification systems, some of which will be dealt with in greater
detail in later sections, are enumerated below :
- Preliminary Classification by Soil types or Descriptive Classification.
- Geological Classification or Classification by Origin.
- Classification by Structure.
- Grain-size Classification or Textural Classification.
- Unified Soil Classification System.
- Indian Standard Soil Classification System.
4.5.1 Preliminary Classification by Soil Types or Descriptive Classification
Familiarity with common soil types is necessary for an understanding of the fundamentals of
soil behaviour. In this approach, soils are described by designations such as ‘Boulders’, ‘Gravel’,
‘Sand’, ‘Silt’, ‘Clay’, ‘Rockflour’, ‘Peat’, ‘China Clay’, ‘Fill’, ‘Bentonite’, ‘Black Cotton Soils’, ‘Boulder
Clay’, ‘Caliche’, ‘Hardpan’, ‘Laterite’, ‘Loam’, ‘Loess’, ‘Marl’, ‘Moorum’, ‘Topsoil’, and ‘Varved
Clay’. All of these except the first nine have already been described in Chapter 1.
Boulders, gravel and sand belong to the category of coarse-grained soils, distinguished
primarily, by the particle-size; these do not exhibit the property of cohesion, and so may be
said to be ‘cohesionless’ or ‘non-cohesive’ soils. The sizes are in the decreasing order.
‘Silt’ refers to a soil with particle-sizes finer than sand. If it is inorganic in nature, it is
called ‘Rock flour’ and it is non-plastic, generally. It may exhibit slight plasticity when wet and
slight compressibility if the particle shape is plate-like. Organic silts contain certain amounts
of fine decomposed organic matter, are dark in colour, have peculiar odour, and exhibit some
degree of plasticity and compressibility.
‘Clay’ cosists of soil particles smaller than 0.002 mm in size and exhibit plasticity and
cohesion over a fairly large range of moisture contents. They may be called ‘lean clays’ or ‘fat
clays’ depending on the degree of plasticity. These are basically secondary products of
weathering, produced by prolonged action of water on silicate minerals; three of the major clay
minerals are ‘Kaolinite’, ‘Illite’ and ‘Montmorillonite’, Organic variety of clay, called ‘Peat’,
containing partially carbonised organic matter, is recognised by its dark colour, odour of decay,