Description and classification of soils and rocks / Описание и классификация почв и горных пород

From an engineering viewpoint, the ground beneath a site can conveniently be divided into the categories shown in Table 2.1, which are based upon generalizations of its expected behaviour in construction works.
These broad generalizations are, of course, limited in accuracy. But they give the geotechnical engineer a good basis on which to consider, at the start of a project, both the likely construction problems and the methods of investigation which might be used. In practice, it is found that the ground varies continuously beneath a site, and it is not often possible to find sharp transitions from one type of material to another. This then, calls for more refined, systematic, description and classification of soils and rocks.The history of soil classification and description has been described by Child (1986). Early attempts to divide soil into different categories used laboratory testing, typically particle size distribution. Casagrande (1947) considered that much of this ‘textural soil classification’ was unreliable, because it did not always reflect the effects of silt and clay on the engineering behaviour of the mass. On the basis of his experience he considered that only the Airfield Classification System, which was based on the results of particle size analysis and plasticity (Atterberg limit) tests (see Chapter 8), was suitable for an assessment of soil for airfield pavement design. It was noted that, with experience, soil might be classified on the basis of visual/manual description alone; Casagrande therefore suggested what he termed ‘descriptive soil classification’. <...>