between the named soil types correspond broadly to significant and identi-
fiable changes in engineering characteristics. Particle size analysis is there-
fore employed for primary classification, to distinguish between gravels,
sands and fine-grained silts and clays. A triangular chart for initial descrip-
tive comparison and classification of soils from their particle size distribu-
tion is shown in Fig. 2.2.
Particle size analysis is insufficient for the complete classification of
fine-grained soils or coarser soils where the matrix includes a proportion
of plastic fines, i.e. clays. Secondary classification by degree of plasticity is
then necessary, using consistency limits expressed in terms of percentage
water content by mass, w.
Theliquid limit,wL, is the water content defining the change in soil
state, i.e. consistency, from plastic to liquid; the plastic limit,wP, defines the
change-point below which a soil is too dry to exhibit plasticity. The range
of water content over which a soil displays plastic behaviour is expressed
by the plasticity index,IP, with IPwL wP. Secondary classification is
determined through IPandwLusing classification charts.
Common classification systems include the Unified Soil Classification
System, used in the USA, and the British Soil Classification System for
Engineering Purposes. In the latter system soils are divided into groups,
each of which is denoted by a symbol, usually comprising two letters. The
first letter refers to the dominant soil constituent, i.e. G, S, M and C for
gravels, sands, silts and clays respectively. The second or qualifying letter
provides descriptive detail based on, for example, particle size distribution
for coarser soils, e.g. SW for well graded sand, or on degree of plasticity
44 EMBANKMENT DAM ENGINEERING
Fig. 2.1 Soil particle size classification systems (after Head, 1980)
