where clay fines are present, e.g. I for intermediate, H for high etc. (BSI,
1999).
2.2.3 Phases in soil: soil porewater: effective stressA soil may constitute a two- or three-phase system comprising solid soil
matrix or skeleton and fluid, either water or gas or both. Water may exist
in the soil in a variety of forms. Apart from being the main constituent of
the liquid phase, water may also exist in a gaseous phase as water vapour
and in the solid phase as adsorbed water. All mineral particles tend to
form physico-chemical bonds with water, resulting in a surface film of
adsorbed or fixed molecular water. This is most significant with the finest-
grained soils as a result of their relatively high specific surface and the
mineralogical composition of clay particles. The associated electrical phe-
nomena occurring at the clay particle–water interface are primarily
responsible for the cohesion and plasticity identified with clay soils.
Free wateris the term used to describe that portion of the total pore-
water which obeys the normal laws of hydraulics. Provided that this water
is present in the soil pores as a continuous liquid phase, Bernoulli’s law
NATURE AND CLASSIFICATION OF ENGINEERING SOILS 45
Fig. 2.2 Descriptive soil identification and classification chart (after Head,
1980)