It has already been pointed out that the bulk of the serious soil problems in road engineering
are associated with fine grained materials. It has been shown that if fine grained materials
have a high LL and a high PI, they will show excessive swelling and shrinkage as well as a
low bearing capacity when wet. The question now is why fine grained, cohesive soils are so
different from sands and why is there such a large variation in behaviour when different
types of fine grained cohesive soils are compared with each other. The reason for this is that
cohesive soils are mainly built up from clay minerals.
Clay minerals are distinguished by both their mineralogy and their particle size. Most particles
are less than 2 microns (0.002 mm) in diameter. Mineralogical, they are all silicates built of
two basic building blocks, the silicate tetrahedron and the aluminium or magnesium
The tetrahedron and the tetrahedron sheet are already shown in figure 11. The sharing of
the oxygen ions in the tetrahedron sheet is done in such a way that the tetrahedrons all sit
on a triangular base with their points in the same direction. The centres of the tetrahedrons
generally contain a silicon ion (Si4+) but occasionally this is replaced by an aluminium ion
(Al3+) which, although it has not the same electrical value, has almost the same size as the
silicate ion. This phenomenon is called isomorphous substitution and will be discussed in
greater detail later on.
Each oxygen ion at the base of the tetrahedron belongs to two tetrahedrons. The oxygens at
the tips however are often linked with a hydrogen ion (H+) to form a hydroxyl ion (OH-). The
tetrahedron sheet may therefore be considered as a layer of silicon ions between a layer of
oxygen and a layer of hydroxyl ions.
The principle of the octahedron is shown in figure 12.