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Thermohydromechanical behaviour of soils and soil structure interfaces 211

The sign convention considered in the following coincides with that employed
thus far. That is, contractive strains, downward displacements, compressive stresses and
increases in angles in the anticlockwise direction are considered to be positive.

5.3 Characterisation of soils
5.3.1 Mineralogy and textural organisation of soils
Mineralogy is a primary attribute of soils because it influences the size, shape and
properties of their solid particles. These properties, together with those of the pore
fluid, characterise the overall behaviour of soils by means of interactions between the
phases that constitute the material (e.g. solid and fluid). In this context, soil minerals
can be classified in clay and nonclay minerals. An example of a soil structure composed
by clay minerals is reported in Fig. 5.1.
Clay minerals belong to the family of phyllosilicates and are mostly characterised
by a small platy shape, but may also be characterised by a needle or a tubular shape.
These minerals are made of two structural units: the silicon tetrahedron, producing a
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silica sheet Si 4 O 10 Þ by the sharing of three of the four oxygen ions in each tetrahe-
ð
dron, and the aluminium or magnesium octahedron, forming an octahedral sheet by
the sharing of oxygen or hydroxyls (Mitchell and Soga, 2005). The sharing of the
aforementioned ions is achieved via strong bonds of primary valence type. Weaker
Van der Waals bonds hold together the structural units in various ways depending on
the type of clay, forming clay minerals (or lamellae), such as kaolinite, illite and smec-
tite. Clay particles (or stacks) are formed by various lamellae. Clay aggregates are

Figure 5.1 Schematic of the two main types of water in saturated soils: (i) free water, mainly in the
interaggregate space and (ii) adsorbed water located in the interparticle and interlamellar spaces.
Redrawn after Cekerevac, C., 2003. Thermal effects on the mechanical behaviour of saturated clays: an
experimental and numerical study. EPFL Lausanne.