Page 237 - Analysis and Design of Energy Geostructures

212 Analysis and Design of Energy Geostructures

formed by various particles. Three classes of pore space can thus be determined by the
textural organisation of clays: (1) an interlamellar space, with average size of
1.5 2.5 nm; (2) an interparticle porosity among connected clay clusters, with an aver-
age size of 20 150 nm; and (3) an interaggregate space between aggregates, defining a
pore size of 1.5 16 μm(Touret et al., 1990; Robinet et al., 1996). The structure of
clay minerals results in a residual negative charge on the surface of the particle that is
balanced by the adsorption of cations from solution (Mitchell and Soga, 2005), result-
ing in so-called adsorbed water. The mechanical characteristics of adsorbed water are
quite similar to those of the solid particles. For the previous reason, absorbed water is
often considered a part of the solid particles (Hueckel, 2002). Cations in excess of
those needed to neutralise the negative charge of the particle and the associated anions
are present as salt precipitate, or in water solution when water is present (Di Donna
and Laloui, 2013). The adsorbed cations try to diffuse away but are tightly held to the
surface. The charged surface and the relative distributed charge in the adjacent phase
are termed diffuse double layer (Chapman, 1913). Only at a given distance from the
clay particles can water be considered free to move and is thus often termed free water.
The water located in the interlamellar and interparticle space is typically absorbed,
while that located in the interaggregate space is free (cf. Fig. 5.1). The amount of free
and adsorbed water depends on the distribution and size of the pores and the chemical
properties of the water.
Nonclay minerals are primarily characterised by a bulky shape. Quartz is probably
the most abundant nonclay mineral occurring in soils, while feldspar and mica occur
in smaller percentages (Mitchell and Soga, 2005). In many cases, nonclay minerals are
relatively inert.
Based on the above, interactions between clay minerals are in most cases pre-
dominantly chemical in nature. In contrast, interactions between nonclay minerals
are predominantly physical in nature. Because of the intrinsically different features of
clay and nonclay minerals, the behaviour of soilsismarkedlyinfluencedbythe clay
mineral fraction. The widely established soil classification depending on the size of
the particles constituting the solid matrix of such materials allows the distinguishing
of gravels, sands, silts and clays (ASTM D2487, 2017). The gravel, sand and most of
the silt fraction of soils are composed of nonclay minerals. The clay fraction of soils
is composed of clay minerals. Therefore in coarse-grained soils (i.e. sands and grav-
els), interactions between particles are predominantly physical in nature and self-
weight forces are dominant; in contrast, in fine-grained soils (i.e. clays and silts),
interactions between particles are predominantly chemical in nature and surface
forces increasingly govern interactions with regards to self-weight forces for a
decreasing particle size (Laloui, 1993; Mitchell and Soga, 2005). In general, for a
given soil mineral and electrolyte, the magnitude of surface forces is proportional to