Environmental Engineering Reference
In-Depth Information
Figure 18.5
Artificial drainage in poorly drained soils. Mole
drainage runs perpendicularly to tile drains.
may initially be rapid but it generally decreases with time and approaches a constant
value. The infiltration can be shown on a graph of cumulative infiltration versus time.
COLLOIDAL PROPERTIES
CLAY MINERALS
The weathering of primary minerals in rocks and loose, transported deposits (e.g. glacial
tills, loess, etc.) produces a range of weathering products. These products are transformed
during the process of soil formation (see pp. 398-400). Of great importance in the soil are
the new clay-sized minerals, or
clay minerals
, which are formed from the weathering
products. 'Clay' has two different but related meanings. It refers to the size fraction of
less than 0ยท002 mm diameter and also refers to secondary clay minerals which are
synthesized from chemical weathering. These distinctive minerals have colloidal
properties, i.e. the very small particles carry an electric charge. It is also possible in soils
to have clay-sized particles consisting of disintegrated fragments of rock; such 'rock
flour' does not have colloidal properties. Clay minerals are aluminosilicates, formed from
the fusion of silica and alumina. The silica is in the form of a sheet of silica tetrahedra.
Figure 18.6a shows the silicon (Si) atom at the centre of a tetrahedron bounded by four
oxygen atoms (O). The alumina unit is shown in Figure 18.6b. It consists of an
aluminium atom (Al) equidistant from six oxygens (O) or hydroxyls (OH). In the silica
sheet the three oxygens at the base of the tetrahedron are shared by two silicons of
adjacent units. The sheet can be visualized as two layers of oxygen atoms with silicon
atoms fitting into the holes between. In the alumina unit each oxygen is shared by two
