Geoscience Reference
In-Depth Information
surface cracking. The infiltration rate then falls as pores
fill with water, as cracks close up owing to swelling clays,
and as structure starts to collapse in the wet state.
Infiltration rates can vary from over 50 cm of water per
hour in coarse permeable sands to as low as 0·02 cm of
water per hour in low-permeability clays.
Infiltration rates of soils can be measured in the field
by means of a commercial infiltrometer or a home-made
device. The commercial infiltrometers are often double-
ring, with the ability to maintain standard moist
conditions in the outer ring. In home-made infiltrometers
the vessel can be plastic piping or a tin. Three broad
techniques are available. The first is to note the time
required for a volume of water, say 250 ml, to infiltrate
completely. The second is to construct a scale on the
inside of the pipe or tin, add 250 ml of water to the
container, and note the time taken for unit amounts, say
50 ml, to infiltrate. The level is then topped up after each
reading. A third set of methods involves an inverted bottle,
with a suitable air intake, so that the level of water in the
pipe or tin is maintained at a constant level. In this case
the scale is on the bottle. The latter two methods are
designed to maintain a constant head of water. The rate
of infiltration 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.
(a)
Oxygen
Silicon
(b)
Hydroxyls
Aluminium
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. Of
great importance in the soil are the new clay-sized
minerals, or
clay minerals
, formed from the weathering
products. 'Clay' has two different but related meanings. It
refers to the size fraction of less than 0·002 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', which 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 19.4a
shows the silicon (Si) atom at the centre of a tetrahedron
bounded by four oxygen atoms (O). The alumina unit is
shown in
Figure 19.4b
. 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 aluminium
ions, forming sheets of two layers of oxygen (or hydroxyl)
in close packing, but only two-thirds of the possible
octahedral centres are occupied by aluminium.
Clay minerals are formed by the silicon-oxygen and
aluminium-oxygen structural units being bonded
together so that sheets of each result. Clay minerals thus
have a platy, crystalline structure. In the soil other ions,
usually of similar size, can take the place of silicon and
aluminium by a process of
isomorphous substitution
.The
different types of clay minerals are determined by three
features: the ways in which the silica and alumina sheets
