Page 211 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
206 Geotechnical Engineering

10.7.4 pH-Dependent Charge
The electrical charge or zeta potential of clays increases with increasing pH or
alkalinity as hydrogen ions are removed from the clay to make water. A qualita-
tive indication of pH of a solution of different salts can be obtained by
considering their reactions with water: For example, NaCl þ H 2 O ¼ NaOH þ HCl,
a strong acid and a strong base, so the solutions are near neutral.
CaCl 2 þ H 2 O ¼ Ca(OH) 2 þ 2HCl, a strong acid and a weak base. Ice-melting
salts that contain this chemical are highly corrosive to automobiles. (The same
argument applies to MgCl 2 .)

Hydrated lime, CaCO 3 þ H 2 O ¼ Ca(OH) 2 þ H 2 CO 3 , a weak acid and a strong
base, which makes lime an effective stabilizing agent, particularly for expansive
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smectite clays. Furthermore H 2 CO 3 ¼ H 2 O þ CO 2 ; as carbon dioxide gas escapes,
the weak acid, which is the same as in carbonated soft drinks, turns to water.

10.8 SUMMARY OF ATTRACTIVE AND REPULSIVE FORCES
INVOLVING WATER AND SOIL MINERALS

The several interactions involving water and soil minerals can be summarized as
follows:
Water molecules are linked by hydrogen bonds in ice. When ice melts the
hydrogen bonds become more random, which allows closer packing and a
higher density so that ice floats on liquid water. This property is unique to water.
The geometry of the water molecule causes it to be dipolar, which in turn leads
to clustering of water molecules around ions in solution.
Mineral surfaces, in particular clay mineral surfaces, usually carry a negative
charge that attracts water dipoles as adsorbed water, and cations with their
associated water molecules that are part of the diffuse double layer.
Ions and water in the double layer can be moved with d.c. electricity to remove
water from difficult clays.
Suppression of the double layer by drying or by the addition of multivalent
ions allows particles to come close enough that they attract one another
through van der Waals forces so that they flocculate.
Lime stabilization combines the flocculating potential of Ca 2þ ions with an
increase in the negative pH-dependent charge on smectite clay particles.

Problems

10.1. Draw a diagram of the hydrological cycle.
10.2. What are three principal ways that precipitation is disposed of after falling
to the earth?

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