Page 143 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
138 Geotechnical Engineering

develops radial cracks as water is extracted from the soil. The expanding lime
paste then injects into the cracks and disperses into the soil. Meanwhile the net
overall volume change is nil.

Another benefit is that as quicklime hydrates, it releases considerable heat that
helps to dissolve the lime and speed up the chemical reactions. Drilled quicklime is
used to stabilize expansive clays in landslides and under foundations, as shown in
Figs. 5.5 and 21.1.

6.5.4 Difficulty with Clay that Is Not a Mineral

The importance of the crystalline nature of clay minerals for engineering is
emphasized by problems that can occur with a noncrystalline clay called
allophone. Allophane clays derive from easily weathered volcanic basalt in areas
of heavy rainfall where drying is not an option. Allophane is a gel, a colloid that
has no regular arrangement of ions and can contain more water than solids.
In fact, the weight of water per unit volume may be as high as 3 to 6 times the
weight of the solids. Occurrences are on islands of the Pacific, including areas of
Japan and Hawaii.

Allophane soils are difficult because they are sensitive, meaning that disturbing
them destroys what little structure they have and turns them into soup. Lime
stabilization requires modifying ionic charges on crystals and is not effective.
These soils sometimes are dried in a kiln and crushed to make aggregate.

6.6 NON-CLAY MINERALS

6.6.1 Importance of Non-Clay Minerals

While most emphasis is placed on clay minerals in soils, non-clay minerals such as
quartz play an important role and dominate granular soils, especially sand.
Whereas most of the strength of a clayey, cohesive soil is strongly influenced by
interactions of clays with water, most of the strength of granular soils relates to
grain sizes, shapes, and packing, and the mineral composition is less significant.
The discussion of non-clay minerals therefore is closely tied to grain size.

6.6.2 Minerals in Gravel, Cobbles, and Boulders

Large soil particles are essentially pieces of rocks. The particles are too large to be
moved by wind, and in a desert subjected to wind erosion they are left behind as a
lag concentrate or ‘‘desert pavement.’’ Transportation by water results in rapid
rounding-off of corners because of the mass and corresponding energy dissipated
during bed impacts, so the dominant particle shape is highly rounded. This is in
contrast to particle shapes produced in a rock crusher, which are highly angular.

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