Geology and ground models - Practical Engineering Geology - page 50

ed pri-
marily on dominant grain size, as in Table 3.2. In terms of soil
mechanics and geotechnical behaviour, granular soils are generally
distinguished from clays in that they are non-cohesive and are com-
paratively inert chemically, although some authors would dispute the
term cohesive being applied to clay, as discussed in Chapter 5.
Granular soils include everything down to silt size and even some
clay-sized soil where this is derived from mechanical breakdown
(rock

Sediments and detrital/clastic sedimentary rocks are classi

flour is clay-size material, generally quartz, produced by glacial
abrasion). Gravel and larger grain sizes are usually made up of rock
(lithic) fragments rather than mineral grains. In continental regions
where granitic rocks dominate, sand and silt are often made up pre-
dominantly from quartz, the most resistant mineral from granite. In
areas where chemical decomposition is inactive, feldspar might also
survive (arkosic).
Many clays are distinct from granular soils, not only because of
their grain size and shear behaviour, but because they comprise a
new series of minerals with their own structure and chemistry;
they are derived from other rocks but not only as detrital mineral
grains but as precipitants from solution (Eberl, 1984). Other clays
are primary, associated with igneous or hydrothermal activity or
are formed by transformation of other minerals. Clays comprise
three groups: phyllosilicates, weakly crystalline aluminosilicates
and hydrous oxides of iron, aluminium and manganese. Selby
(1993) provides a very useful review of clay mineralogy, as well
as their engineering properties.
The most common clay minerals are the phyllosilicates, which, like
mica, comprise sheets or layers of silica and sheets of alumina. The
various clay mineral species owe their differences to the ways that
sheets are arranged; substitutions of other cations are made into the
structure, which produces distortion of the crystal lattice and the
bonding between sheets. The type of clay that will be formed at any
location depends primarily upon the source rock and climatic effects.
Kaolinite and illite are relatively inactive clays and commonly pro-
duced by the weathering of granitic rocks. Montmorillonite (major
member of the smectite class) is much more active and absorbs water
readily, thereby swelling dramatically. When it dries it shrinks, and
these characteristics have important consequences for engineering.
Smectites also tend to have low shear strengths when wet
( Chapter 5). They are commonly the result of weathering in basalt in
tropical areas and formdif cult but productive soils inmany countries,

including Australia, Africa, India and the USA. Soil types include
vertisols and black cotton soils.
Aluminosilicate clays such as allophone develop from volcanic
ash with silica content below a critical level and are important
soils in New Zealand, Japan and Indonesia (Selby, 1993). Like