Page 139 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
134 Geotechnical Engineering

is quite variable, so expansive clays were referred to collectively as ‘‘montmor-
illonite group minerals.’’ Smectite now is used for these minerals. This is a very
old name that Pliny used with reference to cleansing, probably because
expansive clay is highly absorptive and is used as fuller’s earth to remove
lanoline from wool. Smectite clay has similar uses in geoenvironmental
engineering.

6.3.15 Tests to Measure Clay Expandability
Because of the unruly nature of the d 001 spacing of smectite clays, they are treated
with an organic chemical to stabilize the layer spacing before being examined by
X-ray diffraction. The more common chemical is ethylene glycol, the main
˚
component in automotive antifreeze. This fixes the d 001 spacing at 17.6 A. This
treatment was used for Fig. 6.6.
Other tests to measure the physical character of clay at different moisture contents
are more commonly used to characterize its expansive character. These tests,
which are described later in this text, are a basis for classification of soils for
engineering purposes.

Another approach is to measure the cation exchange capacity of a soil,
which indicates the amount and kind of interchangeable cation. This is a wet
chemical analysis, and the pH must be adjusted to neutral to obtain consistent
results.

Figure 6.6
X-ray
diffraction chart
for a loess soil
containing quartz
and expansive
clay mineral
(smectite).
The larger the
individual grains
the sharper the
peaks.

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