Page 164 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 159

Figure 7.6
Chart for
evaluating the
shapes of
individual soil
grains from their
profiles, 1.0
representing the
approach to a
sphere.

not take into account the shortest grain dimension, it tends to overestimate
sphericity of flat particles such as mica.

7.6.2 Special Problems with the Shape of Mica Grains
Especially troublesome, is that mica particles are flat and also are springy, so
compacting a soil with a high content of mica is like trying to compact a bucket of
springs. Although micaceous soils are not common, their behavior is such that
they are given a special category in some engineering classifications, and the glitter
is not gold.

7.7 TEXTURAL CLASSIFICATION OF SOILS

7.7.1 Describing Different Proportions of Sand þ Silt þ Clay
The first step in characterizing grain sizes in a soil is to take the soil apart and
assign the component parts to size grades, namely gravel, sand, silt, and clay. Next
let us describe the products when we put them back together. A naturally
occurring gravel deposit almost inevitably will contain some sand, and a naturally
occurring silt deposit almost inevitably will contain some clay, so when does it
stop having ‘‘silt’’ for a soil name and start being a ‘‘clay’’?

‘‘Clay’’ therefore can mean either (a) clay mineral, (b) clay size, or (c) a deposit or
soil that is mainly clay but also contains other minerals and grain sizes. Engineers
tend to use a term such as ‘‘clay’’ interchangeably for its several meanings, and
should be certain that it is used in a context that ensures that everybody will know
what it means.

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