Page 153 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
148 Geotechnical Engineering

Soil scientists prefer to make the separation between sand and silt at 0.020 mm or
20 mm. However, as shown by the loess and sand soils in Fig. 7.2, the natural
boundary may be closer to the No. 200 sieve (0.074 mm) or even slightly larger.
Geologists sometimes use 1/16 mm ¼ 0.067 mm, sometimes rounded off to
0.06 mm. However, the occurrence of a natural break in the general vicinity
tends to diminish the influence on constituent percentages.

7.3.5 The Silt-Clay Size Boundary
The most widely accepted size definition of clay is particles that are finer
0.002 mm or 2 mm. An earlier definition was based on the resolving power
and eyepiece calibration of a light microscope at the U.S. Bureau of Soils,
and set the boundary at 0.005 mm (5 mm). Later mineralogical investiga-
tions showed that this boundary is too high, but meanwhile it became estab-
lished and still is occasionally used in geotechnical engineering. The 0.005 mm
size also requires less interpolation from measurements that routinely are
made after 1 hour and 1 day testing time. This is discussed in more detail in
section 7.4.6.

7.3.6 Silt-Clay Boundary Based on Physical Properties
Another approach is to define clay on the basis of its plasticity or moldability with
water, as silt is crumbly while clay is sticky and can be molded into different
shapes. These relationships are quantified by two simple tests called Atterberg
limits. These tests and the relationship to engineering soil classifications are
discussed in Chapter 12. The limits define a moisture content range over which a
soil can be molded. This range is the plasticity index, which is a fundamental soil
property in geotechnical engineering.

In order to avoid possible confusion between the two approaches, a clay content
based on particle size may be referred to as clay-size material.

7.4 MEASURING PARTICLE SIZES

7.4.1 General Approach to Size Measurement
Some shortcuts are in order because so many particles must be measured in order
to obtain statistical reliability. One shortcut is to use sieves and screen a
representative soil sample. Another approach that is used for particle sizes too
small to be separated on sieves is to disperse the soil in water and make a
determination based on the sedimentation rate, with largest particles settling the
fastest.

One of the most important steps in analysis is obtaining a representative
soil sample. Large samples are spread out on a flat surface and ‘‘quartered,’’

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.