Page 151 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
146 Geotechnical Engineering

While a histogram is instructive, an accumulation curve is easier to plot and is
almost universally used in engineering. Modes occur on an accumulation curve
where slopes are steepest, and component soil percentages are indicated where the
curve flattens out.

Example 7.2
Large samples of glacial till often contain a mix of different component soils. What are
component percentages in the glacial till in Fig. 7.2?
Answer: The first steep section of the curve is at 41%, which therefore represents one
component soil. The second break is at 60% so the difference is 60 – 41 ¼ 19%, which
represents a second component. Similarly, the third break at 90% defines 90 – 60 ¼ 30% for
a third component, and a fourth component makes up the remaining 10%. The three
components percentage are 41 þ 19 þ 30 þ 10 ¼ 100%. The respective soils are (a) mainly
clay plus some silt, (b) all silt, (c) mainly fine sand, and (d) a mixture of coarse sand and
gravel.

7.3 DEFINING SIZE GRADES

7.3.1 Making the Grades

Not all sand particles are exactly the same size, which means that ‘‘sand’’ must
cover a range of particle sizes, the only requirement being that they are smaller
than gravel and larger than silt grains. Natural size boundaries occur between
gravel and sand, between sand and silt, and between silt and clay, but the
boundaries are transitional and somewhat arbitrary, and different organizations
have adopted different definitions.

Gravel particles require a higher water velocity to be moved than sand, and wind
does not move them at all. Sand particles move by bouncing, or saltation, and silt
grains are mainly carried in suspension, as the mud in muddy water or the dust
in air. Clay particles are so fine that they are very slow to settle out of suspension
and consist of separate mineral species, the clay minerals.

7.3.2 Sieve Sizes

Soils are separated into size grades by sieving, or sifting through a series or ‘‘nest’’
of standardized wire mesh sieves arranged from the coarsest down to the finest.
Common sieve sizes used in engineering are listed in Table 7.1.

A sieve is a wire fabric, so the sieve number does not describe the size of the
opening but designates the number of wires per inch or millimeter. As a matter of
convenience some size grades are defined on the basis of standard sieve sizes:
gravel, for example, commonly designates particles that are coarser than 2 mm,
which is the size of the opening in a No. 10 (wires to the inch) sieve.

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