Page 152 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
Particle Size and Gradation 147

No. (wires per inch) Opening, mm Comment Table 7.1
Standard sieve sizes
(Lid) ––
used in geotechnical
4 4.75 Gravel
engineering
10 ..... ..... ..... ..... ...... . 2.0 ... ...... Size separating gravel and sand
20 0.85
40 0.425
60 0.25 Sand
140 0.106 #
200 ... ..... ..... ...... ..... ... 0.075 ..... .... Size separating
sand from silt
(Pan) –– Silt and clay fall on
through and collect in the pane

One complication is that sieve openings are not round; they are approximately
square. Spherical particles can pass through regardless of their orientation, but
few soil grains are spheres. Sieves therefore are vigorously shaken or vibrated for a
prescribed time in a sieve shaker in order to achieve reproducibility of the data.

7.3.3 Details of the Gravel-Sand Size Boundary

Although the most common size boundary between sand-size and gravel-size
particles is 2 mm, this size separation is not universal, even within geotechnical
engineering.

The Unified Soil Classification System used in earth dam and foundation
engineering makes the separation at the No. 4 (3/16 in.) sieve, and material
from 4.76 to 2 mm in diameter is considered ‘‘very coarse sand.’’ These and other
size boundaries are indicated in Fig. 7.2 Because the boundaries differ, it is
important that they be defined or included on graphs showing the particle size
distribution, as indicated by the vertical lines and grade names across the bottom
in Fig. 7.2.

7.3.4 The Sand-Silt Size Boundary

As silt particles are fine enough to be carried in suspension they show little or no
rounding of corners, whereas sand particles typically are abraded and rounded at
the corners and edges from having been transported and bounced along by wind
or water. However, the boundary is transitional, and for convenience it often is
defined on the basis of a sieve size. In geotechnical engineering practice the
boundary between sand and silt usually is that of a 200-mesh sieve opening,
0.075 mm or 75 mm (micrometers). The earlier designation was ‘‘microns.’’) Sand
therefore presents a range in particle sizes between 0.074 and 2 mm diameter,
a size ratio of 27.

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