Page 148 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
7 Particle Size and Gradation

7.1 GRAIN SIZES AND ORDERS OF MAGNITUDE

7.1.1 Size Ranges or Grades
The individual particles that make up a soil vary in size by orders of magnitude.
For example, the size difference between a 0.002 mm clay particle and a 2 m
diameter boulder is 6 orders of magnitude, or about the same as between a
Volkswagen and the Moon. It therefore is convenient to define particle size grades
by defining discrete ranges in particle sizes that define clay, silt, sand, gravel,
cobbles, and boulders. Each size grade covers a range in particle sizes––that is, all
gravel particles obviously are not the same size. ‘‘Clay’’ thus defined relates to a
range in particle sizes without regard to their mineralogy. However, because of a
relationship between weatherability of different minerals and particle size, most
clay-size particles are composed of the special group of minerals designated as clay
minerals. A particular soil therefore will consist of varying percentages of clay,
silt, and sand sizes with occasional coarser material.

7.2 GRADATION CURVES

7.2.1 Logarithmic Grain-Size Scale
Because of the broad range in particle sizes that can make up a particular soil,
sizes are conveniently plotted on a logarithmic scale. The advantage becomes
apparent by comparing Fig. 7.1, where sizes are plotted on a linear scale, with
Fig. 7.2, where the size distribution for the same glacial till soil is plotted
logarithmically. Figure 7.2 also shows the huge variations in particle sizes between
some common soils deposited by wind, water, and ice.

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