Figure 13-13.—Apparatus for grain size distribution by hydrometer analysis.
HYDROMETER ANALYSIS
You learned in the EA3 TRAMAN that a soil is
considered susceptible to frost when it contains 3
percent or more by weight of particles smaller than
0.020 mm in diameter. To determine whether or not a
soil contains an excessive amount of that size particle,
you must perform a particle-size analysis of the
materials passing the No. 200 (0.074-mm) sieve. You do
this by hydrometer analysis. This TRAMAN discussion
identifies the items used for a hydrometer analysis and
will briefly summarize the procedures used. For a full
discussion of the procedures, you should refer to
NAVFAC MO-330 or to ASTM D 422. You also should
review the topic of hydrometer analysis on page 15-24
of the EA3 TRAMAN before continuing with the
following discussion.
Apparatus
Figure 13-13 shows some of the items that you will
need to perform a hydrometer analysis. Another item
you will need is an ASTM hydrometer (fig. 13-14).
ASTM hydrometers are graduated by the manufacturer
to read in either specific gravity or in grams per liter and
are calibrated at a standard temperature of 68°F (20°C).
Other needed items are a dispersion cup and stirrer (fig.
13-15), a thermometer accurate to 1°F (0.5°C), and a
clock, or watch, with a second hand.
Sample Preparation and Test Procedure
Samples for hydrometer analysis are taken from
representative material finer than the No. 10 sieve. The
approximate size of the sample varies according to the
type of soil being tested. For clays and silty soils, you
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