Page 155 - Geotechnical Engineering Soil and Foundation Principles and Practice

Particle Size and Gradation
150 Geotechnical Engineering

Figure 7.4
Sampling theory
in sedimentation
analysis: at a
particular
sampling depth
the suspension
contains a
representative
sample of all sizes
smaller than the
size that will settle
to that depth.

7.4.3 Sedimentation and the Percent Finer
A sedimentation analysis automatically measures the amounts finer than a specific
grain size. This is illustrated in Fig. 7.4: after a certain time all particles larger than
a certain depth have settled a calculated distance and therefore cannot occur at
depths shallower than that distance. On the other hand, finer particles remain
suspended and therefore are measured.

After each hydrometer reading the hydrometer is removed so that particles will
not settle on the bulb. Removal stirs a small portion of the upper part of the
suspension, but the effect is small so long as particles move horizontally and
not vertically relative to the suspension—as the instrument is removed, the level of
the suspension goes down, and when it is replaced the level goes back up.
The depth to the center of volume of the submerged part of the hydrometer
is the effective sampling depth that is used in the calculations, and depends on
the depth of sinking. This depth is obtained from a calibration chart or table,
Table 7.2.

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