Permeability Test for Granular Soils (Constant-Head Method) - Soil Properties: Testing, Measurement, and Evaluation - page 263

Figure 18-3 Permeability versus Void Ratio Curve

In practice (such as in a commercial laboratory), it will, of course,
be necessary to perform three permeability tests at different void ratios
on the same sample. In a college laboratory, however, a single laboratory
period is generally not enough time for students to perform the required
three tests. To expedite matters, it is recommended that each student
group take part of the sample, compact it to a dry density different from
that of any other group (and therefore to a different void ratio), and
perform a permeability test on it. When all groups have completed their
tests, the results (permeability and void ratio) can be pooled to plot the
graph of permeability versus void ratio.

Comprehensive reports of permeability tests normally include not only
a graph of permeability versus void ratio for the given soil, but also other
pertinent information, such as sample identifications, densities and dry
unit weights of tested specimens, and so on. Generally, these data are
contained in the form on page 265.
It should be noted that permeability determined in a laboratory
may not be truly indicative of in situ permeability. There are several rea-
sons for this, in addition to the fact that soil in the permeameter does
not exactly duplicate the condition of soil in situ , at least not for granu-
lar soils. For one thing, flow of water in a permeameter is downward,
whereas flow in soil in situ may be more nearly horizontal or in a direc-
tion between horizontal and vertical. Indeed, permeability of natural
soils in the horizontal direction can be considerably greater than in the
vertical direction. In addition, naturally occurring strata in in situ soils
will not be duplicated in a permeameter. Also, the relatively smooth

SUMMARY