Permeability Test for Granular Soils (Constant-Head Method) - Soil Properties: Testing, Measurement, and Evaluation - page 252

There are several factors that influence the permeability of a soil:
the viscosity of its water (which is a function of temperature), size and
shape of the soil particles, degree of saturation, and void ratio. (The void
ratio, customarily denoted by e , is the ratio of volume of voids to volume
of solids.) The void ratio has a significant influence on permeability. For
a given soil, permeability is inversely proportional to soil density. This
is intuitively obvious if one considers that the denser a soil, the more
tightly its particles are packed, the smaller will be the void space and
void ratio, and the lower will be the tendency for the soil to allow water
to move through it (i.e., its permeability). Hence, permeability is directly
proportional to void ratio.
In view of the foregoing, each value of permeability for a soil should
be associated with a particular void ratio. Normally, when a perme-
ability value is needed, it is for the permeability of the soil in situ .Per-
meability is often, however, determined by laboratory tests; and to be
representative of the soil's in situ permeability, tests should be per-
formed on “undisturbed” samples. In the case of granular soils, however,
it is extremely difficult to obtain undisturbed samples. Therefore the
recommended procedure is to perform permeability tests on three soil
specimens of the same sample, with each specimen having a different
void ratio. A relationship between void ratio and permeability can be
established for a given soil by plotting a graph of permeability on a log-
arithmic scale versus void ratio on an arithmetic scale. Then, whenever
the value of the permeability of that soil in situ is needed, a sample can
be taken at the project site, its void ratio determined, and associated
permeability obtained from the graphical relationship.
This chapter describes how to determine the coefficient of permeabil-
ity by a constant-head method for laminar flow of water through granular
soils only. To limit consolidation influences during testing, this procedure is
restricted to disturbed granular soils containing not more than 10% soil
passing a No. 200 sieve. Chapter 19 describes the falling-head method for
determining the coefficient of permeability; this method may be used to test
both fine-grained soils (such as silts and clays) and coarse-grained soils.

Permeameter (including constant-head filter tank and manometer
tubes): a specialized device for determining soil permeability (see
Figure 18-1)
Large funnels (with spouts 25 mm in diameter for 9.5-mm maxi-
mum particle size and 13 mm in diameter for 2.00-mm maximum
particle size)
Timing device
Thermometer
Compaction equipment
Vacuum pump or water-faucet aspirator
Balance (with accuracy to 1.0 g)
Miscellaneous apparatus: scoop, 250-ml graduate, jars, mixing
pans, etc.

APPARATUS AND
SUPPLIES