4. PERMEABILITY. Field permeability tests measure the coefficient of
permeability (hydraulic conductivity) of in-place materials. The
coefficient of permeability is the factor of proportionality relating the
rate of fluid discharge per unit of cross-sectional area to the hydraulic
gradient (the pressure or "head" inducing flow, divided by the length of the
flow path). This relation is usually expressed simply:
Where Q is discharge (volume/time); A is cross-sectional area, H/L is
the hydraulic gradient (dimensionless); and K is the coefficient of
permeability@ expressed in length per unit time (cm/sec, ft/day, etc.). The
area and length factors are often combined in a "shape factor" or
"conductivity coefficient." See Figure 13 for analysis of observations and
Table 15 for methods of computation. Permeability is the most variable of
all the material properties commonly used in geotechnical analysis. A
permeability spread of ten or more orders of magnitude has been reported for
a number of different types of tests and materials. Measurement of
permeability is highly sensitive to both natural and test conditions. The
difficulties inherent in field permeability testing require that great care
be taken to minimize sources of error and to correctly interpret, and
compensate for, deviations from ideal test conditions.
a. Factors Affecting Tests. The following five physical
characteristics influence the performance and applicability of permeability
(1) position of the water level,
(2) type of material - rock or soil,
(3) depth of the test zone,
(4) permeability of the test zone, and
(5) heterogeneity and anisotropy of the test zone.
To account for these it is necessary to isolate the test zone.
Methods for doing so are shown in Figure 14.
b. Types of Tests. Many types of field permeability tests can be
performed. In geotechnical exploration, equilibrium tests are the most
common. These include constant and variable head gravity tests and pressure
(Packer) tests conducted in single borings. In a few geotechnical
investigations, and commonly in water resource or environmental studies,
non-equilibrium "aquifer" or "pump" tests are conducted (a well is pumped at
a constant rate for an extended period of time). See Table 15 for
computation of permeability from variable head tests.
(1) Constant Head Test. This is the most generally applicable
permeability test. It may be difficult to perform in materials of either
very high or very low permeability since the flow of water may be difficult
to maintain or to measure.