Environmental Engineering Reference
In-Depth Information
Figure 6.54.
Effect of compaction water content on permeability (Lambe and Whitman, 1981).
Reprinted by permission of John Wiley and Sons Inc.
permeating water can affect soil dispersivity (see Chapter 7) and to a certain extent the
permeability.
Type of flow.
The basic assumption in calculations using Darcy's Law q
kiA is that
the soil is saturated, and the flow is laminar. This is generally the case for flow through
and beneath dams but flow in medium and coarse gravels may become turbulent in some
cases, e.g. around dewatering wells.
6.3.2
Laboratory test methods
6.3.2.1
Permeameters
Most permeability testing of soils is carried out using constant head or falling head per-
meability apparatus. As shown in
Figure 6.55
the constant head test is best suited to high
to medium permeability soils, sand and gravel, and the falling head test to low to very low
permeability soils, silts, fine sands and some clays.
For very low permeability soils, the flow rates are too low to carry out testing in a con-
ventional permeameter, and it would be more usual to do the testing in an oedometer cell
(e.g. a Rowe cell), or to determine the permeability indirectly from consolidation test data.
Many texts in soil mechanics include details of falling head and constant head tests so they
are not repeated here. Head (1985) gives detailed instructions on laboratory procedures, as
do Standards Association of Australia (1980) and American Society for Testing and Materials
D2434. The USBR (1980) also gives detailed instructions for the test in their “
Earth Manual
”.
Some common errors and problems which can arise with permeability testing include:
(a) leakage past the sample in the test mould;
(b) variability in test results for compacted soils due to difficulty in maintaining uniform
water content and density in sample preparation;
