Permeability Testing in Unconsolidated Materials
by: Sebastien Fortin, E.I.T., M.Sc.
Laboratory Methods
Laboratory tests of permeability are usually determined using an apparatus called permeameter (Figure 2). There are two common permeameter tests: the falling head permeability test and the constant head permeability test. Which test is used depends upon the type of soil tested. For soils of high permeability (e.g. sands and gravel) a constant head test is normally used. For soils of intermediate to low permeability, a falling head test is normally used.
Figure 2. Laboratory set up for constant head test (a) and falling head test (b) (adapted from Dominico & Schwartz, 1990).
In the constant head test, a constant total head difference is applied to the soil specimen, and the resulting quantity of seepage can then be measured. In this test, a valve at the base of the sample is opened and the water starts to flow (Figure 2a). After a sufficient volume of water is collected over the time of the test, the volumetric flow rate Q is calculated. Hydraulic conductivity is then determined with Darcy's Law of the form:
where L is the length of the sample, A is the cross-sectional area of the sample, and h is the constant head shown on Figure 2a. This method works well for coarse-grained soils, but with clays and silts, the quantity of seepage is much too small to be accurately measured.
The falling head test is different in that it does not fix the total head difference across the soil specimen. Instead, a standpipe is connected to the inflow, and the water level in this standpipe is then allowed to drop as water flows through the specimen. In this test, the head is measured in the standpipe of the permeameter, along with the time of measurement (Figure 2b). For a sample of length L and a cross-sectional area A, the conductivity is determined by
where a is the cross-sectional area of the standpipe and (t1-t0) is the elapsed time required for the head to fall from h0 to h1. This method will not work well for coarse-grained soils, because they are so permeable that the head drops too rapidly to be accurately measured.
For compressive soils (e.g. silts and clays) the hydraulic conductivity is significantly influenced by the degree of compaction (in-situ density) of the sample. It is therefore important to record the in-situ density of the soil during sampling and to recreate these field conditions in the permeameter. For this reason, fine-grained soils are best tested in a triaxial testing apparatus, where the confining stress can be controlled accurately.
Forward to Field Methods for the Saturated Zone.
Return to Methods of Permeability Testing.
Consult list of References on Permeability Testing.
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