Permeability method for clay

Method

The falling (variable) head permeability test is the most appropriate for clay.

Why: clay has very low permeability ($k \approx 10^{-7}$ to $10^{-10}$ m/s). In a constant head test the flow would be too small to collect and measure accurately in a reasonable time. In the falling head test, a standpipe of small area $a$ is used, so even a tiny quantity of flow gives a measurable fall in head.

Expression

For a sample of length $L$ and area $A$, with head falling from $h_1$ to $h_2$ in time $t$:

$$k = \frac{2.303\,a\,L}{A\,t}\log_{10}\frac{h_1}{h_2}$$

where $a$ is the standpipe area. (A consolidation test can also be used for clays, using $k = c_v m_v \gamma_w$.)