In the design of engineering projects, one of the most important soil properties of interest to the soils engineer is permeability. To some degree, permeability will play a role in the design of almost any structure. For example, the durability of concrete is related to its permeability. In designs that make use of earthen materials (soils and rock, etc.) the permeability of these materials will usually be of great importance.
Soils are permeable (water may flow through them) because they consist not only of solid particles, but a network of interconnected pores. The degree to which soils are permeable depends upon a number of factors, such as soil type, grain size distribution and soil history. This degree of permeability is characterized by the coefficient of permeability.
A number of different methods for determining the coefficient of permeability for soils exist, including in-situ methods and laboratory methods. In the laboratory, two common tests are generally used to determine this soil property. These two tests are the falling head permeability test and the constant head permeability test. Which test is used depends upon the type of soil to be tested. For soils of high permeability (sands and gravels) a constant head test is used. For soils of intermediate to low permeability, a falling head test is used. As we were testing sand we used a constant head permeability test.
By carrying out the constant head permeability test we can determine the coefficients of permeability of given sand over a range of unit weights. During the test we can also observe the phenomenon of piping.
The coefficient of permeability, k, is a product of Darcy’s Law. Darcy established an empirical relationship for the flow of water through porous media. This relationship has evolved into Darcy’s Law, which states:
Q = A . k . i
Q = flow rate (volume/time)
i = hydraulic gradient (unitless)
A = cross-sectional area of flow (area)