Permeability Testing in Unconsolidated Materials
by: Sebastien Fortin, E.I.T., M.Sc.
Introduction
Both in environmental site characterization and in geotechnical engineering projects, one of the most important soil properties of interest to the engineer is permeability. To some degree, permeability will play a role in the migration of contaminant and on the design of almost any structure. For example, compacted clay liners are often used to minimize potentially contaminated seepage from the base of tailings impoundments. In designs that make use of earthen materials (soils and rock, etc.) the permeability of the materials will usually be of great importance.
To illustrate the importance of permeability in environmental site characterization and geotechnical design, consider the following applications where knowledge of permeability is required:
The rate of flow to wells from an aquifer is dependent on permeability;
The migration of contaminant seepage through a saturated or unsaturated soil profile is dependent on permeability;
The design of earth dams is very much based upon the permeability of the soils used;
Permeability influences the rate of settlement of a saturated soil under load;
The performance of landfill or tailings impoundment liners is based upon their permeability;
The stability of slopes and retaining structures can be greatly affected by the permeability of the soils involved; and
Filters to prevent piping and erosion are designed based upon their permeability.
Soils are permeable (i.e. 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 on a number of factors, such as soil type, grain size distribution, water content, degree of compaction and stress history. The ability to transmit water is characterized by the coefficient of permeability (or hydraulic conductivity).
The coefficient of permeability, K, is a product of Darcy's Law (1856), which establishes the following empirical relationship for flow through saturated porous media:
It should be noted that while geotechnical engineers mostly use the term coefficient of permeability, this same parameter is usually referred to as hydraulic conductivity by hydrogeologists and environmental scientists.
Flow in the unsaturated zone differs significantly from flow in the saturated zone. In the unsaturated zone, the pressure head is negative, meaning that it is less than atmospheric pressure. The water content is less than the porosity because some of the void space is filled with soil gas (air). In saturated porous media, water can move through the entire cross-sectional area of the pore space. However, as water is replaced by air, it can only move through the reduced cross-sectional area occupied by the remaining water. This has the effect of lowering the hydraulic conductivity.
The water content of the porous medium (also expressed as the degree of saturation) is variable and a function of the pressure head. The more negative the pressure head (or the higher the suction) the lower the water content. The exact relationship depends on the soil type. Similarly, the hydraulic conductivity decreases with decreasing water content and increasing suction. The result is that Darcy's law (presented above) becomes non linear. Special testing methods therefore had to be developed and adapted to measure the hydraulic conductivity of a soil in the unsaturated zone in order to predict the movement of water.
Please contact Sebastien Fortin if you would like to feature an interesting example of a permeability testing study for a mining project on this website.
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