Permeability Testing in Unconsolidated Materials
by: Sebastien Fortin, E.I.T., M.Sc.
Methods of Permeability Testing
Saturated hydraulic conductivity (Ksat) is a critically important soil property. As a result, many methods have been developed over time for field and laboratory measurement of Ksat (e.g. Klute, 1986). Unfortunately, these methods often yield substantially dissimilar results, as Ksat is extremely sensitive to sample size, flow geometry, and soil physical and hydraulic characteristics. In addition, most Ksat measurement methods are neither appropriate for all applications nor accurate for all soil types and conditions (Bouma, 1983). The literature (e.g. Dun and Philips (1991) shows that a large portion of the subsurface flow often occurs in a small portion of the soil volume, indicating that the spatial hydraulic characteristics of soils are highly variable. Ksat measurements should therefore be evaluated carefully to ensure that the Ksat values obtained are both accurate and appropriate for the intended use.
Because of its dependence on water content, a complete description of hydraulic conductivity in unsaturated materials is very difficult. However, there are many applications in which only the saturated hydraulic conductivity is required. These include estimates of infiltration capacity for input in hydrogeologic models, evaluation of the suitability of soils for constructing liners, evaluation of the potential leakage from tailings impoundments, and several others.
Several methods are available to the engineer/scientist for determining soil permeability (Figure 1). These methods vary in complexity and applicability and exist both for saturated and unsaturated conditions. In general, the hydraulic conductivity of a soil can be determined by the following approaches:
Empirical Methods (based on grain diameter and grain size distribution);
Laboratory Methods;
Field Methods in the Saturated Zone;
Field Methods in the Vadose Zone.
Field tests are by far the most reliable since they permit the testing of larger volumes of soils. In the field, the permeability of a soil can either be measured in existing monitoring wells (for the saturated zone) or from the surface or sub-surface (open holes) using infiltration methods (for the vadose zone). Pump and/or slug tests (slug in or slug out) are typically used for determining the permeability of the soil aquifer material by recording the drawdown (or change in well head) over time. Special methods such as the dual tube (DT) direct push-in methods can also be used to determine the permeability of a soil without the need of a piezometer.
Figure 1. Summary of testing methods for determining soil permeability.
The following sections discuss in more detail the various methods of permeability testing presented in Figure 1. The final section of this paper is on soil permeability testing in the vadose zone. A list of References on Permeability Testing is available for the reader interested in digging deeper into the details of the methods presented above.
Forward to Laboratory Methods.
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