This test method is used to measure one-dimensional flow of aqueous solutions (for example, landfill leachates, liquid wastes and byproducts, single and mixed chemicals, etc., from hereon referred to as the permeant liquid) through initially saturated soils under an applied hydraulic gradient and effective stress. Interactions between some permeant liquids and some clayey soils have resulted in significant increases in the hydraulic conductivity of the soils relative to the hydraulic conductivity of the same soils permeated with water (1). This test method is used to evaluate the presence and effect of potential interactions between the soil specimen being permeated and the permeant liquid on the hydraulic conductivity of the soil specimen. Test programs may include comparisons between the hydraulic conductivity of soils permeated with water relative to the hydraulic conductivity of the same soils permeated with aqueous solutions to determine variations in the hydraulic conductivity of the soils due to the aqueous solutions.

Flexible-wall hydraulic conductivity testing is used to determine flow characteristics of aqueous solutions through soils. Hydraulic conductivity testing using flexible-wall cells is usually preferred over rigid-wall cells for testing with aqueous solutions due to the potential for sidewall leakage problems with rigid-wall cells. Excessive sidewall leakage may occur, for example, when a test soil shrinks during permeation with the permeant liquid due to interactions between the soil and the permeant liquid in a rigid-wall cell. In addition, the use of a rigid-wall cell does not allow for control of the effective stresses that exist in the test specimen.

Darcy's law describes laminar flow through a test soil. Laminar flow conditions and, therefore, Darcy's law may not be valid under certain test conditions. For example, interactions between a permeating liquid and a soil may cause severe channeling/cracking of the soil such that laminar flow is not maintained through a test specimen containing large open pathways for flow.

Interactions that may clog the pore spaces of test soils (for example, precipitation) may occur during permeation with some permeant liquids. Flow through test soils may be severely restricted in these cases. In cases where the measured hydraulic conductivity is less than 1 × 10^-12 m/s, unsteady state analysis may be used to determine the hydraulic conductivity of test soils (2).

Specimens of initially water-saturated soils (for example, undisturbed natural soils) may be permeated with the permeant liquid. Specimens of water unsaturated soils (for example, compacted soils) may be fully saturated with water or the permeant liquid and then permeated with the permeant liquid. Specimens of soils initially partly or fully saturated with a particular liquid (for example, specimens collected from a containment facility subsequent to a period of use) may be fully saturated and then permeated with the same or another liquid. The use of different saturating and permeating liquids can have significant effects both on the results and the interpretation of the results of a test (1). Selection of type and sequence of liquids for saturation and permeation of test specimens is based on the characteristics of the test specimens and the requirements of the specific application for which the hydraulic conductivity testing is being conducted in a test program. The user of this standard is responsible for selecting and specifying the saturation and permeation conditions that best represent the intended application.

Hydraulic conductivity of a soil with water and aqueous solution can be determined using two approaches in a test program for comparisons between the hydraulic conductivity based on permeation with water and the hydraulic conductivity based on permeation with aqueous solution. In the first approach, specimens are initially saturated (if needed) and permeated with water and then the permeating liquid is switched to the aqueous solution. This testing sequence allows for determination of both water and aqueous solution hydraulic conductivities on the same specimen. Obtaining water and aqueous solution values on the same specimen reduces the uncertainties associated with specimen preparation, handling, and variations in test conditions. However, such testing sequences may not represent actual field conditions and may affect the results of a test. In the second approach, two specimens of the same soil are permeated, with one specimen being permeated with water and the other specimen being permeated with the aqueous solution. The specimens are prepared using the same sample preparation and handling methods and tested under the same testing conditions. This approach may represent actual field conditions better than the first approach, however, uncertainties may arise due to the use of separate specimens for determining hydraulic conductivities based on permeation with water and the aqueous solution. Guidelines for preparing and testing multiple specimens for comparative studies are provided in Practice E691. The user of this standard shall be responsible for selecting and specifying the approach that best represents the intended application when comparisons of hydraulic conductivity are required.

Termination criteria used in the test method are based on both achieving steady-state conditions with respect to flow and equilibrium between the chemical composition of the effluent (outflow) relative to the influent (inflow).

Intrinsic permeability can be determined in addition to hydraulic conductivity using results of permeation tests described in this standard.

The correlation between results obtained using this test method and the hydraulic conductivities of in-place field materials has not been completely determined. Differences may exist between the hydraulic conductivities measured on small test specimens in the laboratory and those obtained for larger volumes in the field. Therefore, the results obtained using this standard should be applied to field situations with caution and by qualified personnel.

While not required for determining the hydraulic conductivity of soils with aqueous solutions, soil chemical properties such as pH, electrical conductivity, exchangeable metals (cations), and cation exchange capacity as well as the mineralogical composition of the soil may be useful in the interpretation and explanation of the test results.

Note 1-The quality of the result produced by this standard is dependent of the competence of the personnel using this standard and the suitability of the equipment and facilities. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of these factors.