The hydraulic conductivity function (HCF) is fundamental to hydrological characterization of unsaturated soils and is required for most analyses of water movement in unsaturated soils. For instance, the HCF is a critical parameter to analyze the movement of water during infiltration or evaporation from soil specimens. This is relevant to the evaluation of water movement in landfill cover systems, stiffness changes in pavements due to water movement, recharge of water into aquifers, and extraction of pore water from soils for sampling.

Examples of HCFs reported in the technical literature are shown in Fig. 1(a), Fig. 1(b), and Fig. 1(c), for clays, silts, and sands, respectively. The decision to report a HCF in terms of suction or volumetric water content depends on the test method and instruments used to measure the HCF. The methods in Categories A and C will provide a HCF in terms of either suction or volumetric water content, while the methods in Category B will provide a HCF in terms of suction.

1.1 These test methods cover the quantitative measurement of data points suitable for defining the hydraulic conductivity functions (HCF) of unsaturated soils. The HCF is defined as either the relationship between hydraulic conductivity and matric suction or that between hydraulic conductivity and volumetric water content, gravimetric water content, or the degree of saturation. Darcy’s law provides the basis for measurement of points on the HCF, in which the hydraulic conductivity of a soil specimen is equal to the coefficient of proportionality between the flow rate of water through the specimen and the hydraulic gradient across the specimen. To define a point on the HCF, a hydraulic gradient is applied across a soil specimen, the corresponding transient or steady-state water flow rate is measured (or vice versa), and the hydraulic conductivity calculated using Darcy’s law is paired with independent measurements of matric suction or volumetric water content in the soil specimen.

1.2 These test methods describe a family of test methods that can be used to define points on the HCF for different types of soils. Unfortunately, there is no single test that can be applied to all soils to measure the HCF due to testing times and the need for stress control. It is the responsibility of the requestor of a test to select the method that is most suitable for a given soil type. Guidance is provided in the significance and use section of these test methods.

1.3 Similar to the Soil Water Retention Curve (SWRC), defined as the relationship between volumetric water content and matric suction, the HCF may not be a unique function. Both the SWRC and HCF may follow different paths whether the unsaturated soil is being wetted or dried. A test method should be selected which replicates the flow process occurring in the field.

1.4 These test methods describe three categories of methods (Categories A through C) for direct measurement of the HCF. Category A (column tests) involves methods used to define the HCF using measured one-dimensional profiles of volumetric water content or suction with height in a column of soil compacted into a rigid wall permeameter during imposed transient and steady-state water flow processes. Different means of imposing water flow processes are described in separate methods within Category A. Category B (axis translation tests) involves methods used to define the HCF using outflow measurements from a soil specimen underlain by a saturated high-air entry porous disc in a permeameter during imposed transient water flow processes. The uses of rigid-wall or flexible-wall permeameters are described in separate methods within Category B. Category C (centrifuge permeameter test) includes a method to define the HCF using measured volumetric water content or suction profiles in a column of soil confined in a centrifuge permeameter during imposed steady-state water flow processes. The methods in this standard can be used to measure hydraulic conductivity values ranging from the saturated hydraulic conductivity of the soil to approximately 10^-11 m/s.