5.1 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.

5.2 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.

FIG. 1 Experimental HCFs for Different Soils: (a) k-ψ for Clays; (b) k-θ for Silts; (c) k-θ for Sands (3-14)

5.3 A major assumption involved in measurement of the hydraulic conductivity is that it is used to quantify movement of water in liquid form through unsaturated soils (that is, it is the coefficient of proportionality between liquid water flow and hydraulic gradient). Water can also move through soil in vapor form, but different mechanisms govern impedance of a soil to water vapor flow (diffusion). Accordingly, the HCF is only applicable in engineering practice for degrees of saturation in which the water phase is continuous (that is, no pockets of “unconnected” water). Although this depends on the soil type and texture, this approximately corresponds to degrees of saturation greater than 50 to 60 %.

5.4 The HCFs of soils may be sensitive to the porosity, soil structure, compaction (compaction gravimetric water content and dry unit weight), effective stress, temperature, and testing flow path (wetting or drying). However, not all engineering problems need to account for the effects of these variables. Out of the test methods listed in Section 4, there is not a single method that is best suited to measure the effects of all of these variables. In addition, the different tests may have a wide range in testing times. Table 1 is provided as a guide for selection of the best test for a given soil and application. Test times for low plasticity, silty clays are provided as a baseline reference. Testing times for coarse-grained soils are typically on the order of 1 to 2 days.

5.5 A full investigation has not been conducted regarding the correlation between HCFs obtained using the laboratory methods presented herein and HCFs of in-place materials. Thus, results obtained from the test methods should be applied to field situations with caution and by qualified personnel.

Note 1: The quality of the result produced by this standard depends on the competence of the personnel performing the test and the suitability of the equipment and facilities used. 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 ensure reliable results. Reliable results depend on many factors. Practice D3740 provides a means of evaluating some of these factors.

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.

1.5 The methods of data analysis described in these test methods involve measurement of the water flow rate and hydraulic gradient, and calculation of the hydraulic conductivity using Darcy’s law (direct methods) (1).2 Alternatively, inverse methods may also be used to define the HCF (2). These employ an iterative, regression-based approach to estimate the hydraulic conductivity that a soil specimen would need to have given a measured water flow response. However, as they require specialized engineering analyses, they are excluded from the scope of these test methods.

1.6 These test methods apply to soils that do not change significantly in volume during changes in volumetric water content or suction, or both (that is, expansive clays or collapsing soils). This implies that these methods should be used for sands, silts, and clays of low plasticity.

1.7 The methods apply only to soils containing two pore fluids: a gas and a liquid. The liquid is usually water and the gas is usually air. Other fluids may also be used if requested. Caution shall be exercised if the liquid being used causes shrinkage or swelling of the soil.

1.8 The units used in reporting shall be SI units in order to be consistent with the literature on water flow analyses in unsaturated soils. The hydraulic conductivity shall be reported in units of [m/s], the matric suction in units of [kPa], the volumetric water content in [m³/m³] or [%], and the degree of saturation in [m³/m³].

1.9 All observed and calculated values shall conform to the guide for significant digits and rounding established in Practice D6026. The procedures in Practice D6026 that are used to specify how data are collected, recorded, and calculated are regarded as the industry standard. In addition, they are representative of the significant digits that should generally be retained. The procedures do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the objectives of the user. Increasing or reducing the significant digits of reported data to be commensurate with these considerations is common practice. Consideration of the significant digits to be used in analysis methods for engineering design is beyond the scope of these test methods.

1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.11 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.