Standard Test Methods for Measurement of Hydraulic Conductivity of Unsaturated Soils

Importancia y uso:

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.

Subcomité:

D18.04

Volúmen:

04.09

Número ICS:

13.080.40 (Hydrological properties of soil)

Palabras clave:

column test; hydraulic conductivity function; permeameter; soil water retention curve; unsaturated soils;