Standard Test Methods for Measurement of Hydraulic Conductivity of Coarse-Grained Soils

Importancia y uso:

5.1 These test methods are used to measure one-dimensional vertical flow of water through initially saturated coarse-grained, pervious (that is, free-draining) soils under an applied hydraulic gradient. Hydraulic conductivity of coarse-grained soils is used in various civil engineering applications. These test methods are suitable for determination of hydraulic conductivity for soils with k > 10^–7 m/s.

Note 2: Clean coarse-grained soils that are classified using Practice D2487-17 as GP, GW, SP, and SW can be tested using these test methods. Depending on fraction and characteristics of fine-grained particles present in soils, these test methods may be suitable for testing other soil types with fines content greater than 5 % (for example, GP-GC, SP-SM).

5.2 Coarse-grained soils are to be tested at a void ratio representative of field conditions. For engineered fills, compaction specification can be used to provide target test conditions, whereas for natural soils, field testing of in-situ density can be used to provide target test conditions.

5.3 Use of a dual-ring permeameter is included in these test methods in addition to a single-ring permeameter for the rigid wall test apparatus. The dual-ring permeameter allows for reducing potential adverse effects of sidewall leakage on measured hydraulic conductivity of the test specimens. The use of a plate at the outflow end of the specimen that contains a ring with a diameter smaller than the diameter of the permeameter and the presence of two outflow ports (one from the inner ring, one from the annular space between the inner ring and the permeameter wall) allows for separating the flow from the central region of the test specimen from the flow near the sidewall of the permeameter.

Note 3: Sidewall leakage has been reported to have significant influence on flow conditions for coarse-grained soils due to presence of larger voids at the boundary and higher void ratio in this region of the specimen. Three modifications that have been used to reduce this effect in rigid wall permeameters include: i) placing a piping barrier (for example, caulk rings along every approximately 25-mm length of sidewall), ii) spreading a layer of bentonite and petroleum jelly mixture along the entire surface area of the sidewall, and iii) using a closed-cell neoprene liner attached to the inside wall of the permeameter.

5.4 Use of a flexible wall permeameter is included in these test methods in addition to the rigid wall permeameters. The flexible wall permeameter reduces potential adverse effects of sidewall leakage on measured hydraulic conductivity of the test specimens and allows for application of hydrostatic confining stress conditions on the specimen during the hydraulic conductivity test. Confining stress allows for representing field conditions (that is, simulating stress states in the subgrade that may affect values of k).

5.5 Darcy's law is assumed to apply to the test conditions, flow is assumed to be laminar (see Note 4), and the hydraulic conductivity is assumed to be considered independent of hydraulic gradient. The validity of these assumptions may be evaluated by measuring the hydraulic conductivity of a specimen at three different hydraulic gradients. The discharge velocity (v = k × i) is plotted against the applied hydraulic gradient. If the resulting relationship is linear and the measured hydraulic conductivity values are similar (that is, within 25 %), then these assumptions are considered valid.

Note 4: Previous studies suggest that the limit between turbulent flow and laminar flow for soils occurs for Reynolds numbers between 1 and 10 (1 and 2)3. A formulation for Reynolds number (and division for laminar and turbulent flow conditions) for flow through packed beds has been reported (3). The formulation is presented for uniformly graded, spherical particles in Eq 1.


where:

Re*   =   Reynolds Number for packed bed flow, D   =   granule or particle diameter (m), v   =   superficial fluid velocity (that is, Darcy velocity) through bed (m/s), ρ_f   =   fluid density (kg/m³), μ   =   liquid viscosity (dynamic viscosity) (Pa s), and n   =   porosity of bed (expressed as a ratio).

Provisions are provided in (3) for establishing equivalent particle diameter for use in this equation for nonuniform particle size distributions and nonspherical particles.

Note 5: Using sufficiently low gradients has been demonstrated to be important for obtaining representative results. Hydraulic gradients less than 0.05 have been reported (4). Using a long test specimen (on the order of 1.5 m) has been reported as an effective method for achieving appropriately low hydraulic gradients for materials with k > 0.01 m/s.

Note 6: The quality of the result produced by this standard is dependent of the competence of the personnel performing it 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 result in reliable values. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

Subcomité:

D18.04

Referida por:

D4520-18, D6270-20, D7760-18, D4511-11R20, F2952-22, E1266-20, D7492_D7492M-16AR24, D8298_D8298M-23, D7294-13R21, D8037_D8037M-16, D5126-16E01, D5084-24, D5084-24, D8297_D8297M-23

Volúmen:

04.08

Número ICS:

93.020 (Earthworks. Excavations. Foundation construction. Underground works)

Palabras clave:

constant head; gravel; hydraulic conductivity; permeameter; sand;