ASTM D7928-21e1
(Test Method)Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis
Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis
SIGNIFICANCE AND USE
5.1 Particle-size distribution (gradation) is a descriptive term referring to the proportions by dry mass of a soil distributed over specified particle-size ranges. The gradation curve generated using this method yields the distribution of silt and clay size fractions present in the soil based on size definitions, not mineralogy or Atterberg limit classification.
5.2 Unless the sedimentation sample is representative of the entire sample, the sedimentation results must be combined with a sieve analysis to obtain the complete particle size distribution.
5.3 The clay size fraction is material finer than 2 µm. The clay size fraction is used in combination with the Plasticity Index (Test Methods D4318) to compute the activity, which provides an indication of the mineralogy of the clay fraction.
5.4 The gradation of the silt and clay size fractions is an important factor in determining the susceptibility of fine-grained soils to frost action.
5.5 The gradation of a soil is an indicator of engineering properties such as hydraulic conductivity, compressibility, and shear strength. However, soil behavior for engineering and other purposes is dependent upon many factors, such as effective stress, mineral type, structure, plasticity, and geological origin, and cannot be based solely upon gradation.
5.6 Some types of soil require special treatment in order to correctly determine the particle sizes. For example, chemical cementing agents can bond clay particles together and should be treated in an effort to remove the cementing agents when possible. Hydrogen peroxide and moderate heat can digest organics. Hydrochloric acid can remove carbonates by washing and Dithionite-Citrate-Bicarbonate extraction can be used to remove iron oxides. Leaching with test water can be used to reduce salt concentration. All of these treatments, however, add significant time and effort when performing the sedimentation test and are allowable but outside the scope of this test method. ...
SCOPE
1.1 This test method covers the quantitative determination of the distribution of particle sizes of the fine-grained portion of soils. The sedimentation by hydrometer method is used to determine the particle-size distribution (gradation) of the material that is finer than the No. 200 (75-µm) sieve and larger than about 0.2-µm. The test is performed on material passing the No. 10 (2.0-mm) or finer sieve and the results are presented as the mass percent finer of this fraction versus the log of the particle diameter.
1.2 This method can be used to evaluate the fine-grained fraction of a soil with a wide range of particle sizes by combining the sedimentation results with results from a sieve analysis using D6913 to obtain the complete gradation curve. The method can also be used when there are no coarse-grained particles or when the gradation of the coarse-grained material is not required or not needed.
Note 1: The significant digits recorded in this test method preclude obtaining the grain size distribution of materials that do not contain a significant amount of fines. For example, clean sands will not yield detectable amounts of silt and clay sized particles, and therefore should not be tested with this method. The minimum amount of fines in the sedimentation specimen is 15 g.
1.3 When combining the results of the sedimentation and sieve tests, the procedure for obtaining the material for the sedimentation analysis and calculations for combining the results will be provided by the more general test method, such as Test Methods D6913 (Note 2).
Note 2: Subcommittee D18.03 is currently developing a new test method “Test Method for Particle-Size Analysis of Soils Combining the Sieve and Sedimentation Techniques.”
1.4 The terms “soil” and “material” are used interchangeably throughout the standard.
1.5 The sedimentation analysis is based on the concept that larger particles will fall through a fluid faster than...
General Information
Standards Content (Sample)
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.
´1
Designation: D7928 − 21
Standard Test Method for
Particle-Size Distribution (Gradation) of Fine-Grained Soils
1
Using the Sedimentation (Hydrometer) Analysis
This standard is issued under the fixed designation D7928; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1
ε NOTE—Summary of Changes section was editorially added in June 2021.
1. Scope* 1.5 The sedimentation analysis is based on the concept that
larger particles will fall through a fluid faster than smaller
1.1 This test method covers the quantitative determination
particles. Stokes’ Law gives a governing equation used to
of the distribution of particle sizes of the fine-grained portion
determine the terminal velocity of a spherical particle falling
of soils. The sedimentation by hydrometer method is used to
through a stationary liquid. The terminal velocity is propor-
determine the particle-size distribution (gradation) of the
tionaltothesquareoftheparticlediameter.Therefore,particles
material that is finer than the No. 200 (75-µm) sieve and larger
are sorted by size in both time and position when settling in a
than about 0.2-µm. The test is performed on material passing
container of liquid.
theNo.10(2.0-mm)orfinersieveandtheresultsarepresented
1.5.1 Stokes’ Law has several assumptions which are: the
as the mass percent finer of this fraction versus the log of the
particles are spherical and smooth; there is no interference
particle diameter.
between the particles; there is no difference between the
1.2 This method can be used to evaluate the fine-grained
current in the middle of the container and the sides; flow is
fraction of a soil with a wide range of particle sizes by
laminar; and the particles have the same density. These
combining the sedimentation results with results from a sieve
assumptions are applied to soil particles of various shapes and
analysis using D6913 to obtain the complete gradation curve.
sizes.
The method can also be used when there are no coarse-grained
1.6 A hydrometer is used to measure the fluid density and
particles or when the gradation of the coarse-grained material
determine the quantity of particles in suspension at a specific
is not required or not needed.
time and position. The density of the soil-water suspension
NOTE 1—The significant digits recorded in this test method preclude
depends upon the concentration and specific gravity of the soil
obtaining the grain size distribution of materials that do not contain a
particlesandtheamountofdispersantadded.Eachhydrometer
significant amount of fines. For example, clean sands will not yield
measurement at an elapsed time is used to calculate the
detectable amounts of silt and clay sized particles, and therefore should
not be tested with this method. The minimum amount of fines in the percentageofparticlesfinerthanthediametergivenbyStokes’
sedimentation specimen is 15 g.
Law.Theseriesofreadingsprovidethedistributionofmaterial
mass as a function of particle size.
1.3 When combining the results of the sedimentation and
sieve tests, the procedure for obtaining the material for the
1.7 This test method does not cover procurement of the
sedimentation analysis and calculations for combining the
sample or processing of the sample prior to obtaining the
results will be provided by the more general test method, such
reduced sample in any detail. It is assumed that the sample is
as Test Methods D6913 (Note 2).
obtained using appropriate methods and is representative of
site materials or conditions. It is also assumed that the sample
NOTE 2—Subcommittee D18.03 is currently developing a new test
method “Test Method for Particle-Size Analysis of Soils Combining the has been processed such that the reduced sample accurately
Sieve and Sedimentation Techniques.”
reflects the particle-size distribution (gradation) of this finer
fraction of the material.
1.4 The terms “soil” and “material” are used interchange-
ably throughout the standard.
1.8 Material Processing—Material is tested in the moist or
as-received state unless the material is received in an air-dried
state. The moist preparation method shall be used to obtain a
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sedimentation test specimen from the reduced sample. Air-
ThistestmethodisunderthejurisdictionofASTMCommitteeD18onSoiland
Rock andisthedirectresponsibilityofSubcommitteeD18.03onTexture, Plasticity
driedpreparationisonlyallowedwhenthematerialisreceived
and Density Characteristics of Soils.
in the air-dried state. The met
...
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