ASTM D7928-16
(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 amount of silt and clay size fractions present in the soil based on size definitions, not mineralogy or Atterberg limit data.
5.2 Determination of the clay size fraction, which is material finer than 2 µm, 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.
5.3 The gradation of the silt and clay size fractions is an important factor in determining the susceptibility of fine-grained soils to frost action.
5.4 The gradation of a soil is an indicator of engineering properties. Hydraulic conductivity, compressibility, and shear strength are related to the gradation of the soil. However, engineering behavior is dependent upon many factors, such as effective stress, mineral type, structure, plasticity, and geological origin, and cannot be based solely upon gradation.
5.5 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.
5.6 The size limits of the sedimentation test are from about 100 µm to about 1 µm. The length of time required to obtain a stable initial reading on the hydrometer controls the upper range of results, ...
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 or 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 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 a sieve analysis resulting in 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 smaller particles. Stokes’ Law gives a...
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Designation: D7928 − 16
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. Scope through a stationary liquid. The terminal velocity is propor-
tionaltothesquareoftheparticlediameter.Therefore,particles
1.1 This test method covers the quantitative determination
are sorted by size in both time and position when settling in a
of the distribution of particle sizes of the fine-grained portion
container of liquid.
of soils. The sedimentation or hydrometer method is used to
1.5.1 Stokes’ Law has several assumptions which are: the
determine the particle-size distribution (gradation) of the
particles are spherical and smooth; there is no interference
material that is finer than the No. 200 (75-µm) sieve and larger
between the particles; there is no difference between the
than about 0.2-µm. The test is performed on material passing
current in the middle of the container and the sides; flow is
theNo.10(2.0-mm)orfinersieveandtheresultsarepresented
laminar; and the particles have the same density. These
asthemasspercentfinerversusthelogoftheparticlediameter.
assumptions are applied to soil particles of various shapes and
1.2 This method can be used to evaluate the fine-grained
sizes.
fraction of a soil with a wide range of particle sizes by
1.6 A hydrometer is used to measure the fluid density and
combining the sedimentation results with a sieve analysis
determine the quantity of particles in suspension at a specific
resultinginthecompletegradationcurve.Themethodcanalso
time and position. The density of the soil-water suspension
beusedwhentherearenocoarse-grainedparticlesorwhenthe
depends upon the concentration and specific gravity of the soil
gradation of the coarse-grained material is not required or not
particlesandtheamountofdispersantadded.Eachhydrometer
needed.
measurement at an elapsed time is used to calculate the
NOTE 1—The significant digits recorded in this test method preclude
percentageofparticlesfinerthanthediametergivenbyStokes’
obtaining the grain size distribution of materials that do not contain a
Law.Theseriesofreadingsprovidethedistributionofmaterial
significant amount of fines. For example, clean sands will not yield
mass as a function of particle size.
detectable amounts of silt and clay sized particles, and therefore should
not be tested with this method. The minimum amount of fines in the
1.7 This test method does not cover procurement of the
sedimentation specimen is 15 g.
sample or processing of the sample prior to obtaining the
1.3 When combining the results of the sedimentation and
reduced sample in any detail. It is assumed that the sample is
sieve tests, the procedure for obtaining the material for the
obtained using appropriate methods and is representative of
sedimentation analysis and calculations for combining the
site materials or conditions. It is also assumed that the sample
results will be provided by the more general test method, such
has been processed such that the reduced sample accurately
as Test Methods D6913 (Note 2).
reflects the particle-size distribution (gradation) of this finer
fraction of the material.
NOTE 2—Subcommittee D18.03 is currently developing a new test
method “Test Method for Particle-Size Analysis of Soils Combining the
1.8 Material Processing—Material is tested in the moist or
Sieve and Sedimentation Techniques.”
as-received state unless the material is received in an air-dried
1.4 The terms “soil” and “material” are used interchange-
state. The moist preparation method shall be used to obtain a
ably throughout the standard.
sedimentation test specimen from the reduced sample. Air-
1.5 The sedimentation analysis is based on the concept that
driedpreparationisonlyallowedwhenthematerialisreceived
larger particles will fall through a fluid faster than smaller
in the air-dried state. The method to be used may be specified
particles. Stokes’ Law gives a governing equation used to
by the requesting authority; however, the moist preparation
determine the terminal velocity of a spherical particle falling
method shall be used for referee testing.
1.9 This test method is not applicable for the following
1
ThistestmethodisunderthejurisdictionofASTMCommitteeD18onSoiland soils:
RockandisthedirectresponsibilityofSubcommitteeD18.03onTexture,Plasticity
1.9.1 Soils containing fibrous peat.
and Density Characteristics of Soils.
1.9.2 Soils containing less than approximately 5% of fine-
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