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ASTM D7928: Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis
The hydrometer test that sizes the silt and clay fraction too fine for sieves — the successor to the sedimentation half of ASTM D422 and the source of the clay-size percentage used in expansive-soil evaluation.
| Designation | ASTM D7928 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM D7928 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What the ASTM D7928 test measures
ASTM D7928 is the hydrometer analysis for fine-grained soils. It measures the distribution of particle sizes below the No. 200 sieve, into the silt and clay range, by timing how fast particles settle out of a suspension. It replaced the sedimentation portion of the withdrawn ASTM D422 and is the current method for determining the clay-size fraction of a soil.
How the ASTM D7928 test works, step by step
A measured mass of soil passing the No. 10 or No. 200 sieve is soaked in a sodium hexametaphosphate dispersant, mixed, and transferred to a sedimentation cylinder filled with water. A hydrometer floats in the suspension and is read at intervals from one minute to 24 hours as the larger particles settle first and the density of the suspension drops. Each reading, together with the temperature and Stokes' law, gives the percentage of particles finer than a computed diameter. D7928 tightened dispersion, temperature control, and hydrometer calibration compared to D422.
What the numbers mean
The result extends the gradation curve from 0.075 mm down to about 0.001 mm and reports the percentage finer than 0.005 or 0.002 mm, the clay-size fraction. Clay content combined with the plasticity index gives the activity of a clay, a key indicator of shrink-swell potential in expansive soils. Filter and drain designs, dispersive soil evaluations, and lining and cover material specifications also depend on it.
What can throw the result off
Incomplete dispersion of clay aggregates is the largest error and always makes the soil look coarser than it is. Temperature changes during the 24-hour test, soils that flocculate in the cylinder, and organic content that has not been removed also affect the result.
Common questions about ASTM D7928
When do I need a hydrometer test?
For expansive soil evaluation, filter and drainage design, dispersive clay screening, and any classification where clay percentage rather than total fines is specified. Routine fill acceptance rarely needs it.
How long does it take?
The sedimentation runs 24 hours; with preparation and reporting, allow two to three working days.
How we help with ASTM D7928 testing
We run hydrometer analyses paired with ASTM D6913 sieve gradations and Atterberg limits for geotechnical investigations and expansive soil evaluations, reporting the combined curve and the clay fraction the engineer needs.
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