Home / Test methods / ASTM D421
ASTM International · Soils · Laboratory test
ASTM D421: Dry Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil Constants
The preparation practice that splits, air-dries, and pulverizes a soil sample ahead of gradation and Atterberg limit testing so the specimen tested represents the soil that was sampled.
| Designation | ASTM D421 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM D421 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What ASTM D421 covers
ASTM D421 is the practice for dry preparation of soil samples before particle-size analysis and the Atterberg limits. A field sample is air dried, broken up with a rubber-covered pestle so individual particles are separated without being crushed, split to the required test mass, and separated over the No. 10 sieve for gradation or the No. 40 sieve for liquid and plastic limits.
The practice exists because the soil that goes into the sieve stack or the liquid limit cup has to represent what came out of the ground. Crushing particles, drying at too high a temperature, or testing a sample that was never split properly all change the result before the test begins.
What the practice requires
Drying is at room temperature or in an oven not exceeding 60 degrees Celsius, because higher temperatures alter the plasticity of many clays. Aggregations are broken up, not ground. The sample is quartered or riffle split to the mass the test needs, and the fraction passing the No. 40 sieve is set aside for the Atterberg limits while the whole sample is used for gradation.
Why it matters to your project
Most fill acceptance criteria, the plasticity index limit on select fill, the fines content on a drainage layer, the classification in a geotechnical report, are computed from tests that begin with this preparation. Consistent preparation is what makes results from different laboratories comparable.
D421 has been superseded in current ASTM practice by the preparation sections built into ASTM D6913 and D7928 and by ASTM D4318's own requirements, and it is sometimes listed as withdrawn, but a great many specifications and state DOT procedures still reference it by name.
Common questions about ASTM D421
What is the difference between dry and wet preparation?
Wet preparation, ASTM D2217, keeps the soil from ever drying and is used for soils whose properties change on drying, such as some tropical and organic clays. Dry preparation is standard for most temperate soils and is what fill specifications assume.
Does dry preparation affect Atterberg limits?
For most soils, no. For a few, drying lowers the liquid limit noticeably, which is why ASTM D4318 has a wet preparation option and the report states which was used.
How we help with ASTM D421 preparation
Every gradation and Atterberg set our laboratory runs starts with documented sample preparation, with the method noted on the report, so results are traceable and reproducible when they matter in an acceptance decision.
Scheduling & proposals
Need ASTM D421 testing?
Call for same-day dispatch questions, or send project documents for a written proposal.