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D421-85 – Standard Test Method Technical Guide
🔍 Overview and Significance
ASTM D421-85 (Reapproved 2007) establishes the standard practice for the dry preparation of soil samples received from the field for particle-size analysis and the determination of soil constants. This standard is issued under the fixed designation D421 and is approved for use by agencies of the Department of Defense.
This practice is specifically intended for use when test values are desired on air-dried samples, or when it is known that air drying does not have an adverse effect on the test results relative to samples prepared in accordance with Practice D2217 (Wet Preparation). It is the responsibility of the user to establish appropriate safety and health practices prior to use.
⚙️ Apparatus and Sieve Specifications
The standard mandates specific laboratory apparatus to ensure the consistent breakdown of soil aggregations and the accurate separation of particle sizes.
| 🟦 Apparatus | 📏 Specification |
|---|---|
| Balance | Sensitive to 0.1 g |
| Mortar and Pestle | Rubber-covered pestle for breaking aggregations without crushing individual grains |
| Sieve No. 4 | 4.75-mm opening (Conforming to Specification E11) |
| Sieve No. 10 | 2.00-mm opening (Conforming to Specification E11) |
| Sieve No. 40 | 425-µm opening (Conforming to Specification E11) |
| Sampler | Riffle sampler or sample splitter for quartering |
💡 Tip for Grinding: Always use the rubber-covered pestle in the mortar. This technique is critical for breaking up soil aggregations into constituent individual grains without fracturing the particles themselves. Crushing the grains will fundamentally alter the natural particle-size distribution of the soil sample.
📊 Sample Preparation and Material Allocation
The soil sample is exposed to air at room temperature until thoroughly dried. All aggregations are broken up in the mortar with the rubber-covered pestle. A representative sample is selected using the method of quartering or a riffle sampler.
The selected air-dried sample is weighed (recorded as the mass uncorrected for hygroscopic moisture) and separated using a No. 10 (2.00-mm) sieve. The fraction retained on the No. 10 sieve is ground again and sieved a second time. The final retained fraction is washed free of all fine material, dried, and weighed.
| 🎯 Test Requirement | 🟫 Soil / Fraction Type | ⚖️ Required Mass |
|---|---|---|
| Particle-Size Analysis | Sandy soils (passing No. 10) | 115 g |
| Particle-Size Analysis | Silt or clay soils (passing No. 10) | 65 g |
| Soil Constants (passing No. 40): | 220 g total | |
| — Liquid Limit | 100 g | |
| — Plastic Limit | 15 g | |
| — Centrifuge Moisture Equivalent | 10 g | |
| — Volumetric Shrinkage | 30 g | |
| — Check Tests | 65 g | |
📌 Important Note on Hygroscopic Moisture: The standard explicitly requires that the mass of the air-dried sample selected for testing be recorded as the “mass of the total test sample uncorrected for hygroscopic moisture.” This initial recorded mass is critical for the final calculations in the particle-size distribution analysis.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D421?
ASTM D421 covers the dry preparation of soil samples for particle-size analysis and the determination of soil constants, such as Atterberg limits. It is intended for air-dried samples where the drying process does not affect the test results.
💡 What are the required sieves for this standard?
The practice requires a series of square-mesh sieves conforming to Specification E11. The specific sieves required are the No. 4 (4.75-mm), No. 10 (2.00-mm), and No. 40 (425-µm).
⚡ How much material is needed for the soil constants tests?
220 grams of material passing the No. 40 (425-µm) sieve is required. This is allocated specifically as 100 g for the Liquid Limit, 15 g for the Plastic Limit, 10 g for the Centrifuge Moisture Equivalent, 30 g for Volumetric Shrinkage, and 65 g for Check Tests.
📌 Why is a rubber-covered pestle specified for the mortar?
The rubber-covered pestle is required to break up aggregations of soil particles into individual grains without crushing the grains themselves. Crushing the particles would alter the natural particle-size distribution and potentially the plasticity characteristics of the sample.