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ASTM D4318 is the ASTM International standard for determining the liquid limit, plastic limit and plasticity index of soils, the three values usually called the Atterberg limits. The requirements summarised here are those of the D4318-17e1 edition, as set out in the NICET soils performance examination checklist (form dated 09/2020).
Short answer
ASTM D4318 determines two water contents for the fine fraction of a soil, the liquid limit and the plastic limit, each by a defined laboratory procedure. The plasticity index is the liquid limit minus the plastic limit. The methods apply only to the portion of the soil passing the 425-µm (No. 40) sieve.
Definition and scope
The full title is Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. The scope covers three determinations:
- Liquid limit (LL): a water content, reported to the nearest whole number.
- Plastic limit (PL): a water content, reported to at least the nearest whole number.
- Plasticity index (PI): calculated as PI = LL − PL, so it is a difference between two measured values, not a third test.
The standard offers two ways to prepare the specimen (wet or dry) and two ways to obtain the liquid limit (multipoint or one-point). The requesting authority chooses among them. The plastic limit is tested on material prepared for the liquid limit test. Water contents are determined according to ASTM D2216.
How the methods are used in practice
Method A (multipoint) and Method B (one-point)
Both methods use the same liquid limit device, a cup that is repeatedly dropped so that a groove cut in the soil closes. They differ in how many closures are needed and where they must fall.
| Question | Method A (multipoint) | Method B (one-point) |
|---|---|---|
| Blow range for closure | Three closures: 25 to 35 blows, then 20 to 30, then 15 to 25 | Closure in 20 to 30 blows |
| Starting consistency of the mixed soil | Adjusted to a 25 to 35 blow consistency | Adjusted to a 20 to 30 blow consistency |
| Repeat closure | Further closures in the next blow ranges | A second closure must fall within 2 blows of the first |
| Water content | Determined for each closure per ASTM D2216 | Determined for both closures per ASTM D2216 |
In both methods the groove closes about 13 mm (0.5 in.) while the cup is lifted and dropped twice per second.
Wet and dry preparation
The NICET checklist treats wet preparation as the default and sets out the decision points:
- Dry preparation should only be used when the dry preparation method is specified; otherwise use wet preparation.
- In dry preparation the sample is dried at no more than 60 °C (140 °F).
- In wet preparation, drying toward the liquid limit at room temperature or in warm air currents is the required method for samples containing soluble salts.
- Whichever route is used, the mixed sample is kept in a covered container for at least 16 hours and remixed immediately before the test.
Specimen size
The specimen is 150 to 200 g of material passing the 425-µm (No. 40) sieve. For plastic limit, the sample is 20 g or more of liquid limit material. The plastic limit is the water content at which the soil can no longer be rolled into a thread of 3.2 mm (1/8 in.) diameter without crumbling, and it is calculated by averaging the results of two specimens.
Equipment tolerances a laboratory can check
The apparatus requirements below are laid side by side because summaries rarely list them. The values are those in the NICET checklist for D4318-17e1; they describe what to verify in the device, not how to run a test.
| Item | Requirement in the checklist |
|---|---|
| Height of drop | 10 ± 0.2 mm |
| Brass cup | Mass including hanger of 185 to 215 g |
| Device base | Hard rubber with resilience rebound of 77 to 90%; base dimensions 125 ± 2 mm × 150 ± 2 mm × 50 ± 2 mm |
| Motor drive | 2 ± 0.1 revolutions per second |
| Flat grooving tool | Tip width 1.9 to 2.1 mm; tip depth 7.9 to 8.1 mm |
| Drying oven | 110 ± 5 °C |
| Sieves | 200-mm (8-in.) and 425-µm (No. 40) sieves, each with a rim at least 5 cm (2 in.) above the mesh |
| Weighing | Specimen containers weighed to 0.01 g |
| Wear checks | Cup, cup hanger, cam and base are inspected for wear; the grooving tool is inspected frequently |
Where it matters for testing laboratories
D4318 sits next to other soil standards, and the boundaries are easy to blur. Particle size is a different measurement: see the guide to ASTM D6913 sieve analysis, which covers gradation and notes that the ASTM D2487 classification standard draws on both particle-size characteristics and the liquid limit and plasticity index. D4318 supplies the second half of that pair.
For a testing laboratory, the practical point is that the checklist sets numeric tolerances on the device (drop height, cup mass, motor speed, grooving tool dimensions) and the oven, so the condition of each affects what the lab can claim. Records of calibration, the calibration interval and the calibration certificate are how a lab shows that those items stay in tolerance, and an analytical balance is the instrument behind the 0.01 g weighing requirement.
Common misunderstandings
- “It tests the whole soil.” It does not. Only the portion passing the 425-µm (No. 40) sieve is tested; coarser material is removed.
- “The plasticity index is measured directly.” It is calculated: PI = LL − PL.
- “Any drying method will do.” Not for soils containing soluble salts, where the checklist names room-temperature or warm-air drying as the required route in wet preparation, and not for dry preparation, which should only be used when specified.
- “Method B is just a faster Method A.” It uses a different acceptance rule: two closures within 2 blows of each other in the 20 to 30 blow range, rather than three closures spread across 15 to 35 blows.
- “The plastic limit must break exactly at 3.2 mm.” The operator does not attempt to produce failure at exactly 3.2 mm diameter; the thread is reformed and rolled until it crumbles and can no longer be rolled into a thread.
Frequently asked questions
What does ASTM D4318 measure?
It determines the liquid limit, plastic limit and plasticity index of soils. The scope reads “These test methods cover the determination of the liquid limit, plastic limit, and the plasticity index of soils” (AFNOR listing for D4318-17).
What is the difference between Method A and Method B?
Method A (multipoint) uses closures in three blow ranges, 25 to 35, 20 to 30 and 15 to 25. Method B (one-point) uses closure in 20 to 30 blows, with a second closure within 2 blows of the first.
Is the plasticity index measured directly?
No. It is calculated as PI = liquid limit − plastic limit.
Does D4318 test the whole soil sample?
No. It applies only to the portion passing the 425-µm (No. 40) sieve.
Which edition does this guide describe?
D4318-17e1, as set out in the NICET checklist. The AFNOR store listing for the June 2017 edition, D4318-17, shows the status “Standard Cancelled”, so check which current edition your specification or requesting authority calls for.
Sources
- NICET, Performance Examination – Soils: Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils (ASTM D4318-17e1), form dated 09/2020, checked 29 October 2026: nicet.org checklist (PDF).
- AFNOR, standards store listing for ASTM D4318-17, scope summary, checked 29 October 2026: boutique.afnor.org listing.
- Geocomp, test description for the liquid limit, plastic limit and plasticity index of soils, checked 29 October 2026: geocomp.com test page.
