Atterberg Limits: Liquid Limit and Plastic Limit Testing
Published November 2025 · 5 min read · Elipslab Technical Team
What Are Atterberg Limits?
Atterberg limits define the moisture content boundaries at which fine-grained soils (silts and clays) transition between solid, semi-solid, plastic, and liquid states. Three limits are defined:
- Liquid Limit (LL or wL): Moisture content above which soil flows as a liquid under its own weight.
- Plastic Limit (PL or wP): Lowest moisture content at which soil can be rolled into 3 mm threads without crumbling.
- Plasticity Index (PI): PI = LL − PL. Quantifies the range of moisture content over which soil behaves plastically.
These indices are used in soil classification (USCS, AASHTO), foundation design, and fill suitability assessment.
Liquid Limit Test Equipment
Two methods are used under ASTM D4318:
- Casagrande Cup (Method A): The original device — a brass cup on a rubber base, dropped 10 mm with each crank revolution. The liquid limit is the moisture content at which a standard groove closes over 13 mm in 25 blows. Multiple points are plotted and the LL interpolated at 25 blows.
- Fall Cone (Method B / EN ISO 17892-12): An 80 g cone is released from the soil surface; LL is the moisture content at which penetration reaches 20 mm. Preferred in European practice for its better reproducibility.
Plastic Limit Test
No machine is required — the test is entirely manual. Soil is rolled on a glass plate with the palm of the hand until threads of exactly 3 mm diameter form and begin to crumble. The moisture content at crumbling is recorded as PL. Three or more trials are averaged.
Plasticity Chart (USCS Classification)
| LL | PI Position | USCS Symbol |
|---|---|---|
| < 50% | Above A-line | CL (Lean clay) |
| ≥ 50% | Above A-line | CH (Fat clay) |
| < 50% | Below A-line | ML (Silt) |
| ≥ 50% | Below A-line | MH (Elastic silt) |
Atterberg Limit Equipment from Elipslab
Elipslab supplies Casagrande liquid limit devices (manual and motorized), fall cone apparatus, and complete Atterberg limit test sets compliant with ASTM D4318 and EN ISO 17892-12. Manufactured in Ankara, Turkey.