Atterberg Limits Explained: Liquid, Plastic and Shrinkage Limits
The moisture-content boundaries between a soil's solid, plastic and liquid states
Three limits, three state boundaries
Fine-grained soil behaves differently depending on its moisture content: as a brittle solid when very dry, as a plastic (moldable) material at intermediate moisture, and as a liquid-like slurry when very wet. The Atterberg limits (commonly tested per a standard such as ASTM D4318) mark the moisture-content boundaries between these states: the shrinkage limit, plastic limit, and liquid limit, in increasing order of moisture content.
Liquid limit and plastic limit testing
The liquid limit is typically determined using a standardized device (such as the Casagrande cup) that measures the moisture content at which a soil pat closes a specific groove after a set number of drops. The plastic limit is the moisture content at which a soil, rolled into a thread, just begins to crumble at a standard thread diameter (commonly about 1/8 inch, or 3 mm).
The plasticity index
The plasticity index is simply the liquid limit minus the plastic limit — the range of moisture content over which the soil behaves plastically. A high plasticity index indicates a soil that stays workable/moldable over a wide moisture range (often a clay-rich soil); a low or zero plasticity index (the soil crumbles before it becomes plastic) is typical of silts or sands.
Why it matters for construction
Atterberg limits are core inputs to the Unified Soil Classification System and AASHTO soil classification, which in turn inform decisions about a soil's suitability as fill, subgrade, or foundation material, and how it's likely to behave with changes in moisture (shrink-swell potential).
Worked Example
A soil sample has a liquid limit of 42% and a plastic limit of 24%.
Plasticity index = 42 − 24 = 18
Related Calculators
Related Guides
From the Blog
Sources
ASTM D4318
Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.