What Are Atterberg Limits?

The behavior they describe

Fine-grained soils, especially clays, change behavior dramatically with moisture content — brittle and solid when dry, moldable and plastic at moderate moisture, and essentially a liquid slurry when saturated. Atterberg limits pin down the moisture contents at the boundaries between these states for a given soil. Each limit is a water content, expressed as a percentage of the dry soil mass.

The definitions below follow the FHWA's Geotechnical Aspects of Pavements reference manual (Table 5-21), which also describes the limits as approximate and empirical: they are index values that are useful for comparing and classifying soils, not direct measurements of strength.

• Shrinkage limit (SL) — the water content below which no further volume change occurs with additional drying.
• Plastic limit (PL) — the water content at which a thread of soil rolled down to a diameter of ⅛ in (3 mm) crumbles.
• Liquid limit (LL) — the water content at which 25 blows of the liquid-limit device close a standard groove cut in the soil pat over a distance of ½ in (12.7 mm).
• Plasticity index (PI) = LL − PL — the range of moisture over which the soil behaves plastically.

How the limits are measured, in brief

The tests are run on the portion of the soil passing the No. 40 sieve. The liquid and plastic limits are covered by ASTM D4318 and by AASHTO T 89 (liquid limit) and T 90 (plastic limit). The shrinkage limit is a separate test: AASHTO T 92, or ASTM D4943, which measures a dried soil pat's volume by water submersion. ASTM D427, the older mercury method that the FHWA manual still lists, is reported to have been withdrawn.

Each limit comes from weighing a specimen wet and again after oven-drying, so the arithmetic is the same water-content calculation used for any soil sample. The step-by-step calculation from test masses, including the one-point liquid-limit correction and how results are reported, is in the Atterberg limits guide, and the Atterberg Limits Calculator runs it from raw masses.

What the plasticity index tells you

A soil's plasticity, captured by the gap between its liquid and plastic limits, correlates with how it will behave as fill, subgrade or foundation material. High-plasticity clays are often prone to significant shrink-swell with moisture changes, which can affect pavement and foundation performance if it isn't accounted for.

The FHWA manual reproduces a simple scale after Sowers (1979) that ties the plasticity index to a description and to how a dry sample behaves in the hand.

The liquidity index: where the natural moisture sits

The limits also put a soil's natural moisture content in context. The liquidity index, LI = (w − PL) ÷ PI, where w is the natural moisture content, places the in-place soil between the plastic limit (LI = 0) and the liquid limit (LI = 1). A value between 0 and 1 means the soil is in its plastic range at its current moisture; a value below 0 means it is drier than its plastic limit.

Natural moisture content comes from a separate moisture-content test — see What Is Soil Moisture Content?.

Reading a set of results

Suppose a lab reports LL = 43 and PL = 21 for a soil whose natural moisture content is 30%.

• Plasticity index: PI = 43 − 21 = 22, which is in the 15–30 "medium plastic" band of the Sowers scale.
• Liquidity index: LI = (30 − 21) ÷ 22 ≈ 0.41, so in place the soil is plastic, well short of its liquid limit.
• Classification: LL is below 50 and PI is above 7, so if the point plots on or above the A-line of the USCS plasticity chart, the fines classify as lean clay (CL).

Feeding into soil classification

Atterberg limit results, combined with grain-size data from a sieve analysis, are used to classify soils under the Unified Soil Classification System (USCS, ASTM D2487) or the AASHTO system — standard categories that let engineers quickly communicate a soil's likely engineering behavior.

Where the results show up on a project

Pavement designers use PI to decide whether a subgrade needs lime or cement stabilization; foundation engineers use it to flag expansive soils; specifications for base and fill materials often cap the PI of the fines. The FHWA manual also notes that the optimum compaction moisture content is often in the vicinity of the plastic limit.

Common mistakes

• Quoting a plasticity band without its source — FHWA's scale (after Sowers) and other published scales use different boundaries.
• Treating the limits as precise soil properties; they are empirical index values from standardized tests.
• Testing the whole sample instead of the portion passing the No. 40 sieve.
• Classifying a soil from its limits alone; USCS also needs the grain-size split from a sieve analysis.
• Measuring moisture content on too small a sample — see What Is Soil Moisture Content?.