# Atterberg Limits

The liquid limit, plastic limit and plasticity index: what they mean for a fine-grained soil, how the cup and thread tests work, and the one-point method.

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/soil-tests/atterberg-limits/
- Last updated: 2026-11-01
- Standards: AASHTO T 89, AASHTO T 90, ASTM D4318

The water content of a fine-grained soil changes how it behaves. Very wet, a clay flows like a thick liquid; drier, it can be molded like putty; drier still, it crumbles. The [Atterberg limits](https://pavementinteractive.org/glossary/atterberg-limits/) are the water contents at the boundaries between these states. They were proposed by the Swedish scientist Albert Atterberg in the early 1900s and later standardized for engineering use. Highway agencies use them to classify soils (see *Soil Classification*), to judge whether a soil will shrink and swell, and to set limits on the fines in base and subbase materials (Samtani and Nowatzki, 2006[^1]).

The three values used in pavement work are:

- the [liquid limit](https://pavementinteractive.org/glossary/liquid-limit/) (LL), the water content at which the soil changes from plastic to liquid;
- the [plastic limit](https://pavementinteractive.org/glossary/plastic-limit/) (PL), the water content at which it changes from semisolid to plastic; and
- the [plasticity index](https://pavementinteractive.org/glossary/plasticity-index-pi/) (PI), the range of water content over which the soil is plastic.

$$PI = LL - PL$$

A soil with no measurable plastic limit, or one whose plastic limit is equal to or greater than its liquid limit, is reported as **nonplastic** (NP). Clean sands and many silts are nonplastic.

The tests are run on the fraction of the soil passing the 0.425 mm (No. 40) sieve, because it is the fines that give a soil its plasticity. The coarser particles are removed before testing.

## Liquid Limit

The liquid limit is found with a brass cup that is repeatedly lifted and dropped a fixed height by a crank. A pat of moist soil is placed in the cup and a groove is cut through it with a standard grooving tool. The operator turns the crank at a steady rate and counts the blows until the two halves of the soil flow together and close the groove over a short length, about 13 mm (½ inch). A small sample is then taken from the closed groove for an oven-dry water content (AASHTO T 89; Texas DOT, 2024[^2]).

By definition, the liquid limit is the water content at which the groove closes at **25 blows**. Since that rarely happens exactly, the test is repeated at several water contents, and the results are plotted as water content against the logarithm of the number of blows. The points fall close to a straight line, the **flow curve**, and the liquid limit is read where that line crosses 25 blows.

### One-Point Method

Because a multipoint test takes time, both AASHTO and ASTM allow a one-point method. A single determination is made at a blow count close to 25, and the liquid limit is estimated from an empirical relationship:

$$LL = w_N \left(\frac{N}{25}\right)^{0.121}$$

where:

- $$LL$$ = liquid limit, percent
- $$w_N$$ = water content at closure, percent
- $$N$$ = number of blows to close the groove

The correction is small near 25 blows, which is why the method is only allowed within a narrow band of blow counts (Samtani and Nowatzki, 2006[^1]).

## Plastic Limit

The plastic limit test is done by hand. A small ball of the moist soil is rolled between the palm or fingers and a glass plate into a thread. When the thread reaches about 3 mm (⅛ inch) in diameter it is gathered up, kneaded and rolled again. As the soil dries from the handling, it eventually crumbles when rolled to that diameter. The water content at that point is the plastic limit (AASHTO T 90).

The test is simple but operator-dependent: rolling pressure, rate and the judgment of when the thread "crumbles" all affect the result. Laboratories reduce the scatter by training technicians together and by running proficiency samples.

## Using the Results

The plasticity index is the number used most. Some general guidance (Christopher et al., 2006[^3]):

- A **low plasticity index** (below about 10) usually means a silt or a clean granular soil whose strength depends little on water content, though silts can lose strength and pump when wet.
- A **high plasticity index** (above about 20 to 25) usually means a fat clay that is likely to shrink and swell with seasonal moisture changes and to lose strength when wet. See [Shrinking and Swelling Soils](https://pavementinteractive.org/reference-desk/design/design-parameters/shrinking-and-swelling-soils/).
- Base and subbase specifications often limit the liquid limit and plasticity index of the fines so that the layer does not soften when water gets in.

Plotting the liquid limit and plasticity index together on the plasticity chart separates clays from silts, which is how the Unified system classifies fine-grained soils. The liquid limit and plasticity index also feed the AASHTO group index.

A related value, the **liquidity index**, tells you where a soil's natural water content sits within its plastic range:

$$LI = \frac{w - PL}{PI}$$

where:

- $$LI$$ = liquidity index
- $$w$$ = natural water content, percent

A liquidity index near 0 means the soil is near its plastic limit (stiff); near 1 means it is near its liquid limit (soft).

Calculator: Liquid limit, plastic limit and plasticity index — https://pavementinteractive.org/tools/atterberg-limits/

## Standard Test Methods

- AASHTO T 89: Determining the liquid limit of soils
- AASHTO T 90: Determining the plastic limit and plasticity index of soils
- ASTM D4318: Liquid limit, plastic limit and plasticity index of soils

## Sources

[^1]: Samtani, N.C. and Nowatzki, E.A. (2006). Soils and Foundations Reference Manual, Volume I. FHWA-NHI-06-088. National Highway Institute, Federal Highway Administration. Washington, D.C. https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi06088.pdf
[^2]: Texas Department of Transportation (TxDOT) (2024). Liquid Limit of Soils and Base Materials (Tex-104-E). Materials and Tests Division, Texas Department of Transportation. Austin, TX. https://ftp.dot.state.tx.us/pub/txdot-info/cst/TMS/100-E_series/pdfs/soi104.pdf
[^3]: Christopher, B.R.; Schwartz, C. and Boudreau, R. (2006). Geotechnical Aspects of Pavements Reference Manual. FHWA-NHI-05-037. National Highway Institute, Federal Highway Administration. Washington, D.C. https://www.fhwa.dot.gov/engineering/geotech/pubs/05037/05037.pdf