Atterberg Limits

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 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, 2006¹).

The three values used in pavement work are:

• the liquid limit (LL), the water content at which the soil changes from plastic to liquid;
• the plastic limit (PL), the water content at which it changes from semisolid to plastic; and
• the plasticity index (PI), the range of water content over which the soil is plastic.

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, 2024²).

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:

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, 2006¹).

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., 2006³):

• 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.
• 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:

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

AASHTO T 89 · T 90

The liquid limit from a multipoint flow curve or one closure, the plastic limit, and PI.

Liquid limit (T 89)

Liquid limit methodMultipoint flow curve (Method A)One point (Method B)

Liquid limit trials

Trial	Blows, N	Moisture, %
1
2
3
4

Aim for closures from about 15 to 35 blows, spread on both sides of 25.

Plastic limit (T 90)

Plastic limitMeasured (enter the trials)Couldn’t be rolled out: nonplastic

Plastic limit trials

Trial	Moisture, %
1
2
3

Natural moisture content, % (optional)

Gives the liquidity index.

Specification

Maximum liquid limit (optional)

From your specification for the material, if it sets one.

Maximum plasticity index (optional)

From your specification for the material, if it sets one.

CalculateExample values. Moisture contents are percent of oven-dry mass.

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

@misc{pavint-atterberg-limits,
  author = {{Pavement Interactive}},
  title = {Atterberg Limits},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/soil-tests/atterberg-limits/}},
  note = {Accessed [date]}
}