Atterberg Limits: Liquid Limit, Plastic Limit and PI

The Atterberg limits are a set of simple lab tests that describe how a clay or silt reacts to water. Add water to a dry clay and it goes from hard and crumbly, to moldable like modeling clay, to a runny paste. The water contents at those turning points are the limits. Engineers use them to classify soil, to predict whether a subgrade will swell or soften, and to judge whether fines in a base or surface gravel are the binding kind or the troublesome kind. This guide explains each limit, how the tests are run, the plasticity index and what the numbers mean for driveways and roads.

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Where the limits come from

Swedish chemist and agronomist Albert Atterberg defined the consistency limits in 1911. Arthur Casagrande, a pioneer of soil mechanics, later standardized the liquid limit device and the procedures. Today the liquid and plastic limits are run under ASTM D4318 (AASHTO has matching methods), usually on the part of the soil that passes the No. 40 sieve, and the shrinkage limit has its own method, ASTM D4943.

The four states of a fine soil

Water content	State	How it behaves
Below the shrinkage limit	Solid	Drying further does not shrink it
Shrinkage limit to plastic limit	Semi-solid	Crumbles when you try to shape it
Plastic limit to liquid limit	Plastic	Can be molded and holds its shape
Above the liquid limit	Liquid	Flows like a thick slurry

Liquid limit (LL)

The standard device is the Casagrande cup: a brass dish on a cam that lifts it and drops it a set height onto a hard rubber base. A technician mixes the soil with water, spreads it in the cup, cuts a groove down the middle with a grooving tool and turns the crank. The liquid limit is the water content at which 25 drops close the groove along about 1/2 inch (12.7 mm). Since hitting exactly 25 is unlikely, the lab runs several moisture contents, plots water content against the log of the drop count and reads the value at 25. ASTM D4318 also allows a one-point version using a single trial in the 20 to 30 drop range with a correction factor. Much of Europe uses a fall cone instead, which measures how far a standard cone sinks into the soil and depends less on the operator.

Plastic limit (PL)

The plastic limit test is done by hand. A bit of soil is rolled on a flat, non-porous plate into a thread. As it dries from handling, there comes a point where the thread breaks up when it reaches about 1/8 inch (3.2 mm) across. The water content at that point is the plastic limit. If no moisture content lets it reach a 3.2 mm thread, it is reported as non-plastic (NP).

Shrinkage limit (SL)

The shrinkage limit is the water content below which further drying no longer reduces the soil's volume. It is run far less often than the other two.

Plasticity index and other numbers

Plasticity index: PI = LL − PL. The width of the moisture range where the soil is plastic. One common descriptive scale: 0 non-plastic, under 7 slightly plastic, 7 to 17 medium plastic, over 17 highly plastic.
Liquidity index: LI = (w − PL) ÷ (LL − PL), with w as the soil's in-place water content. Near 0 means the soil sits close to its plastic limit; near 1 means close to liquid.
Consistency index: CI = (LL − w) ÷ (LL − PL). LI and CI add up to 1.
Activity: PI divided by the percent of clay-size particles, used to judge how reactive the clay minerals are.

Worked example (made-up numbers)

Suppose a subgrade sample tests at LL = 38 and PL = 21, and the soil in the ground is at 25 percent water. These values are invented.

PI = 38 − 21 = 17, the top of the "medium plastic" band on the scale above.
LI = (25 − 21) ÷ 17 = about 0.24, so the soil is on the stiff side of its plastic range right now.
CI = (38 − 25) ÷ 17 = about 0.76. Check: 0.24 + 0.76 = 1.

A soil like this can be workable when dry but will lose strength if it gets much wetter, which is why drainage and a solid base matter.

What the limits mean for driveways and roads

Subgrade: high LL and PI point to clays that swell, shrink and soften. Options include removing and replacing, lime stabilization, cement treatment, or a thicker base. See paving asphalt on clay soil.
Base course: specs usually cap PI low so the base drains and stays strong when wet. The FHWA gravel roads guide shows base course PI from 0 to 6.
Surface gravel on unpaved roads: a bit of plasticity helps the surface bind into a crust. The same FHWA guide shows surface gravel PI around 4 to 12. More in our gravel road maintenance guide.
Strength tests: the limits do not measure strength by themselves. They are often paired with a sieve analysis for classification and a CBR test for bearing.

A quick field feel (not a test)

You can get a rough sense of a soil's plasticity by rolling a damp pinch between your palms. If it makes a long, thin, flexible thread, there is a lot of clay. If it crumbles right away, it is sandy or silty. That is only a hint; real decisions need lab numbers.

Questions people ask

What are the Atterberg limits?

They are water contents that mark where a fine-grained soil changes behavior: the shrinkage limit, the plastic limit and the liquid limit. Below the plastic limit the soil is crumbly and semi-solid; between plastic and liquid limits it can be molded; above the liquid limit it flows like a thick liquid.

How do you calculate the plasticity index?

Subtract the plastic limit from the liquid limit: PI = LL - PL. For example, made-up results of LL 38 and PL 21 give a PI of 17.

What is a good plasticity index for road base?

Base course is usually kept low in plasticity. The FHWA gravel roads guide shows base course with a PI of 0 to 6, while surface gravel on unpaved roads benefits from some plasticity, about 4 to 12, so it binds. Your state spec governs.

How is the liquid limit test done?

In the Casagrande method, a pat of moist soil goes in a brass cup, a groove is cut with a standard tool and the cup is dropped repeatedly. The liquid limit is the water content at which 25 drops close the groove over half an inch. Labs run several moisture contents and interpolate.

How is the plastic limit found?

A small ball of soil is rolled by hand into a thread on a flat glass or similar surface. The plastic limit is the water content at which the thread starts to crumble when it reaches about 1/8 inch (3.2 mm) in diameter. If it cannot be rolled that thin at any moisture, the soil is non-plastic.

Why do Atterberg limits matter for paving?

High-plasticity clays swell when wet, shrink when dry and lose strength, which can crack and heave pavement. The limits help classify the soil and decide whether it needs removal, lime or cement treatment, or a thicker base.

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