Plasticity index & Atterberg limits

What do the liquid limit, plastic limit and plasticity index tell you about a soil?

They describe the water contents at which a fine-grained soil changes how it behaves. The liquid limit is the water content at which it starts to flow and the plastic limit the water content at which it stops being mouldable and crumbles. The plasticity index is simply the gap between the two. A wide gap means a clay-rich soil that stays workable over a broad moisture range and moves as it wets and dries.

How do you measure the liquid limit?

With a cone penetrometer. An 80 g cone with a 30 degree tip is released onto the surface of remoulded soil in a cup for five seconds, and the water content that gives a penetration of 20 mm is the liquid limit. The standard method takes readings at four different water contents and reads the limit off the plot. The Casagrande percussion apparatus is an accepted alternative where a specification calls for it.

How is the plastic limit determined?

By hand. A small piece of soil is rolled into a thread on a glass plate until it crumbles, and the plastic limit is the water content at which the thread first crumbles at a diameter of 3 mm. It is a simple test but a judgement-based one, so it is carried out on repeat portions and averaged. Soils that will not form a 3 mm thread at any water content are reported as non-plastic.

Which standard is Atterberg limits testing usually carried out to?

Commonly to BS 1377-2 or to BS EN ISO 17892-12, the Eurocode-era standard covering both the liquid and the plastic limit. Both allow the cone penetrometer and the Casagrande apparatus for the liquid limit. Some specifications also accept a one-point liquid limit rather than the full four-point determination, so tell us which standard and which variant your testing schedule names.

How much material do you need, and should it be dried before it is sent?

We need enough to yield roughly 200 to 300 g passing a 425 micron sieve, because the test is carried out on that fraction alone. For a clay that is a small bag; for a sandy or gravelly soil it can take several kilograms to get there. Do not oven dry it first. Pre-drying can change the limits of some clays irreversibly, so send it sealed at its natural moisture content.

What happens if the soil turns out to be non-plastic?

The report says non-plastic and no plasticity index can be calculated. That is a valid result rather than a failed test: clean sands and gravels, and some rock flours and silts, have no plastic range to measure. If your specification sets a maximum plasticity index, a non-plastic result satisfies it. If it sets a minimum, or assumes a cohesive material, the result is telling you something useful about the soil.

How is the plasticity index used to classify a soil?

The liquid limit and plasticity index are plotted together on a plasticity chart. Material plotting above the A-line is described as a clay and material below it as a silt, while the liquid limit sets the descriptive band: low below 35, intermediate 35 to 50, high 50 to 70, very high 70 to 90 and extremely high above 90. That is how a soil becomes, for example, a clay of high plasticity.

What does a high plasticity index mean for foundations and earthworks?

It signals a soil that shrinks and swells appreciably as moisture changes, which matters for foundation depth, for shallow services and for anything built near trees. NHBC guidance on building near trees works from a modified plasticity index, the plasticity index multiplied by the proportion of soil passing 425 microns, and takes volume change potential as high at 40 per cent or more, medium from 20 and low from 10.

Are the Atterberg limits used to decide whether material is acceptable as fill?

Yes, regularly. Earthworks specifications classify fill partly on plasticity, and the Specification for Highway Works treats material with a liquid limit exceeding 90 or a plasticity index exceeding 65 as unacceptable. Plasticity is also read against the natural moisture content, since cohesive fill that is wet relative to its plastic limit will not compact and cannot be trafficked. Grading and moisture content are normally tested at the same time.

How long does the testing take, and what slows it down?

Preparation takes longer than the testing itself. The material has to be wet sieved through 425 microns and the fines recovered and brought back to a workable condition, which for a heavy clay can take several days before any limit is determined. A four-point liquid limit takes longer than a one-point. Sandy soils that yield little material passing 425 microns can need reworking. We confirm the expected timing before the work is booked.

Does UKAS accreditation apply to liquid limit and plastic limit testing?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so it depends on the standard and the variant your specification names, given that the cone penetrometer and Casagrande methods are separate entries. Ask us to confirm the status of the testing you need and the report will state it result by result.

What should we include in an enquiry for Atterberg limits testing?

The material type and a description, the number of samples, the standard and method variant if your schedule names one, your preferred dates and the site postcode if samples need collecting. Say whether the plasticity index alone is required or the liquid limit and plastic limit are to be reported separately. Sample numbers and the preparation each one needs are the main things that move the price.