Atterberg Limits

From GEOtechref

1 Introduction
2 Laboratory Tests
3 Equipment
4 Derived Limits
5 Possible Errors
6 References

Introduction

A fine-gained soil can exist in any of several states; which state depends on the amount of water in the soil system. When water is added to a dry soil, each particle is covered with a film of adsorbed water. If the addition of water is continued, the thickness of the water film on a particle increases. Increasing the thickness of the water films permits the particles to slide past one another more easily. The behavior of the soil, therefore, is related to the amount of water in the system. Approximately sixty years ago, A. Atterberg defined the boundaries of four states in terms of "limits" as follows:

Liquid limit: The boundary between the liquid and plastic states;
Plastic limit: The boundary between the plastic and semi-solid states;
Shrinkage limit: The boundary between the semi-solid and solid states.

Laboratory Tests

By a series of experiments the various Atterberg Limits of a soil sample can be found. The tests only yield usable results when testing cohesive soils.

The Liquid Limit is found using a Casagrande Device, which consists of a standard dish. This is filled with a soil sample of known water content. A standard-size groove of 13mm (½-inch) is made in the sample. The device is agitated by turning a handle, connected to a cam like system, which lifts the dish a distance before dropping it onto a hard surface. The experiment is stopped when the sides of the groove fail. The moisture content at which it takes 25 drops of the dish to cause the groove to fail is the Liquid Limit.

The Plastic Limit is found by the thread test. It is the water content of a soil such that a thread of the soil crumbles when rolled to 3 mm thick.

The Shrinkage Limit of a soil is the moisture content that is just sufficient to fill the void spaces of a soil when the soil is at the minimum volume it will attain by drying. The laboratory test to determine the shrinkage limit is ASTM D427, and is described here. The Shrinkage Limit is much less commonly used than the Liquid Limit and the Plastic Limit.

Equipment

Liquid limit device and grooving tool
Mixing bowl
Drying cans
Spatula
Large glass plate
Drying oven
Distilled water
Evaporating dishes
Balance (0.01 g sensitivity)

Derived Limits

The values of these limits are used in a number of ways like the Plasticity Index (PI), Liquidity Index (IL), and activity. There is also a close relationship between the limits and properties of a soil such as compressibility, permeability, and strength. This is thought to be very useful because as limit determination is relatively simple, it is more difficult to determine these other properties. Thus, knowledge of the Atterberg Limits not only allows us to identify the type of soil present, but also allows us to make empirical correlations for some other engineering properties.

Plasticity Index

The Plasticity Index (PI) is a numerical measure of the plasticity of a soil. The Plasticity Index quantifies the size of the range of moisture contents that the soil has plastic properties. The Plasticity Index is calculated by subtracting the Plastic Limit from the Liquid Limit: PI = LL - PL. Soils with a high PI tend to be predominantly clay, those with a lower PI tend to be silt, and those with a PI of 0 tend to have little or no silt or clay.

Liquidity Index

The Liquidity Index (LI) is used for scaling the natural water content of a soil sample to the limits. It can be calculated as a ratio of difference between natural water content, plastic limit, and plasticity index: LI=(W-PL)/(LL-PL) where W is the natural water content.

Activity

The Activity (A) of a soil is the PI divided by the percent of clay-sized particles present. Different types of clays have different specific surface areas which controls how much wetting is required to move a soil from one phase to another such as across the Liquid Limit or the Plastic Limit. From the activity one can predict the dominant clay type present in a soil sample. High activity signifies large volume change when wetted and large shrinkage when dried. Soil with high activity are very reactive chemically.

Normally, activity of clay is between 0.75 and 1.25. It is assumed that the plasticity index is approximately equal to the clay fraction (A = 1). When A is less than 0.75, it is considered inactive. When it is greater than 1.25, it is considered active.

Possible Errors

General

The specimen is not representative of the soil. The liquid and plastic limits must be determined using the same mixture of soil as that used for determination of natural water content or for other tests.
Specimen is improperly prepared. The specimens must be thoroughly mixed and be permitted to cure for a sufficient period before testing. Erroneous results may be caused by the loss of colloidal material when removing particles coarser than the No. 40 sieve or by testing air-dried or oven-dried soils.
Inaccurate determination of the water contents would greatly affect the computed liquid and plastic limits because of the small quantities of material available for the water content determinations.
Computational mistakes.

Liquid Limit Test

Improperly constructed or adjusted liquid limit device.
Worn parts of liquid limit device especially at point of contact between the cup and the base, or worn tip of grooving tool.
Soil at the point of contact between the cup and the base. Removing the cup for shaping and grooving the sample will insure that the bottom of the cup and the top of the base are clean. Any soil that has dropped onto the base can be removed with one stroke of the back of the hand before replacing the cup.
Loss of- moisture during the test. Erratic and erroneous results may be caused by drying of some soil mixtures unless the test is performed in a humid room.

Plastic Limit Test

Rolling thread under fingers will break the thread prematurely.
Incorrect final thread diameter. A length of 1/8 in. diameter metal rod close at hand will help in estimating this diameter accurately.
Stopping the rolling process too soon. If there is any doubt as to whether the thread has crumbled sufficiently, it is better to roll the thread once more than to stop the process too soon.