Atterberg Limit Test
Introduction
The Atterberg limits tests are a basic measure of the critical water content of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. As a dry, clayey soil takes on increasing amounts of water, it undergoes distinct changes in behavior and consistency. Depending on the water content of the soil, it can exist in four states: solid, semi-solid, plastic, and liquid states. In each state, the consistency and behavior of the soil is different and consequently, so are its engineering properties. Thus, the boundary between each state can be defined based on the change in the soil’s behavior. The Atterberg limit can be used to distinguish between silt and clays. These limits were created by Albert Atterberg, a Swedish agriculturist and were later refined by Arthur Casagrande. (http://en.m.wikipedia.com, 2017). Soils when wet retain water and some expand in volume. The amount of expansion is related to the ability of the soil to take in water and its structural make-up (the type of atoms present). These tests are mainly used on clayey or silty soils since they are the soils that expand or shrink due to moisture content. Thus, these tests are used widely in the preliminary stages of designing any structure to ensure that the soil will have the correct amount of shear strength and not too much change in volume.
As a hard, rigid solid in the dry state, soil becomes a crumbly (friable) semisolid when a certain moisture content termed the shrinkage limit is reached. If it is an expansive soil, this soil will also begin to swell in volume as this moisture content is exceeded. Increasing the water content beyond the soil’s plastic limit will transform it into a malleable plastic mass, which causes additional swelling. The soil will remain in this state until its liquid limit is exceeded, which causes it to transform into a viscous liquid that flows when jarred.
The liquid limit, plastic limit, and the plasticity index of soils are used extensively to correlate with engineering behavior such as compressibility, hydraulic conductivity, shrink-swell, and shear strength. Atterberg defined four possible states of consistency for soils: liquid, plastic, semi-solid and solid. The liquid limit divides the plastic and liquid states and is defined as the water content at which the soil flows to close a standard size groove when shaken in a standardized device. At this water content the soil has approximate shear strength of 2.5 kPa. The plastic limit separates plastic and semi-solid states. At water contents below the plastic limit the soil cannot be molded without cracking. Hence, on an arbitrary basis, depending on the moisture content, the behavior of soil can be divided into four basic states—solid, semisolid, plastic, and liquid—as shown below.
Now, let us take them one at a time!
1. LIQUID LIMIT TEST
The liquid limit is defined as the water content at which a standard groove cut in the remolded soil sample by a grooving tool closes over a length of 13 mm (0.5 in) at exactly 25 blows of the liquid limit cup falling from a height of 10 mm on a hard rubber base. It is very difficult to mix the soil at the right water content, even after a number of trials. However, if different trials are plotted on a semi-logarithmic scale they should lie on a straight line and the liquid limit could be taken as the value of water content where the line crosses the 25 blows mark. For this reason, in a liquid limit test we try to mix the soil at least three different water contents aiming at blow counts above and below 25. The schematic diagram of the liquid limit device is shown below.
Objective(s) of the Experiment
This practical is carried out to determine the liquid limit of a fine-grained soil.
Equipments and Materials Needed
Procedures
Results and Calculations
2. PLASTIC LIMIT TEST
Objective(s) of the Experiment
This practical is carried out to determine the plastic limit of a fine-grained soil sample.
Equipments and Materials Needed
Procedures
3. LINEAR SHRINKAGE TEST
The shrinkage limit (SL) is the water content where further loss of moisture will not result in any more volume reduction. A sample weighing 150g from a thoroughly mixed portion of bulk material passing 425 micron IS sieve is used (www.civilblog.com, 2017).
Objective(s) of the Experiment
This practical is carried out to determine the linear shrinkage of a fine-grained soil sample.
Equipments and Materials Needed
Procedures
Results and Calculations
The linear shrinkage of the soil is to be calculated as a percentage of the original length of the specimen from the following formula: Linear shrinkage = [1- (Length of oven dried specimen / Initial length of specimen)] x 100. The linear shrinkage of the soil is to be reported to the nearest whole number.
Discussion and Conclusion
From these tests, a very important parameter called Plasticity Index can be calculated, and it gives a lot of information about the soil. It is also used in different soil classification systems.
Plasticity Index
The plasticity index (PI) is a measure of the plasticity of a soil. It is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit. Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no clay or silt.
The plasticity index is calculated as follows:
Using the Plasticity Index
Burmister (1949) classified the plasticity index in a qualitative manner as follows:
References
Credits: Samuel Alalade. Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.