Atterberg Limits – its 3 [Limits and Their Procedures]

The Atterberg limits are used for the determination of the properties of cohesive soils at different moisture contents and to perform liquid limit, plastic limit, and shrinkage limit tests, Gilson carries all the equipment needed.

At varying degrees of moisture content, the consistency and behavior of a clayey soil is different as are the engineering properties, and based on a change in the clay’s behavior the boundary between each state can be defined.

To define the limits of soil consistency for the classification of fine-grained soils, the Swedish scientist Albert Atterberg was the first person to define this.

The soil may be in one of four states such as solid, semi-solid, plastic, and liquid depending on the water content of soil and to determine the liquid limit, plastic limit and shrinkage limit of soils Atterberg limits methods are used and these Atterberg limits are defined in ASTM Standard D4318.

Limits of Atterberg Limits:

There are three limits in Atterberg limits methods such as;

1. Liquid Limit
2. Plastic Limit
3. Shrinkage Limit.

Liquid Limit:

The liquid limit is one of the atterberg limits and this test determines the water content at which the behavior of clayey soil changes from plastic to liquid OR It is the minimum moisture content at which a soil flows upon application of very small shear force and this limit is also known as an upper plastic limit.

Determination of Liquid Limit of Soil:

In the laboratory with the help of the standard liquid limit apparatus designed by Casagrande, the liquid limit is determined.

Apparatus;

1. Casagrande apparatus
2. 40 sieve
3. Spatula
4. Containers and tray
5. Electric oven and balance.

Procedure:

1. In the evaporation dish or on a marble plate to form a uniform paste, about 120 g of the specimen passing through 425-micron sieve is mixed thoroughly with distilled water.
2. In the cup, a portion of the paste is placed over the spot where the cup rests on the base, spread into position, and squeezed down and in the soil pat, the groove is cut.
3. At a rate about 2 revolutions per second, the handle is rotated and until the two parts of the soil sample come into contact at the bottom of the groove along with a distance of 10 mm, the number of blows is counted.
4. For water content determination, approximately 10 gram of soil from near the closed groove is taken after recording the number of blows.

When the groove should close in 25 blows, it is difficult to adjust the water content precisely equal to the liquid limit, and by plotting a graph between the numbers of blows, the liquid limit is determined.

Plastic Limit:

The minimum water content at percent on dry bases at which a soil enters from semi-solid state to plastic state is known as plastic limit and this test involves rolling out a thread of the fine portion of soil on a non-porous, flat surface. the plastic limit is one of the limits of atterberg limits.

Determination of Plastic Limit of Soil:

Apparatus;

In the Waterberg limits the plastic limit consist of the following apparatus;

1. 2 x 2 feet of the glass plate.
2. Spatula and No. 40 sieve.
3. Balance and electric oven.

Procedure;

1. The plastic limit is determined with the soil specimen which is passing 425-micron sieve and with distilled water, it is mixed thoroughly until the soil mass becomes plastic enough to be easily molded with fingers.
2. To allow water to permeate through the soil mass, the plastic soil mass should be left for enough time and with about 8 g of this plastic soil mass a ball is formed and with just sufficient pressure it is rolled between the fingers and a glass plate.
3. The soil is re-molded again into a ball when a diameter of 3 mm is reached and until the thread starts just crumbling at a diameter of 3 mm this process of rolling and re-molding is repeated.
4. For water content determination, the crumbled threads are kept and with fresh samples, the test is repeated twice more.
5. As the average of three water contents, the plastic limit as PL is taken.

Form the relation given below the plasticity index is calculated;

PL = LL – PL

Shrinkage Limit:

The maximum water content at which a reduction in water content will not cause a decrease in the mass of soil volume is known as shrinkage limit and this is the lowest water content and one of the atterberg limits.

Determination of Shrinkage Limit of Soil:

Apparatus;

In the shrinkage limit of atterberg limits, there are the following apparatus such as;

1. 12 cm in diameter with a flat bottom a porcelain evaporating dish.
2. 45 mm in diameter and 15 mm in height, with a flat bottom a stainless steel shrinkage dish.
3. 75 x 75 mm two glass plates, having three metal prongs and other of plain glass.
4. 50 mm in diameter and 25 mm in height a glass cup with smooth and level ground.

Procedure;

1. By filling dish to overflow with mercury, the volume V1 of the shrinkage dish is first determined and by pressing a flat glass plate over its top removing the excess and the mass is taken from the dish filled with mercury.
2. The volume of the dish is determined by the mass of the mercury contained in the dish, divided by its density of 13.6 g/cm3.
3. To fill the voids completely and to make the soil pasty enough to be readily worked into the shrinkage dish without the inclusion of air-bubbles, about 50 g of soil passing 425 microns IS sieve is mixed with distilled water.
4. With a thin layer of Vaseline, the inside of the shrinkage dish is coated and in its center, a volume of wet soil of about one-third the volume of the dish is put and by tapping it gently on a hard surface the soil is caused to flow to the edges.
5. To exclude the inclusion of air, the dish is gradually filled by adding more soil in installments followed by gently tapping and all soil adhering to the outside of the dish is wiped off.
6. Then the dish filled with soil is weighed and by subtracting the mass of the empty dish from the mass of the wet soil, the mass M1 of the wet soil pat of volume V1 is determined and the dish is placed in the oven.
7. on drying, the soil pat will have volumetric shrinkage and from the equation given below the shrinkage limit is then calculated;