DETERMINATION OF THE LIQUID LIMIT, PLASTIC LIMIT AND PLASTICITY INDEX OF THE SOIL

TITLE:

DETERMINATION OF THE LIQUID LIMIT, PLASTIC LIMIT AND PLASTICITY INDEX OF THE SOIL.

OBJECTIVES:

• To determine the liquid limit, plastic limits and plasticity index of the soil sample.

• To produce data within the acceptable range of errors as required by the lab instructor. 

THEORY:

The physical state of fine grained soils at particular water content is known as its consistency.

 Liquid limit is defined as the boundary of water between the liquids and plastic state of the soils. The liquid limit is the minimum water content at which the soil is still in liquid states but has a small shearing strength against flowing.

 Experimentally, liquid limit is the minimum water content at which a part of soil cut by a groove of standard dimension flows together a distance of 12 mm on the impact of 25 number of blows.

Plastic limit is defined as the boundaries of water bet....Show More

TITLE:

DETERMINATION OF THE LIQUID LIMIT, PLASTIC LIMIT AND PLASTICITY INDEX OF THE SOIL.

OBJECTIVES:

• To determine the liquid limit, plastic limits and plasticity index of the soil sample.

• To produce data within the acceptable range of errors as required by the lab instructor. 

THEORY:

The physical state of fine grained soils at particular water content is known as its consistency.

 Liquid limit is defined as the boundary of water between the liquids and plastic state of the soils. The liquid limit is the minimum water content at which the soil is still in liquid states but has a small shearing strength against flowing.

 Experimentally, liquid limit is the minimum water content at which a part of soil cut by a groove of standard dimension flows together a distance of 12 mm on the impact of 25 number of blows.

Plastic limit is defined as the boundaries of water between the plastic state and the semi solid state of the soil. It is also defined as the minimum water contents at which possible will just bring to grumble (crack) when rolled into a thread approximately 3 mm in diameter. 

Plasticity index is defined as the range of water contents over which the soil is in a plastic state and is expressed as:

`I_p=W_L-W_p`

Where,

`W_L=`liquid limit

`W_p=`plastic limit

APPARATUS REQUIRED:

For Liquid limit test:

1. Casagrande cup,

2. Flat grooving tool,

3.  Gauge,

4. Water content containers, 

5. Mixing and storage container, 

6. Balance.

For Plastic limit test:

1. Sieves,

2. Glass plate.

3. Drying oven,

4. Container with lid

PROCEDURES:

The liquid limit is determined in the laboratory using the standard liquid limit apparatus i.e, Casagrande liquid limit device. The Apparatus consists of a brass up which could be raised and allowed to fall on a hard rubber base by turning the handle.

The soil paste prepared by mixing some water to the materials passing through the 0.425 mm sieve is placed in the cup and levelled horizontally. The soil in the cup is divided into two halves by cutting a groove along the diameter through the pivot of the cup by means of standard grooving tool. The handle of the liquid limit device is rotated at the rate of 2 revolution per second. Suddenly, the two halves of the soil in the cup gradually starts flowing towards the bottom of the cup. The number of blows required to close the bottom of the groove through a distance of 12 mm is recorded. The test is repeated at least four times and the water content is increased in each test.

A graph is plotted between water content and logarithm of blows and the best straight line fitting the plotted points are drawn.

 To determine the plastic limit, dried soil passing through 0.425 mm sieve is mixed with clean water until it becomes sufficiently plastics to be moulded into a ball. About 2.5 gram of soil paste is formed into thread of about 6 mm in diameter by rolling between the first finger and thumb of each hand. The thread is then placed on a glass plate and rolled with the tips of the finger of one hand until its diameter is reduced to about 3 mm. The thread is then remoulded between the finger and the procedure is repeated until the thread of the soil shears both longitudinally and laterally. The procedure is repeated for three more test and the water content of all the crumbled soil is determined as a whole. The water content so obtained is the plastic limit of the soil. 

If the plastic limit cannot be determined for a given soil or if the plastic limit is equal to or greater than the liquid limit, the soil is said to be non plastic.

OBSERVATIONS:

For Plasticity test,

CALCULATIONS:

The flow curve with water as ordinate and log of number of blows as abscissa is drawn. The water corresponding to 25 number of blows is taken as the liquid limit of the soil.

Thus, from graph liquid limit of soil `=31%`

Now for plastic limit,

Thus, mass of water is `=0.0128-0.0125=0.0003`

Mass of solid is `=0.0125-0.0109=0.0016`

Water content `=0.0003/0.0016*100=18.75%`

Thus, plastic limit is `18.75%`.

Now,

Plasticity index `=W_L-W_p=31-18.75=12.25%`

RESULTS: 

Thus the liquid limit was found to be `31%` and that of plastic limit was found to be `18.75%`.

SOURCES OF ERRORS:

• There may be an error on the computation of moisture content. 

• There may be some error in weight measurement.

PRECAUTIONS:

• It is recommended to use a scale of higher accuracy especially when measuring moisture content of a small mass.

• While drawing, the temperature of oven should not be more than `105^0C`.

CONCLUSIONS:

After through analysis of the atterberg limit of soil sample, it is concluded that the soil is classified as clay.