When water is added to dry soil, it changes its state of consistency from hard to soft. If we add water to a fine grained soil, then water will change its consistency from hard to semi hard. If we continue to add more water then again the soil will change its state of consistency from semi hard to plastic and finally reach a liquid consistency stage. When the soil reaches liquid consistency state, it has remain no cohesive strength to retain its shape under its own weight. It will start to deform its shape. So the amount of water which is responsible for this state of consistency of soil is called liquid limit of soil. In other words we can define liquid limit as
“It is the minimum water content at which the soil is still in the liquid state, but has a small shearing strength against flow.”
From test point of view we can define liquid limit as follows.
“Liquid limit is defined as the minimum water content at which a pat of soil cut by a groove of standard dimension will flow together for a distance of 12 mm (1/2 inch) under an impact of 25 blows in the device.”
Why to Know Liquid Limit of Soil?
Liquid limit of soil is a very important property of fine grained soil (or cohesive soil)
Value of liquid limit is used to classify fine grained soil.
It gives us information regarding the state of consistency of soil on site.
Liquid limit of soil can be used to predict the consolidation properties of soil while calculating allowable bearing capacity & settlement of foundation.
Liquid limit value of soil is also used to calculate activity of clays and toughness index of soil.
How to Determine Liquid Limit of Soil?
There are two different methods are available for determining liquid limit of soil. These two methods are as given below.
Liquid limit test of soil using casagrande apparatus
Liquid limit test of soil using cone penetrometer apparatus
1. Liquid Limit Test of Soil Using Casagrande Apparatus
This test is done using casagrande apparatus. In this test a soil paste is formed using soil sample passing 425 micron sieve and water. This paste is then placed in the cup of casagrande device. The soil pat is then divided into two halves by making a groove using the grooving tool. Then the cup is dropped by turning the crank until two halves of the soil cake come in contact with each other. The test is repeated by adding different amount of water with the soil. Finally by plotting a graph we can determine the liquid limit of soil.
2. Liquid Limit Test of Soil Using Cone Penetrometer Apparatus
Prepare a soil paste by mixing soil sample passing 425 micron sieve & water. Transfer this paste to the cup of cone penetrometer apparatus. Set the penetrometer in such a way that the cone just touches the surface of the soil paste. Then release the cone to penetrate into the soil paste. Repeat this test by adding different amount of water with the soil. Then plot a graph and determine the liquid limit of soil.