# LIQUID LIMIT OF SOIL – WHAT, WHY & HOW?

## What is the liquid limit of soil?

When water is added to dry soil, it changes its state of stability from hard to soft. If we add water to fine grained soil, the water will change its consistency from hard to semi hard. If we keep adding more water then the clay will again change its consistency state from semi hard to plastic and finally reach a liquid consistency state. When clay reaches a state of liquid consistency, it has no cohesive force to maintain its shape under its own weight. It will begin to deform its shape. So the amount of water which is responsible for this state of stability of the soil is called soil liquid border, In other words we can define the liquid limit as:

“This is the minimum water content at which the soil is still in the liquid state, but has a small shear strength against flow.”

From the test point of view we can define the liquid limit as follows.

“Liquid limit is defined as the minimum amount of water at which a pat of soil cut by a groove of standard dimension shall be applied for a distance of 12 mm (1/2 inch) under the effect of 25 blows in the device. will flow along.”

## Why know the liquid limit of the soil?

1. The liquid boundary of the soil is a very important property of fine-grained soils (or cohesive soils).
2. The value of the liquid limit is used to classify fine-grained soils.
3. It gives us information about the stability status of the soil at the site.
4. The liquid limit of a soil can be used to predict the consolidation properties of a soil when calculating the allowable bearing capacity and settlement of foundations.
5. Soil liquid limit value is also used to calculate soil activity and soil hardness index.

## How to determine the liquid limit of the soil?

Two different methods are available to determine the liquid limit of a soil. These two methods are given below.

1. Soil Liquid Limit Testing Using the Casagrande Apparatus
2. Liquid limit test of soil using cone penetrometer instrument

### 1. Liquid Limit Test of Soil Using the Casagrande Apparatus

This test is performed using the Casagrande instrument. In this test a soil paste is made using a soil sample from a 425 micron sieve and water. This paste is then placed in the cup of the Casagrande device. The clay pat is then divided into two halves by making a groove using the grooving tool. The cup is then dropped by turning the crank until the two halves of the clay cake come into contact with each other. The test is repeated by mixing varying amounts of water with the soil. Finally by drawing a graph we can determine the liquid limit of the soil.

Click here to read a step-by-step procedure for a soil liquid limit test using the Casagrande instrument.

### 2. Liquid limit test of soil using cone penetrometer instrument

Prepare a clay paste by mixing a soil sample with a 425 µm sieve and passing water. Transfer this paste to the cup of the cone penetrometer instrument. Set the penetrometer in such a way that the cone touches the surface of the clay paste. Then leave the cone to soak into the clay paste. Repeat this test by adding different amounts of water to the soil. Then draw a graph and determine the liquid limit of the soil.