Liquid Limit Test of Soil Using Casagrande's Method

The liquid limit test is generally used to determine the moisture content in soil, it helps to find the moisture content at which soil transitions from a plastic to a liquid state. It is normally carried out using Casagrande's method, which was created in the early twentieth century by Italian engineer Atterberg. In this blog, we will present the procedure, advantages, results, and limitations of the Liquid Limit test of soil using Casagrande's method.

Procedure:

1. At first, the sample is collected from the field. It is then sieved using a 425-micrometer sieve and materials passing are taken for the tests.
2. Then take a small amount of soil and mix it with water to make a paste. Mix the soil for about 15 to 30 minutes until a uniform mix is obtained.
3. Keep the mixture in an airtight container or under a humid condition for 24 hours to obtain uniform moisture distribution.
4. Now, the soil is remixed and a portion of it is placed in the metal cup with a spatula, level it by using a spatula or a straight edge to have a minimum depth of the soil as 1cm at the point of the maximum thickness. The excess soil, if any should be transferred to the evaporating dish.
5. Cut a groove in the sample in the cup by using the appropriate tool.
6. Turn the handle of the device at a rate of 2 revolutions per second. After each drop, you'll notice the soil in the cup starts to crack along the groove. Count the blows until the 2 halves of the soil come in continuous contact at the bottom of the groove along about 12mm.
7. A sample is taken from the closed portion of the groove for moisture content determination. The remaining soil is removed from the cup to mix with the original paste adding further water, and the test is repeated for several times.
8. Repeat this process with different moisture contents of the soil sample until you have a range of values for the number of blows it takes to close the groove. Record the number of blows (N) and the water content in each case. It is desirable to have the tests in the blow range of 15 to 35. The liquid limit according to this method, is defined as the moisture content at which 25 blows are required to close the groove.
9. Draw the flow curve between log N and w, and the moisture content corresponding to 25 blows is read from the flow curve as the liquid limit of soil.

Advantages:

• Casagrande's Method is the faster way to find the liquid limit of the soil.
• It's a widely followed standard test method in the field of Civil/ Geotechnical engineering and soil mechanics.

Results:

The primary outcome obtained from conducting this test is the determination of the soil limit. This particular value indicates the moisture content, at which the soil transitions from a solid state to a fully liquid state. It holds importance as it aids in classifying and comprehending the behaviour of the soil. To better illustrate this please refer to the accompanying diagram showcasing the flow curve correlation, between N and w specifically highlighting the limit for N=25.

Limitation:

• The accuracy of the test relies on how the sample reflects the soil conditions, at the site. If the sample doesn't accurately represent it the results might not be reliable.
• Determining when the groove fully closes in this method involves some subjectivity as it can vary based on the user’s judgment.
• This particular test works better for grained soils like clays and silts whereas its effectiveness might be limited for grained soils such as sands and gravels.

To summarize, Liquid Limit Test of Soil Using Casagrande's Method is an approach to assess soil behaviour, in different moisture conditions. However, it has limitations concerning sample representation and subjective aspects during testing.