Determine Liquid Limit of Soil Specimen by Casagrande Method

Determine Liquid Limit of Soil Specimen by Casagrande Method

The liquid limit of soil is defined as the point at which the soil displays characteristics of a liquid with minimal shear strength. This is determined by measuring the water content at which the soil can flow to close a groove within 25 blows using Casagrande’s liquid limit device. However, obtaining precisely 25 blows can be challenging, so multiple tests are conducted. In each test, the number of blows required (N) is recorded, and a semi-log plot is created, with the water content (w) plotted against the logarithm of N. The liquid limit is determined by identifying the water content corresponding to N=25 on the semi-log plot.

Equipment for Liquid Limit Test on Soil

The equipment required for conducting a test using Casagrande’s liquid limit device includes grooving tools of both standard and ASTM types, an oven, an evaporating dish or glass sheet, a spatula, a 425 micron IS sieve, a weighing balance with an accuracy of 0.01g, and a wash bottle. This equipment is essential for accurately measuring the liquid limit of soil samples. The grooving tools are used to create grooves in the soil samples, while the oven is used to dry them. The evaporating dish or glass sheet is used to hold the soil samples during the drying process, and the spatula is used to transfer the samples. The 425 micron IS sieve is used to separate particles of the correct size for testing, while the weighing balance is used to measure the weight of the samples. Finally, the wash bottle is used to moisten the soil samples during testing. Together, these tools enable precise measurements of soil characteristics, which are crucial for understanding soil behavior and making informed decisions about construction projects.

Fig: Casagrande’s Liquid Limit Apparatus

Fig: Casagrande’s Apparatus Details and Tools

Procedure of Liquid Limit Test on Soil

The first step in conducting a liquid limit test is to adjust the drop of the cup of the liquid limit device. This can be done by releasing the two screws at the top and using the handle of the grooving tool or a gauge to adjust the drop to exactly 1 cm at the point of contact on the base. After adjusting the drop, the screws should be tightened to hold the setting in place.

The next step is to take about 120g of the air-dried soil sample that has passed through a 425 micron IS sieve. This soil sample should be thoroughly mixed with distilled water in an evaporating dish or a glass plate until a uniform paste is formed. The mixing should continue for about 15 to 30 minutes, or until a uniform mix is obtained.

Once a uniform mix is achieved, the mix should be kept under humid conditions to obtain uniform moisture distribution for a sufficient period. For some fat clays, this maturing time may be up to 24 hours. After maturing the paste, a portion should be taken and remixed thoroughly before being placed in the cup of the device by a spatula. The soil should be leveled by a spatula or a straight edge to have a minimum depth of 1 cm at the point of the maximum thickness. Any excess soil should be transferred to the evaporating dish.

The next step is to cut a groove in the sample in the cup using the appropriate tool. The grooving tool should be drawn through the paste in the cup along the symmetrical axis, along the diameter through the center line of the cup. The tool should be held perpendicular to the cup. Once the groove is cut, the handle of the device should be turned at a rate of 2 revolutions per second. The number of blows should be counted until the two halves of the soil specimen come in contact at the bottom of the groove along a distance of 12mm due to flow and not by sliding.

After determining the number of blows, a representative sample of the soil should be collected by moving a spatula width-wise from one edge to the other edge of the soil cake at right angles to the groove. This sample should include the portion of the groove in which the soil flowed to close the groove. The remaining soil should be removed from the cup and mixed with the soil left in the evaporating dish.

The water content of the mix in the evaporating dish should be changed by adding more water if the water content is to be increased, or by kneading the soil if the water content is to be decreased. In no case should dry soil be added to reduce the water content. The steps from 4 to 10 should be repeated to determine the number of blows and the water content in each case.

Finally, the flow curve should be drawn between log N and w, and the liquid limit corresponding to N=25 should be determined.

Data sheet for Liquid Limit Test

Sl. No.	Observations and Calculations	Determination No.
		1	2	3
Observation
1	Number of blows (N)
2	Water content can No.
3	Mass of empty can (M1)
4	Mass of can + wet soil (M2)
5	Mass of can + dry soil (M3)
Calculations
6	Mass of water = M2 – M3
7	Mass of dry soil = M3 – M1
8	Water content = w = [(6) / (7)] x 100

 Result of Liquid Limit Test

Draw a flow curve between log N and w. Liquid limit (for N=25) =

Fig: Liquid Limit Flow Curve