Determine Liquid Limit of Soil Specimen by Casagrande Method
The liquid limit of a soil is the water content at which the soil behaves practically like a liquid, but has small shear strength. It flows to close the groove in just 25 blows in Casagrande’s liquid limit device.
As it is difficult to get exactly 25 blows in a test, 3 to 4 tests are conducted and the number of blows (N) required in each test is determined. A semi-log plot is then drawn between log N and the water content (w). The liquid limit is the water content corresponding to N=25, as obtained from the plot.
Equipment for Liquid Limit Test on Soil
- Casagrande’s liquid limit device
- Grooving tools of both standard and ASTM types
- Evaporating dish or glass sheet
- 425 micron IS sieve
- Weighing balance accuracy 0.01g.
- Wash bottle.
Fig: Casagrande’s Liquid Limit Apparatus
Fig: Casagrande’s Apparatus Details and Tools
Procedure of Liquid Limit Test on Soil
1. Adjust the drop of the cup of the liquid limit device by releasing the two screws at the top and by using the handle of the grooving tool or a gauge. The drop should be exactly 1 cm at the point of contact on the base. Tighten the screw after adjustment.
2. Take about 120g of the air-dried soil sample passing 425 micron IS sieve.
3. Mix the sample thoroughly with distilled water in an evaporating dish or a glass plate to form a uniform paste. Mixing should be continued for about 15 to 30 min, till a uniform mix is obtained.
4. Keep the mix under humid conditions for obtaining uniform moisture distribution for sufficient period. For some fat clays. This maturing time may be upto 24 hours.
5. Take a portion of the matured paste and remix it thoroughly. Place it in the cup of the device by a spatula and level it by 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.
6. Cut a groove in the sample in the cup by using the appropriate tool. Draw the grooving tool through the paste in the cup along the symmetrical axis, along the diameter through the centre line of the cup. Hold the tool perpendicular to the cup.
7. Turn the handle of the device at a rate of 2 revolutions per second. Count the number of blows 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.
8. Collect a representative sample of the soil by moving spatula width-wise from one edge to the other edge of the soil cake at right angles to the groove. This should include the portion of the groove in which the soil flowed to close the groove.
9. Remove the remaining soil from the cup. Mix it with the soil left in evaporating dish.
10. Change the water content of the mix in the evaporating dish either 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 the dry soil should be added to reduce the water content.
11. Repeat the steps 4 to 10 and determine the number of blows (N) and the water content in each case.
12. Draw the flow curve between log N and w, and determine the liquid limit corresponding to N=25.
Data sheet for Liquid Limit Test
|Sl. No.||Observations and Calculations||Determination No.|
|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)|
|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