To determine the liquid limit of cohesive soil by Casagrande Apparatus.

Object: To determine the liquid limit of cohesive soil by Casagrande Apparatus.

Standard:

AASHTOT90-86

ASTMD423-82

BS1377:part 4

Apparatus:

• Casagrande liquid limit device
• Grooving tool
• Spatula
• Moisture containers and evaporating dish
• Electronic balance-controlled oven
• No. 40 sieve
• Distilled water

Theory:

• The liquid limit of soil is the minimum water content at which soil flows under its own weight. It may also be defined as the water content at the point of transition from a plastic state to a liquid state.
• Liquid limit is the measure of the shear strength of soil and is widely used for soil identification and soil classification.
• Casagrande liquid limit device consists of a brass cup, base, handle, and grooving tool. The fall of the cup is adjusted to 1 cm. The handle is rotated at the rate of 2 rev/sec. The grooving tool cuts a groove of size 2 mm wide at the bottom, 11 cm wide at the top, and 8 mm high. Each blow of the standard groove corresponds to about 1 g/cm² shear strength.

Procedure:

1. Firstly, adjust the fall of the cup to 1 cm.
2. Take about 200 g of cohesive soil sample and pass it through the No. 40 sieve; mix it thoroughly with distilled water in the evaporating dish so that a uniform soil paste is formed.
3. Take a portion of soil paste in the brass cup so that it is almost half filled. Level off the wet soil with the spatula. The maximum depth of the soil in the cup is about 8 mm.
4. Using a grooving tool, cut a groove along the center line of the soil paste in the cup.
5. Rotate the handle at the rate of 2 rev/sec till the two parts of the soil come into contact and the groove in the soil closes. Note the number of blows.
6. Take some soil samples from the grooved portion in the cup and determine its water content by the oven-drying method.
7. Repeat the procedure by increasing the water in the soil paste to get several blows ranging from 10 to 50.
8. Plot the semi-log graph between the number of blows (N) in the log scale on the x-axis and water content (w%) on the y-axis. The graph is approximately a straight line, which is called the flow curve. The slope of the line is called the flow curve.
9. From this graph, note the water content corresponding to 25 blows. This is a liquid limit of the soil sample.

Observations and calculations:

Results:

The liquid limit of the given soil sample is.

Comments: