Liquid limit of a soil is defined as the water content at which the soil mass changes its states from plastic state to liquid state.
Procedure for determining Liquid Limit: The liquid limit determining procedure suggested by Arthur Casagrande is the mostly used and followed procedures for determining the liquid limit of a soil mass. Apparatus Required for determining Liquid Limit: 1. Artur Casagrande Liquid Limit Device which consists of a brass cup attached with a crank at its one end. The brass cup can be dropped on a hard rubber base from a certain height by the help of the crank. 2. Casagrande suggested Groove.
Procedure of performing the Test:
- To perform the test at first the soil is required to place on the apparatus with a flat surface on the top.
- A groove of around 10 mm is then cut into the soil paste on the apparatus by the standard grooving tool.
- The brass cup is then dropped from a height of 10 mm for several times by the help of crank until the groove in the paste meet around 12.7 mm(0.5 inch)
- After meeting for about 12.7 mm at the bottom of the cup, the soil is collected and the respective moisture content of the soil is noted down.
N.B. The moisture content at which the soil pat at the bottom of the cup meets for 12.7 mm for 25 blows in the appartus is termed as liquid limit according to Casagrande.
It is very difficult to perfectly match the liquid limit for 25 blows in the apparatus. Therefore at least three samples for same soil were taken and the test procedures were conducted with different moisture content.
After then a graph were plotted showing log of blows in the horizontal axis and respective moisture content in the vertical axis of the graph.
A straight line is obtained which is termed as flow curve. The slope of the flow curve is termed as flow index.
Moisture content for 25 blows were determined and are termed as Liquid Limit.
Another method is also available in Asia and Europe named as Fall Cone Method for determining of liquid limit.