Arthur Casagrande developed a standard device for the determination of liquid limit suggesting that at the water content of liquid limit a clay soil has a shear strength of approximately 2.5 kPa. The device consists of a metal cup seated onto a hard rubber base and fixed to a crankshaft arrangement with a handle so that the cup can be alternately raised and dropped.
- The soil is first dried sufficiently to be broken up by a mortar and pestle.
- It is then sieved using a 425-micrometer sieve and materials passing are taken for the limit tests.
- The soil sample is mixed thoroughly with water into a smooth thick paste, and stored in an airtight container for 24 hours to allow the water to mix thoroughly with all the soil grains.
- At the time of testing, the soil is remixed and a portion of it placed in the metal cup. The soil paste is leveled off in the cup to give a maximum depth of 8mm.
- A groove is formed in the soil along the midsection of the cup, using a standard grooving tool.
- The handle is turned at two revolutions per seconds and the number of blows is counted until the two parts of the soil come in continuous contact at the bottom of the groove along about 12mm.
- 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.
- The number of blows and moisture content in each trial are recorded. 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.
- A semi-log plot of the points is obtained with a number of blows as abscissa in logarithmic scale as against moisture content in linear scale. A best-fitting straight line is drawn which is termed as flow curve. The moisture content corresponding to 25 blows is read from the flow curve as the liquid limit of soil.