Fall Cone Test
Introduction
The Fall Cone Test, also called the cone penetrometer test is an alternate to the Casagrande method for measuring the Liquid Limit of a soil sample. It is often preferred to the Casagrande method because it is more repeatable and less variable with different operators. Thus, the fall cone method is said to eliminate most of the drawbacks of the Casagrande method, and results in improved accuracy and repeatability.
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Objective(s) of the Experiment
This test is carried out to determine the liquid limit and plastic limit of a soil sample using the fall cone method
Procedures
Liquid Limit
In this test the liquid limit is defined as the moisture content at which a standard cone of apex angle 30° and weight of 0.78 N (80gf) will penetrate a distance d = 20 mm in 5 seconds when allowed to drop from a position of point contact with the soil surface (Figure 1). Due to the difficulty in achieving the liquid limit from a single test, four or more tests can be conducted at various moisture contents to determine the fall cone penetration, d.
A semi-logarithmic graph is be plotted with moisture content (w) versus cone penetration d. The plot results in a straight line. The moisture content corresponding to d = 20 mm is the liquid limit as shown in Figure 2.
Plastic Limit
As in the case of liquid limit determination, the fall cone method can be used to obtain the plastic limit. This can be achieved by using a cone of similar geometry but with a mass of 2.35 N (240gf). Three to four tests at varying moisture contents of soil are conducted, and the corresponding cone penetrations (d) are determined. The moisture content corresponding to a cone penetration of d = 20 mm is the plastic limit.
Discussion and Conclusion
References
Credits: Samuel Alalade. Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.