Falling cone test apparatus as liquid limit determination option in South Africa

Classification of fine soil requires the Atterberg Limits which includes the liquid limit of the material. In the laboratory the plastic limit and linear shrinkage limit can be determined with good repeatability using the standard test methods. However, the liquid limit is the most challenging due to the difficulty of obtaining repeatable results. Two types of liquid limit test apparatus are available in South Africa. Widely known and commonly employed is the Casagrande apparatus. The lesser known test is the falling cone apparatus, currently mainly employed for research, problem soils or other special cases. The author has ten years of experience in commercial soils testing and the falling cone apparatus has been used as the primary liquid limit test. Experience has shown that the apparatus gives reliable and repeatable results and is highly suited for commercial testing. This paper addresses the physical difference between the Casagrande and falling cone apparatuses as well as the differences in the test methods. An overview is given of international and South African specifications for the liquid limit determination. Practical experience with liquid limit determination comparing the Casagrande and the falling cone apparatus results is presented.