The rapid classification of fine grained soils using fall cones
The empirical concepts for the liquid limit and plastic limit of fine-grained soils have been in use since they were originally defined by Atterberg in 1911, introduced into use by Terzaghi and modified by Casagrande. The liquid limit by fall-cone has been the preferred method since 1975 and is currently described in BS1377: Part 2: (1990). It seems a logical step that the plasticity index should also be determined using the fall-cone. It is at present defined as the mathematical difference between two separate tests:- the current thread rolling plastic limit test and the cone penetrometer liquid limit test, both as described in BS 1377: Part 2: (1990). This research, whilst taking into account the findings of others is empirical in nature. Laboratory testing is carried out by the author to produce data for analysis from samples of natural remoulded UK soils. The initial testing programme involves the determination of cone penetration and undrained shear strength by laboratory shear vane. Determination of plastic limit, (thread-roll method), and liquid limit, (fall-cone method), is also carried out for each soil sample. Other tests include clay content, particle density and dry density. Samples are also prepared and tested for clay mineralogy by x-ray diffraction. A correlation is discovered linking density at the cone penetromer liquid limit with plasticity index. Consequently, additional samples are obtained and tested for index limits and density only. A new method of determining plasticity index from fall cone and density determination is proposed. A survey is conducted whereby volunteers are asked to carry out liquid limit and plastic limit tests on standard soil samples. Additionally, extra weighings are required. Analysis of these data enable a direct comparison to be made between plasticity index determiations produced by the colunteers using both the current method and the proposed new method. It is concluded that the new method, in addition to being faster, gives plasticity test results comparable wioth and more reproducible than results obtained using the current method.