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Technical paper: Determining the consistency limits of high plasticity clays by the BS fall cone method

TW Feng, professor of civil engineering, Chung Yuan Christian University, Chung-li, Taiwan. This paper was first published in GE’s April 2004 edition.


The British Standard (BS 1377:1990)recommends the fall cone method as the preferred technique for determining the liquid limit of soils. The BS fall cone test uses a cup to hold the soil specimen. A major concern in preparation is trapping air in the specimen during filling.

In view of this concern, Feng (2000) recommended replacing the cup with a ring of the same dimensions to facilitate specimen preparation and to improve specimen quality. As the difficulty in specimen preparation is increased with decreasing water content, a conventional consolidation ring, 19mm high and 63.5mm in diameter, was used to prepare specimens for fall cone penetrations less than 10mm. A smaller specimen ring has recently been made so that the volume of soil specimen is greatly reduced.

Therefore two specimen rings are now available for the BS fall cone test. One has the same dimensions as the specimen cup and the other is 20mm high and 20mm in diameter. With these two specimen rings, it is proposed that the BS fall cone apparatus be used to determine both the liquid limit and the plastic limit. In addition, this paper describes the results of applying the proposed fall cone method on four high plasticity clay samples having fall cone liquid limits ranging from 70% to 316%. 

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