Survey of methods to determine the plastic limit of soil based on penetration depth and water content in fall cone test

Abstract

Determining the plastic limit of soil is a difficult task in the field of geotechnical engineering. Methods for determining the plastic limit still contain many subjective factors and experimental results have low repeatability. Therefore, it is necessary to research and determine the plastic limit of soil with objective methods to get accurate and highly repeatable results. This article synthesizes previously proposed methods and verifies them. Among the proposed methods, the fall cone method is chosen for research in this article. The implementation of the fall cone method is similar to the experiment to determine the liquid limit. In the previous studies, an assumption was made that the shear resistance at the plastic limit was 100 times greater than the shear resistance at the liquid limit, from which it was deduced that the settlement at the plastic limit by the fall cone method was 2 mm. However, in this paper, the use of the fall cone method is carried out at many different settlement levels when changing the moisture content of the soil sample. The purpose of this is to determine the relationship between penetration depth and the moisture content of each soil sample. This is to determine the penetration of the fall cone at the plastic limit, using the plastic limit value determined from the classical method. As a result of this method, the penetration depth at the plastic limit of the soil according to the relationship between moisture content and the logarithm of penetration depth is in the range of 3.92 mm to 4.31 mm. In other words, this result verifies that the penetration depth at plastic limit is not necessarily at 2 mm, and the shear strength ratio is not equal to 100.