Mohajerani method: Tool for determining the liquid limit of soils using fall cone test results with strong correlation with the Casagrande test

Abstract

The European Standard EN ISO 17892-12:2018 describes the means of determining the liquid limit (LL) of soils using two independent test methods: the Casagrande percussion cup and fall cone test. The first method is characterized mainly by its universality and the second by its simplicity. However, these two methods are known to provide different results. The higher the LL, the greater is the difference between the results. In 2019, Shimobe and Spagnoli established a correlation between the Casagrande and fall cone tests for soils with LL < 150%, based on an analysis of over 100 experimental datasets. However, further studies are required for soils with LL > 150%. Mohajerani method, which combines the universality of the former method and the simplicity of the latter, is applicable for such soils. The main objective of the study was to apply the Mohajerani method to determine the LLs based on fall cone test results and to compare them with those of the Casagrande test (soft base), with focus on soils with LL > 150%. The experimental results showed that for high plasticity soil (containing bentonite, etc.), the differences in the LL between the Casagrande and fall cone tests are significant (ab to relative ratio 30%), and some modification of the fall cone test evaluation is required. This study confirmed excellent agreement between the LL evaluated using the Mohajerani method (i.e., modification of the fall cone test evaluation) and that obtained from the Casagrande test, even for soils with higher LLs. Furthermore, the fall cone test results were used to determine the plastic limit (PL) for those soils with LLs within 20–50% for which the Casagrande LL and standard fall cone LL were correlated (where the latter was not modified by the Mohajerani method). Determination of the PL for these types of soils via the fall cone test results offers credible results comparable to those of the standard thread rolling test.