Difficulties Regarding Determination of Plasticity Index of Silty Soils by use of Casagrande and Fall Cone Methods

Soil plasticity index and thereby the liquid limit and plastic limit are often used in order to classify the soil type and determine soil properties. Generally, two methods, i.e. the Casagrande cup method and the fall cone method are adopted in order to determine the liquid limit of soil. In many countries, the fall cone is the preferred method to determine the liquid limit due to simplicity and comparative reproducibility. However, some researchers state that the liquid limit obtained by using the fall cone method cannot represent the soil plasticity particularly in soil with either very low or high plasticity. In addition, critics of the fall cone method indicate that the liquid limit can be determined by the fall cone even though it is not possible with the Casagrande cup which is why the soil should be noted as non-plastic. This paper presents how the Casagrande cup method is inapplicable for low plasticity silt due to e.g. problems with cutting the groove. The fall cone method is recommended because the results are more accurate and independent of individual execution. However, the fall cone apparatus also has an applicability limit where it is not possible to determine the liquid limit which is why the silt should be noted as non-plastic. This limit occurs when a clear water level is observed due to quick draining.