Flow index-liquid limit relationship by fall-cone tests in clay-sand mixtures

Abstract

The Fall-cone test is a useful method which can be used in evaluation of many soil index and strength identifiers. In this regard, the flow index is calculated as the slope of the plot of water content against natural logarithm of the penetration depth. This is also a fruitful method used to determine the liquid limit of soils, and evaluation of test results may lead to many parameters related to soil plasticity, i.e. using the flow index. In this study, the changes in the liquid limit and plasticity index of a soil as a function of the flow index were investigated. Later, a simple correlation between the plasticity index and the flow index was obtained, and as a result, the plasticity index was directly calculated. The Fallcone test results show that there is an excellent relationship between the plasticity index and the flow index in soils within a wide range of plasticity level and this relationship is significantly affected from mineralogical properties of components of sand-clay mixtures. A linear relationship with R2 = 0.97 (n = 44) was obtained. As a result, both the liquid limit and the plasticity index of the soils are calculated directly based on the results of the Fall-cone test. In addition, this study compared the results of the standard plasticity chart with the new plasticity chart proposed by Vardanega et al. (2022) for the classifying fine-grained soils. It was shown that the new plasticity chart proposed by Vardanega et al. (2022) can also be used in the classification of both fine-grained soils and clay-sand mixtures. The new plasticity chart by Vardanega et al. (2022) provided modifications on standard plasticity chart. The usefulness of the new chart is also verified by the results of this study, experimental results showed that soil classes -for 115 specimens of 125 total- obtained using Fall-cone tests are in accordance with classifications proposed by Vardanega et al. (2022). According to experimental results, the highest flow index was obtained in %100 bentonite soil at 146.47, while the lowest flow index at 7.20 was obtained in a mixture of 90% sand (SW) −10% bentonite. In 100% pure kaolin clay, the liquid limit was 56.09%, the plasticity index was 27.52% and the flow index was 34.55.