Some observations considering undrained shear strength, liquidity index, and fluid/solid ratio of mono-mineralic clays with water–ethanol mixtures

Abstract

The mechanical properties of clays are influenced by the characteristics of the fluid in the pore space. The liquid limit reacts differently to the permittivity, ε, of the fluid: for smectites the slopes are positive, for kaolinite and illite they are negative and smaller. This dissimilarity can be explained by the structural differences between swelling smectites with solvated interlayer cations and nonswelling clay minerals such as kaolinite and illite. Undrained shear strengths, cu, of Ca–smectite, but not Na–smectite, correlate with the actual fluid ratio. Regressing cu against the liquidity index, IL, yields two different regression lines for Na–smectite and Ca–smectite. For the first time it is shown that normalizing cu to the ε of the pore fluid results in a single regression line for both smectitic clay types. As kaolinites and illites possess significantly less exchangeable cations than smectites, this yields significantly smaller ranges for Atterberg limits and reduces the impact of ε on almost pure particle–particle interactions. In addition, the much larger particle sizes of the kaolinite and illite may dominate the undrained shear strengths, as normalization of cu to ε did not change the relationship to either the actual water content or the liquidity index.