Theory of liquid and plastic limits for fine soils, methods of determination and outlook

Albert Atterberg introduced various consistency limits (state transitions) for fine-grained soil in the 1910s. Of these, the liquid limit (LL) and plastic limit (PL) are ubiquitous in geotechnical engineering practice, including their usage for soil classification purposes and in deducing useful geotechnical parameters via correlations. Given it is about 120 years since first introduced, it seems timely to critically review the current state of play regarding various definitions and theory of these index parameters and their standardised testing methods, as described in majorly used codes worldwide. Because different codes allow different LL apparatus types and employ some dissimilar criteria (e.g., in establishing the end-point for the PL test), a change in method or code may produce different results for testing the same soil. These differences are rationalised in terms of the controlling parameter(s). Some potential pitfalls in consistency limits testing are highlighted. Attention then turn to strength-based approaches, mostly employing fall cone (FC) setups, emphasising their unsuitability for determining Atterberg's PL. Considering the PL test's general poor reproducibility, the paper concludes with an alternative way forward, obtaining useful FC index parameters that provide new possibilities for strength predictions and in the classification of fine-grained soils.