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

Abstract

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.

Determining Liquid Limit and Plastic Limit of Clay Soils by Electrical Surface Conduction and Diffuse Double Layer Thickness

“…Liquid limit and plastic limit are major geotechnical properties, as these measurements indicate how much water can be retained in a specific clay sample before it goes through a transition from a plastic state to a liquid state. In other words, the liquid limit is defined as the states where clay soils start to behave like liquid, while the plastic limit is described as a state featuring a specific water content where the soil goes through the transition from ductile (tough) to brittle (loss of toughness) [14,15]. This transition is a gradual change; therefore, the determination of the transition boundary is inherently arbitrary [15].…”

“…There is no standard device or scientific measurements involved in it, and it is also highly dependent on the judgement of the operators. The method has been described as having poor reproducibility [14,22]. Both liquid limit and plastic limit tests require measuring the water content of each sample by oven-drying overnight.…”