Determination of Atterberg Limits: Uncertainty and Implications

Arthur Casagrande made one of the most important contributions to Geotechnical Engineering; ordering and presenting clearly the existing differences between objectives for civil engineering soil classification and soil classification schemes intended for other purposes. However, more than 50 years after the Unified System of Soil Classification (USSC) was proposed, one of the main ideas expressed by Casagrande: “the plasticity chart representation of the plasticity parameters in different soil moistures, belonging to a common geological origin, is a straight line, parallel to the A line,” has received little scrutiny. The main purpose of this technical note is to begin a revision of Casagrande’s proposal under a probabilistic approach, suggesting some modifications to the application of the plasticity chart. Regression analysis is proposed as a valid technique to express the linear behavior of the Atterberg limits for a given soil. The problem considering plasticity index as a probabilistic variable is exposed demonstrating that the correct representation of the plasticity chart is in terms of liquid and plastic limits. Data from the Libertad–Dolores clays formations from Uruguay are presented, demonstrating the application of the proposed changes.