Silt and its misrepresentation on the conventional plasticity charts

In particle size classification systems Silt lies between Clay and Sand. Clays and Sands are easily identified, with well-researched and understood properties. With less research, Silt-like behaviour is not fully understood and cannot be treated as intermediate between Clay and Sand. Silt’s unpredictable drainage behaviour and unstable pore pressure responses can result in poor load-bearing capacity, uneven settlements and unsuitable engineered fill materials; it is especially susceptible to liquefaction, frost heave, erosion, scour, piping and collapse. With unreliable behaviour the classification of Silt should only be assigned with full awareness of its distinctive characteristics. From a theoretical approach to the liquid and plastic limits, invoking the linear law of Clay/Silt mixtures and the deviations from the law, and confirmed by experimental data and published sources, it is established that the conventional plasticity charts misrepresent Silt classification. The ‘Clay above – Silt below’ the A line interpretation is shown to be incorrect. It is recommended that the identification and classification of Clay and Silt be determined independently from the charts, by understanding the behaviour of the Clay→Silt transition: the plasticity/toughness→dilatancy transformation. Greater use of the manual/visual test criteria, described in complementary standards, in particular, plasticity, toughness and dilatancy, would enable development of a proper identification and classification system for these soils.