Fallacy of wide undrained strength range at the Casagrande liquid limit

Abstract

The liquid limit (LL), as originally proposed by Atterberg, is notionally the water content below which fine-grained soil ceases to flow as a liquid. In many countries, the Casagrande-cup approach is the standard procedure for LL determinations. For remoulded fine-grained soil, the undrained shear strength (s u(LL)) corresponding to the Casagrande LL water content (w L(cup)) value can range 0·7–3·9 kPa, but typically 1·0–2·7 kPa, with many researchers agreeing on a value of around 1·6–1·7 kPa. Further, the values of w L(cup), as determined using standard-compliant Casagrande-cup devices, correspond to a specific strength value of ∼1·0 m2/s2, such that the value of s u(LL) reduces with increasing w L(cup). A few research papers have, however, reported significantly higher s u(LL) values (>4 kPa and up to 12 kPa) for some fine-grained soils investigated. This paper critiques these experimental data and debunks this myth by first considering how these superhigh s u(LL) values were deduced and then explaining why they are not correct. For low- to very high-plasticity soils, the overwhelming weight of experimental evidence presented indicates that s u(LL) can range 1–3 kPa and typically decreases from approximately 2·5 to 1·6 kPa for w L(cup) increasing from 20 to 70%.