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%.
Highlights
The liquid limit (LL), originating from the original research of Atterberg (1911a, 1911b) and introduced into soil mechanics by Terzaghi (1926a, 1926b), with the associated percussion-cup test method standardised by Casagrande (1932, 1958), is notionally the water content below which fine-grained soil ceases to flow as a liquid
It has been widely recognised that the value of water content assigned to the Casagrande LL for a given finegrained soil is dependent on the precise characteristics of the test method and the class of Casagrande-cup device employed in its determination
These back-calculated su(LL) values are unrealistically high, given, as cited by Vinod et al (2013a: p. 416), Narain and Ramanathan (1970) described the LL as ‘the extreme limiting water content above which the forces of interaction between particles become sufficiently weak to allow easy movement of the particles relative to each other’. To contextualise these reported su(LL) values, the expected Ss value of ~1·0 m2/s2 corresponding to wL(cup) (Haigh, 2012) might imply an su(LL) range for the investigated fine-grained soils of 1·41–1·76 kPa according to the analysis presented by Vinod et al (2013b), while an su value of ~1·7 kPa might be expected for standard fall-cone LL approaches (Wroth and Wood, 1978)
Summary
For remoulded fine-grained soil, the undrained shear strength (su(LL)) corresponding to the Casagrande LL water content (wL(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 wL(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 su(LL) reduces with increasing wL(cup). A few research papers have, reported significantly higher su(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 su(LL) values were deduced and explaining why they are not correct. For low- to very high-plasticity soils, the overwhelming weight of experimental evidence presented indicates that su(LL) can range 1–3 kPa and typically decreases from approximately 2·5 to 1·6 kPa for wL(cup) increasing from 20 to 70%
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