Undrained anisotropy of Hostun RF loose sand: new experimental investigations

Abstract

The undrained behaviour of loose and overconsolidated Hostun RF sand in triaxial compression and extension tests is described. The samples are isotropically or anisotropically overconsolidated along several constant effective stress ratio paths with various overconsolidation ratios (OCR), up to 24. To minimize the effect of variation of density on the observed undrained behaviour, all tested samples are required to have a nearly identical void ratio before the final monotonic undrained shearing. Isotropically overconsolidated and normally consolidated samples exhibit the same phenomenon of partial static liquefaction, but anisotropically overconsolidated specimens reveal a completely different undrained behaviour. A common pseudoelastic response is observed for a given overconsolidation history. This response is induced by recent stress history in terms of effective stress paths, independent of the OCR during overconsolidation. The initial gradient of the effective stress paths seems to depend solely on the direction of the previous linear stress path history. This paper offers a comprehensive understanding of the mechanism of the induced anisotropy of loose sand created by simple linear stress paths from three different initial stress states in the classical triaxial plane. The pseudoelastic response can be adequately modelled by a simple hyperelastic component of the elastoplastic framework.Key words: induced anisotropy, overconsolidation, instability, laboratory undrained tests, sand, hyperelasticity.