Predicting onset of cyclic instability of loose sand with fines using instability curves

• A hypothesis is proposed to predict triggering of cyclic instability.
• Proposed hypothesis is judged by published and tested cyclic triaxial test results.
• Triggering of cyclic instability can be predicted using instability curve(s).
• Soil lost significant amount of its strength upon triggering of cyclic instability.
• This framework works irrespective of fines contents below a threshold limit.

This study utilises the equivalent granular state parameter, ψ⁎, as a key parameter for studying static and cyclic instability and their linkage. ψ⁎ can be considered as a generalisation of the state parameter as first proposed by Been and Jefferies so that the influence of fines content in addition to stress and density state can be captured. Test results presented in this study conclusively showed that ψ⁎ at the start of undrained shearing and ηIS, the stress ratio at onset of static instability, can be described by a single relationship irrespective of fines content for both compression and extension shearing. This single relationship is referred as instability curve. However, the instability curve in extension shearing is different from that of compression. In this paper, the capacity of the instability curve in predicting triggering of cyclic instability was evaluated experimentally. An extensive series of undrained one-way (compression) and non-symmetric two-way cyclic triaxial tests, in addition to monotonic triaxial tests in both compression and extension were conducted for this evaluation. Furthermore, a published database for Hokksund sand with fines was also used. Test results show that cyclic instability was triggered shortly after the cyclic effective stress path crossed the estimated ηIS-zone(s) as obtained from instability curve(s) irrespective of whether instability occurs in the compression or extension side.