Drained static bias effects on very loose silt tailings

Abstract

Characterising the dynamic behaviour of soils under significant initial static bias is important for the safety of slopes, embankments, and tailings storage facilities (TSFs). However, this is a complicated task as: (i) the case history database is dominated by level ground sites, (ii) published laboratory element testing programs are often primarily carried out without an applied initial bias, (iii) there is uncertainty as to how liquefaction resistance varies with bias and/or density, and (iv) the criterion for selecting liquefaction triggering in such conditions has not achieved consensus. These issues are a particular challenge for filter-stack TSFs where the tailings can be in a very loose state owing to their moist placement, are under significant bias near the lower portion of the slope, and being often predominately siltsized do not fall well into common categorisations of “clay-like” and “sand-like” soils.

This paper presents a series of cyclic constant volume direct simple shear (DSS) tests on samples prepared in a very loose state, tested under a range of static bias values. The significant effects of bias are highlighted by this testing, along with the low magnitude of cyclic-induced excess pore pressure that developed at the point of “runaway” shear strains in the direction of the static bias. These observations are then compared to typical approaches to select a liquefaction criterion from strains and excess pore pressure in nonlinear dynamic analyses.