Investigation into the impacts of mechanical improvement of tailings within the Critical State Soil Framework

A geotechnical investigation was undertaken in tailings and subsequently analysed within the critical state soil mechanics (CSSM) framework. The geotechnical investigation included cone penetration testing with porewater pressure measurement (CPTu) and laboratory testing. The laboratory testing was inclusive of triaxial compression testing in addition to classification and oedometer testing. The tailings samples were retrieved using the mini-block sampling (MBS) technique to minimise disturbance, though the triaxial testing included both reconstituted and testing in MBS recovered samples. The tailings material was split into four groups related to the deposition methodology used. This included mechanically improved tailings, sub-aerially deposited tailings and two types of sub-aqueous deposited tailings.

Using the triaxial test results, four critical state lines (CSLs) were defined based on fines content of 6%, 40%, 50% and 80%. Each CSL has a varying slope and intercept observing to follow a well-documented trend leading towards an Omega Point. These four CSLs were used to define a CSL region within which there is an independent CSL based on fines content, i.e., a tailings with 5% fines content have a different CSL to one of 55% based on the developed functions. The soil behaviour index (Ic) was then used to estimate the fines content of the CPTu results using three methods; Boulanger and Idriss (2014), Agaiby and Mayne (2020) and a site-specific calibration.

The CPTu results were plotted against the modelled CSL region. The results were used to predict the potential of the tailings to behave either contractive or dilative. The results of this assessment clearly demonstrate that whether the tailings material will behave in a contractive or dilative manner is related to placement strategy. Most relevantly, the assessment allowed estimation of the stress progression and at which point the dilative tailings will transition to contractive behaviour upon subsequent upstream loading. The methods used in this paper provide a robust rational framework to demonstrate the effect of various tailings management practices on design outcomes, able to be applied by practitioners for increased understanding of their tailings and hopefully leading to more informed decision making.