Assessing the strength of tailings with high degrees of saturation using cavity expansion theory and cone penetration testing

Prof Adrian Russell

The University of New South Wales

Abstract: Tailings, being mixtures of water and soil-sized particles, and sometimes air, are waste products generated by mining. They are stored on sites, contained by embankments, forming what are known as tailings storage facilities (TSFs). TSFs fail too often, resulting in tailings reducing in strength and, in some cases, turning to a fluid-like material (a phenomenon called ‘liquefaction’) that can spread many kilometres, destroying lives, property and the environment. 

The strength of tailings inside a TSF may be assessed using results of cone penetration tests (CPTs), integrated with cavity expansion theory. The coexistence of air and water in the pore space means the tailings is unsaturated and gives rise to a suction. The suction, and the extent to which air and/or water can drain through the pores, affects a cavity expansion and a CPT result. 

Here an adaptation of a bounding surface plasticity constitutive model is used to describe the behaviour of the tailings. A constant mass condition, relevant to undrained closed-system loading which prevails during fast deformation, Boyle’s law and hydraulic hysteresis are accounted for to capture the changes to pore air and water pressures, and suction, with the change in tailings volume. The cavity expansion problem is solved using this model, considering four possible drainage conditions (constant suction, constant water mass, constant contribution of suction to the effective stress, and a constant air and water mass). It is reasoned that solutions for a closed system are relevant to the interpretations of CPTs when < 15 % of the tailings volume is occupied by air, and that the constant effective stress condition (which is a close approximation to a constant water mass condition) is relevant when larger air volumes are present. 

By considering CPT data it is observed that linear proportionalities exist between effective cone penetration resistances and cavity wall pressures. The cavity expansion results may then be converted to equivalent CPT results and used to construct charts which relate the normalised cone penetration resistance to the initial state parameter. The charts have use for unsaturated conditions and a variety of air volume fractions, as well as saturated conditions when the cone penetration rate is slow enough so drained conditions prevail or fast enough so that undrained conditions prevail.

Bio: Adrian R. Russell is a Professor of Geotechnical Engineering at UNSW Sydney. His expertise is in the development of analytical and semi-analytical techniques in geomechanics, the behaviour of soils and tailings and rock, physical model testing, cavity expansion theory, the CPT and knowledge transfer to industry. Professor Russell is an Australian representative on TC106 and TC221, which are International Technical Committees on unsaturated soil mechanics and tailings within the ISSMGE. He does expert review work on the stability of tailings storages and serves on Independent Tailings Review Boards. He is one of a team of industry leaders updating the AGS guidelines for risk assessments of slopes. He is also on Editorial Boards of Geotechnique, Computers and Geotechnics and the International Journal of Rock Mechanics and Mining Sciences. He was awarded his PhD in 2005 and BE in 1998, each by UNSW Sydney. His first academic appointment was a lectureship at the University of Bristol in the UK (2003-2007). This was followed by a move UNSW Sydney where has been ever since.