Scaling for investigation of steady-state shear strength of coarse mine-waste which cannot be tested using conventional laboratory equipment

Blasted mine waste containing large particles must be scaled down for laboratory testing. Scaling modifies the particle size distribution (PSD) to obtain a reduced maximum particle size. Scaling is a common practice, and although it is known to impact the material’s geotechnical behaviour, there is no consensus in the literature whether the shear strength of scaled samples is higher or lower compared with the shear strength of the original material. The research objective was to understand the changes introduced with scaling that impact shear strength to establish a best practice for building scaled samples for laboratory testing. It was found that several material characteristics at particle level are actually altered when the PSD is modified for testing. Because these are all particle-size correlated it is difficult to isolate them to evaluate their impacts on material strength. The investigation focused on the observation of two key characteristics: the distribution of particle sizes and the distribution of particle shapes. The effect of only PSD alteration on shear strength was evaluated (Publication 1 of 6), isolating the phenomenon from the influence of particle shape and particle crushing using the discrete element method. The shear strength was found to be invariant when altering the PSD if the shape of the particles is constant across sizes and in the absence of crushing. Micromechanical evaluation of the connectivity and anisotropy of the contact and forces network (Publication 2 of 6) explained the invariance as a compensation between contact orientation and branch length anisotropies. A large-scale testing facility was enhanced to reproduce results with high precision (Publication 3 of 6) and was used to investigate the shear strength of a sample of coarse granular material as well as of different scaled models. The analysis of the material used for the investigation revealed that the size and shape of its particles were correlated (Pub- lication 4 of 6), with larger pieces being more platy and discoidal than smaller pieces. The results of both physical testing (Publication 5 of 6) and additional numerical testing considering particle shape characteristics (Publication 6 of 6), suggest that shear strength changes observed when the PSD is modified for scaling are due to the alteration of particle shape polydispersity, inevitably introduced with the alteration of the PSD when particle size and shape are correlated. In the absence of specific particle size-shape information and assuming that the particle size-shape correlation is a continuous function, the results of this study indicate that scaling of materials should prefer substitution of oversized particles by smaller particles (of the maximum allowable particle size) over the scalping technique. They also indicate that scalping should be preferred over geometric scaling (parallel gradation), aiming for minimum alteration of the sample’s particles shape polydispersity. Altering the body of the original PSD, usually representing most of the material by mass, leads to greater potential corruption of the particle shape distribution. Nevertheless, in an ideal situation, the particular particle size shape correlation should be evaluated and the process of scaling should be designed so that the impact on the particle shape distribution is minimised. If the sorting of particles by shape could be conducted for the fractions already sorted by size in the scaling process, the design and construction of scaled samples with a reduced particle size but replicating the distribution of particle shape of the original material should be possible. Those would be “truly” scaled samples.