Size effects assessment of mine waste-rock shear strength combining numerical, laboratory and in situ approaches

Abstract

With the aim of performing stability analyses of waste-rock (WR) piles, the critical shear strength of loose WR material must be characterised. However, due to the presence of oversized rock clasts, shear tests can only be carried out on small samples prepared using grading scaling techniques. In order to test samples similar to the field material, particle size reduction should be minimised. Considering a testing device able to handle samples of characteristic size , the material should be scaled down to a maximum particle size , given by the minimum sample aspect ratio = / allowing a representative elementary volume (REV). However, worldwide geotechnical standards do not agree on minimum values, and its effects on the mechanical behaviour of coarse samples remain poorly understood. Based on numerical, laboratory and large in situ shear testing approaches, this paper presents a comprehensive study on the effects of sample size and grading on the critical shear strength of WR materials. The main objectives are to analyse the minimum required for a REV in shear testing and to study the suitability of the scalping technique to assess the critical shear strength of mine WR. We study this topic through three methodologies: (1) shearing simulations in the frame of the discrete element method, (2) experimental lab tests using medium to large triaxial devices, and (3) a large in situ direct shear test. We cover a wide range of from 4 to 45. The results show that changes in grading through the scalping technique do not affect the critical shear strength for ≥12, which is higher than some widely applied international standards requirements.