Geotechnical stability analyses of mine waste rock (WR) piles require the critical friction angle (ϕcr) of the coarse blasted rock. However, due to the presence of oversized rock clasts, shear strength can only be characterized on small samples prepared using grading scaling techniques, such as scalping. Thus, considering a testing device able to handle samples of characteristic size D, the material should be scaled down to a maximum particle size dmax given by the minimum sample aspect ratio α = D/dmax. However, a practical concern about how far the size scale can be reduced while keeping representative results remains a matter of debate in the geotechnical community. International standards do not agree on the minimum recommended α, and its effects on the mechanical behavior remain poorly understood. This paper aims to investigate the grading effects and sample size effects on ϕcr of WR materials using the scalping technique, to provide insights on the minimum recommended α. Triaxial tests were conducted on loose and dense samples of diameters D = 150 and 300 mm. Samples were scalped from field material having dmax = 75 mm, to allow a range of α from 4 to 30. Additionally, one of the world largest in-situ direct shear boxes (120 × 120 × 38 cm3) was developed to test the same WR material. The results show that scalping is an appropriate technique to assess the critical shear strength of WR. The minimum α for ϕcr assessment in triaxial testing is not sensitive to grading nor sample size, but it is affected by sample density. The aspect ratio was found to be α ≥ 12 and α ≥ 16 for loose and dense samples, respectively. This finding advocates that α values recommended by worldwide standards, such as ASTM D7181-20, might be too low and should be revisited after comprehensive testing.

中文翻译：

---

通过实验室和原位测试分级剥皮和样品量对矿山废石临界剪切强度的影响


矿山废石 （WR） 桩的岩土稳定性分析需要粗爆破石的临界摩擦角 （φcr）。然而，由于存在超大的岩石碎屑，只能在使用分级缩放技术（如剥皮）制备的小样品上表征剪切强度。因此，考虑到测试设备能够处理特征尺寸 D 的样品，应将材料按比例缩小到最大粒径 dmax，即最小样品纵横比 α = D/dmax。然而，在岩土工程界，关于在保持代表性结果的同时可以缩小多少尺寸比例的实际问题仍然是一个争论的问题。国际标准未就最低推荐α达成一致，其对机械性能的影响仍然知之甚少。本文旨在研究使用 scalping 技术对 WR 材料 φcr 的分级效应和样本量效应，以期为最小推荐α提供见解。对直径为 D = 150 和 300 mm 的松散和致密样品进行了三轴试验。从 dmax = 75 mm 的田间材料中剥皮样品，以允许 4 到 30 的α范围。此外，还开发了世界上最大的原位直接剪切箱之一（120 × 120 × 38 cm3）来测试相同的 WR 材料。结果表明，剥皮是评估 WR 临界剪切强度的合适技术。三轴测试中 φcr 评估的最小α对分级或样本量不敏感，但受样本密度的影响。发现松散和致密样品的纵横比分别为 α ≥ 12 和 α ≥ 16。 这一发现表明，ASTM D7181-20 等全球标准推荐的 α 值可能太低，应在全面测试后重新审视。