Experimental Study on the Reliability of Scaling Down Techniques Used in Direct Shear Tests to Determine the Shear Strength of Rockfill and Waste Rocks

The determination of shear strength parameters for coarse granular materials such as rockfill and waste rocks is challenging due to their oversized particles and the minimum required ratio of 10 between the specimen width (W) and the maximum particle size (dmax) of tested samples for direct shear tests. To overcome this problem, a common practice is to prepare test samples by excluding the oversized particles. This method is called the scalping scaling down technique. Making further modifications on scalped samples to achieve a specific particle size distribution curve (PSDC) leads to other scaling down techniques. Until now, the parallel scaling down technique has been the most popular and most commonly applied, generally because it produces a PSDC parallel and similar to that of field material. Recently, a critical literature review performed by the authors revealed that the methodology used by previous researchers to validate or invalidate the scaling down techniques in estimating the shear strength of field materials is inappropriate. The validity of scaling down techniques remains unknown. In addition, the minimum required W/dmax ratio of 10, stipulated in ASTM D3080/D3080M-11 for direct shear tests, is not large enough to eliminate the specimen size effect (SSE). The authors’ recent experimental study showed that a minimum W/dmax ratio of 60 is necessary to avoid any SSE in direct shear tests. In this study, a series of direct shear tests were performed on samples with different dmax values, prepared by applying scalping and parallel scaling down techniques. All tested specimens had a W/dmax ratio equal to or larger than 60. The test results of the scaled down samples with dmax values smaller than those of field samples were used to establish a predictive equation between the effective internal friction angle (hereafter named “friction angle”) and dmax, which was then used to predict the friction angles of the field samples. Comparisons between the measured and predicted friction angles of field samples demonstrated that the equations based on scalping scaling down technique correctly predicted the friction angles of field samples, whereas the equations based on parallel scaling down technique failed to correctly predict the friction angles of field samples. The scalping down technique has been validated, whereas the parallel scaling down technique has been invalidated by the experimental results presented in this study.

Manufacture and Characterization of Fired Bricks from Gold Mine Waste Rocks

The purpose of this research was to evaluate the possibility of using gold mine waste rocks (GMWRs) as alternative raw material for the manufacturing of fired bricks. The feasibility study was assessed through (i) physical, chemical, mineralogical and environmental characteristics of GMWRs; (ii) determination of the natural clay (NC) substitution effect when using GMWRs; (iii) the effect of the firing temperature on the mechanical and physical properties of the fired bricks. Five mixtures of NC and GMWRs were studied. The percentages of substitution of NC with GMWRs varied from 0 to 100%. The brick specimens were fired at 900 °C, 1000 °C and 1050 °C. The results show that increasing the firing temperature improved the flexural strength and density of the bricks, while the substitution of NC with GMWRs caused a reduction in the mechanical resistance of the bricks and an increase in their porosity and, consequently, their water absorption rate. However, the properties of bricks that contained up to 80 wt% of GMWRs and fired at 1000 °C and 1050 °C satisfied the requirements set by the applicable civil engineering and environmental standards. This was found to be an efficient and sustainable solution to mitigate environmental hazards and better manage mining wastes, concurrently producing marketable products from them, which is in accordance with the circular economy concept.

Cylindrical Caved Space Stability Analysis for Extension Prediction of Mining-Induced Surface Subsidence

Subsequent extension of surface subsidence after vertical caving leads to large-scale surface destruction, as well as associated geological hazards. The extension prediction for cylindrical caved space, which appears circular surface subsidence, is still an intractable issue, due to the absence of robust models. To fill such a research gap, this paper provides an analytical model for the depth and orientation where the shear failure of isotropic rocks around the caved space is firstly observed. The anisotropy of surrounding rocks is further involved to enable this model to analyze the slip failure along discontinuities in anisotropic stress state. The prediction for the extension of the surface subsidence in Xiaowanggou iron mine is conducted, and the comparison between the prediction and the observation in satellite images demonstrates the validity of the proposed model. Even though this model cannot provide a definite boundary after extension, the prediction for the orientation surface subsidence extends to contribute to mitigating the effect of geological hazards. Another contribution of this work is to provide guidance to mitigate the impact of surface subsidence on safety and environment, such as filling the interspace between large-sized caved rocks by dumping small-sized waste rocks or backfilling the caved space with waste rocks.