As the global push towards clean energy intensifies, the demand for critical minerals has driven deep excavation in hard rock formations, posing significant challenges related to rockburst and spalling. Spalling refers to explosion-like rock fractures under high geo-stresses. Despite several successful studies and practical models, the mechanisms governing spalling propagation under polyaxial stress states remain inadequately understood, particularly in weaker and high-porosity rocks. This study introduces a novel Crack Mode-Changing Stress (CMCS) concept, which defines the minimum principal stress required to change the crack mode from shear to tensile failures when rock spalls. The concept was validated using cubed sandstone samples containing centric circular holes subjected to a range of loading conditions including uniaxial, biaxial, generalized triaxial compression, generalized triaxial tensile, and true triaxial loading stress states. Our results highlight the significance of the out-of-plane minor principal stress on the crack initiation threshold and the CMCS, emphasizing the need for careful consideration when designing openings in highly stressed environments.

Solving the challenges facing the mining industry is crucial for shaping the global attitude towards clean energy technologies associated with critical minerals extracted from depth. One of these challenges is the well-known explosion-like fractures (rockbursts) or spalling failures associated with the initiation of internal cracks. To prevent such catastrophic failure, shotcrete, as a cement grout, is widely used in tunnel support applications. In areas where the tunnels are constructed within the limestone strata using tunnel boring machines (TBM), drilling, and/or blasting, millions of cubic meters of crushed limestone (CL) in powder form are extracted and landfilled as waste. Given the fact that natural sand consumption as a raw material in the construction industry exceeds previous records, recycling of such excavation material is now becoming increasingly needed. From this perspective, this study aims to utilize crushed limestone as a potentially sustainable alternative to natural sand in shotcrete applications in deep tunnels. Accordingly, several strength characterization and crack initiation determinations through various stress–strain-based models were carried out on cylindrical samples containing different proportions of crushed limestone. By increasing the crushed limestone content in the shotcrete mix, the crack initiation stress (as a measure of the in situ spalling strength) increased as well. The results suggest that the crushed limestone has good potential to replace the natural sand in the shotcrete mixture used in tunnel support applications.

In response to high demands for minerals and in the light of rapid advancement in instrumentation and testing facilities, mines, tunnels, and infrastructures are getting deeper at an annual increasing rate. For instance, Australia’s deepest Gwalia gold mine near Leonora is extending beyond 2 km depth in the hunt for more ore. In Europe, the Gotthard sub-alpine base tunnels exceeding 10 m wide are excavated at depths greater than 2 km. At depth, explosion-like fractures occur which are known as stress spalling, slabbing, or rock bursting due to high-pressure environment, high temperature, and low porosity. Although rockburst and spalling failures have both been observed in the past 50 years and investigated by numerous researchers, this phenomenon is yet mysterious and the physics behind is still not fully understood. Recent researches have suggested that the spalling strength can be related to the crack initiation point. Various methods have therefore been proposed to identify the crack initiation at laboratory scales based on stress-strain response. Limited models are also available based on applied stresses, but only under triaxial conditions. This study aims to compare the volumetric strain method (as a strain-based technique) with the improved Griffith and Hoek-Brown criterion (G-HB) (as a stress-based method) to determine the crack initiation stress threshold under triaxial loading conditions.

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