Microscopic damage and deterioration of carbonaceous slate in cold region subjected to freeze-thaw cycles

Freezing and thawing processes play a crucial role in causing significant deformation and damage to layered soft rocks in cold region due to daily and seasonal temperature fluctuations. However, the frost heave mechanism of the rocks and their mechanical behaviors at the meso-scale still require further investigations. For this, we focused on carbonaceous slate reported in a high-altitude cold region, in terms of mineral composition, content, and microstructure. The strength and failure of mineral grain (MG) interfaces are studied using three-point-bending tests, in order to explore the evolution of mode I fracture toughness and tensile strength with the Dugdale-Barenblatt model and the Weibull distribution model. The results indicate that the damage of slate involves the initiation and propagation of microfracture networks at clay MG interfaces (bedding planes), driven by frost heave pressure at macroscopic and microscopic scales. This process causes the detachment of some MGs, resulting in fracture surfaces with a distinctive pulled-off planar structure. The hydrophilicity of clay MGs, interfacial strengths, and microfracture structures contribute to the freeze-thaw damage. As the number of freeze-thaw cycles increases, the effective area per unit decreases, leading to an exponentially decreasing in mode I fracture toughness and tensile strength at MG interfaces. Approximately 67% strength degradation occurs after 14 freeze-thaw cycles. This provides theoretical basis and experimental methods for better understanding the damage and deterioration behaviors of layered soft rocks in cold region under natural freeze-thaw cycles.