Progressive Damage of Sandstone and Shale Under Heating and Cooling Cycles: Implications for Stratified Rock Slope Stability

Thermally induced cracking and the evolution of mechanical anisotropy in bedded sandstone under high-temperature conditions: a microscale perspective

Monitoring of structures in shaly rock formation

Shaly rock formations, particularly those rich in expansive clay minerals, pose unique geotechnical challenges for infrastructure development. These formations are susceptible to volumetric swelling and deformation when subject to water ingress, mechanical stress and salt concentration, which can lead to progressive structural damage over time. The long-term swelling effect of shale formation in combination of cyclic thermal and environmental changes induce additional stress on the structure in contact with the shale. This paper illustrates the use of a long-term monitoring program aimed at assessing crack growth and crack propagation of an underground structure situated within shale-dominated geological environments in Southern Ontario, Canada. Real-time continuous data was collected over an extended period to reveal any correlations of seasonal thermal and moisture changes and the deformation of the structure. Furthermore, short-term stress responses were determined based on crane live load tests. These provided insight into the dynamic responses of the structure. The analysis of the time-series data led to the establishment of displacement and strain threshold limits, which ultimately enhanced the structure’s long-term performance and safety. The findings highlight the importance of continuous monitoring in predicting long-term structural responses and support the development of early warning systems and mitigation strategies.

Moisture-induced degradation of clay-bearing sandstone

Clay-bearing sandstones used in the construction of historic monuments, churches, and castles are highly susceptible to contour scaling, which manifests as fractures parallel to the exposed facades. Repeated wetting–drying cycles accelerate material degradation through stiffness reduction and hygric deformation induced by swelling clays. This in turn alters the hygro-thermal (HT) properties of the stone. In this study, a scalar damage variable is employed within the framework of continuum damage mechanics to characterize the evolving damage state under the influence of moisture fields. The most critical climatic conditions that lead to degradation are first identified through hygro-thermal simulations. Subsequently, a fully coupled hygro-thermo-mechanical (HTM) model is developed to account for both moisture transport and material degradation. The results demonstrate that contour scaling can occur solely due to moisture cycling, even in the absence of freezing or salt crystallization effects. As such, subsurface zones of damage localization and a water-accumulation layer emerge that align well with on-site observations, pointing at potential damage mechanisms such as contour scaling.

Mechanical and hydraulic properties of fractured Bentheim sandstone at different laboratory-simulated depths

Understanding how rock properties change with depth is crucial for a variety of geoengineering applications. Even rocks that are homogenous at both micro and macro scales, such as Bentheim sandstone, lose this characteristic once fractured. While recent studies have shown how concomitant changes in stress, temperature and pore pressure affects the evolution of intact sample permeability at depths, an equivalent study on fractured material is missing. Therefore, by combining a multi-methodological approach consisting of rock deformation experiments simulating depth conditions up to 4 km, thin section analysis and fluid composition analysis of water samples, the evolution of permeability of fractured Bentheim sandstone is investigated in this study. Results suggests that fine particles produced by the fracturing and the movements along these fractures play a crucial role in permeability evolution. When these particles are removed, the fracture constitutes a preferential pathway and, together with the chemical processes occurring on the rock–fluid system, lead to a 3–7 times reduction in permeability followed by a complete recovery of it after a simulated burial and exhumation path. On the contrary, when these particles are still present within the fracture zone, they impede fluid flow. This causes a slightly reduction of permeability during the burial path followed by almost constant values of permeability throughout the exhumation path. These findings provide crucial information for georeservoir applications and the transfer of results from laboratory experiments to in situ conditions for a correct prediction of hydraulic properties.

Strength Deterioration of Strongly Altered Granite Under Varying Water Content and Seepage Pressure: Experimental Insights for Reservoir Slope Stability

Reservoir landslides pose a persistent threat to the safe operation of hydropower projects, particularly where altered rock masses within water-level fluctuation zones undergo repeated wetting–drying and seepage-induced deterioration. This study investigates the mechanical behavior and long-term strength evolution of altered granite from the Guobu Slope near the Laxiwa Arch Dam in Qinghai, China. Rock masses with four alteration degrees, ranging from complete to slight alteration, were examined through an integrated experimental program involving torsional shear tests, hydro-mechanical coupled triaxial tests, large-scale direct shear and ring shear tests, Brazilian splitting tests, and long-term P-wave velocity monitoring. The results demonstrate that increasing water content progressively weakens the shear strength of altered granite, while elevated seepage pressure further reduces its strength and deformation resistance under hydro-mechanical coupling. Residual shear behavior also shows a clear dependence on water content, indicating that post-peak strength deterioration should be considered in slope stability assessment. Long-term P-wave monitoring further reveals that mechanical degradation is more pronounced during the early stage and gradually approaches a relatively stable state, suggesting a site-specific decelerating deterioration process rather than unlimited strength loss. Based on the experimental results, empirical relationships between shear-strength parameters and water content are established, and long-term lower-bound strength parameters are proposed for altered granite with different degrees of alteration. These findings provide experimental support for understanding the hydro-mechanical deterioration and long-term deformation behavior of reservoir-bank altered rock masses and offer a basis for parameter selection and stability assessment when combined with rock-mass reduction, field calibration, and sensitivity analysis.

Weathering-Induced Strength Degradation and Hydro-Mechanical Controls on the Stability of Multilayered Tropical Rock Slopes

Tropical environments accelerate rock weathering, which can significantly reduce the stability of mineralized slopes. This study investigates the influence of weathering and groundwater conditions on the stability of a strong–weak–strong multilayered rock slope hosting an iron deposit in the Philippines, where intense rainfall and highly fractured rock masses are common. Detailed geological and hydrogeological investigations were conducted to construct two-dimensional numerical models of the slope system. Mechanical characterization of both weathered and unweathered rock masses was performed using field hardness testing and laboratory geomechanical tests. These parameters were incorporated into numerical simulations to evaluate slope stability under varying degrees of weathering, weak-layer thickness, and hydrological conditions. Results indicate that when the middle weak layer is thin, a 50% weathering intensity still allows a stable multilayered slope at an overall slope angle (OSA) of 60°. However, as weak-layer thickness increases, the same weathering degree leads to slope instability. Weathering effects correspond to an average reduction of 10.8 units in the Geological Strength Index (GSI), equivalent to a 50% weathering degree, implying that each 0.2 reduction in GSI represents approximately 1% weathering of the rock mass. Considering weathering-induced strength degradation, the optimally safe slope angle decreases from the originally designed 60° to approximately 35°, maintaining a minimum factor of safety of 1.2. Hydrological analysis further reveals that low rainfall may temporarily increase slope stability due to matric suction induced by capillary effects in clay minerals, although prolonged rainfall generates positive pore pressures that reduce stability. The thickness of the weak layer also produces localized increases in hydraulic gradient due to permeability contrasts, increasing susceptibility to rainfall-induced failure. These findings highlight the importance of quantitatively incorporating weathering and hydrogeological effects in slope design for tropical open-pit mines.