Monitoring of structures in shaly rock formation

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.

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

Wellbore Size Effect and Borehole Instability Response Characteristics of Fractured Sandstone in SP Gas Storage

The SP Gas Storage is situated in the SP-Xingcheng structural belt, where volcanic gas reservoirs are widely distributed and characterized by abundant primary microfractures and pore structures. The developed pores and fractures degrade the petrophysical properties of reservoirs and render volcanic basement rocks highly abrasive. In addition, pore-fracture systems alter the internal stress field of formations, which substantially increases the risk of wellbore instability and the collapse of injection and production wells. This poses great challenges to drilling operations and the safe running of the gas storage in this block. To systematically clarify the wellbore instability mechanism of large-diameter wellbores and address the drilling engineering problems in the study area, a dedicated experimental scheme for large-diameter wellbore stability was designed in this work. Laboratory true triaxial tests were conducted on wellbore stability with different borehole sizes, and basic mechanical parameter tests of reservoir rocks were also completed. This study systematically investigates the evolution of rock mechanical parameters and the surrounding stress-reconstruction mechanism induced by pore-forming unloading and identifies the dominant internal mechanism of wellbore instability under large-diameter conditions. A clear distinction is made between the formation stress redistribution caused by stratum exposure and unloading during drilling and formation stress evolution during the subsequent injection and production of the gas storage. On this basis, the fracture initiation threshold, propagation paths, and morphological evolution in thin interbedded sandstone–mudstone reservoirs are further analyzed. Combined with rock mechanical parameters and in situ stress balance conditions, criteria and quantitative evaluation methods for wellbore instability discrimination are finally established.

Study on stability assessment of buckling failure and rainfall-induced instability mechanism for steeply inclined rock slope

To address challenges associated with the stability assessment and engineering control of buckling failure in steeply inclined rock slopes, this study takes the southern slope of the Longyu Open-Pit Mine as a case study and adopts an integrated approach combining theoretical modeling, field monitoring, and numerical simulation. First, under the assumptions of coordinated deformation and small strain, a differential equation governing the behavior of the surface rock layer is derived. This leads to the formulation of a safety factor defined as the ratio of the critical to the actual slope length. The analysis indicates that the slope remains stable when the thickness of the rock layer exceeds 10 m and the elastic modulus is greater than 32 GPa, thereby establishing a robust mechanical model for buckling failure. Second, based on field monitoring data of displacement and strain, orthogonal testing and factor sensitivity analyses are conducted. The results reveal the following ranking of influential factors: rock layer thickness ( R = 0.38+39.8 %) is an exceptionally sensitive positive factor; cohesion ( R = 0.25) is a highly sensitive positive factor; unit weight and groundwater level are significantly sensitive negative factors; whereas the influence of elastic modulus is negligible ( R = 0.03). These insights provide a clear priority hierarchy for monitoring and stability control measures. Finally, by incorporating real-time rainfall intensity data, FLAC3D simulations demonstrate a strong negative correlation between rainfall intensity and slope stability. Heavy rainfall is identified as a critical threshold triggering stability failure. Under such conditions, the maximum slope displacement increases by 175 %, reaching 5.5 m, and the shear strain increment develops into an arc-shaped sliding surface. The slope interval between 1360-1390 m is identified as the core risk zone, while the interval from 1290-1310 m acts as a key shear outlet. This clarifies the evolutionary pathway and key focus areas for preventing rainfall-induced instability. The findings of this study offer a solid theoretical foundation and practical technical support for the monitoring, risk warning, and engineering management of similar steeply inclined slopes.

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.

Performance Evaluation of a Tunnel–Slope System

Intense rainfall and the resulting increase in ground saturation can significantly modify the mechanical performance of rock masses in natural slopes, particularly when fractured material is present. Extended infiltration reduces shear strength along discontinuities and increases pore-water pressures, raising the probability of large-scale landslides. When a tunnel is built within or near an unstable slope, the response of both structures becomes coupled, and this tunnel–slope interaction has proven to be an important aspect in the design and safety assessment of underground infrastructure in mountainous regions. This study evaluates the static and seismic performance of a tunnel–slope system in a fractured shale–limestone slope that failed after heavy rainfall. Since ground exploration was limited, the observed failure was reproduced through a back-analysis within a performance-based design (PBD) framework to calibrate representative geomechanical parameters. These parameters were then used in three-dimensional finite difference models to simulate the tunnel construction process and the seismic response of the system. During construction, the interaction between the tunnel and the slope was found to be minor. Under seismic loading, however, the simulations revealed notable interaction effects: slope displacements accumulate in the zone where the tunnel runs closest to the unstable critical section, and the stresses in the tunnel lining increase as a result of both the interaction with the slope and the curvature of the alignment. These results indicate that tunnel–slope interaction should be explicitly considered in the analysis and design of underground infrastructure whenever the tunnel lies within about four diameters of an unstable slope.