Hydraulic-fracture transmission across lithologic interfaces governs fracture-height growth and reservoir connectivity in shale–sandstone interbedded reservoirs, yet the coupled effects of mechanical contrast, interface properties, layer geometry, and operational parameters remain insufficiently quantified. A two-dimensional plane-strain hydraulic-fracturing model was developed in ABAQUS by coupling Biot poroelasticity, cohesive-zone damage, and fracture-fluid flow. The model explicitly represents thin, alternating continental shale–sandstone layers, lithology-dependent in situ stress and stiffness, and cohesive interfaces; its implementation was assessed against the KGD solution and published layered-rock fracture morphologies. Under the simulated conditions, increasing the shale-to-sandstone elastic-modulus ratio from 0.4–0.5 to 0.6–0.8 reduced the number of penetrated layers from eight to six. Increasing tensile strength from 4 to 16 MPa reduced the number of penetrated layers from ten to six and the final fracture length from 32 to 21 m, while increasing the maximum aperture from 6.75 to 9.02 mm. A sandstone interlayer thickness of approximately 3 m marked a transition in the present parameter set rather than a universal threshold. Sandstone-centered perforation and higher injection rates promoted vertical connectivity, whereas very high fluid viscosity increased near-wellbore aperture but restricted long-distance fracture-height growth. These results provide a mechanics-based framework for optimizing perforation placement and stage-specific fluid design in continental interbedded shale reservoirs.
Overburden rotation and mining disturbance promote deep coal fracture and spalling, potentially inducing dynamic hazards, while the coal–rock thickness ratio largely governs the composite’s dynamic response and failure severity. Based on the geological conditions of a typical deep coal mine, SHPB tests were performed on coal–sandstone composite specimens with four systematically varied coal-to-rock ratios (C:R = 1:0, 2:1, 1:1, and 1:2). High-speed photography and the digital speckle correlation method (DIC) were integrated to capture displacement, strain, energy, and fragmentation fields throughout progressive dynamic compression. Experimental data revealed four findings: (1) Crack development follows a sequential evolution process of initiation, propagation, and failure. Higher impact air pressure accelerates crack development and coalescence, resulting in a higher degree of specimen fragmentation and fewer large blocks. Within the tested range of coal–rock ratios, an increase in the rock proportion accelerates coal fracture, which is attributed to the significantly higher density and hardness of rock compared to coal. (2) Energy evolution consistently follows three stages: absorption, accumulation, and dissipation. Under identical impact pressure, a higher rock ratio elevates equivalent stiffness and wave impedance, leading to monotonic increases in peak stress, peak strain, absorbed energy, and dissipated energy. (3) The fragmentation degree exhibits a pronounced dependence on impact pressure. Specifically, for the pure coal specimen (C:R = 1:0), when the impact pressure increases from 0.3 MPa to 0.7 MPa, the mass percentage of coarse debris (>30 mm) drops from 73.37% to 18.57%, whereas that of fine particles (<4 mm) rises from 15.69% to 35.24%. (4) Under identical impact conditions, a higher rock proportion leads to increasing trends in all measured indicators, including peak stress, strain, and energy accumulation and dissipation, which are consistent with the superior mechanical properties of the rock. Based on these observations, it can be inferred that the wave impedance mismatch and stiffness ratio at the coal–rock interface play a key role in controlling stress wave transmission/reflection and strain incompatibility; however, the individual contribution of each factor warrants further dedicated investigation.
Understanding fatigue damage evolution of brittle rocks under cyclic loading is critical for assessing the stability of deep underground geo-systems, yet the loading-history dependence of damage indicators remains insufficiently studied. In this work, cyclic loading–unloading, multistage constant-amplitude loading, and constant-amplitude-cyclic loading tests were conducted on brittle rock specimens. The results show that the secant modulus exhibits an initial increase followed by degradation, while cumulative residual strain develops in three characteristic stages. Energy components (total, elastic, and dissipated energies) increase progressively with cycle number, with dissipated energy showing accelerated growth prior to failure. Fatigue damage was quantified using the modulus method, residual strain method, energy method (EM), and a newly proposed residual deformation–energy method (REM). A comparative analysis demonstrates that the EM provides stable damage characterization under cyclic loading–unloading and multistage loading, whereas the proposed REM more effectively captures damage evolution under constant-amplitude-cyclic loading. These findings clarify the loading-history dependence of fatigue damage metrics and provide guidance for selecting appropriate damage indicators in laboratory evaluation and engineering applications.
The dynamic compression fracture of brittle rock under seepage pressure is a critical issue for deep underground engineering. It directly influences the stability and safety of the surrounding rock during blasting or seismic loading. However, research on the mechanisms of microcrack evolution under these coupled conditions is still lacking. The relationship between microcrack evolution and macroscopic mechanical properties also remains poorly understood. This study develops a micro–macrofracture model grounded in the wing microcrack propagation framework, integrating both mechanical and chemical interactions between free water and rock. Mechanically, it incorporates seepage pressure, dynamic Stefan force, and dynamic fracture toughness. Chemically, it accounts for the effects of saturated water on rock mechanical parameters. This model characterizes the total stress–strain constitutive behavior of brittle rock under varying seepage pressures during dynamic compression failure, encompassing both strain-hardening and strain-softening phases. This result is validated against experimental data. It accounts for the influence of seepage pressure on both the initial crack and the newly formed wing crack. The seepage pressure weakens the wedging force F W on the initial crack while enhancing the seepage tensile force F P on the wing crack, which constitutes the seepage pressure-driven crack growth mechanism. Furthermore, under the combined effects of dynamic loading and free water, the dynamic Stefan force F S and the dynamic fracture toughness K ICD serve as the mechanism for inhibiting crack growth. The combined influences of seepage pressure, confining pressure and initial crack characteristics on the dynamic mechanical behavior of brittle rock under seepage pressure are discussed.
Taking argillaceous sandstone collected from foundation pits as the research object, laboratory tests including drying-wetting cycles, uniaxial compression, cyclic disturbance, and acoustic emission (AE) monitoring were conducted to investigate the deterioration and fatigue damage evolution laws. The results show that, under the influence of drying–wetting cycles, the compressive strength and elastic modulus of argillaceous sandstone both decline following a power-function attenuation trend, while the ultimate fatigue strain under cyclic disturbance rises, and the spatial distribution of AE events grows more scattered. In contrast, a higher upper-limit stress ratio yields a lower ultimate fatigue strain and induces more significant spatial clustering of AE events along the shear failure plane. After being subjected to drying-wetting cycles, the fatigue failure threshold of argillaceous sandstone remains stable within the range of 75%–80% σ . Fatigue life presents a log-linear correlation with the upper-limit stress ratio, and varies in accordance with a power function against the number of drying-wetting cycles. On this basis, a three-dimensional quantitative model was established to characterize the fatigue damage evolution of argillaceous sandstone. The research findings can provide a scientific basis for the design, construction, and stability control of foundation pits in red-bed soft rock.