We performed stick–slip tests on planar rock joints under different normal stresses to compare the effects of alternating shear rates (ASR) and constant shear rates (CSR). ASR tests included shear rate ratios from 1.5 to 50. We assessed how alternating shear rates influence frictional behaviors and microseismic activity. Testing results show that at normal stresses above 3 MPa, CSR tests exhibit larger mean recurrence intervals and stress drops than the high-rate segments of ASR tests at the same shear rate. Healing rates derived from stress drops increase with normal stress, while those based on friction drops show the opposite trend. Under normal stresses ≤3 MPa, ASR healing rates display a non-monotonic dependence on shear rate, contrasting with the steadily decreasing trend in CSR. Acoustic emission measurements reveal that ASR high-rate segments generate higher AE energies and amplitudes than CSR. Lab test-based fitting shows that parameters a and b in rate-and-state friction (RSF) formulation vary under ASR. We propose a modified RSF model in which these parameters explicitly depend on normal stress and shear rate alternation, it explicitly captures the effect of shear rate alternation on parameters a and b in RSF model.
With the depletion of shallow resources, the excavation and blasting of deep rock formations have become essential in mining. However, this process faces challenges due to the complex mechanisms of rock damage evolution under high ground stress. Most current research on cut blasting focuses on shallow conditions and lacks systematic analysis of the influence of ground stress. This study investigates the damage evolution induced by cut blasting in deep, complex stress fields through theoretical analysis and numerical modelling. An analytical model is first developed to describe the stress field around the blast-hole, integrating quasi-static ground stress and dynamic blasting loads. The reliability of the numerical approach is validated using the GPGPU-parallelized Hybrid Finite-Discrete Element Method (HFDEM) and empirical formulas for peak particle velocity (PPV). A parametric study is then conducted to explore the effects of key factors, such as ground stress levels, hole spacing, and blast-hole radius, on damage evolution. The results show that rock damage in deep cut blasting arises from the combined influence of ground stress and explosive stress waves. High ground stress suppresses the expansion of damage and fracture zones. While hydrostatic pressure leads to uniform zone expansion, non-hydrostatic pressure causes directional growth. A relationship between rock damage, PPV, and blast centre distance is established, offering insights into safe PPV thresholds under varying ground stress conditions. As ground stress increases, the effective blasting area decreases, and unexploded zones within the trench cavity expand, reducing blasting efficiency. To address this, two optimization strategies are proposed: reducing hole spacing and increasing void radius. Comparative analysis indicates that both methods enhance crack penetration between holes, but increasing the void radius proves more effective, yielding gentler, more stable vibrations in surrounding rock and better control over crushed zone size. These findings offer quantitative insights into the damage evolution mechanism of deep cut blasting and provide a reference for optimizing blasting parameters in deep rock excavation projects. Due to the simplification of the 2D modelling approach, the results should be combined with field conditions for practical application.
In deep underground engineering (e.g., tunnels, mining, hydropower), dynamic disturbances like blasting and mechanical vibrations frequently induce shear slippage along structural planes in rock masses, particularly in anisotropic layered limestone under true triaxial (3D) stress, leading to instability; however, quantitative analysis of 3D multidirectional disturbance-coupling effects remains insufficient. Compared with conventional unidirectional disturbance conditions, multidirectional disturbances under true triaxial stress may induce stress redistribution, directional interaction, and asymmetric crack propagation, fundamentally affecting the shear instability behavior of rock discontinuities. This study investigates the shear failure behavior of limestone intermittent structural planes under true triaxial stress with varying ratios of disturbed stress amplitudes in different directions. Using a self-developed dynamic-static coupled true triaxial shear testing system, the effects of normal-to-lateral (An/Ap) and shear-to-lateral (Aτ/Ap) disturbance stress amplitude ratios on mechanical behavior, energy evolution, and fracture mechanisms are analyzed. The disturbance amplitude ratios were introduced to characterize the interaction intensity among multidirectional disturbances under true triaxial stress conditions. Acoustic emission (AE) monitoring is employed for precursor analysis of failure. Key findings include: 1). Shear capacity of limestone rock discontinuities decreases significantly with increasing disturbance stress amplitude ratios. When An/Ap increases from 0.2 to 4, the failure shear stress decreases by 7.9
Discontinuity roughness governs the shear strength, deformability, and hydraulic behavior of rock masses. Photogrammetry (including UAV surveys) and LiDAR now provide dense three-dimensional (3D) digital elevation models (DEMs) of joint surfaces; however, the practical translation of 3D-only descriptors into the two-dimensional (2D) indices embedded in engineering practice—such as Z2 and the joint roughness coefficient (JRC)—remains limited, and profile-based estimates depend strongly on scanline selection and observation scale. Here, we present a DEM-based framework that (i) applies a convergence-based, overlap sampling procedure to establish representative numbers and spacing of 2D profiles and 3D sub-surfaces, and (ii) provides an explicit conversion from 3D descriptors to Z2. Scale effects were quantified on three 1 m natural discontinuities using Z2 and fractal dimension (FD); representative estimates were achieved at profile spacing of approximately 1.7–3.4
As global oil and gas exploration advances into deep-sea and deep-earth reservoirs, the precise prediction of in situ rock mechanical parameters at depth has become a critical challenge for development optimization. Traditional macro-scale experiments are constrained by sample size and data volume, while nanoindentation technology, though capable of obtaining micro-scale continuous data, struggles to guarantee in situ conditions. Consequently, there is an urgent need to establish a set of in situ measurement techniques for downhole testing. This paper proposes an integrated framework for predicting in situ mechanical parameters, combining multi-scale experiments, numerical simulations, deep learning, and spatial interpolation algorithms. Nanoindentation experiments and numerical simulations were conducted to construct an inversion dataset encompassing load–displacement curves, elastic modulus, maximum load, maximum indentation depth, residual indentation depth, temperature, pressure, and other parameters. A Oliver–Pharr theory-constrained deep neural network establishes a single-point indentation FEM-DNN inversion model, achieving an accuracy of up to 97.19
Seismic-induced instability of large-scale hydraulic slopes poses a severe threat to engineering safety, and conventional anchor cables are generally inadequate for large deformation and high energy absorption demands. To address this challenge, two novel energy-absorbing anchor cable devices—integrating constant-resistance (CR) energy absorption and yielding-buffering mechanisms—were developed based on the working principles of CR apparatuses and spring/hydraulic buffering structures. The quasi-static pull-out tests were conducted using scaled models to evaluate their mechanical performance. The results demonstrate that both devices exhibit a distinct dual-stage energy absorption characteristic, namely, “constant-resistance sliding to pressure-yielding yield”, with the CR component serving as the primary energy dissipator. The peak load provided by the CR component of this study is approximately 25.2 kN. For the spring-integrated CRLD composite energy absorbing anchor cable (CS device), the ultimate yielding load ranges from 16.3 to 39.2 kN, yielding a total energy absorption of 0.82 1.80 kJ, the CR component contributes 70.4–95.2
The cracking and damage characteristics of hot dry rock (HDR) subjected to cyclic thermal loading under different confining pressures are essential when constructing heat storage with high efficiency. In the present study, a series of physical and tensile mechanical tests were carried out using hollow cylindrical rock samples to investigate the properties of granite, which were subjected to cyclic thermal loading under two conditions (unconfined and confining pressure conditions). A wide variety of influencing factors were taken into consideration in the experimental study, including heating temperatures (100 °C–550 °C), thermal loading methods (water and liquid nitrogen cooling), confining pressure, and the number of thermal loading cycles. In order to obtain a deeper understanding of granite’s cracking and damage mechanism under different constraint conditions during cyclic thermal loading treatment, a grain-breakable model considering thermal effect was established by employing the block discrete element code, UDEC. The results demonstrated that granite samples showed a significant reduction in P-wave velocity and an increase in saturated water absorption after cyclic thermal loading treatment under unconfined condition, more pronounced at higher heating temperature. Cyclic thermal loading treatment caused an obvious reduction in granite tensile strength for both unconfined and confining pressure conditions. The decrease in tensile strength mainly occurred in the first few cycles, most pronounced in the first cycle, for all testing temperatures. The numerical findings indicated that the mesoscopic cracking characteristics of HDR during cyclic thermal treatment were highly determined by heating temperature and constraint condition, as well as cooling liquid and thermal loading cycles. The initiation and propagation of mesoscopic cracks were the result of the combined effects of heating temperature, constraint stress, and the number of thermal loading cycles. The existence of confining pressure limited the initiation and propagation of mesoscopic cracks in granite and weakened the influence of thermal treatment at low stress values and test temperatures, while it promoted the generation of thermal cracks and led to the loss of integrity of the hollow cylindrical rock sample at high confining pressures and temperatures.
Mechanical cutting in hard rock encounters challenges, such as severe pick wear, low breaking efficiency, and intense equipment vibration. Fracturing techniques demonstrate potential in improving rock cuttability by creating fracture interface (FI). Confining pressure is a critical factor governing the cuttability enhancement effect of FIs. In this study, the influence pattern of FI on rock cuttability under various confining pressures was revealed through cutting experiments, and the regulation mechanism of confining pressure on this FI effect was clarified. The results show that as the confining pressure increases, the cutting specific energy consumption first increases and then decreases, while the spalling block volume gradually increases. FIs weaken the peak cutting force and also induce the deflection of the fracture path, expanding the spalling block volume. As the confining pressure increases, this FIs’ effect gradually intensifies, with their influence on the spalling block volume being the most significant, with a maximum increase of 160
The Sichuan Basin is a major tight gas province, where the Upper Triassic Xujiahe Formation hosts significant continental shale gas resources. Understanding the frictional behavior of Xujiahe shale faults is essential for safe field development, yet systematic experimental data remain scarce. In this study, shale gouge from the third member of the Xujiahe Formation (Zhongba Gas Field) was tested under double direct shear configuration through velocity-stepping (VS) and slide-hold-slide (SHS) experiments at normal stresses of 1–4 MPa and shear velocities of 1–36 μm/s. The VS results show that the friction coefficient (μ) decreases with increasing normal stress due to enhanced compaction, but μ reflects frictional strength, not stability. Stability is evaluated by the rate-state parameter (a−b), and all tests yield positive (a−b) values, indicating velocity-strengthening behavior. The (a−b) values exhibit a non-monotonic dependence on normal stress, generally increasing from 1 to 2 MPa and decreasing at 4 MPa. Critically, reducing velocity step amplitude from the high-span (36–6-1 μm/s) to the low-span (25–5-1 μm/s) sequence systematically increases (a−b), demonstrating that smaller shear rate perturbations enhance fault stability. Both sequences display a "sawtooth" fluctuation pattern in (a−b) across successive steps. The SHS tests reveal that healing rate (β) decreases with normal stress and peaks at 5 μm/s, while relaxation rate (βc) shows stress-dependent reversal trends: at 4 MPa it follows β, whereas at 1–2 MPa it exhibits the opposite behavior, suggesting competing mechanisms across stress regimes. These findings provide quantitative constraints for fault stability in shallow and near-wellbore zones, but extrapolation to deep reservoir conditions requires independent verification due to potential shifts in (a−b) with depth. Overall, this study establishes a mechanistic basis for optimizing injection schedules to mitigate induced seismicity risks in the Zhongba Gas Field.
Oil shale is characterized by ultra-low porosity, permeability, and low organic maturity. The high heating temperature required for kerogen conversion and the low oil recovery remain the major challenges hindering the commercial development of oil shale. This study proposes a supercritical CO2 (SC-CO2) fracturing-assisted catalytic pyrolysis strategy; the complete fracture-transport-conversion process was systematically investigated using the pressure monitoring, X-ray CT reconstruction, fracture-surface topography, tracer-based XRF/SEM–EDS analysis, catalytic pyrolysis, and two-dimensional NMR characterization. Under identical laboratory boundary conditions, SC-CO2 fracturing reduced the average breakdown pressure by approximately 45.9
Ensuring the mechanical stability of salt caverns for deep underground energy storage is currently challenged by an insufficient understanding of the intrinsic mechanisms governing energy evolution and damage accumulation under varying loading rates. This study clarifies the loading-rate-dependent energy evolution and fatigue failure behavior of salt rock through uniaxial cyclic loading–unloading tests. A novel set of parameters for characterizing energy behavior is systematically defined. These include the equivalent energy amplitude, failure probability index, energy ratcheting index, energy history parameter, discrete energy spectrum, and historical energy-memory index. The findings demonstrate a rate-induced energy retardation effect, indicating that high-rate loading improves elastic storage while inhibiting viscous dissipation. A strong linear dependence is identified between the damage evolution rate and the reciprocal of the damage state in double-logarithmic space, quantitatively proving that the rate-induced energy lag effect amplifies the driving force for failure. These defined energy behavior parameters provide a multidimensional perspective for quantifying the overall energy excitation level of salt rock, its resistance to cyclic plastic flow, and the dynamic energy distribution within the frequency domain, respectively. Additionally, a nonlinear energy-based damage evolution model is proposed, utilizing the normalized historical energy memory index. The model encapsulates the three-stage nonlinear damage behavior, and experimental validation reveals significantly enhanced predictive accuracy and a superior capacity to identify critical failure characteristics in comparison to conventional theoretical models.
Calcitic marble undergoes simultaneous microstructural and macroscopic changes under high temperatures. To elucidate its thermal aging damage mechanism, this study subjected specimens to cyclic thermal aging at 400 °C with varying cycle counts. At the microscopic level, grain evolution was characterized using polarized light microscopy, pore-fracture network parameters (porosity, coordination number) via μ-CT, and surface fractal dimension/roughness via ImageJ processing. At the macroscopic level, compressive strength and ultrasonic wave velocity were measured synchronously. The results show a stage-dependent evolutionary trend (initial rapid degradation followed by stabilization) for ultrasonic wave velocity, compressive strength, pore-fracture network parameters, and fractal dimension under cyclic thermal aging. By correlating fractal dimension and open porosity with compressive strength, a three-level model (thermal cycles–microstructure–macroscopic performance) was established to quantify the cross-scale coupled thermal aging damage mechanism. The core mechanism is thermal stress-induced cross-scale synergistic degradation, involving sequential granular fragmentation and pore-fracture interconnection, with stabilization reflecting dynamic damage equilibrium. A thermal aging damage prediction model and a damage variable fitting equation correlated with compressive strength were also developed. These findings provide a scientific foundation and theoretical support for structural safety assessment of marble in engineering and thermal damage investigations of analogous rocks.
The formation of fracture networks plays a critical role in enhancing shale reservoir permeability and improving the efficiency of in-situ conversion processes. However, the mechanisms governing the formation and evolution of complex fracture networks under high-temperature conditions remain poorly understood. In this study, high-temperature in-situ X-ray computed tomography (CT) was employed to investigate the fracture evolution behavior of shale samples with different bedding orientations under stepwise heating. Based on three-dimensional reconstructions, fracture structures at successive temperature stages were extracted, and key structural parameters were quantitatively analyzed to systematically characterize fracture network evolution. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were integrated to examine fracture evolution from the perspectives of mineralogical changes and microstructural development. The results demonstrate that shale fracture evolution is governed by coupled thermal–mechanical–chemical processes. With increasing temperature, the dominant mechanism transitions from physically induced deformation to organic matter pyrolysis. Quantitative tracking of individual fractures shows that fracture network development is initially dominated by pre-existing natural fractures. At elevated temperatures, thermally induced fractures increase markedly in number and density and progressively connect with natural fractures, leading to the formation of a highly connected fracture network. This cooperative evolution fundamentally enhances shale reservoir permeability and underpins the effectiveness of in-situ conversion processes. These findings can guide in-situ heating operations, helping to optimize heating rates and fracture layout to improve fracture network control and overall process efficiency.
This work investigates the hydromechanical behaviour of Opalinus Clay (OPA) shale as a caprock material for geological storage of CO _2 . In addition to intact OPA, the hydromechanical response of crushed material is evaluated to simulate damaged zones around injection wells or faults. An initial experimental campaign was conducted to establish a recompaction protocol and to determine the main hydromechanical properties of crushed OPA, from both sandy and shaly-rich facies. The acquired hydromechanical and sealing properties of recompacted OPA were then compared to intact sandy-rich OPA under the same effective stress conditions. Although comparable bulk density and porosity were achieved, the recompacted material remained significantly weaker, exhibiting an elastic modulus approximately one order of magnitude lower than that of the intact material. The water permeability of the recompacted samples approximately one order of magnitude higher, with an average value of 4 × 10^-19 m ^2 , compared with 2 × 10^-19 m ^2 for intact sandy-rich OPA. Constant rate gaseous CO _2 injection tests resulted in comparable levels of breakthrough pressure, with average pressure equal to 1.60 MPa for the recompacted and 2.17 MPa for the intact OPA material. To further analyse and interpret the hydromechanical and sealing response of the caprock material, X-ray tomographies of intact, pre-fissured and crushed OPA samples were acquired at different levels of confining and pore water pressure. The observed microstructural evolution of crushed OPA during mechanical compaction and water resaturation, highlighted the role of mineralogical and structural heterogeneity in the material’s self-sealing response.
The accurate and real-time acquisition of rock strength parameters is very important for underground engineering construction. It is difficult for traditional laboratory testing methods to restore the rock strength properties in the field environment. The drilling test method provides a new way to obtain rock strength parameters in situ. However, the influence of confining pressure on drilling parameters and rock strength parameters should be considered under the conditions of high stress in deep surrounding rock. Based on this fact, digital drilling tests of specimens with different strengths under true triaxial conditions are carried out in this paper. The response rules of drilling parameters and cutting energy to changes in confining pressure and the response rules of cutting energy to variations in rock strength are clarified. An inverse model of equivalent compressive strength of rock while drilling is established, and the strength strengthening efficiency of 5–25 MPa confining pressure on different types of specimens is determined. Based on the above research, this study proposes a test method of surrounding rock strength while drilling under true triaxial conditions, providing a theoretical foundation for the in situ measurement of surrounding rock mechanical parameters in underground engineering.
Hydraulic fracturing in crystalline rocks is governed by the interaction of in situ stress, fluid pressure, and rock microstructure, yet the processes preceding fracture initiation and propagation remain insufficiently resolved. This study investigates hydraulic fracturing in a cubic specimen of Mrákotín granite subjected to controlled true-triaxial stress conditions. The experimental design integrates strain-controlled loading, stepwise fluid injection, acoustic emission (AE) monitoring, and active ultrasonic velocity measurements. The experiments identify a distinct preparatory phase before macroscopic breakdown, characterised by low-energy AE activity during the gradual increase in injection pressure (Pinj). These precursor signals indicate early-stage hydraulic opening and microcrack nucleation near the injection interval. The breakdown pressure (Pb) marks a transition to rapidly evolving hydraulic connectivity, followed by fracture propagation at a stabilised propagation pressure (Pprop). During fracture growth, AE activity is dominated by numerous low-energy events, contrasting with the higher-energy signals observed during purely mechanical loading and stress-hold periods. Post-fracturing analyses reveal persistent elastic anisotropy, modified pore-size distributions, and increased hydraulic conductivity. Ultrasonic measurements performed on a spherical specimen demonstrate that hydraulically induced fractures do not fully close under hydrostatic confining pressures up to 120 MPa, indicating a fundamental difference between hydraulic and thermally induced damage mechanisms. The combined use of active ultrasonic monitoring and passive AE recording enables time-resolved discrimination between fracture initiation, stable propagation, and fluid-driven reactivation. Ultrasonic data track the continuous evolution of elastic stiffness, whereas AE constrains the spatio-temporal development of microcracking. The novelty of this study lies in the quantitative integration of both datasets under controlled hydraulic loading, allowing identification of transient fracture stages not resolvable by either method alone. These laboratory results provide process-based constraints for hydro-mechanical model calibration and improved assessment of fracture growth dynamics relevant to induced seismicity mitigation in geothermal and unconventional reservoirs.
Accurate estimation of in situ stresses is essential for safe design of underground civil and mining engineering structures. Traditional in situ stress measurement methods often require extensive time, resources, and access to test location. Moreover, with these methods, it is challenging to consider the effect of high uncertainty due to limited number of tests. A laboratory-based method deformation rate analysis (DRA) is considered as promising approach allowing the use of core samples to infer in situ stress conditions and the consideration of variability and uncertainty. The key to the successful use of DRA method is the accurate detection of inflection point on stress vs strain-difference curve signifying the transition associated with prior stress states. Traditionally, this point has been determined through subjective visual assessment. Therefore, recent studies have searched for standardized methods to reduce subjectivity. This paper introduces an objective and transparent repeatable mathematical model for the detection of inflection point. First model parameters were fine-tuned using the results of DRA experiments conducted on sandstone samples. Then, the constructed model was implemented on unseen data or validation data collected from the current literature. The results show that the model presents acceptable accuracy for the detection of inflection point independent from lithologies, measurement tools, stress path, and preloading conditions.
This paper reveals the mechanisms through which fracture roughness governs the permeability reduction process driven by microbial-induced calcium carbonate precipitation (MICP) and its efficacy in underground space fractures. Three fracture surfaces with different roughness grades were selected, namely, smooth (JRC = 0), slightly rough (JRC = 3.51) and rough (JRC = 6.74). This study explored the variations in seepage reduction performance under three different roughness levels by considering three core grouting factors: flow rate, cementing solution concentration, and the volume ratio of bacterial to cementing solution. This study adopted mass, coverage area ratio, average thickness and filling ratio of induced CaCO3 to assess seepage reduction performance. Some interesting phenomenon could be found. First, increased fracture roughness contributes to enhance the seepage reduction effect by MICP technology for fracture within a certain range of apertures. Fracture roughness enhances CaCO3 precipitation efficiency through modifying slurry migration characteristic and offering additional microbial attachment sites, thus improving seepage reduction performance. Second, the extent to which fracture roughness influences the effectiveness of permeability reduction varies depending on these three examined grouting factors. The fracture roughness significantly affects seepage reduction at low flow rate, relatively high cementing solution concentrations, and a low ratio of bacterial to cementing solution. Third, this study established the relationship between the induced CaCO3 mass within fractures and hydraulic conductivity. In addition, the differences in precipitation efficiency among the three roughness levels have been compared. The comparison in two aspects reveals that fracture roughness could enhance CaCO3 precipitation efficiency.
Understanding how bedding-controlled fractures initiate and evolve in thermally treated shale is important for clarifying the deformation and failure mechanisms of anisotropic rocks. However, the fracture response of laminated shale is difficult to characterize, because it is jointly governed by matrix strength, bedding-plane weakness, and thermal effects. In this study, the thermo-mechanical failure behavior of Gulong shale from the Songliao Basin was investigated by integrating elevated-temperature uniaxial-compression tests, real-time acoustic-emission monitoring, and thermally coupled discrete-element modeling. Specimens with bedding angles of 0°, 30°, 45°, 60°, and 90° were tested at 100, 130, and 160 °C, and acoustic-emission waveform parameters and spatial localization results were used to characterize the temporal and spatial evolution of damage. The results show that bedding orientation is the primary factor controlling strength anisotropy and dominant failure mode. Intermediate bedding angles promote bedding-related slip and mixed tensile–shear damage, whereas the 0° and 90° specimens mainly exhibit splitting-dominated failure. Kernel density analysis of the log-transformed RA–AF distributions indicates systematic migration of the dominant acoustic-emission populations with bedding angle, reflecting a transition from relatively abrupt matrix cracking to more progressive frictional and bedding-related damage activity. The RA–AF distributions are, therefore, used to characterize the dominant fracture tendencies at different stages rather than to classify every acoustic-emission event deterministically. The calibrated discrete-element model captures the principal stress–strain characteristics, macroscopic failure patterns, and normalized damage-evolution trends inferred from acoustic-emission monitoring by separately representing the intact matrix and bedding interfaces. The numerical results further indicate that increasing temperature changes contact-force redistribution and damage accumulation before and during loading, while bedding orientation governs the preferred localization and coalescence paths of fractures. These findings establish a consistent relationship among macroscopic mechanical response, acoustic-emission activity, and numerical fracture evolution, and clarify the combined effects of bedding orientation and temperature on damage localization and failure behavior in Gulong shale.
To investigate the grain-scale mechanisms of strainburst, a three-dimensional grain-based discrete element model is developed under a true-triaxial unloading stress path. By adopting staged damping control, the accuracy of ejection kinetic energy is improved. Combined with multilevel crack identification and energy tracking, quantitative analysis of microcrack evolution and energy partitioning are performed. On this basis, the effects of grain size and burial depth are further examined. The proposed PFC3D-GBM effectively captures the multilevel failure characteristics of strainburst and reproduces the progressive transition from small grain ejection to spalling and violent fragment ejection. Most of the energy is dissipated through local sliding and damping during strainburst, and only a small amount of excess energy is converted into the kinetic energy of ejected particles. The kinetic energy conversion ratio is generally within 0.5
