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
Earthquakes are a key trigger for the instability of anti-dip bedding rock slopes (ABRSs). However, the seismic failure mechanisms of ABRSs with various slope angles remain poorly understood. In this study, the seismic response, damage evolution, and progressive failure of ABRSs with various slope angles (the stratum dip angles of slope models are 70°, with slope angles of 40°, 50°, 60°, and 70°) were investigated via four independent shaking table tests. The results reveal that the variation range of AAF_X caused by elevation changes is generally greater than that caused by horizontal changes. The AAF_X under natural waves is generally greater than that under Sine waves. Through multimonitoring data, the progressive failure process of ABRSs under earthquakes can be divided into four stages: crack initiation, crack propagation and potential sliding surface initiation, sliding surface formation and rock mass bending toppling, and shattering failure. Moreover, the critical damage threshold for the gentle angle slope (40° and 50°) is 0.4 g, and critical failure occurred at 0.6 g. This corresponds to the toppling–shallow sliding failure mode. The critical damage threshold for the steeper angle slope (60° and 70°) is 0.3 g, and critical failure occurred at 0.5 g. This corresponds to the toppling–shear overturning failure mode. These research results not only provide a theoretical basis for understanding the formation mechanism of anti-dip landslides under earthquakes but also help support the seismic protection design of anti-dip rock slopes.
Rockfalls represent a hazard to infrastructure, particularly for slopes along major roadways. Previous rock slope monitoring studies have linked rockfalls to precursors, including pre-failure deformation and the presence of previous, smaller spatially co-located precursory rockfalls. However, it has not been definitively established that apparent precursory rockfalls are not simply a product of coincidental co-location on a slope with a high level of background rockfall activity, and it is unknown whether precursory rockfalls cause an increase in the rate of pre-failure deformation. This study seeks to evaluate the possibility that precursory rockfalls are caused by coincidental co-location with future rockfalls and assess the interactions between precursory rockfalls and pre-failure deformation. We evaluated previously developed rockfall databases across four slopes and identified 120 rockfalls above a volume threshold of 0.1 m3. For each of these rockfalls, we determined whether there were any associated precursory rockfalls. A simulation was created to represent the “null” condition, where all rockfalls were assigned random locations according to a uniform distribution. The simulation results confirm that the number of precursory rockfalls is significantly higher relative to the null simulation, consistent with the idea that precursory rockfall occurrence is driven by mechanical interaction between adjacent rockfall blocks rather than being random in nature. Additionally, the displacement rate of rockfalls with precursory rockfalls was recorded over time. It was found that the rate of main block displacement does not often substantially increase after precursory rockfalls occur, suggesting that precursory rockfalls do not consistently contribute to pre-failure deformation.
In view of the problems of poor permeability of coal seams, low gas extraction efficiency in high-gas mines in China, and the reliance on experience in parameter selection of traditional hydraulic infiltration measures, multi-variable information redundancy, and difficulty in accurately identifying core influencing factors, this study takes 150 sets of field application data (50 sets each of hydraulic punching, cutting, and fracturing) as samples. Covering multiple geological conditions, such as coal seam burial depth, coal seam hardness, coal seam thickness and gas pressure, principal component analysis (PCA) was adopted to conduct multivariate statistical analysis, combining multiple test methods to quantitatively reveal the core influencing parameters and weight characteristics of the three types of measures. The results show that the parameters of the three types of measures all meet the conditions applicable to principal component analysis; the core principal components were coal seam burial depth, coal seam hardness, and coal seam thickness. The order of the contribution rates of the principal components was consistent as coal seam burial depth > coal seam hardness > coal seam thickness, and the cumulative contribution rates of the top three were 90.446–93.038
Anchor systems experience progressive performance degradation during long-term service, and incomplete inspection data further complicate time-dependent reliability assessment and long-term maintenance decision-making for anchored slope. To address this issue, based on field lift-off test and inspection data of 651 anchors, a Weibull hazard model considering interval-censored and right-censored sample is established to characterize stochastic degradation of anchor tension. On this basis, a three-dimensional mechanical model incorporating stochastic degradation is developed to quantify the time-dependent failure probability of anchored slope. Based on the reliability evolution, a resilience metric is introduced, and a bi-objective optimization model considering resilience and life-cycle cost is established. Subsequently, a genetic algorithm is employed to identify optimal maintenance strategies by determining maintenance timing, intensity, and type at different service stages. Finally, a life-cycle analysis framework integrating performance degradation, reliability evolution, and resilience assessment is developed for anchored slope. Result shows that the system reliability evolution exhibits three distinct stages, including slow degradation, accelerated deterioration, and a high-risk stage, with a critical transition point identified at approximately 30 years. Periodic predictive maintenance effectively delays performance degradation, suppresses instability development, sustains long-term high reliability, enhances resilience by approximately 35
Layered rock masses, widely encountered in mining, construction, transportation, tunneling, and hydraulic engineering, are highly susceptible to vibration‑induced damage and instability under dynamic disturbances such as blasting and earthquakes. Based on thin‑plate vibration theory, this study establishes a dynamic model for a homogeneous rectangular rock plate with clamped edges on all four sides. Modal analysis identifies the 1st-, 5th-, and 6th-order modes as the dominant effective vibration modes of the hard rock plate. The 1st mode governs the initial failure locations, while the 5th and 6th modes control the direction and trend of crack propagation. The failure follows a distinct temporal sequence: tensile–shear cracks first appear at the midpoints of the four sides and rapidly extend along the plate boundaries; subsequently, tensile cracks develop at the plate center, with the primary crack preferentially propagating along the long central axis and a secondary crack extending along the short central axis, eventually forming a characteristic “十‑✱” fracture pattern. Shear failure at the four corners acts as a supplementary mode, synergistically interacting with the tensile–shear failure at the side midpoints and the tensile failure at the center. This mechanism reveals the intrinsic linkage between vibration modes and damage evolution, providing a predictive framework for dynamic damage initiation and propagation in layered rock masses. The findings can be directly applied to stability assessments of underground tunnels by forecasting crack evolution under operational vibrations.
Sustained human and robotic exploration of the Moon, Mars, and beyond requires a thorough understanding of planetary regolith, the granular surface material covering rocky bodies. Limited availability of returned extraterrestrial samples makes terrestrially manufactured regolith simulants essential for technology development in mobility, excavation, in situ resource utilization, and infrastructure. Designing and using simulants involves trade-offs between compositional fidelity, physical property fidelity, safety, and material availability. This paper provides an overview of best practices and considerations in simulant development, informed by work at the University of Central Florida’s Center for Lunar and Asteroid Surface Science (CLASS). Key factors in simulant design, including mechanical, thermal, and electromagnetic properties, are discussed alongside standard characterization methods. The influence of environmental conditions and preparation protocols on experimental outcomes is also examined. Many of these lessons are not widely published, so this work aims to provide researchers, engineers, and technologists with guidance for creating high-fidelity, well-characterized simulants and experimental protocols. Recommendations include advancing standardized testing procedures, developing open-access simulant property databases, and building large-scale test facilities to improve reproducibility and relevance. A systems-level, interdisciplinary approach to simulant design and testing is critical for ensuring that terrestrial experiments accurately represent extraterrestrial surface conditions, supporting safe and effective planetary exploration and resource utilization.
The existing two-way four-lane highway tunnel cannot meet the growing traffic demand, necessitating the expansion and reconstruction of the existing tunnel to enhance its traffic capacity and safety. Based on a highway tunnel expansion project, this study investigates the in-situ mechanical behavior of small clearance and large section highway tunnel expansion through numerical simulation and model test. In numerical simulations, the patterns of circumferential deformation around the tunnel and stress evolution in the support structure of a two-hole, eight-lane tunnel were compared under four different expansion excavation methods. Considering deformation around the tunnel, proximity effects, stress in the support structure, and ease of construction, the three-bench method offers the greatest advantages for in-situ expansion excavation in existing tunnels. Specifically, the maximum horizontal and vertical displacements for the three-bench method were 4.54 mm and 15.18 mm, respectively, while the maximum tensile stresses in the primary support and secondary lining were 4.48 MPa and 1.27 MPa, respectively. At the same time, based on data from triaxial tests on Grade V surrounding rock and similar materials used in the support structure, model tests of tunnel expansion excavation using the three-bench method were conducted. By comparing the test results for vault settlement with those from numerical simulations, the reliability of the numerical simulation results was verified. Furthermore, model tests have further demonstrated that the impact of the following tunnel on the advance tunnel is primarily concentrated at the arch roof, inverted arch, and arch ribs of the advance tunnel—key locations where stress concentration and release occur during the expansion excavation process. The simulations and tests have revealed the mechanical behavior of large-section highway tunnels during in-situ expansion excavation, which holds significant implications for the construction of similar projects.
A comprehensive understanding of mechanical behaviors of shale under the coupled effects of true-triaxial stress and structural anisotropy is crucial for reservoir development. In this study, true-triaxial tests are conducted on Changning shale with bedding planes perpendicular (region I) and parallel (region II) to the maximum principal stress. The stress–strain curves and volumetric-shear strain trajectories show strong dependence on stress conditions and bedding orientation. Under true-triaxial stress conditions, a dominant macroscopic shear band forms within the bedding-containing plane in region I, while macroscopic shear bands develop within the bedding-free plane in region II. Microscopic thin-section analysis reveals that the main shear band within the bedding-containing plane is accompanied by bedding-controlled failure bands in region I, while it within the bedding-free plane is accompanied by multiple sets of parallel failure bands (en-echelon fractures) in region II. Shale exhibits pronounced strength anisotropy, with higher strength in region II than in region I. The difference in deviatoric stress between regions I and II is positively correlated with hydrostatic pressure and negatively correlated with the Lode angle. The main difference between isotropic and anisotropic criteria lies in the deviatoric function. Accordingly, three simple methods for anisotropic deviatoric functions are proposed in this study, including corner point method, variable parameter method, and anisotropy coefficient method. The effectiveness and accuracy of these three methods are verified using the strength data of Changing shale, San Francisco Bay mud, and Santa Minica Beach sand.
Deep metal mine shafts are subjected to complex stress and seepage conditions, making fractured surrounding rock prone to hydro-mechanical instability. Clarifying the interaction between seepage-pressure difference and lateral principal stress difference is essential for understanding the deformation, dilatancy, and failure of deep granite. True-triaxial stress–seepage tests were conducted along two paths, separately varying seepage-pressure difference and lateral principal stress difference while holding the other constant. A dimensionless hydraulic deviatoric ratio was introduced to characterize the competition between directional hydraulic driving and lateral stress anisotropy. Increasing seepage-pressure difference promotes pressure transmission, hydraulic activation, and crack opening, whereas increasing lateral principal stress difference intensifies stress anisotropy and shear-related damage. Across the tested ranges, the corresponding peak stress reductions were 28.18
Wellbore strengthening (WBS) is widely applied in field operations, yet most studies focus on water-based drilling fluids, and the WBS behavior under supercritical CO2 (ScCO2), which mitigates shale hydration and can increase the rate of penetration, remains insufficiently understood. Therefore, a fully coupled thermo-hydro-mechanical (THM) WBS model of ScCO2 first is developed that explicitly incorporates the temperature–pressure response characteristics of ScCO2, elastic–plastic deformation, and damage evolution. The effects of wellbore pressure, temperature, fracture length, and bridging sealing location on WBS are evaluated, and responses are compared with those of water-based fluids. Results indicate that increasing the ScCO2 wellbore pressure enhances fluid-induced compression within fractures near the wellbore, markedly improving pressure-bearing capacity. Cooling the formation with ScCO2 enlarges the fracture opening and increases the space available for plug materials. Longer fractures improve the compaction of the wellbore and, by through the formation of a "stress cage", further boost WBS effectiveness. And the bridging sealing location closer to the wellbore wall yields larger gains in pressure-bearing capacity. Relative to water-based fluids, ScCO2 requires lower injection pressure to place sealing materials, yet achieves higher pressure-bearing capacity, and water-based fluids tend to sustain larger fracture opening under comparable conditions. Global sensitivity analysis indicates the following parameter importance: for pressure‑bearing capacity, it is: wellbore pressure > bridging sealing location > fracture length > temperature; for fracture opening, it is: wellbore pressure > fracture length > temperature > bridging sealing location. These findings provide mechanistic guidance for the design of WBS during ScCO2 drilling.
The scale effect in rockfill materials leads to distinct differences in mechanical characteristics between scaled and prototype particles, primarily due to the inherent size dependence of particle crushing strength (PCS). This study presents a factorial design to clarify how microstructural heterogeneity and contact conditions govern the size effect of PCS. Discrete element simulations, validated against experimental results for limestone particles ranging from 20 to 60 mm through comparisons of force–displacement responses, Weibull statistics, and power-law size-strength relationships, are adopted to assess the relative importance of controlling factors. Based on 1920 single particle crushing simulations, elastic modulus is identified as the dominant factor affecting the size‑effect coefficient, followed by internal porosity. Joint effects between (i) elastic modulus and particle morphology, and (ii) the size ratio of elementary balls and particle morphology, also contribute non-negligibly to variations in PCS size dependence. Further analysis of the underlying mechanisms shows that particles with higher elastic modulus develop smaller contact areas and stronger internal stress concentrations, which in turn increase the variability of PCS and intensify the size effect. In contrast, for larger particles, compact pore clusters act as critical internal flaws and therefore lead to a more pronounced size effect with increasing internal porosity. These findings provide new insights into the governing factors of the size effect of PCS and may contribute to the development of more comprehensive constitutive models for crushable materials.
