Investigating the mechanical behavior and microstructural evolution of granite under high temperatures is crucial for optimizing fracturing strategies and ensuring reservoir sustainability in enhanced geothermal systems (EGS) at the Qiabuqia geothermal field, China. This study conducted triaxial compression tests on granite from the Qinghai Gonghe Basin under temperature from 25 ℃ to 300 ℃, examining the effects of temperature and confining pressure on the mechanical properties and energy evolution of the granite. Additionally, X-ray diffraction (XRD) analysis and nanoindentation tests were employed to assess changes in micro-mechanical properties and mineral compositions. Furthermore, fracture mechanics principles, incorporating thermal stress effects, were utilized to calculate the initiation pressure of reservoirs at an engineering scale for geothermal development in the Qinghai Gonghe Basin. The results indicate that the compressive strength and elastic modulus of Gonghe granite increase with temperature up to 200 ℃ due to the enhancement of mineral mechanical properties and thermal densification, but significantly decrease at 300 ℃ due to thermal damage and fracture propagation. Energy analysis reveals that the granite undergoes a transition from brittle to ductile behavior under high-temperature conditions. The proportion of energy dissipation during deformation increases with temperature. The increased proportion of quartz, coupled with its high thermal expansion coefficient and elastic modulus, generates intense thermal stress at the interfaces between quartz and adjacent minerals. The development and propagation of transgranular fractures around quartz are critical factors influencing the macroscopic mechanical properties of granite. This study provides a good understanding of the effects of high temperature on granite performance and its engineering significance in reservoir development, emphasizing the role of thermal stress in reducing fracturing pressure and promoting fracture propagation.

The strain energy storage index (W_ET) is a crucial index for evaluating rockburst proneness. Interestingly, when conducting tests to obtain W_ET, variations exist in the shape of coal or rock specimens. However, whether shape factors affect W_ET has not been theoretically and experimentally verified. In this study, to investigate the independence of W_ET from specimen shape effects, its rationality was first theoretically derived based on the linear energy storage (LES) laws of rock, indicating that W_ET is influenced by the energy storage coefficient (ESC) of the rock. Two typical rock materials (granite and red sandstone) with different rockburst proneness were selected to verify the migration effect of cubic and cylindrical specimens on W_ET via uniaxial compression tests. The experimental results revealed that the mechanical behavior characteristics of rocks were affected by the shape of cylindrical and cubic specimens, whereas the W_ET and ESC were opposite. Furthermore, the practical W_ET values closely approximate the theoretical values of energy storage-dissipated ratio predicted by the LES law, converging to the peak-strength strain energy storage index (). Based on the LES law, the influence of specimen shape on W_ET and was further discussed, concluding that W_ET and are independent of specimen shape effects. Furthermore, the is more stable than W_ET and reflects the relative magnitude of energy storage and dissipation during the entire pre-peak of rock. Thus, the peak-strength strain energy storage index can be used as a substitute for W_ET in evaluating the rockburst proneness of rock.

The phase-field method (PFM) has emerged as a robust tool for fracture simulation; however, applying this technique to rock materials poses significant challenges, particularly in accurately modeling the propagation of multiple cracks in the presence of complex three-dimensional (3D) mixed-mode loading involving tensile, tensile-shear, and compressive-shear cracks. To address these limitations, this study aims to introduce an enhanced PFM that integrates frictional effects and Lode angle dependence while unifying the volumetric deviatoric (VD) and spectral decomposition (SD) methods. The proposed model incorporates a modified driving force for 3D compressive-shear cracks by embedding a triple shear energy strength (TSES) criterion within the energy decomposition framework. This refinement guarantees that crack behavior remains physically realistic under compression-dominated loading while effectively preserving well-established tensile fracture mechanisms. The validation of the numerical implementation is also conducted through both analytical verification against theoretical solutions and 3D finite element simulations of fissured rock and heterogeneous specimens. Furthermore, numerical case studies demonstrate the model's ability to effectively capture the 3D propagation of multiple cracks and replicate realistic true 3D mechanical responses. The findings present valuable insights and practical guidelines for the application of PFM in rock engineering.

Soil erosion induced by rainfall on slopes poses a significant threat to land sustainability and ecological balance. Enzyme-induced calcium carbonate precipitation (EICP), as an emerging environmentally friendly biomineralization technology, can form a stable crust layer on slopes, effectively reducing rainwater infiltration and enhancing soil erosion resistance. This study designed rainfall erosion model tank tests using soybean urease and cementation solution. The treatment effects were evaluated through macro and microscopic indicators, and the hydrological response of the slope under different rainfall conditions was analysed. The results indicate the calcium carbonate content (CCC) and crust thickness of the slope gradually increase while tend to saturate with treatments. The slope gradient exhibits a controlling influence on the crust distribution, with a systematic downslope shift in the peak thickness zone as the gradient increases. At the microscopic level, with the increase of treatment cycles, the pore volume is significantly reduced, and the particle surface is extensively coated with CaCO₃ precipitates. From a geomorphological perspective, untreated slopes develop rapid and deep gully networks, while treated slopes transition to smoother and more stable surfaces. Under high rainfall intensity, the erosion amount for the slope with ten cycles of treatment reduced significantly, and the maximum gully width and depth exhibit a decreasing trend with erosion amount. The surface runoff rate reaches the optimal performance after seven cycles of treatment, where a continuous uniform CaCO₃ crust significantly increases the runoff rate. The relationship for erosion, runoff rate, and infiltration coefficient with more treatments reflects a coordinated trend.

Hazardous geophysical granular flows, such as debris flows and rock avalanches, can exert intense impact forces on obstacles and threaten downstream structures located in their paths. Installing protective structures can mitigate damage, but quantifying their influence on flow evolution and impact loading remains challenging. This study investigates the interactions of granular shock waves (GSWs) generated in front of two cylindrical obstacles with varying spacings through chute experiments and discrete element modeling. Impact pressure sensors were mounted on the upstream surface of each cylinder and on the chute bed to measure dynamic impact pressures in the GSW region. Granular flow velocity and depth were obtained using image processing. Results demonstrate that cylinder spacing significantly influences the geometric characteristics of GSWs. Runup increases with steady-state Froude number (Fr_steady) but decreases as spacing narrows. The granular vacuum length grows with bed slope but decreases significantly with decreasing cylinder spacing. Impact pressures on the cylinders and the chute bed increase linearly with Fr_steady. Low-frequency power spectral density (PSD) is positively correlated with Fr_steady, whereas centroid frequency and pressure impulse counts exhibit low sensitivity to Fr_steady. The dimensionless impact pressure coefficient (α) decreases nonlinearly with increasing Froude number (Fr). At low Fr, α values for dry granular flows are lower than those for debris flows, but the difference diminishes at higher Fr. These findings may improve our understanding of granular flow-obstacle interactions and might help to design protective structures.

During geotechnical construction, flawed rock masses experience dynamic cyclic disturbances, leading to cumulative deformation and progressive damage. Consequently, elucidating the fracture mechanisms under cyclic loading is crucial for ensuring the safety and prolonged operation of deep underground engineering. This study investigated the mechanical responses of the surrounding rock at different locations by conducting triaxial tests on flawed granite using three distinct cyclic loading and unloading paths. Based on the maximum tangential stress criterion, a fracture mechanics model for open flaws was developed to analyze the intrinsic influence of confining pressure and flaw inclination on crack initiation behavior. The results indicate that graded unloading of confining pressure significantly weakens the flawed rock mass, reducing its peak stress to only 77.5 % of that observed under constant confining pressure. Conversely, flawed rock masses exhibit a substantial increase in bearing capacity under increasing graded cyclic loading, achieving a peak stress 19.3 % higher than that under cyclic disturbance loading. At a constant confining pressure of 40 MPa, the type of disturbance loading has no significant effect on the failure mode. The flawed granite specimens form a nearly V-shaped shear failure zone along the open flaw. However, confining pressure unloading induced a more complex shear-tensile composite failure mode in the specimens. The crack initiation angle increases nonlinearly with confining pressure, but decreases gradually as the flaw inclination angle (β) increases. These findings provide valuable insights for the safe construction of deep underground engineering.

Borehole instability in heterogeneous rocks poses a significant challenge in geo-energy engineering. The deformation and failure around boreholes are heavily mediated by the inherent heterogeneity of rocks. Here, we examined borehole breakout under hydrostatic pressure through both laboratory tests and numerical simulations on sandstone samples. Laboratory experiments demonstrated symmetrical V-shaped failures across various borehole diameters. To replicate these observations, we developed a heterogenous UDEC Voronoi model where the material heterogeneity was interpreted by assigning Weibull-distributed inter-grain contact parameters. The rigorous-calibrated numerical modeling can effectively capture the microscopic damage process and match the observed macroscopic failure modes. Simulations showed that reducing the borehole diameter increases the critical hydrostatic pressure required for borehole failure and prompts a shift from tensile to shear-dominated failure behavior. While stress anisotropy primarily governs the overall breakout morphology, rock heterogeneity influences the specific locations of crack initiation, leading to localized stress concentrations that shape the ultimate failure patterns. These results provide valuable insights into borehole stability in heterogeneous rocks and guide engineering design and pertinent risk assessment.

Evaluation of compressive strength in underground lining structures is critical for ensuring structural integrity and safety. Traditional assessment methods are often destructive, time-consuming, and impractical in confined environments such as tunnels and utility corridors. This study introduces an automated, nondestructive approach to visualize and estimate the compressive strength of underground concrete lining using hyperspectral imaging (HSI) combined with deep neural network (DNN) models. High-dimensional spectral data of concrete lining are assembled and trained to develop two DNN-based regression models, namely the Mono-Spectrum Deep Neural Regressor (MS-DNR) and the Segmented-Spectrum Deep Neural Regressor (SegS_DNR). Utilizing the SegS_DNR model, two-dimensional (2D) compressive strength distribution heatmaps were generated for visualization and assessment of strength variations. The SegS_DNR model demonstrated excellent predictive performance, achieving a coefficient of determination () of 0.925 and a Residual Prediction Deviation (RPD) of 5.28 on the testing set for compressive strength estimation. The idea is further validated in site by investigating the capability of identifying the defect regions of the tunnel concrete lining, namely the cracked, spalling, and leaking areas, and demonstrated promising performance in comparison with experienced inspectors on site. This approach offers a contact-free technique for automated structural health monitoring, contributing to safer and more sustainable underground maintenance practices.

During unconventional energy extraction, substantial volumes of fluid are injected into low-permeability reservoirs to facilitate hydraulic fracturing, creating an extensive network of fractures that enhance fluid mobility. However, such large-scale fluid injection can lead to the initiation and propagation of fractures, potentially triggering detectable seismic events that pose risks to human life and infrastructure. To better understand these processes, in situ dynamic scanning imaging of hydraulic fracture propagation and water-rock interactions in tight sandstones has been conducted using X-ray computed tomography (CT). Our experimental findings reveal that fluid infiltration weakens rock strength, thereby promoting rock failure. Under the influence of fluid injection, microfractures undergo a continuous cycle of generation, expansion, and coalescence, ultimately forming interconnected hydrological pathways. These pathways are critical for the sustained propagation of fractures within the rock. CT imaging highlights a positive feedback loop between fracture growth and the enhancement of fluid diffusion. Notably, the rock at the dry-wet interface of the fluid front is particularly susceptible to fracturing. Additionally, the rates of fracturing vary among different fractures and tend to progressively decrease as the fractures extend deeper into the rock.

Thermal cycling and stress fatigue are recognized as principal factors that induce the Kaiser effect of rock in deep earth rock engineering. Nevertheless, existing scholarly investigations about the mechanical properties of rocks subjected to the synergistic effects of these perturbations have remained insufficient. In this study, conventional triaxial compression tests, multistage equal-amplitude fatigue (MEF) and multistage variable-amplitude fatigue (MVF) tests were conducted on marble subjected to different numbers of thermal cycles, integrated with nuclear magnetic resonance (NMR) and depth-sensing indentation (DSI) micro-monitoring methods, and the rock constitutive equation was established from the perspective of statistical microscopic damage. The results indicated that the increasing number of thermal cycles significantly weakened the physical and mechanical properties of marble, as evidenced by degradations in strength, deformation, and energy parameters. The reversible deformation evolutions of the rock under two stress paths were diametrically opposed. DSI results revealed that the microcellular mechanical parameters of hornblende and dolomite exhibited greater variability, although both conform to Weibull distribution functions. Additionally, NMR analysis showed that the porosity of the marble was 1.6% initially and increased to 3.3%, 4.1%, 5.8%, and 10.9% after 2, 4, 6, and 8 thermal cycles, respectively. The coupled thermal-mechanical damage constitutive model can effectively describe the deformation behavior of marble under complex perturbations, with distribution parameters m₀ and T₀ decreasing linearly with the number of thermal cycles.