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.

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.

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.

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.

This study integrates unconfined compression tests with high-resolution computed tomography (CT) to analyze the pore heterogeneity, crack propagation, and failure modes of red sandstone specimens with diameters ranging from 10 mm to 100 mm. Key findings include: (1) With increasing specimen size, crack initiation stress (CI), damage stress (CD), and unconfined compressive strength (UCS) initially increase and then decrease, (2) In smaller specimens, stress concentration due to pore heterogeneity leads to splitting failure and lower strength, (3) In medium-sized specimens, friction dominates crack propagation, causing shear failure, while increased fragment rotation enhances energy dissipation, yielding highest strength, and (4) In larger specimens, cracks tend to propagate along bedding planes, reducing energy dissipation and then weakening strength. These results provide insights into the reverse size effect on sandstone strength and have implications for engineering applications.

Excavation-induced retaining wall deflection (RWD) significantly influences the safety of surrounding built environment. To predict the three-dimensional RWD in heterogeneous strata, a new partial differential equation (PDE) is derived in this study, and two prediction models are proposed, i.e. the physics-informed neural network (PINN) model and the data-driven PINN model. As a physical constraint, the new PDE is crucial to the loss functions of these models. Then, the validity of the models is verified and analysed using a subway deep-foundation pit. The results show that the training times of both models are controlled within 900 s, which is a significant reduction compared to that of the conventional numerical model. In addition, the prediction accuracy of the data-driven PINN model is higher than that of the numerical model, while that of the PINN model is slightly lower than that of the numerical simulation. However, in contrast to the data-driven PINN model, the PINN model can identify irregular soil interfaces in heterogeneous strata to learn the deflection continuity conditions at irregular interfaces and realize RWD prediction in non-uniform distributed strata. In practical applications in foundation pit engineering, the selection of the PINN and data-driven PINN models can be conducted according to the in situ distribution conditions of the strata to enable the early prediction of potential RWD, thereby providing a reliable basis for the further optimisation of retaining structures design.

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.

This study investigates the influence of mean stress and Lode angle on the mechanical behavior of porous sandstone. Sandstone specimens were tested using a newly developed true-triaxial loading apparatus under five constant Lode angle conditions and seven different mean stresses, covering a transition from brittle to ductile regimes. Based on the experimental results, three types of stress-strain responses were identified, transitioning progressively from Type 1, through Type 2 to Type 3 as the mean stress increases. Type 1 response represents typical brittle behavior, characterized by prominent shear fractures. Type 2 response corresponds to the brittle-ductile transition behavior, exhibiting non-penetrating shear fractures. Type 3 response is associated with ductile behavior, characterized by no visible shear fractures. The deviatoric stress initially increases and then decreases with increasing mean stress, forming a cap surface in the meridian plane. A generalized failure criterion is subsequently developed, capable of accurately characterizing this strength response. Furthermore, the brittle-ductile transition behavior is found to be significantly dependent on the Lode angle. Finally, the brittle-ductile transition boundary is described, incorporating the dependence of Lode angle.

Liquid nitrogen (LN₂)-assisted fracturing has emerged as a promising technique to enhance the productivity of hot dry rock (HDR) geothermal reservoirs. To elucidate the progressive mechanical degradation and fracture mechanisms of granite under cyclic thermal shocks, this study integrates ultrasonic testing, acoustic emission (AE) monitoring, three-dimensional profilometry, and uniaxial compression testing. Damage evolution was assessed through velocity attenuation, waveform distortion, and AE characteristics, while microcrack propagation and fracture morphology were analyzed using scanning electron microscopy and surface topography reconstruction. The degradation process exhibits a distinct cycle-dependent transition, evolving from tensile microcrack initiation during early cycles to shear-dominated failure during prolonged cycling. In Phase Ⅰ (1-3 cycles), initial thermal stresses induce axial tensile microcracks, leading to sharp decreases in P-wave velocity (53.45 %) and amplitude (40.55 %). Frequency analysis reveals a narrowing and convergence of secondary bands, whereas the fracture surfaces exhibit low undulation, dominated by tensile failure. In Phase II (3-20 cycles), shear-dominated damage progressively develops, as cyclic cooling enhances crack connectivity. AE activity intensifies sharply, correlating with macroscopic shear crack networks. Fracture surfaces evolve toward step-like morphologies, with roughness parameters increasing by up to 277.43 %, indicative of composite tensile-shear failure. Cyclic LN₂ cooling significantly lowers crack initiation stress and fracture energy, while promoting crack density and surface roughness. These findings provide critical insights into the mechanisms of LN₂-induced fracture enhancement, highlighting its potential to optimize HDR reservoir stimulation strategies.

The time-dependent failure of surrounding rock in deep engineering is essentially controlled by the evolution of microcracks, with the pre-existing fracturing state induced by excavation playing a crucial role in the subsequent time-dependent fracturing process. From the perspective of microcrack development, it is a continuous, dynamic process. Therefore, taking the microcrack propagation process as the fundamental principle, this paper proposes a novel three-dimensional (3D) time-dependent model for hard rock that can depict the entire fracturing process within a unified theoretical framework. This developed model discards the traditional tri-modal partition method based on deformation, and instead adopts an analysis approach centred on time-dependent tensile and shear fracturing. The results show that the time-dependent deformation of hard rock is the macroscopic manifestation of the progressive evolution of microcracks over time. Under true triaxial stress, the growth tendency of cracks in hard rock is orientation-dependent throughout the entire loading process. This developed model provides a mechanical explanation for key time-dependent fracture characteristics observed in true triaxial creep tests, including the anisotropy of time-dependent deformation and the preferred orientation of macroscopic failure plane, and provides a novel framework for elucidating the time-dependent failure process of hard rock.