ISRM Suggested Methods for rock stress estimation—Part 1: Strategy for rock stress estimation☆

Coal-rock masses located in water-rich environments, such as heavy water mines, and water-isolated coal-rock pillars installed underground, are consistently subjected to erosion from acidic groundwater. In this study, we employed various techniques including acoustic emission (AE), digital image correlation (DIC), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to investigate the evolution of mechanical properties and AE damage characteristics of coal-rock combinations under water-chemical corrosion. Moreover, we analyzed the microscopic deterioration mechanism. Our results reveal that as the acidity of the solution increases, the cumulative energy and AE count of the specimen decrease to varying degrees. The AE ringing counting process can be categorized into stable, active, and sharp development stages. Moreover, as the pH value of the solution decreases, the AE b-value follows a “V" type variation. The dynamic b-values demonstrate localized sudden drops, indicating the occurrence of large-scale cracks. The internal cracks within the coal-rock combination transform from shear cracks to tensile cracks, corresponding to a transition in failure mode from shear failure to cleavage failure. Furthermore, a detailed analysis of the deterioration mechanism of the coal-rock combination under water-rock interaction was conducted. Under on-site groundwater conditions (pH = 6.24), the overall degradation of uniaxial compressive strength (UCS) and elastic modulus reached 13.37% and 28.73%, respectively. The physical, chemical, and mechanical effects of the water-chemical solution induce microstructural changes within the coal-rock combination. Consequently, the total percentage of macropores (>1 μm) in the sandstone section increased by 8.85%, reaching 35.3%, thereby further deteriorating its mechanical properties. In contrast, the proportion of macropores in the coal section only increased by 0.95%. Therefore, water-rock action has a more severe damaging impact on the sandstone section compared to the coal section of the combination.

The relationship between initiation pressure and breakdown pressure, two vital parameters in hydraulic fracturing operations, requires further investigation, with the explicit mechanism underlying breakdown pressure yet to be revealed. This study presents an enhanced particle discrete element model with a regular-shaped borehole, wherein the pressure-updating equation of the pipe network flow model is improved to incorporate comprehensive fluid–solid coupling. Through simulation of the mesoscopic process of hydraulic fracturing initiation and propagation, the characteristic curve of borehole fluid pressure is obtained. By comparing the curves of borehole pressure and fluid flow into the fracture, the physical meaning of breakdown pressure is explained at the mesoscopic level. The results show that the breakdown pressure fundamentally arises from the unstable propagation of the fracture, with its numerical value corresponding to the borehole fluid pressure when the fluid flow into the fracture reaches the injection rate for the first time. Additionally, employing fluid–solid coupling modeling, the impacts of fluid viscosity, injection rate, and borehole diameter on breakdown pressure are investigated. Finally, the evolution of fracture morphology under different injection rates and elastic moduli is analyzed.

Numerical back analysis based on incomplete in situ stress measurements is widely used to estimate the three-dimensional (3D) in situ stress fields of large deep underground caverns. However, the variable collinearity caused by complex geological environments has rarely been considered. This paper proposes a numerical back analysis method for 3D in situ stress fields based on stepwise regression (BSSR). In BSSR, the collinearity between independent variables is associated with the surface topography, and insignificant variables caused by variable collinearity are eliminated through stepwise regression. BSSR was applied to the underground powerhouse of a hydropower station for validation, and the results showed that it can reliably estimate the 3D in situ stress field despite the complex geological environments. Multidimensional mathematical models of in situ stress fields were established with the improved predictive ability and clear physical meaning, which can quantify the contributions of six geological actions and three stress sources. The findings of this study can help with understanding the formation mechanism of in situ stress fields for large, deep underground caverns in complex geological environments and provide a useful reference for optimizing excavation schemes and support designs.

Cemented waste rock backfill (CWRB) is an important composite in supporting technology for safe and green production in mining. However, the mechanical performance of CWRB is highly correlated to cement consumption. This study utilized industrial graphene oxide (GO) and fly ash (FA) to create a high-performance, low-cost, and environment-friendly material for backfill. The results show that the GO-FA hybrid improves the microstructural matrix of hydrated cement by generating nucleation effects that refine the microstructure to a denser state and promote pore-filling effects that form barriers at pore throat junctions. This significantly reinforced the compressive strength (up to 53.6%) of the CWRB specimens. The reinforcing efficiency is more pronounced when using fine particle size distributions due to the lack of skeleton effect of gangue aggregates. By monitoring the acoustic emission and analyzing fractal properties, it can be further revealed that GO improves the integrity of CWRB material during loading. Compared to traditional backfill materials, the study found that 0.008 wt% industrial GO can assist FA to lower the consumption of cement by over 20% and improve the engineering properties by 2.6–53.6% at the same time. This research further proves that GO is promising for CWRB composites reinforcement and solid waste recycling.

Evaluating the similarity between measured and predicted stress states is critical for the numerical back analysis of three-dimensional in situ stress fields. However, conventional nontensor approaches process stress components or the magnitude and orientation of principal stresses separately, which may yield contradictory results among multiple indices. To address this issue, we propose the Euclidean distance similarity (EDS), which is a dimensionless single index based on spatial distance. As a case study, EDS was applied to a site with complex geological conditions for verification. EDS can be derived in the form of stress component or principal stress. EDS exhibited a positive correlation with the initial difference and magnitude error of principal stresses and changed periodically with the Euler angle of rotation around the principal stress. Moreover, three magnitude errors and three Euler angles had different effects on EDS. Finally, EDS can be classified into five levels (i.e., extremely high, high, normal, low, and very low) according to the magnitude errors of principal stresses and Euler angles, where the “normal” level is recommended as a general error standard. The thresholds of EDS differ according to the initial stress level. These results are a valuable contribution to the similarity evaluation of stress tensors.