During the excavation of large-scale rock slopes and deep hard rock engineering, the induced rapid unloading serves as the primary cause of rock mass deformation and failure. The essence of this phenomenon lies in the opening-shear failure process triggered by the normal stress unloading of fractured rock mass. In this study, we focus on local-scale rock fracture and conduct direct shear tests under different normal stress unloading rates on five types of non-persistent fractured hard rocks. The aim is to analyze the influence of normal stress unloading rates on the failure modes and shear mechanical characteristics of non-persistent fractured rocks. The results indicate that the normal unloading displacement decreases gradually with increasing normal stress unloading rate, while the influence of normal stress unloading rate on shear displacement is not significant. As the normal stress unloading rate increases, the rocks brittle failure process accelerates, and the degree of rocks damage decreases. Analysis of the stress state on rock fracture surfaces reveals that increasing the normal stress unloading rate enhances the compressive stress on rocks, leading to a transition in the failure mode from shear failure to tensile failure. A negative exponential strength formula was proposed, which effectively fits the relationship between failure normal stress and normal stress unloading rate. The findings enrich the theoretical foundation of unloading rock mechanics and provide theoretical support for disasters prevention and control in rock engineering excavations. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The use of abrasive waterjets (AWJs) for rock drilling offers advantages in urbanized areas, locations that are vulnerable to damage, and piling operations. However, the overall operational cost of AWJ systems remains high compared to that of conventional drilling methods, which constrains the long-term industrial application of AWJs. For instance, the abrasive costs account for over 60% of the total process cost, but the recycling of abrasives remaining after drilling could significantly reduce machining costs. In this study, the post-impact characteristics of abrasives were explored, aiming to enhance their recyclability. The physical properties and particle distribution of used abrasives vary depending on the jet energy, ultimately affecting their recyclability and recycling rate. The particle properties of used abrasives (particle size distribution, particle shape, and mean particle size) were compared under different waterjet energy variables (standoff distance (SOD) and water pressure) and test conditions (dry and underwater). Furthermore, the collision stages of the abrasive particles within a waterjet system were classified and analyzed. The results revealed that abrasive fragmentation predominantly occurred due to internal collisions within the mixing chamber. In addition, an attempt was made to optimize the waterjet parameters for an economical and efficient operation. The findings of this study could contribute to enhancing the cost-effectiveness of AWJ systems for rock drilling applications. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

In this study, a novel microwave-water cooling-assisted mechanical rock breakage method was proposed to address the issues of severe tool wear at elevated temperatures, poor rock microwave absorption, and excessive microwave energy consumption. The investigation object was sandstone, which was irradiated at 4 kW microwave power for 60 s, 180 s, 300 s, and 420 s, followed by air and water cooling. Subsequently, uniaxial compression, Brazilian tension, and fracture tests were conducted. The evolution of damage in sandstone was measured using active and passive nondestructive acoustic detection methods. The roughness of the fracture surfaces of the specimens was quantified using the box-counting method. The damage mechanisms of microwave heating and water cooling on sandstone were discussed from both macroscopic and microscopic perspectives. The experimental results demonstrated that as the duration of the microwave irradiation increased, the P-wave velocity, uniaxial compressive strength (UCS), elastic modulus (E), tensile strength, and fracture toughness of sandstone exhibited various degrees of weakness and were further weakened by water cooling. Furthermore, an increase in the microwave irradiation duration enhanced the damaging effect of water cooling. The P-wave velocity of the sandstone was proportional to the mechanical parameters. Microwave heating and water cooling weakened the brittleness of the sandstone to a certain extent. The fractal dimension of the fracture surface was correlated with the duration of microwave heating, and the water-cooling treatment resulted in a rougher fracture surface. An analysis of the instantaneous cutting rate revealed that water cooling can substantially enhance the efficiency of microwave-assisted rock breakage. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

Assessment of potential groundwater recharge sites and sustainable water resource management in semi-arid crystalline rock terrain is a challenging task. Globally, analysis of remote sensing satellite imagery data for delineation of groundwater potential zones over sheared crystalline hard rock terrains has been fairly successful. But there is no existing study present at our disposal which discusses the factors controlling the inconsistent groundwater potentiality that exits along the shear zones. This study attempts to analyse the major geological factors controlling the irregular groundwater potentiality of shear zones within older crystalline rock terrain. Therefore, the study area selected for this analysis is the Purulia district of West Bengal, NE India, composed mostly of Precambrian metamorphic rocks i.e., quartzite, granite gneisses, porphyroclastic granite-gneiss, quartzo-feldspathic-granite-gneiss, mylonitic granites, quartz-biotite-granite gneiss, quartzites, carbonatites and phyllites. Satellite imagery study of IRS-P6 LISS IV standard FCC image reveals the presence of two bifurcating shear zones namely North Purulia Shear Zone (NPSZ) and South Purulia Shear Zone (SPSZ) over the study area. Careful analysis of rock structure, different lithotypes, soil thickness, electrical resistivity tomography data and water table data with an emphasis on high water table fluctuation, shows a strong spatial relation between the potentially good groundwater recharge zones and the branching/confluence sites of shear zones present in the study area. The study constructs an attempt to demonstrate the relationship between shear zone conjunctions and significant groundwater recharge sites in Precambrian crystalline fractured-rock aquifer system.

To achieve the loading of the stress path of hard rock, the spherical discrete element model (DEM) and the new flexible membrane technology were utilized to realize the transient loading of three principal stresses with arbitrary magnitudes and orientations. Furthermore, based on the deep tunnel of China Jinping Underground Laboratory II (CJPL-II), the deformation and fracture evolution characteristics of deep hard rock induced by excavation stress path were analyzed, and the mechanisms of transient loading-unloading and stress rotation-induced fractures were revealed from a mesoscopic perspective. The results indicated that the stress-strain curve exhibits different trends and degrees of sudden changes when subjected to transient changes in principal stress, accompanied by sudden changes in strain rate. Stress rotation induces spatially directional deformation, resulting in fractures of different degrees and orientations, and increasing the degree of deformation anisotropy. The correlation between the degree of induced fracture and the unloading magnitude of minimum principal stress, as well as its initial level is significant and positive. The process of mechanical response during transient unloading exhibits clear nonlinearity and directivity. After transient unloading, both the minimum principal stress and minimum principal strain rate decrease sharply and then tend to stabilize. This occurs from the edge to the interior and from the direction of the minimum principal stress to the direction of the maximum principal stress on the epsilon 1-epsilon 3 1-epsilon 3 plane. Transient unloading will induce a tensile stress wave. The ability to induce fractures due to changes in principal stress magnitude, orientation and rotation paths gradually increases. The analysis indicates a positive correlation between the abrupt change amplitude of strain rate and the maximum unloading magnitude, which is determined by the magnitude and rotation of principal stress. A high tensile strain rate is more likely to induce fractures under low minimum principal stress. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

To investigate the long-term stability of deep rocks, a three-dimensional (3D) time-dependent model that accounts for excavation-induced damage and complex stress state is developed. This model comprises three main components: a 3D viscoplastic isotropic constitutive relation that considers excavation damage and complex stress state, a quantitative relationship between critical irreversible deformation and complex stress state, and evolution characteristics of strength parameters. The proposed model is implemented in a self-developed numerical code, i.e. CASRock. The reliability of the model is validated through experiments. It is indicated that the time-dependent fracturing potential index (xTFPI) at a given time during the attenuation creep stage shows a negative correlation with the extent of excavationinduced damage. The time-dependent fracturing process of rock demonstrates a distinct interval effect of the intermediate principal stress, thereby highlighting the 3D stress-dependent characteristic of the model. Finally, the influence of excavation-induced damage and intermediate principal stress on the time-dependent fracturing characteristics of the surrounding rocks around the tunnel is discussed. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).