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Unloading processes are common in natural systems. Intense unloading activities can alter the frictional equilibrium of faults and induce their instabilities. Understanding the slip behavior of faults under stress unloading conditions is helpful in guiding engineering practices. We conducted a series of direct shear experiments under linear-unloading normal force conditions considering the influences of initial normal forces, initial shear forces, and normal unloading rates on planar and rough granite fractures. The experimental results showed that planar fracture exhibits sudden slip events during normal unloading, while rough fracture mostly displays stable sliding behavior. The planar fracture demonstrates an exponential increase in sliding distance and velocity at the end of each slip event. The rough fracture usually exhibits a quasi-static stage before rapid slip events. In addition, the accumulative sliding distance at the slip activation moment (at the first moment when sliding velocity is greater than 0.05 mm/s) for the planar fracture decreases with lower normal unloading rate, larger shear force and larger normal force, while its variation trend for rough fracture is opposite. These findings provide valuable insights into fault slip behavior under stress unloading, aiding in mitigating associated risks in engineering applications. [ABSTRACT FROM AUTHOR]

The geometric properties of fracture surfaces significantly influence shear-seepage in rock fractures, introducing complexities to fracture modelling. The present study focuses on the hydro-mechanical behaviours of rough rock fractures during shear-seepage processes to reveal how dilatancy and fracture asperities affect these phenomena. To achieve this, an improved shear-flow model (SFM) is proposed with the incorporation of dilatancy effect and asperities. In particular, shear dilatancy is accounted for in both the elastic and plastic stages, in contrast to some existing models that only consider it in the elastic stage. Depending on the computation approaches for the peak dilatancy angle, three different versions of the SFM are derived based on Mohr-Coulomb, joint roughness coefficient-joint compressive strength (JRC-JCS), and Grasselli's theories. Notably, this is a new attempt that utilizes Grasselli's model in shear-seepage analysis. An advanced parameter optimization method is introduced to accurately determine model parameters, addressing the issue of local optima inherent in some conventional methods. Then, model performance is evaluated against existing experimental results. The findings demonstrate that the SFM effectively reproduces the shear-seepage characteristics of rock fracture across a wide range of stress levels. Further sensitivity analysis reveals how dilatancy and asperity affect hydraulic properties. The relation between hydro-mechanical properties (dilatancy displacement and hydraulic conductivity) and asperity parameters is analysed. Several profound understandings of the shear-seepage process are obtained by exploring the phenomenon under various conditions.

Photogrammetry, reconstructing three-dimensional (3D) models from overlapping two-dimensional (2D) photos, finds application in rock mechanics and rock engineering to extract geometrical details of reconstructed objects, for example rock fractures. Fracture properties are important for determining the mechanical stability, permeability, strength, and shear behavior of the rock mass. Photogrammetry can be used to reconstruct detailed 3D models of two separated rock fracture surfaces to characterize fracture roughness and physical aperture, which controls the fluid flow, hydromechanical and shear behavior of the rock mass. This research aimed to determine the optimal number of scale bars required to produce high-precision 3D models of a fracture surface. A workflow has been developed to define the physical aperture of a fracture using photogrammetry. Three blocks of Kuru granite (25 cm × 25 cm × 10 cm) with an artificially induced fracture, were investigated. For scaling 3D models, 321 markers were used as ground control points (GCPs) with predefined distances on each block. When the samples were well-matched in their original positions, the entire block was photographed. Coordinate data of the GCPs were extracted from the 3D model of the blocks. Each half was surveyed separately and georeferenced by GCPs and merged into the same coordinate system. Two fracture surfaces were extracted from the 3D models and the vertical distance between the two surfaces was digitally calculated as physical aperture. Accuracy assessment of the photogrammetric reconstruction showed a 20–30 μm digital control distance accuracy when compared to known distances defined between markers. To attain this accuracy, the study found that at least 200 scale bars were required. Furthermore, photogrammetry was employed to measure changes in aperture under normal stresses. The results obtained from this approach were found to be in good agreement with those obtained using linear variable displacement transducers (LVDTs), with differences ranging from 1 μm to 8 μm.

The infrared radiation temperature (IRT) variation concerning stress and crack evolution of rocks is a critical focus in rock mechanics domain and engineering disaster warning. In this paper, a methodology to extract the key IRT features related to stress and crack evolution of loaded rocks is proposed. Specifically, the wavelet denoising and reconstruction in thermal image sequence (WDRTIS) method is employed to eliminate temporal noise in thermal image sequences. Subsequently, the adaptive partition temperature drift correction (APTDC) method is introduced to alleviate temperature drift. On this basis, the spatial noise correction method based on threshold segmentation and adaptive median filtering (OTSU-AMF) is proposed to extract the key IRT features associated with microcracks of loaded rocks. Following temperature drift correction, IRT provides an estimation of the thermoelastic factor in rocks, typically around 5.29×10−5 MPa−1 for sandstones. Results reveal that the high-temperature concentrated region in cumulative thermal images of crack evolution (TICE) can elucidate the spatiotemporal evolution of localized damage. Additionally, heat dissipation of crack evolution (HDCE) acquired from TICE quantifies the progressive failure process of rocks. The proposed methodology enhances the reliability of IRT monitoring results and provides an innovative approach for conducting research in rock mechanics and monitoring engineering disasters.

Geometric morphological features significantly influence the shear mechanical properties of rock fractures, which are quantitatively assessed using roughness indicators. However, it remains uncertain whether fractures with identical roughness indicators exhibit similar shear mechanical behavior. This study employed three-dimensional scanning and the roughness index θ*max/(C + 1) to investigate the roughness of fracture profiles oriented differently on the fracture surface. The findings confirm that natural fracture profiles can have similar roughness levels but distinct morphologies. Consequently, a methodology was proposed to construct fractures with identical roughness but differing morphologies, based on 10 fracture profiles exhibiting varying roughness gradients. Numerical models of rough fracture samples were established, and direct shear tests were conducted using the discrete element method (DEM). The results reveal significant variability in shear strength, shear deformation, and macro-microscopic failure characteristics among fracture samples with identical roughness levels. As the θ*max/(C + 1) of fractures increases, more morphological asperities contribute to resisting external shear forces, although both the mean values and variability of shear mechanical indicators across fracture samples demonstrate an increasing trend. The distribution of fracture morphological features notably influences the characteristics of contact force distribution, as well as the scale and orientation of microcrack development within fracture samples. The contribution of different segments of fractures to resisting external shear forces correlates closely with their specific morphological feature distribution, particularly the location where θ*max/(C + 1)​ occurs. [ABSTRACT FROM AUTHOR]

A rapid and appropriate evaluation of the crack asymptote coefficients is essential for estimating the fracture growth in rock materials with inherent cracks and discontinuities. In the present study, the extended finite element method (XFEM), implemented with free coarse mesh, is used in conjunction with the finite element over‐deterministic (FEOD) method to determine the stress intensity factors (SIFs) and T‐stress for mixed‐mode I/II in‐plane loading of cracked rock specimens. Then, the generalized strain energy density (GSED) criterion is employed to estimate both fracture load and crack initiation angle for two types of rock pre‐cracked specimens. The predictions of the GSED criterion are then benchmarked against the experimental results, here obtained from three‐point bend tests on Neyriz marble. It is shown that such an approach to evaluate the fracture load dramatically reduces the computational cost and effort, meanwhile guaranteeing high accuracy and robustness. Highlights: XFEM is combined with FEOD to obtain stress fields around the crack tip.A new method for extracting test samples from rock cores is introduced.Various mixed‐mode fracture tests are conducted on rock samples.The results are satisfying even with unstructured coarse mesh. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Unloading processes are common in natural systems. Intense unloading activities can alter the frictional equilibrium of faults and induce their instabilities. Understanding the slip behavior of faults under stress unloading conditions is helpful in guiding engineering practices. We conducted a series of direct shear experiments under linear-unloading normal force conditions considering the influences of initial normal forces, initial shear forces, and normal unloading rates on planar and rough granite fractures. The experimental results showed that planar fracture exhibits sudden slip events during normal unloading, while rough fracture mostly displays stable sliding behavior. The planar fracture demonstrates an exponential increase in sliding distance and velocity at the end of each slip event. The rough fracture usually exhibits a quasi-static stage before rapid slip events. In addition, the accumulative sliding distance at the slip activation moment (at the first moment when sliding velocity is greater than 0.05 mm/s) for the planar fracture decreases with lower normal unloading rate, larger shear force and larger normal force, while its variation trend for rough fracture is opposite. These findings provide valuable insights into fault slip behavior under stress unloading, aiding in mitigating associated risks in engineering applications.

Identifying the real fracture of rock hidden in acoustic emission (AE) source clusters (AE-depicted microcrack zone) remains challenging and crucial. Here we revealed the AE energy (representing dissipated energy) distribution rule in the rock microcrack zone and proposed an AE-energy-based method for identifying the real fracture. (1) A set of fracture experiments were performed on granite using wedge-loading, and the fracture process was detected and recorded by AE. The microcrack zone associated with the energy dissipation was characterized by AE sources and energy distribution, utilizing our self-developed AE analysis program (RockAE). (2) The accumulated AE energy, an index representing energy dissipation, across the AE-depicted microcrack zone followed the normal distribution model (the mean and variance relate to the real fracture path and the microcrack zone width). This result implies that the nucleation and coalescence of massive cracks (i.e., real fracture generation process) are supposed to follow a normal distribution. (3) Then, we obtained the real fracture extension path by joining the peak positions of the AE energy normal distribution curve at different cross-sections of the microcrack zone. Consequently, we distinguished between the microcrack zone and the concealed real fracture within it. The deviation was validated as slight as 1–3 mm.

Abstract An accurate understanding of the heat transfer of water through rock fractures is essential for the extraction and utilization of thermal energy from high‐temperature rock masses. A systematic numerical simulation based on the double‐rough‐walled model has been presented to investigate the shear effect on convective heat transfer in rough rock fractures. On the basis of the modified successive random additions algorithm, four different self‐affine surfaces were generated and utilized to establish the 3D double‐rough‐walled fracture models. The fluid flow and heat transfer were simulated by directly solving the Navier–Stokes equation and energy conservation equation, respectively. The combined effects of shear and surface roughness on the heat transfer were investigated. The results show that the heat within rough‐walled fractures transfers preferentially along the main fluid flow channels, and the areas of fast and slow thermal transmission fit well with the high‐ and low‐flow regions, respectively. As shear advances, the heat transfer coefficient firstly increases, then decreases slightly and finally stabilizes within a certain range, in which stabilization occurs earlier in fracture with a larger joint roughness coefficient. The effect of surface roughness on heat transfer shows an opposite trend during shearing. When the shear displacement is small, the enhancement effect of surface roughness that provides larger heat transfer areas dominates the heat transfer. As shear displacement continues to increase, this enhancement effect will be gradually weakened until the decreasing effect that bumps on the rough‐walled surface hinder the fluid flow dominates the heat transfer.

Mining activities are key triggers for strong mine earthquakes and even rock bursts in coal mines. This study explores the impact of mining speed on the overlying strata's deformation and energy release through theoretical analysis, numerical simulation, and the digital speckle method. The temporal and spatial evolution characteristics of the impact energy during mining are simulated. The digital speckle method illustrates a positive correlation between rapid mining and increased fracture block degree of overburden rock and roof separation, confirming that accelerated mining speed extends the fracture distance of the stope. Furthermore, numerical simulations establish that both the energy associated with overlying rock breaking and the frequency of energy occurrence events are amplified during rapid mining, in contrast to slow mining. This observation corroborates that escalating mining speed augments the energy dispensed by the breaking of the upper rock. Consequently, this escalation induces a transformation in the energy levels of mine earthquakes, culminating in a heightened incidence of large-energy mine earthquakes. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress. [ABSTRACT FROM AUTHOR]

An accurate understanding of the heat transfer of water through rock fractures is essential for the extraction and utilization of thermal energy from high‐temperature rock masses. A systematic numerical simulation based on the double‐rough‐walled model has been presented to investigate the shear effect on convective heat transfer in rough rock fractures. On the basis of the modified successive random additions algorithm, four different self‐affine surfaces were generated and utilized to establish the 3D double‐rough‐walled fracture models. The fluid flow and heat transfer were simulated by directly solving the Navier–Stokes equation and energy conservation equation, respectively. The combined effects of shear and surface roughness on the heat transfer were investigated. The results show that the heat within rough‐walled fractures transfers preferentially along the main fluid flow channels, and the areas of fast and slow thermal transmission fit well with the high‐ and low‐flow regions, respectively. As shear advances, the heat transfer coefficient firstly increases, then decreases slightly and finally stabilizes within a certain range, in which stabilization occurs earlier in fracture with a larger joint roughness coefficient. The effect of surface roughness on heat transfer shows an opposite trend during shearing. When the shear displacement is small, the enhancement effect of surface roughness that provides larger heat transfer areas dominates the heat transfer. As shear displacement continues to increase, this enhancement effect will be gradually weakened until the decreasing effect that bumps on the rough‐walled surface hinder the fluid flow dominates the heat transfer. [ABSTRACT FROM AUTHOR]

Underground space creation and energy extraction, which induce unloading on rock fractures, commonly occur in various rock engineering projects, and rock engineering projects are subjected to high temperatures with increasing depth. Fluid flow behavior of rock fractures is a critical issue in many subsurface rock engineering projects. Previous studies have extensively considered permeability evolution in rock fractures under loading phase, whereas changes in fracture permeability under unloading phase have not been fully understood. To examine the unloading-induced changes in fracture permeability under different temperatures, we performed water flow-through tests on fractured rock samples subjected to decreasing confining pressures and different temperatures. The experimental results show that the permeability of fracture increases with unloading of confining pressure but decreases with loading-unloading cycles. Temperature may affect fracture permeability when it is higher than a certain threshold. An empirical model of fracture hydraulic aperture including two material parameters of initial normal stiffness and maximum normal closure can well describe the permeability changes in rough rock fracture subjected to loading-unloading cycles and heating. A coupled thermo-mechanical model considering asperity damage is finally used to understand the influences of stress paths and temperatures on fracture permeability.

Abstract The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear. This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with different oscillation amplitudes. The experimental results show that the activations of fast slips always correlate with unloading of normal stress. Besides, the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant. The rupture patterns are quantified by stress drop, slip length and slip velocity. With the effect of small oscillation amplitudes, the slip events show chaotic shapes, compared to the regular and predictable style under constant normal stress. When the amplitude is large enough, the big and small slip events emerge alternately, showing a compound slip style. Large amplitude of the cyclic normal stress also widens the interval differences of the slip events. This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures.

Since its introduction, discontinuous deformation analysis (DDA) has been widely used in different areas of rock mechanics. By dividing large blocks into subblocks and introducing artificial joints, DDA can be applied to rock fracture simulation. However, parameter calibration, a fundamental issue in discontinuum methods, has not received enough attention in DDA. In this study, the parameter calibration of DDA for intact rock is carefully studied. To this end, a subblock DDA with Voronoi tessellation is presented first. Then, a modified contact constitutive law is introduced, in which the tensile and shear meso-strengths are modified to be independent of the bond lengths. This improvement can prevent the unjustified preferential failure of short edges. A method for imposing confining pressure is also introduced. Thereafter, sensitivity analysis is performed to investigate the influence of the calculated parameters and meso-parameters on the mechanical properties of modeled rock. Based on the sensitivity analysis, a unified calibration procedure is suggested for both cases with and without confining pressure. Finally, the calibration procedure is applied to two examples, including a biaxial compression test. The results show that the proposed Voronoi-based DDA can simulate rock fracture with and without confining pressure very well after careful parameter calibration.

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress.

The stress background and rock fractures are essential factors affecting the stability of mines. In order to better understand the in situ stress background and rock fractures in the Guangdong Fankou Mine, we use ultrasonic borehole television scanning to measure rock fractures. The results indicate that rock fractures are intensively distributed at depths of −360 m to −450 m below the surface, suggesting the effect of intensive mining activities. The present maximum horizontal principal stress direction is NWW, which is consistent with the regional tectonic stress field direction. Systematic measurement of rock fractures is fundamental for further three-dimensional geological modeling and is significant for mining engineering.

The frictional rupture mechanisms of rock discontinuities considering the dynamic load disturbance still remain unclear. This paper investigates the transitional behaviors of slip events happened on a planar granite fracture under cyclic normal stress with different oscillation amplitudes. The experimental results show that the activations of fast slips always correlate with unloading of normal stress. Besides, the intensive normal stress oscillation can weaken the shear strength which is recoverable when the normal stress return to constant. The rupture patterns are quantified by stress drop, slip length and slip velocity. With the effect of small oscillation amplitudes, the slip events show chaotic shapes, compared to the regular and predictable style under constant normal stress. When the amplitude is large enough, the big and small slip events emerge alternately, showing a compound slip style. Large amplitude of the cyclic normal stress also widens the interval differences of the slip events. This work provides experimental supports for a convincible link between the dynamic stress disturbance and the slip behavior of rock fractures. Highlights: Fast slip events always happen at the unloading stage under cyclic normal stress conditions. The cyclic normal stress weakens shear stress and the slip styles are guided by the oscillation amplitudes. Compound rupture sequences can be observed when the amplitude of cyclic normal stress is large. [ABSTRACT FROM AUTHOR]

With the rapid development of underground engineering, the problem of sudden surge water has become one of the main problems that restrict engineering safety. Grouting method is commonly used to seal the fracture water flow. However, grouting in flowing water streams still faces many challenges. In this study, a rough fracture model is established based on the Barton curve, and the effects of fracture roughness, water flow rate, fracture inclination, and grouting pressure are considered, and orthogonal tests are used to study the diffusion pattern of slurry under the four factors. According to the observed phenomena, there are four main diffusion patterns of the slurry: (1) full slurry coverage with no water passing through; (2) little coverage with watercourses and cavity areas along both sides of the fracture; (3) partial slurry coverage with no obvious watercourses but some cavity areas; and (4) less coverage with larger watercourses and cavity areas. The reduction rate of flow rate at the outlet before and after grouting was used to express the sealing effect. The results show that the degree of influence on the slurry sealing effect from strong to weak is fracture roughness, water flow velocity, grouting pressure, and fracture angle in order. A method to characterize the sealing effect by slurry coverage is proposed based on the final distribution of residual slurry. Considering the mutual influence between different factors, the sealing effect of two factors is analyzed in order to provide reference for engineering practice. [ABSTRACT FROM AUTHOR]

The mining stress formed in the surrounding rock after coal seam mining is the root cause of the damage and fracture of the stope surrounding rock. In order to study the evolution characteristics of mining stress and the fracture process of overlying strata on the mining face, a numerical model was established to simulate the evolution law of stress field and fracture field in the mining process of the working face by taking the 40101 mining face of Gaoliang Coal Mine as an example. The results show that the relationship between rock collapse and failure can be judged by using Mohr-Coulomb criterion to analyze the variation law of the difference between maximum and minimum principal stresses. The correctness of the time-varying evolution of fracture field and stress field in working face mining can be verified by comparing the numerical simulation results with the field measured results.

Abstract Fractures are widely present in geomaterials of civil engineering and deep underground engineering. Given that geomaterials are usually brittle, the fractures can significantly affect the evaluation of underground engineering construction safety and the early warning of rock failure. However, the crack initiation and propagation in brittle materials under composite loading remain unknown so far. In this study, a three‐dimensional internal laser‐engraved cracking technique was applied to produce internal cracks without causing damage to the surfaces. The uniaxial compression tests were performed on a brittle material with internal cracks to investigate the propagation of these internal cracks at different dip angles under compression and shear. The test results show that the wing crack propagation mainly occurs in the specimen with an inclined internal crack, which is a mixed‐Mode I–II–III fracture; in contrast, Mode I fracture is present in the specimen with a vertical internal crack. The fractography characteristics of Mode III fracture display a lance‐like pattern. The fracture mechanism in the brittle material under compression is that the internal wing cracks propagate to the ends of the whole sample and cause the final failure. The initial deflection angle of the wing crack is determined by the participation ratio of stress intensity factors KII to KI at the tip of the internal crack.

In the process of rock excavation, cyclic loading may occur before or after the peak value. In order to obtain the fracture damage evolution characteristics of rock under the pre-peak and the post-peak cyclic loading condition, the fracture tests of three-point bending granite beams were conducted by using the digital image correlation method and the acoustic emission technology. Based on the secant modulus, the acoustic emission energy and effective crack length, the rock fracture damage indexes DE, DAE, and DL were constructed respectively, and the rock fracture damage process was quantitatively analyzed. The results show that the damage index of the rock sample has an obvious rate effect. At the same number of cycles, as the loading rate increases, the damage index decreases. For the pre-peak cyclic loading condition, the fracture process of rock from elastic stage to elastic-plastic stage can be well reflected by DE. Under the condition of post-peak cyclic loading, DL can better represent the fracture damage process of rock from step up to continuous failure.

Fracture surface morphology, mechanical and hydraulic properties are related to fracture size. One of the core issues in the size effect is the representative elementary surface, which determines the size of the test specimens in the laboratory and facilitates the application of test results to the field. In this study, the size effect on surface morphology and permeability of rock fractures is investigated, and the representative elementary surface size for the surface morphology and the permeability was determined based on the asperity height data of fracture surfaces obtained by 3D laser scanning in the field. The roughness of fracture surfaces varies significantly at small fracture surface sizes. As the fracture surface size continues to increase, the fracture surface roughness tended toward a relatively stable state. The critical size for fracture surface roughness stability in the study, which is called the representative elementary surface, is 300–400 mm. The size effect on the permeability of rock fractures was investigated, and the results show that fracture permeability tended toward a relatively stable state with increasing fracture size, which was the same as the fracture surface roughness. The representative elementary surface size for fracture permeability is less than 100 mm in the study, which is also less than that for fracture surface roughness with the same coefficient of variation. This result indicated that a lower coefficient of variation might provide the best estimate of the representative elementary surface for fracture permeability compared with the fracture surface roughness. [ABSTRACT FROM AUTHOR]

Thermal pretreatments of rock, such as conventional heating and microwave irradiation, have received considerable attention recently as a viable method of improving the energy efficiency of mining processes that involve rock fracturing. This study presents a numerical analysis of the effects of thermal shock and microwave heating on the mechanical properties of hard, granite-like rock. More specifically, the aim is to numerically assess the reduction of uniaxial compressive strength of thermally pretreated specimens compared to intact ones. We also compare the performance of these two pretreatments (conventional heating and microwave irradiation) in terms of consumed energy and induced damage. Rock fracture is modelled by a damage-viscoplasticity model, with separate damage variables in tension and compression. A global solution strategy is developed for solving the thermo-mechanical problem (conventional heating) and the electromagnetic–thermo-mechanical problem (microwave heating). The electromagnetic part of the microwave heating problem is solved in COMSOL Multiphysics software Version 6.1 first. The electromagnetic solution is used as an input for the thermo-mechanical problem, which is finally solved by means of a staggered explicit solution method. Due to the predominance of the external thermal sources, the thermal and the mechanical parts of the problem in both cases are considered as uncoupled. Three-dimensional finite element simulations are utilized to study the damage-viscoplasticity model. An ore-shaped three-mineral numerical rock specimen is used in uniaxial compression tests. [ABSTRACT FROM AUTHOR]

Mining activities are key triggers for strong mine earthquakes and even rock bursts in coal mines. This study explores the impact of mining speed on the overlying strata’s deformation and energy release through theoretical analysis, numerical simulation, and the digital speckle method. The temporal and spatial evolution characteristics of the impact energy during mining are simulated. The digital speckle method illustrates a positive correlation between rapid mining and increased fracture block degree of overburden rock and roof separation, confirming that accelerated mining speed extends the fracture distance of the stope. Furthermore, numerical simulations establish that both the energy associated with overlying rock breaking and the frequency of energy occurrence events are amplified during rapid mining, in contrast to slow mining. This observation corroborates that escalating mining speed augments the energy dispensed by the breaking of the upper rock. Consequently, this escalation induces a transformation in the energy levels of mine earthquakes, culminating in a heightened incidence of large-energy mine earthquakes.

Three-dimensional rock fracture induced by blasting is a highly complex problem and has received considerable attention in geotechnical engineering. The material point method is firstly applied to treat this challenging task. Some inherent weaknesses can be overcome by coupling the generalized interpolation material point (GIMP) and the convected particle domain interpolation technique (CPDI). For the media in the borehole, unchanged GIMP-type particles are used to guarantee a homogenous blast pressure. CPDI-Tetrahedron type particles are employed to avoid the fake numerical fracture near the borehole for the rock material. A blasting experiment using three-dimensional single-borehole rock was simulated to examine the applicability of the coupled model under realistic loading and boundary conditions. A good agreement was achieved between the simulation and experimental results. Moreover, the mechanism of three-dimensional rock fracture was analyzed. It was concluded that rock particle size and material parameters play an important role in rock damage. The reflected tensile waves cause severe damage in the lower part of the model. Rayleigh waves occur on the top face of the rock model to induce a hoop failure band.

Suspended particle transport is frequently involved in many geophysical processes and subsurface engineering applications. Although common and important, the effect of liquid cohesion on particle clogging has been overlooked in previous studies. We conduct visualized experiments of dilute suspension flow in a rough fracture and find a dramatic enhancement of clogging by a tiny amount of additional immiscible wetting liquid, even at weight percentage ω ≤ 0.5%. An experimental phase diagram of clogging patterns is obtained in the space of secondary liquid content and flow rate. The combined effect of suspension composition and hydrodynamic condition on the clogging behavior is analyzed to explain transitions of clogging regimes. A theoretical model of agglomerate size is proposed to quantify the capillary cohesion effect. This work improves the understanding of fines migration and particle‐clogging behaviors in the subsurface and paves the way for possibility of controlling particle transport and clogging in various applications. Plain Language Summary: Suspended particle flow and clogging in rock fractures are involved in many subsurface engineering applications and natural processes. Water‐wet particles dispersed in oil are cohesive and tend to agglomerate, clogging flow channels of crude oil to resist further recovery. Despite being common and important, the effect of liquid cohesion on particle clogging has been overlooked in previous studies. Here, we conduct visualized experiments of dilute suspension flow in a rough fracture. We, for the first time, find a dramatic enhancement of clogging by a tiny amount of additional immiscible wetting liquid, even at a weight percentage as low as 0.1%–0.5%. We obtain an experimental phase diagram of clogging patterns in the space of wetting liquid content and flow rate. The transitions of clogging regimes are explained through the combined control of clogging behavior by the suspension composition and hydrodynamic condition. We propose a theoretical model of agglomerate size to quantify the effect of capillary cohesion induced by the additional wetting liquid. The experimental and theoretical results improve our understanding of suspension flow and clogging behaviors and pave the way for the possibility of controlling particle transport and clogging processes in various applications by adjusting multiphase liquid composition and flowrate. Key Points: A tiny amount of immiscible wetting liquid added into dilute suspension can dramatically enhance particle clogging in rough fractureAn experimental phase diagram of clogging patterns is obtained, showing the synergistic control of liquid cohesion and flow erosionA model of particle agglomerate size as a function of secondary liquid content is proposed to explain the cohesion effect on clogging [ABSTRACT FROM AUTHOR]

The distribution law of fluid flow in natural fissures was obtained by simulating the fluid flow in the rock fissures of the fault fracture zone. Based on the theoretical analysis, the velocity distribution model of wedge-shaped flow field was established. It was found that the location of the fracture mainstream zone could be expressed by the maximum inscribed circle of the fracture section. Computed tomography (CT) was used to reconstruct the natural fracture of the dielectric skeleton of the fault, and the maximum inscribed circle of the fracture was calculated by MATLAB to estimate the permeability of the fracture. The correlation between the results to calculate the fracture permeability obtained by the maximum inscribed circle method and the hydraulic diameter method was 0.976, which indicated that it was feasible to calculate the fracture permeability by the maximum inscribed circle of the fracture section instead of the hydraulic diameter. [ABSTRACT FROM AUTHOR]

This research aims to explore the fracturing behaviors of sandstone subjected to pulsed high-voltage discharge (PHVD) under different static pressures. An experimental method of rock fracturing induced by inter-hole PHVD was proposed. The static pressure was applied to the specimens, then the proposed method was applied to induce electrical breakdown testing under static loading. The microscopic fracture morphology of the sandstone was observed. The influences of the direction and level of static pressure on the crack length and fractal dimension of sandstone under the effect of PHVD were discussed. The results indicated that in the absence of static pressure, there are a discharge channel and multiple radial cracks in the sandstone after electric breakdown. The microscopic analysis implies that rock failure in the zone around the discharge channel is mainly influenced by the high temperature; while radial cracks are induced by shock waves. When static pressure is vertical to the discharge channel, tensile and compressive stresses concentrate in different parts around the discharge channel, which can remarkably alter the distribution zone and propagation path of cracks. In addition, the fractal dimension and total length of cracks first decrease, then increase with increasing static pressure. When static pressure is parallel to the discharge channel, the distribution characteristics and propagation direction of cracks are unchanged; however, under this loading mode, circularly distributed hoop tensile strains are generated at the zone around the discharge channel, which enables the fractal dimension and crack length increase with increasing static pressure. [ABSTRACT FROM AUTHOR]

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Our research proposes a unique coupled electro-thermal-mechanical model that takes electric breakdown and heterogeneity into account to show the mechanism of rock fracturing under high-voltage electropulses. Using finite element numerical software, the process of high voltage electrical pulse injection into the rock interior for breakdown is described, and the formation law of plasma channels during the electrical breakdown process is comprehensively analyzed in conjunction with the conductor particles present within the rock. On the basis of electrical, thermal, and mechanical theories, a coupled multi-physical field numerical model of rock failure under the action of high-voltage electrical pulses is developed, and a random distribution model is utilized to simulate the potential occurrence of conductor particles in the rock. Innovative numerical model indicates plasma channel creation in the rock-crushing process. Prior to the formation of the plasma channel, the temperature and stress are approximately 103 k and 10−2 MPa, respectively. Once the plasma channel is formed, the temperature and stress increase abruptly in a short time, with the temperature reaching 104 k and the stress reaching 103 MPa or higher. In addition, it is revealed that the breakdown field strength is the essential factor in plasma channel creation. The heterogeneity of the particles within the rock and the fluctuation in electrode settings are also significant variables influencing the creation of channels. The presented model contributes to a better understanding of the mechanism of rock fragmentation during high-voltage electrical pulses, which has substantial implications for oil exploration and mineral extraction. [ABSTRACT FROM AUTHOR]

• Multi-scale dynamical responses of submerged particle jet impact drilling were investigated. • Correlation and nonlinear analysis of rock fracture were modelled and experimentally validated. • Rock fracture can be characterized as instantaneous stepwise temporal evolution and longitudinal expansion. • Dynamic responses are represented in terms of accumulative growths in macro-scale, radial cracks in micro-scale. Particle Impact Drilling (PID) technology is highly efficient for exploitation of unconventional energy resources in extra-deep and ultra-hard strata. The multi-scale dynamic responses and fracture mechanics analysis of rock formations in drilling using the PID are discussed in this paper. Firstly, rock fracture experiments and penetration performance under submerged particle jet impact were conducted to analyze fracture mechanisms in macro-scale. Then, the rock constitutive relation model and damage model for the submerged particle jet impacts using the damage and failure theory and correlation analysis model were constructed. On this basis, multi-scale dynamic responses and nonlinear analysis of failure performance with submerged particle jet impact were conducted. Our results indicate continuous damage growth in macro-scale and radial cracks development in micro-scale when submerged particles impact the drilled formation in a process of creating a well. [Display omitted] [ABSTRACT FROM AUTHOR]

It is of great importance to determine peak shear strength (PSS) of rock fractures, and data-driven criteria have showed advances in fitting capability in recent years. However, the generalization ability of existing data-driven criteria is limited by dataset size and fracture roughness characterization, which is negative to predictive power and robustness of models. Here we proposed a novel data-driven criterion to predict PSS of rock fractures, with high generalization ability on real experimental data. We first created large-scale low-fidelity dataset by discrete-element modeling, and small-scale high-fidelity dataset by laboratory direct shear tests. The numeric features include normal stress, mechanical properties (including PSS of intact and flat-fracture rock specimens), secondary properties (including internal friction angle, cohesion strength and basic friction angle), and the matrixed feature is topography data. We then established domain adaptation (DA) models for cross-domain knowledge transfer between the low- and high-fidelity datasets, and roughness features were automatically extracted by convolution kernels. The best DA-based model is weighting adversarial neural network, outranking other models by error indicator, and the average relative error on experimental data of new rock types is within 10.0 %. Finally, the sensitivity of input features is investigated, which further proves the promising potential of the developed data-driven PSS criterion of rock fractures in engineering practice. [ABSTRACT FROM AUTHOR]

In the geo-energy industry, fluid injection induces different slip behaviors of a rock fracture, from aseismic creep to dynamic slip. The transition from aseismic creep to dynamic slip is explained by the ratio of the stiffness of surrounding rock and the critical stiffness of the fracture. However, numerous studies suggest multiple controls affecting the slip behaviors, and their joint influences on the slip transition remain unclear. Here we trained a dual-stage attention-based recurrent neural network model using fluid injection experimental data to explore the dominant factor controlling the slip behaviors. Our results showed that the dominant factor changes during fluid injection, and the attention to shear stress dominates the occurrence of dynamic slip. We found that high fluctuations of the attentions to normal stress, shear stress, and water pressure gradient promote the slip transition. Our model was applied to explore the competing process between water pressure front and aseismic creep front while gradually increasing the injection pressure and to reveal the dynamic change in the dominant factor during the growth of cumulative moment release. • A recurrent neural network is built to explore the dominant factor controlling slip behaviors. • The attention mechanism reveals the dynamic change in dominant factor during fluid injection. • High fluctuations of attentions to controlling factors promote the slip transition of rock fracture. [ABSTRACT FROM AUTHOR]

Understanding the nonlinear flow of non-Newtonian fluids in rock fractures is critical for underground engineering (e.g., rock grouting). However, how to evaluate the effect of shear-thinning rheology on flow nonlinearity due to fracture heterogeneity is insufficiently understood. In this study, the Herschel-Bulkley-Papanastasiou (HBP) model is proposed to simulate grout flow in fracture with contact areas. The numerical result indicates that the alterations in contact zones of fracture due to the changes in aperture increase the nonlinear flow characteristics. The decrease of shear-thinning rheology reduces the development of eddy regions under constant boundary, which delays the occurrence of significant inertial effects. The local flow within the fracture is influenced by the change in spatial viscosity caused by the rheological properties of the fluid. A quantitative method is proposed to quantify the viscous dissipation of the shear-thinning fluid based on energy balance, and the results demonstrate that strong viscous dissipation is most important for suppressing the nonlinear flow. The predictive function of the critical Reynolds number for the transition from strong to weak inertial flow regime is established. The results can provide novel perspectives and concepts for evaluating the shear-thinning fluids flow in complex fractured media. [ABSTRACT FROM AUTHOR]

Abstract Suspended particle transport is frequently involved in many geophysical processes and subsurface engineering applications. Although common and important, the effect of liquid cohesion on particle clogging has been overlooked in previous studies. We conduct visualized experiments of dilute suspension flow in a rough fracture and find a dramatic enhancement of clogging by a tiny amount of additional immiscible wetting liquid, even at weight percentage ω ≤ 0.5%. An experimental phase diagram of clogging patterns is obtained in the space of secondary liquid content and flow rate. The combined effect of suspension composition and hydrodynamic condition on the clogging behavior is analyzed to explain transitions of clogging regimes. A theoretical model of agglomerate size is proposed to quantify the capillary cohesion effect. This work improves the understanding of fines migration and particle‐clogging behaviors in the subsurface and paves the way for possibility of controlling particle transport and clogging in various applications.

This work aims to assess both experimentally and analytically the fracture behavior of rock specimens containing sharp V-notches (SV-notches) subjected to mixed mode I/II loading. To this end, firstly, several mixed mode fracture tests were conducted on Brazilian disk specimens weakened by an SV-notch (SVNBD sample), performed in their corresponding center and with various notch opening angles. Secondly, the fracture resistance of the tested samples was predicted using a criterion named MTS-FEM. This approach is based on the maximum tangential stress (MTS) criterion, in which the tangential stress is determined from the finite element method (FEM). Additionally, in the present research, the required critical distance is calculated directly from finite element analyses performed on cracked samples. Comparing the experimental results and the analytical predictions, it is shown that the fracture curves obtained from the MTS-FEM criterion are in agreement with the experimental results. These results are achieved without the need for the calculation of stress series expansion coefficients, as an additional advantage of the proposed approach. [ABSTRACT FROM AUTHOR]

Roughness of rock fractured surfaces is one of the most important factors controlling fluid flow in rock masses. Roughness quantification is of prime importance for modelling the flow of ground waters as well as reservoir fluid mechanics. In this study, with the aid of high-resolution 3D X-ray CT scanning and image processing techniques, the roughness of four different rock types is reconstructed with a resolution of 16.5 microns. Moreover, the correlation and structure functions are used to analyse height fluctuations as well as statistical intermittency of the studied rock fractured surfaces. It is observed that at length scales smaller than a critical length scale, fractures surfaces are correlated and show multifractality. Monofractals are neither intermittent nor correlated; hence, a meaningful link between statistical intermittency and the correlation function of multifractals is expected. However, a model that considers this relationship and predicts multifractal spectra of disordered systems is still missing. A simple power law that can exactly forecast the multiscaling spectrum of rock fracture process zone is being introduced. It is explained how the exponent of this power function λi is related to the crossover length of correlation function ξ, and how this critical length scale can be objectively identified. [ABSTRACT FROM AUTHOR]

Based on the existing research results, a theoretical fracture model of strata under the compound impact of static and harmonic dynamic load is improved, and the fracture characteristic parameters (stress intensity factor, T-stress, and fracture initiation angle) under the two far-field stress are determined according to the crack dip angle. Additionally, the effects of harmonic dynamic load on the distribution of the stress field and the fracture characteristic (the crack initiation angle, the fracture degree, the number of fracture units, and the fracture area) are further calculated and discussed by theoretical model solution, extended finite element simulation, and the secondary development of the simulation module, respectively. The research results show that the far-field stress, stress intensity factor, and T-stress vary in harmonic form with time under the compound impact of static and harmonic dynamic loads. The frequency of dynamic load affects the number of reciprocal fluctuations of stress intensity factor and T-stress as well as the crack initiation time, but has less influence on the crack initiation angle and fracture degree. While the amplitude of dynamic load affects the stress intensity factor, the extreme value of T-stress and fracture characteristics of the crack. This study has theoretical guiding significance for parameters' optimization and realization of resonance impact drilling technology. [ABSTRACT FROM AUTHOR]

The aim of the current study was to establish a validated numerical model for addressing the changes in permeability and reactive transport behavior within rock fractures based on the fluid pH under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions. Firstly, a multi-physics reactive transport model was proposed, considering the geochemical reactions that depend on the temperature, stress, and fluid chemistry conditions (e.g., fluid pH and solute concentrations), as well as the changes in permeability in the rock fractures driven by these reactions, after which the correctness of the model implementation was verified by solving the 1D reactive transport problem as a fundamental benchmark. Secondly, the validity of the model against actual rock fractures was investigated by utilizing the model to replicate the measurements of the evolving permeability and the effluent element concentrations in single granite fractures obtained by means of two flow-through experiments using deionized water (pH ∼ 6) and a NaOH aqueous solution (pH ∼ 11) as permeants under stressed, temperature-elevated conditions. The model predictions efficiently followed the changes in fracture permeability over time measured by both experiments. Additionally, the observed difference in the changing rates, which may contribute to the difference in the fluid pH between the two experiments, was also captured exactly by the predictions. Moreover, in terms of the effluent element concentrations, among all the elements targeted for measurement, the concentrations of most elements were replicated by the model within one order of discrepancy. Overall, it can be concluded that the developed model should be valid for estimating the changes in permeability and reactive transport behavior within rock fractures induced by geochemical reactions which depend on the fluid pH under coupled THMC conditions.

During the exploitation of deep resources, rocks are often in a high-temperature, high-pressure environment. It is of great significance to study the acoustic emission (AE) characteristics of thermal damaged rock under load to improve the accuracy of monitoring in practical engineering. In this paper, sandstone was heated at different temperatures, before a uniaxial compression test was performed and the AE in the process was monitored. The results show that the strength and AE energy of sandstone decrease gradually with an increase in heating temperature. Through frequency domain analysis of the AE waveform at the time of failure, it was found that the frequency and intensity of AE also showed a downward trend with an increase in temperature. In addition, multifractal theory is introduced to deconstruct the waveform data. The multifractal characteristics of the waveforms decrease with an increase in temperature. It provides new parameters for waveform analysis, which can be combined with frequency analysis as parameters to more accurately identify rock failure in engineering practice. The attenuation of AE of thermally damaged sandstone may be related to an increase in porosity and a decrease in elastic energy release. [ABSTRACT FROM AUTHOR]

The time–frequency characteristics of electromagnetic radiation (EMR) waveform induced by rock fracture are very important to the monitoring and early–warning using the EMR method for the mine rockburst. The empirical wavelet transform (EWT), as a waveform time–frequency analysis method, has the advantages of a clear theoretical basis, convenient calculation, and no modal aliasing. To apply EWT to the field of EMR time–frequency analysis, the operation of Fourier axis segmentation of EWT is improved. In detail, the adaptive selection method for a window width of closing operation and the adaptive determination method of segment number of Fourier axis are proposed for EWT. The Fourier axis obtained by short–time Fourier transform (STFT) is used in the EWT process, rather than that obtained by discrete Fourier transform (DFT), taking a better Fourier axis segmentation effect. The improved EWT together with Hilbert transform (HT) applied to the time–frequency analysis for the EMR waveform of rock fracture, and the time–frequency spectrum obtained by EWT–HT can well describe the time–frequency evolution characteristics. Compared with STFT and Hilbert–Huang transform (HHT), EWT–HT has significant advantages in time–frequency resolution and overcoming modal aliasing, providing a powerful tool for time–frequency analysis for the EMR waveform induced by rock fracture. [ABSTRACT FROM AUTHOR]

Semi-circle bending (SCB) test is an important test in the field of rock fracture. In order to investigate the propagation process of 3-D internal cracks under semi-circular bending, numerical simulations of SCB specimens were carried out based on the finite element method and adaptive mesh technique. The results show that the numerical simulation results of this method are in good agreement with the previous laboratory test. The symmetrical loading model belongs to the pure I loading where the value of KI is positively related to the bottom load span S/R. The asymmetric loading model belongs to mix-mode loading. With the increase of unilateral load span S2/R, there is a decline and then a rise of the value of SIF at the same point at the crack tip. The S2/R is an important factor affecting the mechanical properties of rock specimens. The crack propagation path was obtained based on the maximum tensile stress (MTS) criterion. In-plane propagation of the internal crack in the symmetrically loading model leads to failure of the specimen. Wing propagation of the internal crack in the asymmetrically loading model causes the specimen to fracture into two asymmetrical parts. Quantitative analysis of the internal crack propagation of the symmetrical loading model shows that the growth rate of the lower end of the crack is the largest, and the upper end of the crack is the smallest. The above results can provide a reference for the research on the propagation of 3-D internal cracks of SCB specimens. [ABSTRACT FROM AUTHOR]

Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration, design, construction, operation, and maintenance of many rock engineering projects such as hydropower development, oil and gas extraction, and underground waste disposal. With the rapid advancement of global and national strategies such as the "Paris Agreement" and the "Belt and Road Initiative", more and more projects are developed in the complex geological environment with varying geological structures. Shear failure and rock instability are prone to occur in fractured rock masses under the coupled effects of high stress, high pore pressure, and engineering disturbance, which are main sources for engineering disasters such as roof collapse and caving, water and mud inrushes, and induced earthquakes. To solve these problems, extensive research on the coupled shear-flow behavior of fractures has been conducted. However, due to the complex mechanical, hydraulic and geometrical characteristics of single fractures and fracture networks, a large number of outstanding issues related to the impact of the coupled processes on the engineering characteristics of rock masses are still unsolved. The relevant experimental apparatuses and methods remain to be further developed. Therefore, in this review, we analyze and summarize the existing shear-flow experimental apparatuses, classify apparatus configurations, specimen shapes, and testing principles, and compare their advantages and disadvantages. We also summarize the main scientific findings obtained from various experimental apparatuses, aiming to provide a reference for developing new shear-flow experimental apparatuses and conducting related scientific research in the future. [ABSTRACT FROM AUTHOR]

During blasting in deep mining and excavation, the rock masses usually suffer from high in situ stress. The initial seismic energy generated in deep rock blasting and its attenuation with distance is first theoretically analyzed in this study. Numerical modeling of the multiple-hole blasting in a circular tunnel excavation under varied in situ stress conditions is then conducted to investigate the influences of in situ stress levels and anisotropy on the blasting seismic energy generation and attenuation. The case study of the deep rock blasting in the China Jinping Underground Laboratory (CJPL) is finally presented to demonstrate the seismic energy attenuation laws under varied in situ stress levels. The results show that with the increase in the in situ stress level, the explosive energy consumed in the rock fracture is reduced, and more explosive energy is converted into seismic energy. The increasing in situ stress causes the seismic Q of the rock mass medium to first increase and then decrease, and consequently, the seismic energy attenuation rate first decreases and then increases. Compared to the condition without in situ stress, the blasting seismic energy decays more slowly with distance under in situ stress. Then the seismic waves generated in deep rock blasting are more likely to reach and exceed the peak particle velocity (PPV) limits stipulated in the blasting vibration standards. Under non-hydrostatic in situ stress, the generation and attenuation of the blasting seismic energy are anisotropic. The highest seismic energy density is generated in the rock mass in the minimum principal stress orientation. Its attenuation is dependent upon the in situ stress aligning the wave propagation orientation. [ABSTRACT FROM AUTHOR]

Microbial-induced calcium carbonate precipitation (MICP) technology mainly uses carbonates produced by the reaction of microbial activities to repair rocks and soils. Temperature influences microbial metabolism and the kinetics of chemical reactions. In this study, microbial repair experiments on fractured sandstone under different temperatures are carried out. The repair effects are tested with nuclear magnetic resonance (NMR), an X-ray automatic diffractometer (XRD), uniaxial compressive strength (UCS), and a scanning electron microscope (SEM) test. The influence of the temperature on the restorative effects of MICP was discussed. The results show that the repair effect of the Sporosarcina pasteurii is significantly better as the temperature increases. When the temperature reaches 33 °C, the porosity and permeability of fractured sandstone can be reduced by 55.174% and 98.761%, respectively. The average uniaxial compressive strength can be restored to 6.24 MPa. The repair effect gradually weakens with the increase in temperature. However, the Sporosarcina pasteurii can still maintain relatively good biological activity at temperatures from 33 °C to 39 °C. The main form of CaCO3 produced in the process of MICP is calcite. It can fill in the rock pores, and result in reducing the size and number of large pores and improving the impermeability and strength of fractured yellow sandstone. [ABSTRACT FROM AUTHOR]

Small-scaled linear cutting tests were first performed to study the influence of penetration on fracture characteristics by a CCS (constant cross-section) cutter. The results indicate that the increase in penetration (ranging from 2.5 mm to 5.5 mm) effectively increases chip masses between cuts and further promotes cutting efficiency. To further understand the fracture mechanism for various penetrations, 3D numerical simulations were performed using PFC 3D. The numerical fracture characteristics agree well with laboratory tests. In addition, the dynamic stress evolution analysis clearly shows that the increase in rolling force frequently results in stress concentrations in rock specimens. When stresses concentrate into critical values, fractures occur. Subsequently, these fracture propagations frequently result in stress dissipations and decreases in rolling force. Thus, the relation between the fluctuations of rolling force and the rock fractures is revealed. In addition, the increase in penetration results in the promoted stress concentrations. This phenomenon can explain why the increased penetration can result in severer fractures. [ABSTRACT FROM AUTHOR]