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Constructing salt cavern gas storage in the ultra-deep strata more than 2000 m in depth is an important strategic technologic development in China. A primary issue is that higher temperature affects the creep behaviors of the surrounding rock. In this paper, triaxial creep tests of rock salt under multi-stage temperatures were carried out to study the influence of temperature on the creep behaviors of rock salt. Synchronous acoustic emission monitoring was performed throughout the creep test to study the microcracks characteristics of rock salt during creep at different temperatures. The results show that the duration of transient creep decreases with the increase of temperature. The steady-state creep rate increases with the increase of temperature. Temperature rise prompts the creep of rock salt to enter the volume dilatation stage. By analyzing the acoustic emission energy/counts, it is found that the microcracks generated during the creep at high temperature are few, but there are more at low temperature. Microcracks are generated intensively during rising temperatures. By analyzing the characteristics of the acoustic emission spectrum, it is found that when the temperature is low, the cracks generated are mainly intergranular cracks with a larger size, and they are mainly intragranular cracks with a smaller size when the temperature is high. The research in this paper is helpful to further understand the creep properties of ultra-deep rock salt and provides an important basis for the design, construction, and operation of gas storage salt caverns. Highlights: The influence of temperature on the creep of rock salt was studied by performing multistage temperature creep tests. The microcracks characteristics of rock salt during creep at different temperatures were studied using acoustic emission technology. The duration of transient creep phase decreases with the increase of temperature. The steady-state creep rate increases exponentially with the increase of temperature. [ABSTRACT FROM AUTHOR]

Rock-burst is one of the most serious risks associated with hard rock tunnelling and mining at high depths. Monitoring of acoustic emissions emitted by the rock-mass during excavation and their interpretation now permits the early assessment of failure events and makes the safe management of the construction works possible. A reliable set-up of the alarm threshold is thus fundamental for the correct implementation of the procedures planned to minimise rock-burst related risk. This paper focuses on a novel in situ test specifically developed to provide an experimental basis for a more accurate assessment of the alarm threshold during tunnelling, representative of the local geomechanical conditions. The test, thanks to the compression induced by two flat jacks at the tunnel side wall, produces an artificial failure process during which acoustic emissions are measured and correlated to the mechanical response of the rock-mass, without the typical limitations of scale that characterised the laboratory experiments. The new methodology, named the Mules method, was successfully tested during the excavation of some stretches of the Brenner Base Tunnel in the Brixner granite, affected by mild spalling episodes. The case-history is fully described in the paper to illustrate the practical application of the proposed approach. Highlights: A new in situ test is proposed for the assessment of the rock-burst alarm threshold. The rock-mass is brought to failure at the tunnel side wall by two flat jacks and acoustic emissions are monitored. Two tests were carried out during the construction of the Brenner Base tunnel in a granite formation under high cover affected by mild spalling phenomena. The power spectrum density of the signals recorded during the tests displayed its maximum values before the rock-mass failure. Test results interpreted in terms of power spectrum density summation were adopted to set up the rock-burst alarm threshold during tunnel excavation. [ABSTRACT FROM AUTHOR]

The method of backfill in underground mining is important for ground control as well as material recycling and energy efficiency. Even though extensive testing and field studies of backfill have been conducted, less is known about the detailed damage and fracturing that occurs directly at the rock/backfill interface. In this paper, cylindrical specimens containing an inner diameter of backfill and an outer diameter of rock (RB) were tested under triaxial compression. Acoustic emissions (AE) were used throughout testing, and X-ray computed tomography (CT) scanning was conducted before loading was applied and after the specimens had failed. The high-resolution CT images were then converted into point clouds to isolate the fractures and visualize them in three dimensions. The point clouds clearly show that fracturing occurred both in the rock and along with the contact between rock and backfill, while very little fracturing was found to occur in the backfill. Based on the point cloud and AE results, a unique evolution of fracturing is found to occur that includes two stages of shear fracturing in the rock, tensile fracturing along with the rock/backfill interface, and final tensile fracturing in the rock after delamination from the backfill, all of which contributed to the nonlinear stress–strain response. This paper presents a novel approach for investigating the initiation and propagation of 3D fractures in laboratory testing and can offer a useful reference for further studies on the mechanics of bi-material structures. [ABSTRACT FROM AUTHOR]

Microseismic/acoustic emission (MS/AE) location technology is a powerful means to study the spatial evolution characteristics of rock fracture and early warning of geological hazards. This paper investigated the variation in MS/AE location accuracy and the spatial evolution characteristics of granite fracture in complex stress conditions by using the velocity-free MS/AE source location method. Results show that the variation of wave velocity caused by granite fracture is a key factor for the variation of location accuracy. It is expected to improve the location accuracy by dynamically correcting the iterative wave velocity. The evolution process and results of granite microcracks in uniaxial and biaxial stress conditions show consistency and difference. The consistency is that the microcracks start from the edges and corners at both ends of the rock and gradually develop to the central side of the rock. The distribution of AE events changes from scattered to clustered, nucleated, and finally to banded distribution. The difference is that the advance of rock damage strain point and the macroscopic fracture surfaces are mostly perpendicular to the minimum principal stress direction in biaxial stress conditions. This paper is not only a useful supplement to the MS/AE location methods and theories, but also provides a reference for the disaster-causing mechanism of rock instability as well as disaster prevention and control. Highlights: The variation of wave velocity caused by granite fracture is a key factor for the variation of location accuracy. The microcracks induced by stress in granite fracture start from the edges and corners and gradually develop to the central side of the rock. Stress conditions could significantly affect the damage strain point and the final morphology of cracks in granite. [ABSTRACT FROM AUTHOR]

Understanding the shear characteristics and acoustic emission features of bolted joints is crucial for the optimization of support systems and disaster early warning. In this paper, a series of shear tests on rock joints using both full-length anchorage and partial anchorage methods were conducted. The evolving patterns of shear mechanical characteristics and acoustic emission features of bolted rock joints were obtained, elucidating the influence of anchorage methods and revealing the shear failure mechanisms of joints under different anchorage lengths. The results indicate that the "pin effect" of the bolt can be rapidly mobilized in the full-length anchorage compared to the partial anchorage method. The full-length anchorage exhibits higher peak shear stress and fracture shear stress, with maximum differences of 0.38 MPa and 0.08 MPa, respectively, when contrasted with the partial anchorage method. The deformation range of bolts in the partial anchorage method approximately doubles that observed in the full-length anchorage. Acoustic emission feature parameters exhibit a good correspondence with shear stress curves, and their evolution suggests that the most significant damage to bolted joints occurs at the shear stress peak, with the highest energy release observed when bolts fail. Under both partial anchorage and full-length anchorage, with an increase in normal stress or JRC, the b of bolted joint acoustic emissions gradually decreases. Compared to the partial anchorage method, the full-length anchorage demonstrates a higher maximum Hit rate, along with lower maximum energy release, implying more intense interaction between bolts and surrounding rock in the full-length anchorage, resulting in greater damage under the same conditions, whereas the deformation range of bolts is smaller, and the energy released upon failure is lower in the full-length anchorage. Highlights: A series of shear tests on rock joints using full-length and partial anchorage methods. Investigated the effects of anchorage lengths, normal stress, and joint surface roughness on shear behavior and acoustic emissions. The effects of the anchorage lengths on the fracture behavior of bolted joints was revealed. [ABSTRACT FROM AUTHOR]

A set of hydraulic fracturing tests were conducted in prismatic granite specimens to investigate the influence of the triaxial state of stress on the cracking processes. The tests considered three states of stress, in which the horizontal and out-of-plane loads were 2 MPa, while the vertical loads were either 1 MPa, 2 MPa, or 4 MPa. A hydraulic pressure inside pre-fabricated flaws was increased at a constant injection rate until hydraulic fractures initiated and propagated. In order to materialize this state of stress, a test setup was developed to independently apply and control three orthogonal stresses in the prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. More importantly, the setup also allowed one to observe the damage processes visually and through acoustic emission monitoring. This paper (the first of two) looks into the visual (macroscopic) observations, whereas the companion paper focuses on the acoustic emission monitoring. In particular, the present paper looks into the white patching (micro-damage) development, crack initiation, propagation and coalescence, and relates these with fracture initiation and breakdown pressures. From the tests, it was observed that the initiation of visible cracks is, generally, preceded by white patching and occurs at or near the pre-fabricated flaws. The visual analysis showed that the induced fractures were primarily intragranular when the triaxial stresses were same in the three orthogonal directions and intergranular otherwise. Additionally, micro-fracture initiation pressures were obtained based on white patch initiation pressures (visually) and using a graphical method which relates the changes in pressurization rate to fracture initiation. It was observed that the graphical method identifies when the initial damage occurred significantly earlier than the visual method, which indicates that important micro-damage processes may occur without macroscopic evidence. Most importantly, the coalescence patterns and the ratio between the breakdown pressure and the vertical stress are stress-state dependent. In fact, for lower vertical loads (VL = 1 MPa), the ratio between the breakdown pressure and vertical load is approximately 10 and for higher vertical loads (VL = 4 MPa) approaches 1.0, which is in agreement with data observed in the field. [ABSTRACT FROM AUTHOR]

Water–rock interaction is an essential factor in the weakening of coal rock masses. Long-term soaking (LTS) as a type of water–rock interaction is widespread in mining engineering and shaft engineering. In this paper, progressive damage characterization of LST coal samples is obtained by a uniaxial compression experiment. Combining physical properties tests in coal samples and hydrochemistry tests in soaking solution quantitatively characterize changes in mineral compositions and microstructures of samples. The weakening mechanism of LTS coal is discussed at a micro-scale. The results show that mechanical parameters and acoustic emission (AE) signals of LTS coal samples continuously decline during loading. The crack initiation stress σCI is regarded as the threshold value for long-term stability of coal rock masses, and the crack initiation stress ratio σCI/σP in a sample soaked for 360 days decreases by 10.8% compared with a saturated sample. With increasing of immersion time, the total number of cracks decreases while the percentage of shear cracks increases. The deformation failure of a sample transforms from tensile failure predominantly to tensile–shear composite failure. The variation of physical properties in coal samples as well as Ca2+, Na+ and K+ concentrations in soaking solution indicates that the dissolution of albite and calcite as well as the swelling of illite are observed throughout immersion process, which leads to an increased pore-crack number and enhanced connectivity in coal samples. As physical properties continue to evolve, the cementation of pre-existing weaknesses between coal matrix is more easily weakened, promoting the slip between matrixes. Under the effect of axial load, the failure surface of a LTS sample will develop along internal pre-existing weaknesses which is most favorable to induce failure. The study results can provide help for reliability assessment and maintenance of underground bearing structure under LTS. Highlights: Long-term soaking of coal samples with mine water. Effect of long-term soaking on the progressive damage characteristics of coal samples. Quantitative characterization of the evolution process of mineral compositions and microstructures of coal samples during long-term soaking. Study on the weakening mechanism of coal under long-term soaking. [ABSTRACT FROM AUTHOR]

This research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance. Highlights: Numerical modeling study accurately replicates the thermal-hydromechanical behavior of rock salt surrounding a heated borehole. Jumps in brine flow rate during transitions in heater power are observed, with the highest jump during cooling. Acoustic emission patterns suggest pronounced damage to rock salt during the cooling phase. Thermal pressurization and tensile-induced damage are identified as the primary contributors to brine inflow spikes. Findings improve predictions of brine availability, aiding the development of robust containment strategies for radioactive waste disposal. [ABSTRACT FROM AUTHOR]

Thermal–mechanical damage is an important problem threatening the safety of deep rock engineering. In this paper, the effects of coupling damage on the deformation and failure characteristics of rock mass were studied via cyclic loading damage, thermal damage and uniaxial compression acoustic emission (AE) tests, and the microscopic fracture process of the damaged rock mass was numerically simulated. Results showed that during heat treatment, the colour of the sample changed significantly, the mass, the P-wave velocity and the number of mineral species decreased. The peak strength and elastic modulus reach their maximum values at 600 °C and 300 °C, respectively, exhibiting a trend of initial increase and subsequent decrease. The rapid growth period of AE activity increased noticeably with increasing temperature, and the effect of energy accumulation became more significant at higher peak strengths. The failure mode was influenced primarily by the cyclic loading amplitude. In addition, an increase in the stress or temperature after crack initiation leads to a sharp increase in the damage within the rock. Temperature had a more significant effect on the generation of damage than stress. Stress-induced microcracks were concentrated in the weakly bonded particles, whilst temperature-induced microcracks were concentrated in the strongly bonded particles. Highlights: Temperature had a significant effect on the mineral composition of rock mass. The rapid growth period of AE activity was prolonged with the temperature increase. The failure mode of rock mass was mainly influenced by cyclic loading amplitude. Temperature-induced microcracks were concentrated in strongly bonded particles. [ABSTRACT FROM AUTHOR]

In the Tunnel Boring Machines (TBMs) construction of soft rock tunnels, adverse geological conditions, such as water-rich and wet strata are often encountered. Water in strata can change the stress state and weaken the mechanical performance of rock mass, further affecting the tunneling process and rock breaking efficiency. To study the effect of water content on mudstone fragmentation characteristics and cutter force during TBM cutter indentation, the water softening effect is considered in the moisture diffusion-fracture coupling model based on the finite-discrete element method (FDEM). Then, the micro parameters (fracture energy release rate, swelling coefficient and moisture diffusion coefficient) of mudstone are calibrated by uniaxial, triaxial compression and swelling tests. Subsequently, the water content and swelling deformation evolution of mudstone samples during water migration are revealed. Finally, the coupling model is utilized to investigate the fragmentation characteristics and cutter force of mudstone samples with different water contents under various confining pressures during TBM cutter indentation. The fragmentation zone, cutter force, acoustic emission (AE), and crack number during cutter indentation are quantitatively analyzed. Results indicate that the vertical extent of fragmentation range (Ev) decreases with the increase of water content, while the horizontal extent of fragmentation zone (Eh) does not decrease obviously until the water content reaches 3%. The increase of confining pressure reduces the effect of water content on Ev, and the increase of water content also reduces the effect of confining pressure on Ev, indicating that confining pressure and water content have a mutual inhibition effect on Ev. Besides, both the peak value of normal force and the fluctuation range of tangential force for the TBM cutter decrease with the increase of the water content. The AE counts show that the failure pattern of mudstone is mainly shear failure during TBM cutter indentation. Specifically, many shear cracks are generated in the compression zone under the cutter, while few tensile cracks propagate in mudstone on both sides and deeper below the cutter. For mudstone under different confining pressures, the proportion of shear cracks increases with the increase of water content. Moreover, the effect of cutter penetration velocity on mudstone fragmentation characteristics and cutter force is discussed. The results in this paper have theoretical guidance for TBM construction of soft rock tunnels in water-rich and wet strata. Highlights: A FDEM moisture diffusion-fracture coupling model is first applied to study the effect of water content on rock fragmentation by TBM cutter. Shear failure is the main failure pattern of mudstone during TBM cutter indentation. The fragmentation zone and cutter force are greatly affected by water content during TBM cutter indentation. Water content and confining pressure have a mutual inhibition effect on the vertical extent of fragmentation zone. [ABSTRACT FROM AUTHOR]

The excavation of a deep jointed rock mass in a true triaxial stress environment can easily lead to catastrophic consequences, and the failure of a rock bridge is directly related to the stability of the jointed rock mass. Most previous studies have focused on experimental and simulation studies related to parallel joints, while few studies have considered nonparallel joints due to their complexity. In this paper, the mechanical behaviors and acoustic emission (AE) characteristics of nonparallel joints with different rock bridge lengths (RBLs) at different intermediate principal stresses (σ2) are studied. The results show that the peak strength increased with the increasing σ2 and RBL, while the peak strain decreased with increasing σ2 and increased with the increasing RBL. The longer the RBL was, the greater the influence of σ2 on rock failure, and the smaller σ2 was, the more significant the influence of the RBL on rock failure. The increase in the AE energy lagged behind the ringing count, and the cumulative AE counts of the specimens all showed a good pattern of decreasing with the increasing σ2. The longer the RBL was, the more sensitive the AE frequency was to the change in σ2. With the same RBL, the high-amplitude signals in the low-frequency range gradually attenuated as σ2 increased. The "quiet period" in the AE signals during rock failure could be specifically characterized by a comprehensive AE analysis, including AE time–frequency signals, AE ringing counts, AE event numbers, and AE b values. Finally, a modified Mogi–Coulomb-type criterion considering the effects of the true triaxial stress and RBL is proposed. The research results can provide a useful reference and help in the identification of precursory information of joint rock mass failure and early warning of sudden instability failure. Highlights: The failure mechanical behaviors of rock bridge with different lengths under different σ2 are analyzed. The AE amplituary-frequency and time-frequency characteristics of different rock bridge lengths under different σ2 are studied. The possibility that the "quiet period" of AE signal could be used as the precursor information of rock failure is discussed. A modified Mogi-Coulomb type criterion considering the effects of true triaxial stress and rock bridge length is proposed. [ABSTRACT FROM AUTHOR]

Evaluating the ability of coal seams to form fracture networks by hydraulic fracturing is important for the development of coalbed methane (CBM) reservoirs. In this paper, a new index for evaluating coal brittleness was established from the perspective of energy evolution during coal failure. Uniaxial and triaxial compression tests of coal monitored by an acoustic emission (AE) system were carried out and the applicability of the new index and the influence of the confining pressure and cleat orientation on the coal brittleness were analyzed. The pre-peak and post-peak dissipated energies were the essential factors in determining the coal brittleness. The new index can characterize the influence of the external stress and cleat orientation on coal brittleness, and can also comprehensively reflect the mechanical properties of the coal during the pre-peak and post-peak stages. The corresponding AE energy curves can be divided into Rapid Fracture Type, Stable Fracture Type and Plastic Fracture Type. For the Rapid Fracture Type, the accumulation rate of AE energy showed sudden changes when reaching the yield stress and peak strength, which represented high brittleness. The Plastic Fracture Type represented low brittleness, and the accumulated AE energy curves were smooth—first concave and then convex. The brittleness index of coal studied in this paper can provide a new method for selecting the optimal CBM reservoir and optimizing the fracturing scheme. [ABSTRACT FROM AUTHOR]

Damaged rock mass contains a great number of microstructures and defects at various scales, which could influence wave propagation significantly. In this paper, investigation of low-amplitude stress wave attenuation in rocks with different damage degrees was undertaken to determine the effects of parameters including damage degree, heterogeneity and type of waveforms on wave propagation, attenuation and slowness. The numerical method was validated through comparison with laboratory measurements. The damage was induced by conducting uniaxial compression tests on the intact rocks and was defined with the acoustic emission. The results showed that the attenuation ratio of low-amplitude stress wave increases with increasing rock damage degree. The rock heterogeneity was incorporated into the numerical model using a digital image processing technique with the combination of X-ray micro-computerized tomography. We found that the rock heterogeneity has great influence on the wave amplitude attenuation. Three types of waveforms, i.e., half-sine wave, square wave and exponential decay wave, were utilized to analyze influences of waveforms on attenuation characteristics in damaged rock specimens. The attenuation ratio of the exponential decay wave is the highest for rock specimens with the same damage degree, attributed to its highest dominant frequency. Moreover, detailed spectrum analysis suggested that the dominant frequency of the wave signal decreases with increasing damage degree. Based on derived simulating results, an improved classification scheme for determining rock fracturing degree, which groups the rock into zones of slightly fractures, moderately fractured and strongly fractured, was established by considering the relationship between wave amplitude and damage, since wave amplitude, other than wave velocity, is more sensitive to rock damage. The findings in this paper could facilitate a better understanding of wave propagation through rock and provide another means to determine rock fracturing degree. [ABSTRACT FROM AUTHOR]

There are still some major issues in the study of fracture network evolution during multi-cluster fracturing in shale reservoirs, such as the simplistic assumption that reservoir rocks are homogeneous and completely brittle, without considering that real rocks are often aggregates of different minerals and may present ductility and heterogeneity. It is important to study the evolution of the multi-cluster fracture network in quasi-brittle shale reservoirs to improve the stimulated reservoir volume (SRV). For this purpose, a 2D, coupled stress-seepage-damage field multi-cluster fracturing numerical model with global cohesive zone was developed in this paper. We conducted triaxial compression acoustic emission experiments using real shale samples and developed a generic trapezoidal TSL softening model that includes triaxial effects. The globally embedded 0-thickness cohesive elements ensures that hydraulic fractures can propagate randomly and thus reflect stress interference among multiple clusters. At the same time, the X-ray diffraction (XRD) experiment was used to determine the mineral content of rock, and the finite element mesh was then processed using the Weibull distribution probability density function to simulate the mineral heterogeneity of rock. In addition, the dynamic distribution of injection fluid flow during multi-cluster fracturing is implemented based on the Bernoulli equation. The cohesive parameters were validated by Brazilian splitting test, and the model was then used to parametrically study the evolution law of the fracture network and the variation characteristics of the flow rate into each cluster. The results show that using conventional bilinear TSL will result in a larger SRV than trapezoidal TSL, and reservoir heterogeneity may further exaggerate the drainage area when using bilinear TSL model. In addition, compared with a homogeneous isotropic reservoir, a highly heterogeneous reservoir has a more balanced flow rate into each cluster during multi-cluster fracturing, and this can significantly increase the complexity of fracture network. By increasing the number of clusters, it is possible to alleviate the stress interference on some internal clusters, which may also promote frequent fracture merging around the wellbore. Compared with increasing the number of clusters, reducing the perforation diameter can better compensate for the stress interference suffered by internal clusters, and make the flow rate into each cluster more balanced, thus improving the SRV. Highlights: The trapezoidal traction separation law can describe the damage softening behavior of quasi-brittle reservoirs. The fracture network formed using the bilinear traction separation law is more complex than the trapezoidal traction separation law. Heterogeneous reservoirs may form a more complex fracture network than homogeneous reservoirs during multi-cluster hydraulic fracturing. Uniformly increasing the number of clusters with small spacing has a limited effect on the uniformity of the flow rate into each cluster. Reducing the perforation diameter is an effective method to balance the flow rate into each cluster by weakening the effect of stress interference. [ABSTRACT FROM AUTHOR]

The determination of internal material damage is always an arduous challenge. Non-destructive monitoring methods show great potential in quantitatively determining the internal material properties, whereas most of the studies relying on external observations remain in a qualitative stage. They either violate the basic thermodynamic assumptions or are difficult to guide engineering practice. In this paper, following the theory of continuum mechanics, an elastoplastic damage model based on non-destructive monitoring methods (i.e., acoustic emission and ultrasonic wave velocity measurement) has been developed. To capture the continuous and precise damage evolution inside rock mass, P wave velocity obtained by ultrasonic wave measurement was utilised and then considered as an input for the proposed elastoplastic damage model. Triaxial loading test results on six Gosford sandstone samples were analysed first to characterise critical stresses along the stress–strain loading curves, such as crack closure stress, stable crack propagation stress and unstable crack propagation stress. The drop of ultrasonic wave velocity can be seen as an indicator to represent the damage evolution inside rock material. Damage initiation is also closely related to the confining stress and dilation induced volumetric expansion. The test results also suggested that the Drucker–Prager criterion is sufficient to describe the plastic yielding surface and the following material hardening. A non-associated plastic flow assumption was adopted, considering the essence of microcrack shearing in rock failure and the effect of hydrostatic pressure on plastic deformation. A modified Drucker–Prager plastic potential was also introduced to track the orientation of plastic increment with material hardening. A scalar damage variable was derived from ultrasonic wave measurement results to indirectly represent the deterioration of rock properties (modulus). The proposed model was used to match lab test results with high consistency, and the main features of rock behaviour in triaxial loading tests were successfully captured by the model. Finally, the damage evolution of rock samples was analysed, which indicates that damage is dependent on its conjugate force, namely damage energy release rate Y. This study proves that P wave velocity can be an effective approach to measure and forecast the internal damage evolution inside rock mass, which has broad prospects for engineering applications. Highlights: An elastoplastic damage model with non-associated plastic flow and scalar damage variable has been developed. P wave velocity is found to be closely related to the expansion of volumetric strain and damage evolution. [ABSTRACT FROM AUTHOR]

Aiming at the disadvantage that the traditional creep model cannot describe the nonlinear creep acceleration stage (third-order creep stage) of rock. This paper explains the creep process of salt rock from a microscopic perspective based on the Riemann–Liouville type fractional-order calculus operator theory and acoustic emission (AE) theory, and describes the creep process of salt rock with the improved fractional-order derivatives. The results of uniaxial creep damage tests on rock salt specimens under quasi-static loading conditions are given, complete creep damage curves are obtained, and a creep model based on fractional-order derivatives for viscoelastic damage of salt rock is proposed, and finally, the best variable values are fitted to determine the optimum values. The AE characteristic parameter curves were compared with the creep strain curves, and it was found that the AE characteristic curves could predict the time point when the salt rock enters the accelerated creep stage in advance. According to this time point, the model is fitted in sections and compared with the experimental results. The predicted value of the model is in good agreement with the test results, and can better describe the nonlinear accelerated creep stage of salt rock. It is believed that the fractional-order model can simulate the whole process of rock creep well and has good practical application value. Highlights: •Established a viscoelastic-plastic damage creep model of rock salt based on fractional derivative. •Based on AE technique, the creep process of rock salt was explained from the microscopic perspective and the damage evolution was obtained. •The AE characteristic curve can predict the time point when the salt rock enters the accelerated creep stage in advance, and the model can be fitted to the segment according to this time point, which can better describe the nonlinear accelerated creep stage of the salt rock. •Segmental fitting can well simulate the whole process of salt rock creep, has good superiority and reliable, which can predict engineering disaster in advance. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study on the dynamic propagation of fractures in granite samples under various injection schemes and initial stress conditions is carried out using a soundless cracking demolition agent (SCDA). Through the combination of acoustic emission (AE) monitoring, stress measurement and fracture morphology observation, the dynamic fracturing process is systematically analyzed. The stress shadowing effect between the two fractures is studied based on the results of stress monitoring and growth of fractures. The experimental results show that SCDA has a strong fracturing capacity and the stress shadowing effect between the boreholes makes the fractures tend to extend toward the direction connecting the two boreholes in the early stage of fracturing, leading to a more complicated fracture propagation mode. The change of stress within the sample can be divided into three stages, which correspond to the initiation of fractures, the initial formation of surface fractures and the stable propagation towards both sides and the lower part of the existing fractures. The fractal dimension of fractures and the roughness of fracture surfaces are quantitatively evaluated. The results show that the initial stress conditions and injection schemes are the key factors affecting the fracturing results. The smaller the side stress coefficient, the stronger the ability to form complex fractures. The optimal injection time interval varies with the selection of evaluation parameters and the fractal dimension of the sample is the highest when the injection time interval is 9 h. Highlights: The dynamic propagation of fractures under various injection schemes and initial stress conditions is systematically studied. The stress shadowing effect in multi-well fracturing makes fracture modes more complicated. The initial stress dominates the formation of complex fractures while the injection schemes also affects the trajectory. [ABSTRACT FROM AUTHOR]

The time evolution of the electric activity in brittle materials, mechanically loaded at levels approaching the ones causing catastrophic fracture, is studied and modeled in terms of the time-to-failure parameter. The electric activity is considered in juxtaposition to the respective acoustic one. The latter was analytically described in a recently published paper in terms of the F-function. The comparison is implemented to calibrate the outcomes of the Pressure-Stimulated Currents technique by means of the respective ones of a mature sensing technique, i.e., that of the Acoustic Emissions. Moreover, this comparative study aims to reveal possible common qualitative features of these sensing techniques. Using data from three different experimental protocols quite a few hidden affinities between the acoustic and electric signals recorded are revealed. Especially at the very last loading stages, i.e., while macroscopic fracture is impending, the time evolution of both the electric and acoustic emissions is governed by a power law, the exponent of which varies in a relatively narrow interval around unity. Moreover, it is shown that both techniques provide interesting pre-failure indicators, warning about upcoming fracture. At least for some protocols of the present study, it is concluded that the pre-failure indicators of the electric signals precede the respective ones of the acoustic signals. The combined use of data obtained from completely different loading schemes, materials, types of specimens and load application modes offers reliable support to the conclusions drawn while at the same time guarantees their wide applicability. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue damage of the wellbore cement sheath under different cyclic loading conditions. According to the irreversible principle of thermodynamics, a damage evolution model considering both tension and compression fatigue damage is established. With acoustic emission monitoring, uniaxial compression and Brazilian disc splitting cyclic loading failure tests were performed. Based on the test results, the damage theory is verified using the cumulative strain, Young's modulus and acoustic emission characteristics. The research gives a numerical algorithm of the fatigue damage theory to develop as a constitutive equation. With reference to the structural and operating parameters of a gas storage salt cavern in Jintan, a casing-cement sheath-stratum numerical model is established. By inserting the damage constitutive equation into the numerical model calculation, the damage characteristics of the cement sheath are shown. Finally, the gas storage operating parameters that meet the requirements of high injection-production efficiency and long service life are given. [ABSTRACT FROM AUTHOR]

The interaction between a natural fracture (NF) and a hydraulic fracture (HF) has been studied extensively, both experimentally and numerically, to better understand the potential for crossing and arrest of a hydraulic fracture intersecting a natural fracture. However, the actual mechanical interaction between a hydraulic and a natural fracture or a bedding plane has not been studied, particularly under triaxial stress and injection conditions. Analysis of field microseismic data recorded during hydraulic fracturing shows that the bedding plane could slip due to the approaching hydraulic fracture. In this paper, we present the results of some lab-scale experimental work, demonstrating HF/NF interaction with an emphasis on the slippage of a discontinuity surface. Injection pressure, stress applied, and the sample deformation are monitored during the tests. Acoustic Emission (AE) technology is employed to record the AE signals generated during fracture initiation, propagation, and during the sliding of the joint. In addition, strain gauges are used to measure the slippage on the natural fracture. The tests are carried out on 101.6 mm diameter cylindrical samples of PMMA, shale, and granite. The calculated displacement based on the recorded strain clearly shows a jump at the breakdown point, which is accompanied by increased AE activity and stress drop. Analysis of the data clearly shows the occurrence of slippage on the joint in response to an approaching hydraulic fracture. Expectedly, the degree of shear slip varies with natural fracture dip and friction angle. [ABSTRACT FROM AUTHOR]

Moment tensor analyses suggest that shear microfractures dominate the compressive characteristics of most rock materials. However, the microfracture sizes estimated by the traditional shear model are unrealistically large. This paper presents a novel shear model for microfracture size estimation in sandstone specimens based on AE measurements. The sizes of 342 microfractures were estimated via the proposed model; most are found to range from 0.0022 to 1.80 mm with an average microfracture size slightly smaller than the average grain size. The microfracture size distribution characteristics observed here are similar to those observed by microscope for stressed sandstone, suggesting that more realistic microfracture sizes can be obtained with the proposed model than the traditional shear model. [ABSTRACT FROM AUTHOR]

The rapid and accurate measurement of in situ stress fields is one of the essential steps in geological engineering. Indoor acoustic emission tests and the Kaiser effect provide a promising prior peak stress measurement method. This paper aims to qualitatively explore the time delay mechanism of the Kaiser effect under uniaxial compressive stress. Acoustic emission tests were conducted with natural and water-soaked sandstone specimens to examine the time delay phenomenon of the Kaiser effect, and the typical Griffith fracture mechanics theory was adopted to explain the mechanism. The time delay of the Kaiser effect under uniaxial compressive stress was affected by the friction coefficient on the crack surfaces and the initial crack angle β . The time effect was indirectly induced by the changes in the friction coefficient on the crack surfaces, and the friction coefficient was controlled by the water content in the rock specimen. A higher initial friction coefficient led to an earlier and easier time delay of the Kaiser effect. Consequently, from the aspect of the Kaiser effect, it was suggested that when testing in situ stress, measures should be taken to maintain the original moisture contents in the rock cores after they are drilled underground. Otherwise, the in situ stress should be tested within 7 days. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt is widely distributed in the dam foundation and underground cavern of the Baihetan hydropower station, in which columnar jointing and microfractures are developed, the integrity of the rock mass is poor, and the material easily relaxes after excavation. Therefore, it is important to obtain the real mechanical parameters of columnar jointed basalt in rock mechanics problems. In the test cave at the dam site, six in situ samples with a size of 50 cm × 50 cm × 100 cm were successfully made, and in situ true triaxial test were carried out. This paper presents a method for obtaining the strength parameters of the Mohr–Coulomb criterion and Hoek–Brown criterion by true triaxial testing. The peak strength parameters under three states and yield strength parameters under one state were obtained by setting various stress paths for the undisturbed samples and failure samples. The strength characteristics of columnar joint basalt and the crack characteristics of the specimens are analyzed, and the results of the triaxial test and direct shear test are compared. By means of acoustic emission (AE) techniques and surface crack descriptions, the failure process and failure pattern of the samples were obtained. The abundant test results provide strong support for the selection of engineering rock mechanics parameters. [ABSTRACT FROM AUTHOR]

Flame jet-assisted mechanical rock breaking technology is an important method to achieve efficient NECMM (non-explosive continuous mechanized mining) in hard rock mines. Therefore, it is very important to understand the physical and mechanical properties of the spalling pit surrounding rock after flame jet. In this paper, thermal spalling tests were first performed on granite blocks using a flame with a power of 13.6 kW. Then, the mechanical behaviors of the specimens at different distances from the flame jet path were measured. The result showed that the Vp (P-wave velocity), BTS (Brazilian tensile strength), UCS (uniaxial compression strength), and E (Young’s modulus) of granite increased with increasing distance. Meanwhile, the mechanical properties of the specimen at 70 mm were slightly lower than that of the untreated specimen. Therefore, it can be assumed that the damage radius of granite after the flame jet was about 70 mm. In Brazilian test, according to the strain cloud diagram of DIC (digital image correlation) system, the high strain zone expanded from the center to both ends of the specimen, and the strain value gradually increased, eventually forming a through-wall crack along the loading direction. In the uniaxial compression test, AE (acoustic emission) counts within the compacted stage increased obviously as the distance from the specimen to the flame jet path decreased. This phenomenon indicated that the thermal damage of specimens near the center of the spalling pit was more severe, and more microcracks inside the specimens were compacted. The thermal spalling and thermal damage model of the rock under the flame jet was established, which can well explain the changes in mechanical properties of rocks. The fact that thermal damage area of the rock after the flame jet is much larger than thermal spalling area is responsible for the feasibility of flame-assisted mechanical rock breaking. Damage factors based on Vp and E can well describe the weakening of the mechanical properties of granite. The results are important for guiding flame jet-assisted mechanical rock breaking in hard rock mines. [ABSTRACT FROM AUTHOR]

When shield tunnelling is constructed in complex geological conditions using a tunnel boring machine, the disc cutter in the cutterhead easily wears to the failure state, particularly when the ground conditions are heterogeneous. This paper summarises the failure modes of the disc cutter in heterogeneous ground conditions into three categories, based on the observed wear data from field: (1) uniform disc cutter wear, (2) non-uniform disc cutter wear, and (3) breakage of cutter ring. Subsequently, the stress state of a disc cutter in the heterogeneous ground was analysed and the effective factors were investigated. The relationships between friction energy during cutting, working status of the machine and the characteristics of the geological conditions were evaluated. Based on the stress analysis and friction energy, a prediction model was proposed. The proposed model was applied to two field case studies: pertaining to uniform and mixed-face ground conditions, for which the empirical coefficient k for energy transfer was also determined. The preliminary results from this research indicated that the proposed model was valid for both homogeneous and heterogeneous ground conditions. Further case studies provided by co-operators are expected to improve the effectiveness of the proposed model. [ABSTRACT FROM AUTHOR]

Surface roughness significantly affects the shear behavior of rock joints; thus, studies on the asperity damage characteristics and its influence on the shear behavior of joints are extremely important. In this paper, shear tests were conducted on tensile granite joints; asperity damage was evaluated based on acoustic emission (AE) events; and the influence of asperity damage on joint shear behavior was analyzed. The results indicated that the total AE events tended to increase with normal stress. In addition, the asperity damage initiation shear stress, which is defined as the transition point from slow growth to rapid growth in the cumulative events curve, was approximately 0.485 of the peak shear strength regardless of the normal stress. Moreover, 63–85% of the AE events were generated after the peak shear stress, indicating that most of the damage occurred in this stage. Both the dilation and the total AE events decreased with shear cycles because of the damage inflicted on asperities during the previous shear cycle. Two stages were observed in the normal displacement curves under low normal stress, whereas three stages (compression, dilation and compression again) were observed at a higher normal stress; the second compression stage may be caused by tensile failure outside the shear plane. The magnitude of the normal stress and the state of asperity are two important factors controlling the post-peak stress drop and stick–slip of granite joints. Serious deterioration of asperities will stop stick–slip from recurring under the same normal stress because the ability to accumulate energy is decreased. The AE b-value increases with the number of shear cycles, indicating that the stress concentration inside the fault plane is reduced because of asperity damage; thus, the potential for dynamic disasters, such as fault-slip rockbursts, will be decreased. [ABSTRACT FROM AUTHOR]

This paper presents the design, development and application of a new multi-phase high-pressure and elevated temperature rock hydromechanical testing apparatus for the investigation of reservoir and cap rock behaviour in carbon geo-sequestration projects. The triaxial apparatus is designed to support high confining stress, injection pressures and higher temperatures to imitate the natural thermo-hydro-geomechanical conditions of deep underground geological formations. The apparatus also includes an acoustic emission device for the study of the mechanical failure behaviour of rocks under compression. The apparatus is designed to support a range of different rock specimen sizes from 34 to 54 mm in diameter. Since sequestration projects involve the injection of supercritical carbon dioxide, which is extremely sensitive to temperature and pressure, and is highly corrosive in nature, special precautions were taken in the design and manufacture of the apparatus. The data acquisition system is powered and calibrated in accordance with each of the sensors and is guided by a series of in-house developed and commercial softwares for data storage and analysis. The methodology for conducting advanced testing on cap rock and reservoir rocks with the injection of water and supercritical CO is presented with the appropriate theory. Some preliminary tests have been carried out on sandstone specimens sourced from the Melbourne region using the newly designed apparatus and the results are presented in this paper. [ABSTRACT FROM AUTHOR]

A coal and gas outburst is an extreme hazard in underground mining. The present paper conducts a laboratory simulation of a coal and gas outburst combined with acoustic emission analysis. The experiment uses a three-dimensional stress loading system and a PCI-2 acoustic emission monitoring system. Furthermore, the development of a coal and gas outburst is numerically studied. The results demonstrate that the deformation and failure of a coal sample containing methane under three-dimensional stress involves four stages: initial compression, elastic deformation, plastic deformation and failure. The development of internal microscale fractures within a coal sample containing methane is reflected by the distribution of acoustic emission events. We observed that the deformation and failure zone for a coal sample under three-dimensional stress has an ellipsoid shape. Primary acoustic emission events are generated at the weak structural surface that compresses with ease due to the external ellipsoid-shaped stress. The number of events gradually increases until an outburst occurs. A mathematical model of the internal gas pressure and bulk stress is established through an analysis of the internal gas pressure and bulk stress of a coal sample, and it is useful for reproducing experimental results. The occurrence of a coal and gas outburst depends not only on the in situ stress, gas pressure and physical and mechanical characteristics of the coal mass but also on the free weak surface of the outburst outlet of the coal mass. It is more difficult for an outburst to occur from a stronger free surface. [ABSTRACT FROM AUTHOR]

The great success and widespread use of impregnated diamond (ID) bits are due to their self-sharpening mechanism, which consists of a constant renewal of diamonds acting at the cutting face as the bit wears out. It is therefore important to keep this mechanism acting throughout the lifespan of the bit. Nonetheless, such a mechanism can be altered by the blunting of the bit that ultimately leads to a less than optimal drilling performance. For this reason, this paper aims at investigating the applicability of artificial intelligence-based techniques in order to monitor tool condition of ID bits, i.e. sharp or blunt, under laboratory conditions. Accordingly, topologically invariant tests are carried out with sharp and blunt bits conditions while recording acoustic emissions (AE) and measuring-while-drilling variables. The combined output of acoustic emission root-mean-square value (AE), depth of cut ( d), torque (tob) and weight-on-bit (wob) is then utilized to create two approaches in order to predict the wear state condition of the bits. One approach is based on the combination of the aforementioned variables and another on the specific energy of drilling. The two different approaches are assessed for classification performance with various pattern recognition algorithms, such as simple trees, support vector machines, k-nearest neighbour, boosted trees and artificial neural networks. In general, Acceptable pattern recognition rates were obtained, although the subset composed by AE and tob excels due to the high classification performances rates and fewer input variables. [ABSTRACT FROM AUTHOR]

In underground coal mining, high abutment loads caused by the extraction of coal can be a major contributor to many rock mechanic issues. In this paper, a large-scale physical modeling of a 2.6 × 2.0 × 1.0 m entry roof has been conducted to investigate the fundamentals of the fracture mechanics of entry roof strata subjected to high abutment loads. Two different types of roof, massive roof and laminated roof, are considered. Rock bolt system has been taken into consideration. A distinct element analyses based on the physical modeling conditions have been performed, and the results are compared with the physical results. The physical and numerical models suggest that under the condition of high abutment loads, the massive roof and the laminated roof fail in a similar pattern which is characterized as vertical tensile fracturing in the middle of the roof and inclined shear fracturing initiated at the roof and rib intersections and propagated deeper into the roof. Both the massive roof and the laminated roof collapse in a shear sliding mode shortly after shear fractures are observed from the roof surface. It is found that shear sliding is a combination of tensile cracking of intact rock and sliding on bedding planes and cross joints. Shear sliding occurs when the abutment load is much less than the compressive strength of roof. [ABSTRACT FROM AUTHOR]

To avoid hydraulic failure of unlined pressure tunnels, knowledge of minimum principal stress is needed. Such knowledge is only obtainable from in situ measurements, which are often time-consuming and relatively costly, effectively limiting the number of measurements typically performed. In an effort to enable more stress measurements, the authors propose a simplified and cost-effective stress measuring method; the Rapid Step-Rate Test (RSRT), which is based on existing hydraulic testing methods. To investigate the ability of this test to measure fracture normal stresses in field-like conditions, a true triaxial laboratory test rig has been developed. Hydraulic jacking experiments performed on four granite specimens, each containing a fracture, have been performed. Interpretation of pressure-, flow- and acoustic emission (AE) data has been used to interpret fracture behaviour and to assess fracture normal stresses. Our experimental data suggest that the proposed test method, to a satisfactory degree of reliability, can measure the magnitude of fracture normal stress. In addition, a clear correlation has been found between fracture closure and sudden increase in AE rate, suggesting that AE monitoring during testing can serve as a useful addition to the test. The rapid step-rate test is also considered relevant for field-scale measurements, with only minor adaptions. Our findings suggest that the RSRT can represent a way to get closer to the ideal of performing more testing along the entire length of pressure tunnel, and not only at key locations, which requires interpolation of stress data with varying degree of validity. [ABSTRACT FROM AUTHOR]

Determination of the cracking levels during the crack propagation is one of the key challenges in the field of fracture mechanics of rocks. Acoustic emission (AE) is a technique that has been used to detect cracks as they occur across the specimen. Parametric analysis of AE signals and correlating these parameters (e.g., hits and energy) to stress-strain plots of rocks let us detect cracking levels properly. The number of AE hits is related to the number of cracks, and the AE energy is related to magnitude of the cracking event. For a full understanding of the fracture process in brittle rocks, prismatic specimens of granite containing pre-existing flaws have been tested in uniaxial compression tests, and their cracking process was monitored with both AE and high-speed video imaging. In this paper, the characteristics of the AE parameters and the evolution of cracking sequences are analyzed for every cracking level. Based on micro- and macro-crack damage, a classification of cracking levels is introduced. This classification contains eight stages (1) crack closure, (2) linear elastic deformation, (3) micro-crack initiation (white patch initiation), (4) micro-crack growth (stable crack growth), (5) micro-crack coalescence (macro-crack initiation), (6) macro-crack growth (unstable crack growth), (7) macro-crack coalescence and (8) failure. [ABSTRACT FROM AUTHOR]

In this paper, the results of laboratory studies of hydraulic fracture in homogeneous sandstone blocks with man-made interfaces and heterogeneous shale blocks with weak natural interfaces are reported. Tests were conducted under similar stress conditions, with fluids of different viscosity and at different injection rates. The measurements and analysis allows the identification of fracture initiation and behavior. Fracturing with high-viscosity fluids resulted in stable fracture propagation initiated before breakdown, while fracturing with low-viscosity fluids resulted in unstable fracture propagation initiated almost simultaneously with breakdown. Analysis also allows us to measure the fluid volume entering the fracture and the fracture volume. Monitoring of acoustic emission hypocenter localizations, indicates the development of created fractured area including the intersection with interfaces, fluid propagation along interfaces, crossing interfaces, and approaching the boundaries of the block. We observe strong differences in hydraulic fracture behavior, fracture geometry and fracture propagation speed, when fracturing with water and high-viscosity fluids. We also observed distinct differences between sandstone blocks and shale blocks, when a certain P-wave velocity ray path is intersected by the hydraulic fracture. The velocity increases in sandstones and decreases in shale. [ABSTRACT FROM AUTHOR]

This study investigates the fracture mechanism of fluid coupled with a solid resulting from hydraulic fracture. A new loading machine was designed to improve upon conventional laboratory hydraulic fracture testing and to provide a means of better understanding fracture behavior of solid media. Test specimens were made of cement mortar. An extensometer and acoustic emission (AE) monitoring system recorded the circumferential deformation and crack growth location/number during the test. To control the crack growth at the post-peak stage the input fluid rate can be adjusted automatically according to feedback from the extensometer. The complete stress-deformation curve, including pre- and post-peak stages, was therefore obtained. The crack extension/growth developed intensively after the applied stress reached the breakdown pressure. The number of cracks recorded by the AE monitoring system was in good agreement with the amount of deformation (expansion) recorded by the extensometer. The results obtained in this paper provide a better understanding of the hydraulic fracture mechanism which is useful for underground injection projects. [ABSTRACT FROM AUTHOR]

Within a research work aimed to better understand frost weathering mechanisms of rocks, laboratory tests have been designed to specifically assess a theoretical model of crack propagation due to ice segregation process in water-saturated and thermally microcracked cubic samples of Arolla gneiss. As the formation and growth of microcracks during freezing tests on rock material is accompanied by a sudden release of stored elastic energy, the propagation of elastic waves can be detected, at the laboratory scale, by acoustic emission (AE) sensors. The AE receiver array geometry is a sensitive factor influencing source location errors, for it can greatly amplify the effect of small measurement errors. Despite the large literature on the AE source location, little attention, to our knowledge, has been paid to the description of the experimental design phase. As a consequence, the criteria for sensor positioning are often not declared and not related to location accuracy. In the present paper, a tool for the identification of the optimal sensor position on a cubic shape rock specimen is presented. The optimal receiver configuration is chosen by studying the condition numbers of each of the kernel matrices, used for inverting the arrival time and finding the source location, and obtained for properly selected combinations between sensors and sources positions. [ABSTRACT FROM AUTHOR]

The tensile strength of rocks is affected by loading conditions, specifically direct and Brazilian tension, as well as water saturation. But the fundamental reasons for the differences between direct tensile strength σ td and Brazilian tensile strength σ tb , and their degradation induced by water saturation, are not yet fully understood. Here, we report a series of direct tensile and Brazilian tests performed on marble and granite rocks under both dry and water-saturated conditions. We employed the acoustic emission (AE) technique and further analyzed the recorded AE waveforms using a methodology based on the statistical analysis of dominant frequency to infer the intrinsic failure process of studied rocks under different water and loading conditions. Our results show that water saturation causes substantial reductions in both σ td and σ tb , with the reduction in σ tb being greater. A correspondence between AE waveforms distributed in high and low dominant frequency bands (H-type and L-type waveform) and rock failure types is found, that is, the H-type waveforms correspond to the micro-shear failures, and the L-type waveforms correspond to the micro-tensile failures. Based on this, the generation of abundant micro-shear failures is the cause of enhancement in σ tb in rocks compared to its σ td , and the reductions in σ td and σ tb for saturated rocks result from increasing micro-tensile failures. Furthermore, the friction-weakening effect induced by lubricating water films is responsible for the tensile strength reduction of saturated marble and granite under direct and Brazilian tension. The pronounced friction-weakening effect resulting from the widespread friction condition in the Brazilian test facilitates the σ tb loss. In addition to the friction-weakening effect, stress corrosion could also be a secondary cause of the tensile strength reduction of saturated granite. Highlights: The corresponding relationship between acoustic emission waveform types with micro-failure patterns is built. Reasons for the difference in measured tensile strength under direct and Brazilian tension are proposed. Water-weakening effects on the tensile strength of rocks under direct tension and Brazilian loading are revealed. [ABSTRACT FROM AUTHOR]

In deep tunnelling, mining and subsurface energy recovery, the reliable estimate of rough fracture (or fault) strength and the potential for reactivation are of vital importance for the assessment of dynamic geo-hazard, such as fault slip rock-burst and induced earthquakes. In this study, true triaxial loading tests were conducted on pre-fractured granite with different orientations to the maximum principal stress, and the fracture slip process was studied with the aid of acoustic emission and deformation monitoring. The reactivation strength was also compared with the theoretical predictions based on the analytical model. Results show that the critical fracture angle for the rock matrix failure and original macro-fracture reactivation is ~ 51°, above which fault slip occurs along the original macro-fracture at a gradually smaller differential stress. The microscopic analysis indicates that new faulting develops traversing the original fracture which becomes more compacted and closed with insignificant damage when the fracture angle is below ~ 44°. The differential stress required for rough fracture reactivation is well predicted by the single plane of weakness theory. In addition, variation of acoustic emission signals (especially AE energy) and deformation along σ3 direction with loading time are very consistent, which can be used to analyse the preparatory and evolutionary process of the fracture reactivation. The stress rate decreases whilst deformation rate increases both from a steady state value to a very large value as the fracture is gradually reactivated, accompanied by the b value decreasing from 1.4–2 to 0.4–0.6, which can be used as precursors for the dynamic fault slip. The findings in the present study will provide new insights into the mechanics of rough fracture reactivation in deep tunnelling, mining and underground resource extraction. Highlights: True triaxial loading tests are conducted on pre-fractured granite with different fracture orientations to the maximum principal stress. Three different failure modes are revealed for pre-fractured granite specimens with increasing fracture angles from 39° to 57°. The original macro rough fracture is reactivated when the fracture angle is above ~50°. The obtained fracture reactivation strengths are very consistent with the theoretical prediction by the single plane of weakness theory. Acoustic emission hits (counts) and deformation along σ3 are effective indicators to show the preparatory process before fracture reactivation. [ABSTRACT FROM AUTHOR]

To investigate the micromechanical properties of sandstones during brittle and ductile failure, we conducted uniaxial and conventional triaxial compression tests on Zigong sandstone (porosities of 17.71% and 10.40%), facilitated by acoustic emission (AE) monitoring and scanning electron microscopy (SEM) analysis. Increasing confining pressure leads to a change in the failure mode from brittle to ductile. In the brittle regime, the fracture angle decreases with the confining pressure while the peak strength increases. In contrast, the peak strength shows a less increasing trend in the ductile regime. Under low confining pressures, the AE hit rate increases rapidly towards the peak stress and reaches the peak when the macroscopic fractures form in the post-peak stage. Under high confining pressures, the AE hit rate rapidly rises beyond the critical stress (Cʹ) indicating shear-induced dilation, which stays at a high level during the post-peak stage. Thin sections of the failed specimen were prepared to quantitatively investigate the microcrack distribution using SEM. It is found that stress-induced cracks are mostly parallel to the direction of the maximum principal stress. Furthermore, the crack number density and crack density inside the shear band are higher than those outside of it, and both metrics increase with the confining pressure. The failure mode was calculated using bifurcation theory, and the theoretical predictions indicate that specimens under high confining pressure exhibited compaction shear bands. Highlights: The mechanical characteristics, failure modes, and AE characteristics of two porous sandstones under different confining pressures are studied. The direction of stress-induced cracks and crack density are statistically analyzed, revealing the effect of confining pressure on microcracks formation. The consistency between the orientation of microcracks and the macroscopic fracture angle have been revealed. Bifurcation theory is employed to predict the failure mode and the orientation of shear bands. [ABSTRACT FROM AUTHOR]

Microwave energy and in situ stress cooperatively influence the permeability of coal reservoirs in microwave-assisted coalbed methane (CBM) extraction. In this study, the combined influence of microwave energy and load on the failure characteristics of coal reservoirs was investigated using acoustic emission (AE) technology. The failure characteristics of anthracite derived from microwave irradiation during uniaxial compression was analyzed from aspects such as the correlation dimension (D) of the AE time series, b-value, and AF–RA correlation. Then the combined mechanism of microwave energy and in situ stress was explored. The results indicated that microwave energy significantly affected the AE activity of anthracite under loading. Moreover, microwave irradiation lessened the damage intensity and increased the crush degree during loading. After low-energy microwave (≤ 90 kJ) irradiation, the AE signals of anthracite were active only in the unstable crack growth stage, with less damage under load, and mainly characterized by the rapid expansion of large fractures. In contrast, after high-energy microwave (> 90 kJ) irradiation, AE signals penetrate the entire loading process, and the required energy for damage decreased, mostly manifested as small-scale microcracks. Additionally, microwave irradiation changed the failure mode of anthracite under loading from shear failure to tensile failure. In summary, microwave thermal effect reduced the stress threshold for fracture propagation by improving the physical structure of coal reservoirs. Cracks initiated by microwave irradiation could cause in situ stress redistribution, forming stress reduction areas and stress concentration areas, thereby accelerating the expansion of fracture networks. Therefore, the results of this study have great significance for optimizing the scheme of microwave-assisted CBM extraction and improving the CBM production rate through effective utilization of in situ stress. Highlights: Microwave irradiation enhances the AE activity of anthracite under load. Microwave irradiation decreases the failure intensity and increases the damage degree of anthracite. Microwave irradiation changes the failure mode of anthracite from shear failure to tensile failure. The cooperative mechanism of microwave energy and in situ stress during microwave-assisted CBM extraction is discussed. [ABSTRACT FROM AUTHOR]

In this paper, the results of laboratory studies of fracture initiation, early propagation and breakdown are reported. Three experiments were conducted on a low permeability sandstone block, loaded in a polyaxial test frame, to representative effective in situ stress conditions. The blocks were instrumented with acoustic emission (AE) and volumetric deformation sensors. In two experiments, fluids of different viscosity were injected into the wellbore, fluid injection was interrupted soon after the breakdown pressure had been reached. This allowed us to investigate hydraulic fracture initiation. In the third test, fracture initiation criteria were applied to stop hydraulic fracture propagation significantly earlier, prior to breakdown, and as it propagated a short distance from the wellbore. The analysis of AE results shows an increase in AE activity and a change in the AE spatial correlation, during the fracture initiation. This early stage of fracturing correlates strongly with the onset of rock volumetric deformation, and is confirmed by the analysis of ultrasonic transmission monitoring. The rock microstructure, after the test, was investigated by analysis of scanning electron microscope images. These indicated the development of leak-off zone near the wellbore and a dry hydraulic fracture at the farther distance from the wellbore. [ABSTRACT FROM AUTHOR]

Rockburst is a sudden and violent failure of rocks and it often occurs in hard rocks in highly stressed ground. Strainburst is classified as one type of rockburst and it often occurs in rocks near or at the excavation boundary. Deep insight into the strainburst phenomenon is essential for safe underground construction at depth. In this paper, an experimental laboratory study on the strainburst behavior of Beishan granite is presented. Based on in-situ stress measurement data from the Beishan area in China, a series of tests under different unloading rates were performed to investigate the strainburst process using a true-triaxial strainburst test system which was equipped with an acoustic emission (AE) monitoring system. In addition, a high-speed video camera was used to record and visualize the initiation and ejection of rock fragments as well as the sudden dynamic failure (strainburst) of the test samples. AE characteristics associated with the cumulative energy and frequency-amplitude distributions were analyzed. Characteristics of the microscopic structure of a fragment generated from one test were observed using a scanning electron microscope. The experimental results indicate that the degree of violence during failure and the associated AE energy release in the strainburst process are dependent on the unloading rate. When the unloading rate is high, the rock is prone to strainburst. On the other hand, as the unloading rate decreases, the failure mode changes from strainburst to spalling. In addition, the cumulative AE energy is not sensitive to unloading rates greater than 0.05 MPa/s. When the unloading rate is less than 0.05 MPa/s, the cumulative AE energy shows a marked decreasing trend during rock failure. [ABSTRACT FROM AUTHOR]

Surface-parallel slabbing is a failure mode often observed in highly stressed hard rocks in underground excavations. This paper presents the results of experimental studies on slabbing failure of hard rock with different sample height-to-width ratios. The main purpose of this study was to find out the condition to create slabbing failure under uniaxial compression and to determine the slabbing strength of hard rock in the laboratory. Uniaxial compression tests were carried out using five groups of granite specimens. The mechanical parameters of the sample rock, Iddefjord granite from Norway, were measured on the cylindrical and Brazilian disc specimens. The transition of the failure mode was studied using rectangular prism specimens. The initiation and the propagation of slabbing fractures in specimens were identified by examining the relationship among the applied stress, strain and the acoustic emission. The stress thresholds identified were compared to those reported by other authors for crack initiation and brittle failure. It is observed that the macro failure mode will be transformed from shear to slabbing when the height/width ratio is reduced to 0.5 in the prism specimens under uniaxial compression. Micro σ-parallel fractures initiate when the lateral strain departs from its linearity. Slabbing fractures are approximately parallel to the loading direction. Labotatory tests show that the slabbing strength ( σ) of hard rock is about 60% of its uniaxial compression strength. It means that if the maximum tangential stress surrounding an underground excavation reaches about the slabbing threshold, slabbing fractures may take place on the boundary of the excavation. Therefore, the best way to stop or eliminate slabbing failure is to control the excavation boundary to avoid the big stress concentration, so that the maximum tangential stress could be under the slabbing threshold. [ABSTRACT FROM AUTHOR]

In situ stress measurement is not well accepted yet in Indonesia due to the unavailability of technology, high costs and because it is impractical in remote regions. Alternatively, the Kaiser effect of acoustic emission (AE) can be used as a method for determining the stress-state at depth, without creating induced stress and is practical in remote areas. This paper is focused on the development of the AE test. The research has started to study the phenomenon of stress memory in a rock sample, the factors influencing the phenomenon, and finally, to determine the in situ stresses around underground excavations by applying the AE method. It is expected that knowledge in gaining the most important input parameters for maintaining the stability of underground excavations can be well understood and be reliably conducted at a reasonable cost. [ABSTRACT FROM AUTHOR]

Shale oil and gas economic exploitation depends closely on effective hydraulic fracturing stimulations. However, due to the reservoir heterogeneity and in-situ stress variation, the hydraulic fracturing through multiple perforations in shale is confronted with imbalanced fracture growth. To investigate, we reported a new laboratory study that simulates hydraulic fracturing through multiple perforations in shale blocks. The effect of horizontal in-situ stress difference on simultaneous and asynchronous fracture growth from spiral perforations was analyzed. The results show that the in-situ stress difference significantly influences hydraulic fracture initiation and growth, fluid pressure behavior, and fracture orientation. Under a high value of horizontal in-situ stress difference, parallel transverse fractures are initiated simultaneously and growing independently along the direction of σH. Higher injection pressure is required to promote hydraulic fracture propagation with the decrease of horizontal in-situ stress difference. The induced fractures grow asynchronously from two adjacent perforations and later merge into a large tortuous fracture. Besides, the azimuth angle of hydraulic fracture deflects from the direction of σH, and the inclination angle gradually decreases. When the value of σh is close to σH, an inclined longitudinal fracture and multiple mini-transverse fractures are created from spiral perforations, resulting in a prolonged fracturing process and a significant near-wellbore fracture complexity. Highlights: A novel test configuration was developed to investigate multiple hydraulic fractures initiation and propagation from spiral perforations in shale Hydraulic fracture geometry, orientation, perforation breakdown, and fluid pressure behavior showed a clear dependence on stress difference The simultaneous and asynchronous fractures growth from multiple perforations was characterized based on the acoustic emission source location [ABSTRACT FROM AUTHOR]

Brittleness is an intrinsic mechanical property of rock materials that has attracted significant attention to be properly quantified as it plays an important role in characterization of brittle fracturing. Endeavors have led to the establishment of many Brittleness Indices (BIs) for various rock types and widespread engineering applications. Among them, assessing burst proneness as a serious challenge in underground mining has received considerable attention. Parallel to BIs' development, various Bursting Liability Indices (BLIs) have been proposed to specifically assess coal bursting phenomenon. Despite having different names, both BI and BLI in principle have aimed at evaluating the burst–brittleness level of different rocks for different applications. In this study, the principles of burst and brittleness were discussed followed by the development of a novel so-called burst–brittleness ratio (BBR) to assess the relative burst–brittleness of rock types irrespective of their applications. To do so, the proposed BBR was governed by point load testing (PLT) which has significant advantages over the other rock testing methods used in BI estimation such as direct or indirect tensile testing. To examine the suitability of the proposed ratio, three different rock types from various geological origins including coal, granite and sandstone were selected and tested under uniaxial compressive, indirect tensile Brazilian and point loadings. The high-speed imaging technique and Acoustic Emission (AE) were utilized to characterize the cracking process (e.g., failure under shear or tension) and to monitor the real-time failure behavior of samples under different loading conditions. The resulting data revealed that the severity of strength loss in coal samples was significantly higher than that observed in other rock types particularly under uniaxial compression endorsing the validity of the proposed BBR. Highlights: Comparing Brittleness index with Bursting Liability Index along with their pros and cons Development of a new Burst–Brittleness Ratio (BBR) based on UCS and point load strength index Validation of the proposed ratio through point load and UCS tests on granite, coal, and sandstone using high-speed imaging and Acoustic Emission (AE) techniques [ABSTRACT FROM AUTHOR]

The non-persistent jointed rock mass is prone to compressive-shear and tensile-shear failures. Tensile-shear failure has been studied less than compressive-shear failure. This study develops a new shear test apparatus for cubic rock specimens to explore the tensile-shear failure of jointed rock mass under varying joint persistence and compare it to compressive-shear failure. The jointed rock specimens are sheared and monitored using acoustic emission (AE) equipment, an infrared thermal imager, and a high-speed camera to track stress, AE signal, temperature, and deformation. Based on the test results, RFPA2D numerical simulation is conducted to investigate the effect of joint persistence on the compressive- and tensile-shear strength of the rock bridge. Tensile- and compressive-shear stress curves differ greatly. The former may have several stress peaks and peak AE count time does not equal peak strength time. The rock specimen's compressive-shear strength is far higher than its tensile-shear strength, but tensile-shear tests accumulate more AE counts and energy. Tensile-shear failure has a large crack opening and rapid temperature progression. The rock specimen cools before peak strength, which is the opposite of compressive-shear failure. The two tests differ significantly regarding crack development and deformation evolution. The shear strength of the specimens in the compressive-shear and tensile-shear tests decreases approximately linearly as joint persistence increases, and the shear strength of the rock bridges increases and then decreases. Previous research showed that increasing joint persistence increases rock bridge cohesiveness. At relatively high joint persistence, however, the considerable cohesion weakening of the rock bridge caused by tensile cracks near the joint tip should be considered. The findings could contribute to a better understanding of the failure process of jointed rock mass under compressive-shear and tensile-shear stresses. Highlights: Compressive- and tensile-shear tests on cubic rock-like specimens were conducted on a novel test apparatus. Effect of joint persistence on compressive- and tensile-shear failure behavior was investigated. AE, thermal infrared imager, and DIC technologies were utilized to analyze the failure mechanism difference between compressive- and tensile-shear tests. The rock bridge's shear strength increases first and then decreases with joint persistence increasing in both compressive- and tensile-shear tests. [ABSTRACT FROM AUTHOR]

Shear failure often occurs during the construction of the geotechnical engineering projects, especially under dynamic loading condition. To investigate the effect of loading rate and normal stress on the shear responses of rock materials, a bonded-particle model (BPM) was adopted to insight the failure process from a micro-mechanical perspective using the moment tensor. Firstly, we propose a novel calibration methodology for direct shear test, which provides a possibility to effectively derive microscopic parameters of PBM for obtaining the macroscopic properties of rocks considering loading rates under direct shear. Validation results show that the simulation demonstrated good agreement with the experimental results. Then, a series of simulated direct shear tests under different normal stresses ranging from 0 to 20 MPa and loading rate ranging from 10–3 to 10–1 mm/s were performed. The results demonstrate that both normal stress and loading rate have a positive influence on shear strength. However, normal stress has a greater impact on the shear strength compared to loading rate, and the effect of loading rate on shear strength weakens as the normal stress level increases. Increasing the loading rate and normal stress level led to a higher distribution of AE events in the nonlinear stage. At lower loading rates and normal stress, intense AE activity was concentrated around the peak stress point. In contrast, under higher situations, the nonlinear stage is prolonged, resulting in an increased number of AE events in the nonlinear and post-peak stages. Normal stress has a greater impact on the generation and propagation of micro-cracks compared to loading rate. With higher normal stress, the failure mode shifts from a shear-dominated to a mixed mode with tensile micro-cracks in a dominant role. Highlights: A numerical model was established using DEM to investigate the mechanical properties and failure mode evolution of rock specimens under shear loading, taking into account the influence of loading rate and normal stress factor. A novel calibration procedure was proposed for the direct shear model, and the results showed that it effectively improved the calibration efficiency compared with traditional methods, as validated by experimental data. The fracture behavior of the specimen under different strain rates and normal stress states was monitored using numerical simulation based on moment tensor theory of acoustic emission, which allowed for analysis of the shear failure process. [ABSTRACT FROM AUTHOR]

A series of stepwise cyclic tests and short-term creep tests were conducted on Beishan granite to quantify the separated effect of stress corrosion and friction degradation during cyclic loading. An opti-acoustic monitoring platform that combined digital image correlation and acoustic emission techniques was utilized to monitor the rock deformation and fracturing at both the micro- and macro-scale simultaneously. In addition, a micromechanical model was built to study the crack growth of rocks exposed to cyclic and creep loads. Experimental and theoretical results indicated that both the repeated and constant loads play a great role in deformation localization, micro-fracturing, and damage accumulation. A similar law was observed in the evolution of the AE b value and distribution of frequency-amplitude in granite under cyclic and creep loading. However, cyclic loads induced a greater AE count rate in granite than constant loads at low loading levels, while constant loads stimulated a greater AE count rate than cyclic loads at high loading levels. Moreover, cyclic loads promoted more strain localization bands than constant load; so, more macro-cracks, branching, and debris were observed in granite during cyclic loading. These similarities and differences indicated that both the stress corrosion and fatigue mechanisms were highlighted to be responsible for the sub-critical crack growth of rocks during cyclic loading. The development of sub-critical crack growth and the evolution of stress intensity factor at the crack tip within rocks under cyclic and constant loads were well described by the micromechanical theoretical model. Highlights: Laboratory experiments were conducted to distinguish stress corrosion and reciprocating wear from rock fatigue. Real-time observation of rock deformation and fracturing was achieved using opti-acoustic technology. A micromechanical theoretical model was built to characterize the sub-critical crack growth in rocks. [ABSTRACT FROM AUTHOR]

Deep learning technique is an effective method for representation of complex relationships applied in various subjects and areas. Acoustic emission technique, a powerful non-destructive testing method, has been used widely to collect acoustic emission signals and monitor rock deformation and damage. This study proposed a deep learning approach to model the relationship between the remaining time to failure and data monitored by acoustic emission technique and predict failure time of sandstone in the uniaxial compression loading experiment. In the meantime, optimal feature combination of model input and optimal parameters of the model structure were obtained and discussed. Three different types of models were established, including fully connected neural networks, gated recurrent units*** and mixed model combining convolutional neural networks with gated recurrent units. The mixed model's coefficient of determination for sandstone failure prediction under uniaxial compression was 0.9699 and its mean absolute percentage error 11.64%. Its performance surpasses those of the others on tested samples. Furthermore, the metric coefficient of determination exhibits significant improvement compared with previous studies. Therefore, the obtained metric results demonstrate a satisfactory level of accuracy for predicting sandstone failure in laboratory-scale applications. Finally, this work demonstrates the capability of deep learning to predict sandstone failure via acoustic emission monitoring technique and offers the theoretical reference for practical applications at the engineering scale. Highlights: This work investigates a novel method for prediction of sandstone failure under uniaxial compression loading based on deep learning method using acoustic emission data. This work exploits an available and efficient approach for input feature selection without traversing. The average signal level (ASL) is found a critical acoustic emission characteristic for prediction of sandstone failure. In this work, the mixed model performs best comprehensively compared with the others. This work proves the applicability and potential of sandstone failure prediction based on deep learning method via acoustic emission technique. [ABSTRACT FROM AUTHOR]

The shear mechanical properties are the important factors affecting the failure of rock mass. At present, tests measuring the shear mechanical behavior characteristics of rocks under three-dimensional stress are rare. In particular, the influence of lateral stress on the shear failure mechanism of rock is less often considered. The true triaxial test device was used to conduct direct shear, true triaxial stress shear, and confining shear tests under three-dimensional states of stress. Through the three different types of shear tests, the deformation, failure modes, strength, and surface roughness of sandstone were investigated. Focusing on the acoustic emission (AE) characteristics during the failure of sandstone specimens, the influences of lateral stress on the shear failure mechanism of sandstone under true triaxial stress were studied. The test results show that lateral stress can increase the peak strength and residual strength of sandstone, significantly affecting its lateral deformation. Under high normal stress, the weaker the lateral restraint on the rock, the more severe the lateral deformation failure. The analysis found that lateral stress increased the compressive compactness of the mineral particles inside the sandstone and the occlusal hinge strength of the particles. It also restricts the lateral deformation of the sandstone and the germination of the number of internal cracks. Crack propagation is also accelerated under the combined action of high normal stress and lateral stress. Highlights: Direct shear, true triaxial shear and confining shear tests on sandstone specimens were carried out using true triaxial test device. Through the different types of shear tests, the strength, deformation, JRC and failure modes of sandstone were investigated. Focusing on the AE characteristics, the influences of lateral stress on the shear failure mechanism of sandstone were studied. [ABSTRACT FROM AUTHOR]