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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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. [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]

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]

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]

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]

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]

To study the thermal healing effect on stress-induced damage of Beishan granite, the host rock of China's high-level radioactive waste (HLW) geological disposal underground research laboratory, comprehensive experimental works were carried out. The deformation rate analysis test was carried out on granite after different stress levels and different temperature conditions (room temperature and mild temperature, 150 °C). Results showed that the deformation memory to the previous stress loading (stress damage) was fading after mild temperature treatment, and the Kaiser effect was also faded notably that AE activity was suppressed after mild temperature treatment, which implied stress-induced damage was recovered under mild temperature. In addition, stress-induced pores, especially large pores, were decreased markedly after mild temperature treatment characterized by the nuclear magnetic resonance measurement. Overall, the mild temperature possesses the ability to heal stress (σcc to σci, σci to σcd) induced damage. The permeability reduction, strength enhancement, pores decreasing, and the mechanism under mild temperature were jointed analyzed, which is positive for the long-term stability and tightness of the HLW geological repository. Highlights: The healing effect of temperature on stress-damaged Beishan granite was studied. The DME of granite after different stress loaded was fading after 150 °C treated. The AE activity was suppressed and the Kaiser effect disappeared after 150 °C treated. The mild temperature exhibits the ability of healing stress-induced damage. [ABSTRACT FROM AUTHOR]

Understanding the response mechanism of multiparameter rock fractures can improve the accuracy of crack diagnosis. In this study, three types of rock samples with notches were subjected to three-point bending tests. The fracture morphology was observed using scanning electron microscopy, and real-time resistivity and acoustic emission (AE) data were used to describe the crack propagation. The spatial distribution of the electric potential was simulated based on practical crack morphologies and can explain the correlation between rock fracture and rock resistivity or AE. The results reveal that the initiation and propagation of cracks reconstructs the electrical potential distribution characteristics of the rock samples and changed their overall resistivity. The crack growth rate was proportional to the rate of increase in the resistivity rate, and the resistivity increased with nonuniform crack growth. Crack geometry complexity affected circuit connectivity, and a higher resistivity change rate was typically caused by cracks with more uniform propagation. The resistivity variation had the same trend as the fracture toughness, whereas the AE energy exhibited a similar trend to the fracture energy evolution. The cumulative AE count of the granite fracture was the largest, and the peak AE count of coal was larger than that of sandstone. In the main frequency band of 100 ± 25 kHz, a relatively large AE event occurred during crack initiation, and the AE amplitude of granite was the largest. The primary fracture propagation increased the peak AE count, amplitude, energy, and fracture energy release efficiency. During rock fracture, the opening of a microscopic bedding plane and a matrix fracture results in time-varying resistivity and AE characteristics. The complementary electrical and acoustic parameters help describe the details of crack propagation behaviour. Highlights: Cracks restructure the electrical potential distributions in rock samples and change the resistivity. The acoustic emission (AE) energy exhibits a similar trend to the fracture energy evolution. The relationship between the resistivity and AE is mutually verified and complementary. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing, which determines geothermal resource productivity, is one of the critical technical components in the construction of hot dry rock (HDR) reservoirs. Although the mechanical coupling between solid and fluid has been included in algorithms based on the discrete element method (DEM) generally employed to investigate hydraulic fracturing, crystalline rocks of reservoirs are mostly treated as homogeneous isotropic models without considering their petrographic texture. By combining a grain-based model (GBM) and a modified fluid–solid coupling algorithm, a novel hydro-GBM is constructed in this study to analyze the hydraulic fracturing response of polycrystalline rocks. Moreover, acoustic emission (AE) events during fracturing are extracted to describe the characteristics of hydraulic-fracturing-induced seismicity. Under in-situ conditions with high differential stress, the propagation direction of hydraulic fractures is mainly perpendicular to the direction of minimum in-situ stress, with little influence by material heterogeneity and fluid viscosity; under near-hydrostatic in-situ stress conditions, the microcracks along the mineral boundaries increase remarkably, and the fracture pattern tends to be complex, especially when a low-viscosity fluid is injected into the rock. From the Gutenberg-Richter type relationship between the AE event numbers and the moment magnitudes, it is found that large induced seismic events increase with in-situ stress and with fluid viscosity. In summary, the proposed hydro-GBM can well reproduce the propagation behavior of hydraulic fractures influenced by material heterogeneity, and the research results reveal the interactions between petrographic texture, in-situ stress, fluid viscosity, and hydraulic fracturing characteristics, which will provide a valuable reference for on-site reservoir stimulation. Highlights: A novel fluid-solid coupling grain-based model is proposed to simulate the hydraulic fracturing of crystalline rocks. The model well reproduces the propagation behavior of hydraulic fractures influenced by material heterogeneity. Effects of in-situ stress and fluid viscosity on breakdown pressure, fluid pressure field, and seismic event are analyzed to bridge the gaps in the existing studies. Injecting low-viscosity fluid contributes to complex fracture networks in brittle granitic reservoir because of inducing numerous mineral boundary cracks and reducing large seismic events. [ABSTRACT FROM AUTHOR]

Mechanical properties of reservoirs and overlying strata in the condition of underground coal gasification (UCG) are vital to the stability of surrounding rock. In this work, sandstone samples of seam roof were collected and a series of experiments including X-ray computed tomography (CT) scanning, X-ray diffraction (XRD) and uniaxial compression tests with synchronous acoustic emission (AE) monitor were conducted to study crack distribution, propagation, minerals composition and failure modes of sandstone samples after different temperature treatments. As the treatment temperature increases, the threshold ratios of crack initiation stress and rupture stress gradually decline. Tensile failure mode of the biplane symmetric is dominant for sandstone samples treated with 25 °C and 200 °C. Single plane shear failure mode is dominant for 400 °C and 600 °C. Nevertheless, for 800 °C and 1000 °C, comprehensive features of the above temperatures are exhibited, resulting in a complex failure mode. 800 °C is the threshold temperature for failure behavior and uniaxial compressive strength (UCS). The primary fracture morphology plays a critical role in crack propagation and failure mode of sandstone samples. When axial stress reaches 96% of peak stress, peak frequency abruptly declines. It discloses the precursory features of the critical failure of sandstones after different temperature treatments. Meanwhile, various forms of water evaporation, the muscovite and kaolinite decomposition as well as phase transformation of quartz due to high temperature affect the mechanical properties and failure modes of sandstone. The results can guide the safety evaluation and roof disaster prevention of UCG. Highlights: The crack propagation characteristics and failure modes of roof sandstone are investigated in macroscopic and microscopic respects. Sandstone samples with different temperature treatments have various failure behaviors and 800℃ is the threshold temperature. As the treatment temperature increases, the crack initiation stress (30–45% peak stress) and rupture stress (65–80% peak stress) gradually decrease. [ABSTRACT FROM AUTHOR]

A series of mechanical tests and acoustic emission (AE) monitoring are conducted considering the disturbance stress paths, disturbance modes (low disturbance (LD) mode, medium disturbance (MD) mode, and high disturbance (HD) mode) and initial confining pressures (ICPs). The fractal dimensions of the AE temporal distribution ( D t ) and spatial distribution ( D s ) are calculated, analyzed, and compared with the normalized AE count. The "sensitive effect", "inferior sensitive effect", and "delayed sensitive effect" of the disturbance mode can be observed through the temporal–spatial distribution of AE activities under the ICP of 12.5 MPa, 25 MPa, and 37.5 MPa, respectively. According to the normalized AE count, the damage of the disturbance modes under the ICP of 12.5 MPa, from serious to mild in proper order is HD, MD, and LD modes. The rapid increase in D t and the sharp decrease in D s appear in the MD and HD modes, respectively. This is in accordance with the "sensitive effect". Under an ICP of 25 MPa, only the HD mode can cause remarkable damage, as the evolution of the AE parameters does not vary obviously in the LD and MD modes, which can be regarded as the "inferior sensitive effect". Likewise, under an ICP of 37.5 MPa, each parameter presents a significant difference in the late period of DS among the three disturbance modes. This may correspond to the "delayed sensitive effect". The consistency in the occurrences of the discrepancy of the parameters indicates that the damage propagation and mechanical behavior of the engineering rocks are simultaneously dominated by the ICP and disturbance mode. Highlights: A series of mechanical tests as well as the AE monitoring are carried out under the disturbance stress paths. The "sensitive effect", "inferior sensitive effect" and "delayed sensitive effect" of temporal-spatial distribution of the AE activities and its fractal characteristics under the disturbance stress paths are revealed. The damage characteristics of rock under the different disturbance modes and initial confining pressures are comprehensively investigated by the normalized AE count, Dt, and Ds. [ABSTRACT FROM AUTHOR]

Though the exterior cracking process of flawed rocks has been studied intensively, the exterior and interior coalescence process, especially from the aspect of micro-crack development, has not been systematically studied. In the present study, the acoustic emission (AE) techniques and digital image correlation (DIC) are applied together to monitor the damage process of double-flawed granite samples under uniaxial compression. Multiple AE signal characteristics, including cumulative event, ring count, waveform frequency, initial frequency (IF), and rise angle (RA) are utilized to analyze the evolution of the coalescence properties before and after the initiation of macro-cracks. The experimental result indicates that the exterior and interior fractures in microscopic and macroscopic scales can be identified from the combination of AE parameters and DIC results. In the early stage of loading, most initiated micro-cracks are attributed to tensile mode based on the analysis of IF and RA values, while the strain concentration cannot be conspicuously observed by the DIC technique. Then, the micro-crack mode changes from tensile to shear or mixed mode with the reduction of the average IF value. With the initiation of macro-cracks, the RA value increases accordingly, and the deformation around the crack pattern can be obviously recognized by the DIC strain field. The property of the coalescence strongly depends on the flaw geometries. When increasing the bridging angle β, the crack mode of the coalescence varies from shear to mixed tensile–shear or pure tensile (β ≤ 90°) and to a more complex mode (β > 90°). Highlights: Multiple AE parameters are used to characterize the damage and cracking process. Eight modes of crack coalescence are observed. The crack initiation and coalescence in different scales can be identified by DIC and AE methods. The crack mechanism is studied by more AE parameters other than AE events and localization. The changes of microcrack mode from tensile to mixed tensile-shear are found. [ABSTRACT FROM AUTHOR]

Layered composite rocks (LCRs) are often encountered in underground excavation in which extensive deformation, buckling and local collapse near the layer interface frequently occur. However, there is limited advance knowledge of the failure mechanisms of LCRs to interpret the field observations due to the relative scarcity of polyaxial compression testing. To that end, this study conducts the true-triaxial experiments with the acoustic emission (AE) monitoring on the LCR specimens, with the emphases being on the effects of mechanical contrast (MC) between the layers on the fracture behaviors under true-triaxial stresses. First, the strength of LCRs is controlled by the weaker layer. Second, at the unstable fracturing phase, high AE event rate is faithfully recorded but intermittently interrupted by macrofracturing in granite but not in sandstone. Third, main failure planes of LCRs appear sub-orthogonally to σ3 (minimum principal stress), taken the form of delamination fracture and exhibited the mode of σ3-transverse-symmetric fracturing. As the distance from the unloaded surface increases, the fracturing transitions from σ3-transverse-symmetric spalling to σ3-transverse-symmetric shearing. Depending on the MC, both the fracture propagation through the interface and the fracture containment within sandstone are registered. Moreover, the interface effect is numerically revealed through a 3D finite-element modeling. This study gets insights into the failure mechanisms of LCRs with a changing mechanical property in individual layers under true-triaxial stresses. Highlights: True-triaxial experiment investigations of layered composite rocks in a brittle-brittle sequence. Quantitative interpretations of acousto-mechanical properties and fracturing behaviors in layered composite rocks. The fracture propagation through the interface is registered, as well as the fracture containment within the sandstone. The delamination fracture along σ3-unloading direction dominates the feature of σ3-transverse-symmetric fracturing. The interface effect is revealed by a 3D finite-element modelling. [ABSTRACT FROM AUTHOR]

Rock sections in intermittent joints are important structures that prevent joints from interconnecting and maintain the stability of the rock mass, and they play a pivotal role in the strength and failure mode of the rock mass. In order to analyze the controlling effect of rock section in intermittent joints, the samples with different rock section angles were prepared and laboratory tests were carried out using Acoustic Emission and Digital Image Correlation (DIC) techniques. The results show that the uniaxial compressive strength of the samples reaches its minimum value when the orientation of the rock section in the intermittent joints coincides with the loading direction. Furthermore, as the angle of the rock section increases, the time share of the sample in the compaction stage increases and the time share in the plastic-yield stage decreases. The samples show the structural characteristics of being difficult to close at the early stage and easy to destroy at the late stage. In terms of the damage mode, the increase in the angle of the rock section reduces the difficulty of the rock section itself being penetrated, and the samples are more easily damaged. Finally, a damage constitutive model considering the compaction stage and initial damage was developed and the effect of rock section angle on the rate of rock damage was analyzed. Highlights: The influence of the angle of the rock section in the intermittent joints on the compressive strength and failure mode of the samples was investigated. Based on acoustic emission, DIC and damage constitutive models, the effect of the angle of the rock section in an intermittent joint on the deformation phase of the sample is quantified. A damage constitutive model considering the initial damage of the intermittent joints and the compaction stage of the rock is proposed. [ABSTRACT FROM AUTHOR]

The progressive brittle failure of high-stress hard surrounding rock masses caused by layered excavation severely threatens the stability of large underground caverns. We summarize the stress path in the potential failure zone of the Shuangjiangkou underground powerhouse during layered excavation, and then design the multistep unloading confining pressure test and the cyclic loading–unloading test with multistep unloading confining pressure. The deformation, strength, acoustic emission, and failure characteristics of granite, as well as the evolution of critical stress thresholds and mechanical parameters are analyzed based on experimental results under different stress paths. The peak strength of the rock significantly depends on the stress path and the amount of unloading confining pressure. The rock failure process under multistep unloading confining pressure is divided into stable and unstable crack growth stages, with the failure mode shifting from tensile cracks to mixed tensile–shear cracks. The evolution modes of critical stress thresholds, cohesion, and internal friction angle with amount of unloading confining pressure differ significantly from that with the equal plastic strain. The former is nearly linear, while the latter is exponentially nonlinear. The evolution of the deformation modulus, cohesion, and internal friction angle confirms the rationality of the cohesion-weakening–friction-strengthening model with degraded deformation modulus for the stability analysis of surrounding rock masses under layered excavation. The experimental methods for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure are also discussed. The results imply that the layered excavation-induced nonlinear deterioration of the deformation modulus and strength parameters of high-stress hard surrounding rock masses should be considered in the stability analysis of large underground caverns. Further, the timely compensation of the excavation-induced confining pressure loss is critical to ensure the stability of high-stress hard surrounding rock masses under layered excavation. Highlights: A stress path was designed to reflect the stress evolution characteristics for rock masses of the large underground cavern under layered excavation; The deformation, strength, acoustic emission, and failure characteristics of granite under the layered excavation stress path were analyzed; The evolution of rock mechanical parameters with unloading confining pressure and the equivalent plastic strain were obtained; An experimental method for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure was discussed. [ABSTRACT FROM AUTHOR]

Understanding the microcracking mechanisms and associated acoustic characteristics involved in the creep of heterogeneous granite at the grain scale is of high importance in practical engineering. To perform this task, a series of creep numerical simulations were conducted on granite with a single pre-existing open flaw using a grain-based parallel-bonded stress corrosion model. We investigated the effect of flaw inclination angle on the creep behavior and discussed the significance of microstructure. The results suggest that the axial strain, secondary creep strain rate and time-to-failure all depend strongly and nonlinearly on the flaw inclination angle. Creep failure is mainly induced by the initiation, propagation and coalescence of microcracks, and the dominant types are inter-grain tension cracks and intra-grain tension cracks. The b value, source types and distribution of acoustic emission events are associated with the flaw inclination angle and driving-stress ratio. Tensile sources account for the majority of the total number of acoustic emission sources, while shear and implosive acoustic emission sources are larger in magnitude. The model with higher heterogeneity creeps at a faster rate under the same applied constant axial stress. The higher number of acoustic emission events at each creep stage of such model indicates that the relatively heterogeneous microstructure provides more paths for the growth of microcracks during creep loading. As the heterogeneity increases, the secondary creep strain rate rises and the b value decreases. The position of the final macroscale shear band of pre-cracked granite is more affected by pre-existing flaw compared to its microstructure. Highlights: An extended grain-based parallel-bonded stress corrosion (GB-PSC) model was proposed to simulate the creep and associated acoustic behavior of granite. Voronoi tessellations to provide a more realistic representation of the microstructure of granite. Effects of open flaw inclination angle on creep behavior of pre-cracked granite were systematically investigated and the significance of microstructure were discussed. The acoustic emission characteristics of mineral boundaries and interiors during creep loading were obtained using the moment tensor analysis. [ABSTRACT FROM AUTHOR]

Monitoring and predicting crack propagation in rock using acoustic emission (AE) technology is integral to a variety of sub-disciplines in the geosciences. The utility of existing AE data, however, is severely limited by prevailing visualization techniques, which suffer from problems of occlusion. Here, we introduce a novel approach to visualize 3D data through time (4D data) using unfiltered individual (AE) event data collected from a granite boulder for a period over 3 years. We implement a 3D extension of Ripley’s K function to evaluate the magnitude of clustering, and use the scale at which clustering is strongest to parameterize three-dimensional kernel density estimation (3DKDE) of AE events. We develop a parallel approach that features a load balancing technique to decrease the computational effort for 3DKDE and hence, reduce execution time. The results from the 3DKDE allow for comprehensible visualization of high AE density areas—best reflecting the actual location of subcritical cracking—and their changes through time, which is a substantial improvement over most existing methods. Our framework is scalable and portable to a variety of other disciplines such as epidemiology, ecology, and any point data by extension. Assumptions and limitations are identified as well as possible future research directions. [ABSTRACT FROM AUTHOR]