Search

Cracking in dryers has long been a challenge in the pulp and paper industry. The acoustic emission (AE) technique, which is capable of supplying information regarding the cracking behaviour of metal objects, can be used to reduce or prevent dryer failure. This study applies AE testing to a paper dryer made of grey cast iron in China. To realise this task, the AE characteristics of the cast iron dryer were examined by accomplishing three main work parts: (i) in the lab part, basic AE signatures from the dryer shell material were characterised; (ii) in the artificial crack test part, AE features from the crack were obtained and the AE location of the crack in the dryer was tested; (iii) in the field part, the basic procedure for testing the paper dryer under hydrostatic test conditions was verified based on the analysis of parts I and II. The results indicate that grey cast iron is a good AE emitter. The tensile test revealed that the AE responses are active and the AE sources include the deformation of pearlite, decohesion and fracture of several graphite flakes and microcrack growth. The AE attenuation in the dryer is not severe and the cracks in the dryer during hydraulic testing exhibit an active AE response. Additionally, an accurate crack location was obtained by using the cylindrical location method. These results can provide useful information for AE inspection on structures made of cast iron. [ABSTRACT FROM AUTHOR]

At present, the leakage rate of the water distribution network in China is still high, and the waste of water resources caused by water distribution network leakage is quite serious every year. Therefore, the location of pipeline leakage is of great significance for saving water resources and reducing economic losses. Acoustic emission technology is the most widely used pipeline leak location technology. The traditional non-stationary random signal de-noising method mainly relies on the estimation of noise parameters, ignoring periodic noise and components unrelated to pipeline leakage. Aiming at the above problems, this paper proposes a leak location method for water supply pipelines based on a multivariate variational mode decomposition algorithm. This method combines the two parameters of the energy loss coefficient and the correlation coefficient between adjacent modes, and adaptively determines the decomposition mode number K according to the characteristics of the signal itself. According to the correlation coefficient, the effective component is selected to reconstruct the signal and the cross-correlation time delay is estimated to determine the location of the pipeline leakage point. The experimental results show that this method has higher accuracy than the cross-correlation method based on VMD and the cross-correlation method based on EMD, and the average relative positioning error is less than 2.2%. [ABSTRACT FROM AUTHOR]

Calculation and characterization of the whole process of internal microscopic damage to surface damage in red-bed soft rock is a theoretical research difficulty and an urgent need for engineering safety protection. However, the current study cannot accurately and directly correlate internal and external damage. Therefore, in this paper, a coupled tensor–DEM–FEM model is proposed to deal with surface damage by indoor triaxial test digital image processing (DIC), internal damage by FJM acoustic emission study, and internal and external damage by moment tensor correlation. The study demonstrates that the whole process damage process of the red-bed soft rock peak front can be divided into six distinct phases, with early damage beginning with the elastic phase; the local strain divergence value begins to spiral out of control during the period of crack acceleration development; the overall acoustic emission intensity distribution is in the range of [−8.5, −6.3] in two dimensions and in the range of [−11, −9] in three dimensions; the R were between −40 and 40, which corresponded to the results of the indoor tests. A model has been developed that allows a direct reflection of the whole damage process. The method can be used to better understand the disaster mechanism and guide engineering practice. [ABSTRACT FROM AUTHOR]

From core observations of shales of the Niutitang formation in the northern part of Guizhou Province, China, calcite was often found to act as a natural fracture filler and affect the extension of fractures in hydraulic fracturing. Therefore, it is crucial to understand the tensile mechanical behavior of shales by calcite veins. In this paper, by computed tomography scanning of shales containing calcite veins of different dip angles from the Niutitang formation, a three‐dimensional numerical model reflecting the internal fine structure of the shale was constructed. Direct tensile numerical simulations were carried out to investigate the effect of calcite veins at different angles on the fine‐scale damage process and mechanical properties of shale. The experimental results show that the tensile capacity of the shale increases with the increase in calcite veins. Depending on the dip angle of the calcite vein, the damage pattern of the shale is divided into three types of damage: pull‐off damage along the calcite vein, jagged damage, and horizontal damage perpendicular to the loading method. At high dip angles, the shale damage is more intense and the fracture network more complex. The temporal and spatial characteristics of the acoustic emissions provide a good indication of the microscopic behavior of the shale specimens during the damage process. The box dimension method was used to calculate the fractal dimension of the shale specimens at the final damage, and it was found that the damage was more intense and the fracture network was more complex at high dip angles, and there was an angular threshold. [ABSTRACT FROM AUTHOR]

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]

It is well known that the Strategic Petroleum Reserve in salt caverns is an important means to solve the potential petroleum shortage. However, salt mines in China are mainly lacustrine-layered structures with many mudstone interlayers. Meanwhile, the acid value of extracted crude oil has been increasing in recent years. The acid erosion of salt cavern interlayers by crude oil would affect the safety of bedded salt cavern oil storage. Therefore, combined with acoustic emission technology, the multistage creep mechanical behaviors of natural mudstone interlayer samples and the mudstone interlayer samples treated by oil erosion are studied in this paper. The internal structures of interlayer samples before and after oil erosion were systematically analyzed by SEM. The results show that petroleum acid (naphthenic acid) plays a significant deterioration role in the mechanical properties of mudstone interlayers during the petroleum reserve process. Compared with the uncorroded samples, the mudstone interlayers after oil erosion show obvious brittle failure characteristics. At low stress levels, the axial strain grows stepwise and the lateral strain is smaller than the axial strain. At high stress levels, however, the lateral creep is obviously higher than the axial creep. A large number of AE signals were generated at the initial loading stage for different stress levels. After the creep stabilized, the AE signals were significantly reduced. During the process of petroleum erosion, the internal pores in rocks continued to develop with the dissolution of many mineral particles. This suggests that crude oil storage in the bedded salt rocks could accelerate the deterioration of the surrounding rocks via erosion. This paper could provide basic research data and a reference for the construction of oil storage in the bedded salt rocks. [ABSTRACT FROM AUTHOR]

Humans have paid a lot of attention to environmental geological challenges in recent years. Landslides, being one of the most prevalent geological disasters, are characterized by their suddenness and destructiveness. Southwest China is prone to landslides and debris flows due to its unique geological structure. This paper uses landslides in southwest China as an example, focusing on research on landslide initiation mechanisms and outlining modern landslide monitoring devices and prediction models. Landslides are caused by external variables such as persistent precipitation, groundwater movement, and significant seismic activity, as well as interior reasons such as fine particle rearrangement and the action of positive pore water. The reduction of the friction coefficient of the shear surface, which is induced by the increase of the shear rate, the supercritical carbon dioxide and superheated steam of the shear zone, and the mineral recrystallization process on the shear surface, all have an important impact on reducing the friction coefficient of the shear surface, is a key factor in the occurrence of highspeed remote landslides. Real-time landslide monitoring using space-air-ground and acoustic emission technology, as well as the creation of machine learning-based forecast models, have aided in the research of landslide development and early warning. [ABSTRACT FROM AUTHOR]

Aiming at the problem of seepage-stress coupling failure instability of cemented gangue-fly ash backfill (CGFB) with defects, the triaxial compression test and acoustic emission (AE) monitoring test of the CGFB were carried out in this paper. The strength characteristics of defective CGFB under seepage-stress coupling were analysed，the variation rules of AE parameters (ring count, b-value and entropy value) in the failure process were systematically studied. It is found that the AE parameters (ring count, b-value, and entropy value) of backfill with defects under seepage-stress coupling have a 'mutation' phenomenon, which can be used as a precursor feature of failure instability of backfill. Combined the grey-cusp catastrophe theory, a failure prediction model of backfill based on AE parameters (ringing count) was established, and the seepage-stress coupling fracture of backfill with defects was effectively predicted, indicating that the model is feasible to predict the seepage-stress coupled failure of backfill with defects. The research results can provide a theoretical basis for stability monitoring and failure prediction of backfill in deep water-rich mines in China. [ABSTRACT FROM AUTHOR]

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

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam's standing sustainability. In the paper, the coal pillar dam body of the 22 layers of coal in the Shangwan Coal Mine is studied (22 represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions. [ABSTRACT FROM AUTHOR]

Coal is the mainstay of China's energy supply. With the gradual progress in China's policy of phasing out backward coal production capacity, the intensive and deep mining of coal has gradually become the new norm. The current mining depth is increasing at a rate of 10~15 m/year. The high crust stress, high gas pressure, high ground temperature, and engineering disturbance stress in deep coal mines can lead to the occurrence of coal–rock–gas dynamic disasters that are complex and show the characteristics of compound dynamic disasters. It is important to understand the evolution and mechanism of deep coal and rock dynamic disasters accurately for the safe development of deep resources. To study the mechanism of occurrence and the evolution of impact–protrusion compound dynamic disasters, we herein analyzed the apparent characteristics of coal–rock–gas compound dynamic disasters in deep mines and obtained the mechanical and acoustic emission characteristics of coal–rock composites through indoor experiments. Then, we conducted in-depth analysis on the non-uniform deformation behaviors and non-uniform stress field of the coal–rock composite and clarified the generation mechanism of local tensile cracks at the coal–rock interface. Subsequently, we established the energy transfer model of the rock–rock–gas composite specimen in the process of dynamic destabilization in the engineering scale mining field and revealed the mechanism of nonlinear energy evolution and release of the coal–rock–gas composite, which has been less reported in previous studies. In this paper, we further clarified the occurrence and development mechanism of coal–rock–gas compound dynamic disasters in the engineering scale mining environment to guide the prevention and control of coal–rock–gas dynamic disasters in deep mines. [ABSTRACT FROM AUTHOR]

The mechanical behavior of coal is significantly influenced by the loading rate and bedding plane. In this paper, the uniaxial compression tests at different loading rates were conducted on coal specimens with vertical bedding (CSVB) and coal specimens with parallel bedding (CSPB), which were prepared by the coal with bursting liability from the Ordos deep mining areas, China. The results show that the uniaxial compressive strengths of CSVB and CSPB are positively correlated with the loading rate. The peak values of acoustic emission (AE) counts and accumulated absolute AE energy of CSVB and CSPB increase with the loading rate, and the AE parameters of CSVB are larger than those of CSPB. The variation rules of input energy density and elastic energy density are similar for CSVB and CSPB, but the variation rules of dissipated energy density in stage IV are different. As the axial load increases, the energy storage rate and energy dissipation rate of both coal specimen types increase and decrease, respectively. The bedding planes of coal and the loading conditions in the field should be fully considered when identifying the bursting liability of coal and forecasting coal burst. This study can provide a reference for coal burst warning and prevention under similar geological conditions in Ordos deep mining areas. [ABSTRACT FROM AUTHOR]

In recent years, the depth, intensity and complexity of coal mining in China has continued to increase. Meanwhile rockburst disasters, especially fault-slip bursts, have become even more severe. Research on precursor characteristics and early warning methods need to be in-depth, as rock burst disasters severely restrict the safety production of coal mines. Stress waves (dynamic loading) caused by the rupture of the roof of overlying rock layers, or the slipping activity of faults act on coal and rock masses, can easily cause the sudden instability of coal and rock masses and induce rockburst disasters in a high in situ stress environment. Therefore, this article is about a study of fracture characteristics of coal mass and its acoustic emission (AE) signal response under dynamic loading. The results show that the dynamic strength presented a "double peak" pattern. It rapidly dripped after two crests appear, and the second crest was higher than the first crest. With increase in bullet velocity (and applied axial static load conditions), the dynamic strength–strain curve shifted upward gradually. The difference between the two peaks was relatively stable. The coal sample with cracks and pores was damaged significantly at the first wave crest, the damage area was concentrated mainly around the ring of the sample. The proportions of the meta-instability stage in total process time increased with increase in impact velocity applied on the samples, which ranged from 3.17 to 5.94%. The axial static load did not show a significant role affecting proportion of the meta-instability phase in whole process time. The AE peak count and energy increased linearly with increase in impact velocity, while they decreased with increase in axial static load. When employing a same impact velocity, the difference between the two peaks increased with increase in axial static load. Because the first wave crest appeared when the coal body ruptured, the accurate rupture time of the coal body can be monitored by the AE count and energy. The paper sheds some light on exploring the fractural evolution process of coal and rock masses under dynamic loading, as well as being a reference basis for the evolutional mechanism of rockburst under a far-field fluctuational load condition. The research results are helpful to further improve the prediction accuracy and control of rock burst in coal mines. [ABSTRACT FROM AUTHOR]

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

This paper presents an experimental study to assess the behaviour of coal samples under tensile loadings to better understand the failure mechanisms and the interactions with the coal characteristics. A set of Brazilian splitting tests were carried out using disk specimens obtained from Tashan Coal Mine in China. The digital speckle correlation method and acoustic emission (AE) were used to capture the deformation localisation and AE characteristics of each specimen during the loading process. The precursor characteristics of AE and the failure mechanism are discussed. It was found that the entire loading process mainly consists of compaction, elastic and post-peak dropping stage without an obvious yielding stage. Two kinds of deformation localisation were observed: central symmetry and axis symmetry. The corresponding AE evolution patterns have different phases, including gradual rise, step rise, transient rise and steady rise. During the subcritical failure stage, AE counts demonstrate a "rapidly increasing + flatten" intermittent feature. The results provide a reference for a better understanding of the damage process of the brittle coal material and its application in ground control design. [ABSTRACT FROM AUTHOR]

Coal bump, a common dynamic disaster in mining of deep coal resources, its assessing and predicting is an important component in safety management. This paper presents a model to assess and predict coal bump risk based on multiparameter indices. A new energy accumulation index S was proposed by considering acoustic emission and electromagnetic emission signal characteristics in mine shocks. Combined with indices E (energy of microseisms) and N (frequency of microseisms) of microseismic monitoring, a static and dynamic coal bump risk assessment and prediction model was established. We studied coal bump events that occurred during extraction in 311305 working face of Bayangale coal mine in Inner Mongolia, China. We obtained the acoustic emission and electromagnetic emission signal distribution and change law, using principal component analysis method and density ellipse to establish the index S. A typical precursory of coal bumps is that AE and EME strength has obvious fluctuation period of 3-4 days, index S showing an obvious decreasing trend, while the time-series curve of the microseismic energy is relatively stable, and the vibration frequency curve has a significant upward trend. After predict the potential coal bump risk and its area of occurrence, large diameter drilling (Φ150 mm) on-site was used to relief pressure concertation in coal seam and roof. The results demonstrate that this model based on multiparameter indices is capable of quantitatively prewarning rock burst risk. [ABSTRACT FROM AUTHOR]

The study of the influence of temperature on granite's physical and mechanical properties is significant to deep geotechnical engineering such as nuclear waste disposal engineering. The granite samples were taken from Beishan of Gansu Province, China, a candidate site for a high-level radioactive waste repository. In this paper, thermal damage on Beishan granite under different temperature (25 °C, 200 °C, 300 °C, 400 °C, and 500 °C) was studied to investigate the thermal effect on physical and mechanical properties. Uniaxial compression tests, ultrasonic velocity, acoustic emission (AE) monitorization and scanning electron microscope were adopted. The results showed that: (1) a temperature threshold of 300 °C was recognized, with the increasing temperature, the peak strength, P-wave velocity, and mass loss ratio of the granite decreased sharply. (2) combining with the scanning electron microscope results, the changes in the growth of thermal cracks due to the heating of granite at different elevated temperatures indicated the cracks in the rock increased and gradually formed a fractures zone eventually leading to rock failure. (3) The granite sample was a brittle splitting failure when the temperature was below 200 °C, the failure of granite sample presented shear failure once the temperature exceeded 200 °C (4) The accumulated AE counts increased when the stress reached σ ci (crack initiation stress) and increased significantly when the stress reached σ cd (crack damage stress). The higher the temperature was, the more seriously the granite samples were damaged. [ABSTRACT FROM AUTHOR]

Rock burst monitoring of heading face is a weak aspect of rock burst monitoring in China; acoustic emission (AE) monitoring is one of the few monitoring technologies used in heading face, but its target signals are small energy events which are easy to be disturbed. Researchers usually focus on the weak AE events but ignore the microseismic (MS) events (different from AE event and caused by a larger scale of coal fracture), while this kind of events can also reflect the pressure situation of heading face and have higher energy value which may become a better indicator for rock burst monitoring of heading face. So, the basic characteristics of MS events in heading face are studied based on a running vibration signal acquisition system, including the occurrence position, main frequency range, maximum amplitude (MA) range, event duration, and relationship with geological structure. This paper provides a development basis of the monitoring method for rock burst monitoring of heading face by using MS events. [ABSTRACT FROM AUTHOR]

Rock burst is a catastrophic dynamic disaster caused by sudden failure and instability of coal, which brings threats to deep coal mining; the AE-charge signals and the fragment distribution are related to both mechanical properties of coal and disaster early warning directly. Hence, the variation of AE and charge induction during coal failure, fractal feature of coal fragments, and their relationship should be studied in depth. In this paper, uniaxial loading test was carried out for coal with bursting tendency samples produced by blocks cored from 800 m depth in Xiaoqing coal mine of the Tiefa Coal Group in northeast China; the fractal characteristics of specimens are obtained by using the statistical fractal method. The mechanics of similarities and differences between acoustic emission and charge signal is investigated by using loading experiments and theoretical analysis. It is found that the fragments of coal have good self-similarity properties; the fractal dimension of the specimens is in the range 2.085–2.521, the maximum range being 2.300–2.468, which is slightly higher than that of rock. The high-amplitude pulses of the acoustic emission and charge are concentrated in the macroscopic fissure development and expansion stage but they have asynchronous characteristics between them. The charge generation process is accompanied by the inhomogeneous deformation and sliding friction; the friction slip is the major one and is analysed theoretically. A theoretical model for the force-electric coupling relationship is established. The statistical results show that both the acoustic emission and the charge signal accumulation have a significantly proportional relationship with the fractal dimension. Both the acoustic emission and charge signal reveal coal breakage evolution process, which will help in obtaining the precursor information on coal failure. Furthermore, the monitoring results can predict the extent of coal mass instability. [ABSTRACT FROM AUTHOR]

Coal and rock mass fracturing is an important technology on exploiting coalbed methane (CBM), because there are large numbers of natural cracks in coal and rock mass, the extension and evolution rule of cracks is very complicated. In order to describe the rule of cracks evolution accurately, different groups of cracks were considered in this paper, and base on mutation theory, the model of coal and rock mass fracturing and energy release are established. Hypothesis that the releasing energy is all transferred to crack propagation and evolution. According to energy conservation principle, calculation method for micro-cracks length and number in coal and rock mass mutation behavior is been established. Taking coal samples which were taken from right lane six, 3# layer, west block three, Zhangchen mine, Jixi, Heilongjiang as research objects, through calculation, the evolution parameters of coal and rock mass micro-cracks are obtained, and the rationality of the theoretical model presented in this paper is verified by laboratory fracturing experiments. After calculation, number error of micro-cracks is about 0.27%, and the energy error is about 5.42%. The new model provides a new method for further study of the evolution rules of micro-crack fracturing parameters of coal and rock mass. [ABSTRACT FROM AUTHOR]

• The interface features of coal-rock combination are obtained and a 3D reconstruction model is established. • The influence of interface inclination on the temporal characteristics of damage and failure of coal-rock combination is analyzed. • The influence of interface inclination on the fracture fractals of damage and failure of coal-rock combination is analyzed. • The effect of interface inclination on the damage and failure of primary coal-rock combination is studied. In recent years, China's coal mines have gradually entered into deep mining. Ensuring safe and efficient extraction of deep coal resources has emerged as a key concern in the coal mining industry. The characteristics of the coal-rock interface play a significant role in influencing the damage and failure of the coal-rock combination. In order to deeply study the influence of interface inclination on the damage and failure law of the primary coal-rock combination, this paper successfully established the three-dimensional reconstruction model of the coal-rock combination with 5 different inclination angles (0°, 30°, 45°, 60°, 90°) by combining CT technology and AVZIO software. Then PFC2D software was used to simulate the damage and failure of coal-rock combination under different interface angles. The results show that: the loading damage characteristics of coal-rock combinations with different dip angles are similar, that is, the fracture evolution of coal-rock bodies is mainly concentrated in the coal bodies, and the damage and failure of coal-rock interfaces are the most severe. In the early stage of the loading damage, the fractal dimension of the coal-rock combination increases with the increase of interfacial inclination, but the growth rate of the fractal dimension shows a decreasing tendency with the increase of interfacial inclination. With the increase of the inclination angle of the interface of the assemblage, the growth rate of the acoustic emission signal and the total cumulative acoustic emission of the coal-rock combination show a tendency of increasing firstly, then decreasing and then increasing again. The porosity near the coal-rock interface is relatively large when the coal-rock combination is damaged by a load. [ABSTRACT FROM AUTHOR]

This study employed a comprehensive approach utilizing X-ray diffraction, scanning electron microscopy (computed), computed tomography (CT) three-dimensional scanning, uniaxial compressive testing, acoustic emission (AE), and digital image correlation to investigate the micromorphology, mechanical properties, macro–microscopic fractal characteristics, failure modes, and mineral composition changes in acid–base coal samples from underground reservoirs. The findings from this study indicate that, in acidic environments, calcite undergoes acidolysis to form calcium chloride (CaCl2), whereas kaolinite reacts with alkaline substances to produce albite (Na2Al2Si2O8). The average elastic modulus of the coal samples treated with strong acids or alkalis decreased by 51.08% and 38.17%, respectively, when compared to the naturally dried samples. After strong acid treatment (pH = 2), the post-peak modulus decreased to its lowest value of 1.78 GPa. The fracture threshold in the plastic phase of the acidic coal samples decreased by approximately 60% when compared to that of the naturally dried samples, with the cumulative AE energy reaching a maximum of 2.46 × 1010 aJ. CT image segmentation revealed that with higher H+ concentrations, the sliced fracture fractal dimension increased from 1.01 to 1.25, indicating enhanced fracture disorder. The evolution of the spatial and energy fractal dimensions of the AE, analyzed using the box-counting method and correlation dimension, showed that in strong acidic and alkaline environments, the spatial fractal dimension before the peak failure was higher, pre-disposing the formation of highly disordered and smaller-scale fractures. During the fracture development stage, the time-series energy correlation dimension of the acid–base coal samples experienced two significant fluctuations, with a rapid decrease before the peak, indicating a transition from disorderly expansion to orderly fracturing. In the final stage of fracturing, the strongly acidic samples predominantly developed microcracks. The stronger the acidity or alkalinity, the more likely it is that high-energy (greater than 104 aJ) AE shear events will occur, with the mixed mode of tensile and shear failure tending to concentrate in the areas of high shear strain, thereby enhancing the macroscopic fragmentation orderliness. This research is crucial in regard to the safety assessment of dam bodies or coal pillars in acidic and alkaline coal mines in Northwestern China, particularly for predicting their stability. [ABSTRACT FROM AUTHOR]

Semi-rigid base is the main component of highway pavements in China and is prone to cracking damage under vehicle loading and environmental effects. In this paper, three-point bending (TPB) tests on prefabricated cracked specimens of CSM beams were conducted to study the damage evolution and fracture characteristics of cement-stabilized macadam (CSM) under different loading rates. The acoustic emission (AE) technique and scanning electron microscope (SEM) analysis were utilized. The results indicate that the fracture energy and fracture toughness of CSM were enhanced at increasing loading rates, and the amplitude distribution of AE events became wider and more energetic. The damage modes of CSM can be classified into interface transition zone (ITZ) damage, cement mortar damage, and aggregate damage. The fracture surfaces were mainly ITZ damage. The brittle structure of CSM was caused by incomplete hydration of cement particles in ITZ. This study can provide a better understanding of the fracture mechanism of CSM at different loading rates. The findings can also guide the design, construction, operation and maintenance of semi-rigid roadbed pavement structures. • The effect of loading rate on the fracture characteristics of CSM was analyzed. • The evolution of fracture damage in CSM three-point bending tests was investigated using AE technique. • The cracking-damage mechanisms of CSM under different loading rates were revealed. [ABSTRACT FROM AUTHOR]

To obtain the influence of anisotropy and energy evolution characteristics on wellbore stability, the acoustic and mechanical anisotropy characteristics of shales are studied through various experiments, including scanning electron microscopy, ultrasonic pulse transmission, and uniaxial compression experiments, with the Longmaxi Formation shale in the southern area of the Sichuan Basin as the research object. The energy evolution characteristics of the Longmaxi Formation shale under different bedding angles are analyzed. The influence of anisotropy on the wellbore stability of shale formation is discussed on this basis. The results show that the acoustic and mechanical parameters, failure mode, and energy evolution characteristics of shale have significant anisotropy. Furthermore, the P-wave and S-wave time differences decrease with an increase in bedding angle. The compressive strength and Poisson's ratio decrease first and then increase with an increase in bedding angle. Meanwhile, the elastic modulus gradually increases with an increase in bedding angle. Rock samples with different bedding angles show diverse failure modes in mechanical tests, including splitting, shear, and shear-splitting failure. The total energy and elastic energy decrease first and then increase with an increase in bedding angle. Finally, the formation anisotropy affects the wellbore stability: the higher the formation anisotropy, the more vulnerable is the wellbore to instability. [ABSTRACT FROM AUTHOR]

Coal and gas outburst disasters in coal seams are becoming more serious as coal mines extend deeper underground in China. Damage evolution has affected gas flow in coal seam greatly, which also controls gas permeation and gas extraction results, and finally, it has tremendous influence on prevention and control of gas disaster accidents. In this paper, the cracking process of coal under compressive stress condition and the permeability variation during the whole process is experimentally studied with the aid of 3D acoustic emission monitoring system. The experimental results suggest that the coal failure and degradation of mechanical properties are essentially related to the propagation and coalescence of induced cracks. Using a damage tensor defined by the direction-dependent crack density, the damage evolution during the whole loading process is studied. It is revealed that the damage evolution is mainly initiated with the appearance of microcracks and accelerated in the post-peak region. During the post-peak region, a similar increase tendency of damage and permeability is noticed, and therefore, a post-peak permeability model is proposed considering the influence of damage evolution on the permeability variation in the post-peak region. Finally, we analyze the gas outburst hazard with coal mining. There exists a transition zone around the vertical stress concentration, and energetic failure may result in this transition zone. [ABSTRACT FROM AUTHOR]

To investigate the tensile properties of weakly cemented rock samples at Shendong Mining Area. Four common weakly cemented rocks are subjected to Brazilian Splitting Test, and both the acoustic emission signals and deformation field are monitored simultaneously. Thus, deformation and acoustic emission features were analyzed in different rock samples. Moreover, the relationship between the evolution of localization belt and acoustic emission features was identified. The results show that the samples displayed similar deformation features and that the evolution of R value is also basically consistent. [ABSTRACT FROM AUTHOR]

Nationwide unification of carbon emission right market in China has long been proposed yet not achieved. Launching of the nationwide trading center at Shanghai was a recent great step towards this target but the regional pilots covering respective zones are still in operation. With all trading data in those markets from their inceptions, this paper investigates the development status and trend of each submarket, based on which, it is revealed that sudden replacement of regional markets by the unified nationwide trading center is infeasible at the present stage, furthermore, feasibility of market merging is analyzed and optimized route for essential unification is designed. This paper contributes to deconstruct the value of carbon emission right and interprets its properties of option value, the equivalence of which value from the perspective of financial investment and enterprise operations is explored and formally established. Applying the geometric Brownian motion and optimal stopping time to model the carbon asset operation, option value of carbon emission right is estimated and marginal return-ability of its consumption is calculated for each region. Based on the analysis regionally, market merging is simulated and optimized route for nationwide market unification is proposed with policy suggestions. [ABSTRACT FROM AUTHOR]

The protection and utilization of groundwater resources in coal mining process is the prerequisite and necessary condition for realizing green mining at the ecological fragile mining area in western China. Among them, the extensive application of water-preserved mining and underground reservoir technology provides the possibility for solving the problem of coal-water co-production. In this paper, with the background that the stability of the coal and rock boundary of underground reservoir changed under the conditions of repeated immersion in water, the sandy mudstone of the No.3-1 coal seam roof of Wulanmulun Mine was investigated. The natural water absorption law of the sample was obtained by non-destructive water immersion test, and the water immersion time of the sample water content reaching saturation state was obtained. According to the immersion time, the sample was dried-saturated and repeatedly immersed in water. It was found that the water content increased exponentially with the number of immersion, whereas the shear strength and the number of water immersion in the sample decreased exponentially. The cohesive force decreased with the increase of the number of immersion times, while the internal friction angle increased first and then decreased, indicating that the uniformity of the material, and the shape of the fracture surface and the shape, size and direction of the fracture caused the internal friction angle to increase. The cycle immersion later was the dominant factor to make the internal friction angle decrease. Based on the relationship between cohesion, internal friction angle and number of dry-saturated cycles, the Mohr-Coulomb criterion was established based on the number of dry-saturated cycles. The acoustic emission counting curve had a strong correspondence with the shear stress curve. The acoustic emission event increased significantly at the moment of sample failure, corresponding to the obvious rising phase of the cumulative acoustic emission counting curve. The sample mainly had a low RA value during the failure process, and the failure time corresponded to a high RA value, i.e. , the common type of crack in the failure process of the specimen was tensile crack, and the moment of failure corresponded to the obvious shear crack. These results show that the strength of coal and rock pillars in the groundwater reservoir boundary changed significantly under the action of water immersion-water loss during the impoundment-pumping process in the groundwater reservoir. It is suggested that the weakening effect of water on the strength of boundary coal and rock pillars should be considered when constructing the underground reservoir. [ABSTRACT FROM AUTHOR]

The concept of state vector stems from statistical physics, where it is usually used to describe activity patterns of a physical field in its manner of coarsegrain. In this paper, we propose an approach by which the state vector was applied to describe quantitatively the damage evolution of the brittle heterogeneous systems, and some interesting results are presented, i.e., prior to the macro-fracture of rock specimens and occurrence of a strong earthquake, evolutions of the four relevant scalars time series derived from the state vectors changed anomalously. As retrospective studies, some prominent large earthquakes occurred in the Chinese Mainland (e.g., the M 7.4 Haicheng earthquake on February 4, 1975, and the M 7.8 Tangshan earthquake on July 28, 1976, etc) were investigated. Results show considerable promise that the time-dependent state vectors could serve as a kind of precursor to predict earthquakes. [ABSTRACT FROM AUTHOR]

In order to comprehend the dynamic disaster mechanism induced by overburden rock caving during the advancement of a coal mining face, a physical simulation model is constructed basing on the geological condition of the 21221 mining face at Qianqiu coal mine in Henan Province, China. This study established, a comprehensive monitoring system to investigate the interrelations and evolutionary characteristics among multiple mechanical parameters, including mining-induced stress, displacement, temperature, and acoustic emission events during overburden rock caving. It is suggested that, despite the uniformity of the overburden rock caving interval, the main characteristic of overburden rock lies in its uneven caving strength. The mining-induced stress exhibits a reasonable interrelation with the displacement, temperature, and acoustic emission events of the rock strata. With the advancement of the coal seam, the mining-induced stress undergoes four successive stages: gentle stability, gradual accumulation, high-level mutation, and a return to stability. The variations in other mechanical parameters does not synchronize with the significant changes in mining-induced stress. Before the collapse of overburden rock occurs, rock strata temperature increment decreases and the acoustic emission ringing counts surges with the increase of rock strata displacement and mining-induced stress. Therefore, the collaborative characteristics of mining-induced stress, displacement, temperature, and acoustic emission ringing counts can be identified as the precursor information or overburden rock caving. These results are in good consistent with on-site situation in the coal mine. [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]

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

In China, preserving historic trees and ancient timber structures has attracted more attention from researchers and institutions. Nondestructive testing (NDT) technology has gained momentum in the application and research in recent years. This paper presents some examples of the application of NDT techniques in the protection of historic trees and ancient timber structures in China. A total of nine NDT techniques were applied, including five techniques of visual inspection, sounding, Resistograph, stress wave transmission velocity, stress wave tomography for both historic trees and ancient timber structures, two techniques of the pulling-test and ground-penetrating radar (GPR) for historic trees alone, and two techniques of acoustic emission (AE) and digital imaging for ancient timber structures. Based on field testing, there is a very good detecting effect for visual inspection and Resistograph, but sounding works unsatisfactorily. The stress wave transmission velocity is found to be excellent in screening for the internal defects of historic tree trunks, but performs badly in screening for the internal defects within the wood members of ancient timber structures. AE is less successful but digital imaging performs well in long-term monitoring and damage warning of wood structures. The stress wave tomography successfully obtains images of internal defects inside tree trunks. The anchorage of some historic trees cannot be quantitatively evaluated with the pulling test if the tree is too large. GPR's resolution and accuracy are very low, which limit its application. Theoretical research on NDT and promotion of NDT technology are still an essential part of the work in the future. [ABSTRACT FROM AUTHOR]

At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

Rockburst is one of the major problems in rock underground engineering and mechanics. To make a risk assessment of rockburst and improve the mine safety index, some evaluation systems for rockburst propensity have been proposed and applied, and progress has been made in the study of high-value mines and single evaluation systems, but these evaluation systems are still immature for deep low and medium grade phosphate mines with more complex influencing factors. Therefore, to solve this problem, this study takes the phosphate mining area of the Sujiapo phosphate mine as the main research object, and combines the characteristics of the deep rockburst of the phosphate mine and its physical and mechanical properties; the evaluation system combines fuzzy mathematical method and multiple evaluation methods to determine the propensity of rockburst of deep mining of Sujiapo phosphate mine. The JSM-500LV SEM and acoustic emission results are used to analyze the microscopic mechanism of rockburst from the morphology and composition of the rocks, and the rationality of the evaluation system is further demonstrated. The study shows that dolomite and phosphorite in the study area are moderate rockburst, and shale has a weak rockburst tendency; because the roof dolomite and phosphorite crystallization degree are higher, the elastic modulus is large. Under the action of external load, the roof dolomite stored more elastic strain energy. Deformation damage consumes less energy. The energy released to the outside world after the damage is large, more likely to occur brittle damage, and the rockburst tendency is high. Therefore, the evaluation system combining multiple evaluation methods can comprehensively determine rockburst in phosphate mines. [ABSTRACT FROM AUTHOR]

In this study, to reduce the consumption of natural sand and improve the utilization rate of desert sand in western China, while preparing 14 groups of samples, desert sand is used to replace natural sand by the ratio of 20%, 40%, and 60%, and steel fiber is mixed with volume fraction 0.5%, 1.0%, 1.5%, and 2.0%. The mechanical properties of the specimens, including compressive strength, splitting tensile strength, and axial compressive strength were tested. Besides, the microstructures of the samples were analyzed by SEM, XRD, and acoustic emission detection technologies to identify the damage process. The results show that the desert sand can refine the microstructure and fill the pores, and it has good comprehensive properties at a 40% substitution rate. The compression properties of specimens are not apparently improved, but the tensile strength and deformation properties are significantly improved. The steel fiber with 1.5 vol% content behaves better, and the 28d compressive strength of the optimized group reaches 58.7 MPa. As a result, the polynomial fitting degree of total AE hits and stress level receives a more incredible goodness (R2) value than 0.96. The strength characteristics of steel fiber-desert sand concrete (SFDSC) can meet the demands of C40 concrete, and this research can provide a reference for engineers using desert sand in their designs. [ABSTRACT FROM AUTHOR]

Natural fractures, like tectoclases, are essential in the formation of shale gas reservoirs and have been the focus of study for shale gas development. Tectoclases provide most storage space for gas and are largely controlled by the paleo-tectonic stress field in shale reservoirs of the Niutitang Formation, northern Guizhou area, China. An accurate prediction of the development and distribution of tectoclases in the reservoirs is of great significance to exploring and developing shale gas sweet spots in the area. Based on geological structure evolution and fracture characterization, this study is focused on factors that control the fracture development in the Niutitang Formation shale reservoirs in northern Guizhou through characterization and modeling of geomechanisms and tectonic movements. A geomechanical model is formulated for the shale reservoirs against the geological background of the area. On this basis, the fractures are predicted by using the acoustic emission data. Numerical simulation results show that the development and distribution of tectoclase is controlled by fault zones, some of which have no obvious turning points with tectoclase in the middle sections being more developed and fragmented than those at the two ends. Some of these have obvious S-shaped turning points where tectoclases are the most developed and fragmented. [ABSTRACT FROM AUTHOR]

To determine the anisotropic mechanical characteristics of carbonaceous slate at the Jinman Mine located in Lancang River Gorge, China, uniaxial compression, acoustic emission (AE) monitoring, and scanning electron microscopy (SEM) tests were conducted. The results show that stress loading directions and bedding effects have a significant impact on strain characteristics. The deformation of slate undergoes compaction, elastic, damage accumulation, and failure stages, and there are significant differences in strain paths. The mechanical properties of grey and carbonaceous slate have significant features, and variations of these properties of carbonaceous slate are more than that of grey slate. The discrete degree is significantly related to the direction of stress loading and different types of slate structures. The AE response intensity is related to the loading mode and slate type. A sudden increase in the AE cumulative ring number near peak stress indicates instability and rupture and is a precursor of rock rupture. The failure patterns and fracture characteristics are significantly correlated to the layered structure of the slate. Slates subjected to vertical loading and parallel loading conditions are dominated by shear fracture and tensile fracture, respectively. The peak strains of gray and carbonaceous slate correspond to brittle failure. The SEM tests indicate that slate samples subjected to parallel loading primarily show a tensile failure compared with slate samples subjected to vertical loading, with fractured sections being of lesser roughness, and scattered fractures and sections being smooth without obvious protrusions or cracks. [ABSTRACT FROM AUTHOR]

Identifying shale geological temperatures is important for understanding the thermal maturity, thermal evolutionary history, and hydrocarbon generation stages of shale formations, and enables efficient shale gas development. This paper investigates the thermal Kaiser effect of shales under different warming rates by studying the thermoacoustic emission (TAE) characteristics of shales in the Lower Palaeozoic Niutitang Formation in the Micangshan area of China. The developmental evolution and microscopic response mechanisms of different types of fractures in shales under high-temperature conditions are explored by combining acoustic emission (AE) characteristic parameters. The memory and sensitivity of the thermal Kaiser effect were also verified by thermal fatigue tests. The results show that there is a significant thermal Kaiser effect during the heating of shale and that its threshold temperature is approximately 180 °C, which is consistent with the palaeotemperatures based on bitumen reflectance (R b) estimation. The heating rate does not have a significant effect on the threshold temperature. At high temperatures, the shale is dominated by small-scale tensile fractures. When the shale is heated to 300 °C, the organic matter reaction rate peaks and the proportion of shear cracks begins to increase. An increase in heating rate is beneficial to the development of large-scale tensile cracks, while inorganic minerals begin to expand due to heat and micro-cracks gradually expand and connect, thus improving the connectivity of the pore channels. In thermal fatigue tests, the shale retains its memory of the threshold temperature when heated to less than 300 °C. Above 300 °C, pyrolysis of organic matter and microcrack expansion interfere with the thermal Kaiser effect, causing the shale to lose its memory of the maximum temperature. Within the same cycle stage, the threshold temperature is significantly higher at high heating rates than at low ones, especially above 400 °C. [ABSTRACT FROM AUTHOR]

Tectonic regime change refers to the process in which the two periods of tectonic movement changes and produce corresponding tectonic regime change surfaces (unconformity surfaces) and other geological responses in the process of basin development and evolution. The change of tectonic movement is the root cause of tectonic regime change. Tectonic regime changes produce different geological responses, which can be used as the signs to identify tectonic regime changes. In order to study the characteristics of tectonic regime changes of Chepaizi Uplift in Junggar Basin and the control on oil and gas migration and accumulation, four stages of tectonic regime changes were identified by using the techniques of the rock acoustic emission (AE) technology, tectonic subsidence analysis, and tectonic stratigraphic sequence analysis, which occurred at the end of Early Permian, the end of Late Triassic, the end of Jurassic, and the end of Paleogene, respectively. The main stratigraphic product of tectonic regime change is the unconformity structure. The unconformity structure has obvious stratification, including the bottom conglomerate layer, the weathered clay layer, and the semiweathered rock layer, and plays an important role in controlling oil and gas migration and accumulation. The weathered clay layer and the hydrolysis layer have a sealing and shielding effect on oil and gas; the leaching layer can become the effective reservoir; at the same time, the leaching layer can provide a channel for the lateral transportation of oil and gas. [ABSTRACT FROM AUTHOR]

Taking the weakly cemented sandstone of Ordos, China, as the research object, the evolution law between the relative stress of weakly cemented sandstone and the multiparameters of the acoustic emission under different dry-wet cycles was explored, and the critical failure identification mode of weakly cemented sandstone under dry-wet cycle was established. The results show that as the number of dry-wet cycles increases, the wave velocity loss rate gradually increases. Overall, the longitudinal wave loss rate is larger than the shear wave loss rate, indicating that the longitudinal wave is more sensitive to the degradation of weakly cemented sandstone. With an increase in the number of dry-wet cycles, the crack is mainly caused by the main crack penetration failure, and the secondary crack is significantly reduced. The fractal dimension decreases with an increase in the dry-wet cycles and reaches its maximum at 0 dry-wet cycles, which means that 0 dry-wet cycles witness the most complex morphology of fractures within the weakly cemented sandstone. This finding indicates that the dry-wet cycle inhibits the generation and expansion of fractures. The event rate appears to be close to 0 before the rupture, and then the platform oscillates, followed by a sudden increase. The acoustic emission b value is relatively high during the initial stage and then decreases, which is the initial damage process. The elastoplastic phase rises again, the peak stage decreases rapidly, and the weakly cemented sandstone undergoes unstable damage. The change in the acoustic emission entropy value is exactly the opposite of the b value change law. When the weakly cemented sandstone reaches the critical failure state under different dry-wet cycles, the relative stress value is 95%. The test results provide new methods and a basis for the damage evolution mechanism and fracture prediction of weakly cemented sandstone under dry-wet cycles. [ABSTRACT FROM AUTHOR]

The automatic modulation recognition (AMR) enables the receiver to automatically recognize the modulation type of the received signal for achieving correct demodulation. However, the noise and interference in the underwater acoustic channel greatly influence the features extracted from the signals. In this letter, we proposed the optimizing autoencoder (OAE) and the evaluation enhanced K-nearest neighbors (EEKNN) algorithms. The combination of OAE and EEKNN not only improves the feature discrimination but also avoids the misjudgment caused by abnormal samples and realizes accurate and efficient AMR. The experimental results of the data measured in the South China sea show that the proposed method successfully recognizes eight modulation types. The recognition accuracy is up to 99.25%, and the recognition time is only 3.48 ms. [ABSTRACT FROM AUTHOR]

More than 50% of China's CBM is contained in the fractured soft and low-permeability coal. This medium has characteristics of low strength, ductile failure, multiple fractures and low permeability, which could lead to HFs were short if direct fracturing technology was adopted. Indirect fracturing is one of the new technologies to realize the high CBM production. The core scientific problem is to study the constitutive relationship of plastic damage and fracture-seepage coupling of the coal, then HFs area under the influence of multiple factors was analyzed. In this paper, the coal discontinuities are the main channel for the HFs propagation by hydraulic fracturing and acoustic emission experiments. 3D network model of coal discontinuities was established based on CT data. The fluid-solid coupling constitutive equations were established to reflect characteristics of mixed mode ductile fracture of discontinuities and plastic damage-seepage of coal matrix. On these bases, parameters of indirect fracturing were optimized through numerical simulation in order to maximize the HFs area in the coal. The results show that, compared with the brittle continuum, the ductile failure and the mixed mode fracture process consume lots of the hydraulic energy, which is the root cause of the small HFs area. In addition, the area decreases in the form of power or exponential function with the increase of coal permeability and the distance between well and interface, but increases in the form of power function with the increase of injection rate, elastic modulus ratio of roof to coal, interface strength, and difference between maximum and minimum stress. By controlling the above factors in the sensitivity critical intervals, the target of large HFs area in the coal can be realized. The above conclusions have been applied to the fractured soft and low-permeability coal region, which makes the CBM production increase by 20 times. • 2%–30% of the hydraulic energy is used for the propagation of HFs in the coal. • Coal discontinuities induce HFs to form an energy-consuming mixed fracture mode. • Plastic damage of coal matrix will aggravate hydraulic energy dissipation. • Distance between well and interface, injection rate are main controlling factors. • Stress, elastic modulus, and mixing ratio also affect the HFs area in the coal. [ABSTRACT FROM AUTHOR]

The ecology of arid mining areas in western China is facing serious problems because underground coal mining destroys overlying aquifers, causing groundwater to shift along fracture zones and groundwater levels to drop. Therefore, it is important to construct strategically coal mine goafs as underground water reservoirs to store and utilize mining-affected water resources. During the long-term operation of underground water reservoirs, the stability of coal pillar dams is affected by the repeated rise and fall of mine water levels. To understand the repeated effects of water, nondestructive water immersion, ultrasonic velocity, uniaxial compression, and acoustic emission (AE) tests were performed to study the mechanical properties and damage mechanisms of coal samples under repeated drying–saturation conditions. The ultrasonic velocity test mainly achieved two functions: selecting consistent coal samples and providing evidence of changes after each drying and saturation process. Coal samples were grouped according to the number of drying–saturation processes (DSPs) to investigate and compare the mechanical and AE parameters. The post-peak phases of full stress–strain curves present perpendicular, L-shaped, and stepped forms, among which the latter two show obvious residual strength. The AE count, cumulative AE count, and stress can jointly determine the various stages and stress thresholds of the sample failure process (or crack development process), except the boundary between elastic stage and stable crack growth stage. Multiple immersions in water reduces the fractal dimension of broken blocks; that is, the degree of fragmentation decreases and the quantity and mass of large-size blocks increase. [ABSTRACT FROM AUTHOR]

Featured Application: This work is highly significant for hydraulic fracturing construction and interpretation in the field. Hydraulic fracturing is an important means for the development of tight oil and gas reservoirs. Laboratory rock mechanics experiments can be used to better understand the mechanism of hydraulic fracture. Therefore, in this study we carried out hydraulic fracturing experiments on Triassic Yanchang Formation tight sandstone from the Ordos Basin, China. Sparse tomography was used to obtain ultrasonic velocity images of the sample during hydraulic fracturing. Then, combining the changes in rock mechanics parameters, acoustic emission activities, and their spatial position, we analyzed the hydraulic fracturing process of tight sandstone under high differential stress in detail. The experimental results illuminate the fracture evolution processes of hydraulic fracturing. The competition between stress-induced dilatancy and fluid flow was observed during water injection. Moreover, the results prove that the "seismic pump" mode occurs in the dry region, while the "dilation hardening" and "seismic pump" modes occur simultaneously in the partially saturated region; that is to say, the hydraulic conditions dominate the failure mode of the rock. [ABSTRACT FROM AUTHOR]

Rock creep characteristics play an important role in geological engineering and have important physical implications when evaluating the long-term stability of rock structures. In this study, a series of uniaxial compression and creep experiments were performed on red sandstone specimens from Jiangxi Province, China. During experiments, stress, strain, acoustic emission (AE) parameters, and waveform files were recorded continuously. First, an AE creep model with a damping factor of red sandstone specimen was established based on the theory of statistical mechanics and damage mechanics. The intrinsic relationship between the cumulative AE events and the strain in the accelerating creep stage was revealed by a creep model derived from the inversion of experimental data in the decelerating and steady creep stages. Then, using the singular value decomposition method for cumulative AE events, quantitative identification of the accelerating creep stage of red sandstone was achieved. The evolution characteristics of b value in the creep process of red sandstone specimens are revealed. The AE signals in the decelerating creep and steady creep stages had abrupt waveforms, whereas the AE signals in the accelerating creep stage had mixed abrupt and continuous waveforms. Therefore, quantitative identification of the accelerating creep stage was achieved from the perspective of AE waveform characteristics. Finally, the relationship between the wave characteristics of AE signals and the length of microcracks in the rock was identified, and the wave characteristics of AE signals at different creep stages were considered from the perspective of elastic wave dynamics. [ABSTRACT FROM AUTHOR]

The crack initiation stress (CI), a threshold stress at which brittle rock starts to become damaged during compression, has been widely studied. It can be used to estimate the spalling strength of tunnels in brittle rock masses. Many methods based on stress–strain curves and acoustic emission data have been proposed to identify the CI, and these methods have good statistical consistency. In the present study, the influence of varying the Poisson's ratio on the value of the CI is analyzed using the crack volumetric strain (CVS) method, and the physical meaning of the lateral strain response (LSR) method is explained. Based on the LSR method, a more stable lateral strain interval response (LSIR) method is proposed to determine the CI. We analyzed the CI of granite samples from two deep boreholes in the Beishan area, the site of China's underground research laboratory (URL). The results show that the mean CI values determined by our proposed method are in good agreement with the results from the LSR method. The standard deviation and correlation analysis show that the CI values obtained by the proposed method have a smaller dispersion and a higher independence than those from the LSR method. [ABSTRACT FROM AUTHOR]

The development of reliable early rockburst warning models for underground mines is a challenging task considering the complex nature of rockburst phenomena. In this paper, a real-time, integrated multi-system, early rockburst warning model is developed to achieve the quantitative prediction of the rockburst probabilities at a specific time. The model input parameters are data derived from microseismic (MS) and acoustic emission (AE) monitoring systems. The early warning model is based on fuzzy comprehensive evaluation method (FCEM), confusion matrix and maximum membership degree principle. These tools integrate the advantages of both multi-system and multi-parameter into one unified model. During the early warning procedure, six early warning indices representing temporal, spatial and energy parameters are rationalized. Next, the weight of each index is determined from confusion matrix leading to a score value. Real-time early warning results can then be determined from the maximum membership degree principle. The results show that the proposed integrated MS-AE early warning model has the advantage of combining multiple systems and indices, and it has higher prediction accuracy than individual systems. The early warning model is successfully demonstrated with a case study from the Wudong Coal Mine in China. The proposed model improves the rockburst early warning efficiency and provides a solid foundation for ensuring the safety of personnel and equipment in underground mining. [ABSTRACT FROM AUTHOR]

Rocks in cold regions tend to experience exacerbated degradation under the combined effects of environmental and anthropogenic factors, which may arise from, for example, temperature fluctuation, mechanical excavation, and blasting. Activities related to rock support or open-pit slope optimization in cold regions require a complete understanding of the failure mechanisms of rock under the complex conditions. This paper quantitatively documents the impact of combined cyclic mechanical load and freeze-thaw cycles (i.e., the effect of stress "history") on the microstructural evolution and mechanical degradation of three porous sandstones with distinct porosity values (from 3.9 to 14.1%). The three sandstone samples were collected from different geological regions in China. The microstructural evolution of the tested samples was quantitatively analyzed using the low-field Nuclear Magnetic Resonance (NMR) technique. To investigate sample degradation arising from the impact of the stress "history", the cyclic-loaded and freeze-thaw cycled samples were eventually compressed to failure, during which an acoustic emission system was used to monitor microseismic activities. The results of the study show that the porosity of all tested sandstone samples was increased after cyclic load, with a much more rapid and further increase in porosity observed for samples being subsequently treated under the freeze-thaw cycles. More interestingly, the Chuxiong sandstone with relatively small porosity values were much more sensitive to the impact of cyclic load compared with the Linyi sandstone, exhibiting a somewhat larger increase rate in porosity. However, the Linyi sandstone with larger initial porosity values exhibited a relatively large increase rate in porosity under the multiple freeze-thaw treatments. The multiple freeze-thaw treatments mainly resulted in the development of relatively large pores. The results of the uniaxial compression tests show that the strength reduction of the samples being solely treated by freeze-thaw cycles was within the range of 5–10%, whereas it was within the range of 20–40% for those samples subjected to the combined cyclic load and freeze-thaw cycles. • Combined effects of cyclic load and freeze-thaw cycles on the degradation of sandstones are investigated. • Cyclic load leads to the enlargement of relatively small pores. • Freeze-thaw cycles result in the development of relatively large pores. • The UCS of the tested rock decreases within the range of 20 - 40% under the combined forcing. • The Poisson's ratio of the tested rock increases after the freeze-thaw cycles but decreases under the combined forcing. [ABSTRACT FROM AUTHOR]

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