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As the most common dynamic disaster of coal rock in deep coal mining, rock burst seriously affects the safe and efficient mining of coal resources. Accurate and timely to predict the rock burst is the best way to control and prevent it. In order to study the precursory information characteristics of coal rock impact to establish an efficient and accurate dynamic prediction model. The triaxial compression test of coal bursting liability performed to get the acoustic emission characteristic parameters of coal failure process, meanwhile the monitoring values of microseismic parameters of typical impact mine face were selected and compared with the AE parameters obtained in the laboratory, then based on the similarity of them, the relationship between laboratory-scale and engineering-scale impact acoustic information representation had been built. It shows the “stepwise” increase of accumulated AE energy during coal sample compression and the “lack of earthquake” phenomenon before the occurrence of large energy seismic events have a good correlation, both of them could be used to revealed the energy breeding process of coal and rock mass. The same energy index has the same evolutionary trend for the occurrence of large energy events at different scales, and it can be used as an early warning index of rock burst risk after quantification. In addition, the static factors and stress dynamic factors affecting the occurrence of rock burst in working face are selected, and the dynamic weights are given respectively by combining the microseismic parameters obtained, which modified by using the Bayesian probability, then a static cooperative comprehensive evaluation and early warning model of rock burst is constructed. Furthermore, the model is used to evaluate and warn the danger of a mining face in Henan Province, and the results are better than those of the traditional evaluation model, which shows the engineering applicability and accuracy of the model.

Understanding the failure mechanisms of rocks that are exposed to different water contents is important for rock stability in rock engineering applications, and the quantitative analysis of rock behavior is necessary for predicting and preventing the occurrence of rock failure due to water effects. Mechanical tests using real-time acoustic emission (AE) technology were carried out to reveal the damage evolution in sandstone rocks in a dried state, natural state, and saturated state, which includes a quantitative analysis of AE characteristics and cracking properties. The testing results indicate that with the growth of water content, sandstone rocks show a decreasing trend in strength and tend to experience gentle damage with relatively fewer fractures. The crack morphology of the main fracture surfaces is quantitatively described, including a fractal dimension calculation and cracking length measurements. As the water content rises, when rock failure occurs, a higher AE b-value can be obtained, revealing an increasing proportion of large-scale cracks. The fractal dimension of the acoustic emission hit rate shows that the evolution of rock damage and deformation has self-similarity, that is, the transformation from order to disorder to order, and it is affected by different water contents. The AE waveforms of the sandstone have two dominant frequency bands (0~75 kHz and 75~150 kHz) no matter which water-bearing state they are in. The increase in rock water content has resulted in the decline of AE waveforms located in the range of 200–300 kHz, whereas the rise of AE waveforms is located in the range of 0–50 kHz. The findings of this study deepen our understanding of the mechanism behind rock failure and provide a meaningful reference for disaster assessment and control. [ABSTRACT FROM AUTHOR]

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Rock fracture and rock instability at high temperatures are serious threats to the safe and efficient exploitation of deep geothermal resources. The electric potential (EP) can provide valuable information to monitor and forecast these issues. In this work, compression-shear failure tests were performed to monitor the EPs of granite samples after thermal treatment at 25 °C, 200 °C, 400 °C and 600 °C. The temporal response and non-extensive statistical characteristics of EPs subjected to different thermal treatments were analyzed. The precursory information of the EPs was studied by exploring the change in Tsallis entropy q and variance with the damage variable. The effect of thermal damage on the response mechanisms of EPs was studied using fracture surface scanning, thermogravimetry-infrared radiation (TG-IR) and scanning electron microscopy (SEM) tests. The results show that the thermal treatment affects the EP response by changing the mechanical properties, failure behavior and microstructure. With increasing treatment temperature, the average EP value gradually decreased. The probability density distributions (PDF) of the EPs under different treatment temperatures were consistent with a q -Gaussian distribution, and q increased with increasing treatment temperature. According to the critical theory, the damage state of samples is reflected by the evolution of q and variance based on the EPs and can provide precursory information for instability failure. With increasing treatment temperature, the initial thermal damage becomes more severe, the fractal dimension and roughness of the fracture surface increase, and the unstable propagation of microcracks and precursory points appears earlier. • The weakening effect of thermal damage on the response of EPs is analyzed. • The different treatment temperatures affect the response mechanisms of EPs. • The PDF of EPs after thermal treatments is proved to be consistent with q -Gaussian distribution. • A novel precursory method based on q value of EPs is proposed for predicting the granite failure. • The q value and variance can reflect the evolution of damage state in the granite samples. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology 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.)

Understanding the failure mechanisms of rocks that are exposed to different water contents is important for rock stability in rock engineering applications, and the quantitative analysis of rock behavior is necessary for predicting and preventing the occurrence of rock failure due to water effects. Mechanical tests using real-time acoustic emission (AE) technology were carried out to reveal the damage evolution in sandstone rocks in a dried state, natural state, and saturated state, which includes a quantitative analysis of AE characteristics and cracking properties. The testing results indicate that with the growth of water content, sandstone rocks show a decreasing trend in strength and tend to experience gentle damage with relatively fewer fractures. The crack morphology of the main fracture surfaces is quantitatively described, including a fractal dimension calculation and cracking length measurements. As the water content rises, when rock failure occurs, a higher AE b-value can be obtained, revealing an increasing proportion of large-scale cracks. The fractal dimension of the acoustic emission hit rate shows that the evolution of rock damage and deformation has self-similarity, that is, the transformation from order to disorder to order, and it is affected by different water contents. The AE waveforms of the sandstone have two dominant frequency bands (0~75 kHz and 75~150 kHz) no matter which water-bearing state they are in. The increase in rock water content has resulted in the decline of AE waveforms located in the range of 200–300 kHz, whereas the rise of AE waveforms is located in the range of 0–50 kHz. The findings of this study deepen our understanding of the mechanism behind rock failure and provide a meaningful reference for disaster assessment and control.

According to the steady state of fault and energy balance, we provided a new idea to observe the precursors for a stressed fault. The meta-instability (or sub-instability) state of a fault is defined as the transition phase from peak stress to critical stress of fast instability (earthquake generation) during a full period of slow loading and fast unloading. The accumulative deformation energy begins to release in this stage. Identifying its deformation before fast instability would be beneficial to obtain premonitory information, and to evaluate the seismic risks of tectonic regions. In this study, we emphasized to analyze deformation process of the meta-instable stage with stain tensor data from a straight precut fault in granite at a slow loading rate, and observed the tempo-spatial features during the full deformation process of the fault. Two types of tectonic zones and instabilities occur on the stick-slip fault. The low- and high-value segments in the volume strain component appear along the fault strike with a load increment. The former first weakens and then becomes initial energy release segments; the latter forms strong stress-interlocking areas and finally turns into the initial region of fast instability. And there are two stages in the entire instable process of the fault: the initial stage is associated with the release of the low volume strain segments, which means fault pre-slips, slow earthquakes or weak earthquakes. The second one characterizes a strong earthquake through the release of high volume strain parts. The rupture acceleration in the first stage promotes the generation of the second. Moreover, fault instability contains two types of strain adjustments along the fault: the front-like strain change along the transition segments from low- to high- strain portions with volume strain release, and the compressive strain pulse of fault instability after the volume strain release extends to a certain range with loading increment. In laboratory experiments, the front-type strain occurs about 12 seconds before fast fault instability; the compressive pulse initiates within less than 0.1 second, and then the fault turns quickly into a dynamic strain adjustment, which appears quasi-synchronously between different measurement points, and, finally, an earthquake is generated.

Impact risk assessment is a work that must be carried out in driving and stoping stage in mining coal seam with impact risk to ensure the safety of driving and stoping working face. As one of the methods to evaluate the impact risk, ground acoustic emission monitoring has been widely used in the monitoring and evaluation of the impact risk of coal seam. Ground acoustic emission monitoring system was used to monitor the driving process of opening hole in the first section of working face at the 21st floor in the south area of the second level of a mine in real time, and then to evaluate the impact risk. A method was proposed to classify the impact risk by using ground acoustic emission anomaly index. Combined with the actual occurrence of rock burst, the analysis of ground acoustic emission data shows that the continuous increase or silence of single channel activity and the sudden increase of two or more channels activity indicate the increase of impact risk, and can be used as the ground acoustic emission precursor information before the occurrence of rock burst. According to the statistics, the accuracy rate of ground acoustic emission monitoring is 66.6%, which indicates that ground acoustic emission monitoring has a good effect on impact risk assessment.

The coal-rock combination body is prone to rock burst and surface subsidence under loading, and adopting effective monitoring methods are valuable. In this paper, cyclic loading and unloading (CLU) tests were conducted to investigate the response and precursor of pressure stimulated currents (PSCs) in coal-rock combination samples. The results indicate that the PSCs of the coal-rock combination samples have good response characteristics to stress. Furthermore, the PSCs between the two components are significantly larger and more stable than those of the coal body, which can clearly reflect the failure under a progressive failure mechanism and are related to the difference in the resistivities and microstructures of the components. During the CLU tests, the PSCs of each cycle are well fitted by the Gaussian distribution and have a memory effect. During the loading or unloading process of each cycle, the variation in PSCs with stress conforms to the power function relationship with exponent k. As the cycle number increases, the rates of increase in PSC under stress are gradually weakened, and the indexes of PSCs including exponent k increase or decrease sharply first and then remain stable or even decrease, which is due to the loading history. The stable state of these indexes with obvious inflection points in later cycles of CLU tests can be considered a precursor of coal-rock failure. The PSC flow direction between the two components is exactly opposite to that in the coal body. The results provide new ideas for the monitoring and prediction of coal-rock combination body failure, which is helpful for the prevention of geological disasters in mines. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology 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.)

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

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology 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.)

A fault can be both the inrush channel and the water source. In order to better understand how such a mine disaster develops, a fluid–solid coupling analysis model and a Forchheimer, nonlinear, non-Darcian flow model were used to study an inrush in the Hongcai coal mine. Numerical simulation was used to reconstruct the dynamic process of the inrush. Then the coupled evolution of the stress, displacement, and seepage fields were analyzed and the precursor information characteristics were summarized. The results showed that the inrush was the result of excavation disturbance and hydraulic pressure in the fault fracture zone. Before the inrush occurs, the stress of the surrounding rock drops after continually increasing, the displacement of the surrounding rock increases sharply after a stable build-up, and the gradually increasing pore water pressure of the surrounding rock decreases rapidly just before the inrush. During this process, the multi-field information in the aquiclude continuously changes. During the inrush, groundwater flows into the roadway from top to bottom along the fault fracture zone. The pressure in the runoff path continuously decreases and the flow velocity generally increases. However, when the water initially enters the channel and roadway, the flow velocity first decreases due to the change in flow direction, and then increases again, and the velocity and pressure gradually stabilizes after the water finally enters the roadway. These results help explain how flooding caused by a water-rich fault inrush evolves and provide a scientific basis for determining the characteristics of appropriate inrush precursors. [ABSTRACT FROM AUTHOR]

In order to effectively extract the precursor information of containing macroscopic structural coal body in destabilizing failure, the authors have carried out the experimental research on the failure characteristics and charge signal law of intact coal samples and coal samples with a prefabricated large different-dip crack under uniaxial compression. The results show that with the decrease of pre-crack angle( the angle between the crack and the axial direction), the peak stress of coal samples decreases gradually, and the time reaching the peak stress de-creases, and the coal samples change from the tensile splitting failure to shear slip failure. At the same time, the new wing cracks are transformed from the wing cracks of the vertical pre-crack surface to the wing cracks of the parallel pre-crack surface. The pre-crack coal samples are more prone to rupture due to the existence of macroscopic cracks, and there are significant charge signals before the stress peak. Therefore, the precursory information of pre-crack coal samples is more easily captured than that of intact coal samples.With the pre-crack angle changing from 60° to 45° to 30°, the time of the first high-value charge signal in coal samples cracking process appears gradually advances,respectively 68.5% σc and 65.5%T,60.5% σc and 47.0%T,37.5% σc and 30.0% T( T for the whole destruction time of coal samples), and the amplitude of charge signal after the peak stress failure stage decreases gradually, as well as the mutation points in the charge accumulation curve increase and the first mutation point is advanced. The existence of pre-crack can affect the charge laws of coal samples by changing the deformation fracture process and the form of coal samples. Therefore, the charge signal related characteristics of pre-crack coal samples can be applied to detect the macroscopic geological structures and infer the occurrence of joints or faults of coal or rock mass reasonably. The study may provide new methods and approaches for the prediction of coal and rock dynamic disasters. [ABSTRACT FROM AUTHOR]

According to the current research status that precursory information is difficult to be identified and extracted during the failure process of coal containing gas loading, the acousto-charge monitoring device of coal containing gas loading has been developed by the authors to obtain the acoustic emission and charge precursory information in whole loading process. The experimental results show the acousto-charge monitor system can capture charge and acoustic emission signal generated by coal containing gas. Part of charge signal changes ahead of acoustic emission signal during the process of loading. Acousto-charge signal is related to crack propagation of inner coal. Free charge and the acoustic emission produced in coal have essential difference. The former is produced under the comprehensive additive effect of piezoelectric effect,friction effect, charge separation because of new surface produced by microfracture, gas migration carrying charge and so on, however, the latter is the process of inner coal crack propagation and release strain energy in pulse wave form. The study demonstrates that the application of acousto-charge monitoring technology to mine dynamic disaster early warning is feasible through the division of acousto-charge precursory information abnormal area of coal containing gas under failure. [ABSTRACT FROM AUTHOR]

Abstract: Research on precursory information about the scale invariance of seismicity before large earthquakes has been an interesting topic for geophysicists in recent years. However, in some cases, it is difficult to capture this precursory information. Our study results show that seeking a pertinent selection of methodology is really needed for the purpose of gaining precursory information in the application of the analytical methods related to scale invariance. We investigated the scale invariance of the interevent time series of the seismic sequences for the Minle and Songpan regions in China by applying the methods of Hurst analysis, local scaling property, correlation dimension, and generalized dimension spectrum. It is indicated that there are clearly precursory changes of local scaling property before the Minle M6.1 and Songpan M7.2 earthquakes, as well as precursory changes of the Hurst exponent and generalized dimension spectrum before the Songpan M7.2 earthquakes, while there are no clearly precursory changes of the Hurst exponent and generalized dimension spectrum before the Minle M6.1 earthquake, as well as the precursory change of the correlation dimension before the above large earthquakes, which signifies that there is a difference in the capability for capturing precursory information among the above four methods. This result suggests that the selection of an appropriate methodology is quite necessary to obtain the precursory information on the scale invariance in the use of the above four methods. If we do not select an appropriate methodology, we might not obtain any precursory information. [Copyright &y& Elsevier]

A new method for precursory information extraction, i.e., slope-difference information method is proposed in the paper for the daily-mean-value precursory data sequence. Taking Tangshan station as an example, the calculation of full-time-domain leveling data is made, which is tested and compared with several other methods. The results indicate that the method is very effective for extracting short-term precursory information from the daily mean values after the optimization is made. Therefore, it is valuable for popularization and application. [ABSTRACT FROM AUTHOR]

It has become seismologist’s common view to attach importance to the study of the characteristics about the relationship among the space distribution of precursory anomalous stations, active structure, deep rock character, and different kinds of earthquakes. In this paper, the information characteristics of ground fluid precursors are analyzed with a few examples of earthquakes. The result shows that information characteristics of ground fluid precursors mainly demonstrate, temporally stage and acceleration pattern, specially, swarm and concurrence feature. It is a key scientific problem, we propose, to give deep study on the stage and concurrence pattern for realizing the seismogenic process as well as making relatively correct prediction to the potential earthquake focus and the occurrence time. [ABSTRACT FROM AUTHOR]

Due to strain energy releases sharply, fault slip is often accompanied with sudden stress change around fault structures, especially on the fault surface. In this study, the slippage of a reverse fault structure induced by coal mining in Yima mining area, Henan province, China is studied by employing physical and numerical simulation. The stress evolution around this fault is analyzed to further understand the mechanism of coal bursts around the fault structures. The results show that the normal stress is relatively greater than shear stress on the fault surface, and the normal and shear stress on the fault surface near the coal seam are greater than that far away from the coal seam. It is interesting to find that the variation of normal stress and shear stress on the middle of fault is opposite in the physical model. To in-depth understand this phenomenon, the numerical simulation method is employed to investigate the stress variation and dynamic energy evolution on the fault surface before, during and after fault slip by applied different horizontal stress on the lateral surface of model. The results show that the opposite variation of normal and shear stress is induced by the greater horizontal stress. This phenomenon can be considered as a precursory information for major fault slip. The horizontal stress is the main driving force of the fault slip induced by the coal seam mining. In this study, it is also suggested that the existence of extra-thick conglomerate rock and reverse fault in the high horizontal stress environment could cause large area of roof movement and provide the dynamic pressure source for the occurrence of coal bursts in Yima mining area. • The reason why the horizontal loading is the main driving force of the fault slip has been found out. • The fault slip precursory information in terms of fault stress is found out. • The dynamic environment of coal bursts from combined effect of extra-thick rock and F 16 fault is studied. • The reason why the coal burst accidents occur so frequently in Yima mining area is analyzed. [ABSTRACT FROM AUTHOR]

• Compound EPB shield machine was developed to simulate the tunnel excavation. • Soluble 3D printing of an irregular cave solve the problem of model prefabrication. • Ground settlement model can be constructed by laser scanning technology. • Mechanical force provided by the machine are related to precursory information. The shield tunnel, when running through urban karst areas, may induce engineering hazards, such as water inrush, ground collapse, and shield drooping. To reveal the water inrush mechanism triggered by confined karst caves during shield excavation, we developed a model test system for studying the evolution of water inrushes, which could be used to simulate the shield excavation process. The compound EPB shield machine allows the adjustment of parameters, including the cutter rotation speed, torque, excavation speed, supporting force, tunneling footage and excavation height. At a 1780 mm excavation distance, the deflection of the front end can reach the millimeter level. A special cutter with an automatic protection mechanism was designed to avoid low rotation speeds and high jacking forces, thereby effectively protecting the cutter. Moreover, several new and advanced test techniques were developed, including the soluble 3D printing of an irregular karst cave model and ground settling reconstruction using laser measurements. Based on these test devices and advanced techniques, a model test of the water inrush in the Jinan metro was conducted using the EPB shield machine, in which the variations of displacement, stress, and seepage pressure were analyzed. The results showed that the apparatus performed stably and reliably. [ABSTRACT FROM AUTHOR]

To study the precursory information of water inrush from impermeable rock mass failure, a true triaxial geomechanical model test apparatus was developed. The test apparatus designed is 5.5 m in length, 4.0 m in width and 2.5 m in height. It is mainly composed of a steel frame system, a hydraulic control system, a water pressure loading system and a data monitoring system. The main feature of the true triaxial model test apparatus is its ability to apply high, uniform compressive loads in all three principal directions to perform tests under true triaxial static loading conditions. The apparatus can also be used to study the solid-fluid interaction of the surrounding rock mass and water. A test sample can be placed into and removed from the apparatus automatically and conveniently by hydraulic control software. A model test of the water inrush in the Xiema Tunnel, which is located in Chongqing, China, was carried out by using this apparatus. The size of the experimental sample was 1500 × 1000 × 1000 mm. The variations in displacement, stress and seepage pressure were investigated. The test results indicate that the apparatus is stable and reliable. [ABSTRACT FROM AUTHOR]