Coal Burst Research Articles

Study on the evolutionary characteristics of coal burst proneness under the action of multiple mining disturbances

AbstractCoal burst is a serious hazard in deep coal mining. To investigate the coal burst proneness evolution during multiple mining disturbances (MMD), this paper implemented a range of uniaxial cyclic loading–unloading tests coupled with acoustic emission (AE) measurement to simulate the loading–unloading effects of the MMD. The maximum load was used to simulate the highest load that the coal has endured in history, the loading–unloading rate was used to simulate the mining speed, and the loading–unloading cycles were used to simulate the number of mining disturbances. Finally, the burst proneness indexes of the damaged coal sample are measured and compared with the origin coal sample. Analysis on the experiment results suggest that, the burst proneness increases when the maximum load is 30% of the uniaxial compressive strength (UCS), interestingly, when the low loading–unloading rate is applied, 50% of the UCS enlarge the burst proneness as well. However, it decreases when the maximum load is 70% of the UCS. Sensitivity analysis indicates that the maximum load, loading–unloading rate, and loading–unloading cycles exhibit decreasing influence sensitivity to the elastic modulus, as well as to the UCS and duration of dynamic fracture. Similarly, the influence sensitivity of maximum load, loading–unloading cycles, and loading–unloading rate to the elastic strain energy index decreases, as well as to the bursting energy index. To ensure the safety of coal mining, it is crucial to avoid the disturbances that would put the coal under low loading and low loading rate.

Seismic Mitigation Effect of Overlying Weakening Strata in Underground Coal Mines

Artificial construction of a weakening zone over the roadway is an essential method for preventing coal bursts and rock bursts caused by strong mining tremors. However, concerning the seismic absorption and load reduction capabilities of an artificial structural weakening zone, the degree of rock mass damage to the roadway under weakening zone protection remains unclear. This study employed principles of elasticity and UDEC (Universal Distinct Element Code) to explore the seismic attenuation and load reduction capabilities of the weakening zone. The results indicate that the absorbing ability of the weakening zone increases exponentially with its weakening coefficient. Under the same dynamic load disturbance, when the weakening coefficient rises from 0.00 to 0.99, the sidewall displacement from the elastic wave source side changes from 0.400 m to 0.228 m. The total number of cracks in the roadway-surrounding rock, and the ranges of overstressed zones decreased linearly. The critical threshold of the roadway resisting the mining tremor disturbance increased. In particular, when the mining tremor is located directly above the roadway, the initial deformation of the roof is the largest, and the cumulative deformation of the rib is greater than the roof. By creating a weakening zone with a coefficient exceeding 0.95, the roadway remains unaffected by the 20 MPa dynamic loading. The study provides a theoretical basis for controlling coal burst that is triggered by mining tremors.

Change Characteristics of the Advance Stress and Strata Fracture Structure of Spatial Isolated Island Formed by Roof Drainage

Large-energy microseismic events, coal bursts, and other abnormal mine stress are often observed in spatial isolated island areas in the roof water-rich gob-side working face during mining operations. In view of this problem, numerical simulation and on-site microseismic monitoring are employed to study the advance abutment stress and strata fracture structure in the spatial island area formed by ‘roof drainage + goaf’ during mining and the method of eliminating the spatial island area. The results show that the lateral stress is distributed in a step-like pattern, and the drainage area and goaf are superimposed to form a high-stress space island area before the mining of the working face along the goaf. The distribution pattern of the advance abutment stress in the spatial island area changes from ‘C’ type to ‘S’ type during the mining process. The strata structure of the roof water-rich gob-side working face is either a single-peak arch or double-peak arch, and the arch peak corresponds to the boundary of the drainage area. The method of local hydraulic fracturing can eliminate the influence of spatial island areas and improve the safety of roof water-rich gob-side working face mining.

Numerical investigation of seismic wavefield characteristics induced by a tensile source in a heterogeneous mining environment

Mining-induced tremors serve as the main dynamic stress source of coal bursts which represent one type of the most critical mining disasters in underground coal mines. To evaluate coal burst risks and comprehend the response of the coal-rock mass to seismic waves, microseismic monitoring stands as the principal method. By inferring the zones with elevated coal burst risks based on the seismic event clusters, valuable insights can be gained. However, because of limited seismic monitoring results generated from an individual seismic event, the recognition of the corresponding dynamic disturbance process remains ambiguous. The dynamic disturbance process presents immense complexity due to the focal mechanism and heterogeneous medium, particularly under the effect of structural heterogeneity induced by large-scale mining operations. In this study, forward numerical modelling was employed to gain the comprehension of seismic wave propagation affected by extraction zones. The findings revealed heightened intricacy of the wavefields and increased displacement concentrations around the working face. The characteristics of peak particle velocity (PPV) and displacement spectra were investigated in the near- and far-field zones, respectively. Owing to the existence of the extraction zone, both a significant amplification effect and the redistribution of PPV were observed around the near-field zone. The intricate wavefield caused by the focal mechanism and extraction zones resulted in more scattered distributions of the low-frequency spectral level, which can potentially cause an overestimation of source intensity. However, the impact of the complex wavefield on the corner frequency was limited. Through the analysis of the PPV attenuation law, as well as the distribution of both seismic moment and source radius, this study offered valuable insights for seismic risk assessment. The forward modelling approach facilitated the identification of high-risk potential areas resulting from seismic events, which deepened our comprehension of the dynamic disturbance process in the complex underground mine environment.

Experimental investigation into damage and failure process of coal-rock composite structures with different roof lithologies under mining-induced stress loading

Coal burst is essentially a destructive manifestation of stratigraphic structure. Although the failure mechanism of the structure has been extensively studied using experimental and numerical methods, few studies have considered the critical role of the mining stress environment and the roof stratum in structural instabilities. In this paper, uniaxial static-cyclic loading tests were performed on a series of composite specimens with different roof lithologies. The objective is to investigate the damage and failure process of composite specimens under mining-induced stress conditions, particularly the mechanical response and damage evolution to coal and rock under cyclic disturbance. The results indicate that the mechanical behaviour of the composite specimen is closely related to the mechanical properties and interactions of the components, and the coal and rock show various characteristics of deformation and damage under cyclic loading with increasing static stress. A dynamic elastic strain energy (DESE) index based on the energy conversion of the specimen was proposed, which is more consistent with the trend of the P-wave velocity and thereby can well reflect the damage state of the specimen. The variation in the acoustic emission (AE) energy rate implies that the composite specimens experience progressive damage before failure, along with AE events spatially clustered in the failure plane of the weak roof and near the upper and lower boundaries of the coal body. The failure pattern of the composite specimen changes with increasing roof strength from initial structural instability due to tensile splitting or shear failure of the roof to the ejection of localized coal chips near the interface, which is attributed to the lateral tensile stresses on the coal near the interface caused by the mechanical difference and frictional constraint under cyclic unloading.

Evolution Laws of Stress–Energy and Progressive Damage Mechanisms of Surrounding Rock Induced by Mining Disturbance

The rock burst induced by the mutual disturbance of mining and excavation is significantly influenced by high static load stress and external dynamic load disturbance. In this paper, the evolution characteristics and progressive damage mechanism of surrounding rock in the process of mutual disturbance of mining and excavation are systematically studied. The results show that the evolution of surrounding rock stress can be roughly divided into three stages: rapid rise in the early stage, continuous rise and step-like decline in the middle stage, and slow rise in the late stage. In the process of parallel mining, the overlying rock movement above the goaf shows the sequence of horizontal penetration of tiny fissures—fracture intensification transition to stratification—non-coordinated caving of middle–low overlying rock—obvious horizontal cracks in the upper key layer. Only under the quasi-static loading mining action does the upper key layer not reach the breaking condition. The wave side of the heading face which is close to the focal point is affected by the dynamic load disturbance, the acceleration duration is short, and the attenuation is relatively fast, so it is the area prone to the earliest impact failure in the face of mining disturbance. The conclusion is helpful to deepen the understanding of the coal burst mechanism of mutual disturbance of mining and excavation.

Research on the initiation pressure criterion of directional hydraulic fracturing in coal mine

Directional hydraulic fracturing (DHF) is more and more widely used in coal mines in China for hard roof and coal burst control. The key to this technology is to determine the crack initiation pressure that affected by the shape of the artificial notch and the stress state around the fracturing hole. Reasonable and simple formula for fracturing pressure calculation is essential since the fracturing pump used in coal mines is usually limited by the harsh conditions and hardly replaced once selected. Based on the superposition principle, the simplified 2D model of DHF was established as the elliptical hole with the internal pressure and solved by using the complex functions method. The analytical solution of tangential stress on the inner surface was obtained meanwhile the corresponding criterion of fracturing pressure can be set up. Considering the characteristics of DHF in coal mines, we further got a simplified formula that controlled by the ratio of major to minor axis of the ellipse-like notch, the ratio of the minimum to the maximum principal stress, as well as the tensile strength of the rock. The formula also gave a guide to the design of the notch that major diameter should be at least twice the minor diameter, and the optimal solution for the ratio is to 2～4 and recommended 4, which can resist the initiation pressure to a large extent affected by the in-situ stress. Once the pressure of the fracturing fluid is high enough to satisfy the equation cracks would arise at the tips of the notch along the major axis which belongs to mode Ⅰ crack and would grow unsteadily and rapidly. A PFC simulation model was used to verify the analysis, the results of which are very consistent with the theoretical solutions.

Investigations into the failure mechanisms of coal bursts based on a real burst event in the Tangshan coal mine

To investigate the failure mechanisms of coal bursts in the longwalls of underground coal mines, a case study based on the real coal burst in the Tangshan coal mine occurring on 2 August 2019 was carried out to analyze the cause and basic theories of the coal burst and, thus, propose approaches and methods to mitigate the damage. The coal burst occurred mainly due to the sliding of coal from the longwall ribs and the simultaneous uplifting of the floor. As a result of the longwall retraction, the roof above the coal seam was too stable to collapse, forming a long hanging roof behind the longwall. Because the longwall acted as a fulcrum, the hanging roof acted like a saw, exerting massive loads on the longwall. As a result, the roof in front of the longwall tended to slope upward, leaving the underlying coal seam unconfined. Due to the horizontal stresses, the longwall ribs collapsed into the longwall, causing the coal seam to break out. These findings could help improve the fundamental understanding of the failure mechanisms of coal breakouts and, thus, aid in the development of measures to prevent such occurrences.

Detection of Coal Bust Risk in a Kilometer Depth Workface with Hard–Thick Roof Based on Active–Passive Seismic CT

Practice and theory research proved the “square effect” during longwall mining with hard–thick strata lying on the coal seam, which could cause severe underground pressure and even dynamic disasters, such as coal burst; therefore, it became a key area and stage that need special attention. The combined active–passive seismic computed tomography (CT) was introduced to detect the abutment stress concentration in the coal seam. The results of active–passive CT inversion show that the “square effect” appears as early as the workface ahead of the theoretical position of 50 m with a 100 m significant influencing zone, which provides a strong guarantee for accurate risk evaluation of coal burst. Precursor information before the “square effect” can be identified based on the everyday total energy of mining tremors, which had period peaks in this case. The everyday average energy of mining tremors indicates that the seismic type during the “square effect” stage belonged to a foreshock–mainshock pattern. The combination of the seismic events and CT can improve the accuracy of coal burst danger distinguishing notably. The research can provide worthwhile guidance for the monitoring and prevention of coal burst hazards in similar conditions.

Mechanism of the coal bursts in the working face during mining of steeply inclined and extra thick coal seam

Coal bursts caused by mining-induced seismicity occur in the working face in steeply inclined and extra thick coal seams (SIETCSs). This article presents a case study in SIETCS through discrete element simulation as well as field investigation. The evolution of the coal burst was studied from the perspective of stress distribution, crack propagation and coal fragment ejection. The results showed that dynamic failure mainly occurred in the coal near the roof. The deep bottom coal and the top coal near the roof are under high static stress due to the clamping effect of the roof and floor and the rotation of the deep roof. The coal burst in the working face was triggered by mining-induced seismicity. The coal burst occurred when the sum of the static stress and the dynamic stress caused by the mining-induced seismicity exceeds the sum of the ultimate strength of the coal and the support strength, which was called the ‘clamping-rotation-dynamic loads’ mechanism. During the coal burst, the cracks developed rapidly into the deep bottom coal and the top coal on the roof side. The results can serve as an important reference for the coal burst failure mechanism in the working face in SIETCSs.

Variation of seismicity using reinforced seismic data for coal burst risk assessment in underground mines

Variation of seismicity using reinforced seismic data for coal burst risk assessment in underground mines

Coal Spray Rate Prediction Based on Factor Analysis and Neural Network (NN) Algorithm

Given the many factors that affect coal bursts as well as complex nonlinear relationships, this study analyzes the main factors that affect coal bursts in coal seams. Central Sulawesi Province. Using NN algorithms to predict coal bursts. This algorithm uses factor analysis to subtract the attributes of the original high-dimensional sample and obtains three common factors to maintain the correlation characteristics of 82.227% of the original data that had 10 sets. factors affecting coal burst rates can be used as training datasets as well as the last five datasets used as testing data and inputted into the rapid miner application to support NN algorithms to predict results. By comparing the prediction results where the NN algorithm has a non-explosion threshold value of 3.194% and an explosion threshold value of -3.230% and is more suitable for predicting coal burst explosions.

A new empirical chart for coal burst liability classification using Kriging method

A new empirical chart for coal burst liability classification using Kriging method

Failure mechanism and control of the coal bursts triggered by mining-induced seismicity: a case study

Failure mechanism and control of the coal bursts triggered by mining-induced seismicity: a case study

Peak-strength strain energy storage index for evaluating coal burst liability based on the linear energy storage law

The strain energy storage index WET was widely used to evaluate coal burst liability, but the scientific evidence for selecting the unloading stress level interval (around 80% of peak strength) remains lacking, and WET can not reflect the energy storage and dissipation ratio (ESD ratio) of the whole pre-peak stage for coal materials. In this study, these two key problems in WET calculation and application were solved based on the linear energy storage (LES) law. The LES law was defined as the linear relationship between the elastic strain energy and input strain energy for solid material during loading. Using the LES law, the elastic strain energy and dissipated strain energy of at 10 types of coals were calculated precisely, and ideal ESD ratio and general ESD ratio at any stress level will be obtained subsequently. The results also show that WET is extremely close to the ideal and general ESD ratio, which proves that the selecting stress level of WET calculation is scientific and reasonable. Furthermore, the general ESD ratio converges to the peak ESD ratio (namely peak strain energy storage index WETP) as stress level increases. Compared with WET, WpET not only reflects the ESD ratio of coal materials over the whole pre-peak loading stage, but also exhibits excellent stability. Consequently, WpET is suggested as a new evaluation index of coal burst liability.

Detection and targeted control of regional stress field for coal burst prevention

Detection and targeted control of regional stress field for coal burst prevention

An energy-based quantitative coal burst proneness rating system for development roadways

Coal burst is one of the principal hazards in development panels of underground coal mines. This study develops a practical approach to quantitatively estimate coal burst proneness in development roadways. A rigorous numerical and statistical study is conducted to establish a novel burst proneness rating (BPR) system based on energy release characteristics. The rating system includes eight geological and geotechnical factors and covers a wide range of inputs. A dataset including 18 field cases (9 burst and 9 no-burst cases) was used to validate the proposed rating system and to determine a critical BPR value (i.e. the threshold value of burst cases). The probability hat{p}_{burst} of coal bursts was then obtained based on BPR logistic regression. The computed rating values using the BPR system can distinguish burst and no-burst cases that are in line with the scope of the eight input parameters in this study. The model results also showed that the mining depth was the most significant contributor of the energy release, along with roof elastic modulus and coal mass conditions. These results suggested that an environment that consists of relatively weak and brittle coal under high vertical stresses and surrounded by a relatively soft strata could promote unstable failures such as coal bursts.

A Numerical Analysis of Coal Burst Potential After the Release of the Fault-Slip Energy

A Numerical Analysis of Coal Burst Potential After the Release of the Fault-Slip Energy

A Method to Determine the Chain Pillar Width considering Coal Burst and Goaf Ignition Dual-Hazard Management

Abstract High rock stress and ground temperature pose great threats to the routine production of longwall top coal caving (LTCC) panels. In this risky condition, the width of the chain pillar is considered a factor adjustable for controlling coal burst and goaf ignition hazards. However, a contradiction, as suggested by longwall experience, is that narrowing the pillar helps coal burst prevention but negatively leads to higher self-ignition potentials, while widening the pillar restrains goaf ignition but increases the likelihood of coal burst. This paper conducted a case study on a longwall panel from Tangkou Mine, China. The paper first analysed stress, elastic strain energy, and goaf temperature variation with varying pillar widths, by which the coal burst risk index δr and goaf ignition risk index Qs were defined and correlated to pillar width D. Further, a pillar width determination method considering coal burst and goaf ignition dual-hazard management was developed by means of the operating point principle. By this method, a reasonable width range was defined by plotting both correlation curves δr=fD and Qs=gD on a chart, followed by optimal width determination according to the intersection of both curves and further verification via a field trial.

Failure mechanism and control of coal bursts triggered by mining induced seismicity in steeply inclined and extra thick coal seam

With the increase in mining depth, coal bursts have become a major challenge in the safe mining of steeply inclined and extra thick coal seams (SIETCSs). Based on a typical mining induced seismicity triggered coal burst (MSTCB) in SIETCS, a large-scale numerical model was developed using the Universal Distinct Element Code. The numerical model was calibrated and validated by laboratory results and field observations. The stress evolution, crack development and ejection velocity patterns in the MSTCB were analysed, and the effect of mining induced seismicity vibration velocity on the MSTCB was discussed. The results show that a triangular static stress concentration zone is formed in the coal on the roof side. And the high-energy mining induced seismicity leads to high dynamic stresses in the coal at the roof side rib and top of the headentry. Coal bursts occur under the superposition of static and dynamic stresses. The MSTCB results in tensile failure near the headentry surface and shear failure in the depth. The vibration velocity has a significant effect on the roof side rib and top of the headentry, while it has only a slight effect on the working face rib and bottom of the headentry. The dynamic stress and ejection velocity in the roof side rib and top of the headentry are positively correlated with the vibration velocity. Finally, measures for MSTCB prevention were proposed. The findings presented in this study can provide guidance for the prevention and control of MSTCBs in SIETCSs.