Rockburst Hazard Research Articles

Microseismic Temporal-Spatial Precursory Characteristics and Early Warning Method of Rockburst in Steeply Inclined and Extremely Thick Coal Seam

Early warning of a potential rockburst risk and its area of occurrence helps to take effective and targeted measures to mitigate rockburst hazards. This study investigates the microseismic (MS) spatial-temporal precursory characteristic parameters in a typical steeply inclined and extremely thick coal seam (SIETCS) with high rockburst risk and proposes three spatial/temporal quantification parameters and a spatial-temporal early warning method. Analysis results of temporal parameters show that the sharp-rise-sharp-drop variation in total daily energy and event count can be regarded as a precursor for high energy tremor. The appearance of peak values of both energy deviation (≥20) and event count deviation (≥1) can be regarded as precursors that indicate imminent rockburst danger. A laboratory acoustic emission (AE) experiment reveals that precursor characteristics obtained from the study can be feasibly used to warn the rockburst risk. The spatial evolution laws of spatial parameters show that the high energy density index of MS (EDIM), velocity, velocity anomaly regions correlate well with stress concentration and rockburst risk areas. The field application verifies that the temporal-spatial early warning method can identify the potential rockburst risk in a temporal sequence and rockburst risk areas during the temporal early warning period.

Geophysical and analytical determination of overstressed zones in exploited coal seam: a case study

A variety of geophysical methods and analytical modeling are applied to determine the rockburst hazard in Polish coal mines. In particularly unfavorable local conditions, seismic profiling, active/passive seismic tomography, as well as analytical state of stress calculating methods are recommended. They are helpful in verifying the reliability of rockburst hazard forecasts. In the article, the combined analysis of the state of stress determined by active seismic tomography and analytical modeling was conducted taking into account the relationship between the location of stress concentration zones and the level of rockburst hazard. A longwall panel in the coal seam 501 at a depth of ca.700 m in one of the hard coal mines operating in the Upper Silesian Coal Basin was a subject of the analysis. The seismic tomography was applied for the reconstruction of P-wave velocity fields. The analytical modeling was used to calculate the vertical stress states basing on classical solutions offered by rock mechanics. The variability of the P-wave velocity field and location of seismic anomaly in the coal seam in relation to the calculated vertical stress field arising in the mined coal seam served to assess of rockburst hazard. The applied methods partially proved their adequacy in practical applications, providing valuable information on the design and performance of mining operations.

Rock Burst Precursor Electromagnetic Radiation Signal Recognition Method and Early Warning Application Based on Recurrent Neural Networks

The fracture-induced electromagnetic radiation (FEMR) method is a promising geophysical method for the monitoring and early warning of coal-rock burst disasters. At present, the time-series characteristics of FEMR data are primarily used for rock burst monitoring and early warning. However, these rock burst precursor signal identification and hazard warning methods need further development. The progress made in the field of deep learning provides a new method for the identification of rock burst warning signals and the realization of intelligent early warning. In this paper, based on a deep learning algorithm, a method for identifying rock burst precursor FEMR signals is proposed. Based on bidirectional long short-term memory recurrent neural networks, this method trains and validates the model by analyzing a large number of normal FEMR signals and rock burst FEMR precursor signals. The input of the model is the FEMR data sequence, and the corresponding output is the hazard identification result. Upon the completion of training and validation, the model can perform automatic/intelligent shock hazard precursor signal recognition and quickly and accurately provide rock burst hazard early warning without requiring parameter adjustment and manual intervention. The results obtained showed that the rock burst precursor signal recognition method based on the recurrent neural network responds well to the rock burst hazard and can capture information regarding impact hazards in advance. Therefore, it is of great significance with regard to accurate rock burst monitoring and early warning.

Fractal Analysis of Fragmentation Distribution of Rockbursts Induced by Low-Frequency Seismic Disturbances

Low-frequency seismic disturbances frequently induce violent rockburst hazards, seriously threatening the safety of deep excavation and mining engineering. To investigate the characteristics and mechanisms of rockbursts induced by seismic disturbances, in this study a series of true triaxial experiments, including the moderate seismically induced, the weak seismically induced, and the self-initiated rockburst experiments under different conditions were conducted. The fractal geometry theory was applied to study rockbursts and the fractal dimensions of fragmentation distribution of different types of rockbursts were calculated. The results show that the fragmentation distributions of both the seismically induced and self-initiated rockbursts exhibit fractal behaviors. For the moderate seismically induced rockbursts, as the static stresses (i.e., the maximum and minimum static stresses) and disturbance amplitude increase, the fractal dimension increases, whereas, as the disturbance frequency increases, the fractal dimension decreases first and then increases. Under similar static loading conditions, the moderate seismically induced rockbursts have the largest fractal dimension, followed by the self-initiated rockbursts, and the weak seismically induced rockbursts have the smallest fractal dimension. There is a linear relationship between the average fractal dimension and kinetic energy of these rockbursts, implying that the fractal dimension can serve as an indicator for estimating rockburst intensity. Furthermore, from a fractal point of view, the energy input, dissipation, and release of these rockbursts are all linear processes.

Seismic activity reduction with the use of blasting and passive seismic tomography control, a case study from copper ore mine, Poland

Deep copper ore mines in Poland have been struggling with seismic hazard since almost fifty years ago when the first rock burst occurred. Increasing exploitation depth and mined-out space make the mining conditions constrained and severe causing the seismic activity to grow substantially. Consequently, rock burst preventive activities have to be incorporated into mining technology to provide work safety. To date, the group winning blasting has posed the most commonly used preventive measure, so its efficiency in seismic energy reduction is of significant importance for the rock burst hazard mitigation. The more energy blasting works provoke to release the safer the work environment gets. The article aimed to assess the efficiency of such an energy reduction, which was approached in two ways. Firstly, the drop of energy and in turn its reduction efficiency was assessed simply using a percentage of provoked tremors’ energy and number. Next, the analysis employed passive seismic tomography results to find if provoked tremors took place in high-velocity zones. The analysis made it possible to verify the assumption that the more provoked energy occurs in such zones the more successful preventive blasting is.

Influence of water content on seismic activity of rocks mass in apatite mining in Khibiny

The authors analyze connection between the rock mass water content due to surface water (precipitation, snow melting and water bodies) and rock bursts in mines. The seismological monitoring performed from 2001 at the surroundings of the Apatite Circus and Rasvumchorr Plateau apatite deposits identified fluctuation of the general seismic background from year to year and seasonably. For these deposits, increased water content of rocks is seen as a natural trigger for rock mass seismicity during intensive rains and snow melting. The 15 year-long seismic monitoring of the apatite deposits under study has revealed regularity in a seasonal growth of seismicity during snow melting and precipitations, mainly in spring months. Particularities of geodynamical event occurrence allow a conclusion on the ultimately stress state of the upper earth’s crust in whole in the deposits’ vicinities. Reactivation of tectonic disturbances and weakened rock mass zones occurring due to relief of normal compression during infiltration can be a potential principal mechanism of seismic activation of the rock mass during the increased water content periods. The origin of the events observed is linked with the dynamic release of ultimate rock mass stresses when geomechanical conditions change under the action of hydrostatic pressure on tectonically disturbed zones. The authors make a conclusion on necessity to apply the revealed regularities of impact of natural factors on rockburst hazard when planning mining operations and performing geomechanical monitoring.

Physical and mechanical properties of placers and rocks in the Rakovsky and Bolotny stream channels in the Vedugin gold occurrence area

The information on the rock mass behavior in the course of mineral mining is of vital necessity for exploration and mining companies. In engineering decision-making as well as for the prevention of possible difficulties, it is required to have the most comprehensive data on physical and mechanical properties of rock masses. The aim of this study is determination of physical and mechanical properties of placer rocks in the Rakovsky and Bolotny stream channels in the Vedugin gold occurrence area, as well as assessment of elasticity and brittleness of ore and rocks to define their susceptibility to rockbursting. It is found that bedding rocks in Rakovsky and Bolotny placers down to a depth of 87 m below ground surface are rockburst-non-hazardous. The research findings are usable in elaboration of project documentation for mining Rakovsky and Bolotny placers, as well as in selection of technologies for supporting rock mass and for actual mining of placer reserves. In the latter case, it is possible to make reasonable decisions on support design and geotechnology parameters so that to combine mining safety with the quality of mineral extraction as early as the project design. It is recommended that Burkandya company undertakes continuous monitoring and prediction of rockburst hazard as mining is advanced to deeper levels.

Ground Support Design of Underground Openings in Seismically Active Mine: A Case Study from a Lead and Zinc Underground Mine

Ground control is multi-tasking and continuous process for the stability of underground openings. Balya underground lead and zinc mine is an important case study area due to its deepest working condition and the highest production amount among Turkish underground metalliferous mines. Mining-induced seismicity needs to be investigated especially for deep as well as tectonically active mine sites. Increasing depth will increase the stress around an underground opening that is a key factor of rock-burst hazard. Rock burst potential can be estimated from analytical approaches based on the relation between rock mass strength and stress around openings; however, one of the best way is to monitor the mining-induced seismicity. The support system is designed based on the actual geotechnical conditions determined from the excavated working face prior to support applications in the mine. Empirical methods are then used to make a decision about the support system. If the location is designated as a seismically active region based on micro-seismic event results, support components are selected as a dynamic support system that must absorb the energy released from the surrounding rock masses. The purpose of this study is to investigate seismically active locations according to mining-induced seismicity and the support system design principles for that locations to protect the openings against seismic loads.

The Rock Burst Hazard Evaluation Using Statistical Learning Approaches

The great threat and destructiveness brought by a rock burst make its prediction and prevention crucial in engineering. The rock burst hazard evaluation at project locations is an effective way of preventing rock burst since currently real‐time prediction is not available. Since different control factors and discrimination conditions of rock burst were accepted by conventional risk determination methods, the rock burst risk determination in the same area may produce conflicting results. In this study, Naive Bayes statistical learning models based on different model prior distributions representing highly complicated nonlinear relationship between rock burst hazard and impact factors were built to evaluate the rock burst hazards. The results suggested that the Bayes statistical learning model based on a Gaussian prior has the strongest performance over four preset prior distributions. Combining the rock mechanics parameters measured in the laboratory and the stress data collected on the project sites, the proposed model was successfully employed to evaluate the kimberlite rock burst risk of a diamond mine in Canada. The Bayes statistical learning model exhibits its robustness and generalization in rock burst hazard evaluation, which can be generalized for similar engineering cases with enough supported data.

Numerical Simulation of Mining‐Induced Stress Evolution and Fault Slip Behavior in Deep Mining

The ground pressure distributes significant variation in underground mining near fault. Fault reactivation is an important factor to induce the rock burst. Therefore, characterizing geological settings in mining areas by the geological information can improve the accuracy of simulation. To investigate the characteristic of mining stress evolution and reactivation of the F16 reverse fault during the retreat Mining‐Induced s in Yima coalfield, a three‐dimensional digital elevation model based on GIS platform was applied. The 3D geological model includes three working faces, and F16 fault was constructed by AutoCAD software. Then, the 3D geological model was imported into the FLAC3D code to simulate the potential of mining‐induced fault reactivation. The simulation results illustrate that the footwall of F16 fault is a high stress concentration area. Affected by F16 fault and the huge thick gravel rock in the roof, the coal seam near the fault accumulates a large amount of elastic strain energy, which increases the potential of rock burst hazards in the process of mining.

Microseismic monitoring, analysis and early warning of rockburst

Rockburst hazard is one of the most outstanding problems in deep rock mechanics, which has become the bottleneck for deep underground engineering in China. Rockburst occurs with some evident precursors, however, the rockburst mechanism is not clearly revealed. The acoustic emissions (microcracks) occur before the failure of rockmaterial, and rockburst is the sudden failure of rock mass caused by the development of microcracks. Therefore, it is effective to study the microcrack precursors for the surrounding rock of deep-buried tunnels by the microseismic monitoring system, and to warn of the rockburst. A method for monitoring and warning of rockburst in deep underground engineering is proposed based on the study of rockburst mechanism and in situ experience for many years. Main points of the proposed method are numerical simulation based on the monitoring, warning of rockburst based on the numerical simulation, and control of rockburst based on the warning. The proposed method was applied and improved successively in the deep-buried tunnels of the Jinping II Hydropower Station and the Hanjiang-to-Weihe River Diversion Project, which may prevent the occurrence of rockburst or decrease the incidence of rockburst. The proposed method is fast, accurate and intelligent by integrating the microseismic monitoring, artificial intelligence, big data and large-scale numerical analysis, and may be promoted to other deep underground engineering.

Spatio-temporal hierarchical cluster analysis of mining-induced seismicity in coal mines using Ward's minimum variance method

Mining-induced seismicity is one of the most dangerous physical phenomenon's accompanying coal and ore extraction in underground mines and often resulting in rock burst. In many cases, this seismicity can be grouped together, forming spatial clusters of events related to some structures in mines such as faults or pillars. However, as shown in this paper, the usual seismicity in coal mines is mainly related to current mining process and there are no distinct spatial or temporal groups of seismic events, but rather they form continuous cloud of events related to mining activity. Therefore, we introduce new spatio-temporal metric together with Ward's minimum variance method to obtain hierarchical clustering analysis of mining-induced seismicity. Constructing metric in this way made it possible to assess seismic hazard concurrently in space and time. The clustering method is utilized to a case study of mining-induced seismicity from coal mine with high seismic hazard and where high energy mainshock occurred. In each cluster, magnitude distribution of seismic activity is calculated, showing in some cases, a significant departure from Gutenberg-Richter distribution of seismic events, which can be important in seismic hazard analysis. As a measure of accuracy of each cluster locations we have introduced bootstrapping procedure. We have shown that location errors of seismic activity affect each cluster center location of the order of a few tens of meters. The presented cluster analysis of mining seismicity provides new measures to determine group of seismic sources and high stressed areas in the rock mass in mines allowing for determination of seismic and rockburst hazard.

The Impact of the Coexistence of Methane Hazard and Rock-Bursts on the Safety of Works in Underground Hard Coal Mines

Several natural threats characterize hard coal mining in Poland. The coexistence of methane and rock-burst hazards lowers the safety level during exploration. The most dangerous are high-energy bumps, which might cause rock-burst. Additionally, created during exploitation, safety pillars, which protect openings, might be the reason for the formation of so-called gas traps. In this part, rock mass is usually not disturbed and methane in seams that form the safety pillars is not dangerous as long as they remain intact. Nevertheless, during a rock-burst, a sudden methane outflow can occur. Preventing the existing hazards increases mining costs, and employing inadequate measures threatens the employees’ lives and limbs. Using two longwalls as examples, the authors discuss the consequences of the two natural hazards’ coexistence. In the area of longwall H-4 in seam 409/4, a rock-burst caused a release of approximately 545,000 cubic meters of methane into the excavations, which tripled methane concentration compared to the values from the period preceding the burst. In the second longwall (IV in seam 703/1), a bump was followed by a rock-burst, which reduced the amount of air flowing through the excavation by 30 percent compared to the airflow before, and methane release rose by 60 percent. The analyses presented in this article justify that research is needed to create and implement innovative methods of methane drainage from coal seams to capture methane more effectively at the stage of mining.

Seismic activity induced by exploitation in vicinity of mined-out space, case study from deep copper ore mine in Poland

Rock bursts are one of the most difficult to recognize and assess natural hazards in deep copper ore mines in Poland. The occurrence of rock bursts is conditioned by a number of complex geological and mining factors. The impact of each of the factors on the state of seismic hazard is sometimes impossible to be determined. The correct recognition of the rock mass condition and the causes of rock burst threat gives a chance for advance preparation of adequate prevention, and thus for eliminating or reducing the threat to the level of tolerated risk. In Poland’s copper ore mines, continuous observation of seismic activity is carried out, which is expressed in the number of tremors and their energy over a specified period of time. Moreover, the effectiveness of prevention measures and, in particular, of the active ones, which are aimed at provoking seismic energy emissions through properly executed winning blasting works, is calculated and analysed. Reducing the amount of energy accumulated in the rock mass reduces the risk of rock bursts. Exploitation is carried out in a way that avoids parallel approach of the operational front towards the goafs, drifts, and faults with drops greater than the height of the mined deposit. However, over forty years of mining of the copper ore deposit, which has created the large areas of mined-out space mean that mining works are increasingly carried out under restrained conditions. There are unfavourable geological and mining situations in the exploitation fields, disrupting rhythmic development and an even front line. The specific structure of the copper ore deposit in Poland makes it possible to conduct multidirectional cutting and to introduce local changes in the adopted directions of exploitation. Cases of operational fronts approaching goafs occur often in the vicinity of old mining infrastructure and when there is the need to mine remnant parts of the deposit not previously intended for exploitation or not mined yet. Generally, exploitation in the direction of goafs or yielding zones is limited to the cutting and yielding works. The purpose of the study was to analyse the seismic and rock burst hazard in an exploitation field located at a depth of 850 m in a copper ore mine, over a period of 5 years, when mining was carried out towards goafs. Seismic activity and the effectiveness of rock burst prevention measures were assessed and analysed. The active prevention i.e. the group winning blasting works was an inseparable element of the excavation technology. It was found that when the operational front progressed towards adjacent goafs, there were symptoms of an increase in rock mass pressure causing the destruction of roof rocks and, as a consequence, deterioration of roof stability, squeezing of floor, relaxation of the side walls and increase of seismic activity, which posed the potential threat of rock bursts.

Numerical simulation of the possibility of seismic event occurrence in hard rock mine

This article presents the problem of seismic and rockburst hazard in underground hard rock mines. Types of rockbursts and their mechanisms as well as the causes of dynamic phenomena are discussed. The possibility of using numerical modeling to simulate the occurrence of a high-energy tremor, as a result of sudden shear rupture, is also shown. Results of numerical simulations are presented for a model mining field in a hard rock mine at high depth, in which the deposit is mined in a room-and-pillar mining system. General geological and mining conditions characteristic for Polish underground copper mines in Legnica-Glogow Copper Mining District belongs to KGHM Polska Miedz S.A. were adopted in the models. The typical lithological cross-section of the polish copper mines is characterized by rigid, high-strength rock layers in the roof, capable of accumulating elastic energy, while layers with much lower strength parameters are present in the floor. In the analyzed mining field, the remaining undisturbed rock, with a width of 40 m, was left. Numerical simulations were conducted in a plane strain state by means of Phase2 v. 8.0 software. An elastic-plastic model with softening was accepted for the rock mass. The results of numerical modeling showed that sudden (one computational step in the model) collapse of roof strata may occur over the excavated space on the edge of the remaining undisturbed rock, as a result of exceedance of shear strength, above all. This may cause a very high-energy tremor, and under the appropriate conditions, may result in the rockburst phenomenon.

Experimental study on mechanical properties and failure behaviour of the pre-cracked coal-rock combination

The mechanical property of roof is critically important for underground coal mining activities. The existence of hard roof that cannot cave naturally after coal extraction results in an unconsolidated goaf with large voids, and the sudden failure of this type of roof in uncontrolled manner will cause server windblast or rockburst hazard. To address this hazard, pre-existing fractures are created in roof by hydraulic fracturing or blasting to weaken the hard roof and promote its natural caving. The effectiveness of these roof weakening techniques is highly dependent on the geometry of pre-conditioned fractures and its impact on the mechanical behaviour of combined roof-coal strata, which has not been well-explored yet. Therefore, 21 groups of uniaxial compression testing on combined rock-coal specimens with pre-existing cracks were carried out, and the effect of fracture length and angle on the mechanical properties and failure modes of combined rock-coal strata were investigated. It can be concluded that longer cracks and/or cracks with angle closer to 30° tend to have the following characteristics. (1) The more gentle the stress-strain curve of the combined coal and rock tends to be, the larger the decrease range of its elastic modulus, peak strength and peak strain, and the smaller the strain interval corresponding to the elastic-plastic stage. The results show that the artificial fracture weakens the mechanical properties of composite coal and accelerates the process from micro-crack to macro failure. (2) When the specimen enters the plastic stage, the acoustic emission jumps sharply; the cumulative number of micro-cracks and the cumulative energy released during the whole loading process gradually decrease, and the number of micro-cracks and the released energy at the moment of failure also decrease gradually. (3) The crack-initiating stress for combined coal-rock specimens gradually decreases via one of three typical modes: preferential crack initiation of coal, simultaneous crack initiation of combined coal-rock and preferential crack initiation of rock, which also matches with coal failure, rock-coal failure and rock failure in the combined specimens. Thus, to promote the continuous fracture of the roof under loading, the length and orientation of the pre-conditioned cracks must be controlled and optimised.

The Seismic Source Parameters of Tremors Provoked by Long-Hole Destress Blasting Executed During the Longwall Mining of a Coal Seam Under Variable Stress Conditions

The underground mining of coal seams in the Upper Silesian Coal Basin is carried out at great depths and mostly in the presence of remnants or edges of other surrounding coal seams, i.e. under the condition of high stress level in the rock mass. Therefore, this mining is accompanied by rockburst hazard and suitable preventive action is required. Long-hole destress blasting plays an important role and is commonly applied in rockburst prevention in underground hard coal mines. Estimated blasting effectiveness is important when designing rockburst prevention. It is commonly estimated on the basis of the seismic energy of a provoked tremor. The seismic source parameters have already been considered for this purpose. Additional information about the effects of long-hole destress blasting could be contemplated in the planning of active rockburst prevention. The seismic source parameters of tremors provoked by long-hole destress blasting have been calculated and are presented in this article. Destress blasts were performed during the longwall mining of coal seam no. 506 in one of the hard coal mines in the Upper Silesian Coal Basin. They were executed from the longwall face, in order to destress the rock mass ahead of it. Parameters of the blasts were variable and they were modified according to geological and mining conditions and the observed level of rockburst hazard. The seismic source parameters have been determined for tremors provoked directly after firing explosives and for tremors occurring in the waiting time, and they have been compared with each other.

Use of Neural Networks to Forecast Seismic Hazard Expressed by Number of Tremors Per Unit of Surface

The seismic and rock burst hazard should be considered as one of the most important hazards in Polish hard coal and copper ore mines. Over the last several years, Upper Silesia has witnessed a constant process of limiting the extraction of hard coal. Despite a significant decrease in production, no reduction in the number of registered high-energy tremors with energy higher than 105 J. is observed. Therefore, it is important to develop the existing methods for assessing the state of seismic hazard and seeking new ways to estimate the level of this threat. The article presents the results of research aimed at determining the possibility of using artificial neural networks to forecast the level of seismicity induced by mining operation. The data used in the conducted research came from the area of operation of a heavily seismic USCB mine. The presented research results showed that in the case of the discussed area, the use of the new tool, which is an artificial neural network, allowed to obtain good results of the forecast of the number of tremors. For the data set in question, the neural network with optimal architecture is composed of twelve neurons in the entrance layer, two neurons with bipolar characteristics in the hidden layer and one neuron with linear or bipolar characteristics in the initial layer. The results of calculations made for the highly seismic area of mining works carried out in a hard coal mine confirmed the possibility of using neural networks to estimate the changes in the size of induced seismicity associated with the deposit operation.

Evaluation of Rockburst Hazard in Deep Coalmines with Large Protective Island Coal Pillars

As the cover depth of coalmines in China increases, large protective coal pillars are widely used to support the overlying strata during mining. After the longwall panels on both sides of the protective pillar are mined, the protective pillar becomes an island pillar. The high static and dynamic stress accumulated in a protective island pillar may cause overall rockburst hazards in the gateway between the protective island pillar and the longwall panel being mined. Based on a rockburst accident that occurred in the Longyun coalmine, Shandong province, China, this study elucidates the occurrence mechanism of this type of rockburst. Firstly, a mathematical model is proposed to evaluate static stress distribution in the protective coal pillar. The calculation results indicate that large elastic energy is accumulated in the large protective pillar and overall rockburst hazards is very likely to be induced. To reduce rockburst risk, rockburst index is defined and long-borehole de-stress measures were adopted in this study. Then, dynamic stress evolution in the protective island pillar during mining the longwall panel was analyzed using PFC2D. The results show that the peak stress in the protective pillar increased continuously from 28 to 67 MPa with the working face advancing, indicating high rockburst risk in the protective pillar. The results of the mathematical model and numerical analysis agreed well with the rockburst accident that occurred in Longyun coalmine. This study paves the way for the design of large protective pillars based on overall rockburst prevention in deep coalmines.

Geomechanical aspects of development of closely spaced ore deposits under conditions of rockburst hazard

Geomechanical aspects of development of closely spaced ore deposits under conditions of rockburst hazard