Use of mining and seismological parameters as premonitors of rockbursts

Comprising one of the deepest mines in the world, the gold mining region of the Kolar Gold Fields (KGF) in southern India is known to be confronted with the acute problem of rockbursts that pose a hazard to workmen and cause disruptions in the production schedule. In combination with mining parameters, seismological data of these rockbursts events systematically monitored over the years using a regional seismic network have been employed to examine and model premonitory characteristics of the events. Linear empirical relations are established relying on correlations seen between the seismic energy released due to a rockburst, total tonnage of ore mined out and total number of rockbursts, as well as seismic events in some shallow and deep workings currently active at KGF. The deterministic model, although not free from certain limitations at present, has been applied to predict rockburst activity with some success.     

Dynamic buckling mechanism of pillar rockbursts induced by stress waves

Rockbursts are sudden and violent rock failures that can lead to huge production and equipment losses,injury or death of mining workers.Buckling has been regarded as one of the key mechanisms of rockbursts,which are often induced by dynamic loads from mining excavations,such as drilling and blasting in underground mining.The paper attempts to investigate the dynamic buckling mechanism of pillar rockbursts in underground mining,by considering rockbursts as a dynamic stability problem of underground rock structures.The results include:(1) A new explanation of the "sudden and violent" phenomenon of rockbursts,characterized by exponential growth of the amplitudes of transverse displacement responses,even in the presence of rock damping;(2) Identification of the critical role in inducing rockbursts of dynamic loads that bear frequencies approximately double the natural pillar frequency;(3) The greater influence on rockburst occurrence of the amplitude of dynamic component relative to the static component of loads;and(4) Quantification of the relative effects of stress waveform of dynamic loads on pillar rockbursts,which are in decreasing order if other parameters remain constant:rectangular,sinusoidal,and exponential waveforms.Application examples are provided and limitations of the approach are discussed.This research is motivated by the on-going and ubiquitous occurrence of rockbursts in underground excavations all around the world.In contrast to conventional methods that use rock specimens or rock materials to study rockbursts,this investigation emphasizes the structural effects on rockbursts,which has potential applications in hard rock mining engineering.     

大规模开采矿山地震视应力和变形与区域性危险地震预测

 大规模开采的矿山存在复杂的诱导地震活动,在高应力硬岩条件下具有产生区域性危险地震和岩爆灾害的可能性,震源的地震视应力和变形参数是地震机制和地震活动性预测研究的重要参数,对此开展研究具有重要意义。针对冬瓜山铜矿深井开采诱导地震活动,对地震视应力和位移在矿区开采过程中的时空特征开展研究,发现地震活动具有区域性分布特征,其时空分布与开采活动及矿区矿柱分布具有较强的对应关系。以此为依据,结合矿区地质构造和地应力状态,研究该矿区区域性地震机制,指出可以利用天然地震复杂震源的凹凸体理论来解释该矿区区域性地震成核机制,从而提出该矿区区域性危险性地震成核概念模型,并针对该矿地震活动的特征讨论识别成核区的方法。针对该矿区域性岩体破坏实例,分析地震视应力和变形参数时间序列特征,讨论潜在区域性地震成核区的地震活动危险性和岩体破坏性态的评价与预测。研究结果在工程实践中得到实际应用。     

Delineation of rockburst fractures with ground penetrating radar in the Witwatersrand Basin, South Africa

The depth of mining in the gold mines of the Witwatersrand Basin, South Africa is continuously increasing, with several mines already stoping at depths exceeding 3500 m. Due to the high stresses imposed on the rock mass in these deep level mines, a high potential for rockbursts exists. Rockbursts cause injuries and deaths to the workers and lost production time. One of the techniques for reducing the severity of damage of face-bursts (a class of rockbursts where the mining face is ejected into the excavation) is preconditioning. Ground Penetrating Radar (GPR) is used to quantify the change in fracture pattern with preconditioning. It is found that both the intensity and depth to which fracturing occurs ahead of the mining face increased, thereby delineating the area of influence of the preconditioning blast. This allowed the establishment of the optimum preconditioning methodology. To further enhance the understanding of the controlling factors involved, a rockburst was simulated in the tunnel sidewall of a crosscut in a deep level mine. GPR is again used to depict the fracture pattern before and after the synthetic rockburst. The radar scans show a new set of blast-hole parallel fractures, whilst nearer the sidewall, dilation and a slight increase in length of pre-existing fractures occurred. In both cases it is shown that GPR represents a useful tool for rock mechanics practitioners in defining the most suitable strategies for minimizing the damaging effects of rockbursts.     

A review of mining-induced seismicity in China

Active seismicity and rockbursting have been an emerging problem in Chinese mines. The distribution and characteristics of mining-induced seismicity in China, and its monitoring, mitigation, and research, are reviewed in this paper. Mining at depth and the activity of current tectonic stress field are the two major factors leading to rockburst hazards. Three critical depths, i.e., critical initiation depth, roof upper-bound depth, and floor lower-bound depth, have been identified based on hypocenter data of seismic events in coalmines. A strong correlation between rockbursts and gas outbursts in coalmines has also been established, and it is recommended to use this correlation for rockburst and gas outburst hazard assessment and warning interchangeably. We find that the key problems of rockburst hazard mitigation in China are the lack of mine seismicity-monitoring networks in most mines, and the need for improvement of the accuracy of the monitoring systems for mines that have been equipped with such systems. Because the demand for minerals resources is extremely high and the mining activities are progressing deeper and deeper, an increasing trend of mining-induced seismicity hazards in China may be anticipated for the near future. Mining-induced seismicities are hazards, but at the same time they have been found useful for studying geophysical problems in deep ground, particularly in the field of earthquake prediction. With the enforcement of relevant laws for the mining industry and the continuous effort to study rockburst problems using rock mechanics and geophysics principles and methods, it is believed that new approaches for rockburst hazard control and mitigation can be developed.     

Use of machine learning algorithms to assess the state of rockburst hazard in underground coal mine openings

The risk of rockbursts is one of the main threats in hard coal mines. Compared to other underground mines, the number of factors contributing to the rockburst at underground coal mines is much greater. Factors such as the coal seam tendency to rockbursts, the thickness of the coal seam, and the stress level in the seam have to be considered, but also the entire coal seam–surrounding rock system has to be evaluated when trying to predict the rockbursts. However, in hard coal mines, there are stroke or stress-stroke rockbursts in which the fracture of a thick layer of sandstone plays an essential role in predicting rockbursts. The occurrence of rockbursts in coal mines is complex, and their prediction is even more difficult than in other mines. In recent years, the interest in machine learning algorithms for solving complex nonlinear problems has increased, which also applies to geosciences. This study attempts to use machine learning algorithms, i.e. neural network, decision tree, random forest, gradient boosting, and extreme gradient boosting (XGB), to assess the rockburst hazard of an active hard coal mine in the Upper Silesian Coal Basin. The rock mass bursting tendency index W_TG that describes the tendency of the seam–surrounding rock system to rockbursts and the anomaly of the vertical stress component were applied for this purpose. Especially, the decision tree and neural network models were proved to be effective in correctly distinguishing rockbursts from tremors, after which the excavation was not damaged. On average, these models correctly classified about 80% of the rockbursts in the testing datasets.     

Prediction of rockbursts by analysis of induced seismicity data

This paper describes a technique for predicting the occurrence of strong rockbursts in mines. Both the kinetic theory of solid strength and the “rigid inclusion” model form the basis of this technique. The proposed precursor characteristics of the strong rockburst preparation process, when applied in retrospect to the North Ural Bauxite Mines induced seismicity data base show high efficiency as well as insight into the rupture initiation process.     

中国矿震受区域应力场影响的探讨

对中国矿震受区域应力场影响问题,探讨了矿震与固体潮之间的相关性,发现矿震发生与月球固体潮有较好的相关性;探讨了矿震与其附近区域天然地震在时间上的相关性,发现天然地震较矿震具有滞后性;探讨了矿震空间分布规律,发现在平面分布上发生矿震的矿井也属于天然地震的多发地带,而且几乎都分布在构造活动性强的地震带内;在深度分布上,存在着发生矿震的临界深度。矿震的发生受区域应力场的影响,区域应力场是影响矿震发生的重要因素。     

Discussions on rockburst and dynamic ground support in deep mines

The paper is a summary of discussions on four topics in rockburst and dynamic ground support.Topic1 is the mechanisms of rockburst.Rockburst events are classified into two categories in accordance with the triggering mechanisms,i.e.strain burst and fault-slip burst.Strain burst occurs on rock surfaces when the tangential stress exceeds the rock strength in hard and brittle rocks.Fault-slip burst is triggered by fault-slip induced seismicity.Topic 2 is prediction and forecasting of rockburst events.Prediction for a rockburst event must tell the location,timing and magnitude of the event.Forecasting could simply foresee the probability of some of the three parameters.It is extremely challenging to predict rockbursts and large seismic events with current knowledge and technologies,but forecasting is possible,for example the possible locations of strain burst in an underground opening.At present,the approach using seismic monitoring and numerical modelling is a promising forecasting method.Topic 3 is preconditioning methods.The current preconditioning methods are blasting,relief-hole drilling and hydrofracturing.Defusing fault-slip seismicity is difficult and challenging but has been achieved.In very deep locations(3000 m),the fracturing could extend from the excavation face to a deep location ahead of the face and therefore preconditioning is usually not required.Topic 4 is dynamic ground support against rockburst.Dynamic ground support requires that the support system be strong enough to sustain the momentum of the ejecting rock on one hand and tough enough on the other hand to absorb the strain and seismic energies released from the rock mass.The current dynamic support systems in underground mining are composed of yielding tendons and flexible surface retaining elements like mesh/screen and straps.Yielding props and engineered timber props are also used for dynamic support.     

Innovation and future of mining rock mechanics

The 121 mining method of longwall mining first proposed in England has been widely used around the world.This method requires excavation of two mining roadways and reservation of one coal pillar to mine one working face.Due to considerable excavation of roadway,the mining roadway is generally destroyed during coal mining.The stress concentration in the coal pillar can cause large deformation of surrounding rocks,rockbursts and other disasters,and subsequently a large volume of coal pillar resources will be wasted.To improve the coal recovery rate and reduce excavation of the mining roadway,the 111 mining method of longwall mining was proposed in the former Soviet Union based on the 121 mining method.The 111 mining method requires excavation of one mining roadway and setting one filling body to replace the coal pillar while maintaining another mining roadway to mine one working face.However,because the stress transfer structure of roadway and working face roof has not changed,the problem of stress concentration in the surrounding rocks of roadway has not been well solved.To solve the above problems,the conventional concept utilizing high-strength support to resist the mining pressure for the 121 and 111 mining methods should be updated.The idea is to utilize mining pressure and expansion characteristics of the collapsed rock mass in the goaf to automatically form roadways,avoiding roadway excavation and waste of coal pillar.Based on the basic principles of mining rock mechanics,the“equilibrium mining”theory and the“short cantilever beam”mechanical model are proposed.Key technologies,such as roof directional presplitting technology,negative Poisson’s ratio(NPR)high-prestress constant-resistance support technology,and gangue blocking support technology,are developed following the“equilibrium mining”theory.Accordingly,the 110 and N00 mining methods of an automatically formed roadway(AFR)by roof cutting and pressure releasing without pillars are proposed.The mining methods have been applied to a large number of coal mines with different overburdens,coal seam thicknesses,roof types and gases in China,realizing the integrated mode of coal mining and roadway retaining.On this basis,in view of the complex geological conditions and intelligent mining demand of coal mines,an intelligent and unmanned development direction of the“equilibrium mining”method is prospected.     

Numerical modeling for rockbursts:A state-of-the-art review

As the depth of excavation increases,rockburst becomes one of the most serious geological hazards damaging equipment and facilities and even causing fatalities in mining and civil engineering.This has forced researchers worldwide to identify different methods to investigate rockburst-related problems.However,some problems,such as the mechanisms and the prediction of rockbursts,continue to be studied because rockburst is a very complicated phenomenon influenced by the uncertainty and complexity in geological conditions,in situ stresses,induced stresses,etc.Numerical modeling is a widely used method for investigating rockbursts.To date,great achievements have been made owing to the rapid development of information technology(IT)and computer equipment.Hence,it is necessary and meaningful to conduct a review of the current state of the studies for rockburst numerical modeling.In this paper,the categories and the origin of different numerical approaches employed in modeling rockbursts are reviewed and the current usage of various numerical modeling approaches is investigated by a literature research.Later,a state-of-the-art review is implemented to investigate the application of numerical modeling in the mechanism study,and prediction and prevention of rockbursts.The main achievements and problems are highlighted.Finally,this paper discusses the limitations and the future research of numerical modeling for rockbursts.An approach is proposed to provide researchers with a systematic and reasonable numerical modeling framework.     

Abatement of mercury pollution in the small-scale gold mining industry: restructuring the policy and research agendas

This paper critiques contemporary research and policy approaches taken toward the analysis and abatement of mercury pollution in the small-scale gold mining sector. Unmonitored releases of mercury from gold amalgamation have caused considerable environmental contamination and human health complications in rural reaches of sub-Saharan Africa, Latin America and Asia. Whilst these problems have caught the attention of the scientific community over the past 15-20 years, the research that has since been undertaken has failed to identify appropriate mitigation measures, and has done little to advance understanding of why contamination persists. Moreover, the strategies used to educate operators about the impacts of acute mercury exposure, and the technologies implemented to prevent further pollution, have been marginally effective at best. The mercury pollution problem will not be resolved until governments and donor agencies commit to carrying out research aimed at improving understanding of the dynamics of small scale gold mining communities. Acquisition of this knowledge is the key to designing and implementing appropriate support and abatement measures.     

深埋硬岩矿床岩爆控制研究

冬瓜山铜矿是我国开采的首家有岩爆倾向的深埋硬岩金属矿山，对此开展了岩爆控制研究。应用该矿床典型岩石的变形全过程试验及峰值荷载变形条件下的松弛试验等室内试验结果和现场岩爆调查资料，分析了该矿矿岩的岩爆倾向性和井巷岩爆特性，从岩爆控制角度，采用数值分析方法以能量释放率为衡量指标，对采场结构参数、开采步骤等进行了优化。在此基础上，提出该矿床开采采用控制能量释放、减少能量储存、合理支护和进行岩爆监测等岩爆控制的原则和措施。     

Some examples of the impact of metalliferous mining on the environment: a South African perspective

Metalliferous deposits have played and continue to play a vital role in the economy of South Africa, gold being the most important. The shallow early mines, now abandoned, give rise to subsidence. As a result, the erection of buildings on the undermined land in the old reef area, which runs east to west through the city of Johannesburg, has been controlled by the Government Mining Engineer, including the permissible heights of buildings in relation to the depth of the mining. More recently, the controls have been relaxed when development proposals have been accompanied by sound methods of mine stabilization. Various types of subsidence are discussed, including both the development of sinkholes, which can take place rapidly and have catastrophic results and loss of life, and dolines, associated with dewatering activity. Induced seismicity has also been associated with gold mining. Investigations showed that the seismic events originated near working faces and occurred around the time of blasting. With magnitudes rarely exceeding 3, rarely is there significant surface damage. The conditions in which acid mine drainage related to the oxidation of sulphide minerals develops and the associated problems are also discussed. Case histories are presented to illustrate the environmental problems related directly and indirectly to the mining. Electronic Publication     

Observation of mining-induced faults in an intact rock mass at depth

Stoping of a highly-stressed remnant in a deep South African gold mine was accompanied by considerable seismic activity and some significant rockbursts. The larger seismic events were registered some 60 km away at a World-wide Standard Seismograph Network (WSSN) station. Several shear ruptures were encountered subsequently in a tunnel driven to follow the mining faces.A careful study based on detailed exploration of two of these ruptures proved them to be the source of two of the larger rockbursts. The ruptures were essentially planar features of considerable areal extent with shear displacements of up to 100 mm of a normal dip–slip sense.Certain striking features were revealed by a scanning electron microscopic study of some of the fresh cataclastic rock-flour which formed part of the comminuted filling of these ruptures. These provided strong evidence for extremely violent fracturing phenomena in the faulting process. A proper understanding of these phenomena could provide useful insight into the source mechanism of rockbursts and the control of the accompanying damage.Because of the many similarities in morphology of the faults and in the nature of the seismic signals it could be argued that the rockburst rupture is a microcosm of the pristine earthquake. Understanding of the mechanics of the rockburst shear rupture might therefore have some relevance in the study of the earthquake source.     

In the Shadow of the Gold Mines: Migrancy and Mine Housing in South Africa

The profitability of the gold mining industry has long been based upon the exploitation of the vast reserves of African migrant labour drawn from South Africa and the sub-continent. A central institution of the migrant labour force has been and continues to be the vast housing complex, where the majority of the industry's 700 000 migrant workers are housed in single-sex barrack-like accommodation under the tight control of the mining industry. Although in recent years the gold mining industry has attempted to use alternative housing strategies for its married migrant workers, today the barrack-like housing complex remains the most important institution of labour control in South Africa. This paper examines the mine housing complex and the social control mechanisms deeply embedded in the design and social organisation of migrant workers' housing in South Africa. Although the new Government of National Unity is committed to improving the housing circumstances of all South Africans, mine workers face a housing problem inherited from segregation and apartheid which requires particular attention from employers, the government and the unions representing the mine workers.     

孙村煤矿－1 100 m水平深部煤岩冲击倾向性组合试验研究

冲击地压灾害防治是深部开采面临的重要难题之一。对孙村煤矿-l100m水平(埋深1310m)延伸时煤岩的冲击倾向性进行了试验研究。为探求煤-岩层相对厚度变化对深部煤岩体冲击倾向的影响，进行了7组不同高度比的煤岩组合试件力学性质与动态破坏特性的试验研究，获得了深部煤岩冲击倾向性评判结果；研究了浸水对深部煤岩强度与冲击倾向的影响，为该矿防治冲击地压提供了试验依据。     

正交各向异性板裂屈曲岩爆机制与控制对策研究

高应力硬岩板裂化破坏现象普遍存在于工程地质和采矿工程现场,而以此诱发的板裂屈曲岩爆也给深部资源的安全高效开采带来了严峻挑战。为了分析深部高应力硬岩板裂屈曲岩爆的力学机理与控制对策,对板裂化岩体建立了正交各向异性薄板力学模型,推导出双向受力条件下板裂屈曲岩爆临界荷载值;在解出弯曲形变势能的基础之上,依据能量法求得了薄板压曲状态下的挠度值。提出采用充填法的控制对策以防治板裂屈曲岩爆的发生,并推导出充填体所需的围压值。研究结果表明:(1)针对竖向层状岩体中发生的板裂屈曲岩爆现象,应对板裂化岩体建立正交各向异性薄板力学模型进行相关力学机理的研究;(2)轴向应力的增加不仅促进了板裂化破坏的形成,还加剧了板裂屈曲岩爆发生的可能性;(3)在一定范围内,板裂体在压曲作用下的水平挠度值随板厚的减小而增大,且当长高比为E_2/E_1~(1/4)时,有最大挠度值;(4)在对采空区进行充填时,较小的充填体围压值便可以有效抑制板裂屈曲岩爆的发生。     

矿山地震活动多重分形特性与地震活动性预测

矿山地震活动可直接揭示矿山岩体活动状态和危险性水平。利用冬瓜山铜矿矿山地震监测数据,采用广义关联积分计算地震震源空间分布的分形维,利用Gutenberg-Richter关系中的b值与地震事件能量分形维关系计算地震能量分形维,并结合开采活动及矿山结构特征,研究矿山地震活动的分形结构特性和时域谱特征。研究结果表明,矿山地震活动具有多重分形特性,它与开采活动及矿山结构特点之间存在相关性;只有当计算空间区域的地震分形结构较均匀时,其分形维才能有效应用于地震活动性预测。这些空间通常对应于采掘类型少和结构单一的空间区域。地震空间分布分形维D∞反映了矿山地震活动状态,其时域谱具有较好的主震前兆特征,可以作为地震预测的参数;D2-D∞时域谱的地震前兆性较差,较难预测主震。     

基于采动顶、底板岩层损伤的冲击地压预测

为探索微震法预测冲击地压的原理和应用技术,在装备高精度微地震监测设备的煤矿,开展采掘过程连续的岩体破裂现场监测,使用自主研发的采动岩体破裂规律分析和冲击地压预测软件MapRAS进行预测研究和工程应用。发现采动过程岩体微破裂在顶板和底板的深度扩散是产生冲击地压的大概率事件;提出采动顶、底板深度损伤是冲击地压成核重要因素的观点。建立应用微震数据辨识顶、底板采动破裂损伤深度的函数关系式和算法。分析显示顶板和底板深度损伤存在联动,与顶板关键层的周期破断及其后效相对应,反映出顶板、底板的加–卸载过程,在华亭煤田多显现为巷道底板破断型冲击地压。经工程应用检验,预测效能较高,应用效果良好。