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.     

Rockburst management in Canadian hard rock mines

Rockburst is a complex mining-induced phenomenon that remains difficult to be quantified.In particular,geological conditions contributing to rockbursts are more variable than those in many conventional engineering approaches.Faults,intact rock properties,and other rock mass conditions vary,sometimes,within small distance.Stress levels that store energy or drive structures can change with geological complications.For example,faults may channel stress between the void and the discontinuity,and/or alter the local stress direction.High-strength rock masses may be very massive at shallow depth,allowing for large/stable excavations.The same rock type may store excessive strain energy at depth or high mineral extraction,and lead to rockbursts.More robust ground support systems that can bear dynamic loading are now commonplace in mining industry.A typical approach would be able to evaluate rockburst potential,and selectively deploy more robust ground support systems.Rockburst resistant support is more expensive in installation than most support systems designed for gravity loading.Thus,an excessively conservative risk assessment can affect both direct support costs and productivity.As mines continue to go deeper,selective use of burst-prone support may not be an optimal choice,and pervasive installation may become necessary.This implies a need to maintain a reasonable installation speed,and the time to get to deep orebodies is a major economic factor.This paper shows field examples of rockbursts to highlight some of the complex issues related to the rockburst.A multi-tiered risk mitigation approach is used for rockburst conditions.Examples of risk reduction strategies are given and some examples from a deep hard rock mine are used to illustrate the relative success of the strategies.Given that some residual risks to worker safety remain,discussion is presented on where more research or process improvements would be beneficial.     

Comprehensive prediction of rockburst based on analysis of strain energy in rocks

Strain energy is a key factor of inducing rockburst in mine. The occurrence of rockburst depends not only on the property of storing strain energy in rocks, but also on the environment of strain energy accumulation in mining. This investigation has been carried out on the property of strain energy stored in rocks by experiment and of which distributed around the mining pits of Linglong gold mine in China by using three-dimensional numerical modeling. In comparison with the pre-existing criteria of rockburst in literature, a comprehensive prediction of rockburst has been made so far as the ores are extracted at the depth of −570 m. The study shows that strain energy analysis with the aid of numerical modeling is useful in prediction of rockburst.     

Practices on rockburst prevention and control in headrace tunnels of Jinping II hydropower station

Rockburst problems induced by high in-situ stresses were prominent during construction of the headrace tunnels of Jinping II hydropower station. The rockbursts occurred in various forms, and it is necessary to take pertinent measures for integrated prevention and control of rockbursts. In view of the rockburst characteristics during tunnel construction of Jinping II hydropower station, the engineering geological conditions were presented, and the features, mechanisms and forms of rockbursts observed during construction were analyzed in detail. A large number of scientific researches, experiments and applications were conducted. Multiple measures were adopted to prevent and control rockbursts, including the prediction and early warning measures, stress relief by blasting in advance, optimized blasting design and optimized tunnel support in the tunnel sections prone to strong rockbursts. The effectiveness of these prevention and control measures was evaluated. Experiences have been accumulated through a great number of helpful explorations and practices for rockburst prevention and control. A comprehensive rockburst prevention and control system has been gradually established.     

Preliminary engineering application of microseismic monitoring technique to rockburst prediction in tunneling of Jinping Ⅱ project

Monitoring and prediction of rockburst remain to be worldwide challenges in geotechnical engineering.In hydropower,transportation and other engineering fields in China,more deep,long and large tunnels have been under construction in recent years and underground caverns are more evidently featured by ＂long,large,deep and in group＂,which bring in many problems associated with rock mechanics problems at great depth,especially rockburst.Rockbursts lead to damages to not only underground structures and equipments but also personnel safety.It has been a major technical bottleneck in future deep underground engineering in China.In this paper,compared with earthquake prediction,the feasibility in principle of monitoring and prediction of rockbursts is discussed,considering the source zones,development cycle and scale.The authors think the feasibility of rockburst prediction can be understood in three aspects：（1） the heterogeneity of rock is the main reason for the existence of rockburst precursors;（2） deformation localization is the intrinsic cause of rockburst;and（3） the interaction between target rock mass and its surrounding rock mass is the external cause of rockburst.As an engineering practice,the application of microseismic monitoring techniques during tunnel construction of Jinping II Hydropower Station was reported.It is found that precursory microcracking exists prior to most rockbursts,which could be captured by the microseismic monitoring system.The stress concentration is evident near structural discontinuities（such as faults or joints）,which shall be the focus of rockburst monitoring.It is concluded that,by integrating the microseismic monitoring and the rock failure process simulation,the feasibility of rockburst prediction is expected to be enhanced.     

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.     

Rockburst characteristics of several hard brittle rocks:A true triaxial experimental study

Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation.Rock lithology is a control factor of rockburst.In this paper,rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks,i.e.granodiorite,granite,marble,basalt,sandstone and limestone,under one-free-face true triaxial loading conditions.With the use of high-speed cameras,an acoustic emission(AE)system and a scanning electron microscope(SEM),rockburst of different rocks was investigated.The results show that the strainbursts of granodiorite,granite and marble were accompanied by tensile splitting near the free face,and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release.For basalt,sandstone and limestone,failure was primarily dominated by shear rupture.The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release;while no burst failure occurred on limestone due to its relatively low peak strength.Rock strength,fracturing and fragmentation characteristics,and failure modes of different rocks can significantly affect rockburst proneness and magnitude.The AE evolution coupled with SEM analysis reveals that the differences in the inhe rent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.     

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.     

Studies on the evolution process of rockbursts in deep tunnels

This paper focuses on the evolution processes of different types of rockbursts occurring in deep tunnels. A series of laboratory tests and in-situ monitoring in deep tunnels excavated by tunnel boring machine (TBM) and drill-and-blast (D&B) method have been conducted to understand the mechanisms and processes of the evolution of different types of rockbursts, including strain rockburst, strain-structure slip rockburst, immediate rockburst and time-delayed rockburst. Three different risk assessment methods are proposed to evaluate the intensity and potential failure depth of rockbursts. These methods can be applied before excavation and the results can be updated according to the real-time information during excavation. Two micro-seismicity based real-time warning systems have been established for predicting various intensities of rockbursts, such as slight, moderate, intensive and extremely intensive rockbursts. Meanwhile, the probability and intensity of the rockburst are also given. The strategy for excavation and support design has been suggested for various intensities of rockbursts before excavation. The strategy for dynamic control of the rockburst evolution process is also proposed according to the monitoring results. The methodology has been successfully applied to rockburst risk reduction for deep tunnels at Jinping II hydropower project. The results have illustrated the applicability of the proposed methodology and techniques concerning rockbursts.     

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。     

Analysis of the damage mechanism of strainbursts by a global-local modeling approach

Strainburst is the most common type of rockbursts. The research of strainburst damage mechanisms is helpful to improve and optimize the rock support design in the burst-prone ground. In this study, an improved global-local modeling approach was first adopted to study strainburst damage mechanisms. The extracted stresses induced by multiple excavations from a three-dimensional (3D) global model established by fast Lagrangian analysis of continua in 3 dimensions (FLAC3D) are used as boundary conditions for a two-dimensional (2D) local model of a deep roadway built by universal distinct element code (UDEC) to simulate realistic stress loading paths and conduct a detailed analysis of rockburst damage from both micro and macro perspectives. The results suggest that the deformation and damage level of the roadway gradually increase with the growth of surrounding rock stress caused by the superposition of mining- or excavation-induced stresses of the panel and nearby roadways. The significant increase of surrounding rock stresses will result in more accumulated strain energy in two sidewalls, providing a necessary condition for the strainburst occurrence in the dynamic stage. The strainburst damage mechanism for the study site combines three types of damage: rock ejection, rock bulking, and rockfall. During the strainburst, initiation, propagation, and development of tensile cracks play a crucial role in controlling macroscopic failure of surrounding rock masses, although the shear crack always accounts for the main proportion of damage levels. The deformation and damage level of the roadway during a strainburst positively correlate with the increasing peak particle velocities (PPVs). The yielding steel arch might not dissipate kinetic energy and mitigate strainburst damage effectively due to the limited energy absorption capacity. The principles to control and mitigate strainburst damage are proposed in this paper. This study presents a systematic framework to investigate strainburst damage mechanisms using the global-local modeling approach.     

Tempo-spatial characteristics and influential factors of rockburst:a case study of transportation and drainage tunnels in Jinping Ⅱ hydropower station

Jinping Ⅱ hydropower station is located in a high in-situ stress region in Southwest China. During the excavations of the transportation and drainage tunnels, more than 460 rockburst events were recorded in the transportation tunnel and 110 in the drainage tunnel, which has a serious and negative influence on the tunnels' construction and the safety of staff and equipment. In the paper, the characters of rockburst patterns are analyzed for the transportation and drainage tunnels. The results are illustrated...     

地下硐室岩爆的相似材料模拟试验研究

提出了模拟岩爆的新的相似系数Ｅ／λ。指出不同Ｅ／λ的材料，可模拟不同类型、不同震级的岩爆。研制出了模拟岩爆的系列脆性破坏材料，可应用于不同模拟比例、不同类型和不同震级的岩爆模拟试验。对圆形硐室的岩爆进行了模拟试验研究，证明所研制的相似材料可行，试验还证明存在岩爆发生的临界荷载，并验证了理论分析的结果     

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.     

岩体结构分析与电磁辐射监测相结合的岩爆预测技术——以乌兹别克斯坦卡姆奇克隧道为例

地下洞室施工期间的岩爆预测对保证施工人员安全,合理安排施工进度具有重要意义。提出一种岩体结构分析与电磁辐射监测相结合的岩爆预测方法,实施过程中先通过开挖面及其附近岩体结构的观察,初步判断可不可能发生岩爆,然后对初步判断可能发生岩爆的部位采用便携式电磁辐射仪进行监测,通过电磁辐射能量及脉冲的变化进一步预测会不会发生岩爆及岩爆的强度。该方法综合考虑了岩爆发生的岩体结构条件和岩爆孕育过程中的能量转换及岩体微破裂频次,具有易于现场技术人员掌握、测试方便等优点,实践证明是地下洞室施工期间岩爆预测的可行方法。结合"中亚第一长隧"--乌兹别克斯坦卡姆奇克隧道,系统介绍了岩体结构分析与电磁辐射监测相结合的岩爆预测方法中涉及的岩爆发生的岩体结构条件、电磁辐射重点监测部位及监测方法、岩爆的电磁辐射判释指标及基准值等主要内容。     

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。     

TBM施工岩爆微震监测的准确率及适用性研究

深埋隧道岩爆是困扰TBM施工安全和进度的重要因素,为此微震监测系统逐渐引入TBM施工对岩爆进行监测预警。然而,TBM施工微震监测的工程案例还很少,其监测预报的准确性和适用性存在争议。以新疆ABH工程和陕西引汉济渭工程为依托,对岩爆微震监测预报的等级和位置进行现场跟踪验证和分析,将岩爆按风险高低划分为Ⅰ、Ⅱ、Ⅲ共3个等级,给出了不同风险等级岩爆预测的准确率和适用性。结果表明：岩爆微震监测预报的准确率随着岩爆风险等级的增强而提高,对Ⅰ、Ⅱ级岩爆基本可以实现准确定位;引汉济渭工程Ⅰ级岩爆预测的准确率达到78.4 %,14.0%的Ⅰ级岩爆被预报为Ⅱ级,Ⅱ级岩爆预测准确率为55.2%,26.1%的Ⅱ级岩爆被预报为Ⅰ级;ABH工程Ⅱ级岩爆预测准确率为53.3%,46.7%的Ⅱ级岩爆被预报为Ⅲ级;Ⅲ级岩爆预测等级和定位的准确性都比较低。综合考虑岩爆预测的准确性以及施工安全、施工速度和防控措施的因素,建议：TBM工程仅存在Ⅲ级岩爆风险时,无需引入微震监测;存在Ⅰ级、Ⅱ级岩爆风险时,宜采用微震监测,以确保施工安全。     

岩爆预测的概率模型

基于岩石单轴抗压破坏特征和强度概率分布规律，首次引入岩爆烈度是岩石概率破坏表现的观点，建立岩爆烈度与岩石单轴压缩强度概率密度函数的对应关系，进而确定不同应力强度比条件下各级岩爆烈度发生概率的计算式。该预测模型能够反映洞室岩爆发生的基本规律，并可以预测不同应力条件下的各级岩爆的发生概率。工程实例分析表明该预测模型是有效的。     

Rockburst response in hard rock owing to excavation unloading of twin tunnels at great depth

This study aims to investigate the rockburst characteristics of hard rock during the successive excavation unloading of twin circular tunnels subjected to high active stresses. The entire evolution process of the rockburst phenomena around the tunnels is reproduced. The numerical results indicate that the unloading rates, burial depths, and presence of structural planes between the twin tunnels play important roles in the occurrence and damage degrees of rockbursts. The failure intensity and dynamic responses are aggregated with the increase of the unloading rate of the subsequent adjacent tunnel. The rockburst damage degree is exacerbated with increasing buried depth, and the rock response of the twin tunnels becomes more sensitive to the dynamic disturbance (as compared to a single tunnel at a great depth). The presence of a structural plane between the twin tunnels has both favourable and unfavourable effects on the stability of the surrounding rock. When the structural plane is parallel to the maximum tangential stress, the dynamic disturbance from the adjacent tunnel can be attenuated by the structural plane or rock joints via reflection and scattering, thus reducing the dynamic response between the twin tunnels. However, for those structural planes oblique to the maximum tangential stress, a violent rockburst is more prone to be induced, owing to the integrated response to shearing and sliding along the structural plane, and slabbing from the excavation unloading process. It is also found that the effect of the structural plane on the rockburst response is largely dependent on the burial depth.

     

局部能量释放率与微震相结合的岩爆动态预警指标

基于锦屏二级水电站引水隧洞、排水洞施工过程中的大量的微震信息以及不同等级的岩爆实例,运用数值计算与微震信息相结合的手段的对岩爆灾害进行研究。结果表明,局部能量释放率可以作为高地应力条件下隧洞开挖过程中围岩能量释放的指标,将其与微震监测数据相结合可以对深埋隧洞开挖过程中岩爆的强度、发生位置及范围进行预警:当局部能量释放率指标达到4×10⁵ J/m³以上,同时最大能量微震事件的能量值大于10⁵ J时,将发生强烈岩爆;当局部能量释放率指标介于10⁵~4×10⁵ J/m³之间,最大微震能量值在10⁴~10⁵ J之间时,具有中等或轻微岩爆风险;当局部能量释放率低于10⁵ J/m³,微震能量均值小于10⁴ J时,无岩爆风险。最后在施工过程中对上述预警指标进行运用,有效的抑制了岩爆灾害的发生。