Investigation of overburden behaviour for grout injection to control mine subsidence

This paper describes a field and numerical investigation of the overburden strata response to underground longwall mining, focusing on overburden strata movements and stress concentrations. Subsidence related high stress concentrations are believed to have caused damage to river beds in the Illawarra region, Australia. In the field study, extensometers, stressmeters and piezometers were installed in the overburden strata of a longwall panel at West Cliff Colliery. During longwall mining, a total of 1000 mm tensile deformation was recorded in the overburden strata and as a result bed separation and gaps were formed. Bed separation was observed to start in the roof of the mining seam and gradually propagate toward the surface as the longwall face advanced. A substantial increase in the near-surface horizontal stresses was recorded before the longwall face reached the monitored locations. The stresses continued to increase as mining advanced and they reached a peak at about 200 m behind the longwall face. A numerical modelling study identified that the angle of breakage (i.e., the angle of the boundary of caved zone) behind the longwall face and over the goaf was 22–25° from vertical direction. This is consistent with the monitoring results showing the high gradient of stresses and strains on the surface 150–320 m behind the mining face.     

Structure and deformation measurements of shallow overburden during top coal caving longwall mining

Mining induced pressures are strong and overburden failure areas are large in top coal caving longwall mining, which constrains high production and safety mining. By employing the combination of the full view borehole photography technique and the seismic CT scanner technique, the deformation and failure of overlying strata of fully mechanized caving face in shallow coal seam were studied and the failure development of overburden was determined. Results show that the full view borehole photography can reveal the characteristics of strata, and the seismic CT scanner can reflect the characteristics of strata between the boreholes. The combined measurement technique can effectively determine the height of fractured and caved zones. The top end of the caved zone in Yangwangou coal mine employing the top coal caving longwall mining was at the depth of 171 m and fractured zone was at the depth of 106-110 m. The results provide a theoretic foundation for controlling the overburden strata in the shallow buried top coal caving panel.     

Fractured zone height of longwall mining and its effects on the overburden aquifers

As mining depth becomes deeper and deeper, the possibility of undermining overburden aquifers is increasing. It is very important for coal miners to undertake studies on the height of fractured zone during longwall mining and the effects of longwall mining on the underground water while mining under surface water bodies and underground aquifers. In order to study this problem, piezometers for monitoring underground water levels were installed above the longwall panels in an American coalmine. Large amounts of pre-mining, during mining and post-mining monitoring data were collected. Based on the data, the heights of fractured zones were obtained and the effects of longwall mining on the underground water were studied. The results demonstrate that when the piezometer monitoring wells had an interburden thickness of less than 72.7 m, the groundwater level decreased immediately to immeasurable levels and the wells went dry after undermining the face of longwall. The height of the fractured zone is 72.7–85.3 m in the geological and mining conditions. The results also show that the calculated values of fractured zones by the empirical formulae used in China are smaller than the actual results. Therefore, it is not always safe to use them for analyses while mining under water bodies.     

Challenges of mining the first right-handed longwall panel in a new reserve block in Pittsburgh Seam

In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh Seam are introduced. The longwall headgate T-junction experienced very high face convergence(up to 61 cm), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded. Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter,and borehole pressure Cell(BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment,were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.     

Research on Mineral Resource Prediction by GA-ANN

1　 IntroductionThe research objectives of mineral engineer-ing are usually large systems. Compared withprocessing industry,the mineral industry has itsown features,i.e.the operation objective is natu-ral occurrence of mineral and rock,the operationsite continually moves,and mineral production isa complex process composed of multiple links andprocedures.Therefore,there are two points thatshould be emphasized in the study on mineral en-gineering optimization[1] :1 ) The research on temporary an…     

岩性圈闭综合预测技术在油气勘探中的应用

运用沉积相、测井地质学、地震信息储层预测等方法和技术,结合地震、测井信息及地质资料,建立地震相、测井相和沉积相的关系,摸清砂岩成因及分布规律是建立预测岩性圈闭模型,指导油田勘探的有效途径。实践表明,该项技术在油气勘探中取得了较好的应用效果。新发现莱10-斜7、莱2南、广2南等5个岩性圈闭,提供可钻探井位4口,完成探井2口,新增探明石油地质储量62×104t。     

Review of coal and gas outburst in Australian underground coal mines

Outburst of coal and gas represents a significant risk to the health and safety of mine personnel working in development and longwall production face areas. There have been over 878 outburst events recorded in twenty-two Australian underground coal mines. Most outburst incidents have been associated with abnormal geological conditions.Details of Australian outburst incidents and mining experience in conditions where gas content was above current threshold levels are presented and discussed. Mining experience suggests that for gas content below 9.0 m³/t, mining in carbon dioxide (CO₂) rich seam gas conditions does not pose a greater risk of outburst than mining in CH₄ rich seam gas conditions. Mining experience also suggests that where no abnormal geological structures are present that mining in areas with gas content greater than the current accepted threshold levels can be undertaken with no discernible increase in outburst risk. The current approach to determining gas content threshold limits in Australian mines has been effective in preventing injury from outburst, however operational experience suggests the current method is overly conservative and in some cases the threshold limits are low to the point that they provide no significant reduction in outburst risk. Other factors that affect outburst risk, such as gas pressure, coal toughness and stress and geological structures are presently not incorporated into outburst threshold limits adopted in Australian mines. These factors and the development of an outburst risk index applicable to Australian underground coal mining conditions are the subject of ongoing research.     

Influence of longwall mining on the stability of gas wells in chain pillars

Longwall mining has a significant influence on gas wells located within longwall chain pillars. Subsurface subsidence and abutment pressure induced by longwall mining can cause excessive stresses and deformations in gas well casings. If the gas well casings are compromised or ruptured, natural gas could migrate into the mine workings, potentially causing a fire or explosion. By the current safety regulations,the gas wells in the chain pillars have to be either plugged or protected by adequate coal pillars. The current regulations for gas well pillar design are based on the 1957 Pennsylvania gas well pillar study. The study provided guidelines for gas well pillars by considering their support area and overburden depth as well as the location of the gas wells within the pillars. As the guidelines were developed for room-andpillar mining under shallow cover, they are no longer applicable to modern longwall coal mining, particularly, under deep cover. Gas well casing of failures have occurred even though the chain pillars for the gas wells met the requirements by the 1957 study. This study, conducted by the National Institute for Occupational Safety and Health(NIOSH), presents seven cases of conventional gas wells penetrating through longwall chain pillars in the Pittsburgh Coal Seam. The study results indicate that overburden depth and pillar size are not the only determining factors for gas well stability. The other important factors include subsurface ground movement, overburden geology, weak floor, as well as the type of the construction of gas wells. Numerical modeling was used to model abutment pressure, subsurface deformations, and the response of gas well casings. The study demonstrated that numerical models are able to predict with reasonable accuracy the subsurface deformations in the overburden above,within, and below the chain pillars, and the potential location and modes of gas well failures, thereby providing a more quantifiable approach to assess the stability of the gas wells in longwall chain pillars.     

Volumetric changes in the focal areas of seismic events corresponding to destress blasting

The typical development of total volumetric change in the focal areas of seismic events, corresponding to destress blasting, is characterized as an explosive phase followed by an implosive phase and with alternating additional phases following on from that. In a few cases, a non-typical development of volumetric change was identified, where the first phase was implosive and the second phase, explosive. This development is mainly typical for induced seismic events recorded during mining, not for destress blasting.Seismic events were recorded during longwall mining in the Czech part of the Upper Silesian Coal Basin, where the destress blasting technique is used as a rockburst prevention active measure.Kinematic source processes in the focal areas of selected seismic events were analyzed by the seismic moment tensor inversion method, as well as by studying geomechanical rock mass conditions at the localities of the seismic events. The main goal of the analysis was to attempt to identify the reasons for non-typical development of volumetric changes in these cases. Volumetric changes were analyzed for seismic events with energy greater than 104 J, recorded in the period of time from 1993 to 2009(1109 events). 80%(891) of the recorded seismic events were induced seismic events that were registered during longwall mining and 20%(218) corresponded to destress blasting events. Research shows that the main reason for the non-typical development of volumetric changes in the focal areas of seismic events is an association with destress blasting in the rock mass, which is very close to rock mass overstressing. The detonation of explosives in boreholes, which would dominate the first phase of volumetric changes, probably obscured stress release in the rock mass, as manifested in the first implosion phase of the volumetric changes in this case.     

Impact of geo technical factors on strata behavior in longwall panels of Godavari Valley coal field-a case study

Understanding the cantilevers formed by thick, massive beds in the near-seam overburden above longwall panels and the associated loads and strata fracturing effects developed during caving(main and periodic weightings) are key elements for the successful implementation of longwalls. Such caving mechanisms rely on the geotechnical conditions within the panel. In India, the majority of longwall downtime and/or roof failures were caused by a lack of knowledge on overburden caveability, in particular when the main and periodic weightings will impact the face and longwall support selection to effectively mitigate such weightings. Godavari Valley Coal Fields is no exception as longwall faces were adversely affected due to the presence of thick, massive beds in the near-seam overburden at both Godavarikhani(GDK) 7 and 9 Incline mines. In contrast, overburden weightings were negotiated successfully in GDK10 A and Adriyala Longwall Project(ALP) mines by detailed geotechnical studies, the use of adequate longwall support capacities, and effective operational practices. Thirteen longwall panels with varying face width, at different depths have been extracted under massive sandstone beds of 18 m to28 m thickness at GDK 10 A and ALP mines. This study elucidates that the main roof weighting interval decreases with an increase in face width and attains a constant value with further increases in face width under the same geo-mining conditions. In addition, this study also concludes that with increases in face width, the periodic roof weighting interval decreases and shield loads increase. Similarly with increasing panel width to depth ratio, the main and periodic roof weighting intervals decrease but shield loads again increase. Lastly, the strata behaviour of the longwall face retreated along up-dip direction is demonstrated. The results of this paper improves the mechanistic understanding of the impact of face width,depth and main roof thickness on periodic weighting and associated roof control problems on the longwall face.     

Pillar design and coal burst experience in Utah Book Cliffs longwall operations

Longwall mining has existed in Utah for more than half a century. Much of this mining occurred at depths of cover that significantly exceed those encountered by most other US longwall operations. Deep cover causes high ground stress, which can combine with geology to create a coal burst hazard. Nearly every longwall mine operating within the Utah's Book Cliffs coalfield has been affected by coal bursts. Pillar design has been a key component in the burst control strategies employed by mines in the Book Cliffs.Historically, most longwall mines employed double-use two-entry yield pillar gates. Double-use signifies that the gate system serves first as the headgate, and then later serves as the tailgate for the adjacent panel. After the 1996 burst fatality at the Aberdeen Mine, the inter-panel barrier design was introduced.In this layout, a wide barrier pillar protects each longwall panel from the previously mined panel, and each gate system is used just once. This paper documents the deep cover longwall mining conducted with each type of pillar design, together with the associated coal burst experience. Each of the six longwall mining complexes in the Book Cliffs having a coal burst history is described on a panel-by-panel basis.The analysis shows that where the mining depth exceeded 450 m, each design has been employed for about 38000 total m of longwall panel extraction. The double-use yield pillar design has been used primarily at depths less than 600 m, however, while the inter-panel barrier design has been used mainly at depths exceeding 600 m. Despite its greater depth of use, the inter-panel barrier gate design has been associated with about one-third as much face region burst activity as the double-use yield pillar design.     

A coal rib monitoring study in a room-and-pillar retreat mine

The National Institute for Occupational Safety and Health(NIOSH) conducted a comprehensive monitoring program in a room-and-pillar mine located in Southern Virginia. The deformation and the stress change in an instrumented pillar were monitored during the progress of pillar retreat mining at two sites of different geological conditions and depths of cover. The main objectives of the monitoring program were to better understand the stress transfer and load shedding on coal pillars and to quantify the rib deformation due to pillar retreat mining; and to examine the effect of rib geology and overburden depth on coal rib performance. The instrumentation at both sites included pull-out tests to measure the anchorage capacity of rib bolts, load cells mounted on rib bolts to monitor the induced loads in the bolts, borehole pressure cells(BPCs) installed at various depths in the study pillar to measure the change in vertical pressure within the pillar, and roof and rib extensometers installed to quantify the vertical displacement of the roof and the horizontal displacement of the rib that would occur during the retreat mining process.The outcome from the monitoring program provides insight into coal pillar rib support optimization at various depths and geological conditions. Also, this study contributes to the NIOSH rib support database in U.S coal mines and provides essential data for rib support design.     

Simulation of recovery of upper remnant coal pillar while mining the ultra-close lower panel using longwall top coal caving

With the depletion of easily minable coal seams, less favorable reserves under adverse conditions have to be mined out to meet the market demand. Due to some historical reasons, large amount of remnant coal was left unrecovered. One such case history occurred with the remnant rectangular stripe coal pillars using partial extraction method at Guandi Mine, Shanxi Province, China. The challenge that the coal mine was facing was that there is an ultra-close coal seam right under it with an only 0.8–1.5 m sandstone dirt band in between. The simulation study was carried out to investigate the simultaneous recovery of upper remnant coal pillars while mining the ultra-close lower panel using longwall top coal caving(LTCC). The remnant coal pillar was induced to cave in as top coal in LTCC system. Physical modelling shows that the coal pillars are the abutments of the stress arch structure formed within the overburden strata. The stability of overhanging roof strata highly depends on the stability of the remnant coal pillars. And the gob development(roof strata cave-in) is intermittent with the cave-in of these coal pillars and the sandstone dirt band. FLAC3 D numerical modelling shows that the multi-seam interaction has a significant influence on mining-induced stress environment for mining of lower panels. The pattern of the stress evolution on the coal pillars with the advance of the lower working face was found. It is demonstrated that the stress relief of a remnant coal pillar enhances the caveability of the pillars and sandstone dirt band below.     

Accuracy of forecast of mine tremors location

The Upper Silesian Coal Basin is one of the most active mining areas in the world in respect of seismicity. Underground mining in this area takes place in a special environment with a high degree of risk of unpredictable event occurrence. Especially dangerous are phenomena that occur during the extraction of deposits at great depths in the environment of compact rocks. Deep underground mining violates the balance of these rocks and induces dynamic phenomena at the longwall life (in terms of distance) referred to as mine tremors. The sources of these tremors are located in layers characterised by high strength, especially in thick sandstone strata occurring in the roof of the mined seam. In the paper a discussion is presented about the influence of mining inten-sity (longwall face speed) on the location of mine tremor sources, both in the direction of longwall life (in terms of distance) and towards the surface. The presented material has been prepared basing on the results of tests and measurements carried out at the Central Mining Institute.     

Effects of overburden characteristics on dynamic failure in underground coal mining

Dynamic failures, or ‘‘bumps", remain an imperative safety concern in underground coal mining, despite significant advancements in engineering controls. The presence of spatially discrete, stiff roof units are one feature that has been linked to these events. However, an empirical stratigraphic review indicates that no significant difference exists in the relative commonality of discrete units between bumping and non-bumping deposits. Instead an apparent relationship exists between reportable bumping and the overall stiffness of the host rock. However, this initial study is too simplistic to be conclusive; to weight the relative impact of changes in a single variable, such as the thickness or location of sandstone members, it must be examined in isolation—i.e., in a setting where all other variables are held constant.Numerical modelling provides this setting, and the effects of variability in a stiff discrete member in a hypothetical longwall mining scenario are investigated within the context of three stratigraphic ‘‘types",Compliant, Intermediate and Stiff. A modelling experiment examines changes in rupture potential in stiff roof units for each stratigraphic type as discrete unit thickness and location are manipulated through a range of values. Results suggest that the stiff-to-compliant ratio of the host rock has an impact on the relative stress-inducing effects of discrete stiff members. In other words, it is necessary to consider both the thickness and the distance to the seam, within the context of the host rock, to accurately anticipate areas of elevated rupture-induced hazard; acknowledging the presence of a discrete unit within the overburden in general terms is an insufficient indicator of risk. This finding helps to refine our understanding of the role of individual stiff, strong roof members in bumping phenomena, and suggests that a holistic view of overburden lithology and site-specific numerical modelling may be necessary to improve miner safety.     

近水平露天煤矿端帮靠帮开采方式与剥采比研究

运用露天采矿学的基本原理,提出了端帮靠帮开采的基本方法、实现形式、基本条件以及靠帮开采的意义,给出了靠帮开采剥采比的概念;在单一煤层和复合煤层情况下,推导出了2种端帮靠帮开采方式的靠帮开采剥采比计算公式,建立了不同煤层厚度和覆盖物厚度对靠帮开采剥采比影响趋势预测的数学模型和图示方法.结果表明：在近水平露天煤矿实施端帮靠帮开采,可以降低剥采比,提高煤炭资源回收率和经济效益,同时也丰富了露天开采方法与设计理论.     

The current perspective of the PA 1957 gas well pillar study and its implications for longwall gas well pillars

Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells. The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span. At the time, coal was mined using the room-and-pillar mining method with full or partial pillar recovery, and square or rectangle pillars surrounding the gas wells were left to protect the wells. The study provided guidelines for pillar sizes under different overburden depths up to213 m(700 ft). The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s. The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover. However, under deep cover, severe deformations in gas wells have occurred in longwall chain pillars. Presently, with a better understanding of coal pillar mechanics, new insight into subsidence movements induced by retreat mining, and advances in numerical modeling, it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars. In this paper, the data from the 1957 study is analyzed from a new perspective by considering various factors, including overburden depth, failure location, failure time, pillar safety factor(SF), and floor pressure. The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery. A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining. Through analyzing the data from the 1957 study, the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining. Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings. Results of this study indicate that the guidelines in the 1957 study may be appropriate for pillars protecting conventional gas wells in both room-and-pillar mining and longwall mining under overburden depths up to 213 m(700 ft),but may not be sufficient for protective pillars under deep cover. The current evaluation of the 1957 study provides not only insights about potential gas well failures caused by retreat mining but also implications for what critical considerations should be taken into account to protect gas wells in longwall mining.     

Underground mine stream crossing assessment: A multi-disciplinary approach

Underground mine designs typically try to avoid extraction beneath streams and rivers of any significant size, especially when the overburden rock thickness between the stream bed and the mine is thin.Potential issues with mining beneath streams include excessive groundwater inflow to the mine, weak ground(roof, floor, and pillar) conditions, horizontal stress effects, as well as stream loss and other potential adverse environmental effects. However, there are times when crossing beneath a stream or river is necessary to move into a new area of mineral reserve without creating additional mine access points from the ground surface. Often, stream crossings are completed without thorough assessment, potentially resulting in increased costs, decreased safety, and, in some cases, failure to advance the mine.Selection of the most favorable location(s) to cross the stream must account for numerous factors and the associated assessment often requires a multi-disciplinary approach. Stream crossing investigations often require geological, hydrogeological, geotechnical, and geophysical expertise. Phases of stream crossing investigations include desktop evaluation of maps and aerial photography, stream bed observations, drilling, detailed rock core logging, downhole geophysical surveying, hydraulic conductivity testing(packer testing), geotechnical laboratory testing, assessment, and reporting. The deliverables from a stream crossing assessment typically include geological, geotechnical, and hydrogeological characterization of potential stream crossing locations, classification of favorable and unfavorable crossing locations,recommendations for entry design and pillar sizing, and recommendations for if, and how, to conduct pre-grouting activities. Examples of technical aspects of data collection and assessment are provided based on decades of industry experience conducting stream crossing assessments in various underground mining scenarios.     

Fundamentals of modern ground control management in Australian underground coal mines

Underground coal mining is inherently hazardous, with uncontrolled ground failure regarded as one of only several critical risks for multiple fatality events. Development, implementation and management of overarching systems and procedures for maintaining strata control is an important step to mitigating the impact of ground failure hazards at a mine site operational level. This paper summarised the typical pro-active ground control management system(PGCMS) implemented in various Australian underground coal mines. Australia produces approximately 100 million tonnes a year of metallurgical and thermal coal from approximately 30 of the world's safest longwall mines operating in New South Wales and Queensland. The increased longwall productivity required to achieve both high levels of safety and profitability, places significant emphasis on the reliability of pro-active ground control management for longwall mining operations. Increased depths, adverse geological conditions, elevated variable stress regimes and weaker ground conditions, coupled with an industry wide need for increased development rates continue to make ground control management challenging. Ground control management is not only about ground support and pillar design though but also a structured process that requires a coordinated effort from all levels of the workforce to both minimise the occurrence of adverse geotechnical events and mitigate the potential risks when they do occur. The PGCMS presented in this paper is proven to provide both a safer and more productive mine environment through minimisation of unplanned delays. The critical elements of the method are presented in detail and demonstrate the utility and value of a ground control management system that has potential for implementation in underground coal mining globally.     

采煤面顶板覆岩破坏模拟试验研究

采煤工作面开采过程中"两带"高度值是指导矿井防水煤柱留设、保证矿井安全生产的重要参数,试验以内蒙古某矿3-1煤层首采面为地质模型,通过模拟其开采过程,运用并行电法系统,确定"两带"高度值及其变化特征。同时在给定边界条件和初始条件下,运用数值模拟方法,获得顶板应力变化分布图。综合分析模拟试验结果,判定该工作面回采过程中垮落带高度为40m,导水裂缝带最大高度为80m。根据不同时间电阻率分布图和垂向应力分布云图,可清晰分辨出煤层顶板覆岩破坏的过程和规律,可为煤矿安全生产提供参考。