Search Documents

By P. Oliver

Rockburst conditions were first encountered at Inco's Sudbury area operations in the mid 1930's when mining advanced to below the 610 m (2000 ft.) horizon. Now, mining is being carried out at depths down to 2130 m (7000 ft.) at Inco'a Creighton Mine. While rockburst potentials have increased with depth. improvements in mine planning, methods, support systems and developments in destressing have alleviated the condition to where it is less of a problem now than it was in the early years. Hang of the improvements were developed, of necessity. through observation and reasoning because these were the only tools available at the time. Now, we are making use of stress - measurements, numerical modelling and microseismic source location technology to - help further reduce rockburat hazards.

Jan 1, 1987

By Sean C. Farrell

In underground mining and tunneling, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Australian companies WDS Limited and Cram Fluid Power, the engineering department of J.H. Fletcher & Company was tasked to design and manufacture roof bolting modules to be mounted off both sides of a roadheader, providing in-cycle bolting. The objective was to produce a boom and bolt module that would allow an operator to bolt from a remote location, eliminating the need for miner to work in unsupported ground. The bolt module needed to operate in front of the cutting boom, but retract to a parked position out of the way during the cutting cycle. Due to Australian electrical approval requirements, it was desirable to design the module with minimal use of electrical controls. The need for multiple, complex, drill steel and bolt handling steps led to the need for a custom hydraulics package to automate several of the movements. The result of the design is dual drill and bolt modules mounted to a multistage boom with over 24 feet (7.31 m) of extension. Each unit rotates and tilts to meet the bolt pattern requirements in conjunction with the stow and load requirements. The bolt module builds on previous designs incorporating a modular assembly with each sub-component designed to be replaceable and adjustable independently from other machine components. Each major component is painted a different color to help the operator make the connection visually from the module to the operator?s control valves. Hydraulic package valves were designed to automatically control several of the sub-components during the drill cycle, reducing the number of steps a drill operator would, otherwise, have to do manually. This design removes the operator from working in unsupported roof and away from other inherent hazards, putting them at a safe distance on the operator?s platform. The cutting and bolting equipment has been combined into one machine, which is advantageous in single entry drifts because there is no need to place change equipment. The hydraulic package valves eliminate the need for any electronics, thus avoiding any electrical regulations with respect to the controls. The addition of these modules to the roadheading machine increases miner safety and streamlines production while meeting the ground control support requirements.

Jan 1, 2014

By Zhang Hongzhen

The fast convergence of boundary is a major problem of probability integral method, especially for the prediction of mining under thick alluvium, abandoned mine goaf, etc. To solve this problem and improve the accuracy of probability integral method, a modified model is constructed for surface movement and deformation based on theoretical analysis. In which, three weighted values i.e. 1, -1/t1, 1/t2 are set for the mining movement and deformation of three different major influence radius i.e. r1, R1, R2, respectively, then the parameters are selected as following: (1) Parameters are inversed based on the observation data, in which the subsidence factor q, deviation of inflection point s, displacement factor b are considered as generic parameters, the tangent of major influence angle tanß is modified as tanß2, that is the major influence radius r is modified as r2. (2) Inversion parameters of tangent of major influenceangle tanß or major influence radius r, based on the observation station edge data which beyond major influenc eradius distance from work-face mining boundary. The inversed tanß and r will be used as tangent of major influence angle tanß1 and major influence radius r1, respectively. (3) Other relevant parameters i.e. t1, t2, R1, R2, are calculated with the algorithm of this paper. A comparison experiment between the traditional and modified models has been conducted using the observation data from a observation station, the results show that in its central area, 58.6 percent of observation points accuracy is improved and 36.6 percent is decreased, while in its edge area, 61.5 percent is improved and 15.4 percent is decreased, totally, 68.6 percent is improved and 30 percent is decreased. Therefore, we can draw the conclusion that our modified model can improve the prediction predication accuracy, in particular, the boundary region.

Jan 1, 2014

By Weijia Guo

With the exploitation of coal resources, a large area of land had been affected by surface subsidence, causing conflicts between man and land to become more conspicuous by the day, affecting the life of general public, and constraining further development of the land. This paper predicts the subsidence areas according to the 10-year mining plan of the Jibei mining district as an example. A plan of environmental management was developed based on sciences and practical economics.

Jan 1, 2006

By Pinnaduwa H. S. W. Kulatilake, Biao Shu

The intact rock properties and discontinuity properties for both Devonian Rodeo Creek (DRC) and Devonian Papovich (DP) rock formations that exist in the selected open-pit mine were determined from tests conducted on rock samples. Special survey equipment, which has a total station, laser scanner and a camera, was used to perform remote fracture mapping in the research area. From remote fracture mapping data, the fracture orientation, spacing and density were calculated using more refined procedures compared to what exist in the literature. Discontinuity orientation distributions obtained through remote fracture mapping agreed very well with the results of manual fracture mapping conducted by the mining company. The GSI rock quality system and Hoek-Brown failure criteria were used to estimate the rock mass properties combining the fracture mapping results with laboratory test results of intact rock samples. Fault properties and the DRC-DP contact properties were estimated based on the laboratory discontinuity test results. A geological model was built in a 3DEC model including all the major faults, DRC-DP contact and two stages of rock excavation. The built major discontinuity system of 44 faults in 3DEC with their real orientations, locations and three-dimensional extensions were validated successfully using the fault geometry data provided by the mining company using seven cross-sections. Numerical modeling was conducted to study the effect of boundary conditions and lateral stress ratio on the stability of the considered rock slope. For the considered section of the rock slope, the displacements obtained through stress boundary conditions seemed more realistic than those obtained through zero velocity boundary conditions (on all four lateral faces). Stable deformation distributions were obtained for k0 in the range of 0.4 to 0.7. Because the studied rock mass is quite stable, it seems that an appropriate range for k0 for this rock mass is between 0.4 and 0.7. The displacements occurred between July 2011 and July 2012 due to the nearby rock mass excavation that took place during the same period; they were compared between the field monitoring results available from the mining company and the predicted numerical modeling results and a good agreement was obtained. Overall, the successful simulation of the rock excavation during a certain time period indicated the possibility of using the procedure developed in this study to investigate rock slope stability with respect to expected future rock excavations in mine planning.

Jan 1, 2015

By Lou Gaozhong, Guo Wenbing, Zhao Gaobo, Wang Shuren

"The height of a fractured zone due to high-intensity mining (HIM) plays a vital role in the safety analysis of coal mining under bodies of water. This paper described the characteristics of HIM. The processes of overburden failure transfer (OFT) are analyzed. The state of destruction transfers from a certain stratum to the upper stratum as face advance distance increases, and the processes of OFT are divided into two stages: transmission development stage and transmission termination stage. Through theoretical analysis, the limited suspension-distance and limited overhanging distance of each stratum are proposed to judge the damage of each layer. Based on this, the OFT criteria of fractured zone development are obtained. The mechanical model of strata suspended integrity and overhanging stability are established. Finally, a new method to predict the height of fractured zone due to HIM is put forward based on the processes of OFT, the limited suspension-distance, and the limited overhanging distance. Further, a HIM panel in the Datong coal mine is taken as an example. Theoretical methods are proposed, and engineering analogy is used to predict the height of the fractured zone, which is compared with in-situ measurements reported in the literature. The results show that the in-situ measurements for the height of the fractured zone are in close agreement with the theoretical calculation result. Therefore, the rationality and reliability of the theoretical method proposed to predict the height of fractured zone is verified.INTRODUCTIONThe height of the fractured zone (HFZ) due to the exploitation of coal is a key parameter for the safety analysis of coal mining under a body of water (Wenbing, Youfeng, and Hou, 2012). Accordingly, it is necessary to predict HFZ under the corresponding mining conditions before coal mining."

Jan 1, 2018

By Pinnaduwa H. S. W. Kulatilake

To exploit an underground mine effectively and safely, it is important to have a good understanding of the geotechnical behavior of the rockmass surrounding the underground mine excavation. The aim of this study was to investigate, at the threedimensional level, the geotechnical behavior around a tunnel excavated in a metal mine in China. A three-dimensional numerical model was built by using the 3DEC software package to simulate a tunnel excavation under a high in situ stress condition and by relying on the information available on lithology, geological structures, in situ stress measurements, physical and mechanical properties of intact rock, discontinuities, and interfaces between different rocks. The authors also investigated the effect of the discontinuity network, possible intact rock and discontinuity parameter variability, representation of rock masses as discontinuum or equivalent continuum material and rock support system on the deformation, and stability around the tunnel. A very good agreement was obtained between the collected field deformation monitoring results and the results of the conducted numerical stress analyses with appropriate geomechanical property values.

Jan 1, 2013

By Michael Murphy

A multi-stage triaxial procedure has been developed for the enhanced servo controls and feedback systems at the National Institute for Occupational Safety and Health (NIOSH) rock mechanics laboratory. Due to the difficulty of preparing multiple rock specimens for single-stage triaxial tests at different confinements, multi-stage testing provides a huge advantage if only a few samples of coal measure rocks can be prepared from the drill core. A multi-stage test can provide mechanical properties, post-failure properties, and a complete failure envelope for a single specimen. For the multi-stage test procedure, a parameter used to define the slope of the stress-radial strain curve was developed and monitored in real time in order to successfully predict peak strengths at different confining stages. For this paper, multi-stage tests have been conducted on sandstones of varying strengths, including Berea sandstone which has well-documented properties, and fine-grained and coarse sandstone from a western US coal mine. For the tests conducted on the fine-grained sandstone, it was found that the ramp rate was an important factor in the success of obtaining peak strengths during the multi-stage tests. For each rock type, a table including Young?s modulus, Poisson?s ratio, unconfined compressive strength, friction angle, cohesion, residual friction angle, and residual cohesion are reported. The values from these tables have been used as input parameters for projects at NIOSH requiring numerical model input parameters.

Jan 1, 2014

By Eric Zahl

Researchers at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, Spokane, WA, have collaborated with three Western longwall coal mines in an ongoing effort to develop technologies that will aid in providing safe and stable working areas. The goal of the research described here is to develop a stress monitoring system that will provide immediate information to mine managers for making daily safety decisions as areas of poor ground are mined through. Initial work has focused on answering preliminary questions regarding the reliability and use of stress change patterns. Research is concentrated on monitoring horizontal stress because horizontal stress is transmitted over long distances through stiff strata, thus allowing an extended length of entry to be monitored. This paper presents an explanation of the concept, key results from field tests at two mine sites, and a proposed process for imple¬menting a monitoring system. System design layouts, instrument use, data collection and interpretation methods, and processes to present findings to mine staff are described. Additional validation and correlation with actual failure mechanisms are required before this approach can be recommended at ongoing operations. How¬ever, initial results indicate that this approach shows promise in mines prone to bumps and roof falls associated with large stress changes. In particular, analyses of measured changes in horizontal stresses appear useful.

Jan 1, 2002

By Frederick Carr

1. The application of retreat longwall mining to the North American coal mining conditions is be¬coming more advantageous as more experience is gained. The productivity results being achieved and potential possible illustrates that this system can be the keystone of future underground coal production. 2. In the American coal fields multi-entry is normally practiced as this allows profitable longwall development with existing equipment and systems. Difficulties are being experienced however in the longwall retreat phase by the col¬lapse of multi-entry roadways and excessive use of supplemental support in the following panel tailgates. In some instances the headgate of an operating face can also be in difficulty. 3. Jim Walter Resources Mining Division is developed on a mining plan that calls for three retreat longwalls at each of its four deep mines which have varying depths of cover and immediate roof and floor conditions. The #3 Mine has experienced longwall entry difficulty and there were differing viewpoints with respect to the best development systems to adopt particularly regarding the number of entries and pillar sizes. In an attempt to reconcile these different points of view J.W.R. asked Dr. Wilson to examine the problem and this paper outlines the new approach developed.

Jan 1, 1982

By Liang Dong, Jinxuan Liu, Biao Qiu, Jing Chai

"The Cenozoic unconsolidated strata in the east area of China are composed of clay, sandy clay, fine sand, hardpan, etc. Numerous shafts of the coal mines in these areas have been damaged in various degrees since 1987. The main reason for this disaster is the vertical additional stress on shaft caused by the subsidence of the thick unconsolidated strata. In order to monitor the subsidence deformation of the unconsolidated strata, a sensing network system with multiplexed Fiber Bragg Grating (FBG) for monitoring settlement deformation of the unconsolidated strata in deep borehole is proposed. The field monitoring results show that, soil layers settle or expand with decrease or increase in ground water levels. The thick hardpan generates the main compressive deformation. This FBG strain monitoring method can take place of traditional ground water measuring method. This system is stable and reliable, and the monitoring results can show the subsidence deformation of different layers, which has significance for the predictive maintenance of the shafts.INTRODUCTIONSince 1980s, a specific geologic hazard has occurred frequently on the coal mine shafts in eastern China: non-mining-induced shaft deformation, involving up to 100 shafts located in Jiangsu, Shandong, Anhui and Herran province, distributed in Xuzhou, Datun, Huainan, Huaibei, Yanzhou and Yongxia coal fields. This kind of geologic hazard can lead to shaft wall deformation, water infiltration, cage stuck, water-sand burst or shutting down of coal mines, which seriously risks the coal mines. Zhangshuanglou coal mine, Haizi coal mine and Linhuan coal mine shut down for 2-8 months due to this hazard (Chen, 1997; Cui, 1998). The main reason for this geologic hazard is the subsidence of the super thick unconsolidated strata (Liu et al., 2007; Xu et al., 2006).An innovative structure health ·monitoring and damage estimation method has been developed in recent years, i.e., optical fiber smart material and structure (Nobuo, 2002; Wang et al., 2000). A Fiber Bragg grating (FBG) sensor is a device that can transfer the change of the measured strain or temperature to the change of Bragg wavelength. One of the most important applications of FBG sensors is to monitor the internal stress and strain distribution of material and structure (Philipp et al., 2000; Ansari et al., 1998). Compared to traditional electrical sensors, FBG sensors have many advantages; they are adaptable to harsh environments, abrasion resistant, immune to electrical magnetism (EM) interference, safe and reliable. They also offer integrated sensing and transmission on a single fiber, have simple structure and are convenient to use, which satisfies the requirements of long distance, distributional and long term use in monitoring mining and geotechnical engineering structure. Due to such advantages, the FBG sensor has already been applied in petroleum engineering (Kersey, 2000), rock mechanics testing (Cornelia et al., 2003; Yang et al., 2007; Wei et al., 2008), physical simulation experiments (Chai et al., 2007; Chai et al., 2006; Zhu et al., 2008; Li et al., 2008; Chang et al., 2008), strain sensor (Li et al., 2007; Chai et al., 2008) and construction structure damage diagnosis (Dong et al., 2004; Kerrouche et al., 2008; Whitten et al., 2001)."

Jan 1, 2010

By Hongwei Zhang, Sen Yang, Shimin Liu

"This laboratory study investigates the damage evolution of sandstone specimens under two types of cyclic loading conditions through monitoring and analyzing the variations of elastic moduli and ultrasonic velocities. We adopted the rigidity degradation method for evaluating the damage accumulation through the rock dynamic ultrasonic velocity measurements. Compared to the ultrasonic test, the method of rigidity degradation shows deficiencies in the detection of damage development. The cyclic loading within the elastic limit led to a stiffening trend for the loading process but a softening trend for unloading process. The method of rigidity degradation was unable to describe the damage accumulation in this situation, while P-wave velocity clearly reflected the damage development. Moreover, a simplified model is established for interpreting the behaviors of the modulus variations and ultrasonic velocity. The cyclic loading within elastic limit enhances the anisotropy of rock, which causes a denser packing pattern in the direction of axial stress. The persistent damage accumulation under the cycling with an increasing upper limit is the consequence of the tensile failure in the microstructure of sandstone. INTRODUCTION The damage accumulation due to dynamic (cyclic) loading is highly detrimental in many engineering structures, such as mine openings, petroleum and natural gas boreholes, tunnels, rock foundations, and underground chambers. The sources of cyclic or repetitive loadings can be roughly divided into two types: (1) periodic operation, including drilling, blasting, and mining; (2) random vibration, including earthquakes and traffic loads. The repetitive loading-unloading process opens and closes the micro-pores and micro-cracks within the rock, further inducing the growth of cracks. As expected, the accumulating damage becomes a potential factor to trigger the rapid and violent failures of large-scale engineering rock structure."

Jan 1, 2017

By Francis S. Kendorski

High-extraction mining of coal and other stratified minerals has a predictable effect on the surface and subsurface. Impacts on ground waters in overlying strata and on surface waters, that are associated with overlying sequential zones of collapse, fracturing, aquiclude, constrained strata, and surface disturbances can be anticipated. Since first quantified around 1979 in Bureau of Mines sponsored contract research work and others (and often adopted by state regulators), with zone extents based upon multiples of mined seam height of 24 to over 60 for the fractured zone, much new data have become available. In examining the sources of data and the methods and intents of the researchers of over 65 case histories, it became apparent that the strata behaviors were being confused with overlapping vertical extents reported for the fractured zones and aquiclude zones depending on whether the researcher was interested in water intrusion into the mine or in water loss from surface or ground waters. These more recent data, and critical examination of existing data, have led to the realization that the former "Aquiclude Zone" defined for its ability to prevent or minimize the intrusion of ground or surface waters into mines has another important character in increasing storage of surface and shallow ground waters in response to mining with no permanent loss of waters. This zone is here named the "Dilated Zone." The existence of the dilated zone explains the ratios of up to 60 times the mined thickness. Surface and ground waters can drain into this zone, but seldom into the mine, and can eventually be recovered through closing of dilations by mine subsidence progression away from the area, or filling of the additional void space created, or both. A revised model has been developed which accommodates the available data, by modifying the zones as follows: ? collapse and disaggregation extending 6 to 10 times the mined thickness above the panel ? continuous fracturing extending approximately 24 times the mined thickness above the panel, allowing temporary drainage of intersected surface and ground waters ? development of a zone of dilated, increased storativity, and leaky strata with little enhanced vertical permeability from 24 to 60 times the mined thickness above the panel above the continuous fracturing zone, and below the constrained or surface effects zones, whose thickness or existence is dictated by the zones above and below ? maintenance of a constrained but leaky zone above the dilated zone and below the surface effects zone, whose thickness or existence is dictated by the zones above and below ? limited surface fracturing in areas of extension extending up to 50 ft or so beneath the ground surface

Jan 1, 1993

By Wenbing Guo, Yi Tan, Erhu Bai

"It is important to study the mining technology under structures for raising the coal resources recovery ratio. Based on the geological and mining conditions, the top coal caving harmonic mining technique in thick coal seam beneath the earth dam was put forward and studied. The 5 factors such as the panel mining direction, panel size, panel location, panel mining sequence and panel advance velocity were taken into account in this technique. The dam movement and deformation were predicted after the thick coal seam mining and the effects of mining on the dam were studied. By setting up the surveying stations on the dam, the movement and deformation of the dam were observed during mining. By taking some protective measures on the dam, the top coal caving mining technique in thick coal seam beneath the earth dam was carried out successfully. The study demonstrates that harmonic mining in thick coal seam is feasible under the dam. The safety of the earth dam after mining was ensured and the coal resources recovery ratio was improved.The surface movement and deformation caused by coal mining not only influence the ecology environment in the mining area, but damage the surface structures such as buildings, railroads, highways, water bodies and damsr1-31_ The movement of strata and surface caused by mining had disturbed various aspects of industrial and agriculture productions in the mining arear41. The earth dam is a very important water retaining structures, so it must makes sure the safety of dam if mining under it. Therefore, to study the effect of mining on dam and control measures to ensure safety of mining under the dam are important factors for improving the mining recovery ratio, reliving the mining continuity, preventing or reducing damage to the dam.By means of the harmonic mining technologyrs1, theoretical analysis and field observation, the mining method under an earth dam and the mining influences were studied and analyzed in this paper. The mining of a thick coal seam under the dam was carried out safely and successfully."

Jan 1, 2016

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994

By Stephen C. Tadolini, Minova USA Inc., Peter S. Mills, David R. Burkhard

"Fiber-reinforced spayed concrete has been used for several years in civil and tunneling operations. Research conducted to reduce cure times and increase compressive and flexural strengths resulted in the development of Tekcrete Fast®, a cementitious product capable of obtaining 41 MPa (6,000 psi) compressive strength and 8 MPa (1,200 psi) flexural strength in only 3 hours and reaching 7 day strengths of 62 MPa and 11.7 MPa (9,000 and 1,700 psi), respectively. A single bag product that uses conventional shotcrete and gunite application systems make it a natural crossover product for mining applications. The discovery of incredible adhesion properties and high resistance to chloride permeability helps ensure long-term stability and increases the ease of application. Project results from Disaster City® in Texas and the application for rehabilitating a coal mine belt entry are presented. The case study illustrates the effectiveness of the product in stabilizing a coal mine beltway and adjacent cross-cuts that were subjected to progressive sloughage due to humidity and cyclical loading. INTRODUCTION Original research produced a shotcrete formulation that extends beyond stabilization and can be used to restore the structural integrity of damaged infrastructure. The primary challenge was to develop a shotcrete or gunite that had structural strength within a few minutes to prevent parasitic loading, allowing ingress into damaged structures minutes after applying the mix formulation. This also applies to mining conditions where rapid support of progressive failures can minimize failure. An additional benefit was discovering that the product had extremely high adhesion properties in addition to high flexural strengths. The approach to product design was the development of “single bag” dry mixes that are entirely self-contained, can be rapidly transported anywhere in the field or underground, and can be deployed using simple equipment. Dry mix eliminates issues relevant to high shear water mixing because water is added in the form of a mist at the point of delivery; thus a low water-to-cement ratio can be achieved. However, since there is no onsite powder mixing, the product must have a high degree of inherent homogeneity. Minova offers a commercial version of this material, Tekcrete Fast®, a product that has been successfully deployed in the mining industry and is currently being used in civil, construction, and tunneling operations. The material can rapidly stabilize lose ground in metal, nonmetal, and coal mines. This paper described the technology development, product testing results, and applications and describes a case study to stabilize a coal mine beltway."

Jan 1, 2017

By Ihsan B. Tulu, David F. Gearhart, Gabriel S. Esterhuizen

"Ground deformation and support response was monitored in a longwall gateroad as part of a research project into gateroad stability under changing loading conditions. In this study, instrumentation was installed in a gateroad located between two active longwall panels. The first panel to pass was the second panel in the district, and the second panel to pass the instrumentation sites was the third panel in the district. All of the panels in the district were right-handed. All of the instrumentation was installed in the number one entry, the tailgate, the second panel to pass the instrumentation sites at a mid-pillar location, and at an outby intersection location under about 600 feet of cover. The instrumentation included roof extensometers, cable bolt load cells, hollow inclusion cells, borehole pressure cells, standing support load cells, and convergence sensors, and a multipoint borehole extensometer. The roof extensometers measured roof convergence and sag. The cable bolt load cells measured the roof load applied to the cable bolts in the secondary support array. Hollow inclusion cells measured the magnitude and direction of the principal and shear stresses. The borehole pressure cells measured the stress on a pillar in the entry nearest to the second panel to pass the instrument site. The instrumented wood cribs in the entry nearest the second panel recorded the closure and load on the cribs, and the multipoint borehole extensometer in a pillar closest to the second panel recorded the location and width of cracks that occurred and total expansion of the pillar. This paper describes the effects of the changing stress caused by the first panel as it approached and passed the locations of the instruments on the opposite side of the gateroad system and the later effect caused by the second panel as it approached and passed the mid-pillar instrument location. The first panel passing caused significant loading at the mid-pillar instrument location, but not at the intersection location, while the second panel had passed the mid-pillar location and just arrived at the intersection location when the dataloggers needed to be removed."

Jan 1, 2017

By Ashok Kumar, Rajendra Singh, Petr Waclawik

High-grade coal is locked in the shaft protective pillars at 900 m below surface in the Czech part of Upper Silesian Coal Basin. To optimise recovery of coal from the larger size shaft protective pillar, room and pillar (called bord and pillar in India) is trialled to further develop these on smaller pillars. This development followed a constant gallery width of 5.2 m and 3.5 m height. The monitored pillar size in the first developed panel varied from 860 m2 to 1225 m2 of area. The stability of these pillars (i.e., natural supports) at this depth of cover is observed to be a vital issue to control ground movement and safety of the surface structures. A simple calculation of safety factor revealed that application of an empirical method for estimating size and shape of the pillar is inappropriate due to complex geo-mining conditions of the site. Accordingly, geo-technical instruments are installed in the panel for monitoring the stability of the diamond shaped pillar at different stages of the development. Extensive monitoring of the pillar behaviour by these instruments provided an important set of data for such optimum recovery. An established process of simulation-based pillar strength estimation is often used by the mining industry, but calibration is generally done with the help of empirical formulation. However, for the deep-seated pillar, the strength suggested by the existing empirical formulations (except the CMRI formula) is found to be much different from the instrument observations. Using a study conducted at Czech hard coal mines, an attempt is made to develop a strain-softening-based numerical modelling approach to assess pillar performance at deeper cover. The recorded value and nature of the pillar spalling in the field along with the CMRI formula are used to calibrate the developed simulation approach. An assessment of the stability of the developed pillars using simulated models did not indicate any collapse of the natural supports. This paper briefly presents difficulties in pillar design at great depth of cover and field and simulation results of pillar strength estimation in Czech hard coal mines.

Jan 1, 2018

By Jessica M. Wempen, Michael K. McCarter

"Differential Interferometric Synthetic Aperture Radar (DlnSAR), a satellite-based remote sensing technique, has potential application for measuring mine subsidence on a regional scale with high spatial and temporal resolutions. However, the characteristics of Synthetic Aperture Radar (SAR) data and the effectiveness of DlnSAR for subsidence monitoring depend on the radar band (wavelength). This study evaluates the effectiveness of DinSAR for monitoring subsidence due to longwall mining in central Utah using L-band (24 cm wavelength) SAR data from the Advanced Land Observing Satellite (ALOS) and X-band (3 cm wavelength) SAR data from the TerraSAR-X mission.In the Wasatch Plateau region of central Utah, which is characterized by steep terrain and variable ground cover conditions, areas affected by longwall mine subsidence are identifiable using both L-band and X-band DlnSAR. Generally, using L-band data, subsidence magnitudes are measurable. Compared to X-band, L-band data are less affected by signal saturation due to large deformation gradients and by temporal decorrelation due to changes in the surface conditions over time. The L-band data tend to be stable over relatively long periods (months). Short wavelength X-band data are strongly affected by signal saturation and temporal decorrelation, but regions of subsidence are typically identifiable over short periods (days). Additionally, though subsidence magnitudes are difficult to precisely measure in the central Utah region using X-band data, they can often be reasonably estimated.INTRODUCTIONDifferential Interferometric Synthetic Aperture Radar (DlnSAR) is a satellite-based remote technique that can be used to measure surface displacement over large regions with high spatial resolution. Under good conditions, displacements can be measured with centimeter to subcentimeter accuracy (Buckley, 2000; Massonnet and Feig!, 1998). DlnSAR also has high temporal resolution, and imaging periods typically range from 10 to 50 days (Rosenqvist, Shimada, and Watanabe, 2004; Roth, 2004). In the last two decades, the application of DlnSAR for mine subsidence monitoring has been demonstrated in coal basins in Europe, Australia, China, and the United States (Camec and Delacourt, 2000; Ge, Chang, and Rizos, 2007; Ge et al., 2008; Ismaya and Donovan, 2012; Ng et al., 2010; Perski, 2000; Perski and Jura, 2003; Popiolek and Krawczyk, 2006; Wegmuller et al., 2005; Wright and Stow, 1999; Zhao et al., 2014; Xinglin et al., 2014). Overall, these studies have demonstrated good data resolution, strong relationships between mine development and subsidence, and reasonable agreement between displacements measured by DinSAR and displacements measured by conventional surveys."

Jan 1, 2016

By Tulu B. Ihsan, Mark A. Van Dyke, Ted M. Klemetti

"How well do current modeling procedures calculate the rear abutment extent and loading? Does an improved understanding of the rear abutment extent warrant a change in standing support in bleeder entries? What is the optimal standing support for bleeder entries separated from the longwall startup room by a barrier pillar? To help answer these questions and to determine the current utilization of standing support in bleeder entries, four bleeder entries at varying distances from the startup room were instrumented, observed, and numerically modeled. This evaluation was intended to determine the rear abutment extent and magnitude at various locations to optimize standing support in these entries and those under similar conditions. This paper details observations made by NIOSH researchers in the bleeder entries of a deep cover longwall panel; specifically data collected from instrumented pumpable cribs, observations of the conditions of the entries, and numerical modeling of the bleeder entries during longwall extraction. The primary focus was on the extent and magnitude of the abutment loading experienced by the standing support. As expected, the instrumentation of the standing supports showed very little loading relative to the capacity of the standing supports—less than 25 tons load and 1 inch convergence. The observations of the conditions showed little to no change from before the longwall panel extraction began to when the panel was more than 50% extracted. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the bleeder design. The Flac3D program was used to evaluate these four bleeder entries using previously defined modeling and input parameter estimation procedures. The results indicated only a minor increase in load during the extraction of the longwall panel. The model showed a much greater increase in stress due to the development of the gateroad and bleeder entries, with about 80% of the increase associated with development and 20% with longwall extraction. The Flac3D model showed very good correlation between expected gateroad loading during panel extraction and those expected based on previous studies. The front and side abutment extent modeled was very similar to observations from this and previous panels with similar conditions. The results of this study showed that the rear abutment stress experienced by this bleeder entry design was minimal. The convergence measured in these bleeder entries was small enough not to mandate any additional support beyond development even by U.S. or Australian standards. The pumpable crib load and convergence experienced depends on the geological setting, mining geometries, and timing. The farther away from the startup room, the lower the applied load and smaller the convergence in the entry if all else is held constant. Finally, the numerical modeling method used in this study was capable of replicating the expected and measured results near seam."

Jan 1, 2017