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By Khaled Morsy

The tragic incidents at Sago and Darby coal mines in 2006 pointed out the need for stronger seals to resist the blast effects of violent gob explosions. Mine Safety and Health Administration (MSHA) has published the final rule on Sealing of Abandoned Areas in April 2008. According to the final rule, the mine seals for unmonitored gob areas should withstand an instantaneous pressure of 120 psi. Reinforced concrete (RC) seal is one of the feasible options that can be used to meet the final rule. Concrete is a heterogeneous, cohesive-frictional material and exhibits complex non-linear inelastic behavior under multi-axial state of stress. The increased use of concrete as primary structural material requires the development of sophisticated material models to have an accurate prediction of the material response to a variety of loading situations. The constitutive modeling of concrete has advanced considerably in the past three decades. Choosing a reasonable concrete material model is important in order to obtain an accurate mine seal design. This paper attempts to address the effect of different concrete material models on the response of RC seal subjected to dynamic loads from methane explosion. Three concrete models, from ABAQUS library, were selected to address the effect of non-linear behavior of concrete and damages due to tensile and compressive stresses. In addition, a typical and verified concrete material model was used for conducting 3D finite element models simulating the RC seal by ABAQUS, EXPLICIT ver. 6.6.

Jan 1, 2008

By Mingju Liu

China has been suffering from coal and gas outbursts since 1950s. Nowadays, the situation has become worse than ever with the need to mine deeper coal deposits at higher production rates in a more complex underground mining environment. In the year 2006 alone, 37 outbursts occurred in underground coal mines with a death toll of 232. This paper describes a case study of an outburst event, which occurred in July 2006 during the shaft development of the Mengjin Coal Mine, Henan Province, China at a depth of 768m below surface. Detailed analysis of the possible causes that led to the occurrence of the outburst is reported. These include the study of complex geological features, gas storage conditions, possible improper technical management practice as well as incomplete prevention and control measures. Based on the case study and the observation of outburst occurrence in China in the year 2007, suggestions have been put forward to help prevent and control future coal and gas outbursts.

Jan 1, 2007

By Nic Slatalla

Acoustic emission monitoring techniques are a powerful tool for analysing deformation behaviour and fracture characteristics of rock samples under uniaxial loading. Uniaxial compression tests were carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage develop-ment. All sandstone samples revealed congruent characteristic acous-tic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages proved to be entirely different due to variations in their microstructure.

Jan 1, 2008

By Arka Jyoti Das, Prabhat Kumar Mandal

"Coal mining in India has a strategic importance for the energy security of the country. Presently, the majority of coal production is coming from open-cast mining. Due to the fast depletion of reserves amenable to open-cast mining, future coal production will depend primarily on underground production. In conventional bard-and-pillar/room and pillar mining as it is practiced in India, the percentage of extraction is very low. In fact, within most Indian coalfields, a large number of coal seams extract only 15- 20% of coal, but no pillar extraction is done due to the presence of surface or sub-surface structures leading to huge amount of coal being locked-up in underground. Spontaneous heating and fire, the accumulation of poisonous gases, severe stability issues, unsafe workings and environmental hazards are all major problems associated with these developed coal pillars, so there is a pressing need for technology in the mining industry to extract this huge amount of coal locked up under different constraints. In this study, we propose that the locked-up coal be extracted by replacing the developed coal pillars with artificial pillars. The paper briefly discusses the extraction methodology designed for this and its evaluation through numerical modeling for applied suitability underground.INTRODUCTIONDriven by rising populations and urbanization, an expanding economy and also a quest for improved quality of life, energy usage in India is expected to increase manifold in the coming years. As for India's energy scenario and its projected future plans, there is no doubt that coal will remain a prime source of energy, even over the next few decades.The majority of coal production in India comes from open-cast mining because of large-scale mechanization, but the coal reserves amenable to open-cast mining are gradually depleting - which may impose adverse impacts on the energy sector if underground production does not increase. Underground coal extraction, meanwhile, is going to become more complicated in near future due to the fast exhaustion of easily mineable coal reserves."

Jan 1, 2016

By Karsten Zimmermann

The paper presents a study on the influence of face retreat rate in longwall coal mining on surface subsidence. It shows possibilities to influence dynamic ground movement by using a specific and controlled winning regime. For this, a dynamic stochastic prediction model used by DMT in Germany will be proved and adjusted on the specific character of geology in the Northern Appalachian coal field. Occurring ground movements such as inclination, curvature and deformation can be analyzed under static and dynamic points of view. It is theoretically shown that the face retreat rate has only a small influence on the static deformation values, but directly affects rate and acceleration of subsidence and deformation processes. The study shows that because of low mining depth and high mined thickness, static deformation values have the largest impact to surface objects. This implies that high face retreat rates don?t affect the impact on surface objects.

Jan 1, 2007

By Luis Giraldo

The installation procedure of fully grouted rebar bolts requires that the bolt be pushed through the resin grout until it is fully inserted in the borehole. Resin grout is a highly viscous fluid that is forced to flow through the annulus between the bolt and the borehole as the bolt advances. Variables such as bolt insertion speed, bolt vs. borehole diameter and grout viscosity determine the magnitude of the pressure generated ahead of the bolt tip. Under weak rock conditions, the magnitude of the pressure of bolt insertion is often sufficient to initiate hydraulic fracturing along joints or bedding planes that allow injection of grout into the strata leading to significant loss of bolt encapsulation. The grout pressure front has also been linked to gloving, a bolt installation problem that reduces bolt effectiveness. A series of tests were conducted to characterize the pressure of bolt insertion during bolt installation under various conditions. Analysis of the data shows three distinct stages of pressure build up during bolt installation. Understanding of the mechanisms involved may allow development of strategies for controlling the pressure buildup and its negative impact during roof bolting under weak rock conditions.

Jan 1, 2006

By Kot Unrug

Mine seals are necessary in nearly every mine to isolate mined-out areas from the ventilation network. There is a large number of seals in active mines and that number continually increases as the development spreads across the reserves. The recent accident caused by the failure of seals in the Sago mine resulted in new regulations. These regulations have been set based on a principle of eliminating the risk of failure, rather than the sound engineering and understanding of the conditions that may cause seal failure during an explosion of methane in sealed areas of the mine including the probability of such occurrences. This paper discusses the following subjects related to mine seals: The potential causes of methane explosions and the dynamic effects of pressure wave propagation. Seal materials and methods of construction in view of the interaction of the seal with the host rock environment as well as requirements for gathering geotechnical data. The design of seal dimensions is based on loading conditions, therefore the probable ranges of such loading is reviewed. There is also discussion of possible methods for shielding of seals from pressure pulses following an explosion. Particular seal?s types are examined in reference to the conditions at the seals location. A novel design is presented that will be a low- cost solution for the sealing of mined out areas. Recommendations are given for further research leading to a rational engineering system for design of new seals in coal mines.

Jan 1, 2008

By D. Minney

The quality of resin used in South African collieries is currently controlled according to South African Bureau of Standards, "SABS-1534:2002". It has however been found that resin which passed the SABS tests failed in underground short encapsulation pull tests. An investigation was therefore conducted to determine the characteristics of resins that are currently being used in collieries. A series of in situ short encapsulated pull tests was conducted to determine the effect of expired resin on support performance. The results showed that the longer the time after the expiry date the poorer the resin performance. A series of resin-torque tests has been performed using Anglo Coal's testing facility. The following parameters were investigated on different resins: Spinning speed, Mixing time, and Setting time. An attempt was also made to develop a new quality control procedure for resin. This procedure enables mining personnel to make sure that the quality of resin can provide the required support performance. The effect of spinning speed on resin performance was also investigated. The results showed the resin gelling times of resin decreases as the spinning speed increases. It is found that over-spinning with slow speed roofbolters may result in decreased bond strengths. It was found that a 30-second resin (10-second spin, 30-second hold) should never be spun more than 20 seconds, even with very slow roofbolters (100 rpm).

Jan 1, 2005

By Matt Leeming

Several of the underground mines in the UK require a significant number of cable bolts to be installed, remedially, to control worsening roadway conditions. Often the access is limited and the only power source is compressed air. Machines for this duty are generally designed to perform with 100 psi but regularly only half of this is available. This paper seeks to outline the difficulties this situation causes the Rock Bolting Engineer, highlight the technical requirements for a machine of this type to succeed and detail a solution offered by our engineers.

Jan 1, 2007

By Nikolay Boykov

Subsurface imaging technologies are part of the electromagnetic (EM) geophysical methods. EM technologies such as radio imaging method (RIM), ground penetrating radar (GPR), and drill string radar (DSR) have great application potentials in the mining industry. However, the success for applying the EM methods to subsurface observations is largely dependant on the correct evaluation of the electrical properties (i.e., resistivity and dielectric constant) of the rocks in the domain of interest. The depth of penetration of the EM signal in coal, as well as the electrical contrast between coal and subsurface boundary rocks or anomalies, are critical parameters for any successful radar system. Based on the measured electrical properties of the coal seam, the proper operating frequency of the radar system must be chosen in order to successfully detect features such as the coal-roof rock boundary, dykes, and voids ahead of mining. The electrical properties of the rocks in a coal seam are frequency dependent. Even though there has been a great advancement in the hardware and data processing software in the past twenty years, there have not been many examinations of the high-frequency properties of coal measure rocks. In this paper, the laboratory setups, the testing procedure, and case studies will be presented for a wide spectrum of frequencies for coal measure rock samples collected from various US coal fields. Tests have been conducted to study the different conditions, including moisture content, that affect the EM properties.

Jan 1, 2006

By Freddie Hunter

An underground mining method for extracting an eight metre (26') coal horizon, containing a sandstone layer. A decrease in the thickness of the rock strata between the number two and number three seams of the Sasolburg Coal Field, has resulted in a total mining horizon approximately eight metres (26') thick. This parting varies in thickness and competency and in some areas is impractical to cut with a continuous miner. In addition, the general mining area is highly jointed. The top seam is first mined to a normal bord and pillar plan with a continuous miner. The bottom seam is then mined by selectively undercutting the stone parting constituting the floor of the top seam. In areas where this parting is between 0.75 - 2.50 metres (29.5" - 98") thick, it is suspended from the primary roof support installed when mining the top seam. Secondary, high extraction retreat mining is then undertaken in the coal pillars adjacent to the suspension supported headings in the bottom seam. Left in-situ, the parting provides lateral support to the pillar sidewalls in both the top and bottom seams, enabling high percentage extraction rates in what was previously thought an uneconomically mineable area. The paper proposed, presents the challenges and practical implications of executing the mining method, including the primary roof control of the top seam and the stone parting between scams.

Jan 1, 2003

By Wenbing Guo

Strip pillar mining method is one of the most important techniques for mining subsidence control in China. Mining depth is one of the most important factors that affect subsidence in strip pillar mining method. Six numerical simulation models were set up in this paper to simulate surface subsidence of strip pillar mining at different mining depths. The mining depth in these models ranged from 300 to 800 m. The simulated results were compared and analyzed. The results showed that the subsidence factor of strip pillar mining is related to mining depth. While other things being equal, the subsidence factor of strip pillar mining increases logarithmically with increase in mining depth. Their quantitative relationship was established based on the simulated results.

Jan 1, 2007

By Dennis Dolinar

In this National Institute for Occupational Safety and Health (NIOSH) investigation, numerical modeling is used to evaluate the degree the pillar strength is increased with a rectangular pillar over a square pillar. Three-dimensional models are developed with pillar sizes and geometries typical of underground limestone mines in the United States. Physical properties for the models are based on geotechnical surveys of nearly 30 underground limestone mines. To examine the effects of pillar length on pillar strength with the modeling, a parametric study was conducted where the pillar width was held constant while the length and height were varied resulting in width-to-height ratios ranging from 0.5 to 1.5 and width to length ratios ranging from 1 to 6. There are two aspects to pillar strength and failure. At relatively low stresses, pillars can fail as a result of geologic structure. At greater depths and sufficient extraction ratios, the pillars can also fail as a result of the vertical stresses exceeding the pillar strength. The effects of increasing pillar length regarding both strength aspects are presented in this study. Equations were developed for the increase in pillar strength with both height and length based on the model results. The results from these equations were compared to those from previously developed empirical equations for rectangular pillars. For the more slender limestone pillars with a width-to-height ratio below one, there is little increase in pillar strength for length-to-width ratios above 2 and almost no increase with length for a width-to-height ratio of 0.5. Only with the more squat pillars is there a significant improvement in strength with length. The gain in pillar strength with length for the pillars is explained by the change in the ratio of the area of the pillar affected by the ends to the area of the pillar. For the squattest pillar, the change in strength is also affected by an increase in pillar confinement along the central length of the pillar as the pillar transitions from a square to a long rectangular pillar. With geologic structure, added pillar length can significantly improve the pillar strength, depending on the strike of that structure with respect to the long pillar axis.

Jan 1, 2007

By Wenbing Guo

The stability of strip pillar is the key to successful strip pillar mining. In analyzing the stability of the strip pillar, the width of yield zone must be determined. The determination of the width of yield zone has been an important subject for researchers. In this paper, the numerical method was used to study the relationship between the width of yield zone in strip pillar and coal recovery ratio. The results showed that as the recovery ratio increases, the width of yield zone increases exponentially. Accordingly, the formula for determining the width of yield zone was updated and the effect of changing coal recovery ratio on strip pillar deformation is presented.

Jan 1, 2006

By H. Nan

Using the case of Qianqiu Mine, Yima Coal Mining Administration, theoretical analysis, laboratory physical simulation, and underground observation were performed to study the major parameters for fully mechanized sublevel caving mining method, including drawing spacing, drawing sequence, drawing numbers, and number of shields performing simultaneous drawing. The effects of those parameters on coal recovery and amount of waste rocks were also investigated. The optimal coal drawing technology for this case was obtained.

Jan 1, 2006

By Carl Tom Blevins

Resin was first introduced to U. S. mines in 1971 and first produced in 1973. The original polyester roof bolt resin was very expensive and limited to a single cartridge size and a single slow gel time. Early resin applications were limited to very difficult ground conditions. Today approximately 80% of the over 100 million roof bolts installed in U.S. mines, tunnels, and construction projects employ polyester roof bolt resin. The industry has evolved from hand packing samples for trial testing to producing enough resin, if converted to a .9 inch (22.9mm) diameter cartridge, to encircle the world at the equator approximately three (3) times each year. Manufacturing and design improvements allow for a more consistent product with a wide range of applications at a price that is approximately 30% below our 1974 sales price. An inflation adjusted dollar buys six (6) times more resin in 2006 than in 1974. Modern gel times range from a few seconds to over 15 minutes. Viscosity is matched to bolt type and drilling machine characteristics. Multiple cartridge diameters and a wide range of cartridge lengths are offered by two operating companies. Resin manufacturers have also gone through a period of restructuring and change to meet the challenges of the mining industry. DuPont Fasloc has become Fasloc, Inc., a stand-alone company. Celtite became Fosroc then Minova, as it is known today. During this transformation, several manufacturers have either discontinued production or were absorbed. The industry led the development of the chemical grouting materials specifications contained in ASTM F432-95, Standard Specifications for Roof and Rock Bolts and Accessories. A history of the industry will be presented within a discussion of the evolution of the strength index, speed index, cartridge volume, class designation, and product marking requirements of ASTM F432-95.

Jan 1, 2006

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 Xinxian Zhai

Jining No. 3 Coal Mine, a subsidiary of Yanzhou Coal Company Limited. China, is a super-large modern mine. The coal mine is not only the first one constructed with five-million tons of designed production capacity in China, but also a modern underground coal mine with coal transportation systems by trains and ships, and with a power plant. Based on the hard roof conditions of the sublevel caving coal face in the fault-affected zone, the authors designed rebar and cable bolts for supporting the tailgate that is separated from the gob by a narrow pillar. With extensometers and borescope, strata behavior was systematically studied during tailgate development: (1). Roof convergence is much smaller as well as roof strata separation between inside and outside the anchored zone. (2). The affected zone during tailgate development is approximately 50 m. (3). Because of serious floor heave and obvious rib convergence, the tailgate finally turned into a reversed-trapezoid from the original rectangular shape. (4). There being many joints and fractures in the pillar, coal is crushed from the elastic bedded structure into loose blocky materials. But the tailgate met the service requirements during longwall retreat mining, and demonstrated that the method of roof support was a success. Since then, roof bolting has been widely used for the tailgate of longwall panels at Jining No. 3 Coal Mine. Keywords: coal mine; bolt supporting; tailgate; convergence of surrounding rock; ground control; strata behaviors.

Jan 1, 2007

By Michael Murphy, Khaled M. Mohamed, Berk Tulu

"Failures of coal ribs are major hazards in underground coal mines. For the period of 2010 to 2014, rib falls were the leading cause of groundfall fatalities and accounted for 35% of all groundfall-related fatalities. Currently, the National Institute for Occupational Safety and Health (NIOSH) is conducting a research project to develop an engineering-based design methodology for rib control in underground coal mines to reduce rib fall accidents and fatalities. Numerical models can be used as part of the design methodology provided the models are appropriately calibrated and verified.This paper presents a coal rib model to simulate the mechanism of coal deformation and fracturing that can lead to stress-driven rib falls. The proposed coal rib model was calibrated against published data from an instrumented rib site at the West Cliff Mine in Australia. Fracturing of the coal rib was defined by critical VALUES (of plastic shear strain of the coal material and rib displacement. A critical plastic shear strain of 2.8% and a sudden increase of rib displacement of 30 to 45 mm were sufficient to develop a fracture plane in the rib model. Accordingly, a discontinuous rib deformation was achieved and it compared favorably against field monitoring data.INTRODUCTIONCoal ribs are the vertical walls of coal formed when entries are developed in a coal bed. Failures of coal ribs are a major hazard in underground coal mines. Furthermore, failing ribs can contribute indirectly to roof and floor instability by increasing opening widths across intersections and entryways. Over the last 18 years, there has been a continual occurrence of injuries and fatalities in underground coal mines due to rib falls. For the period of 2010 to 2014, rib falls (excluding longwall face failure and bursts) were the leading cause of ground fall fatalities and accounted for 35% of all groundfall-related fatalities (MSHA, 2015)."

Jan 1, 2016

By B. Macdonald

In 1991, roofbolts were introduced as a primary support for inseam retreat longwall entries at CBDC's Phalen Colliery. Increasing mine depth (+550 m) necessitated replacing conventional inseam steel sets in these entries because they were being severely damaged during wall retreat and subsequently causing gate end problems. Roofbolts had been used in the Sydney coalfield between 1949 and 1963 in room and pillar working, however, some concerns arose with using roofbolts as a primary support in long (2.2 - 3.4 km) single entry roadways. In order to alleviate these concerns, a phased approach was adopted whereby conventional steel densities remained along with roofbolts. The roofbolt design was such that the roofbolts would be the primary support and the steel sets would experience little or no loading. A detailed geotechnical monitoring program was developed by both CBDC and the Cape Breton Coal Research Laboratories technical staffs. The results of this monitoring were used to build-up confidence in roofbolts and subsequently reduce the steel set densities until eventually only roofbolts were used as a primary support. This paper will discuss briefly the phased introduction of rootbolts at Phalen Colliery and more specifically the development of geotechnical data collection and interpretation.

Jan 1, 1994