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By Jinhua Wang

Based on engineering examples and theoretical analysis, the mechanism of chemical consolidation for fractured roadway surrounding rock was discussed. The chemical consolidating materials and three consolidating methods, that is, capsule, forced infusion and spraying, were introduced. Finally the application effects of the chemical consolidating technique to reinforce the fractured surrounding rock of roadways were analysed.

Jan 1, 1996

By Morgan M. Sears, Gamal Rashed, Jim Winfield, Brent Slaker

"Over the past decade, ground falls have resulted in five fatalities, representing over 40% of all fatalities occurring in underground stone mines in the United States. In conjunction with recent high-profile pillar collapses, the need for further ground control research in the nation’s stone mines is evident. Ground conditions are growing more difficult. Therefore, this research is focused on deep cover, steeply dipping, and multi-level underground nonmetal mines. The objective of this paper is to introduce a geomechanical study of pillar development in a steeply dipping limestone deposit. To accomplish this, a numerical model was developed using input from modified laboratory test data and in situ stress measurements provided by the mine to simulate the development of a pillar under these conditions. The model was then verified against field observations and stress mapping. The calibrated model was then used to calculate the estimated stresses in the pillar and immediate roof as development progresses around an instrumented pillar.INTRODUCTIONGround falls represent a significant hazard in the nation’s underground stone mines. In response to recent large-scale pillar collapses, as well as stakeholder interactions, researchers from the National Institute for Occupational Safety and Health (NIOSH) are currently conducting detailed investigations into the complex loading conditions at mines operating in challenging operational environments. These include deep-cover, steeply dipping, and multi-level mines where there is the potential for an increased risk of ground failure. The Pleasant Gap Mine, shown in Figure 1, operates in the Valentine Formation, which is typically light grey, extremely fine-grained limestone, and approximately 21.3 m (70 ft.) thick. The Centre Hall Formation makes up the immediate and main roof members and is approximately 9.1 m (30 ft.) thick, consisting of medium-dark-gray limestone (Murphy et al., 2018). Situated in a syncline, the top of the Valentine Formation dips at approximately 19° with a strike of approximately N54E in the area of interest (see Figure 2)."

Jan 1, 2018

By Behdeen Oraee, Amirhossein Bangian, Kazem Oraee

"Determination of the optimum design of chain pillars has a significant effect on the economics and safety of longwall operations. Most pillar design formulae are based on empirical methods and supplemented by local experience. They therefore lack versatility of application under different geotechnical conditions.In this paper, in order to illustrate the shortcomings of the above, a typical real coal pillar in Tabas coal mine has first been studied and the conventional formulae have been used to determine the optimum dimensions. The results show that a wide difference exists between the predicted and the real field data. The Oraee-Hosseini formula has then been applied to this mine in order to determine the optimum design. The results from this formula demonstrate a close fit between the theoretical VALUES (and those produced by laboratory tests and in practice. It is further demonstrated that the wide discrepancy between the results obtained from the two formulae is attributed to the dissimilarities between geotechnical conditions of Tabas and the original regions whose data were used to devise the empirical formulae.It is finally concluded that the application of numerical simulation methods and experimental equations together with engineering judgment used by the mining design engineer, will provide the most accurate design characteristics.INTRODUCTIONLongwall is an underground mining method which has a high rate of production capacity and mechanization ability. This mining method, which is appropriately applied for soft rock fiatlying bedded ore bodies, gradually became a widespread mining method amongst European coal mines at the beginning of the 19'h century. It was then applied in some of the U.S. coal mines with the development of self advancing support mechanisms in early 1960's (Oraee, 2001).Underground coal production around the world using Longwall mining method doubled during 1990s. The main reasons for this improvement were: the high level of mechanization, increased 10 safety, adequate design of panels, and well designed ventilation systems (Peng, 2006)."

Jan 1, 2010

By Ihsan B. Tulu, Keith A. Heasley, Aanand Nandula

"The objective of this paper is to present the development of a software tool that will assist in an area calculation of the Analysis of Roof Bolting Systems (ARBS) support intensity while incorporating more detailed geology, stress, and intersection span input. In this area ARBS tool, a detailed CMRR distribution can be determined from the available geologic database at the mine. In addition, the overburden and multiple-seam stresses, as obtained from the boundary-element program LaModel, can be converted to a “pseudo-depth” which is then used as the depth input to the area ARBS calculation. Finally, the section-specific intersection spans can be determined from the mine design for input to the calculation. All this detailed area input information for ARBS is handled by the latest version of the Stability Mapping (StabMap) program, which now outputs an area ARBS suggested support density given the required input parameters. This final ARBS grid can be plotted, analyzed and overlaid on the mine map for optimum use in support design and for presentation to production personnel.INTRODUCTIONSince the inception of roof bolts in the 1940s, there has been a lack of a universally accepted roof bolting design method (Mark, 2002; Mark and Pappas, 2012). To address this situation, the National Institute of Occupational Safety and Hazard (NIOSH) developed roof support design guidelines, which were incorporated into a computer program called the Analysis of Roof Bolting Systems (ARBS). These guidelines were based on the effects of roof quality, stress, and mine geometry (Mark, Molinda and Dolinar, 2001). The roof rock quality in ARBS is represented by the Coal Mine Roof Rating (CMRR), the depth is used as a surrogate for the stress, and the intersection span serves to represent the mine geometry (Mark, Molinda and Dolinar, 2001). These three input parameters are used in the fundamental, empirically derived equation of ARBS to calculate a required support density factor, called ARBS, (Mark, Molinda and Dolinar, 2001):"

Jan 1, 2018

By Kazem Oraee-Mirzamani, Nikzad Oraee-Mirzamani, Arash Goodarzi

"Unexpected movement of ground can potentially endanger lives, damage equipment or destroy property. Occupational accidents frequently occur with fatal consequences in developing countries with significant economic dependence on industries such as mining. Therefore, there is increasing need for miners' protection against such hazards. Slope stability and roof support accidents are two of the major causes of fatalities at surface and underground mining operations. According to national codes, employers are obligated to protect workers against accidents; however, these rules fall short of the standards in developed countries. National safety regulations should clearly specify support systems. The International Labor Organization (ILO) prepared two codes of practice, aiming to guide those responsible for improving standards of safety and to provide guidelines for drafting of safety regulations for the coal mine industry and quarry open cast mines. The practical recommendations of these codes in the ground control area have been analysed and the advantages and disadvantages of ILO codes concerning ground control are summarized.INTRODUCTIONTwo million people die every year from work-related accidents and diseases. An estimated 160 million people suffer from work-related diseases, and there are an estimated 270 million fatal and non-fatal work-related accidents per year. In economic terms, the ILO has estimated that 4% of the world's annual GDP is lost as a consequence of occupational diseases and accidents (ILO, 2009).Mining has always been dangerous due to many potential risks such as those associated with using machinery in enclosed spaces, rock falls, blasting operations and explosion of flammable gases, which endanger miners' safety and health. However, it is suggested that implementing new laws along with strict adherence would reduce such risks in the workplace.The severity and frequency of accidents are trending downward in developed countries, where the focus is on large private companies who invest heavily in mechanization and developing economies of scale. But in many parts of the world, particularly in developing countries, minerals are extracted by people working with simple tools and equipment; they work on a smaller scale, are more flexible, and can exploit irregular ore bodies and steep, dipping seams."

Jan 1, 2010

By Xicai Gao

With the intensification and expansion of deep coal mining, the surrounding rock stress environment becomes further deteriorated because of the complicated geological environment and powerful mining effect, so the major deformation of roadway appeared frequently. The proportions of floor heave are increasing and , the most serious problem is the floor heave of preparatory workings. The coal seam of Binchang coal area in Shaanxi province is at a depth of 621.6 ~ 817.0 m, the average thickness of the coal seam is 25 m, and the overburden and coal measure strata are the main regional aquifers and are highly permeable. The properties of water-weakening are obvious in the floor strata. The main preparatory workings or chambers were driven in the coal seam. The severe floor heave of the water chamber appeared during early mining and the amount was 180 ~ 910mm, and seriously affected the normal installation and use of the water chamber. Theoretical study and numerical simulation as well as the in-situ measurements were carried out to investigate the deformation and failure characteristic of the surrounding rock with non-support in the floor and the main reasons which caused the floor heave were determined. A new combined support technology was proposed which consists of overcutting, grouting, backfilling and invertedarch and bolt-grouting. The field monitoring showed that the cumulative convergence of the water chamber was 20mm, the maximum convergence speed is 1.2 ~ 2mm/d, the accumulative convergence of two ribs was 36mm and the maximum convergence speed is 2mm/d. The combined support technology will enhance the support intensity between the roof and floor, the two sides of the entry and improve the loading capacity of the floor effectively and the floor heave can be controlled effectively.

Jan 1, 2013

By Elizabeth Holley, Meriel Young, Gabriel Walton

"The coal mine roof rating (CMRR) was developed by Chris Mark and Greg Molinda to bridge the gap between geological variation in underground coal mines and engineering design. The CMRR accounts for the compressive strength of the immediate roof, the shear strength and intensity of any discontinuities present, and the moisture sensitivity of the immediate roof. The CMRR has been widely used and validated in Eastern US coal mines, but it has seen limited application in the Western US.This study focuses on roof behavior at a Western coal mine. Mine A shows significant lateral geological variation, along with localized faulting and a laterally extensive sandstone channel network. The CMRR is not used to predict roof instability at the mine.It is, therefore, hypothesized that there are other factors that are correlated with roof instability in underground coal mines that could potentially also be included in the CMRR.This hypothesis was tested by collecting 30 CMRR measurements at Mine A. At each measurement location, a binary record of the roof condition (stable or unstable) was made along with parameters, such as depth of cover, thought to also correlate with roof stability. A statistical analysis of the data was performed to determine the parameters in addition to those included in the CMRR, which can be correlated to roof stability.INTRODUCTION AND BACKGROUNDRoof fall is one of the greatest hazards facing underground coal miners (Barczak et al., 2000). In 2017, 91 lost-time injuries occurred due to roof fall in US underground coal mines. A further 48 roof falls occurred with no lost days (MSHA, 2018).The coal mine roof rating (CMRR) classification was developed by Molinda and Mark (1994) to quantify the geological description of mine roof into a single value that could indicate mine roof stability and be used in engineering design. It is widely used in the Eastern US as an input for the analysis of longwall pillar stability (ALPS) in underground coal mines. It provides an excellent start; however, it is arguably not fully comprehensive, nor is it widely used in the Western US."

Jan 1, 2018

By Stanislaw Prusek

In this paper, results of underground measurements conducted in gateroad 3, seam 615, at the Bobrek-Centrum Colliery are presented. The Bobrek-Centrum Colliery is owned by Kompania Weglowa, the largest hard coal producer in the European Union. Yearly production at the colliery, including the longwall system, is about 2 million tonnes. Gateroad 3 was used simultaneously for two retreating longwall faces, No. 1 and No. 2. This working was also employed for the conveyor belt for longwall No. 1 and served as a transport road for longwall No. 2. Approximately 100 meters of gateroad 3 was maintained behind the face of longwall No.1 and then, with the progress of longwall No. 2, it was liquidated. In this paper, we present the results of underground measurements of vertical and horizontal convergence in gateroad 3. We also present the results of numerical modeling for deformation of the working. Finally, we analyse the accuracy of the numerical calculations by comparing them to the actual deformation of gateroad 3.

Jan 1, 2014

By Andre Zingano

The coal seam strength and the failure criteria that represent the behavior of the coal seam are very important for mine section and pillar design. Additionally, mine section and pillar design take into account roof and floor layers, which form the roof-pillar-floor system. Therefore, underground ground control design begins, most importantly, with rock mass characterization. Another issue is the distribution or behavior of the coal seam strength for the whole mine area. Sometimes, the coal seam strength is assumed to be the same for an entire area, but this is not true because the coal seam is heterogenic and anisotropic, and also the coal strength is affected by faults and spacing between fractures. The objective of this paper is to demonstrate the use of the methodology for geomechanics characterization to determine the strength for the Bonito coal seam, and discuss the coal seam strength distribution around the entire mine area. The intact coal strength was determined by laboratory tests that were completed during the last 17 years, including triaxial and uniaxial strength tests. It was noted that there is a strong correlation between all laboratory strength tests with 90% confidence interval. The explanation for this coal seam strength variation is the presence of the discontinuities and their variation in quantity and quality at each location in the mine area, because the coal seam rock classification and its behavior will be affected by the quantity and quality of fractures.

Jan 1, 2014

By Guo Wenbing

"INTRODUCTIONThere is a great amount of coal (about 140 billions tons) left unmined under the surface structures, water bodies, railways (referred to as the “three-body”). Coal mining under “threebody” especially that under surface structures has become a major problem in the mining area. The key problem of mining under surface structures is to control overburden strata and surface movements. At present, the major surface subsidence control methods are strip pillar mining, backfilling mining method, room and pillar mining, grouting of bed separations, harmonic mining, and so on. Wongawilli mining method is a high-efficient shortwall mining method developed based on the room and pillar mining, and named after the first successful trial in Australia “Wongawilli” coal seam. The room and pillar mining equipment such as continuous miner is employed in this method. Its major advantage is flexible face layout. It can be used to mine the small wedge coal blocks and those that are difficult to be mined with retreat longwall mining method. It requires less capital investment, short lead time for production, flexible equipment operation and face move and high productivity. It has been applied in some China’s coal mines.The “Wongawilli strip coal mining technology” is a new coal mining method for mining under surface structures. It combines the strip mining longwall layout and Wongawilli high efficient mining technology. This technology can fully utilize the advantages of both the strip pillar and Wongawilli mining methods, and overcomes the disadvantages such as frequent face move, and low mining efficiency in strip pillar mining, and poor ventilation condition and poor long-term stability problems of coal pillars. In this paper, the Wongawilli strip mining method was designed for Wangtaipu mine and its feasibility was studied using numerical modeling and surface subsidence prediction.Geological and mining conditionsAt present, most of the No.15 coal seam reserves totaling 377.07 million tons in Wangtaipu mine are located under surface structures. In order to mine the coal under the surface structures, the Wongawilli strip mining method was employed. An appropriate panel was selected for trial and the surface movement monitoring lines were set up. The trial area is located in panel XV2214 (East) ( re 1). In this area, the floor elevation of coal seam is +634~646m, the surface elevation is +810~817.5m, and the average depth of coal seam is about 174m.The average panel length is 282m and average panel width is 73.5m. The seam dips 1~3° and is 1.8~2.5m thick with an average of 2.15m. The roof and floor rock characteristics are shown in Table 1."

Jan 1, 2015

By Josh Johnson, Stephen Tadolini, Mark Ferster, Curt Jacobs

"Stillwater Mining Company is the only U.S. producer of platinum group metals (PGMs) and the largest producer outside of South Africa and Russia. The Company controls a considerable portion of the J-M Reef and is currently developing the ""Blitz Project,"" which will provide a main haulage level, ore pass systems, backfill plants, and flow-through ventilation to increase production and ensure sustainable extraction of this critical strategic mineral. While driving two sub-parallel footwall laterals to about 8,125 m (26,000 ft) long, one of the development drifts experienced a massive fall in a geologically disturbed area created by a dike intrusion. Combinations of a void fill material (Tekseal), polyurethane injection, fore poling coupled with steel arches, and supplemental bolting allow safe and efficient advance through this geologically disturbed zone. This paper presents the details of this project.INTRODUCTIONStillwater Mining Company is the only U.S. producer of platinum group metals (PGMs) and the largest producer outside of South Africa and Russia. The Company controls a considerable portion of the J-M Reef and is currently developing the ""Blitz Project,"" which will provide a main haulage level, ore pass systems, backfill plants, and flow-through ventilation to increase production and ensure sustainable extraction of this critical strategic mineral.Tunnel Boring Machines (TBMs), shown in Figure 1, have been used extensively at both the Stillwater Mine and the East Boulder Mine to develop access to the J-M Reef. The J-M Reef is a PGMbearing reef deposit located in the Stillwater Complex near NyeMontana. The mining property and location are shown in Figure 2.The Blitz Project is named after the Blitz mining claims and is developing the Eastern extent of the J-M Reef Project., This consists of a 5.5 m (18 ft) diameter TBM drive on the SOE (1,562 m (5,000 ft) above sea level and east of the shaft collar) and a 4.7 x 4.7 m (15 x 15 ft) drill and blast heading on the 5,600E (1,750 m (5,600 ft) above sea level and east of the shaft collar). The Benbow Decline will be completed running perpendicular to the TBM by drilling and blasting methods. The TBM drift will be used for exploration drilling, ventilation, and will serve as the main haulage level. The 56E will be used for ventilation and exploration drilling. The Benbow decline will serve as the exhaust drift."

Jan 1, 2016

By Anton Sroka, Antoni Tajdus, Axel Preusse, Krzysztof Tajdus

"In Europe, the magnitude and range of mining deformation predicted on the surface have been, in most cases, determined using methods based on Gauss' influence function (e.g., Knothe's theory, Ruhrkohle method, Stochastic method). The increase in the application of numerical methods (finite element method FEM, finite difference method FDM, etc.) provides an opportunity to offer a different approach in describing mining deformations. However, this phenomenon is very complex, and due to numerical limitations, a two dimensional analysis was made (in plane strain) in most cases. Such 2D models could not accurately describe the mining geometry and geological conditions. The authors of this paper carried out the calculations in 3D models which describe a mining situation and the character of the strata in one Upper Silesian Polish mine. Furthermore, the authors include some tips for three-dimensional modeling.INTRODUCTIONThrough the years, many numerical models have been used to describe a phenomenon of underground mining exploitation influence on the ground surface. However, such rock masses are highly disturbed (cracks, fractures, changes in water conditions, etc.), and it is very difficult to suitably describe, in a mathematical manner, such materials. Therefore, many of the numerical models created have been corrected in various degrees.At first, models widely used to describe the behavior of strata in the region of exploitation appeared as elastic models, where scientests assumed that a whole strata behaved in a linearly elastic manner, according to Hook's law. Those models have been modifed to received suitable VALUES (of settlement and range of subsidence troughs. Scientests created this amongst others: elastic models with contact between the rocks layers (Shippam, 1975; Salamon, Chugh and Yang, 1993), elastic models with no tension (Chrzanowska-Szostak, 1988), and elastic models with a contact on the subsidence limit plane (Tajdus K., Tajdus A., 2005). However, in most cases, those models did not bring the expected results and the differences between shapes of measured and the calculated subsidence trough were substantial."

Jan 1, 2010

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 Nengxiong Xu

The WTZ coal mine is located adjacent to an auxiliary dam in P. R. China. The mining-induced ground movements are calculated using a 3-D numerical procedure based on the finite difference method (FDM) to judge whether the extraction of the coal seam will have a negative impact on the dam. First, the values of the rock mass mechanical parameters in the numerical model are estimated using the available literature that relates intact rock and discontinuity properties to rock mass parameters. Then, based on available surface subsidence monitoring data for an area in WTZ mine, the main mechanical parameters of coal and rock masses are calibrated by a back analysis procedure that combines an experimental design technique with numerical simulations. Then the reliability of these rock mass parameters is verified. Finally, according to the planned mining sequence, predictions of mining induced surface subsidence are computed in steps, and the boundary of the ground movement area of each step is determined. The nearest distance from the boundary of the surface movement area to the edge of the dam foundation was found to be about 1347 m. The maximum surface subsidence within the mining area turned out to be 2.7 m, and the maximum settlement point was found to be close to west-south corner of A2 mined area. These findings led to the conclusion that the coal seam mining would not cause dam damage.

Jan 1, 2013

By David Hill

"End of panel longwall face recovery is almost certainly the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. The paper presents Australian experiences and updates the outcomes of an industry sponsored research project on the geotechnical issues associated with conventional longwall recoveries.A number of critical features of the geotechnical environment, support design and mining geometry have a pronounced impact on ground control during face recovery. A model of roof behaviour at the take-off point has been developed and validated. Key geotechnical issues include low roof competency, an adverse weighting environment, geological structure, horizontal stress concentrations at the gate ends and the capacity and. integrity of the shields. All of these are identifiable either at the support design stage or, at worst, prior to the start of powered support removal, which is the critical stage of the recovery process.Key aspects of the geometry and process are the ability to maintain powered support resistance during bolt-up and recovery, the direction of shield removal, the impact of chutes and the speed of the shield removal process.The author presents case studies that illustrate these issues and the associated ground behavior.INTRODUCTIONEnd of panel face recovery is almost certainly the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. However, unlike most geotechnical aspects of coal mining, very little research has historically been undertaken on the issue of conventional longwall recoveries. This is in marked contrast to the design of pre-driven recovery rooms, an issue which has attracted considerable research interest (Thomas, 2008). In 2006, Strata Engineering completed an Australian industry (ACARP) sponsored research project aimed at defining and minimising the geotechnical threats during longwall face recovery (Strata Engineering, 2006). The research project had the overall objective of developing guidelines for the specification of ground control strategies, so as to minimise the likely geotechnical threats relating to the safety, operational costs and production delays associated with the recovery of a longwall face."

Jan 1, 2010

By Ed Van Eeckhout

As part of a U.S. Bureau of Mines-sponsored project on the utilization of "stiff' backflll to minimize potential rock burst hazards, a finite element study was undertaken to predict stresses and displacements around a stope mined up from the 5400 level of the Sunshine mine, Kellogg, Idaho. This stope was to be backfilled using a high-modulus fill. In preparation for that configuration, certain instrumentation was installed near the 5400 level to monitor stress changes. Due to untenable economics, only 2 cuts of the stope were ever made. This paper presents the results of the instrument readings taken during the first cut and compares them with the numerical predictions, as well as presents the modeling results of closure and fill stresses in the stope if it had been mined.

Jan 1, 1984

By Anthony Spearing

There are approximately 68 million rockbolts installed annually in underground coal mines in the USA (Tadolini and Mazzoni, 2006). The most utilized is the passive fully grouted resin rebar bolt, which constitutes almost 90% of the primary support in US underground coal mines. Resin-grouted rockbolts are a very effective and safe support, but their performance still needs to be further optimized and improved. This paper investigates their performance and suggests operating conditions to optimize the performance of resin-grouted rockbolts. In addition, this paper discusses a small-scale test that can also be used to evaluate the performance of rockbolts that is quicker, easier, and less costly than short encapsulation tests.

Jan 1, 2014

By Kevin J. Ma, John Stankus

"In order to access remote reserve areas, some U.S. coal mines have to maintain aged underground entries for a great distance. However, high humidity, warm temperature, and time dependent deterioration can cause progressive roof deterioration and unexpected roof falls can, pose a great challenge to ground control engineers. With an active belt structure in place and limited space, re-bolting becomes very costly, less effective, and, sometimes, impractical and unfeasible. To gain long-term entry stability and serviceability, operators typically rehabilitate the aged belt entries by installing standing steel set supports. In the last several years, Keystone Mining Services, LLC, (KMS) has assisted many coal mines with their belt entry rehabilitation projects, evaluated the ground condition of various aged belt entries, and designed different standing steel set support systems. This paper presents a case study of a large-scale roof fall that occurred at an aged belt entry in a mine located in an eastern coalfield, analyzes root causes of excessive deformation of square sets that were installed in an adjacent entry, evaluates the adequacy of an existing rehabilitation square set, and develops remedial recommendations for future rehabilitation practice. Based on the case study, the paper outlines design guidelines for rehabilitation steel sets that include field evaluation, engineering considerations, design assumptions, steel structural analysis, and field installation quality control. INTRODUCTION With gradual depletion of shallow coal reserves, some underground coal mines have to maintain long-term entries (belt, track, return airway, etc.) for a great distance in order to access remote coal reserve areas. However, due to high humidity, warm temperature, and aging in the belt entry, roof strata weathering and pillar degradation become more and more severe. Progressive roof deterioration poses a great challenge to ground control engineers. Pillar rib sloughs at certain locations, entry width increases in some areas, immediate roof sags, immediate roof rock falls out between bolts, existing bolts lose functionality due to corrosion, cutters develop deeper in corners, and strata separation develops deeper up into the main roof. As a result, unexpected roof falls occur in a random manner resulting in loss of production, damage of belt structure, and even injuries to underground personnel."

Jan 1, 2017

By Peter Fischer, Heinz-Jiirgen Kateloe, Anton Sroka, Axel Preusse

"Since mining in Germany has a long history, there are still many locations where the surface fracture problem has to be solved. Shallow mine workings from an earlier era can affect the surface in a discontinuous way. In many cases, due to light overburden load, those mine rooms did not collapse and a downfall can suddenly happen for many reasons, e.g., in connection with a changed load entry into the subsurface or by additional influence of water. This results in ground movements, which in extreme cases, appear as surface fractures.Depending on the use of the surface, ground movements caused in such a way constitute an endangerment by law. Particularly, within the southern Ruhr district, surface fracture and subsidence endangered ranges are determined on the basis of an empirical method that resulted from a systematic evaluation of surface fractures and rock disclosures. This method indicates maximum ranges within which impacts from abandoned shallow mine workings must be assumed.With a special emphasis on a risk management approach, RWTH Aachen University adapted the model of stochastic media to the conditions of abandoned shallow mining to compute and illustrate potential ground movements due to the collapse of shallow mine workings. Further investigations were concerned with the analysis of characteristic parameters which can serve for the assessment of the surface fracture probability in a comparative view of different cases. In recent years, more than 50 surface fractures in the southern Ruhr district were analyzed in order to develop a risk assessment of surface fracture endangered ranges over shallow mine workings. The results that have been obtained so far are described in this paper.INTRODUCTIONAbandoned underground mine workings of shallow depth can pose a threat to the surface. Due to small overburden rock loads, shallow excavations often remain intact for decades after the operations have ceased and may collapse abruptly (Braemer et al., 2005; Hollmann and Nuerenberg, 1972; Preusse et al., 2007). Several approaches for prognosis of surface fracture resulting from 97 subsurface collapse deal with this topic (Loebel and Sroka, 2001; Tajdus and Sroka, 2009)."

Jan 1, 2010

By Yoginder Paul Chugh, Jason E. Tinsley

"Over the last five years, the authors have compiled a database of Short Encapsulation Pull Test (SEPT) evaluations from Interior Basin (Basin) coal mines. Typically, SEPT data are analyzed to obtain peak load carrying capacity per unit length of resin encapsulation designated as the ""Grip Factor,"" or GF. In a previous paper, ""An Analysis of Short Encapsulation Bolt Pull Test (SEPT) Data From Interior Basin Coal Mines,"" authors analyzed tangent value of the load-displacement curve as Anchorage Stiffness at 50% (AS-50) of the peak load, or GF value, to assess reinforcement potential of an installed bolt system. This paper attempts to correlate the secant AS VALUES (of SEPT on 6 ft (1.8 m) long installed bolts at a lower load of about four tons from several different mines in the Basin with their MSHA-approved primary support requirements. Since the current regulatory environment requires plan changes dynamically to ensure that roof falls are minimized, the authors hypothesize that the practiced roof control plan must meet the minimum requirement of adequately supporting immediate roof strata. The analyses are based on 181 SEPT studies performed by Minova professionals. Analyses indicate a macrolevel correlation for both roof support density and roof support cost that may have the potential to be used by mine operators to compare their performance with other mines in the area. Limitations of the analyses include ignoring mining depth, mining height and required secondary supports as part of the requirements for a roof control plan. The authors recommend additional analyses to develop this concept further to develop guidelines that can be confidently used by coal companies.INTRODUCTIONPartially- and fully-grouted bolts are extensively used in coal mines for roof support as part of MSHA-approved roof control plans. Since grouted bolts have a larger area of contact with the rock, they can develop much greater anchorage capacity than conventional expansion shell (point-anchored) bolts. Load transfer characteristics and interaction mechanics between steel rebar, grout and rock mass around the bolt hole for grouted bolts are generally assessed in the laboratory or field though short-encapsulation pull tests (SEPT), which have been extensively researched by Aziz and Jalalifar (2005), Aziz (2004a), Serbousek and Signer (1987), Signer (1990), and Mark et al. (2002), among others. In a SEPT, about 12 in. (30 cm) of bolt length is typically encapsulated in resin and a pull test is performed to collect load-displacement data prior to failure and, in some cases, post-failure. A typical load-displacement data set for a bolt is shown in Figure 1. SEPT characteristics are designated by peak load carrying capacity, or grip factor, which is the peak load carrying capacity per unit length of resin encapsulation. SEPT is routinely used to assess anchorage performance characteristics of different lithologies overlying a coal seam to identify type of bolt, bolt diameter, length and spacing across and along an excavation. Anchorage stiffness, or AS, at any load may also be calculated as the tangent or secant value of the slope of the load displacement curve at a particular load level. Anchorage Stiffness characteristics of bolts from SEPT data have typically not been considered in any depth until recently (Chugh et al., 2014; Chugh et al., 2015)."

Jan 1, 2016