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By Mingju Liu

In this research, we measured fractal characteristics of over ten typical outburst areas on different scales and related them to their outburst proneness, in attempts to provide new methods for outburst prediction. It was shown that both faults and folds follow the power law on coal basin scale, mine scale or mining district scale, whichever it is. Fractal dimensions are appropriate indices to describe geological structures quantitatively. Preliminary relationships have been established between outburst proneness and fractal dimension. Comparisons have been made among fractal characteristics and degrees of outburst proneness on all three scales. The results indicate that multi-scale assessment of outburst proneness can be made based on fractal measurements independently or supplemented by other assessment methods.

Jan 1, 2000

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms in strata around a longwall gate road system at two sites in an underground coal mine near Ham, WY. Of particular interest was timedependent (on the order of several months) roof deformation. Strata above and below the coal seam are very weak, carbonaceous mudstones that have cohesionless planes of weakness oriented along bedding, which dips approximately 8° to 16° at N 49° E. An extensive array of instrumented roof bolts, roof extensometers, and biaxial stressmeters were installed during development mining at these two sites under variable depth of cover [I83 to 335 m (600 to 1,100 ft)], seam dip (10° to 16°), and top coal thickness [estimated to be 0.30 to 0.61 m (1 to 2 ft) on the downdip side of the entry]. Bolt load and roof deformation histories during the entry development periods were compared. Bolt loads at site 2 were less than those at site 1, which is consistent with the amount of overburden. Deformation magnitudes at site 2 were similar to those at site 1, but deformation rates were approximately 44% greater and nearly constant over time, whereas the deformation rates at site 1 decreased exponentially over time. Also, most deformation occurred above the bolt horizon at site two, but extensometer measurements at site 1 showed that a significant amount of deformation occurred within the bolt horizon. The most significant factor that influenced bolt tension and timedependent strata deformation was strength and Young's modulus of the strata. Finite-difference models support this conclusion.

Jan 1, 1996

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

The objective of extensive underground experimentation on three longwall coal faces was to improve the stability of mechanised longwall faces through investigation of the relations between support resistance and convergence and resultant deformation of roof and face strata. In particular the influence of high setting and yield pressures on several strata and support parameters was studied in great depth. Increased setting pressures resulting in an increase in setting load densities from 0.15 MN/m2 to O.45 MN/m2 were shown to reduce convergence, face spalling roof flaking, lateral movement of roof and floor, extrusion of the face wall and progressive failure of the coal wall ahead of the face. This was shown to result mainly from the change in roof strata deformation from a wedge shaped compression zone with tension at the face edge to an even beam shaped compression zone over the face area. Debris thickness above the support canopy and below the base was observed to be the one of the most variable parameters which influenced the support characteristics particularly the rate of load acceptance by supports and the roof to floor convergence. A setting load density of 0.4 MN/m2 to 0.5 MN/m2 was found to be optimum under soft to moderately strong sandstone roofs. The authors demonstrated that use of guaranteed set valves with powered supports contribute to good strata control, reduce face down-time and increase face productivity.

Jan 1, 1984

By Qingxiang Huang

Shallow seam coal field has the largest coal reserve in China. Mining in shallow depth causes serious problems. One of them is subsurface dewatering. In this paper, the physical simulating model was performed to study overburden movement and aquiclude stability in the shallow seam mining of Yushuwan coal field, China. According to the characteristic of clay aquiclude in the overburden, the proper simulation materials and proportions of mixture for simulating the plastic clay aquiclude layers and brittle bedrock layers were determined by the stress-strain tests. The physical simulating model was carried out to simulate the failure and caving process of the roof and overburden. Based on the simulating tests, it was found that the movement of clay aquiclude follows the movement of the underlying bedrock layers. The basic caving mechanism of the overburden roof strata was also presented. The tests also showed that the best way to control the stability of aquiclude is to reduce the subsiding gradient Ts. This research provides a foundation for further study on mining dewatering and water conservation.

Jan 1, 2006

By Tetsuro Esaki

This paper describes the present condition of mining-induced damages in Japan and the characteristics of them. The change of mining operation and environmental condition has caused various significant impacts on the ground environment: subsidence, cave-in, springing out of ground water etc. The characteristics and the influences of these phenomena are discussed by investigation of the actual conditions and by fundamental studies. Some counter measures are also introduced.

Jan 1, 1989

By D. S. Choi

This paper describes an improved source location technique for microseismic events related to coal mining. A system of non- linear distance equations is solved with the use of the Newton-Raphson method. This computational method is- advantageous when the number of geophones is limited. In addition, this paper includes the results of the analysis of data from several instruments which characterize both mine seismicity and environmental noise such as vehicle traffic. Analysis of the mine seismicity has contributed to a better understanding of caving above a longwall gob and of what constitutes stable ground conditions.

Jan 1, 1990

By Jim Sandford

The South Bulga Colliery longwall operation, owned and operated by Cyprus Coal Australia. Oakbridge Pty. Ltd., and located near Singleton, NSW, is currently operating under an average 80-ft-thick (24-nt) sandstone unit residing immediately above the coal seam, resulting in sudden and severe face weightings. To date, nine severe weightings have occurred at the property (within Panels 3, 4. and 5), three of which resulted in face closures accompanied by significant equipment damage and several weeks of downtime. The remaining six events nearly resulted in face closures, incurring single-pass vertical convergence of up to 8 in (200 mm) along the pan line. In an effort to mitigate and/or eliminate the occurrence of these severe weighting events, mine management has established an in-house program to immediately address the geotechnical and operational mechanisms underlying rapid weightings, as well as specific measures to mitigate these events on future panels. In support of this in-house effort. NSA Engineering Pty. Ltd., is assisting the engineering staff at South Bulga in developing a more aggressive face monitoring program. The aim of this program is to provide weighting occurrence predictions sufficiently ahead of time to ensure that operational controls are optimized prior to attempting to mine through potential weighting areas. This paper reviews the occurrence of weightings to date at the South Bulga Colliery as well as the latest developments in shield monitoring technologies to forecast these events in near-real time.

Jan 1, 1999

By Yi Luo

The efforts to mitigate longwall subsidence influences on various residential and farming structures by many coal mining companies have been successful in the recent years (Luo, et al, 2003). The successes were largely dependent on the accurate subsidence predictions, correct assessments of potential influences, appropriate design and implementation of the mitigation measures. However, such efforts for protecting large industrial structures are still very rare. In this paper, the successful assessment and mitigation of the subsidence influences associated with a very intensive dynamic longwall subsidence process on a large workshop are presented.

Jan 1, 2005

By Juan J. Monsalve, Nino Ripepi, Richard Bishop, Jon Baggett

"Terrestrial laser scanning (TLS) is a useful technology for rock mass characterization. A laser scanner produces a massive point cloud of a scanned area, such as an exposed rock surface in an underground tunnel, with millimeter precision. The density of the point cloud depends on several parameters from both the TLS operational conditions and the specifications of the project, such as the resolution and the quality of the laser scan, the section of the tunnel, the distance between scanning stations, and the purpose of the scans. One purpose of the scan can be to characterize the rock mass and statistically analyze the discontinuities that compose it for further discontinuous modeling. In these instances, additional data processing and a detailed analysis should be performed on the point cloud to extract the parameters to define a discrete fracture network (DFN) for each discontinuity set. I-Site Studio is a point cloud processing software that allows users to edit and process laser scans. This software contains a set of geotechnical analysis tools that assist engineers during the structural mapping process, allowing for greater and more representative data regarding the structural information of the rock mass, which may be used for generating DFNs. This paper presents the procedures used during a laser scan for characterizing discontinuities in an underground limestone mine and the results of the scan as applied to the generation of DFNs for further discontinuous modeling.INTRODUCTIONAccording to the Mine Health and Safety Administration (MSHA), between 2006 and 2016, the underground stone mining industry had the highest fatality rate in 4 out of 10 years, compared to any other kind of mining (MSHA, 2016). Additionally, during that same time, 40% of the fatalities were due to ground control issues, such as roof and rib collapses and pillar bursts. The National Institute for Occupational Safety and Health (NIOSH) developed guidelines for designing underground stone mines (Esterhuizen, Dolinar, Ellenberger, and Prosser, 2011). However, these guidelines do not apply to all underground stone mines.Monsalve, Baggett, Bishop, and Ripepi (2018) present a case study of an underground limestone mine experiencing a structurally controlled mode of failure. The authors analyze different methods to study that type of instability. They conclude that the integration of terrestrial laser scanning (TLS) with discrete element modelling (DEM) can be used to prevent rock falls in underground excavations to enhance worker safety. However, an adequate rock mass characterization and structural mapping must be performed in order to generate reliable models that allow the engineer to have a better understanding of the rock mass behavior."

Jan 1, 2018

By Bruce K. Hebblewhite

Underground pillar extraction continues to be practiced in Australia in various forms, in spite of the dominance of the longwall method, which accounts for over 90% of underground production. Australia has been a leader in various forms of pillar extraction, particularly since mechanisation was introduced to the industry during the middle of the 20th century. Historical developments have included a range of thick seam mining pillar extraction methods; the widely adopted Wongawilli method; early adoption of the use of mechanized breaker line supports; and a number of more recent partial extraction systems including what is known as rib stripping. Current pillar extraction practice varies from total extraction panels to various forms of partial extraction, driven by either surface subsidence constraints or by underground conditions. This paper reviews a number of different pillar extraction methods and mining layouts, and also highlighting a number of fundamental geotechnical principles which have been found to be critical for safe pillar extraction ? often through lessons learned the hard way, as a result of back analysis of different accident scenarios. These principles include the all-important understanding of the location of the goaf (gob) edge at any point in the operation, and the management of that goaf edge; the impact of geological structures on local and regional stability; the critical importance of extraction lifting sequences; the role of stooks (remnant pillars); and a number of other factors. The paper will also briefly reference the educational and training practices adopted in Australia aimed at the continuous improvement of pillar extraction safety.

Jan 1, 2011

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By Ross W. Seedsman

The in situ behaviour of claystone floors associated with the Wallarah, Great Northern and Fassifem Seams has been studied using a comprehensive suite of stress and displacement monitors. The instrumentation was supplemented by in situ and laboratory testing of the claystone materials. The instrumentation demonstrated that the presence of claystone floors is associated with floor heave, extra rib spall, and settlement of the pillar into the floor. Horizontal floor extensometers showed that the depth of extrusion of clay was limited to 1-2 in. A pore pressure transducer installed in a claystone floor demonstrated the possible existence of short-term undrained behaviour of the claystone. Long term swelling of destressed claystone was also documented. Settlement of pillars into claystone floors can be analysed using simple elastic theory. The assumption of plane strain is valid for most claystone thickness/pillar width aspect ratios. Floor heave can be analysed using bearing capacity concepts. However, it is considered that heave is the result of local bearing capacity failure in the rib spall zone. The advantage of this model is that the bearing capacity equation can be applied without resort to unrealistically high factors of safety.

Jan 1, 1992

By Lawrence R. Artler

The North American Coal Corporation, its subsidiaries and other coal companies, have been mining the Pittsburgh #8 seam in Eastern Ohio for several decades. Underground mining by the "room and pillar" method, and more recently by longwall methods, have been practiced. This presentation deals with ground control problems and subsequent solutions to those problems for both mining methods. But first, let me briefly discuss some pertinent points about the geologic conditions of the coal seam near the southwest edge of the Pittsburgh Coal Basin. The Eastern Ohio region of the Pittsburgh Coal Basin is situated along the western edge of the unglaciated Allegheny Plateau. The local terrain is quite steep, with topographic relief of 430-500 feet being common. The coal thickness in the region ranges from 36"to 120", with the average being 60" in the areas being currently mined. Regional dip of the seam is toward the southeast in an amount of 10 to 40 feet per mile. A cross-section of the mineable seam generally consists of the main bench (60"), a rider coal (6"-12"), and a claystone parting (6"-12") for a total excavated height of 6-7 feet. Greater excavated height occurs where the fireclay floor is so soft and friable that movement of mechanical equipment breaks up the first 4-8 inches. Generally, the main bench and rider coal thin and become more variable toward the southwestern portion of the mining area. The rider coal also grades laterally into a coaly carbonaceous shale. In those areas where the roof coal can no longer be counted on to help support the immediate top, some of the main seam (6") is left to protect the overlying 8'-12' of mudstone/claystone strata. These strata are virtually non-bedded, highly slickensided units and are "hung in suspension" by roof bolts from a more competent limestone stratum above. These mudstone/claystone units are very susceptible to moisture and thus to the weathering cycles. The rider coal or 6" of the main bench is left in place to "seal" the mudstone/claystone units in Mains and Submains development. For many years, it was thought that the strength of the limestone member was detrimental in attempts to fully extract the seam by conventional means; however, longwall methods that have been more recently employed have broken the "myth" that the Redstone Limestone prevents full extraction of pillars.

Jan 1, 1984

By Klaus Opolony

The German coal industry operates a multi-seam extraction system, with average working depths currently around 1,000 meters. The gate roads serving the longwalls are often used for a second time - a practice which has the advantage of allowing several panels to be extracted with a minimum of roadway drivage work. This preference for re-using gate roads is based on a number of factors: such an arrangement a) prevents the strata pressures in the subsequent panels from being adversely affected by the presence of support pillars, b) helps achieve the target of full mineral extraction and c) allows the roadways to be used for air conditioning and ventilation purposes as part of an advancing longwall system. This paper explains the planning procedure required for a rectangular rockbolted roadway, using as an example a gate road in the 3 meter-thick D2/C seam at Lippe colliery, and describes the subsequent operating experience. Here special attention is focused on modem procedures and techniques for mine roadway planning. The investigation also examines the part played by the mine ventilation system and the available machine technology, which are particularly important parameters for the design of the T-junction areas and support system. Keywords: multiple seam extraction, longwall mining, roadway drivage, roof bolting, roadside pack system, hydraulic frame type support.

Jan 1, 2002

By Ihsan Berk Tulu, Keith A. Heasley, Chris Mark

"The Analysis of Retreat l\.fining Pillar Stability (ARJ\1PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover. To this end, the National Institute for Occupational Safety and Health (NIOSH) and the West Virginia University (WVU) l\.fining Engineering Department have been working together to improve both the ARJ\1PS and the Lal\.fodel programs.This paper compares and analyzes the overburden loads calculated by ARJ\1PS 2002, the new ARJ\1PS 2010, and the Lal\.fodel program with regard to the retreat pillar line, the gob, and the barrier pillars. The analysis shows a number of distinct differences between the ARJ\1PS and Lal\.fodel loading distributions. Ultimately, the programs, with their distinctive load distributions, are used to analyze a database of 52 deep cover pillar retreat case studies, and the ability of each program to discriminate between successful and unsuccessful retreat pillar plans is evaluated.INTRODUCTIONThe Analysis of Retreat l\.fining Pillar Stability (ARJ\.f PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover.Pillar recovery accounts for less than 10% of the coal produced from underground coal mines; however, in the period between 1989 and 1996, it accounted for more than 25% of all ground fatalities (l\.fark et al., 2003). In the past 15 years, the safety of retreat mining operations has greatly improved. l\.fark (2009) explains three steps promoted by l\1SHA and NIOSH for safer pillar recovery:1) Global stability through proper pillar design2) Local stability through proper roof support3) Worker safety through proper section managementThe ARJ\1PS and Lal\.fodel (Heasley, 1998) programs have been playing an important role in guiding mine operators in designing pillars to ensure the global stability of a retreat panel."

Jan 1, 2010

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

By P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993

By Andre Cezar Zingano

"The objective of this paper is to estimate the coal seam strength and the pillar safety factor for low coal seams using back analysis and rock mass characterization. The coal seam strength is very important to estimate both pillar capacity and mine section design. There are two approaches to estimating coal seam strength: (i) by geomechanics characterization of the boreholes cores; and (ii) by back analysis from pillars successes and failures, or some in-situ tests. For low coal seams (1.2 m, or 47 in.), the confinement of the center of the pillar is very important to defining global pillar strength because the low seam height increases the confinement of the pillar core, even in small pillars. We used in this case study one underground coal mine in the Parana State where the coal seam thickness is 0.8m the pillar height is 1.3m, the entry width is 5m and seam depth is 130m. A panel test was prepared to study the pillar behavior for use of the retreat mining method. The designed pillar size was 6x5m and 1.3m high with a predicted safety factor of 1.0 based on the Hoek and Brown failure criterion is 5.3 MPa. Once no pillar failure was observed, the pillar safety factor was assumed at 1.0 and the coal seam strength was backcalculated. The back analysis estimated the coal seam strength to be 6.2MPa. The confinement of the pillar center directly affects the coal pillar strength, and also the coal seam strength estimated by back calculation.IntroductionThe coal seam strength is very important to estimating pillar strength and mine section design. There are two approaches to estimating coal seam strength: (i) by geomechanics characterization of the coal seam using borehole cores; and (ii) by back analysis from pillar successes and failures, or some in-situ tests like panel test or ultimate pillar capacity (Salamon and Munro, 1967; Maleki, 1981; Peng and Dutta, 1992). For the first approach, lab tests (mainly triaxial tests) and borehole geomechanics descriptions (quality and quantity of the discontinuities) determine the coal seam strength by the Hoek and Brown failure criterion (Hoek and Brown, 1997). For the second approach, a panel test to study the pillar behavior can be built to define coal seam strength by back calculation. The safety factor for the pillars in the test is 1.0, based on the estimated coal strength from the geomechanics characterization. In this case, the probability of failure is 50%. If some pillar failure occurred, and the real pillar size was measured, the back analysis could be applied to define pillar strength and the coal seam strength."

Jan 1, 2015