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By Guy Reed, Russell Frith, Kent Mctyer

"Coal pillar design has historically been based on assigning a design factor of safety (FoS) or stability factor (SF) to coal pillars according to their estimated strength and the assumed overburden load acting on them. Acceptable FoS VALUES (have been assigned based on past mining experience, and at least one methodology includes the determination of a statistical link between FoS and probability of failure (PoF).The role of the pillar width-to-height (w/h) ratio has long been established as having a material influence on both the strength of a coal pillar and its potential mode of failure. However, there has been significant professional disagreement on using both FoS and w/h ratio as part of a combined pillar system stability criterion, as compared to using FoS in isolation. The argument is that, as w/h ratio is intrinsic to pillar strength, which, in tum, is intrinsic to FoS, it makes no sense to include w/h ratio twice in the stability assessment. At face value, this logic is sound.However, this paper will argue that there is a valid technical reason to bring w/h ratio into system stability criteria (other than its influence on pillar strength), as it is related to the post-failure stiffness of the pillar, as measured in situ, and its interaction with overburden stiffness.When overburden stiffness is bought into pillar system stability considerations, two issues emerge. The first is the width-to-depth (W/H) ratio of the panel, in particular whether it is sub-critical or super-critical from a surface subsidence perspective. As a minimum, this directly relates to the accuracy of the pillar loading assumption of full tributary area loading. The second relates to a reevaluation of pillar FoS based on whether the pillar is in an elastic or non-elastic (i.e., post-yield) state in its as-designed condition, as relevant to maintaining overburden stiffness at the highest possible level.The significance of the model presented is the potential to maximise both reserve recovery and mining efficiencies without any discernible increase in geotechnical risk, particularly in thick seams and higher depth of cover mining situations. At a time when mining economics are, at best, marginal, removing potentially unnecessary design conservatism without negatively affecting safety is of interest to all mine operators and is an important topic for discussion amongst the geotechnical community."

Jan 1, 2016

By Keith A. Heasley, Christopher R. Newman

"Originally developed in 1993, LaModel has aided mining engineers and researchers alike in improving the design and safety of underground single- and multiple-seam mining operations (Heasley, 1998). Through the years (Heasley, 2011), the use and capabilities of the LaModel program have grown immensely while adapting to ground control concerns and problems within the mining industry. Presently, MSHA specifically mentions the use of LaModel for the analysis of, “complex non-typical roof control plans,” defined as either room-and-pillar retreat mining greater than 1000 feet, bump- or bounce-prone mines, or other criteria considered unusual by the District Manager (Stricklin, 2013).With an ever increasing number of users and an increase in available solution options and complex analyses, there is now a significant demand for better education and training in the practical application and detailed analysis using LaModel. This paper presents the development of an electronic user’s manual and comprehensive online training course created in an open online learning environment in order to better inform and train industry professionals as well as engineering students and new users. The development of both the user’s manual and online training course will ultimately increase the ground control design effectiveness of mining engineers, leading to safer and more productive mine designs.INTRODUCTIONOver the past 20 years, mining engineers and researchers have used the LaModel program for improving the design and safety of single- and multiple-seam mining operations. LaModel uses a laminated, displacement-discontinuity, boundary-element method which simplifies the overburden into a stack of frictionless homogeneous laminations and limits the detailed analysis to the seam itself. This approach is fundamentally simpler than either finite-element, finite-difference, or discrete-element approaches; and, therefore, the program provides a significant reduction in computational time and efficiently calculates the displacements, stresses, pillar safety factors, multiple-seam interactions, subsidence, etc. for thin-bedded deposits such as coal, salt, potash, limestone, etc. (Heasley and Salamon, 1996; Heasley and Agioutantis, 2001). The laminated overburden model in LaModel is different than previous homogeneous elastic formulations and more realistically represents the natural flexibilities of the stratified sedimentary overburden typically associated with coal seams (Heasley, 2008)."

Jan 1, 2015

By Michael Murphy

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

Jan 1, 2014

By Gamal Rashed

This research is divided into two parts: the first part of this research focuses on understanding coal mine bump mechanisms, and the second part focuses on mitigating the violent failure associated with a bump. Many researchers believe that the coal bumps happen because of sudden loss of constraint between pillar/ roof and pillar/floor or the hammering effect due to the breaking of thick, massive strata above the coal seam, creating a sudden impact load on the pillars and causing them to bump. However, very little work has been done to explore and understand the consequences of these proposed mechanisms for coal bumps. In the first part of this research, computer simulation of coal specimen behavior have been used to explore two issues: 1) How would an instantaneous loss of constraint or a small sudden impact load cause violent failure? 2) How does the strength of the coal specimen control coal mine bumps? Eliminating or reducing the violent failure is the main objective of the second part of this research. The coal specimen needs to be softened either in the rib zone or the core zone and then its mode of failure studied to see whether it is violent or not. Instantaneous loss of constraint and the hammering effect are associated with sudden release of elastic energy, increase in the kinetic energy, and very rapid transient vertical stresses of the coal specimen. These are three of the major proposed mechanisms for bumps. The potential for violent failure increases with increasing the unconfined compressive strength, UCS, of the coal specimen. Failure of the rib zone is not the main factor for the violent failure of a coal specimen. The energy stored in the core zone is the key factor for the violent failure.

Jan 1, 2013

By Uwe Wyink, Guenther Thiele, Isabel Gollnick, Klaus Thyrock

"The application of a thin spray on liner was implemented in longwall T-junction sections in 2009 for the first time. The development of the material and the application is based on a technical cooperation between RAG Deutsche Steinkohle AG and BASF Corporation. The report gives an overview of the first trials. The underground test occurred in a seam with a height of 1.5 m (5 ft) in a depth of 1,100 m (3,609 ft). The roadway is dual-use entry and the roof strata is composed of shale and sandy clay. In some sections geological faults impact the surrounding strata. The poor conditions induce separating and loosening of roof rock. Several remedial actions have to be implemented for decreasing deformations at the face entrance.The application of thin spray on liner (TSL) provides simplification and cost reduction of supporting work at longwall T-junction. The installation of TSL to avoid roof and coal fall operations for lagging are omitted.INTRODUCTIONFor safe operation of face retreat in dual-use roadways, the processes of securing and stabilizing the face end area are very important in order to minimize deformation and stress. Avoiding roof and coal fall are the main concerns for safe operation. For that reason different methods of reinforcement are tested in underground operation. In this paper the application of TSL is presented.The specifications of TSL are described, followed by the case study results. The underground study is located in one mine of RAG Deutsche Steinkohle. The extraction of the hard coal in this mine is characterized by great depth and high stress. There are poor geological and geotechnical conditions. The roadway used for the underground application was headed with the support system combination type A, which will be described later in the paper. After the first face retreat, deformation of steel arch support and bagging destroyed the roadway, so for the second face retreat remedial actions had to be undertaken. For this a new system of reinforcement, simplifying the supporting work at the longwall T-junction was tested. The reinforcement consisted of different parts that are described in the following report. Furthermore two different versions were applied. One method is the operation with TSL and the other one is the use of concrete for sealing. Both methods are compared in the text."

Jan 1, 2010

By Donovan J. Benton

Photogrammetric methods are advancing rapidly and show considerable promise as a ground control research tool. The ability to quickly capture three-dimensional geometry, especially changes in geometry, in underground and laboratory settings is a significant advance from point distance measurements. Photogrammetry is being applied in both of these settings as part of the NIOSH ground control research. This paper describes these applications and the equipment and software being employed. It also describes tests conducted to quantify the accuracy of these systems. Accuracy varied with system specifics and procedures. A factory-calibrated camera produced results within 1/32 of an inch, plus 0.04%. A user-calibrated camera was found to be less accurate, at 1/20 of an inch, plus 0.24%. These calibrations demonstrate that photo grammetry can produce research quality measurements in laboratory and mine settings.

Jan 1, 2014

By Robert J. Coutts, Dan A. Payne

"The Broadmeadow mine is a punch longwall mine owned and operated by BHP Billiton Mitsubishi Alliance (BMA) in the Bowen Basin of Queensland, Australia. In 1999, an exploratory adit was driven as part of the feasibility study for the establishment of the mine. Subsequent development of the mine in 2004 started some distance away from the initial test adit. Ten years later, the trial adit would be used as the headgate of the 5'h longwall panel. However, multiple roof falls had occurred in a heavily jointed area over a distance of 1, 000 ft (300 m) during the past 10 years. Due to the immediate coal roof, low stress environment and time, some of these falls had progressed to 38 ft (11.5 m) above the mining floor. As a result of the extensive work involved and safety implications of trying to recover the roof falls conventionally, a strategy was developed to fill these roof falls with a cement based product from the surface, down boreholes, and redrive the roadways through the consolidated roof fall material. This paper describes the process of rehabilitating these roadways, the results when the roadway was redriven and the headgate performance as the longwall mined through the area.INTRODUCTIONBMA operates 7 open cut and 2 underground mines in the Bowen Basin of Queensland, Australia producing over 50 mt per year of high quality coking coal. As a result, BMA has established over 150 miles (250 km) of open cut highwall.Broadmeadow Mine is a punch longwall mine located 22 miles (35 km) north of Moranbah in Queensland, Australia on the western limb of the Bowen Basin Coal Measures (Figure 1). Broadmeadow Mine commenced development in 2004 and began longwall operations in 2005. Broadmeadow is currently completing the fifth longwall panel and produces over 4 Mt per year of high quality coking coal. Mining is undertaken in the Goonyella Middle Seam, which is 21 to 25 ft (6.5 to 7.5 m) thick in the current mining area.The mine layout required the use of an old exploratory gateroad as the headgate of its 5th longwall. This gateroad had experienced numerous large roof falls and required extensive rehabilitation."

Jan 1, 2010

By William G. Pariseau, Michael K. McCarter

"The challenge of estimating mine-wide subsidence and linkages to seismicity over tabular deposits is addressed by a special finite element technique (dual node - dual mesh). Subsidence and mine-induced seismicity begins near the face when caving occurs and propagates to the surface as extraction reaches a critical extent. Thus, the challenge is to obtain details at the face at the meter scale and also at the surface over the whole mine at the kilometer scale. Interactions between old and new sections of a mine are automatically taken into account with this technique. The finite element method is well established technology based on fundamentals of physical laws, kinematics and material laws. With this technique, no empirical ""scaling"" or fitting computer output by input data ""adjustment"" to mine measurements is necessary. Capability is demonstrated for doing practical whole-mine subsidence analysis from first principles. Mine-induced seismicity is shown to correlate well with face advance and element failure.INTRODUCTIONSeismicity associated with coal mining has been under study for several years by a team at the University of Utah. Team members include mining faculty, research geophysicists and students from the Departments of Mining Engineering and Geology & Geophysics. Mines in central Utah that have been studied include the Trail Mountain Mine (Pariseau, 2015; Pariseau and McCarter, 2015), the Beehive Mine, also known as the Des-BeeDove (Pariseau, 2014), and the Crandall Canyon Mine (Pariseau, 2015). These mines were developed from outcrops in the Wasatch Plateau coal field west of Price, Utah, where the topography is characterized by . steeply incised canyon drainages and high plateaus with relief of a thousand meters or so.Seismicity associated with coal mining in central Utah has been of interest for many years and includes studies in the Book Cliffs to the east of Price, Utah, where mining is still active (Smith et al, 1974; Agapito, Goodrich and Moon, 1997; Arabasz et al, 2001; Arabasz et al, 2002; Fletcher and McGarr, 2005; McGarr and Fletcher 2005; Maleki, 2005) and to the northwest (Ellenberger et al, 2001). Results of many micro-seismic studies are summarized perhaps best by lannacchione, et al (2005) in stating, ""Deviations from normal strata response can provide useful stability information.""This study concerns a fourth underground coal mine in the southern portion of the Wasatch Plateau coalfield, hereafter referred to as the MINE. Focus is on mining from 2004-2008 with the objective of examining mining in relation to seismicity."

Jan 1, 2016

By C. D. Peters

The Bureau of Mines has been researching remote sensing analysis of an underground coal mining area in Utah and computer-aided methods of defining ground control hazards in this mining area. As a result of these studies, a preliminary potential hazards map was developed for the test site. This map is based primarily upon a lineament (linear feature) analysis of Landsat satellite data. Other data used were paleochannel location data, resistivity data, and information from surface and underground field checking of the lineaments and ground control problems in the mine. Lineaments proved to be reliable indicators of hazardous areas, although the type and degree of hazard varies with different lineament trends and the presence of other conditions such as lineament intersections and intersections with other geologic structures or with paleochannels. Lineaments were observed to match faults, joints, and/or joint trends in the area. The paleochannel data, derived from drill hole data, provided basic information about potentially hazardous areas in the mine by suggesting where paleochannels could cause instability in the roof rock. The ground control problem data served largely to define the relative degree of hazard associated with lineaments and the types of hazards encountered in the mine. Resistivity data helped confirm the presence of fractures where they were expected to occur both from surface geology and from the lineament data.

Jan 1, 1986

By Gamal Rashed

Coal bump is a sudden manifestation of the release of strain energy stored in a coal pillar (Peng, 2008). It is a very serious mining hazard that adversely affects the safety and productivity of the mines. Several case histories in mining have described coal pillars or faces of coal failing violently with an accompanying ejection of debris and broken material into the working areas of the mine (Campoli, 1987; Osterwald, 1962; Peperakis, 1958; Garvey and Ozbay, 2013). These failures resulted in large losses of life and total collapses of entire mine panels (Chase, Zipf, and Mark, 1994; Zingano, Koppe, and Costa, 2004). There are questions in the literature about whether or not the coal itself plays a significant role in bump. Rice (1935) mentioned that structurally strong coal is one of the key factors for bump; however, the strength of the Pocahontas #3 coal seam is low, and it bumped. Moreover, it is not obvious in the literature whether the strength of the tested coal samples is the inherent strength or if it is influenced by the shape or the size of the specimen. The main objective of this paper is to answer the following question: To what extent does the mechanical properties of the coal have an impact on the potential for violent failure? The best approach to answer this question is to compare the mechanical properties of two kinds of coals from bump-prone (Utah coal-Sunnyside seam) and non-bump-prone (WV coal-Sewickley seam). This comparison includes uniaxial compressive strength, modulus of elasticity, shear strength (cohesion and friction angle), and the burst index.

Jan 1, 2014

By David R. Burkhard

"Comparative roof beam performance, as measured by delamination within the bolted horizon, was used to evaluate two types of roof bolt drill hole geometries in a development setting. Short encapsulation pull tests had demonstrated a higher grip factor in drill holes featuring helical grooves in the side walls in comparison to drill holes featuring continuous smooth to slightly rough walls. The study incorporated the two geometries during development mining in which fully grouted mechanically anchored bolts were installed as primary roof support.bolts were installed as primary roof support.The study site, in an entry adjacent to a longwall panel, was instrumented with extensometers and data collected during longwall retreat. Statistical analysis was conducted to validate null and alternative hypotheses used to compare drill hole geometries. Analysis yielded no statistically significant difference between drill hole geometries. Inspection of the data does reveal less delamination within the smooth walled site in comparison to the site containing grooved hole geometry.INTRODUCTIONFrom their introduction as a means of ground control over sixty years ago, roof bolts today are the primary mechanism of roof support utilized throughout mining. Whether through suspension, keying, or beam building, roof bolts are employed to reinforce the inherent strength of the strata comprising the immediate roof with the goal of creating a competent bolted horizon of sufficient load bearing capacity to be self-supporting for a given mine's design life, stress regime, and geomechanical properties.For grouted bolts in sedimentary strata, axial loading due to vertical rock movement and bending due to horizontal rock movement are the principal means of loading (Signer, 2000). Regardless of the type of bolt and extent of grouting employed, the success of any roof bolting design is determined, to a large extent, by the effectiveness of load transfer between the rock, grout, and bolt.At San Juan Mine, an underground longwall mine located in the northwestern corner of New Mexico, a number of different roof bolt types, grades, and lengths have been used over the years in an effort to optimize primary roof support. One common denominator shared throughout has been the use of full length grout. Load transfer for full length grout is achieved primarily through mechanical bonding between the grout and rock. It is this interaction that is the focus of the study described in this paper. The study assessed and compared two types of bolt hole geometries employed during gate road development at San Juan. Underground instrumentation and evaluation of the bolted roof provided the means of assessment and comparison."

Jan 1, 2016

By Essie Esterhuizen, John Ellenberger, Dennis Dolinar

"Underground stone mines in the United States use the room-and- pillar method of mining. The stability of the roof between pillars determines the mining dimensions and size of equipment that can be used. The findings of a survey of roof span stability issues and design practices are presented, and the observed factors contributing to roof instability are discussed. Identified stability issues are evaluated, such as maximum roof span, horizontal stress, and roof beam stability. The results of field observations, roof monitoring, and numerical analyses are used in the evaluation. The importance of the first roof beam thickness and its impact on roof stability and support requirements is presented. An evaluation of the current experience with stone mine roof spans relative to international experience is presented. A step-by-step design procedure is suggested which emphazises the need to collect adequate geotechnical data, understand the immediate roof stability, select a mining direction, and meet support requirements.INTRODUCTIONIn room-and-pillar mines, the roof between the pillars is required to remain stable during mining operations for haulage as well as access to the working areas. Roof and rib falls account for about 15% of all reportable injuries in underground stone mines (Mine Safety and Health Administration, 2009). Therefore, the stability of the roof is an important safety consideration. The size of the rooms in underground stone mines is largely dictated by the size of the mining equipment. Large mining equipment is used, requiring openings that are on average 13.5 m (44 ft) wide by approximately 7.5 m (25 ft) high to operate effectively (Esterhuizen et al., 2007). The dimensions of the roof spans are largely predetermined by the equipment requirements, and design is focused on optimizing stability under the prevailing rock conditions. If the rock mass conditions are such that the desired stable spans cannot be achieved cost effectively, it is unlikely that underground mining will proceed. In addition, large roof falls can extend across the full width of a room and may extend more than 5 m (16 ft) above the roof line. These large falls are a safety hazard and can have a significant impact on mine access and production. The National Institute for Occupational Safety and Health (NIOSH) research into stone mine roof stability has focused on identifying both the causes of roof instability and techniques to optimize stability through design."

Jan 1, 2010

By Andrew Lewis, Amarit Nitaramorn, Alexander Garcia, Peter Altounyan

"Indonesia is a major coal producer, the vast majority being sourced from opencast operations. Underground production to date has been constrained by unfavorable ground conditions and limited local experience. However, an underground trial in East Kalimantan has now shown that it is possible to successfully extract coal and mine safely, despite the technical challenges.A trial drift mine has been worked to a current maximum depth of 160 m (525 ft) in a dipping coal seam. The 3 m (10 ft) thick coal seam is overlain by very weak mudstone and sandstones. The mudstone forming the immediate seam roof strata proved a particular challenge given its adverse reaction to water and overall friability.Rock bolts were used as roadway support, the system being carefully selected to optimize effectiveness. An innovative method of partial extraction was trialed, which demonstrated the potential for productive mining despite the ground conditions. Roof falls have occurred within the working area, and roadways demonstrate reasonable stability over time.Throughout the course of the trial, strata behavior monitoring has been an important aspect, and a comprehensive ground control monitoring system is in place. This has enhanced the ability to increase the level of support where required and to react to deteriorating conditions. In addition, monitoring information has allowed a quantitative assessment to be made of the technical feasibility of future full-scale mining.INTRODUCTIONWith current production at over 200 million tons, much of this destined for overseas markets, Indonesia is the second largest exporter of coal (Ritschel and Schiffer, 2007). The majority of the coal is sourced from the Kalimantan coalfields. Mining is predominately opencast, yet in recent years there have been attempts made to introduce underground operations. This move would permit continued access to good quality seams that are beyond the reach of viable opencast mining."

Jan 1, 2010

By Claude A. Goode

Large quantities of high grade coal exist in thick seams in the United States. Many of these thick seams are too deep to be surface mined and do not lend themselves readily to underground extraction. To develop the new mining technology required to effectively mine thick seams, the Department of Energy has entered into a Cooperative Agreement with Mid- Continent Resources, Inc. to demonstrate the multilift longwall mining technique in a 28 foot-thick seam at their L.S. Wood No. 3 mine near Redatone, Colorado. The seam will be extracted in two lifts, with a 5-foot natural partition of coal left between the upper and lower lifts. Proper selection of face supports is critical to the success of this project. Specifications were defined for the roof supports, and it was concluded that the supports selected must provide full coverage, yield at the legs, and have a 60-inch push- out cantilever that can be set at any position under full load. A Hemscheidt two-leg shield was judged as the best qualified candidate and was selected for the multi-lift trial.

Jan 1, 1981

By Ismet Canbulat, John Naylor, Jason Emery, Peter Craig, Andy Sykes

"Longwall mining is by far the most common method of underground coal extraction in Australia. The industry trends and expectations are placing increasing emphasis on the reliability and productivity of these operations. The longwalls are becoming wider and longer while retreat rates are continuously increasing due to significant improvements achieved in longwall equipment reliability and automation. This increased longwall productivity is placing significant emphasis on the reliability of longwall panel development.Although there have been significant improvements in the reliability of continuous miners and hydraulic drill rigs, the traditional resin encapsulated bolt installation is still the principal method used in all major coal-producing countries around the world. Anglo American realised an opportunity existed to introduce an alternative roof bolt installation technique called “spin to stall” in Australia. Spin to stall was first introduced in Anglo American Coal in South Africa where it has been successfully used for over a decade, though implementation of South African spin to stall resin in Australia has proven to be near impossible due to a significant difference in geotechnical conditions, mining method, and, subsequently, roof bolting equipment. Therefore, a new spin to stall development project was initiated between Anglo American Coal and Jennmar Australia (SPIN2STALL®). This paper summarises the journey of this project, the results, and the successful implementation of spin to stall achieved at Grasstree Mine.INTRODUCTIONThe Anglo American Coal business unit operates Grasstree and Moranbah North underground longwall mines in the Bowen Basin of Central Queensland, with a third longwall mine, Grosvenor, currently being developed, and two more (Aquila and Moranbah South) in the project pipeline. The vision for Anglo American is to achieve “Zero Harm” through the effective management of safety at all operations, while for the coal business unit, at the same time to achieve industry-leading operational performance. Currently, both Grasstree and Moranbah North longwalls are operating at industry best practice rates. This is placing increased pressure on the development process to allow such high productivity levels to be sustainable."

Jan 1, 2015

By D. W. H. Su

This paper presents the results of ill situ horizontal stress measurements using a Minifrac system in several northern Appalachian coal mines. The effect of stress magnitude and orientation on longwall gate entry stability was analyzed using a series of three-dimensional finite element analyses. The best headgate stability is achieved if the maximum horizontal stress is aligned with the direction of longwall retreat or if the panel is oriented such that the maximum stress relaxes over the headgate due to the presence of the newly formed gob. The degree of horizontal stress damage to longwall gate entries depends largely on the stress magnitude and roof geology. Accurate assessments of support or design techniques which may mitigate longwall headgate or tailgate instability in areas of thinly laminated roof and high horizontal stresses must account for the in situ horizontal stress state and site specific roof geology. The ability to measure stresses and map geology is thus essential to prevent ground control failures at the longwall headgate.

Jan 1, 1995

By Guy Reed, Russell Frith

"The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database.This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength “index.” These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of super-critical overburden behaviour as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases.A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.INTRODUCTIONThe inspiration for this paper is founded in three statements from two eminent persons in the field of coal pillar strength research, Jim Galvin and Essie Esterhuizen, as follows:Both Salamon and Munro and [the University of New South Wales] based the derivation of their pillar strength formulae on a criteria that the diameter of a panel of pillars, W, had to at least equal the depth of mining, H. This was thought to result in full tributary loading. It is now known that there are some mining environments included in both the South African and Australian databases in which mining span must exceed depth by a considerable margin in order to achieve full deadweight loading. Hence, it is logical to conclude that these data points may have contributed to pillar strength being overestimated by Salamon and Munro and UNSW (underline added by authors). Normally, this should be of no consequence because it is reflected in the probability of design success associated with any given safety factor (Galvin, 2006)."

Jan 1, 2018

By Boqiang Wu, Ganggang Xu, Xiaodong Wang, Hai Wang, Shibin Zhu

"Using a paste-like slurry as a filling from the ground surface to the goaf, artificial pillars can be built in the cavity goaf to support the goaf roof to prevent roof collapse and ground subsidence. If the accumulation laws of paste-like slurry are known well, the filling technology can be optimized. The laws can establish the foundation for finding out the best methods for building artificial pillars and filling technology. In order to understand the accumulation laws of paste-like slurry in the cavity goaf, Aeolian sand with wide distribution in northwestern China was selected as aggregate, and Portland cement was used as cementitious material to prepare a paste-like slurry. A test model of the cavity goaf was built to simulate the filling process by pump from the ground surface to the goaf to study accumulation laws of the paste-like slurry. The results show that the whole process of expansion and accumulation of the paste-like slurry could be divided into stages: free expansion, fill tightening expansion, and stable expansion. The angle of repose of the accumulation body of the paste-like slurry is constant in the stage of stable expansion, and it is the final angle of repose of the accumulation body. The angle of repose of the accumulation body is highly relevant to the slump of the paste-like slurry, and it is unrelated to the distance between the floor and roof. Under certain conditions of the cavity goaf, reducing the slump of paste-like slurry is one of the effective ways to obtain cost-effective pillars.INTRODUCTIONA large number of cavity goafs of coal mines were left in northwestern China; they were formed in the mining process by room-pillar or strip mining. Safety coal pillars were used to support the roof to keep long-term stability in the goaf, so the ground subsidence was not obvious (Fu, Deng, and Zhang, 2011; Xie, 2014). With gradual weathering, destruction of coal pillars, or additional load from ground surface buildings and structures, there could be a risk of instantaneous collapse of the roof in the goaf. This would affect the safety of existing structures on the ground and adjacent mines (Qu et al., 2017; Li, Xiang, and Jia, 2011; Tan, Guo, and Zhao, 2016; Tan et al., 2017). For disaster prevention of the falling cavity goaf, the current common practice is full filling (Hu et al., 2008; Liu, Zhu, and Huang, 2015; Pan and Wu, 2010). However, for buildings and structures with lower deformation requirements, full filling is excessive. For example, it is acceptable for goaf treatment under general roads and industrial sites to control the maximum deformation and keep the deformation continuous (Liu, 2011; Liu, 2012)."

Jan 1, 2018

By Winton J. Gale

The subsidence over a longwall panel at Tahmoor Mine in the Southern Coalfield of NSW, Australia, was found to be approximately twice the size it had been in previous measurements. An investigation into the potential causes was conducted using computer modeling together with hydrological characterization and detailed geotechnical characterization of the strata. The abnormal subsidence was found to be consistent with localized weathering of joint and bedding planes above a depressed water table adjacent to an incised gorge. The study showed that other factors such as variation in stress field, joint zones, variation in rock strength and topographic factors did have sufficient impact to induce the abnormal subsidence. The results have significant implications to subsidence prediction in areas that may be prone to the phenomenon found at Tahmoor. Key indicators of the potential for this form of abnormal subsidence are presented.

Jan 1, 2011

By Christopher Mark

Mines with exceptionally low-strength roof (UCS <3,500 psi and CMRR <40) are much more likely to struggle with roof falls than other mines. Weak-roof is a particular problem for many room and pillar mines in the Midwestern and Northern Appalachian coal basins. Traditional roof support techniques are often not up to the challenge at these operations. This paper focuses on two mines, one operating in the Upper Freeport seam and the other in the Herrin No. 6 seam. Together, the two mines had more than 300 roof falls during a recent 5 year period. Each has experimented with a variety of roof bolt types and lengths, and with different supplemental supports. Detailed statistical analysis was conducted to determine which support combinations have proven to be most effective. Geology, horizontal stress, and time are also important at both mines. Successful control techniques have included; ? Identifying particularly troublesome lithologic roof units; ? Installing longer and stronger roof bolts; ? Installing supplemental support in intersections, and; ? Limiting the duration that a panel remains open. The lessons learned from these mines, and others like them, could help improve ground control safety for the entire U.S. mining industry.

Jan 1, 2004