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By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By Bruce K. Hebblewhit

Australia is well endowed with extensive reserves of thick underground coal scams, particularly in the range of 4.5m to 9m thicknesses. (For the purposes of this paper, thick scams are defined as being greater than 4.5m thick.) At the present time, the thickest, high production, high productivity underground mining operation in Australia operates at a maximum of approximately 4.8m using single pass longwall methods. In a number of instances, conventional' height operations are mining thick seam coal, effectively sterilising considerable reserves - either because of financial or technical risk concerns, or in some cases poorer quality coal at some horizons in the scam, UNSW has been involved in thick scam mining research over many years, most recently through an ACARP -funded project, jointly with the CMTE in Australia. A key part of that project involved the assessment of the relative geotechnical risks associated with the various methods under consideration. The methods were: single pass longwall; multi-slice descending longwall; soutirage and related caving methods (including the Chinese Longwall Top Coal Caving method (LTCC)): and hydraulic mining. A formal risk assessment process was adopted to review the risks, relative to a conventional 4m high longwall operation. This paper presents the findings of that risk review and discusses the geotechnical issues (and some possible solutions for risk management) which need to be dealt with in order to bring these methods to fruition in the context of the Australian coal industry environment.

Jan 1, 2001

By J. K. Whyatt

A comprehensive survey of mine seismicity and rock bursting during development of two sublevels at the Lucky Friday Mine, Mullan, ID, USA, was conducted to better define rock failure mechanisms and sources of ground control hazards. Survey data included rock burst damage reports, seismic event locations and magnitudes and, for the most energetic events, first-motion information. Several subsets of this database, including large seismic events, rock bursts, and microseismic activity near the face, were analyzed. Elements of the large-event and rock burst data sets were nearly independent. That is, there was no relationship between the risk posed by a seismic event and its energy, although such a relationship is well established for the mine as a whole. All data sets showed that certain geologic features appear to control the spatial distribution of events and the spatial distribution of rock burst risk. The data also suggest that. within the scope of this study, the greatest risk of injury occurs when a pocket of heightened rock burst risk is first encountered and that this risk is controlled when miners adapt their practices to these conditions. Recognition of the role of particular geologic features in the spatial distribution of rock burst hazards provides an opportunity for anticipating, rather than only reacting to, a changing level of rock burst hazard. However, much work will be required to make good on this promise.

Jan 1, 1998

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Zuoliang Xiu

Experience indicates that rockbolting is the most efficient and cost effective means of supporting gateroads of retreating longwalls. However, gateroads supported solely by rockboltiug currently accounts for less than 16% of the 4500km of annual gatemad drivage in China According to a stated target of the Ministry of Coal Industry, this percentage should be at least 50% by the year 2000. Since the learning stage of the technology which is indicated by low drivage rates and frequent roof falls in rockbolted gateroads must be overcome , strategies for the application of rockbolting to gateroads have become the focus of discussions on this topic. This pap presents the recent development on longwall gateroad rockbolting technology in China The development work has concentrated on understanding the mechanism of rockbolting; establishing new design procedures of bolting parameters fix gateroad support upgrading the consumable; optimizing bolt installation procedures; adopting better roofbolting machines; and developing adequate secondary reinforcement technique. The authors believe that this development work together with the on-going wok in other aspects of gaterosd rockbolting, as discussed in this paper 9 represent the rational strategies for the application of rockbolting technology to a wider range of longwall gateroads in Chinese d mines .

Jan 1, 1997

By Stanley J. Michalek

Good planning is critical to operating a safe and efficient mining operation. As the shallow coal scams become depleted, attention turns toward deeper mineable seams or remnant coal reserves. It is becoming more common to place a mine in a scam that is below or adjacent to existing mine workings. Significant research has been conducted regarding the ground control issues of multiple scam mining. However, safety aspects of the operation may not be fully considered. Safety issues include working below or adjacent to a flooded mine and controlling gas Communication between the active mine and the nearby mine. This paper will discuss the safety issues a mine operator should consider when working under or adjacent to mines that may be flooded with water.

Jan 1, 2000

By W. G. Pariseau

A new approach to the direct inclusion of the effects of joints and geologic structure in rock mechanics analyses is applied to an elastic analysis of stability of a Lucky Friday undercut and 4111 longwall stope (WL) using the finite element method (FW). The approach is base+ on the concept of equivalent properties of a heterogeneous test volume, but does not depend on the assumption that a test volume is a representative volume element. Comparison with conventional FW results show greater variability in fields of stress, strain, displacement, safety factor and energy densities. Stability requires a knowledge of rock mass strengths as well as elastic moduli.

Jan 1, 1989

By Gregory M. Molinda

An investigation was conducted to determine the nature and frequency of coal mine roof failure beneath valleys. A mechanism for this failure, and suggestions for controlling this problem are presented. Hazardous roof conditions identified in a number of mines were positively correlated with mining activities beneath stream valleys. Mine maps with overlays of unstable roof and locations of stream valleys show that 52% of the instances of all unstable roof in the surveyed mines occurred directly beneath the bottom-most part of the valley. The survey also showed that broad, flat-bottomed valleys were more likely to be sites of hazardous roof than narrow-bottomed valleys. Evidence of valley stress relief was found beneath a number of valleys in the form of bedding-plane faults and low-angle thrust faults. This type of failure, previously believed to be only a shallow phenomenon, was found at mining depths as great as 300 ft. In situ horizontal stress measurements in a mine beneath a valley and the adjacent hillsides confirmed valley stress re1ief. Underground mapping showed that roof falls in excess of 100 ft in length and up to 25 ft high closely aligned with the valley axis. Numerical analysis of 13 valleys overlying one mine property showed the effect of the valley excavation on horizontal and vertical stress. Thickness of cover, valley shape, and the orientation of the valley relative to the maximum regional horizontal stress all influence the "valley effect" on roof stability.

Jan 1, 1990

By David Oyler

Unusual circumstances may require that a longwall retreat into or through a previously driven room. The operation can be completed successfully, but there have been a number of spectacular failures which exposed miners to serious roof fall hazards. To help determine what factors contribute to such failures, the National Institute for Occupational Safety and Health (NIOSH) compiled a comprehensive international database of 130 case histories. The cases include five failures where major rock falls occurred in front of the shields, and six even more serious failures involving major overburden weighting. This suggests two room failure mechanisms. The first is a roof fall type failure caused by loading of the immediate roof at the face as the fender narrows. The second is a weighting type failure caused by the inability of the roof to bridge the recovery room and face area, and affecting rock well above the immediate roof. The data indicate that the roof fall type of failure is less likely when intensive roof reinforcement (bolts, cables, and trusses) is employed together with higher-capacity shields. The overburden weighting failures, in contrast, occurred when the roof was weak and little standing support was used.

Jan 1, 1998

By M. Karmis

mining factors and representative ground movement parameters. Based on this analysis, the previously developed prediction methods were modified and tested for their applicability in the Appalachian coalfield. Finally, in order to facilitate the implementation of the prediction methods, as well as the data processing of the monitoring program, a number of computer software were developed and will be discussed in the paper. Surface deformations above underground mines and their prediction and control have been the topics of an extensive research effort by Virginia Polytechnic Institute and State University over the past few years. During the initial stages of this research, a number of case studies were collected from the literature, the mining industry and various government and state agencies and, based on the analysis of this information, appropriate prediction methods were proposed (Goodman, 1980; Webb, 1982; Hasenfus, 1984; Karmis et al., 1984). Although the prediction methods were based on sound concepts, it became evident that their accuracy and implementation were a reflection of the size and quality of the collected case studies. In this respect, the data bank was rather insufficient and posed certain questions regarding the development of subsidence information due to inappropriate length and spacing of monuments used in subsidence surveys, density and accuracy of such surveys, lack of monitoring horizontal movements, etc. In an effort to alleviate these problems, a comprehensive monitoring program was initiated in the coal mines of southwest Virginia, which included: - Two longwall and five room and pillar mines. - In total, seven longwall and sixteen room and pillar panels. - Monitoring lines in excess of 35,000 feet. - Vertical as well as horizontal measurements. A high accuracy digital computer tacheometer was used in all surveys and the type and spacing of all monuments were in accordance with well accepted guidelines. The data from this monitoring program completed the initially collected data bank and, thus, enabled a much needed statistical analysis between

Jan 1, 1987

By D. J. Reddish

This paper illustrates how numerical modelling has been utilised to predict water and gas pathways around active longwall panels using two case studies. This information has proved invaluable m the design of methane drainage schemes and for risk assessments undertaken at the mining planning stage. The modelling approach described has now been applied successfully to several situations in UK coalmines in order to optimise the position, length and orientation of methane drainage boreholes. The first case study describes the numerical modelling of the stress and shear plane development mund a longwall panel. The simulation was used to predict potential gas pathways through regions of low stress confinrment which are conducive to fluid flow. The conclusions from the modelling were utilised to allow an increase in the volumes of gas drainage and allow the efficient mining of the panel. A detailed discussion is provided on the model construction, determination of input paramters and optimisation of the borehole positions Details are provided on how the models allow a prediction of the fracture patterns, stress distributions and zones of reduced permeability around an active longwall panel. The second case study describes the numerical modelling of a fault adjacent to a face line drivage of a panel that had just started to be retreated. An inrush of water had occurred and a concern was whether further mining would lead to more extensive inrushes or whether the water inrush was associated with more local, limited capacity water bearing features. The role of a fault that was positioned close to the locality of the mine workings was examined. Fracture patterns stresses, strata permeability and water flows were predicted using mechanical and hydro-mechanical coupled models.

Jan 1, 2003

By David Newman

Coal refuse impoundments and underground mines are frequently sited in close vertical and horizontal proximity in the valleys of the Southern Appalachian coalfield. The steep V-shaped valleys provide the volume necessary for slurry storage against a coarse refuse embankment that is constructed across the valley mouth. The presence of coal outcrops on the valley walls provides economic access to mineable coal seams that may be located under several thousand feet of overburden beneath the adjacent ridges. In some instances where the underground mine precedes the slurry impoundment, the type of mining (partial or complete extraction) and pillar centers were selected without consideration for the potential future use of the valley as an impounding structure. The high standard established for the design and stability analysis of the coarse refuse embankments is clearly illustrated by the absence of an embankment failure in the thirty-one years since Buffalo Creek. However, the recent Martin County slurry spill focused attention on a thorough geotechnical analysis to prevent a connection between the flowable slurry and the mine void. The overburden, roof, pillar, floor, and outcrop barrier stability are examined to quantify the break through potential. The strength and physical properties of the consolidated fine refuse are also important since this material is often the most well-defined and quantifiable barrier separating the slurry and mine void. Because each site is unique, the appropriate analysis ranges from the calculation of roof, pillar, and floor safety factors to detailed numerical modeling of the mine workings and the determination of ground deformation and strains resulting from mine subsidence. The method of refuse disposal may be altered to slurry cells where the accuracy of the mine maps, extent of mining, type of mining (development or retreat) cannot be reliably verified or the break through analysis does not produce an acceptable level of risk. Case histories from the Southern Appalachian coalfield are used to illustrate the application of methodologies to quantify the break through potential of coal refuse impoundments.

Jan 1, 2003

By John C. Stankus

From research started six years ago, a new design criteria has been developed for mine roof bolt systems This design criteria precisely and quantitatively determines parameters such as bolt length, installed tension, and spacing that will produce the Optimum Beaming Effect (OBE) OBE, a new roof control term, is defined as the roof beam that has no separation above and within the bolted range with the shortest bolt passible Utilization of this design criteria has shown that in many cases shorter roof bolts with higher installed tension can produce a much stronger beaming effect (Stankus, 1995) This paper will first present a brief deception of how the design criteria was formulated From there, actual case studies where the design criteria was utilized will be discussed In conclusion, current and future research efforts will be presented

Jan 1, 1997

By Thomas M. Barczak

Wood cribs are used extensively to stabilize mine openings by providing resistance to deflections of the immediate roof and floor and by supporting the weight of unstable rock masses. While the unit costs of these supports are relatively low, their extensive use results in significant costs to coal mining. The Bureau of Mines has developed a model that computes the load capacity of wood cribs as a function of the displacement of the crib structure induced by mine roof and floor convergence. This permits comparison of the loading characteristics of various crib constructions and enables systems to be designed with consideration of the load conditions imposed by the mine environment. The design method matches the stiffness, strength, and stability of the crib structure with expected rock mass behavior to determine a crib design and employment spacing that will provide the lowest cost support.

Jan 1, 1993

By Syd S. Peng

A total of 209 cases of subsidence data over longwall panels in 16 US coal seems have been collected and built into a subsidence database. The database is developed under MS Windows environment. It uses a very user-friendly menu driven system. The empirical formulae for a number of commonly used subsidence parameters have been derived from those collected subsidence data.

Jan 1, 1995

By Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By Marc T. Filigenzi

Two of the major ground control safety issues confronting underground mine operations today are shaft pillar stability and the failure of rock around active mine openings. Failure of a mine shaft can lead to the entrapment of workers. Failure of rock around active underground mine openings can lead to roof falls, which in turn can result in worker injuries and fatalities. Finite-element analysis has proven to he a reliable method for predicting stresses and displacements around underground mine openings. However, this is a complex and time consuming technique and is not used as often as it could be in mine planning. The purpose of this paper is to demonstrate one technique developed at the Spokane Research Center that allows the user to create a finite-element model of a two-dimensional section of an underground mine in a relatively straightforward manner. This model is then used to calculate stresses, displacements, and safety factors around mine openings. With this information, mine planners can evaluate the stability of mine openings as well as the stability of pillars and mine shafts. This analysis will help develop mining sequences and layouts that minimize stresses in that section of the mine. This, in turn, should minimize the occurrence of shaft failure, roof falls, and other hazards associated with underground mining.

Jan 1, 1997

By Daniel W. H. Su

A floor-bearing capacity test apparatus was designed and constructed to determine the true bearing capacity of soft floor materials beneath coal pillars or longwall face supports. The apparatus has been employed in entries up to eleven feet in height. Results of in-mine floor-bearing capacity tests suggested that the ratio of the soft fireclay thickness to the bearing plate width and the moisture content in the fireclay were the most critical parameters affecting the ultimate bearing capacity. Foundation engineering principles were applied to interpret the field test results to estimate the floor-bearing capacity beneath longwall face supports.

Jan 1, 1993

By Craig Collins

The objective was to develop a Real Time Drilling Display System for Rotary Roof Bolting using Analyzing Software to display mapped holes as they are being drilled in real time. The Analyzing Software was developed to analyze and interpret drill sensor data produced during a rotary drilled roof bolt hole by using algorithms and drill sensor data trends to find and locate roof separations or voids. After this software was validated through lab and field testing, its application needed to be tested in a production environment. However, the current method used to record the drill sensor data proved to be cumbersome for the mine to use on a continuous production basis. With different more practical mediums being explored and considered, a Drilling Display System was developed that will both map holes in real time and record the data for later review. With the Analyzing Software's current level of accuracy and with the convenience offered by the real time Drilling Display, the Drill Display System was implemented along with a test mine's current roof control plan and tested as a roof separation "Early Warning System". After installing the Drilling Display System onto a machine, input and suggestions from the mine environment were considered and design changes were made. The real world application provided a design direction that made the information produced by the Analyzing Software both convenient and significant for the test mine. Also, using the Drilling Display System increased the number of drilled holes that were both mapped and recorded. The results from studying these recorded data files lead to refinements in the current algorithms used by the Analyzing Software which increased the overall accuracy and capability of the system A fairly accurate representation of void or separation locations in the mine roof can be determined fromsensor data recorded during production bolting cycle. The immediate feedback on the local roof condition that the Drilling Display System provides, amounts to a quicker and safer alternative to the scratch hole and the video bore-scope examinations that arc performed after the bolting is completed. This project has shown that making the alternative method of examining the roof condition practical for the mine to use is just as important as the accuracy of its results. Based on the results of this field test, this or other methods of reviewing the analyzed drill hole data will be explored.

Jan 1, 2004