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By Bob Thomas

The roof drill operator productivity can be enhanced by the introduction of control technology The operators control station can be improved by considering human factors such as handle forces and control location. The smaller electrical controls can be located much easier than the traditional hands on valves. The operators control can he improved by using processors with programmable subroutines that assist the operator in the decision making process. For example to increase feed rate - to slow rotation - to avoid clogged steels - etc. The overall affect is to improve drilling and bolting consistency. The control system uses a dedicated processor packaged as an intrinsically safe control station. The roof drill operator is assisted in the drilling; arid bolting by a closed loop feedback system. The processor monitors the drilling parameters such as feed force, feed rate, rotation torque, RPM, drill position, and vacuum condition Based on the drilling parameters the processor executes pre-determined condition statements intended to enhance drilling The drill operator has an active display to alert him to the real time machine status. For example if the feed were to slow or delay due to a high vacuum level the display would show "high vacuum". The dedicated processor is available on the arm feed style machines, The prototype machines used programmable logic controllers to develop the feedback relations. The ability to record drilling parameters was extremely helpful in this development project. The extended use of the recording feature will aid operators in mapping the mine roofs Interpreting the recorded drilling parameters can reveal a void in the top or hard competent drilling The prototype machines also showed the importance of vacuum monitoring This feature is extremely valuable in softer materials, The benefits of the control system range from efficient use of the consumable bolting materials to uniform operator performance.

Jan 1, 1999

By Brian E. Travis

Lightweight polymer grids have recently been introduced to the mining industry. They are currently being used for roof and rib control, longwall shield recovery, safety guarding or barricade, highwall screening, internal reinforcement of gunnite/shotcrete applications, sub-base reinforcement, construction of steepened slopes, retaining walls, mine reclamation (i.e. erosion control, turf reinforcement), and in numerous mining waste related projects. These polymer mining grids utilize a strong, lightweight polymer, usually special grades of polypropylene or polyethylene. High tensile strengths are achieved by molecular orientation of these polymers in the manufacturing process. The first step in the manufacturing process is to extrude the polymers into thick continuous sheets. These sheets are then punched, and drawn into open grid structures. Aperture size and configuration are controlled by the geometry of the punched hole and the degree to which the grids are stretched. The drawing process is the key to the molecular orientation required for the tensile strengths. These long chain polymer molecules in their natural state have a random orientation. When stretched, the molecules are pulled into a parallel alignment, thus creating a lightweight product with tensile strength comparable to steel. The finished polymer mining grid product is a viable alternative to conventional supplemental roof control meshes such as chain link fence and welded wire. Polymer mining grids are a fraction of the weight, have comparable load support qualities, and will not rust or corrode in even acidic or alkaline conditions. Specific flame retardant additives can also be incorporated to make the polymer grid self extinguishing for use in underground "gassy" environments.

Jan 1, 1992

By Peter Zhang

As simpler and easier reserves are exploited to depletion, underground coal mining continues in more complex geologic settings and conditions. Roof bolting for long-term entry stability is always a challenge. For long-term entries of a mine such as belt, track entries and return airways, roof could be under-supported if one uses the same bolting plan as in short-term entries or even seem over-supported with increasing use of longer, stronger bolts along with mesh, pans, straps or trusses. In most underground coal mines, roof deteriorates over time with cutters developing deeper in entry corners; roof sagging and popping out between bolts; entry width increasing due to rib sloughage; and separations developing deeper up into the roof. In deteriorated long-term entries, roof falls could occur at certain locations and re-bolting could be costly or sometimes unfeasible. Therefore, it is important to choose an appropriate bolting plan during development based on mining and geologic conditions if the entries will be used for the long-term. In both short-term and long-term entries, primary roof bolting is critical in building a strong and effective beam. But in long-term entries, supplementary bolting is necessary in certain cases to ensure roof stability can still be maintained even if a roof failure develops beyond the primary bolted horizon. This paper deals with roof bolting for long-term entries based on studying the progressive roof failure and roof falls under different geologic conditions. The paper presents bolting considerations in terms of bolt type, length, diameter and spacing as well as bolting strategy to minimize the risk of roof falls in long-term entries.

Jan 1, 2011

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 Mark G. Colwell, Russell Frith

"An Analytical Model for Coal Mine Roof Reinforcement (AMCMRR) has been developed. AMCMRR utilizes a Factor of Safety (FOS) approach, which is commonly used in all forms of engineering. The starting point in the development of AMCMRR was an existing analytical roof behavior and roof support design methodology/model, originally developed by Colwell. This technique was used successfully in the Australian underground coal industry for roof support evaluation and design prior to and during the course of the research project, and when used, it was essentially calibrated on a site by site basis.It has long been recognized that bolts and longer tendons can modify the behavior and load bearing capacity of the reinforced roof via the concept of beam building. The break-through in developing AMCMRR was combining the original analytical model with a comprehensive database (associated with Australian collieries) to effectively quantify this reinforcing effect, and this in turn provided the ""platform"" by which this analytical model can be calibrated for the entire Australian underground coal industry.This paper focuses on the application and use of AMCMRR and the analyses undertaken to quantify the reinforcement beam building offers to the immediate roof. Furthermore, this paper demonstrates that a combined empirical and analytical approach is currently the most practical way of developing credible geotechnical design tools for the Australian or any other underground coal industry.BACKGROUNDBy the start of 2006 the ALTS (Analysis of Longwall Tailgate Serviceability-Colwell et al., 2003) and ADRS (Analysis and Design of Rib Support-Colwell, 2006) design methodologies were being routinely and widely used within the Australian underground coal industry. As a result of their success, Colwell Geotechnical Services commenced a research project entitled, ""The Future Development and Integration of ALTS & ADRS for Improved Underground Roadway Design. "" The project (which became known as the ALTS 2006 Project) was funded directly by several of the major coal producers as well as individual Australian collieries."

Jan 1, 2010

By Rodger Fry

All coal deposits contain anomalous geologic conditions which have an impact on roof control, Run-Of--Mine (ROM) coal quality and production costs. To remain competitive in today's coal market and provide a safe working atmosphere, mine operators need to know in advance of mining the geologic conditions which prevail in the coal deposit. In order to identify geologic anomalies within a coal seam and surrounding roof rock in advance of mining, the Radio Imaging Method (RIM) survey procedure has been developed. This paper describes the RIM procedure and how it is used to predict various geologic conditions within a coal deposit. Figure 1 illustrates the RIM in-mine and surface surveys which can be used in an underground mine. [ ] RIM is an electromagnetic (EM} wave process which consists of in-mine and surface survey techniques. Both techniques are used in underground coal mines to detect faults, interbedding, sandstone channels, and thinning coal. Thesurface survey can also be used to determine seam correlation and continuity in strip and underground mining. In-mine and surface surveys work by propagating EM waves along straight paths through the coal. As the RIM signal travels along a ray path, it interacts with the seam material and surrounding rock. This interaction causes the signal to lose energy thereby decreasing its strength along the path. The amount of energy loss per unit path length depends upon the coal seam height and type of surrounding roof and floor rock. A reading of the signal level is taken at the end of the ray path at the receiver location. This reading is used to determine the loss (attenuation) rate of the signal along the path. The signal level data is compiled and analyzed through computerized algorithms which construct images of any anomaly present. This graphical depiction of the coal provides a basis for further geologic interpretation. By comparing the images reconstructed by the RIM analysis to in-mine geologic mappings made while mining through RIM study zones, it has been confirmed that RIM can help detect the following; Rapidly Thinning Coal Incompetent Roof Rock Poor Floor Lithology

Jan 1, 1984

By Hamid Maleki

A novel ground control program has been developed at Plateau Mining Company through five years of geotechnical investigations. The study was initiated to determine the causes for roof stability problems and to develop alternative support and monitoring systems for increased safety and productivity. From systematic roof fall observations, extensive geologic mapping, roof drilling and monitoring ground movements along gate entries of four longwall panels, a sand- stone channel system was detected and shown to impact roof stability. In particular, underlying mudstones up to seven feet thick experienced instability due to differential compaction and load transfer from the gob. Roof bolter operators were trained to report roof lithology for mapping channel positions into the roof and for selective use of longer bolts. This boosted operator morale, and improved ground conditions. Monitoring rates of roof movement is used in these areas for detection of instability and for application of secondary support: This has significantly increased safety and productivity.

Jan 1, 1987

By Chris Pritchard

Falconbridge Ltd., a major base metal producer in the Sudbury Basin of Ontario, Canada, identified a need to find a safe and cost effective replacement for welded-wire mesh used to support loose ground during underground nickel mining. Safer and lower operating costs are a must to maintain mine viability in lower base metal price environments. Stringent material strength parameters were put in place to determine if a spray-on liner was acceptable for testing and application trials. Falconbridge Ltd. took a methodical approach to the concept of a spray-on liner as a screen replacement proving the strength characteristics of the liner as a safe, cost effective, and suitable method of supporting ground. Materials testing was undertaken at Laurentian University to further understand the capacity of the material Laboratory and field tests indicated excellent adhesion and good tensile strength. Further, it maintains significant tensile strength even after elongation of over 25%. Compact, hydraulic mixing and spraying equipment have been developed and successfully utilized in the Fraser Mine of Falconbridge's Sudbury Operations Currently the Tekflex material has been approved as a wire-mesh replacement and is undergoing full-scale production trials in selected locations at Fraser Mine.

Jan 1, 1999

By Daniel W. H. Su

Surface subsidence induced by multiple-panel coal extraction was calculated with finite element stress analysis. The use of nonlinear material behavior and GAP elements, which provide a realistic representation of shearing along existing planes of weakness, allows the finite element program to accurately reproduce observed subsidence profiles. A reduction factor of 1/6 is applied to the measured rock strength and modulus for use in the finite element model. The models satisfactorily predict a significant change in maximum subsidence and subsidence profile associated with an increase from subcritical to near critical panel width. Modeled shearing along planes of weakness is in good agreement with available time domain reflectometry field observations and is demon¬strated to have a very profound effect on the maximum subsidence and the shape of the subsidence profile. When both material nonlinearity and planes of weakness are incorporated, the finite element model appears to possess great potential for accurately predicting surface subsidence and subsurface strata deformation in environments where little subsidence information is available.

Jan 1, 1990

By Ismet Canbulat

The cut out distance in the continuous miner sections in South African collieries is limited to 12 m. However, this distance can be extended to 24 m by the permission of the Department of Mineral and Energy (DME). This paper investigates the risk of roof Calls associated with the extended cut out distances permitted by the DME. Extensive underground experiments have been conducted using sonic probe extensometer under various geological conditions to identify the effect of cutting distance and tinge on the stability of the workings and the performance of the support. The results showed that that the most critical parameter with respect to ground control aspects of extended cut out distance is the bord width, which determines the amount of deformation that will take place in the roof. This was confirmed by the numerical modelling. The results also showed that a significant portion of the maximum deformation in the roof takes place before the ratio of cut out distance to bord width exceeds two. This indicates that once the face is extended more than twice the bord width, the roof will stabilise, assuming that no geological discontinuity is present and that there will be no further geometrical changes in the area as a result of for example the development of an intersection.

Jan 1, 2000

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By Mark Colwell

This paper summarizes the results of a research project whose goal was to provide the Australian coal industry with a chain pillar design methodology readily usable by colliery staff. The project was primarily funded by the Australian Coal Association Research Program and further supported by several Australian longwall operations. The starting point or basis of the project was the Analysis of Longwall Pillar Stability (ALPS) methodology. ALPS was chosen because of its operational focus; it uses tailgate performance as the determining chain pillar design criterion rather than simply pillar stability. Furthermore, ALPS recognizes that several geotechnical and design factors, including (but not limited to) chain pillar stability, affect that performance. There are some geotechnical and mine layout differences between United States and Australian coalfields that required investigation and, therefore, calibration before the full benefits offered by the ALPS methodology could be realized in Australia. Ultimately, case history data were collected from 19 longwall mines representing approximately 60% of all Australian longwall operations. In addition, six monitoring sites incorporated an array of hydraulic stress cells to measure the change in vertical stress throughout the various phases of the longwall extraction cycle. The sites also incorporated extensometers to monitor roof and rib performance in response to the retreating longwall face. The study found strong relationships between the tailgate stability factor, the Coal Mine Roof Rating, and the installed level of primary support. The final outcome of the project is a chain pillar design methodology called Analysis of Longwall Tailgate Serviceability (ALTS). Guidelines for using ALTS are provided.

Jan 1, 1999

By P. R. Sheorey

Four case studies of special geomining conditions from Indian coalfields are discuss- ed. These cases relate to (a) pillar extraction in two seams simultaneously under a difficult roof (Satpura I and II). (b) depillaring in an inclined seam resulting in rib instability (Sudamdih), (c) depillaring resulting in floor lift and side spalling (Jarangdih) and (d) contiguous pillar extraction under hard incavable strata. Solutions to these problems are given based on rock mechanics principles involving stress analysis, classification techniques, past experience and engineering judgement.

Jan 1, 1988

By Robert L. Schmidt

The support of underground openings by water is a natural phenomenon. Surface sinkholes, such as those that occur with some frequency in Florida, are attributed to a lowering of the water table resulting in the draining of existing limestone cavities. This withdraw the natural support and causes the surface subsidence. The role of water as a ground support medium was demonstrated in a mining situation during Bureau of Mines research on borehole mining of phosphate in St. Johns County, Florida. The borehole mining tool (BHT) incorporates a water jet nozzle and slurry eduotor in a single down- hole tool. The tool is lowered through a bore- hole into the ore zone, and an underground cavity is eroded by the motion of the water jet with the slurry simultaneously pumped to the surface. In addition to phosphate, the system has been successfully used in coal, oil sands, and uranium. The Florida phosphate ore is located at a depth of approximately 250 ft in strata that produce high water inflows. These factors preclude extraction by conventional methods, either surface or underground, but they present an ideal setting for borehole mining. Previous borehole mining research had led the researchers to believe that the system would only be effective in a void cavity, that is, a cavity where the water level is maintained below the water jet nozzle, so that the water does not impede the action of the let. It was soon discovered, however, that the void cavity presented ground control problems. The overlying strata started to cave soon after the support provided by the water was removed. This was an unacceptable situation as it adulterated the ore and threatened surface subsidence. Further tests wherein the size of the cavity -as reduced by cutting only a 30° arc also resulted in caving. A new concept, air shielding, was then introduced and tested. With this system, the water jet cutter was surrounded with a shroud of compressed air, thereby gaining the advantages of cutting in air while retaining the roof support afforded by a water-filled cavity. This arrangement proved to be highly successful. The air-shrouded jet was able to cut a cavity radius of about 18 ft, and the system production was 35 tph, which compared favorably to the production previously attained in void cavities. The borehole phosphate experience proves the efficacy of remote mining in a water filled opening. This system neutralises the hazards of heavy ground, and may, as in the case of the phosphate deposits, be the only viable alternative. It opens up the possibility of application in a wide range of deposits not only those amenable to hydraulic mining. Remote mechanical System might be developed using present robotics technology and then operated in water-filled openings.

Jan 1, 1986

By David B. Plumeau

The Cayuga Mine, a deep rock salt mining, operation, changed from a standard room and pillar mine design to a yielding pillar/pressure arch design five years ago. In general the roof conditions improved. However, entries adjacent to the abutment pillars continued to fail. It was noticed that where abutment pillars were accidentally "notched" (by driving cross- cuts one cut into the pillars), the abutment entries had better roof conditions than where there were no "notches". Experiments with intentionally notched abutment pillars showed that notching did improve abutment entry conditions. During the past four years notch configuration has been varied to suit local ground conditions. Recently, "pre-cutting" of notches has proven itself as a means of protecting abutment entries under severe ground conditions. As an added bonus, the notches improve productivity by providing more places to work and reduce costs because it is not necessary to bolt the notches.

Jan 1, 1981

By Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001

By Thomas L. Hutchinson

The mechanical design of a roof support is basically a matter of a statics and dynamics problem, assuming of course, that the imposed loads and mining conditions are known. Here in the Appalachian coal fields the general conditions are known well enough to allow the majority of the proposed new faces of longwall to be furnished with standard commercially available supports equipped with suitable variants and options. While not trying to negate the primary importance of thoroughly investigating a potential face and the selection of the correct equipment for that face, it still becomes clear that an equally important aspect of the design of an adequate ground control program has in the past too of ten been overlooked, having been overshadowed by the ?endless? selection process of the roof supports proper. The equally important aspect which this paper discusses is the overall design of a mining system, which design incorporates not only the size and capacity of the equipment but the following points as well, namely: Equipment adaptability to mining scheme, Equipment va. human constraints, Operation at the designed levels, and Coordination of operation, maintenance and support design. Unless the coordination of these factors is considered in the design and selection of the equipment, a lopsided and inefficient operation is possible. How can we afford to put an 8-10 million dollar investment up to chance as to whether it will yield optimum results or not?

Jan 1, 1981

By H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

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 Vasundhara

A chance detection of a seismic event of unexplained origin in the mining district of Appin in New South Wales. Australia, led to a detailed investigation of recording and analyzing seismic events occurring in the area. Multiple events were recorded each day on the seismometer stations. The larger events were strong enough to be recorded on a number of stations spread out in the district. Background investigation of seismicity stimulated due to engineering activities revealed that similar cases of mining triggered seismicity have been reported previously from many mining districts around the world. If the tectonic stresses are close to failure in a region then a small perturbation of the stress state may lead to instability in the region which may manifest itself as seismicity. The parameters that control the occurrence of such siesmicity include tectonic stress magnitudes and orientations. geological weaknesses or structural features and the amount of stress contribution due to mining. This paper reports on the seismic investigative study conducted in the Appin area and considers its association with the geological layout of the region and the mining activity.

Jan 1, 2001