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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 Ke Yang

"Engineering practices have shown that the skip-fault longwall mining is frequently used when faults with offsets larger than 1 Om are present. Since the skip-fault longwall mining not only reduces the recovery rate of coal reserves, but also increases the cost due to developing another setup room on the opposite side of the fault and moves the longwall into it. The coal pillar so created induced residual stress in and thus affects the safe mining of adjacent coal seams. In order to investigate the influence on panel layout and retreat by a l 6.5m offset normal fault, the similar material simulation model was constructed based on the geological and engineering conditions of No.13 coal seam in the western district of Liuzhuang coal mine employing the down-dip longwall mining. The characteristics of deformation, movement and failure of the overlying strata and abutment pressure when the face cut through the hangingwall and footwall of a normal fault were obtained. There were significant differences between the hangingwall and footwall as conformed by the equilibrium structure model. The results showed that superposition of the tectonic stress from a normal fault of large offset and that induced by mining is one of key factors that lead the severe pressure difficult to control during panel retreating from the hangingwall to footwall. Accordingly, the technical solutions for a fully mechanized mining face directly cutting through the fault are presented and optimized, and the applications of the technical solution in Panels 171301and171303 of Liuzhuang mine enabled them to complete the mining safely and efficiently. INTRODUCTIONFaults have been one of the important technical factors in underground coal mining l1-2i. Engineering practice has shown that the methods of mining through the faults depend on the seam thickness, seam dip angle, offset of fault throw, equipment height, rock strength, and advancing direction l3-5l. In the fully-mechanized longwall mining, the face normally proceeds to cut through the fault directly if the offset of the fault less than 5m, and to skip Uump) the fault if the offset is larger than 5m. In the former case, the most important problem is to efficiently control or avoid roof fall accidents (i.e., crushing, falling, pushover, and rock burst). In the latter case, the most important problem is very expensive to drive a new setup room and move the face equipment to it. The skipped coal pillar may poses problems for mining in adjacent seams."

Jan 1, 2016

By Richard N. Campbell

The world?s ever-expanding demand for energy and coking coals has resulted in the underground mining industry looking at taking on increasingly marginal and geotechnically challenging deposits. The drivers for this change include a reduced availability of open cut reserves and a lack of remaining ?good coal leases?. The lack of plum deposits is forcing some companies wanting to enter the coal supply market to purchase and operate in higher risk, lower margin areas. As geotechnical professionals, we are expected to develop strategies to facilitate safe and productive mining in ground that once would have been avoided. To be successful in these reserves, we are required to use every tool available, from exploration data to longwall automation, and invent or reinvent some new tools along the way. This paper presents geotechnical case studies of challenges in longwall mining and some of the methods developed and used to allow longwall mining to succeed where others said we would fail.

Jan 1, 2014

By Gregory Hinshaw

Underground mine bolting machines have evolved since inception in the 1940's due to the need for increased safety and productivity. However, major design innovations historically came from changes in the mining plan and the mine environment. During the last ten years, productivity needs have outpaced the evolution of existing designs. J. H. Fletcher & Co. has responded with a commitment to research and development which has fostered several new designs. The most recent machine built is a walk through crawler frame powering four drilling units simultaneously with semiautomatic computer control using two operators. This machine utilizes technology to create a safer yet more productive roofbolter for continuous mining sections utilizing place changing methods.

Jan 1, 1998

By Clare Offner

In 1998, the Australian Coal Association Research Program (ACARP) awarded funding to the School of Mining Engineering at the University of New South Wales to establish a rockbolt testing facility to research anchorage mechanisms associated with fully encapsulated rock bolts and to develop standardised testing procedures for rockbolts. This facility is still under development but has already been used to evaluate the effect of bolt profile and confinement on the load transfer characteristics of a resin encapsulated rock bolt. This paper describes the test facility and presents some of the results of development testing to date.

Jan 1, 2000

By Naj Aziz

A series of laboratory experiments were conducted on a variety of bolt types to examine the load transfer capacities of different profiled bolts in short encapsulation push testing. A 75 mm section of 150 mm long bolt specimen was anchored in a 75 mm long stainless steel tube using full resin encapsulation. Six types of different profiled bolts and two non - profiled bolts were tested. Bolts with higher profile were in general found to have greater shearing resistance and higher stiffness than low profile bolts. Widely spaced profiles allowed greater displacement at peak shear strength, and bolts with no profiles produced very link load transfer capability. Rough surfaced plain bolts showed a significant load transfer capability in comparison to a factory supplied smooth surface bolt which supports the belief that rusted bolts have higher lnad transfer capability that un-rusted bolt surfaces.

Jan 1, 2003

By Thomas Rymer

Mathematical modelling of mine subsidence measured at 21 locations in Pennsylvania and northern West Virginia allows for the refinement of current methods of predicting maximum land subsidence associated with longwall coal mining. This new method (RYBAD model) is an extension of that provided by Tandanand and Powell (1984; TP Model). The RYBAD model accomplishes more accurate subsidence predictions (error typically less than 10 percent) for a greater range of mining and geological parameters through the recognition of the following: 1) subsidence increases at a decreasing rate (curvilinear relationship) with greater overburden thickness up to a critical value beyond which subsidence decreases; 2) subsidence is strongly influenced by the stratigraphic proximity of strong rocks to the coal (size of potential caving zone); and 3) subsidence index is related to overburden thickness by a family of curves, each for incremental variations in the effective percentage of strong rock in the overburden.

Jan 1, 1988

By B. T. Wells

This work concerns the stress fields encountered around mining tunnels. A critical review is undertaken of the various analytical assessments of the support requirements for mining tunnels. Comments are made regarding degree and type of support together with method and timing of installation. The results have been used to indicate the type of support required in different mining and geological conditions.

Jan 1, 1981

By John Bullock

Limestone and dolomite quarries were mapped to determine specific failure mechanisms for the various geologic and structural conditions. Algorithms for analyzing these mechanisms were incorporated into a software package together with ranges of physical properties such as joint shear strength, angle of internal friction, and cohesion for specified lithologic units. Reliability options using Monte Carlo sampling and sensitivity studies are included to evaluate alternative highwall designs. The package is designed to facilitate progressive stability evaluation of quarry highwalls as mining continues. This package includes an optimum design protocol that can be followed to avoid the potential of massive failure. The program was developed in conjunction with the quarry industry and is demonstrated using case studies.

Jan 1, 1993

By A. Bortolussi

The paper discusses the state of the progress reached in the development of the diamond wire tool for cutting hard rocks. To the advances of this relatively new technology a substantial contribution has been provided by the research being carried out at the University of Cagliari on samples of granite rocks from different Sardinian stone quarries. On the basis of field experiments and laboratory test data, the mechanism of diamond wire cutting has been assessed. In particular the influence of the rock features and operational variables an tool performance (cutting rate and bead wear) has been elucidated.

Jan 1, 1992

By Holger Witthaus

The conditions of great mining depth and multi-seam¬longwalling require a special method of rockbolting, It can be described as a control loop which is based on the elements of planning, drivage and measurement. Different planning elements such as the results of the observation of physical models, numerical programmes for calculation of stress-distribution and support success, lab and underground support testing and the knowledge of strata behaviour underground including geotechnical classification are used to get best results. The increasing geotechnical information collected during drivage of roadways are used to estimate and minimise the risks of support-overload. The repeated plan/acutal comparison during the use of roadway and continuously monitoring of support and roadways provided the opportunity of optimising support design during the process of drivage and if necessary in front and after of the longwall face.

Jan 1, 2000

By M. E. Duncan Fama

The aim of the paper was to model numerically some aspects of the extraction of coal pillars by split and fendering. A two-dimensional elasto-plastic plane strain model with strain softening and interface modelling was used. The model was able to simulate the improvement of conditions experienced in lifting from the fender, when the roof adjacent to the fender has caved. In contrast, when a stook was left, providing support to a span of uncaved roof, the model predicted the fender would suffer considerably more deformation and strain.

Jan 1, 1992

By K. F. Unrug

A new testing technique for the determination of the rock strength from portable rock tester is proposed. It is a quick and economical method that can determine the rock strength with reasonable accuracy in different conditions. This method is based on the application of portable rock tester working at the principle of forcing steel ball into the rock surface. The correlation between laboratory measured compressive and/or tensile strength and rack tester reading has been determined in terms of the three-controlling parameters: (1). diameter of indentation &'the testing sample, (2). diameter of indentation-on the reference bar, (3). Brine11 hardness number of reference bar.

Jan 1, 1990

Retreat longwall method with re-use of gateroad has been traditionally employed in Japan, mainly because of the advantage of higher recovery. But the Miike Colliery has newly introduced the interpanel-pillar system to maximize the production efficiency. The Department of Mining, Kyushu University, in cooperation with the Miike Colliery and the Coal Mining Research Center (CMRC), has conducted research into the optimum design of interpanel-pillar, because this system may be adopted in other coal mines as well. Field measurements of the deformation of the gateroads and of stress distribution in the pillar show that the designed layout of the panel and pillar and the supporting system are proper from the viewpoints of gateroad maintenance and spontaneous combustion in the pillar.

Jan 1, 1990

By Vincent Scovazzo

Cost-effective underground mine configurations have been designed in highly fractured rock masses where the stone possesses a high material strength. A limestone mine in the eastern United States is used to demonstrate procedures for the geotechnical design of one such operation. The mine discussed is located in both limbs and the axis of a recumbent fold that fractured the rock mass. In a highly fractured rock mass, where the host rock has high strength, a design can be developed to alleviate intact rock failure. This is possible because in most cases as the stress levels needed for intact failures are approached, the stresses are quickly dissipated due to movement along discontinuities, which represent planes of weakness. As a result most underground failures in highly fractured rock are due to sliding or toppling. Even when intact rock fails in a highly fractured rock mass, it is the movement along an existing joint or other weakness plane that often initiates or causes this failure. It is therefore important to obtain the characteristics of weakness planes that are the discontinuities within the mass. The required information on weakness planes, geology, and strength of materials is discussed and recommendations for obtaining this information are presented. A discussion is provided on failure modes and the use of developed information for their evaluation. The impact on geotechnical design is demonstrated through pillar sizing procedures; however, this approach can also be applied to mine roof and pillar foundations.

Jan 1, 2002

By Tom Wynne

In order to shorten the time required and improve safety for a longwall face move, a 36-ft wide longwall recovery room was predriven. The room was supported with specialty bolts, trusses and concrete cribs. Rock mechanics instrumentation was implemented to evaluate the adequacy of the whole recovery system. This included instrumented roof bolts, entry convergence, load cells, and abutment stress in the surrounding pillars. This paper describes the methodology of support and instrumentation designs, and monitoring and evaluations. The results indicated that this method offers the safest and yet faster longwall recovery so far. But the rock mechanics monitoring also indicated some improvements are desirable in the truss system and concrete crib layout.

Jan 1, 1993

By Alexander Kattner

Surface fractures in urbanized areas show the risk potential of near-surface mining. This kind of mining subsidence damage occurs mainly in areas where hard coal was mined near the surface with very little overlying rock strata. In North Rhine-Westphalia (NRW), the mining authorities assess areas that are at risk for caveins by means of envelopes constructed according to Hollmann and Nürenberg (1972). Drafting abandoned underground structures depends on the assessment of historical documents such as mine plans, mine titles, and operation plans of prior mines in this area. A research and development project establishing a comprehensive Geo Information System (GIS) for hazard prevention was undertaken in cooperation with the mining authorities of NRW. It aims to display the areas endangered by abandoned, shallow underground mines for the purpose of hazard prevention. A tool was programmed using ArcGIS as a result of this project. It is used to digitize underground workings from preserved mine plans, calculate outcrop lines of coal seams and display hazardous areas. In the future, a database of shallow mine workings will be established. The tool will be used for a systematical mapping of shallow workings and assessment of endangered areas.

Jan 1, 2011

By John L. Hill

Cutter roof failure is a specific type of ground control problem which frequently results in massive roof failure. It is a cannon occurrence in coal mines of the Northern Appalachian Coal Sasin, causing delays in production and posing a safety hazard to mine personnel. The Bureau of Mines is conducting research on the causes of cutter roof failure togain a basis from which to prevent its occurrence and to support such roof when failure does occur. Research conducted in a coal mine of central Pennsylvania has revealed a correlation between the occurrence of clastic dikes and fonation of cutter roof failure. Inmine mapping of ground conditions showed an increase in roof failure in areas of high frequencies of clastic dikes. Rock pressure monitoring around clastic dires registered the greatest amount of roof loading near the inter- section of dikes with the rib. Load cells measurim horizontal pressure changes in the roof indicated that the greitest pressure changes were occurring perpendicular to entry headings when clastic dikes were present. Analysts of rock pressure nwnitiring shows that the roof behaved as two cantilever beams when severed by a clastic dike. Additional roof supports such as trusses and cribbing were found to effectfvelv suooort the roof in areas of clastic dikes and prevent cutter roof iailure. However, there lnethads were only successful when employed shortly after mining.

Jan 1, 1984

By Kevin Beauchamp

In many cases traditional support designs cover the many variations in ground conditions and bolt performance by a conservative factor of safety. The result is that most bolts are ineffectual most of the time and yet the response to occasional roof collapses is an even more conservative design. The probabilistic analysis presented in this paper takes several important parameters of a support design into account such as span, bolt strength, bolt length, water pressure, seismic accelerations, joint orientation, bed thickness and joint strength and allows the effect of variations in these parameters to be evaluated simultaneously. The analysis demonstrates the application of the probabilistic approach to both blocky, wedge-dominated conditions and bedded deposits dominated by laminations. All mine excavations have a designed stand-up time. By taking into account of the possible variations in the design parameters explicitly, it is possible to produce a support system designed to ensure stability over a given period of time. Rather than use a factor of safety criterion, the approach uses a percentage reliability or probability of failure. By understanding the probability of failure of the support system, it is possible to improve the performance of ground support without any increase in cost or conversely to decrease the cost while maintaining the performance of ground support. This approach reduces reliance on empirical experience and leads to a better understanding of the key factors that influence stability and can provide a much more accurate and optimized assessment of support design and performance. The result is a more cost-effective support system and we hope that a broader application of this approach to ground support in underground mines will lead to a greater acceptance in the industry of probabilistic analyses and of terms such as Probability of Failure and Reliability of Support.

Jan 1, 2003

By D. N. Cleaver

Like other influence function methods, the SWIFT subsidence prediction program, developed within the Mineral Resources Engineering Department at the University of Nottingham, requires calibration to regional data in order to produce accurate predictions of ground movements. Previously, this software had been solely calibrated to give results consistent with the Subsidence Engineer's Handbook (NCB, 1975). This approach was satisfactory for the majority of cases based in the United Kingdom, upon which the calibration was based. However, in certain circumstances within the UK and, almost always, in overseas case studies, the predictions did not correspond to observed patterns of ground movement. Therefore, in order that SWIFT, and other subsidence prediction packages, can be considered more universal, an improved and adaptable method of regional calibration must be incorporated. This paper describes the analysis of a large database of case histories from the UK industry and international publications. Observed maximum subsidence, mining geometry and Geological Index for several hundred cases have been statistically analysed in terms of developing prediction models. The models developed can more accurately predict maximum subsidence than previously used systems but also, are capable of indicating the likely range of prediction error to a certain degree of probability. Finally, the paper illustrates how this statistical approach can be incorporated as a calibration system for the influence function program, SWIFT.

Jan 1, 1995