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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 Gennaro G. Marino

Site conditions at several shallow room and pillar mines in Illinois are described and compared with the charac¬teristics of the subsidence profiles at the ground surface. The shape and magnitude of the subsidence profiles were found to be closely related to the thickness of the soil and rock over-burden the percent extraction of the coal, and the shape of mine pillars. The room and pillar mines were located at a shallow depth (from 112 to 220 ft), beneath flat to gently rolling terrain. Thickness of the coal seams ranged from 5 to 9 ft and at all sites was underlain by underclays. This information has been developed in order to improve techniques for evaluating the subsidence potential at sites within the Illinois coal basin. he results presented form a small portion of the work carried out by the University of Illinois with Illinois Abandoned Mined Lands Councils and the U. S. Bureau of Mines at the Minneapolis Research Center in Minnesota (Marino and Mahar, 1985). Presented in the paper are the modes of mine failures observed for Illinois room and pillar mines, an overall summary of the site geologic and mining condi¬tions, case histories of each mode of failure, a comparison of sag profiles, and then an empirical relationship between the site conditions and the maximum sag subsidence. The most ideal case histories are presented to illustrate fundamental behavior. A brief of the Paper is given in a summary and conclusions section.

Jan 1, 1984

By S. Y. Liu

Based on the underground measurements, physical model test of simulated materials in the laboratory and finite element analysis, this paper demonstrates the pattern and process of roof fall over the tip-to-face area in the fully mechanized coalfaces. The main factors that affect the stability of the immediate roof are thoroughly studied including the fracture position and tilt angle of the main roof, the tensile strength (Rt) and cohesive force (c) of the immediate roof, and type and resistance of the powered support. Based on these factors, this paper proposes the basic process of immediate roof breakage a. follows: As the longwall farm advances, the main roof breaks and then rotates. A tensile fracture zone will be formed in the immediate roof under the fracture line of the main roof. A caving zone (plastic zone) will be formed in unsupported area between coalface and canopy tip. If these two zones overlap the immediate roof will be completely unstable. Under this condition, powered support will have difficulties lo control the rod. Therefore it is necessary to study the process of immediate roof breakage and look for effective methods for controlling the roof.

Jan 1, 1990

By Patrick S. Artrip

A deep mining operation in the Lower Kelly (Imboden) seam in Wise County, Virginia experienced severe ground control conditions in its attempt to develop reserves situated under increasingly higher cover conditions. Ultimately one of only three mines capable of providing access to a large reserve area had to be abandoned. A thorough geotechnical exploration program was carried out to allow characterization of the critical roof and floor rock strata in accordance with the modified geomechanics classification system. The principal roof and floor rock stratigraphic sequence types were defined and mapped throughout the reserve area along with other critical geological factors. A ground control study based on the geotechnical findings provided a quantitative analysis of the causes of previous entry instability. Further analysis of pillar and entry stability identified configurations which would remain stable for the variety of strata types and range of overburden depths within the remainder of the reserve. Mine designs based on the geotechnical evaluation have, to-date, provided the stability required for long-term future development - of the reserve area. However, some areas of difficult ground were encountered which could not be anticipated with the drill data available at the time of the original study.

Jan 1, 1993

By Thomas Barczak

Longwall shields provide essential ground control in Iongwall mining, yet a high percentage of shields are operating at less than peak capacity and many at well below the rated support capacity due to defective hydraulic cylinders or malfunctions in other hydraulic components. Leg pressure data are currently collected on state-of¬the-art longwall shields, but typically are not analyzed to evaluate shield performance. The National Institute for Occupational Safety and Health (NIOSH) Shield Hydraulics Inspection and Evaluation of Leg Data (SHIELD) program is a Visual Basic software system that is designed to analyze leg pressure data and identify shields that are not performing to rated specifications. The program analysis is configured to detect the following conditions: (I ) loss of leg pressure, (2) imbalance in leg pressure, (3) low set pressure, (4) low yield pressure, and (5) full extension of the bottom stage. Other performance assessment measures include the percentage of the support capacity utilized (ratio of peak load to yield load), the percent of time that a shield operates at yield load, the ratio of the set load to the yield load, and the amount of support capacity that is lost due to leaking cylinders. Historical record keeping will allow the user to select a particular shield and review the performance record as developed by the program for that shield. In addition to these performance assessment measures, the software will include an animated description of shield hydraulic systems that will describe the operation and significance of each hydraulic component and the impact of component failures on the shield's capability to provide the required roof support. A general overview of the SHIELD program is provided in the paper together with an example analysis of a 2.5-year-old Australian longwall face.

Jan 1, 2002

By Keith A. Heasley

Pillar recovery in deep coal mines with competent roof and floor can concentrate stresses and generate hazardous bumps. Actual calculation of the geologic strain energy released in association with these coal bumps has been rare, although energy release values have been utilized successfully for indicating burst potential in numerous hard- rock mines. This paper advances the current knowledge of energy release calculations as applied to coal bumps through an extensive analysis of an actual bump occurrence. The U.S.B.M, displacement- discontinuity code, MULSIM/NL, is used to produce a numerical model which generates stress, displacements and energy values associated with a pillar retreat section. This model is calibrated with actual field data, and then dissipated energy values from the model are examined as to their suit- ability for indicating bump potential. Ultimately, it is determined that prudent application of dissipated energy calculations can be used as a tool to investigate the coal bump potential of a mining plan or cut sequence.

Jan 1, 1990

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 Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002

By M. V. Kurlenya

The experience of application of geomechanical investigations for substantiation of extraction of undermined ore deposit in the north of Siberia is discussed. The finite element method was used for forecasting the state of massif during the development of mining works. The results of calculations were compared with the experimental data of observations in situ for evaluation of authenticity of numerical modelling.

Jan 1, 1990

By Lance R. Barron

The U.S. Bureau of Mines, in cooperation with a central Utah coal .mine operator, began a study in July 1988 into longwall gateroad designs applicable to deep, bump- prone mine conditions. Prior to the study, four adjacent longwall panels, incorporating a variety of panel entry configurations using "yielding' chain pi liars, experienced severe bumps and coal outbursts at the face and in the tailgate pillars during panel extraction, resulting in several lost-time accidents and a fatality. To quantify those factors primari1y responsible for bump occurrences during panel retreat, a field investigation was initiated to confirm the anticipated performance of a two-entry gateroad system employing a large, nonyielding chain pillar design. Hydraulic borehole pressure cells and roof-to-floor closure [convergence) measurement stations were installed to monitor gateroad abutment loading and entry closure during panel extraction. Study results indicate that very high confinement of the coal seam by strong roof and floor members existing across the entire property, in conjunction with overburden depths in excess of 1.600 feet, provides for bump-scale energy storage in not only gate pillars, but along the entire periphery of the active mining area. Insufficient gate pillar designs were also identified as possibly enhancing the occurrence of bumps in the tailgate and adjacent face areas.

Jan 1, 1990

By Rao Pothini

Measurement of pressure distribution under the base plate of the Hemscheidt 2-leg 800-ton shield was performed underground for two shifts using 12 pressure cells installed in two rows, one each under each split base plate. Pressure distribution was uneven depending on contact condition. In general the highest base pressure occurred at 1/3 of the base plate contact length from the rear edge. The resultant was near vertical, and the pressure under the tail side split base plate was always higher than the head side. For comparison, pressure distribution on the canopy and under the base plate was also measured under controlled conditions at the USBM's Mine Simulator Testing Facilities in Pittsburgh, PA. A total of 45 tests were conducted under several simulated contact conditions. The results showed that the actual contact area is highly dependent on the compliance of the immediate roof and floor due to the sloped canopy tip and a base lifting device under the base tip. The loading distribution tends to change in magnitude and profile as the leg pressure increases. The base lifting device exaggerates the high toe loading on the base plate characteristics of this type of shield support. This paper describes in detail the testing methods, data analysis results obtained, and recommended changes.

Jan 1, 1992

By Anthony T. Iannacchione

Underground bituminous coal mining is a large and significant industry within the Commonwealth of Pennsylvania, and it has a legacy of environmental consequence. For example, in 2008, 13.5 acres (5.5 hectares) of land were undermined by longwall mining every day. Laws and regulations have been promulgated requiring companies involved in this industry to remedy mining damage caused to homes, businesses, and land; replace impaired water supplies; and repair impacts to streams and wetlands. The regulations and technical standards set by the Pennsylvania Department of Environmental Protection (PA DEP) have been put in place to assure that coal operations comply with ACT 54 of the Bituminous Mine Subsidence & Land Conservation Act and other related legislation. These regulations and standards require operations to address impacts when a mining permit is submitted and as permits are modified and extended. One of the special provisions of ACT 54 is the requirement by the PA DEP to produce an assessment of the surface impacts of mining every five years. The University of Pittsburgh was contracted by the PA DEP to perform the 3rd assessment, which spanned from August 21, 2003, to August 20, 2008. This paper details the findings related to surface structure undermined during this period. A total of 3,735 structures were tracked. Four-hundred-fiftysix reported effects were found, and each required additional information and analysis. The majority of the reported effects were associated with eight longwall mines. Characteristics of the structures were analyzed and discussed, with special attention to those with reported effects. The number of undamaged and reportedly damaged structures (i.e., dwelling, barn, commercial building, etc.) undermined during the assessment period were listed. Lastly, the resolution status and type of resolution were analyzed for all structure reported effects.

Jan 1, 2011

By John P. Dunford

Ground support continues to be the most important safety consideration in underground mining. The Bureau of Mines has developed a yielding steel post with a capacity of 45 tons and b c1osure allowance of 24 inches in a 6 to 7 foot seam height. The post mechanism in a three piece unit consisting of a top and bottom leg made from standard steel pipe and a separate foot bracket. It develops its load carrying ability when the lower pipe, with an attached interference ring, is forced into the larger top leg. The walls of the top leg are deformed radially and along the axis. Yield continues until the unit bottoms out. Other advantages of the post is that due to its simplicity it can be made at the mine shop level and its cost should be about one-hundred dollars. Prototype units were tested in the laboratory. Based on these results, modifications were made that would make the post perform better as a mine support. Two field tests were conducted. The first test used the units as supplemental support for the wood crib rows in a longwall tailgate. The second and larger field teat had 84 posts placed on salvaged track rails. They were installed on two foot centers in the middle of the entry. Overall the supports functioned well during use in the headgate entry, but problems with the quality of some of the pipe caused localized failures. However, these problems can be attributed to one manufacturer of pipe and through tighter specifications can be eliminated. The use of the posts with the rails is showing additional benefits as a ground support system. By providing continuous support between the posts a beam is created that has much more load bearing. The additional support can reduce floor heave and in some cases eliminate the need for the rib side crib line.

Jan 1, 1986

By Christopher Mark

The demographics and safety record of the pillar retreat segment of the U.S. coal industry was analyzed using statistics collected by the Mine Safety and Health Administration. Pillar recovery is practiced primarily by mines in Appalachia and the Midwest. Using 1993 data, the accident rates and productivity of a large sample of pillar re- mines were found to be similar to other room-and-pillar mines in the same geographic areas. Pillar recovery apparently accounts for about 10% of all U.S. underground production, but has been associated with about 25% of the mf and rib fatalities during 1989-96. However, of the 28 fatalities that were analyzed, only 4 occurred for which no citations were issued for violations of mining law. Nearly one- half of the fatal incidents occurred during the mining of the last lift or pushout. All four no-citation incidents occurred during the removal of the last lift during a "Christmas tree" extraction sequence.

Jan 1, 1997

By Duk-Won Park

A vast majority of the operating longwall sections use shield-type face supports to provide ground control in the United States. As a co-operative research program between the University of Alabama (UA) and Jim Walter Resources, Inc.(JWR), a joint research program is underway to study interaction of shields with surrounding rock masses. A series of analyses on the response of shields are performed using monitored data at a longwall face in a deep coal mine. In this paper, various effects of parameters, such as leg pressure, setting load, developing load, maximum loading rate, pressure in capsule, and convergence between canopy and base, are discussed and recommended as stability indices. A microcomputer program is being developed for instantaneous interpretation of the shield monitored data, and for giving warnings of potential problems. The preliminary results define the effects of the setting load, developing load, and rate of developing load or leg closure rate. The objective of the study is to understand the shield characteristics and their interaction with surrounding strata in real time, then develop a reliable program for maintaining longwall stability.

Jan 1, 1992

By David H. Tang

Similar massive pillar failures were observed in two underground coal mines with different configurations of mine workings and overburden depths. Finite element models were used to analyze the causes and mechanisms of the pillar failure. The results of the analyses confirms the underground observation. For Case No. 1 the failure of coal pillar is mainly caused by a combined effect of smaller coal pillars under a large overburden and the interaction of multiple seam mining. For Case No. 2, the pillar failure was initiated by the weak roof rock but the ultimate pillar failure was caused by the splitting of pillars into very small size.

Jan 1, 1984

By David P. Conover

The effects of stress distribution on pillar stability were evaluated through rock mechanics studies conducted in three underground room-and-pillar coal mines. Results of the three field instrumentation programs are presented to illustrate the use of borehole pressure cells to determine vertical stress distribution in coal pillars at various stages of mine development. Vertical stresses and stress changes are determined using methods developed at the Bureau of Mines. Measured stress distributions are compared against theoretical distributions and possible explanations for observed behavior are discussed. Results indicate that stresses and stress changes calculated from pressure cell data are inconsistent with theoretical values; however, the data are useful for qualitative evaluation of trends over time. Calculated values are found to be sensitive to coal properties and initial setting pressures of the cells.

Jan 1, 1989

By Gift Makusha

Surface subsidence associated with coal mining activities in the Mpumalanga Province of South Africa changes the natural environment in several ways, and current challenges for mining companies include rehabilitation of the natural environment and the prevention of further degradation. To monitor the spatial and temporal evolution of surface subsidence, traditional fieldbased monitoring approaches such as GPS and spirit leveling are employed at a number of locations. However, the resulting measurements are point-based, and frequent revisitations are necessary to map the evolution of surface subsidence basins over time. To address these limitations, differential interferograms derived from repeat-pass satellite-borne synthetic aperture radar (SAR) systems were tested for their ability to measure and monitor surface deformation. The SAR data was captured by the European ERS-1 and ERS-2 satellite and covered a timeframe between 2008- 09-15 and 2008-09-15. The resulting interferograms revealed several features indicative of surface subsidence. Ground truth data confirmed the presence of a subsidence basin, detected using differential interferometry techniques during the 35-day period between April 12, 2008 and May 17, 2008; a maximum vertical deformation of 3.2 cm (1.3 in) was recorded. Interferometric monitoring revealed an eastward migration of the subsidence basin between June 2, 2008 and September 15, 2008 with an additional 4.7 cm (1.9 in) of subsidence observed. This migration coincides with the advance of the working face of the mine during this period. Finite element modeling will be performed in order to model the rates and evolution of surface subsidence haloes as underground mining advances. These presented results demonstrate the ability of interferometric synthetic aperture radar techniques to measure surface subsidence as well as the monitoring of the evolution of subsidence basins over time. This implies that the technique could be included, together with traditional field-based surveying techniques, in an operational monitoring system. With knowledge on deformation rates and subsidence basin evolution, informed decisions on current and future infrastructure development can be made and remedial actions and prevention strategies can be formulated for the problems associated with environmental degradation.

Jan 1, 2011

By Jim Galvin

4 and 6 point timber chock constructions are used extensively on both a systematic and spot basis by Australian longwall operators to support tailgate roadways. In 1996, the School of Mining Engineering at UNSW published design and performance criteria for timber chocks constructed from Australian hardwoods. The research was funded by the Australian Coal Association Research Program (ACARP). In 1997, ACARP awarded funding for a stage 2 testing program. This testing focussed on examining the effect of using thinner timber elements in chock constructions to improve OH&S and comparing the full-scale performance of traditional 4 point chocks to Link-n-Lock constructions. It was established that chocks constructed from thinner elements have failure loads much less than thicker elements. However up to the failure loads of the thinner elements there was no significant difference in the response to convergence of chocks constructed from 25, 50 or 100mm thick elements. Chocks constructed from 150mm thick elements have a similar ultimate strength to the 100mm thick elements. However, they have a significantly lower resistance to load, thereby permitting more convergence to occur. There is more potential for chocks comprised of thin elements to be constructed 'out of plumb' and so suffer eccentric loading. The full scale testing program established that at any given level of convergence up to 10% strain, the timber in a Link-n-Lock chock generates twice the support resistance of the same quantity of the same timber in an equivalent size 4 pointer chock. On the basis of support resistance per cubic metre of timber, there is only a marginal difference in the performance of 1.0m Link-n-Lock chocks and 1.2m Link-n-Lock chocks constructed from select Blue Gum. The cost per tonne of support provided by a Link-n-Lock chock is effectively half of that provided by a 4 pointer chock of equivalent dimension and timber. Up to a strain level of 4%. Link-n-Lock chocks constructed from 1.2m long elements are slightly more cost effective than the same chocks constructed from 1.0m long elements. Thereafter, there is no significant difference in cost per tonne of support resistance. Beyond a strain level of about 4%, Link-n¬Lock chocks constructed from landscape grade Blue Gum are just as cost effective as Link-n-Lock chocks constructed from structural grade 4 Blue Gum.

Jan 1, 2000

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