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By Axel Preusse

Underground coal mining can cause seismic events. There is currently no way to predict such phenomena reliably, in particular stronger events. That is why, according to the current state of the art, empirical methods are employed in this field. Some assessment criteria, which relate to the geological and mining situation and either stimulate or rather help to avoid seismic events, are presented in this paper. These criteria are examined for a practical mining situation in Germany, with a view to estimate possible seismic events.

Jan 1, 2009

By Don Kay

A two year study into the effects of coal mining induced subsidence on cliff lines was undertaken by the Department of Mineral Resources with the support of a grant under the National Energy Research, Development and Demonstration Program. A new monitoring technique, using an Electronic Distance Meter and acrylic reflectors attached to the cliffs, provided safety for the surveyors and extensive and accurate three dimensional displacement data on the movement of the cliffs as they were being undermined. The study report included many graphs showing the measured vertical and horizontal displacements, differential subsidence, tilts and strains and included analysis of these movements with various parameters, such as, seam thickness, depth of cover and distances between the reflectors, longwall face and the edge of the chain pillar. Some of these graphs are presented in this paper and could be used, with caution, to predict subsidence movements around other cliff lines, after allowing for differences in geology and mining layout. A mechanism is suggested as an explanation for the observed high horizontal movements around the cliff lines. Sixteen percent of the overall lengths of cliffs that were undermined during the monitoring study fell and the contributions of various factors that appeared to influence the cliff falls are discussed. It is concluded that no single factor dominated as the major cause of the cliff falls and, often, many factors combined to cause cliff face instability.

Jan 1, 1992

By Frank E. Chase

The Harris No. 1 Mine, located in Boone County, WV, has been longwalling the Eagle Coalbed for over 30 years. Harris has experienced numerous interactions associated with the extensive room-and-pillar and longwall mining operations which have been conducted in the overlying No. 2 Gas Coalbed. The problems have included roof falls, excessive rib sloughage, and gateroad and bleeder entry closure. A detailed evaluation of the multiple seam experiences at Harris No. 1 Mine was conducted as part of the National Institute for Occupational Safety and Health (NIOSH) nation-wide multiple seam mining case history data base. One observation from the Harris gateroad case histories was that smaller, critically loaded, upper seam pillars seemed to cause more severe ground conditions than did wider pillars. The LaModel program was used to investigate this supposition, and the results confirmed that "critical" sized pillars do transmit the highest amounts of stress to adjacent seams. In addition, the data suggest that the probability of a major multiple seam mining interaction increases when the depth of cover is 1,000 ft or greater and when the Eagle seam pillars have a Analysis of Longwall Pillar stability factor less than 1.50.

Jan 1, 2005

By Peter Cain

In response to the appearance of excessive convergence of bolted gateroads in the front abutment zone of retreating coal faces at Phalen Colliery, staff from the CANMET Cape Breton Coal Research Laboratory in Sydney, N.S. initiated a programme of roof profile measurements in the front abutment zone of longwall headgates. The results of these measurements allowed a characteristic deformation curve to be determined for each headgate, and the early identification of areas showing an unstable deformation trend. These areas were then reinforced with supplementary support. The monitoring programme was adopted and enlarged by the rock mechanics staff of the Cape Breton Development Corporation, and has been continuing for a period of approximately two years. A sufficient quantity of data now exists to be able to make an assessment of the effects of a number of mining parameters on the performance of the gateroad in the front abutment zone. The following paper describes the method of measurement, illustrated by typical results, and goes on to relate variations in rate and patterns of movement to geology, interaction, method of support and pre-mining stresses.

Jan 1, 1996

By A. W. Khair

This paper presents an analysis of fracture depth and intensity due to subsidence over the longwall panel. Sonic reflection techniques were utilized to determine fracture depth, and a variation of p-wave velocity was used as an indication of the fracture intensity. Instrumentation has been tested for consistency and accuracy first in the laboratory using small scale models, then applied in the field in two mine sites with different mining geometry and geologic conditions. A new sonic viewer was utilized for sonic velocity measurements. Regular hammering method was on acoustic source. The results were concurrent with monitored horizontal strain profiles and measured open fractures over the longwall panels.

Jan 1, 1989

By Youzhen Zhang

The drilling boom is the key component of a roof bolter, and it has significant impact on the performance, safety, and reliability of the overall working device. In this paper, the experimental modal parameters of a drilling boom at low frequencies are identified with the Laplace wavelet correlation filtering method. Based on the obtained experimental parameters, a finite element model of the drilling boom is revised using an optimization algorithm to improve its accuracy. The dynamics of the drilling boom are analyzed through its modal and harmonic response. As a result, an improvement for the drilling boom is proposed to fix the weak links and enhance the dynamic performance.

Jan 1, 2012

By Murali M. Gadde

Perimeter mining is a special form of very productive partial extraction method practiced at some mines in the United States. In the Illinois Basin mines where all major coal seams have weak underclay in the immediate floor, experience shows that any long-term ground control issues will arise due to floor failures rather than instability of the coal pillar itself. While there is a significant amount of work done on floor stability associated with regular pillars, no work has been done so far on the stability of perimeter pillars. This paper reports the results of a comprehensive study that was conducted for the first time on floor stability during perimeter mining in the Illinois Basin mines. A special plate template, which mimicked a common perimeter pillar configuration was created to conduct in-situ bearing capacity tests at five mines in three states of the Basin. Additionally, because of the complex shape of pillars created in perimeter mining that defies any closed-form solution, floor bearing capacity was studied using three-dimensional nonlinear numerical modeling. Based on these in-situ and numerical studies, some guidelines are provided for perimeter pillar stability analysis.

Jan 1, 2009

By Ming-Gao Qian

How to monitor and control the support and surrounding strata system on a longwall face effectively is one of the major research problems in ground control. Based on observations on movements of the support and surrounding strata of face No. 2703 of Dongpang colliery, the relationship between the downtime related to support and surrounding strata and other influential factors is analysed systematically in this paper. Monitoring indices and control measures are then determined by taking corresponding downtime as an objective function. When the monitoring method, developed from the above findings, was used on face No. 2703, the downtime due to support and surrounding strata was remarkably reduced, and the face advanced normally.

Jan 1, 1992

By Stephen Iverson

The National Institute for Occupational Safety and Health (NIOSH) is currently performing research to help mine operators minimize the amount of loose or damaged rock surrounding a blasted opening. Improperly designed blast rounds can result in excessive wall rock damage that may reduce the rock mass competency and increase ground support requirements. Unnecessary blast damage can cause many safety and operational concerns including an increased potential for injury from rock falls and other hazards resulting from sub-optimal blast designs. This paper focuses on the ground control and safety implications of blast damage in underground mines. A reduction in rock quality due to blasting can increase the amount of ground support required. Case study data collected by NIOSH in underground metal mines during standard drill and blast production rounds included blast vibration monitoring, detailed three-dimensional laser scanning of the as-built excavation, and geotechnical mapping of development headings.

Jan 1, 2007

By Robert W. Bruhn

An investigation was male of ground response to pillar retreat mining in a 1.7 meter thick seam at 108 meter depth at a site in northern West Virginia. This paper describes mining-related stress changes and movements at nine level, displacements within the overburden, ground surface subsidence and groundwater level variations. Subsidence approached 40 per cent of the nine height ad continued more than a year after mining. Water levels declined la certain deep-lying strata.

Jan 1, 1984

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By Hideki Shimando

Periodic weighting and windblast which are caused by the dynamic effects of overburden in a longwall panel significantly affect longwall mining This study describes past experience with wind blast at Miike Colliery, proposes a new loosening system using hydraulic fracturing and moistening, and also discusses strength deterioration in hard-to-collapse roof rocks due to water in order to solve the problems caused by the effects of dynamic roof rocks behavior in a longwall face

Jan 1, 1998

By David Newman

Multiple seam underground coal mining is ubiquitous in Appalachia. Many coal properties have from two to ten or more economically mineable coal seams. Because of the ability to penetrate 600-feet to 1,000-feet or deeper into the mountainside leaving panels of web pillars separated by barrier pillars, highwall mining is a hybrid between surface and underground mining. As a result, many of the ground control problems commonly associated with multiple seam underground mining exist with multiple seam highwall mining. These include: i) superjacent mines, subsidence and caving propagating to overlying seams and ii) mines, pooled water and the potential for inundation resulting from a iii) breakthrough between an active highwall mine and abandoned underground mines in the same or superjacent seams, the presence of overlying or underlying mine workings where iv) pillar columnization in the active mine is not possible, and v) the question of whether to mine ?top-down? or ?bottom-up.? Case histories are presented from southeast Ohio where the Middle Kittanning No. 6 and Lower Kittanning No. 5 seams are successfully highwall mined with 20-feet to 40-feet of largely shale interburden. A highwall miner was caught in a highwall collapse in the Middle Kittanning No. 6 seam when it was overmining auger workings in the Lower Kittanning No. 5 seam. Multiple seam interaction caused by the overmining transferred overburden stress onto the Lower Kittanning No. 5 seam. The overburden concentrated through web pillars initiated auger web pillar failure, resulting in roof collapse and subsidence in the Lower Kittanning No. 5 seam that propagated up to the Middle Kittanning No. 6 seam. A second case history involves multiple seam highwall mining in southeast Kentucky. The A3 Rider, A3, B2, and C1 seams are being highwall mined around abandoned underground mines in the A3 and B2 seams and augering in the A3 Rider. Because of spoil storage constraints for the surface contour mines, the highwall miner follows the active pits in each seam so a standard sequence of ?top-down? or ?bottom-up mining does not exist. The planning and engineering design considerations associated with each case history is presented along with geotechnical information. In the case of the highwall collapse a back analysis is presented of the stability and safety factors for the auger webs, highwall miner web pillars, and barrier pillars.

Jan 1, 2009

By John Latilla

Exxaro Mining?s Matla Coal situated in Mpumalanga, South Africa operates two shortwall faces on the No 4 and No 2 seam while the No 5 seam has also been longwalled in the past. Severe strata loading events have lead to a number of serious roof falls on the wall face (facebreaks) resulting in safety threats as well as considerable production losses. In 2002 the No 4 seam shortwall suffered a facebreak restricted mainly to the tailgate side of the mining panel due to a thick, near surface dolerite sill not breaking properly. This was partly due to the premature suspension of longwalling in the overlying No 5 seam. The resultant overloading lead to severe pillar damage and subsequent bord collapse. Surface pre-split blasting was successfully used to relieve the pressure as well as to pre-condition the dolerite above the following two panels. The shortwall on the No 2 seam has experienced mid-face loading due to No 4 seam chain pillars lying over this area and creating a secondary cantilever effect. Pre-split blasting of the solid rock above the No 4 seam pillars has resulted in a significant reduction in the number of facebreaks. The panel layout of the No 2 seam was subsequently changed so that the chain pillars on both the No 4 and No 2 seam are superimposed whereas previously they were offset.

Jan 1, 2007

By Thomas M. Barczak

This report chronicles the historical development of longwall mining in the United States and speculates on future developments to the turn of the century. Five eras of technological development during the modern period are described and analyzed. These eras discuss the development of (1) mechanized extraction, (2) self advancing roof supports, (3) high capacity roof supports, (4) shield supports, and (5) system automation. Current trends are analyzed in terms of: longwall utilization, production capability, support capacity, face widths, and new technological developments. From these analyses, the future of longwall mining to the year 2000 is speculated. It is concluded that longwall mining will continue to grow in importance during the next decade and that the next major technological milestone will be the realization of a fully automated longwall mining system.

Jan 1, 1992

By Daniel W. Su

This paper presents the results of a detailed geotechnical study on the potential multiple seam interaction between the No. 2 Gas (upper) and the Powellton seams, which are separated by an average interburden of 45 ft. The interburden consists of 4 ft of clay shale below the No. 2 Gas seam, 38 ft of massive sandstone, and 3 ft of shale above the Powellton seam. The No. 2 Gas seam was mined about 60 years ago, and many water-pooled areas were projected to be present. The deepest overburden over the No. 2 Gas seam is about 750 ft. To evaluate the potential interaction between the two mine works and to determine the pillar safety factor, Analysis of Multiple Seam Stability (AMSS) software was initially used to determine pillar dimensions to obtain a minimum safety factor of 2.0 for areas under pooled water, as well as a minimum safety factor of 1.5 for areas not under pooled water. Furthermore, to assess the quality of the interburden rock strata, two surface core holes were drilled, and rock cores were tested in the laboratory for their strength and elastic moduli. Using the experimentally determined strengths and moduli as part of the input parameters, the RocScience Phase2 finite element program was employed to model the stress field scenarios and estimate the safety factors of the interburden rock strata before and after Powellton seam mining. The pillar safety factors derived from the Phase2 analysis were in excellent agreement with those obtained from the AMSS analysis. Phase2 results also predicted that the effect of the Powellton seam mining, first mining only, would reach a maximum of 19 ft above the Powellton seam, which led to the proposal of drilling 25-ft deep test holes to detect water, instead of the 45-ft deep test holes mandated by MSHA. The proposed test-hole height was accepted by both Mine Safety and Health Administration (MSHA) District 4 and the state of West Virginia. Mining of the Powellton coal seam commenced in February 2011 and continued for eight months. To date, no 25-ft deep hole has encountered water under the water-pooled zone, which validates the RocScience Phase2 finite element analysis. Also, inmine observation of excellent pillar stability and roof condition confirms the validity of the AMSS analysis.

Jan 1, 2012

By Alan A. Campoli

The mining industry has long been searching for an effective structural application that prevents weathering and falls of loose rock, debris, and sediment. J-Crete is a fiber-reinforced, engineered, cementitious composite rock application that is designed, tested, and successfully utilized as such a material. Laboratory evaluation of J-Crete material properties has shown the final compressive strength to be in excess of 7,300 psi (50.34 MPa), with 24 hour strength in excess of 1,500 psi (10.34 MPa). The flexural tensile strength is in excess of 600 psi (4.1 MPa) and is provided through a unique combination of polyvinyl alcohol (PVA) fibers activated by a proprietary liquid additive that prevents them from clumping and enables even distribution throughout the grout. Tensile bond strength in excess of 350 psi (2.4 MPa) at 28 day cure tightens the strata and holds loose material in place. Practically impervious to gas and liquids, J-Crete protects strata against long-term weathering, does not require screening to be effective, and has proven to be a successful waterproofing material. J-Crete has demonstrated the potential to alleviate coal mine roof cutters in soft, wet clay formations. J-Crete reinforces coal mine ribs by bonding directly to the rib, conforms to the strata, strengthens the rib, and eliminates sloughing due to long-term weathering; stabilizes coal mine entries by bonding directly to the brow and roof, and creates a monolithic shell with double reinforced corners.

Jan 1, 2012

One of the problems associated with the maintenance of roadways in underground coal mines is that of dealing with roadway closure. Sidewall weakening technique is based on the transference of the high stress zone further away from the vicinity of roadway. This transferral is carried out by weakening parts of sidewalls of roadway by cut- ting slot, drilling a series of parallel holes or blasting during development stage. In the paper, the effect of sidewall weakening on the stability of roadway is studied by finite element analysis and field experiment. The finite element analysis shows that sidewall weakening can control the development of yield zone around a roadway, improving the stability of roadway. It is also shown that if in- situ horizontal stress is lower or higher than the in-situ vertical stress, it is difficult to obtain roadway stability by sidewall weakening. The field experiment shows that roadway closure can be controlled by sidewall weakening with blasting.

Jan 1, 1990

By Matt Leeming

Several of the underground mines in the UK require a significant number of cable bolts to be installed, remedially, to control worsening roadway conditions. Often the access is limited and the only power source is compressed air. Machines for this duty are generally designed to perform with 100 psi but regularly only half of this is available. This paper seeks to outline the difficulties this situation causes the Rock Bolting Engineer, highlight the technical requirements for a machine of this type to succeed and detail a solution offered by our engineers.

Jan 1, 2007

By Anil K. Ray

This paper discusses in situ studies conducted at an Illinois Basin room-and-pillar coal mine to evaluate the performances of three different types of bolts under similar geologic conditions. The bolt types considered were the fully grouted passive rebar (FGPR), the resin-assisted mechanical anchor (RMAB), and the fully grouted tension rebar (FGTR). During the evaluations, the three types of bolts were installed in a single entry covering a span of three contiguous pillars with almost identical geologic conditions. The performances of the three types of bolts were investigated by instrumentting the bolts prior to installation as primary supports on cycle. Additional data on roof behavior was collected with extensometers and shear meters placed near the instrumented bolts. Excluding some time lapse right after installation, the performance evaluation of the three bolt systems was based on the data collected for over a year. Based on this study, it was found that the axial loads measured in the different bolt types did not show any identifiable patterns under the prevailing geological conditions at the mine. Therefore, the assumption that active bolts must be superior to passive ones is not always valid. Even though the studied mine did not have any thinly laminated, weak rocks in the immediate roof, the passive bolts performed as well as the active systems.

Jan 1, 2012