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By Christopher C. Woomer

Cyprus Shoshone Mine uses the longwall method to extract a deep, thick, pitching coal seam in the Hanna Basin of South Central Wyoming. The immediate, and main roof rock consists of weak, thinly-bedded, silty mudstones with weak, interbedded fine-to medium-grained sandstone. Tailgate ground control has been a critical factor impacting productivity at the mine. A gateroad condition mapping program for the 11 left longwall gateroads indicated potentially severe ground control problems for the tailgate. It was predicted that the existing, secondary support pattern of wood cribs would not provide adequate support capacity. As predicted, severe deterioration of the tailgate occurred in the form of a roof cutter, sloughing ribs, and failing wood cribs. Productivity was affected and additional support was required. Shoshone personnel evaluated several options for providing the additional support needed in the tailgate. The additional support system and placement method had to satisfy several requirements with respect to access, materials handling, placement method, set-up time and support capacity. Approximately 6,000-ft. of deteriorating tailgate had to be supported. The initial wood crib support system left a 6-ft. walkway for access and construction of the additional support. This access was further reduced as the roof loaded the wood cribs. This eliminated the possibility of mechanized material handling to the construction locations. All materials would have to be transported by hand. The additional support system had to be placed quickly to stay ahead of, and keep pace with, the required longwall advance rate. It had to incorporate the existing wood cribs and provide at least 500 tons of extra support capacity. And most critically, the system could not affect longwall ventilation. Longwall coordinators and engineers made the decision to use a low density, pumpable cement known to the industry as TeksealTM, to provide the system required. A 200 psi ultimate strength mix was decided on to provide the required load capacity. The existing cribs were formed with 1-in. by 6-in. boards and brattice cloth to provide the containment. To overcome the access limitations, three boreholes were drilled from the surface to the tailgate on 2,000-ft centers. A mobile pumping station was established on the surface and the TeksealTM was pumped 900-ft. down the boreholes through a 1.5-in. steel pipe, then as much as 1,800- ft. along the tailgate entry through 1.25-in. miner spray hose. The materials required for the TeksealTM supports could all be carried into the construction locations by hand. As a direct result of incorporating relatively new methods of pumping high yield, low density, cementitious grouts, the Shoshone Mine reduced downtime due to tailgate ground control problems by approximately 70% in comparison with previous Longwall panels. The longwall set three monthly production records while mining the 11 left longwall under the deepest cover, steepest pitch, and most extreme ground control conditions ever encountered at the mine.

Jan 1, 1995

By Jun Lu

There are many factors affecting the strength of pillar, including width-to-height (W/H) ratio, mechanical properties of coal and surrounding rocks, property of partings in the coal, and the interface property between coal seam and the roof/floor. In this paper, the effect of varying pillar?s width-to-height ratio (W/H=3, 5, 7, and 10) and coal/rock interface cohesion (Cj =50 psi, 150 psi, and 300 psi) on pillar strength has been studied. The confinement effect of W/H ratio and interface shear strength is investigated by studying their effects on the average minimum principal stress developed in the pillar. The deformation behavior of the coal pillar is investigated. It is found that the pillar?s transition from strain softening to strain hardening behavior depends on its W/H ratio and the interface shear strength.

Jan 1, 2008

By Ke Yang

In Chinese coal mines, extraction of deep, highly gassy soft coal seams sandwiched between soft floor and soft roof in complicated geology is commonly encountered. How to safely and efficiently excavate such seams is a very significant and important problem. It has been a focused research area in recent years. By combining the characteristics of fully mechanized longwall mining methods such as large cutting height and mining along the dip, an integrated method and technologies for the Zhangji Coal Mine were developed. This technology includes the design of a complicated geological exploration system, optimization of mining method and working face parameters, control of rock pressure and high levels of gas, reliability model of the production system, and selection and compatibility of equipment. Engineering practice shows that the integrated technology was successful in mining these complicated seams. This study not only enriches the mining method, but also improves the coal mining practice, especially ensuring both safety and productivity under complicated geological conditions.

Jan 1, 2009

By Robert A. Siokler

Approximately 75 percentage of the earth's land surface is comprised of shale or shale-like materials. Shale itself is composed of the residue from an almost infinite variety of weathered parent materials which have undergone deformations ranging from intense to negligible. Under standard test procedures, shales exhibit a very wide range of physical properties particularly if the test specimen is subjected to a change in moisture content, when the original test values may vary from 0 to 100 percentage. It is this wide variation in physical property values compounded by its ubiquitous presence that makes shale such a problem material to engineers. In reviewing the literature relating to shale, it is evident that the "problem" has been approached from many directions. In order to understand why shales behave as they do, investigators have sought mineralogical, geological, chemical, electrical, and physical answers. Each study yields valuable academic information, but few contribute much of value to the engineer in the field. Many valid relationships between shale strength or competency and clay type, clay content, organic content, water content, grain size, cementation, density, porosity, etc, have been established. Yet it is very difficult to extend these understandings to the in situ condition with results that can be used by the field engineer. One major reason earlier work has not produced better information lies in the ambiguity of the definition of shale. It is apparent that no one laboratory parameter can be successfully used to provide sufficient engineering information to properly describe in situ conditions. For example; shale with swelling clays will produce low strength values provided: the swelling clay is in sufficient quantity, the clay is dispersed throughout the medium involved, and there is a low value of cementation. Shales with high unconfined compressive strengths have been found with very low shear strengths. Sandy shales often provide as many ground control problems as clay shales. Shales of similar nature may provide good roof conditions in one part of a mine yet in another part of the same mine the apparently identical shale may cause substantial problems. A clear need exists to classify shale in a manner which will enable the engineer to better predict the strength and durability of shale-like materials. Studies at the University of Missouri-Rolla (UMR) have focused on finding parameters which influence durability and competency, while being discretely discernible through uncomplicated testing. Initial research concentrated on distinguishing shale8 on the basis of standard physical tests such as: unconfirmed compression, triaxial shear, and slake durability. As results accumulated, however, it became evident that moisture content played a much greater role in determining competency than had been recognized previously. As a result the focus of the research turned to developing tests which would evaluate the influence of moisture on shale behavior. The above work then became a foundation for a second phase of investigation in which a suite of standardized tests were developed to provide a classification procedure. From this study five tests were identified as being critical: Moisture Activity, Atterberg Limits, Submersion, Unconfined Compression, and Shore Hardness.

Jan 1, 1986

By C. Dinis da Cama

In order to reduce the complexity of mechanisms influencing rock- fragmentation by blasting a simulation approach is proposed, using the capabilities of micro-computer interactive graphics. Situations involving cratering and bench blasting are simulated, and both jointed and intact rocks can be subjected to explosive action, taking into consideration its breakage mechanisms. Size analysis curves relating rock distribution before and after the blasts are determined and compared with field data.

Jan 1, 1984

By Arnold Hammons

Automated temporary roof support (ATRS) systems have been incorporated into roof bolters since the 1970?s. During roof bolting, many injuries have occurred from large pieces of draw rock falling inby the ATRS beam and tipping back into the operator?s area. Other injuries have occurred due to smaller, sharp rocks falling onto the operator?s hand and arm during initiation of roof bolt drilling. These injuries are attributed to weak immediate roof, sometimes called roof skin failure. Even though ATRS systems support the roof and provide a measure of protection against roof skin failure, additional hazards sometimes do exist immediately inby and outby the supported roof. Design innovations such as the ATRS mounted rocker pad deflector systems have been designed to improve the roof bolter operator safety. This paper will inform the reader of the hazards, machine evolution through different design solutions, and the current state of the art in preventing roof skin failure injuries when roof bolting.

Jan 1, 2009

By Christopher Mark

A coal burst is defined as the sudden, violent ejection of coal or rock into a mine opening. Coal mines in the North Fork Valley (NFV) of Colorado?s Gunnison River have a long history of coal bursts in a wide variety of settings. These have included longwall and room-and-pillar, development and retreat, and single- and multiple-seam mining. In contrast to other coalfields, where bursts are typically associated with strong, massive sandstone and extremely high stress, many bursts in the NFV have occurred beneath shale roof and some distance from the most highly stressed locations. The goal of this paper is to attempt a systematic study of the entire burst history in the NFV, in order to gain insight into their characteristics and causes. The primary sources of information are the reports of investigations conducted by the Mine Safety and Health Administration (MSHA) Roof Control Division over the past 15 years. This paper provides details on the geologic mapping conducted as part of these investigations, as well as a regional plot of burst locations using a Geographic Information System. The mapping indicates that many of the NFV bursts have been associated with structural features, including low-angled faults and well-defined zones of high-density jointing that exhibit little or no displacement. Some of these features appear to extend for several miles, impacting more than one mine.

Jan 1, 2012

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 Da Zhao Gu

An important problem in rock engineering is to control the properties of artificial rock. A compaction technique for casting artificial rock specimens with controlled properties is described, i.e. using a compression-testing machine to compress the mixture. Uniform density is necessary to achieve uniformity of properties of the specimens, and uniform density can be obtained by using the specific pressing method presented. Rebound and borax content are important to the technique and have been studied as well.

Jan 1, 1992

By Dan Payne

The Crinum mine has recently completed workings in the southern area of the mine. Over the 10 years of longwall production 40 million tonnes have been extracted and the mine has experienced over 40 longwall face, 3 main gateend, and 2 tail gateend roof falls as well as 5 roadway roof falls away from the longwall. Weak roof (less than 10MPa (1450psi) in the bolted horizon) has been the principal roof control issue at the mine. However, weak, highly cleated coal, water inflow from overlying aquifers, some minor structure and a diatreme in the main headings have also contributed to the challenges at the mine. This paper describes the geotechnical experience over the 10 years, the mine?s approach to addressing the issues and the relative success of these approaches.

Jan 1, 2008

By Yoginder P. Chugh

Subsidence may be observed above partial extraction shallow room-and-pillar mines where the coal seam(s) is associated with weak and thick claystone in the floor. This paper presents results of subsidence studies at such a mine. During the 26-month study period, a time dependent subsidence causing a maximum vertical movement of 50 mn was observed. After a subsidence rate of 6.0-6.5 m/month for the first two months after development of mine workings, the rate decreased to 0.8-0.9 mm/month after 17 months. However, presumably due to settlement of barrier pillars, it increased therafter to 1.7 mm/month for the next 9 months. Angle of draw, based on zero subsidence, was calculated to be about 29 degrees. Maximum tensile strain and compressive strain values observed were 0.040 pct and 0.039 pct respectively. A hypothesis was fomlated to correlate surface subsidence movements with observed underground movments. The data collected to date fits reasonably well within the formulated hypothesis.

Jan 1, 1989

By Daniel Su

Approximately 90% of the primary roof bolts used in CONSOL?s Pittsburgh seam coal mines is 2-piece, 8-foot long, 7/8? (or ¾?), high-strength, resin assisted, mechanical shell tensioned bolt. The remaining 10% is the 2-piece, 8-foot long, 7/8? combination bolt, which employs 4 feet of fast resin to anchor the deformed rebar at the top of the hole. In either system, a 1-3/8? hole is required to accommodate the mechanical shell and the coupler. The cheaper torque-tension rebar system, which is typically fully-grouted with 2-speed resin and which typically provides low bolt tension, is not used in CONSOL?s Pittsburgh seam mines. Rather, it is used in CONSOL?s Central Appalachia mines, where the roof rocks have CMRR rating of 50 or higher. Fully-grouted headed rebars, on the other hand, are used in CONSOL?s room-and-pillar mines. The selection of tensioned versus non-tensioned, fully-grouted bolts for coal mine primary roof support has been debated for the past thirty years. Many researches have been conducted to understand the theoretical bases behind the two systems. This paper describes CONSOL?s experience of using a new fully-grouted, mechanical shell tensioned bolt in the Pittsburgh seam with the development of low insertion force resin.

Jan 1, 2007

By Andre Zingano

The variation in pillar behavior at underground coal mines working in the Bonito coal seam is observed in both research and mining company case studies. There is no consistency in pillar behavior, as it varies from mine to mine. In some cases, it is possible to find rib yields for both excavation modes and pillar sizes. This paper?s objective is to investigate pillar behavior for different load conditions and pillar sizes. Three-dimensional numerical simulation models considering these factors were built. A matrix was then assembled to compare pillar behavior between loading conditions, as well as between different pillar sizes with the same loading condition. The results showed some pillar rib plasticity due to vertical loading and stress relief independent of the pillar size and load. For the same load conditions, the yielding thickness is the same for different pillar sizes.

Jan 1, 2012

By Michael Alber

Coal mining by the longwall method typically causes seismic events. This paper describes the research on mining induced seismicity around two specific longwalls at 1100m depth under several old longwalls. A local network featuring 21 geophones has been installed, underground core drilling and an extensive rock testing program were executed. A rock mechanical model featuring a multi-layer strata was created for use with numerical analyses. Seismic events were localized and three different event types are described. Focal analyses yielded distinct modes of failure. Those types are explained by different failure mechanisms, namely fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses allows evaluating the findings from seismology approaches. Conclusions are drawn that the orientation of longwall with respect to the major horizontal stress orientation plays an important role in mine induced seismicity.

Jan 1, 2008

By Anthony Iannacchione

Coal mine bursts have represented a major hazard for U.S. mining operations for more than 90 years. During this time, many prevention controls have been developed and tested. This paper reviews 11 prevention control techniques. Although coal mine bursts are not common events in most underground coal mines, their occurrence almost always requires a change in mining practice. Over the many years of dealing with these hazards, specialized requirements for layouts and novel extraction sequences have been developed on a site specific basis. The keys to mitigating risks are to properly assess the coal burst hazards and to possess the knowledge and skills to prevent or remediate their occurrence.

Jan 1, 2008

By David A. Newman

Barrier pillars created during mine development provide additional reserves during mine closure and can be the sole access to reserves previously abandoned because of mining height or out of seam dilution limitations. The safe and successful mining of barrier pillars requires a thorough knowledge of the magnitude and distribution of the vertical stress field in addition to the strength and physical properties of the coal, immediate roof, and immediate floor strata. Quantifying the vertical stress field is complicated by the combination of the overburden stress, abutment stresses transferred onto the barrier pillar from adjacent gob areas, and in multiple seam mining, stresses transferred from abandoned subjacent mines. These variables make the extraction and development of barrier pillar reserves a site specific consideration. Three case histories are presented in which barrier pillar development and extraction provides; i) an access point and the means to ventilate a large area of low seam reserves left by a prior operator after developing only the thicker areas of a multiple split reserve, ii) the means to increase total recovery of a deep, thick seam longwall reserve, and iii) opportunities for the development and retreat mining of a two mile long barrier pillar in thick seam reserves.

Jan 1, 1995

By Michael Alber

Resin grouted rock bolts are the major support measure in coal mining. They are rather easy and foolproof to install, take immediately load and are the least time consuming support measure for controlling gateroad deformation. The use of only resin grouted bolts allows fast gateroad development which is essential for economical resource exploitation. The design of reinforcement / support systems for coal mining typically bases on empirical methods, which work fine when encountering known or usual ground conditions. However, coal mining approaches depths where little experience exists and numerical modeling is often the only method to estimate ground behavior. This paper discusses the geotechnical properties relevant to numerical model and focuses particularly on input values for modeling resin grouted rock bolts for safe gate design.

Jan 1, 2006

By Andrzej Walentek

This paper presents the results of numerical modeling of the fractured zone around longwall gateroads. The numerical computations have been based on the results of underground measurements of the fractured zone around longwall gateroads, which were performed for various geological and mining conditions and measured with the use of the endoscopic method with a digital camera. Having results of the underground investigations, trials of duplicating them by the Finite Element Method using software PHASE and the Hoek-Brown failure criterion were performed. In order that the fractured zone predicted by the model-based computations closely simulate that measured underground, some selected parameters in the Hoek-Brown failure criterion were modified based on the earlier determined relations used for prediction of the roof-to-floor convergence in longwall gateroads. As a result, the model-based computations produce results very close to those obtained from the underground measurements. Determination of the fractured-rock zone around the mine roads is of great importance to mining practice, especially in planning and designing supports for longwall gateroads. The influence of the front abutment pressure causes the fractured rock zone around the roads to increase and results in an increase of support load. Moreover, proper determination of the fractured rock zone is one of basic elements in the process of planning roof bolting for mine workings.

Jan 1, 2009

By John C. Stankus

At the 13th Conference on Ground Control in Mining, a paper was presented detailing a successful test in which a new truss system was utilized, in lieu of wood cribs, for tailgate support in the Pittsburgh seam (Stankus et al., 1994). Since that time, additional tests using this same truss system in the Pittsburgh and other seams are now complete. Also, several mines are now using trusses for full panel tailgate support with no cribs. Through an extensive instrumentation program, much data and new information has been gained from these tests and the full panel usage. From this data, not only have new concepts pertaining to tailgate support been developed, but also for headgate, pillar configuration, primary and supplemental support. In this paper, an update will be presented summarizing the results of these various tailgates.

Jan 1, 1995

By Floyd Varley

In deep and highly-stressed coal mining, advanced design practices are used to minimize the potential for damaging events to occur. Despite these design efforts, the hazard of coal mine bumps can not be completely eliminated. This paper will review current and historic work practices used internationally to minimize the hazard to miners from mining in conditions which could produce a bump. These site specific work practices have a common base in monitoring for conditions which could result in a bump and application of controls to prevent the occurrence or minimize the severity of an event where miners are working. Monitoring practices range from purely observational to semi quantitative. Control methods attempt to initiate events in a controlled manner, absorb the energy of a potential event or reduce the potential for or magnitude of an event by altering the stress conditions of the mine workings. Building from the experiences of other coal fields, an empirical framework to manage the potential hazard of coal mine bumps can be developed to enhance the level of safety provided by prudent mine design.

Jan 1, 2008