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By Thomas M. Barczak

The initial Support Technology Optimization Program (STOP), Version 1.0, was released at the 19th Gmund Control Conference. This original program has since been updated to Version 2.3 which was released in May, 2001. This paper describes the additional features of Version 2.3, focusing primarily on the following three aspects: (I) including uncontrolled convergence into the design requirements for standing roof supports, (21 the addition of design procedures for cable bolts as an alternative secondary roof support, and (3) the addition of new standing roof support technologies. Another new feature which should facilitate the development of design criteria for standing roof support is the capabilily to define ground reaction curves through convergence measurements alone wthout having to make support loading measurements. There are several other new features provided in Version 2.3 which are only briefly addressed in the paper. These include: (I) additional graphics capabilities to facilitate rapid assessment of support comparisons. (2) an East-West designation for support selection to more accurately provide cost and support availability data, (3) additional safety measures including a safety factor to identify support design near the peak support capacity, checks to see if the support dimensions comply with aspect ratio requirements, and measure of roof coverage for the design layout of the support system These new features of STOP provide more capabilities to design and analyze secondary roof support systems for conditions whlch could not be fully analyzed in the original version of the program.

Jan 1, 2001

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 A. Wahab Khair

This paper presents results of the experiments carried out in Beth Energy No. 33 Mine for the purpose of alleviating problems associated with cutter roof. Cutter roof problems have delayed the advance of the entry development considerably and maintenance cost of these entries was very high due to the requirement of very extensive and heavy artificial supports. The study includes assessment of the problem, investigation of the geology and lithology of the mines, study of in-situ stresses and corrective measures to combat the problem. Two approaches were selected as the corrective measures: 1) implementing yield pillar concept in order to reduce the effects of in-situ horizontal stresses, 2) designing an effective roof reinforcement system and application procedure. Results of this study revealed that roof strata is weak and in-situ stresses are very high at the problem area which results in development of cutter roof. Implementation of yield pillar concept was found to be ineffective because cutter roof often developed at the entry face concurrent to the development. However, proper design and implementation of roof reinforcement system stabilized the roof and eliminated roof caving during both entry development and retreat of longwall face, despite existence of the cutter roof, at a much lower cost.

Jan 1, 1992

By Gerald I. Finfinger

Identifying the properties of overlying rocks in underground mining operations is important to ensure the appropriate roof support design is used to maintain stability of the mine entries. Recently J. H. Fletcher & Co. developed a monitoring and control system for roof bolters for the underground mining industry. The system records the drilling parameters used during roof bolt drilling and the information can provide insight into the physical properties of the roof strata. The parameters include thrust, rotational speed, torque and velocity and the measurements are collected every 0.1 second during the operation. The drilling parameters were analyzed to determine the application of identifying the strength of rocks being drilled from the measurements. The data was converted into the specific energy of drilling which is a measure of the amount of energy required for removing a given unit of rock during a drilling operation. The laboratory studies completed to date indicate a fairly high correlation between the specific energy of drilling and the unconfined compressive strength of the rocks that were drilled. Additionally, the drilling parameters were shown to be effective for identifying the presence of fractures or bed separations between rock ]avers. The thrust, torque and specific energy of drilling were all good indicators for identifying the fractures or separations. Regardless of the drilling parameters used during the drilling experiments, the location of the fractures were identified, In order to determine the application of the drilling parameters for identifying rout rock properties, two series of experiments were conducted. The first series of experiments used three "manufactured" roof layers that had various rock samples embedded in concrete blocks. The rock samples included three types of sandstone, marble. and argillite. Another concrete block was poured with foam inserts to simulate large bedding separations (2 to 8-in). Two other manufactured blocks were constructed using high-strength concrete with cardboard layers embedded to simulate smaller fractures or bedding separations. The size of the cardboard layers varied from 1/8- to 1-in thick. One of the blocks had the cardboard layers embedded at an inclined angle to determine the effect of the orientation or, the drilling parameters. A series of experiments was conducted with the rotational speed and the penetration rate held constant and the thrust and torque allowed to vary. The information collected from the experiments is used to determine the application of the drilling parameter measurements for identifying rock properties, fractures and bedding separations.

Jan 1, 2000

By Francis Kendorski

A longwall coal mine in Appalachia about 1,500 ft deep encountered a fault while developing a new longwall panel. The fault extended from mining depth to the surface near a secondary road and drainage. The fault was located inside the anticipated angle of draw within the mined panel and gob. The fault extended vertically out and up, away from the panel, caved zone, and gob at nearly the angle of draw: the fault very nearly following the angle of draw. It was initially thought that "fault reactivation" could possibly occur. Fault reactivation is the phenomenon of having mining subsidence localized along a fault leading to a "reactivation" of the fault and shearing and displacement along the fault. Such fault reactivation would disrupt and deform the fault plane beyond the normal angle of influence of the subsidence trough, and may provide a conduit for any ground and surface waters to reach the mine. We contacted all operators of longwall mines in Appalachia to determine if any Appalachian longwall mines had ever experienced fault reactivation, and teamed that none had experienced the phenomenon. After studies of possible water intrusion quantities and rates based upon in-fault pump tests, which indicated that water intrusion rates should be manageable, and the prior experience that faults in this particular area were usually barriers to water flow, mining proceeded with caution and monitoring. Mining was successful with no noticeable increase in water inflow rates, and no measurable off-setting of the fault exposure on the surface. It can be concluded that the fault did not reactivate due to its relationship to the mining sequence.

Jan 1, 2003

By T. L. Dobecki

As a means of demonstrating the effectiveness of reflection seismology in determining continuity of coal seams, three U. S. field projects are reviewed. The three projects involve coals of varied thickness (2-14m) and age (Pennsylvanian to Eocene) from three coal areas of the U. S. (Pennsylvania, Wyoming, and Washington). Each project also employed its own particular seismic technique, recording system, and seismic energy source although all are considered state-of-the-art, high resolution, digital seismic surveys. Project 1 (thin, Pennsylvania coal) sought detection of sand channels using dynamite and standard in-line (2-D) seismic technique. Project 2 (thick, Wyoming underground coal gasification) involved a gas-explosion (Dinoseis) source with areal (3-D) acquisition methods. Project 3 (thick, Washington underground coal gasification) employed a shotgun-type source and standard in- line methods. Data processing was handled by different contractors for each project. Each project was successful in accomplishing its own particular objective; however, data quality and interpretation seem to be more a function of thickness of the target seam, complexity of the overburden, and processing contractor than a seismic source, acquisition scheme (2-D versus 3-D), or recording instrumentation.

Jan 1, 1981

By Matthew J. DeMarco

Research personnel at the U.S. Bureau of Mines have conducted longwall gate stability studies at four western U.S. mines attempting to employ yield pillars for improved ground conditions. Located in the Book Cliffs and Wasatch Plateau coalfields of north-central Utah, these operations are generally characterized by multiple-seam mining, cover depths averaging 450 - 600 m (1,500 - 2,000 fl), occasionally extending beyond 760 m (2,500 ft), abrupt cover depth variation due to the overlying canyon/mesa topography, and massive, rigid sandstone roof units. Such conditions encourage severe gate and panel coal bumping, as well as various types of roof instabilities and occasional floor-heave problems. To alleviate these adverse conditions, several mines have employed yield pillars to-reduce hazard-inducing excessive gateroad stress concentrations. This paper describes the evolution of gateroad designs at four of these operations, the specific mining conditions involved, and the general stability performance attained by using yielding gate systems. Mine experiences are "compared" in terms of basic setting conditions, and the general concepts of yielding system applicability and success potential are summarized.

Jan 1, 1993

By Bruce K. Hebblewhite

Underground pillar extraction continues to be practiced in Australia in various forms, in spite of the dominance of the longwall method, which accounts for over 90% of underground production. Australia has been a leader in various forms of pillar extraction, particularly since mechanisation was introduced to the industry during the middle of the 20th century. Historical developments have included a range of thick seam mining pillar extraction methods; the widely adopted Wongawilli method; early adoption of the use of mechanized breaker line supports; and a number of more recent partial extraction systems including what is known as rib stripping. Current pillar extraction practice varies from total extraction panels to various forms of partial extraction, driven by either surface subsidence constraints or by underground conditions. This paper reviews a number of different pillar extraction methods and mining layouts, and also highlighting a number of fundamental geotechnical principles which have been found to be critical for safe pillar extraction ? often through lessons learned the hard way, as a result of back analysis of different accident scenarios. These principles include the all-important understanding of the location of the goaf (gob) edge at any point in the operation, and the management of that goaf edge; the impact of geological structures on local and regional stability; the critical importance of extraction lifting sequences; the role of stooks (remnant pillars); and a number of other factors. The paper will also briefly reference the educational and training practices adopted in Australia aimed at the continuous improvement of pillar extraction safety.

Jan 1, 2011

By Dennis R. Dolinar

A room and pillar coal operation in central Pennsylvania was experiencing roof cutters and long running roof falls caused by high horizontal stresses. The roof conditions created hazards for the miners, and caused several production panels to he abandoned prematurely. The mine requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in applying the "advance and relieve" mining to reduce the affects of the horizontal stress during panel development. The "advance and relieve" plan that was developed called for removing a pillar on one side of the panel as it was being advanced, thus creating a cave. The caving then relieved a portion of the horizontal stress across the panel. Because the horizontal stress direction is key to the success of this method, stress mapping as well as mining experience was used to establish the direction of the horizontal stress. Subsequent field measurements provided an estimate of the stress magnitude. Three panels were mined using this technique, and good roof conditions were achieved in all these panels. Instrumentation was used to monitor the stress changes in the roof created by the cave. Stress relief of over 1.000 psi was measured 120 ft from the cave and to depths of 20 ft into the roof. The lateral extent of the stress relief zone across the panels appears to be about 400 to 500 ft. This case history has provided much insight into the practical application of the "advance and relieve" method discussed in this paper.

Jan 1, 2000

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 Keith Heasley

An eastern longwall mine was required to mine through a set of three entries located midway down the length of a panel. Of course, the mine desired to design a successful support system for the mine-through area at the least cost. To further complicate the design process, the mine-through entries were located directly under the end of a previously mined, overlying longwall panel and the entries were close enough to be subject to stress interactions from the overlying seam. In order to accurately evaluate the multiple-seam stresses and modify the mine-through support system appropriately, the boundary element program LaModel was used to model the stress and displacements in the two mines. Using the results of the model's stress analysis, the support density was systematically varied throughout the mine- through entries for optimum sup@ efficiency and economics. During the actual mining of the cross panel entries, the longwall experienced few, if any, additional delays, and overall, the support and mine through of the cross-panel en- tries at Harris #1 was very successful, saving the mine confineable time and money.

Jan 1, 2003

By M. Bouteldja

A finite element modelling approach is developed for the evaluation of the mechanical performance of cable bolts supports in underground mine structures. The modelling approach permits the simulation of any arbitrary number or pattern of cable bolts, that can be either partially or fully grouted, with or without end plates. The modelling technique used treats each cable bolt as a set of bar elements representing the steel cable. The bond stiffness characteristics of the grout bolt interface are represented by shear springs uniformly distributed along the cable length. Coupled with a simple linear elastic analysis, this technique can serve as a dedicated numerical model for cable bolted excavations. The application of the model is illustrated with two case studies: one for a hard rock mine and another for a coal mine.

Jan 1, 1999

By Kanaan Hanna

The integrity factor approach was developed by the Bureau of nines to assess the stability of mine pillars and has been applied primarily to longwall chain pillars. Recently this approach was applied to a room-and-pillar mine in the Illinois Coal Basin. A special configuration of hydraulic borehole pressure cells was installed in a pillar to determine the in situ strength-and- stress relationship of the coal pillar and to verify the applicability of the integrity factor approach. Results indicate that the integrity factor approach is applicable to the assessment of pillar st6b:lity and nay bs a realistic and effective approach for room-and-pillar mine design.

Jan 1, 1988

By John L. Hill

Over 50 mines of the Appalachian and Illinois Basins are presently experiencing poor ground conditions believed to be caused by overlying stream valleys. The Bureau of Mines is conducting research into the causes of this problem with the aim of developing a predictive method for determining the safe limits of mining beneath stream valleys. In addition, the factors that control instability in these regions will be analyzed to provide design guidelines for adjusting support requirements. This paper presents the theorized causes of poor ground conditions beneath stream valleys and compares the theories with the results of a Bureau study of the Birchfield Mine No. 1 and a preliminary investigation of the Davidson Mine, both of southern West Virginia. The Birchfield Mine excavated four main entries beneath a 50-ft wide stream with only 50 ft of overburden. The valley flood plain is over 500 ft wide, flanked by valley wall slopes of 25 deg and relief of over 950 ft. In-mine monitoring of strata movement and detailed geologic mapping revealed that roof rock strength was greatest under high cover and weakest along outcrops and beneath the valley, with localized areas of decreases in rock mass quality. To improve the rock mass quality beneath the stream and reduce the threat of inundation, an extensive drilling program was undertaken to inject grout into the overburden from the surface. As a result, mining beneath the stream was practically routine from a ground control perspective, and water was not encountered until mining had advanced to a point beneath the floodplain that had not been grouted. The only ground control problems which were encountered were entirely a result of localized poor rock mass quality. In contrast to the Birchfield Mine, the Davidson Mine experienced failure of intact roof rock at a depth of 300 ft beneath a V-notch shaped valley with no change in rock mass quality. Finite element analysis was conducted to study the premining in situ stress at the two sites. The results are discussed within this paper and provide insight into the causes for the types of failure seen beneath valleys of varying shapes and depths.

Jan 1, 1988

By Gregory J. Chekan

Developing a coal seam that has been influenced by previous mining in seams either above or below can result in severe ground control problems. In many instances, interactions between operations are inevitable. but improvements in ground stability can usually be achieved by-predicting potential problem areas in advance and adjusting the mining plans accordingly. This report investigates four case studies involving interactions between operations that used both room-and-pillar and longwall mining methods. In each case, geologic and mine design parameters are addressed, and those parameters found critical to the interaction are further defined in relation to observed ground problems.

Jan 1, 1990

By George Gardner

The rapid growth of highwall mining in the Appalachian coalfields has resulted in unique safety concerns. Due to the concentration of activity at the base of the highwall and the potentially destabilizing influence of the mining process, proper mine planning and continual monitoring of the ground conditions is essential to ensure a safe and productive operation. This will also help to minimize the need to recover equipment that has become entrapped underground, and the subsequent hazards associated with a recovery operation. Adequate geotechnical site investigation and rational design methods should enable mine planners to properly account for the site-specific conditions that influence web and abutment pillar, roof, floor, and highwall stability. This paper examines the growth of highwall mining, provides a general overview of the various safety concerns, and reviews the modes of highwall failure as they relate to a highwall mining operation. Gcotechnical considerations that should be taken into account before and throughout the mining operation are emphasized. The importance of closely examining and monitoring highwall conditions for subtle signs of highwall instability will be discussed.

Jan 1, 2002

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 Hani S. Mitri

The behaviour of grouted cable bolts used for rock reinforcement as a means of ground control in hard rock mining is investigated. The study utilizes an analytical model which is based on the variational principle to calculate the shear bond stress distribution along the embedded length of cable bolt. In this paper, a comprehensive parametric study which covers both geometric and mechanical design parameters involved is presented. Special emphasis is placed on studying the characteristics of the grout model represented by Winkler springs. This involved varying the maximum shear strength of the grout, its residual strength and the slip occurring between the cable and the borehole wall.

Jan 1, 1990

By P. E. Christensen

Due to ever-increasing stripping ratios associated with the No. 8 Seam at the San Juan Mine surface operation, BHP Minerals decided to implement longwall mining for the extraction of the substantial future reserve base. In preparation for longwall developments, BHP opted to first develop a small-scale pilot operation within the No.8 Seam on an adjacent lease to address geotechnical/geological concerns related to ground control, subsidence, caving, panel and entry design, and operational issues related to entry development and proper face equipment selection. Room-and-pillar mining methods are currently being utilized to develop a full extraction panel for a wide range of geological, geotechnical, and geomechanical evaluations aimed at characterizing and assessing the mineability of the No.8 Seam. Variable roof and floor conditions, coupled with a multi-bench coal seam, require that a number of mining horizons, opening geometries, and roof and rib support fixtures be evaluated relative to expediting gate and panel developments. Issues related to development of timely caves in areas subject to overlying sand channel deposits will also be addressed through underground in-seam stress and deformation measurements, and interburden and surface subsidence monitoring. This information will ultimately be used to evaluate face support and gate and panel pillar/entry development requirements. The current depositional model for the property, developed from an aggressive drilling, highwall mapping, and lineament evaluation program, will also be refined during the Pilot Mine study through extensive in-trine mapping, additional roof and floor drilling and physical property testing, and determination of the in situ principal horizontal stress field.

Jan 1, 1998

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982