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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 Haile Manteqi

In longwall coal mines, estimation of roof strata condition, sequence of loading, and supporting requirements are of a high degree of importance, and they are directly related to production and safety. Also, a prediction of the initial caving distance is rather complicated to calculate, because there are many parameters that effect caving process?roof and floor strata conditions, thickness of immediate roof, depth of cover, excavation geometry, and magnitude, and direction of principal stresses. This paper presents the results of 3D numerical modeling of the initial caving mechanism using the finite difference code FLAC3D at the E1 longwall panel of the Parvade 1 underground mine located in the Tabas area in the central part of Iran. The initial caving distance obtained was 10.4 m. Also, the results of numerical modeling have been compared to the analytical methods, such as Peng?s method. The results show good agreement with each other.

Jan 1, 2012

By David Conover

Technological advancements in electronic sensors and mine-wide monitoring systems have improved remote monitoring and analysis of underground mining information. The effective use of . these systems for ground control applications, however, remains a challenge facing the mining industry. The U.S. Department of Interior, Bureau of Mines is conducting research, utilizing real-time acquisition of rock mechanics data, to provide mine operators the means to rapidly identify and manage potentially hazardous ground conditions. The Bureau has installed a remote monitoring network in a Colorado coal mine to record Pressures from instruments installed in the roof and pi1lars of gateroads and from transducers on shield-legs along the face. The field data are being monitored at the Denver Research Center, 150 mi from the mine. A computer at the Center analyzes the field data and generates graphic displays, which permit detailed examination of mine-wide ground conditions in real-time. For the first time, Bureau researchers are able to perform real-time monitoring and analysis of rock mechanics data while mining is in progress. The data collection and display capabilities of the system were effectively used to identify and evaluate face stability problems at the mine.

Jan 1, 1990

By Leigh J. Wardle

This paper describes a numerical method for calculating the three¬-dimensional stresses and displacements induced by tabular excavations in a rock mass consisting of parallel layers of distinct material properties. The method is based on the superposition of numerical solutions for elementary displacement discontinuities in a parallel layered rock mass. The method can be used to analyse complicated excavation layouts such as those encountered in room-and-pillar or longwall mining. The method can automatically take into account the effect of the earth's surface. The method offers an economic and efficient solution to stress analysis problems which cannot be adequately modelled using two¬-dimensional methods and for which the expense of three-dimensional finite element or general boundary element analysis cannot be justified.

Jan 1, 1984

By Christopher Mark

Severe dynamic multiple seam interactions can occur when active mine workings are subsided by underlying mining activity. The most dramatic events are usually caused by longwall mining or pillar recovery beneath open, overlying entries. But pillars in abandoned workings can also fail and cause subsidence and damage an overlying mine. This paper describes a case history in which an apparent ?pillar squeeze? in the abandoned workings of a lower seam was initiated during retreat mining in an upper seam. The event subsequently extended more than 1,500 ft and ultimately closed the upper seam mine. Analysis indicated that the pillars in the lower mine were adequately sized for the lower seam mining and were not affected by the initial development above them. When retreat mining in the upper seam had progressed several hundred feet, however, the pillars located directly beneath the pillar line were overloaded, and an extensive squeeze initiated in the underlying workings. The squeeze, in turn, subsided the overlying workings, causing widespread rib falls and roof instability. Previous examples of dynamic multiple seam interactions resulting from delayed subsidence of underlying workings have been reported in the literature. This is, apparently, the first recorded incident in which pillar recovery in an active mine triggered pillar failure in an underlying mine, which then triggered a dynamic interaction that impacted the active overlying seam.

Jan 1, 2012

By M. M. Zarnan

This paper presents the results of s study involving characterization of ground subsidence of abandoned lead-zinc mines in northeastern Oklahoma. Two coordinated efforts were undertaken: (1) collection and analysis of available data such as driller's logs, geologic cross-sections, structure contour naps and stratigraphic sections, and (2) development of a numerical (finite element) technique for prediction of collapse and mine subsidence. An in-depth analysis of a major collapse in the region was conducted. The factors which contributed to this collapse were identified.

Jan 1, 1989

By Christopher Mark

The Bureau of Mines has recently completed a comprehensive study of coal strength. More than 4000 individual test results from over 60 seams were extracted from the literature and combined in the most complete data base of the uniaxial compressive strength of coal ever assembled. In addition, more than 100 case studies of in-mine pillar performance were available in the Analysis of Retreat Mining Pillar Stability (ARMPS) data base. Statistical analysis of this wealth of data has yielded valuable results. The data shows clearly that the "size effect" is related to coal structure. The widely-used Gaddy formula, which predicts a significant strength reduction as the specimen size is increased, was found to apply only to "blocky" coals. For friable coals, the size effect was much less pronounced or even non-existent. Case histories of failed pillars are the best available data on in situ coal strength. This study found no correlation between the ARMPS stability factor of failed pillars and coal specimen strength. Pillar design was much more reliable when a uniform coal strength was used in all case histories.

Jan 1, 1996

By Yuanwci Shi

This paper discusses the results of research on the interaction between roof strata and shield supports including the active and passive supporting functions of the shield supports which prevents roof strata separation; pressure distribution on the canopy and under the base plate as measured in the laboratory and underground; comparison between various shield structures regarding their effectiveness on roof and floor control; and determination of optimum support capacity under given geological and mining conditions including hard-tocave roof

Jan 1, 1997

By Thomas M. Barczak

This report examines practical considerations in longwall support selection that have evolved from Bureau of Mines ground control research. The scope of discussion includes trends in support selection, analysis of shield designs, performance testing, and an examination of support and strata interaction theories and their relevance to in situ support loading. Application recommendations are made for two-leg shields. four-leg shields, and chock-shields. Two-leg shields account for 67 pct of the current active longwall installations and are used in a variety of applications. Automated control systems are likely to continue to grow in utilization. The next major change in shield design is likely to be the application of wider supports. The most effective method of performance testing longwall shields is by controlled displacement of the canopy relative to the base. Controlled force application does not control the load transfer through the shield and may not produce maximum loading in all support components. Theories of estimating support capacity based upon a detached roof block that must be maintained in equilibrium by the powered support are being challenged by concepts based upon the equivalent stiffness of the total ground supporting system.

Jan 1, 1990

By Mike Faure

Hercules cribs make use of timber loaded parallel and perpendicular to the grain. Maximum use is made of the orthotropic properties of timber resulting in crib- having superior strength, high initial stiffness and controlled post yield deformations. Blocks and timbers can be combined in many different ways resulting in thousands of different geometries. Each geometry has a specific combination of block on block, timber on timber and block on timber contact area which results in a unique performance. Each crib can be fingerprinted by defining four parameters relating to its load/deformation curve. If the geometry is known, the performance can be predicted with a high degree of accuracy. If the mine support requirements are known, the crib geometries can be designed which met theme requirements in the most cost effective manner Hercules cribs come closest to satisfying mine support requirements. Due to the flexibility of the concept and their superior strength, they can be employed in most mining situations to enhance safety, improve productivity, reduce working costs or improve profitability.

Jan 1, 1993

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 Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The response of the surrounding rock mass to the creation of mining excavations determines the ultimate load on a pillar support system. In conditions where the ground is relatively soft and weak, the full overburden weight can be transferred to the pillar system. However, in stiffer and stronger rocks, a greater portion of the overburden load is transferred to the unmined coal barriers or abutments, and the pillar stress is reduced. This paper makes use of numerical models to examine the interaction between typical pillar systems and the surrounding rock mass for weak and strong geological conditions at various spans and depths of cover. The concepts of structural failure and functional failure of pillars are used to assess pillar performance when pillars are deformed beyond their peak resistance. The results show that the span-to-depth ratio is an important factor in determining the pillar stress and the ultimate deformation of pillars. The ultimate pillar strain appears to be closely related to the functional success of pillar systems.INTRODUCTIONThe National Institute for Occupational Safety and Health (NIOSH) was requested by Congress to study the safety of retreat pillar mining following the Crandall Canyon Mine disaster that took place near Price, Utah in August, 2007. As a part of this study, an investigation was made to better understand how the rock mass responds to the creation of mining excavations and how the loads are distributed among the pillars and the surrounding abutments or barriers. The outcome of these investigations contributed to the development of a modified loading model for the ARMPS-2010 (Mark, 2010) method ofretreat mining pillar design.Structural and Functional Failure of PillarsPillar design is typically conducted by estimating the pillar strength and stress, and then sizing the pillars so that an adequate margin of safety exists between the expected strength and stress. The pillar strength can be defined as the maximum resistance of a pillar to axial compression (Brady and Brown, 1985). If a pillar is loaded beyond its peak strength, load shedding or yielding can occur, and the pillar is considered to have failed as a structure. Structural failure generally refers to a loss of loadcarrying capacity."

Jan 1, 2010

By W. Smith

The U.S. Bureau of Mines (USBM) is using laboratory and computer-based techniques to investigate the post- failure behavior of yielding pillars and pillar reinforcement techniques to control entry deformation. Poorly dimensioned yield pillars, can cause adverse ground conditions. Laboratory studies to evaluate various reinforcement techniques for controlling yield pillar deformation were conducted. A cracking and crushing model is used to evaluate potential tensile and compressive failure planes within pillars of various widths to assess the degree of yielding and stress redistribution. The use of an elasto-plastic model to evaluate the effect of various reinforcement on yield pillars is also discussed. These procedures effectively account for the residual pillar strength, important in yield pillar design.

Jan 1, 1995

By Daniel Su

This paper describes recent ground control challenges encountered at the longwall recovery areas of one of Bailey Mine?s two longwall faces and the proactive measures taken by CONSOL management and engineering to mitigate them. The challenges derived primarily from the following causes: (1) irregular roof coal and draw slate thickness, (2) presence of thick sandstone at the recovery area and extremely weak rocks below the sandstone, (3) close proximity to a regional anticline, (4) low shield set pressure and tip load, and (5) ineffective primary roof support that led to progressive failure of the headgate roof. The presence of the anticline contributed to extensive fracturing of the sandstone. Excessive pot-out in advance of the shields frequently occurred near mid-face due to the presence of thick draw slate and irregular roof coal near the recovery area. This, in turn, resulted in face heights that exceeded the maximum reach of the shields. Subsequently, during longwall meshing, when the advance rate slowed considerably, fractured sandstone would often break loose, causing the shields to yield, thereby reducing the height and inhibiting shearer movement. To improve the headgate and face conditions within the longwall recovery area, several actions were taken, including: (1) assessing site-specific roof geology by conducting in-mine roof coring around the recovery areas and by mapping immediate draw slate and roof coal thicknesses within surrounding entries, (2) increasing pump pressure and thus shield set pressures (3) diligently monitoring shield set pressures and longwall extraction horizon, (4) installing additional cable bolts and roof screening to prevent progressive failure of the immediate headgate roof, (5) fully-grouting supplemental cable bolts to improve headgate support stiffness, and (6) gluing the headgate roof within the recovery areas with polyurethane. As a result of these collective efforts, longwall recovery delays at Bailey Mine were reduced from 21 days in 2006 to nearly zero in 2009.

Jan 1, 2009

By Christopher Mark

"Many problems in rock mechanics are limited by our imperfect knowledge of the material properties and failure mechanics of rock masses. Mining problems are somewhat unique, however, in that plenty of real world experience is generally available and can be turned into valuable experimental data. Every pillar that is developed, or stope that is mined, represents a full-scale test of a rock mechanics design. By harvesting these data, and then using the appropriate statistical techniques to interpret them, mining engineers have developed powerful design techniques that are widely used around the world. Successful empirical methods are readily accepted because they are simple, transparent, practical, and firmly tethered to reality.The author has been intimately associated with empirical design for his entire career. But where his past papers have described the application of individual techniques to specific problems, the focus of this paper is the process used to develop a successful empirical method. A six-stage process is described:1. Identification of the problem, and of the end users of the final product2. Development of a conceptual rock mechanics model, and identification of the key parameters in that model3. Identification of measures for each of the key parameters, and the development of new measures (such as rating scales) where necessary4. Data sources and data collection5. Statistical analysis6. Packaging of the final productEach of these stages has its own potential rewards and pitfalls, which will be illustrated by incidents from the author’s own experience. The ultimate goal of this paper is to provide a new and deeper appreciation for empirical techniques, as well as some guidelines and opportunities for future developers."

Jan 1, 2015

By Thomas M. Barczak

With the development of the shield support, the primary requirement for successful ground control in longwall mining is to provide stable gate road and bleeder entries. Wood cribbing has been the dominant form of secondary support. However, the cost and limited availability of timber, along with the poor performance of softwood crib supports, has forced western U.S. mines to explore the utilization of other support systems. The recent success of cable bolts has engendered much interest from western operators. Several innovative freestanding support systems have been developed recently including: (1) "The Can"' support by Burrell Mining Products International, Inc., (2) Hercules and Link-N-Lock wood cribs and Propsetter by Strata Products (USA) Inc., (3) Variable Yielding Crib and Power Crib supports by Mountainland Support Systems, (4) the Confined Core Crib developed by Southern Utah Fuels Corporation; and (5) the MEGA prop by MBK Hydraulik. This paper assesses design considerations and compares the performance and application of these alternative secondary support systems.

Jan 1, 1996

By Stephen Tadolini

The annulus thickness, for bolts anchored with resin, effects the systems final performance. Recent developments in the manufacturing process of these support systems, coupled with improved resin characteristics, have resulted in design changes to minimize hole drilling requirements and reduce resin usage. The amount of resin to properly anchor a 7/8-SRD (Special Rolled Dimensions) bolt in a 25.4 mm (1-in) diameter hole has been reduced to about one-third of the resin required to anchor a No. 7 rebar bolt (22 mm (7/8-in) diameter) traditionally installed in a 35 mm (1-3/8-in) diameter borehole. More importantly, laboratory and field tests, indicate that higher bond strengths and anchorage capacities can be realized with reduced annulus systems. These factors have a direct bearing on reinforcement efficiency and subsequent roof support performance.

Jan 1, 1998

By James J. Scott

Research and development has been carried out on the patented Dynamic Rock Anchor, tradenamed DYNA-ROK and DYNA-ROK PLUS Anchors, which are manufactured and marketed by Ingersoll-Rand Company. This paper will present the results of field testing of this revolutionary new concept in ground control. Geologic conditions will be described where the plastic sleeve of the DYWA-ROK anchor alone is sufficient to provide anchorage and also, where in softer ground conditions, the DYWA-ROK PLUS is applied. The rationale for making the decision as to the type of fixture to be employed, the length of plastic tube to be used and the quantity of resin are explained. Pull test data will be shown from eight different coal seams in the Eastern United States where the product has been installed.

Jan 1, 1990

By Yi Luo

In the study of the topographical effects on surface movement due to underground longwall mining, the factors that affect the incremental horizontal displacements are identified using 10 collected cases. Based on this study, the angle of surface natural slope is the major contributor to the incremental horizontal displacement while the total movement is also playing an important role in some cases.

Jan 1, 1996

By Richard D. Begley

This paper summarizes the development of a mechanistic model to predict longwall mining induced surface subsidence. The model was originally developed from data acquired from a recent field study conducted in Northern West Virginia. Additional data has been collected from published sources and used to refine the model. The original model utilizes a negative exponential equation to predict the subsidence profile after the maximum possible subsidence is predicted based on the following input parameters: mine geometry, angle of draw and expansion factors for the deformation zones that have been identified above a longwall excavation. The model refinement involved the development of relationships between physical properties of the overburden strata and angle of draw in addition to the expansion or bulking factors. Therefore the refined model now requires only mine geometry and a typical geologic column from the site to predict the subsidence profile. This paper provides the methodology that enables the model to determine the angle of draw and expansion factors breed on physical properties of the site geology i. e. percent of strong rock and Rock Quality Designation of the overburden strata. In addition :comparisons of predicted results with measured data for over H) different sites with various geological and geometrical parameters located throughout the United States will be presented.

Jan 1, 1989