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By Andrew P. Jarosz

The present computerized mine planning systems use CAD-like and Data Base Management System (DBMS) environments for the handling of numerical, textural, and graphical data. These two environments are usually integrated, that the data provided by the DBMS are linked to any graphical element rendered by the CAD system. The subsidence monitoring, analysis and prediction process should be integrated with the mine design process. It should allow for the assessment of impacts of mining on surface, surface utilities, and structures. It should also provide for the measures to control mining, that will create only acceptable levels of negative impacts on the surface. The common data base, and graphical capabilities of the CAD interface, may be used with great advantage for both systems. The findings of the subsidence engineering modulus should be directly incorporated into the mine design system, and vice versa. In this paper the author suggests guidelines for the development of the comprehensive computer system, able not only to predict the vertical and horizontal components of the ground movement, but also assess the induced surface damages, and costs related to subsidence prevention and compensation.

Jan 1, 1995

By S. L. Huang

The location and shape of potential failure surfaces in soil slopes were directly determined based on the stresses induced in the soil mass and the strength of the soil itself. Stress calculations were carried out by the finite element method with the Drucker-Prager nonlinear material model. The Mohr-Coulomb failure criterion and Mohr's circle stress analysis were then applied to the calculated stresses to determine the characteristics of the failure surface.

Jan 1, 1989

By John D. VandeKraats

Yielding rock bolt support systems have been developed to accommodate ground movement in shifting ground such as in coal operations; in creeping ground such as salt, trona, and potash; and in swelling ground associated with some clays. These systems, designed to remain intact despite ground movement, should enhance mine safety and help contain costs in areas where rebolting of rigid non-yielding systems is typically required. Four such systems were tested in straight tensile pulls in the laboratory. They include the Slip Nut System from Dywidag Systems International USA, Inc., Ischebeck's bolt mounted Titan Load Indicator, Rocky Mountain Bolt Company's Yielding Cable Bolt, and a rock bolt installed variation of the yielding steel post developed by RE/SPEC Inc. The first two systems are currently marketed products and the latter two are prototype systems. Each system responds to load and displacement by yielding in a unique manner. All are designed to yield at predetermined loads. A description of each system and its yield function is provided. Each system was tested over its prescribed yield range in a test machine. At least five tests were performed on each system. Each system yielded and continued to provide support according to its design. Each shows promise for ground control use in shifting or creeping rock. This work helps to illustrate the comparative differences in performance between these specialized systems and the applications where they may be most useful.

Jan 1, 1996

By Y. M. Jiang

A users friendly PC Computer Program has been developed to implement a systematic design of the powered support. The required input data are: (1) thickness, RQD, and compressive strength of the immediate roof; (2) mining height; and (3) type, thickness, and tensile strength of the main roof. Several important parameters for support selection under a given geological condition are generated through the implementation of the program, including type of support most favorable for the geologic condition, minimum inclination angle of the legs with respect to the horizontal axis, suitable setting load ratio of the front to rear leg for the 4-leg supports, optimum setting load, and support capacity.

Jan 1, 1989

By Stephen C. Tadolini

The U.S. Bureau of Mines, in cooperation with the Cyprus- Plateau Mining Company, has conducted research to provide an alternative to traditional secondary support systems in a two-entry, yield pillar gate road. As recent as two years ago, it was impossible to imagine a gate road supported solely with internal high-strength resin-grouted cable supports that would virtually eliminate the necessity for crib or concrete external supports. The support system evaluated consisted of 1.6-m (5-ft) full-column resin-grouted bolts and 4.8-m (164) long cable supports installed in conjunction with wire mesh and "monster-mats." Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during first and second panel extractions. The stress and loading histories for the panels and yield pillar were monitored to evaluate the stress transfer and pillar performance in conjunction with the roof and floor behavior. The test results indicated that the designed support system successfully maintained the roof during the extraction of two longwall panels and dramatically reduced the cost of secondary support. This paper will describe the theory of a cribless support system, the advantages of cable supports, and present the pillar, roof, and floor measurements made to assess the support performance during longwall retreat mining.

Jan 1, 1995

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 E. D. Doney

The Kerr-McGee Coal Corporation's Galatia Mine's first longwall mining system was implemented on May 3; 1989. Start-up of the longwall face represnted the successful conclusion of an effort begun nearly four years earlier. This paper-presents a case history of the processes that led to the longwall's implementation, and the subsequent mining of the first 7,900 foot long panel.

Jan 1, 1990

By Rob Thomas

Adequate load transfer is considered essential for optimum ground support and is increasingly important in weak roof and/or high stress environments. If adequate load transfer is achieved not only can this have a positive impact on the resulting roof behavior, but it may also allow the use of lower and hence more economic densities of roof support. A wide variety of post-groutable cable bolts are currently in use in the Australian coal mining industry, each of varying capacity, design, and grouting methodology. Though all of the cables are supplied with the appropriate steel specification, strength, and modulus data, the Australian coal mining industry does not have access to a comprehensive and comparative database on the corresponding load transfer performance, which is a critical problem. On the basis of the above, this paper describes the methodology and results of a laboratory pull testing program that aims to assess the impact of cable bolt design on load transfer, and specifically, the impact on the capacity and stiffness of the anchorage. As part of the testing program, 19 cables were tested including (1) plain-strand cables installed in holes drilled with 28 to 55 mm diameter drill bits and (2) bulbed and nutcaged cables installed in holes drilled with 42 to 64 mm diameter drill bits. Other parameters assessed included the significance of smooth and profiled wire, wire packing, and expanded geometry bulbs. The main results of the testing indicate that in comparison to plain-strand cables, if the cable is bulbed or nutcaged, the capacity of the anchor can increase by up to 400%, and the peak stiffness by between one and two orders of magnitude. The results also indicate that increasing hole diameter may have a positive impact on load transfer in the case of bulbed or nutcaged cables and a negative impact on plain-strand cables. No conclusive results were obtained with regard to the impact of grout strength on load transfer.

Jan 1, 2012

By Morgan M. Sears

In recent years, one large mine collapse and numerous smaller bump events have resulted in a renewed interest in coal bump research. With numerical modeling, the Energy Release Rate (ERR) calculation quantifies the ?release? of the gravitational potential energy of the rock mass into the environment as mining progresses. This release of energy can occur passively in the form of heat and sound or dynamically in the form of pillar bumps or rock bursts. From the initial application of a calculated energy release rate in the deep hard-rock mines of South Africa, the ERR was found to have a significant correlation with the risk or potential of damaging coal bumps or rock bursts. In this research, an ERR calculation is incorporated into the modern LaModel program to facilitate the analysis for potential coal bumps. Initially the ERR calculations in LaModel are verified using a case study of cut sequences originally modeled using the MULSIM/NL program. Then, the ERR calculations are applied to a bump-prone mine in Southern Appalachia were a number of different pillar recovery cut sequences were used in order to minimize the risk of bumps.

Jan 1, 2009

By John Ellenberger

The nature of a stone deposit is rarely defined through exploration and engineering design, but more through the actual experience encountered as the mine develops; hence, issues with ground control are often at the forefront for a new mine. Ground control problems account for over 15 percent of lost-time injuries in underground stone mines. The objective of this work is to present actions taken during early development of a stone mine to mitigate the obvious effects of high horizontal stress on ground control. The mine was developed from a box cut with relatively little cover, and high horizontal stresses were not anticipated. Early recognition of possible high horizontal stress resulted in mapping of the mine using the NIOSH-developed Roof Fall Risk Index (RFRI) to provide a baseline to objectively analyze roof conditions. A program of borescoping the mine roof provided valuable roof beam thickness data and found that the roof beam was of minimal thickness, approximately 10 percent of drift width. As the mine developed, it became apparent that ground control problems related to high horizontal stress existed. A concentrated effort was made to improve ground conditions by implemeting a number of mitigation techniques. Actions taken by the mine included systematic roof support, reorientation of mining direction, a lower roof horizon, reduced span widths and reduced mining height in crosscuts to prevent running falls. These strategies resulted in signifcantly improved ground stability.

Jan 1, 2012

A model was developed to facilitate secondary tailgate support selection based on analysis of over 100 case studies, compiled from two different surveys of operating longwall coal mines in the United States. The ALPS (Analysis of Longwall Pillar Stability) program was used to determine adequacy of pillar design for the successful longwall case histories. A relationship was developed between the secondary support density necessary to maintain a stable tailgate entry during mining and the CMRR (Coal Mine Roof Rating). This relationship defines the lower bound of secondary support density currently used in longwall mines. The model used only successful tailgate case history data, with adequate ALPS SF according to the CMRR for each case. This model facilitates mine design by predicting secondary support density required for a tailgate entry depending on the ALPS SF and CMRR, which can result in significant economic benefits.

Jan 1, 1996

By Hakan Gurgenli

Many coal seams have weak shale immediate roofs that cause ground control problems. Therefore, it is important to know the properties of these shales so that preventive measures can be developed in advance. However, it is sometimes hard to understand this rock type due to its unpredictable behavior. A good example of this situation is found in two shale coal mine roofs located in the Illinois coal basin (Herrin No. 6 seam). Although, these mines are located very close to each other, one of them has many roof falls, whereas the other has no measurable ground control problems. The aim of this paper is to explain why and how one shale roof behaves worse than the other. Laboratory tests, including point load strength index, slake durability index, moisture activity index, weatherability index, water content determination, swelling strain and x-ray diffraction tests were performed to analyze the shale properties. According to these analyses, some useful correlations were found between the engineering properties, moisture-sensitivity indices, and weathering properties. Finally, mine and support design considerations based on the test results were discussed and recommendations to prevent ground control problems were given.

Jan 1, 2006

By A. T. Iannacchione

Numerical simulation of coal pi1lar loading has traditionally been a difficult task due to the unique and highly variable properties of coal and the inability of numerical procedures to duplicate these properties. This simulation was based upon material properties developed from laboratory tests of coal cubes and examined against coal pillar strength data developed from actual underground measurements. The coal cube measurements indicated the coal followed a nonlinear Mohr- Coulomb failure criterion. The in situ measurements showed that a pillar yield zone developed similar to Wilson's hypothesis and, at high confinements, the pillar core seemed to followed a pseudo-ductile behavior. The strain-softening material model, based on a 1inear Mohr- Coulomb failure criterion, simulated coal cube and pillar behavior at moderate load conditions. However, at high loads the models were inaccurate, due to an inadequate failure mechanism and/or to changes in the pillar constraints at the coalbed interface.

Jan 1, 1989

By Peter (Yunqing) Zhang

Longwall recovery under weak roof conditions could have considerable delay if the immediate roof is weak and deteriorates during screening. A pre-driven longwall recovery room is a feasible solution to minimize the impact of longwall move on production. In this longwall recovery project, a 16-ft recovery room was pre-driven to cover screening, and the shearer cut another 16 ft outby to reach the final recovery line. The pre-driven recovery room was carefully monitored as the face was approaching it. The longwall face operated very well as it cut into the recovery room. The recovery process took about 13 days, and considerable time was saved compared with the previous panel. The support in pre-driven recovery room was designed to handle both front abutment pressure and weak immediate roof conditions. Pumpable standing support was used to support the cantilevered roof as the fender pillar narrowed and collapsed when the face cut into the recovery room. To ensure the stability of the recovery room, 3-D computer modeling was used to evaluate the support system. Abutment pressure, critical fender pillar width, bolting system, pumpable cribs and shield support were considered in the model. The recovery room was carefully monitored during longwall recovery. Roof-to-floor convergence and crib convergence were recorded every shift as the longwall face was approaching the recovery room. Roof, floor and pillar activities and crib performance were also observed as the face cut into the recovery room. This paper presents the support design for the recovery room, evaluation of the support system through computer modeling, and details of the monitoring results.

Jan 1, 2006

By Floyd D. Varley

Outbursts, the sudden and violent out- pouring of material into mine openings, is a ground control problem increasingly encountered in the United States as deeper seams are developed. Outbursts, caused by several mechanisms, are the result of the loading of ground support systems be- yond their designed capacity to failure, manifested in a violent release of stored energy. There are several types of outbursts typical to deep underground coal mines. - Outbursts of the coal face due to the presence of pressure abutments in close proximity to the face. These outbursts are usually accompanied by liberation of large amounts of methane. - Instantaneous convergence of the ribs and floor due to overloading and failure of ribs which lengthens the beam of floor strata sometimes resulting in a chain reaction failure of the mine floor. - violent failure of the confined core of a pillar due to rapid loading beyond the yield point of the pillar.

Jan 1, 1986

By Dao Hong Quang

This paper describes the numerical analysis of some geotechnical factors influencing the application of Longwall Top Coal Caving (LTCC) method. The study was aimed at comparing the differences in stress distribution in the chain pillars of the face ends and caving mechanism of the top coal in LTCC method between the flat and inclined thick coal seams. The outcomes from this analysis will benefit mine planners in designing the location of roadway development in the next longwall panel. In addition, results from the study will extend the understanding of caving mechanism in top coal section when applying the LTCC method (with the direction of the longwall face is down dip, retreating along strike) in inclined coal seams. The results demonstrate a clear effect of dipping factor on cavability of the top coal and on the stress distribution in the chain pillar of the face ends.

Jan 1, 2008

By Peter Zhang

The immediate roof at the test site consisted of laminated clay shale that is likely to be susceptible to weathering. With seasonal dry-wet cycles in the mine ventilation air, the roof in the intake air would deteriorate and lose integrity over time. In many instances, the weathering-prone roof would gradually scale off piece by piece between bolts and over the entry corners. In some long-term entries, roof weathering could develop up to 2-3 ft deep with columns of bolted roof hanging under the upper intact roof. If wire mesh is used, the roof bags down and reduces the entry height, and the weight of the bagged roof tends to pull the bolts out and poses a safety hazard. An effective measure is always sought to support this type of weak roof that is sensitive to moisture degradation. One solution is to seal the roof shortly after entry development to prevent moisture penetration so that roof integrity can be maintained for the long term. In this application, a polymer-based sealing material was tested in the Rockspring mine for weathering protection and reinforcement. The material is a one-component polymer powder for spray application onto the roof and rib. The sealant material is mixed with water in the spraying nozzle to apply to the roof and rib as a paste. The product sets within 5-10 minutes and forms a membrane as thin as about 1/8-in. The membrane will quickly develop tensile and bonding strength within minutes and completely cure within a few days. Through sealing, bonding and lateral reinforcement, the integrity of the weak roof can be maintained. The material has been tried in a long-term evaluation over several months in a set of main entries in the Rockspring mine owned and operated by Rockspring Development, Inc., affiliate of Foundation Coal. Preliminary evaluation has shown that the material is an effective and economical solution to ensuring the long-term stability of weathering-prone roof.

Jan 1, 2009

By Sean Harvey

One of the manifestations of the dynamically induced failures in deep coal mines is the violent ejection of coal from the ribs. A possible protection against such failures is the use of a specifically designed dynamic support system with reinforcement units having the sufficient capacity to absorb the kinetic energy of the dynamic failure and a skin support that can adequately contain the disintegrated coal. With this method of mitigation in mind, a recently introduced yielding rock bolt, trade name Roofex, has been identified (Harvey and Ozbay, 2009) as a possible energy absorbing component of a dynamic support system. According to the manufacturer?s specification, the Roofex bolts can yield 300 mm (or more) displacement while holding 80 kN pull load. In order to assess the performance of this support unit in coal ribs, a series of in situ pull tests performed in two coal mines in Colorado. The results show that, under conventional pull test loading conditions, Roofex yielding bolts perform very close to their claimed specifications, thus showing a potential for their use as the main energy absorbing component of a dynamic support system. Further studies are required to confirm the in situ performance of these bolts under dynamic loading conditions and also to develop a containment component for a dynamic support system for the mitigation of dynamic coal rib failures.

Jan 1, 2009

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 Y. Luo

A PC based computer model has been developed by the authors for predicting the surface subsidence due to underground coal mining. Its reliability, comprehensiveness and user friendliness demonstrate that it is a good tool for the mine operators, government agencies and scientific researchers alike.

Jan 1, 1989