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By Hamid Maleki

Having developed a methodology for estimating mining-induced seismicity resulting from slip along geological discontinuities in 2003, in this paper, the importance of measurements are high-lighted in the study of caving and seismic source mechanisms as applied to mine designs. Geotechnical data is analyzed from four case studies to address cave condition, load transfer, and potential effects on stability and resource recovery in four Utah mines using measurements and observations conducted over the last three decades. Cave conditions are evaluated on the basis of measurements in the gob of a western U.S. operation mining under the massive Castlegate Sandstone and indirectly using the results of measurements and boundary-element modeling at panel boundaries at a mine where strata behave elastically. Together with subsidence measurements and microseismic monitoring, regional caving and load transfer in multiple-panel extractions are addressed while the effectiveness of barrier pillars in reducing stress and seismic response where mining under massive stratigraphic units is analyzed using measurements and three-dimensional computer modeling.

Jan 1, 2006

By Hamid Maleki

Roof falls are one of the leading ground control problems that adversely influence productivity and worker safety in underground mines, construction sites, and nuclear waste repositories. To control roof stability problems, the operator needs to have practical methods for measuring changes in strata conditions and criteria for defining the timing of secondary support installation. The objective of this study was to develop measurement techniques and criteria for assessing roof conditions at the mining face. An integrated rock deformation and tomographic imaging method was developed and applied at two sites in a western U.S. trona mine. It was shown that rock damage occurred at a deformation of 30 to 60 mm (1.2 to 2.4 in) (2 to 4 pct strain) and was associated with a significant (16 pct) reduction in wave velocity. Thus, supplementary support may be installed when there is a measured change in wave velocity of at least 16 pct. Tomographic measurements can be obtained in rocks rapidly, thus providing a powerful tool for analyzing growth of failure and for installing supplementary support systems in a timely manner.

Jan 1, 1996

By Ted Klemetti

Roof deterioration in weak moisture-sensitive rock leads to roof falls in coal mines. An observation site was set up in intake air at an underground coal mine in central Ohio to evaluate the time-dependent deterioration of moisture-sensitive roof rock. By periodically collecting and weighing the rock falling from the roof, the National Institute for Occupational Safety and Health (NIOSH) was able to quantify the deterioration of the moisture-sensitive shale roof. The measurements showed that more roof rock is falling in the dry winter months. Movements in an intersection were monitored in order to determine if deformation correlated with seasonal changes in temperature and humidity. The rate of intersection roof-to-floor convergence was found to be cyclical and related to seasonal changes, with more convergence during the months of April through September. At the study site intersection, the roof beam did not show deflection indicating movement most likely due to mine floor deformation. The bolt load also increased in the warmer months before shedding load, possibly due to rock failure. Temperature and relative humidity sensors, placed in holes in the roof show that mine air does not mix with the air in the holes. Consistent saturation levels of relative humidity were measured in the holes and it appears that high levels of moisture may have migrated into the bolt holes from roof shales. A roof sealant applied to a portion of the test roof appears to be preserving the roof and reducing the amount of rock fall into the entry, while the screened area virtually eliminated rock falling from the roof.

Jan 1, 2009

By Gerald L. Stolarczyk

Safe and efficient coal extraction requires advanced radar subsystems that can be mounted on cutting drums of coal cutting machines. A look-ahead radar (LAR) must solve the radio geophysics problem of transmitting an electromagnetic (EM) wave through at least 20 feet (6.1 meters) of coal to an air- or water-filled void. The reflected EM wave must be processed in the radar electronics to determine the physical distance to the void. Horizon sensing enables selective cutting near the undulating sedimentary rock boundaries of the coal bed to address coal quality and ground control issues. Both sensors require subsystems for dynamic electric power generation, real-time measurement of drum rotation angle, detection of reflected EM waves, and processing to determine distance through coal and RF modem transmission of measured data to the mining machine. This paper describes the development of the subsystems and field test results.

Jan 1, 2006

By Peter Swanson

A seismic monitoring network has been installed in western Colorado (USA) in the vicinity of three underground coal mines to (i) distinguish and characterize seismic activity as either mining related or naturally occurring, (ii) implement a real-time event monitoring and notification tool, and (iii) collect data for use in research studies aimed at quantifying impacts from mining-related and natural seismicity. These potential impacts include dynamic rock mass failures such as coal bumps as well as strong shaking in the vicinity of critical structures such as impoundment dams, reservoirs, mine seals, mine openings, and steep slopes. Examples of two damaging seismic events are presented and the mining and geologic factors attending these dynamic failures are described.

Jan 1, 2008

By Yi Luo

In dealing with various subsidence cases, accurate predictions of mining induced subsidence are the basis for correct assessment of subsidence influences and subsequent mitigation efforts. There is a good demand for a reliable tool to perform the subsidence predictions and related tasks. A comprehensive and integrated subsidence prediction model, CISPM, developed by the senior authors in the later 1980?s and early 1990?s (Peng and Luo, 1992) is one of such tools. Built on solid mathematical models and calibrated by a large number of collected subsidence cases, CISPM provides a rich set of features needed in subsidence engineering and research works. Its reliability and usefulness have been proven by numerous field applications. This upgrade of CISPM will not only preserve its original features and reliability but also provide some new needed features and a more user-friendly working environment.

Jan 1, 2008

By Takashi Sasaoka

The PT. Adaro Coal Mine targeted in this research is located in South Kalimantan, Indonesia. The coal production of this mine is increasing year, with about 35 M tons of coal produced in 2006. This mine faces many ground control problems, especially instability of the footwall. Large-scale landslides occur often in the footwall; this may be because of weak rock and the effect of footwall rock weathering. Hence, prompt measures must be developed and followed in order to preserve the stability of the footwall and make this mine safe. Additionally, the final pit design is now being reconsidered because of the increasing demand for and high price of coal. With these issues in mind, this paper describes a solution for improving the stability of the footwall and discusses the final pit design based on the results of an on-site investigation and numerical analysis.

Jan 1, 2008

By Takashi Sasaoka

About 200 M tons of coal were imported into Japan in 2007 and Indonesia was its second largest exporter, accounting for about 30 M tons (12% of the total imported coal). Indonesia produced about 200 M tons of coal, about 75% of which was exported in 2007. As the Indonesian government has decided on an energy policy wherein 33% of the country?s primary energy demand will be met by coal by the year 2025, the domestic demand for coal has increased dramatically in recent years. Over 99% of the coal produced in Indonesia comes from surface mines. However, the conditions of surface mines are worsening each year: the stripping ratio is increasing, approaching the economic ratio, and the infrastructure for coal from inland mining areas is insufficient for such intense mining operations. To meet the demand for coal in Indonesia and the rest of the world, underground mines have to be developed. However, mining engineers and/or miners in Indonesia do not have a lot of experiences, expertise and know-how concerning the plans and operations of an underground coal mine. In addition, the industry faces a lot of problems which include poor conditions such as a steep coal seam, groundwater problem, stability of highwall, etc. Under these situations, a mine targeted in this research attempts to develop a new underground mine from an open-cut highwall because the stripping ratio in the surface mining area has almost reached the limitation of economic value. This paper discusses the possible development of a new underground coal mine from an open-cut highwall by means of field observation and numerical analysis.

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 D. J. Reddish

An understanding of the mechanisms that cause large seismic events in a deep mine provides a foundation for designing effective measures to reduce the incidence of these events, as well as reduce the potential for damage arising from these events. The work described in this paper focuses analytical efforts on major recognizable and controllable factors that influence mining-induced seismicity and serves to screen out many of the more random aspects of individual events. Information used in this study was developed in the course of a long-standing rock burst research program conducted by the U. S. Bureau of Mines in cooperation with mines in the Coeur d'Alene Mining District and regional universities. Information was collected on 39 seismic events having local magnitudes greater than 2.5 that occurred between 1989 and 1994. Five major types of events were developed from these data that describe all but two of the 39 large events. The characteristic mechanisms, tint-motion pattern, damage pattern, and relationships to mining and major geologic structures were defined for each type of event. This information provides a foundation for assessing whether or not changes in mining activity aimed at reducing one type of rock burst will increase the incidence of other types.

Jan 1, 1996

By F. E. Chase

Mobile roof supports are shield-type support units mounted on crawler tracks. Mobile roof support (MRS) units are used during retreat mining, and they eliminate the setting of roadway, turn, and crosscut breaker posts which are required during pillar recovery operations. Mobiles are a more effective ground support than timbers, and their usage enhance the safety of section personnel and reduces material handling injuries. MRS usage is rapidly increasing, and approximately 40 U.S. coal mines have successfully employed this relatively new technology. This paper is in response to increasing requests from operators, State and Federal Regulatory Agencies, and others on the practical aspects of MRS usage in underground coal mines. During this investigation, nearly half of the U.S. mines which have utilized mobiles were visited. This report depicts the more common pillar extraction methods which operators have found successful. The Christmas tree and outside lift methods are illustrated and discussed. Roof control plans that do not require breaker posts or allow pillar extraction with fewer than four mobiles are also examined. In addition, operators' experiences with setting pressures, loads, and rates of loading during pillar extraction are addressed. Mining and support strategies to more effectively control hillseams, weak roof, and gob overrides which have entrapped equipment are also discussed.

Jan 1, 1996

Characterizing coal mine roof rock is extremely important for hazard assessment, reinforcement, and entry design. The Coal Mine Roof Rating (CMRR) is an innovative rock mass classification that has found acceptance in the coal mining industry. A disadvantage of the original CMRR is that it cannot be determined in advance of mining, because it requires underground observations. An entirely new set of procedures have now been developed to determine the CMRR from exploratory drill core, as described in this paper. The new procedures employ Point Load Tests (PLT) to determine strength parameters that account for approximately 60% of the rating. Both diametral (parallel to bedding) and axial (perpendicular to bedding) PLT are conducted. The diametral tests allow estimates of bedding plane cohesion and rock anisotropy, both of which are critical to estimating susceptibility to horizontal stress. Traditional core logging procedures are used to determine discontinuity spacing and roughness. To ensure compatibility with the original CMRR, the new rating scales were verified by comparing drill core results with nearby underground exposures. To assist mine planners in using the new CMRR procedures, a large database of strength ratings of roof rocks was assembled through extensive point load testing and logging. Over 2000 point load tests (both axial and diametral) have been made on 30 common coal measure rock types from mines representing most U.S. coalfields. This database has been merged with the popular core rating system of J.C. Ferm into a field guide for geologists and engineers. By using the pictorial classification system of Ferm to identify the rock, typical CMRR can be compared with values calculated using the new CMRR procedures.

Jan 1, 1996

By Thomas M. Barczak

The intent of this paper is to make mine operators and engineers aware of parameters which affect the loading behavior of passive roof support systems. The strength of a passive roof support element alone is not a meaningful measure of support performance. Passive roof support elements and the surrounding strata act as a system, responding to changes in the physical mine environment due to the extraction of coal and associated redistribution of stresses. The load- ing of a passive roof support element is a function of the stiffness of the roof support structure; a stiffer structure will react a higher load to a converging mine roof than will a more flexible structure. If the stiffness if the support structure is not properly designed, excessive loading will result due to the displacement of the mine roof, causing premature failure of the support. Ideally, the yield strength of a passive support needs to be no greater than that required to support the dead weight of the immediate mine roof, providing the post yield characteristics of the support are compatible with the irresistible convergence of the overburden strata and the support system continues to provide this resistance after reaching its yield load. Idealized design considerations for passive roof support systems in terms load-displacement characteristics discussed. The stiffness and yield characteristics of wood concrete cribbing, as determined controlled tests in the Bureau's Roof Simulator, are compared preliminary recommendations passive roof support utilization also provided.

Jan 1, 1987

By S. Yamashita

Recently in Japan, unexpected degradation (surface subsidence) at old underground quarry sites, such as 'Ohya-Ishi' (pumice tuff) pit field has often been occurred. At these quarry sites, the excavation was started 20 or 30 years ago with mechanical chain cutter and the Room and Pillar method, and stopped 7 or more years ago. Therefore, a reliable method has been required for an estimation of long term mechanical properties of the rock and for an observation of a stability of rock-pillars or structures. In this study, creep tests under uniaxial compressive load were carried out on the 'Ohya-Ishi', with the aim of estimating the long term stability of the rock. From these results, a creep mechanism of the rock was discussed using the failure mechanism hypothesis proposed by Bieniawski and by others. On this basis, critical time to failure was predicted by the relationship between the logarithm of life time and stress ratio of the sustained axial creep load. Additionally, outline of the monitoring system of AE/MS activity in the field, which have set by the Ohya Aria Consolidate Public Corporation and the others to predict or to prevent the degradation of those residual pillars, is to be reported. Key words : Ohya-ishi, Pumice tuff, Mine pillar, Long term stability, Creep, Critical time

Jan 1, 1996

By R. D. Yancich

A case study of longwall mining surface subsidence was performed at a mine in Southwestern Pennsylvania. The mine operated in the Pittsburgh coal seam which averaged 70-72 inches with 6-8 inches draw slate in the longwall location. The average overburden depth ranged from 625 8eet to 660 feet. Three adjacent longwall panels were studied and classified as Panel 1, Panel 2, and Panel 3. The following is information on each and can be seen on the General Mine Map (Fig. 1).

Jan 1, 1984

By W. G. Pariseau

A crown pillar cave occurred at the Ropes Mine in December, 1987, that resulted in closure of the mine. Production was resumed soon afterwards with the formulation of a rock mechanics plan designed to address several issues of safety and stability. The plan proved to be successful and shows what can be accomplished in the realm of applied rock mechanics where a cooperative atmosphere exists.

Jan 1, 1989

By Wenbing Xie

At Renlou Coal Mine, Huaibei Mine Area, the track entry deformed significantly after longwall mining in an overlying multiple seams. The supporting characteristics of primary roof bolts and shotcrete with wire mesh are closely related to the movement of the mine?s surrounding strata. A case study of a track entry deformation was monitored during the longwall mining of a selected overlying panel. The measured data showed the unique entry deformation pattern during the longwall mining in the overlying seam. Underground extensometer measurement showed that entry stability was mainly affected by the periodic roof weighting of longwall mining in the overlying seam. The displacement of the deeper strata occurred a head of the immediate vicinity of the entry. Thereafter, deformation of the roof and floor strata was in the form of shearing of the shallower strata. Due to the use of heavy-duty roof bolting with wire-mesh, the roof-tofloor and rib-to-rib convergences were small, but the floor heave was large, greater than 80% of the roof-to-floor convergence. The measured displacement from extensometers varied with the position of the monitored cross-sections and depth into the roof. There were several deformation patterns, such as shearing movement, compression, and expansion, and the variations between compression and expansion. Strata compression at the entry surface was always accompanied with shearing movement at the deeper strata. During the longwall mining, the whole surrounding strata of the entry moved toward the opening. Underground practical experience indicated that prior to mining, the entry was supported very well in excellent conditions. After mining, however, it deformed greatly. The current entry support technology was not effectively control the severe deformation incurred in entries driven in soft strata due to the following reasons: (1) little is known about the entry deformation behavior of the strata surrounding the opening and (2) the degree of interactions between entry deformation and roof bolting with wire mesh is not well understood. Consequently, effective supporting systems cannot be properly selected to control its deformation and maintain a stable supported surrounding structure.

Jan 1, 2012

By J. Willan

This paper addresses the problem of designing a destress blasting procedure to reduce the incidence of rockbursting in hard rock mines. A review is made of the developed theory and practice of destress blasting in rockburst-prone ground. Specific reference is made to a recent survey of past and current practice with particular reference to Ontario mining operations. The need is identified for a more scientific basis for destress blast design and monitoring. Reference to developments in blasting technology as well as rock mechanics are then made to support a proposed, integrated design procedure and research strategy.

Jan 1, 1984

By Theresa M. Bodus

The strength of roof shales is, in part, a function of the preferred orientation of clay minerals within them. Therefore, analysis of clay fabric under both air-dried and hydrated conditions should be helpful in understanding roof shale failure. Core samples were collected from the Energy, Anna, and Lawson shales, which are locally present as roof shales of the Herrin No. 6 coal of southern Illinois. The Clay Fabric Index (CFI) was measured for thirty core samples, using X-ray diffractomtry, to investigate the relationship between clay fabric and roof collapse in coal mines. Greater CFI values indicate a weaker preferred orientation among clay minerals, which generally results in a weaker inter-grain bond. Average CFI values for air-dried samples were: 0.15 for Anna shale; 0.20 for Energy shale; and 0.37 for Lawson shale, .After the introduction of water, average CFI values increased to: 0.28 for Anna shale; 0.22 for Energy shale; and 0.43 for Lawson shale. The laboratory results indicate the following: [l] under air-dried conditions, Anna shale is the most stable, followed by Energy and Lawson shales; and 121 under hydrous (mine) conditions, Energy shale is the most stable, followed by Anna and Lawson shales. In mine environments where roof failures involving Energy shale have occurred, planar trends delineated by a relatively high density of small (0.2-4 cm diameter) iron concretions have been reported along the failure surface. Radial cracks have been observed around unsealed anchor bolt holes in the Anna shale. Instability of the Lawson shale is ubiquitous in coal mines. In the laboratory, similar physical changes were noted in the shale samples after the introduction of water, including [l] increased hydration around the perimeter of iron concretions in Energy shale, [2] radial extension cracks in Anna shale, and 131 extreme slaking of Lawson shale. Measured increases in the CFI reflect the observed physical changes of the samples upon the introduction of water, which probably reflect changes in strength that may result in roof failures in hydrated mine environments.

Jan 1, 1989

By Mao Bai

In predicting the integrity of under-mined structures, surface horizontal displacements and curvatures are frequently of greater importance than the more homogeneous vertical displacements. Accurate evaluation of mining induced subsidence and horizontal displacement further requires that the predictive model adequately represents the subsurface geology, mining geometry and extraction sequencing. The following documents an extension of the Subsidence Prediction and System Identification method (SPASID) to the evaluation of horizontal strains. Comparisons are made between predictions from SPASID and two other empirical methods against measured data from a case study. The system identification method is illustrated to predict horizontal displacements and strains most consistently with the in situ data.

Jan 1, 1989