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By Vincent A. Scovazzo

On March 12, 1994, the 1,100-foot deep 2 Yard South panel of the 1l0 year-old Retsof Mine collapsed. At the time of this occurrence, the Retsof Mine was characterized as the world's largest, most productive salt mine. The collapse produced a seismic event and resulted in a sinkhole and small lake forming on the surface. An existing highway bridge was rendered unusable. Water entered the mine and despite valiant efforts by the miners and the mining company, the entire mine was flooded and lost. John T. Boyd Company (BOYD) studied the incident to evaluate design procedures, and assess the short- and long-term environmental impacts of the collapse. This paper presents our analysis of the cause of the collapse and our review of the rock mechanics design of 2 Yard South panel. Our review is, in part, based on a rock mechanics monitoring program of 2 Yard South. The monitoring program was established by the mining company as the panel was developed and continued even after the panel became unstable.

Jan 1, 1997

By Don Kay

A two year study into the effects of coal mining induced subsidence on cliff lines was undertaken by the Department of Mineral Resources with the support of a grant under the National Energy Research, Development and Demonstration Program. A new monitoring technique, using an Electronic Distance Meter and acrylic reflectors attached to the cliffs, provided safety for the surveyors and extensive and accurate three dimensional displacement data on the movement of the cliffs as they were being undermined. The study report included many graphs showing the measured vertical and horizontal displacements, differential subsidence, tilts and strains and included analysis of these movements with various parameters, such as, seam thickness, depth of cover and distances between the reflectors, longwall face and the edge of the chain pillar. Some of these graphs are presented in this paper and could be used, with caution, to predict subsidence movements around other cliff lines, after allowing for differences in geology and mining layout. A mechanism is suggested as an explanation for the observed high horizontal movements around the cliff lines. Sixteen percent of the overall lengths of cliffs that were undermined during the monitoring study fell and the contributions of various factors that appeared to influence the cliff falls are discussed. It is concluded that no single factor dominated as the major cause of the cliff falls and, often, many factors combined to cause cliff face instability.

Jan 1, 1992

By Abouzar Vakili

Longwall Top Coal Caving (LTCC), as the most attractive method for thick coal seams, has been suffering from insufficient geomechanical understanding for the past few years. Cavability without doubt is the most critical geomechanical issue associated with the top coal caving method. A discontinuum method has been used for comprehensive modeling of LTCC operation by using the Universal Distinct Element Code (UDEC). In addition Particle Flow Code (PFC) and Phase2 were used as support for the caving mechanics analysis. Although the previous researches of gravity flow in caving operations neglected the effect of continuous span expansion, it has been observed in this study that face advancement has a great impact on roof symmetrical behavior. The amount of symmetry fluctuates between different face distances. Horizontal displacement seems to be the best indication for roof symmetrical evaluations. Two critical caving conditions have been modeled and parameters such as horizontal displacement, caving angle and recovery rate have been compared thoroughly. Results show that unsymmetrical behavior on the roof is closely related to face stability.

Jan 1, 2007

By Ghassan Skybey

Australian coal mines uses state of the art technology in coal rib reinforcement. This paper highlights the development over the past 15 years in coal rib support technology, along with the advantages and disadvantages of each system. It also outlines the current trend in using coal rib reinforcement in speeding up the dowels installation to improve the current low drivage rate. An innovative cuttable dowel is discussed which improves the drivage rate. This new dowel is self anchoring it uses no resin anchor it has a 5 tonnes (5.51 tons) anchor capacity and can be fully installed in approximately 10 seconds.

Jan 1, 1995

By Yi Luo

Multiple-seam mining operations induce surface subsidence basins different from those caused by mining in a single coal seam, and also disturb the mining operations in the other coal seams. A computer program, CISPM-MS (Comprehensive and Integrated Subsidence Prediction Model for Multiple Seam Mining), has been developed to predict the final surface movements and deformations as well as the interactions associated with multi-seam coal mining operations. The program applies a newly developed subsurface subsidence prediction model that is able to consider the geological stratification variations. In predicting surface subsidence, the surface movements and deformations caused by the individual mining operations as well as the interactive effects are all considered. A case study is conducted in this paper for validating the program.

Jan 1, 2012

By Takashi Sasaoka

Chain Conveyor Cutter (CCC) is a chainsaw-type machine and originally developed for the construction of underground walls and/or ground improvements. CCC was developed from the accumulated technology of coal mining, combining the cutting technology of a shearer and the fuctions of chain conveyor. The CCC can cut harder soils and soft rocks, and vertically mixes the cut soils/rocks with cementing material selected through the discharging port installed at the bottom of the machine. In Indonesia, highwall mining trials have already been conducted in several coal mines. However, full-scale and regular operations have not been done yet due to the ground and machine control issues from their geological conditions. In order to overcome these problems, the application of CCC to highwall mining systems was proposed, and its applicability and technical issues to be solved were investigated in this research. This paper discribes the characteristics of CCC system and discusses the application of CCC to highwall mining systems.

Jan 1, 2012

By Keith A. Heasley

In the past, numerous displacement-discontinuity (DD) models of stratified rock masses have been proposed using a homogeneous isotropic elastic overburden. In this paper, it is postulated that a laminated overburden model might be more accurate for describing the displacements and stress in these stratified deposits. In order to investigate the utility of using a laminated overburden in a DD model, the fundamental mathematical basis of the laminated model is presented and graphically compared with the fundamental behavior of homogeneous isotropic elastic overburden and with field data. Specifically, the stresses and displacements surrounding an idealized longwall panel as determined from the laminated overburden model are presented and compared with results from the homogeneous isotropic overburden and with measured abutment stress data. Also, the remote displacements and surface subsidence as calculated by the laminated overburden model are compared with homogeneous isotropic calculations and with measured subsidence data. The results of this investigation show that the laminated overburden is more supple, apt to propagate displacements and stress further, and better able to fit observed data than the classic homogeneous isotropic overburden. Ultimately, it is suggested that the laminated model has the potential to increase the accuracy of displacement and stress calculations for a variety of mining situations.

Jan 1, 1996

By Qingxiang Huang

Shallow seam coal field has the largest coal reserve in China. Mining in shallow depth causes serious problems. One of them is subsurface dewatering. In this paper, the physical simulating model was performed to study overburden movement and aquiclude stability in the shallow seam mining of Yushuwan coal field, China. According to the characteristic of clay aquiclude in the overburden, the proper simulation materials and proportions of mixture for simulating the plastic clay aquiclude layers and brittle bedrock layers were determined by the stress-strain tests. The physical simulating model was carried out to simulate the failure and caving process of the roof and overburden. Based on the simulating tests, it was found that the movement of clay aquiclude follows the movement of the underlying bedrock layers. The basic caving mechanism of the overburden roof strata was also presented. The tests also showed that the best way to control the stability of aquiclude is to reduce the subsiding gradient Ts. This research provides a foundation for further study on mining dewatering and water conservation.

Jan 1, 2006

By A. W. Khair

Laboratory studies indicate that the specimen size influences the compressive strength of coal significantly. In the past, the diameter/height ratio of coal and rock specimens with different geometry and its influence on failure characteristics was discussed (Khair, 1993; Khair, 1994; Khair and Xu, 1994). In the current attempt, tests are designed and carried out under uniaxial condition in the laboratory to investigate the influence of specimen size. Cubical specimens of coal with edge dimension ranging from 5 cm (2 in.) through 20 cm (8 in.) were prepared according to the standards and tested uniaxially. Results show that compressive strength of coal is dependent on specimen size until certain level and later the change in specimen size would not yield significant change in compressive strength. Cubical specimens were chosen for testing because they are not only in close resemblance to underground pillars, but they are also the most stable shape under loading. The failure characteristics of these specimens were analyzed and their applicability to pillar design aspects in coal mines is studied. A detailed analysis of the physical and mechanical properties of the tested coal seam is also presented.

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 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 Kot F. v. Unrug

The present combination of coal market prices and reserves, situated in the Central Appalachian Coal Field, is such that mining of smaller coal reserve blocks has become economically attractive. In above drainage deposits, and particularly under mountain ridges, overburden depth can be changing within a wide range. Mines operating in these deposits are usually developing mains along the center of the ridge, with panels projecting from the mains towards the outcrop. Due to changing topography, proximity of the outcrop, and related cover depth, the panels have various lengths. In this reported case study, a mine in Pike County, Kentucky, along the Kentucky ?West Virginia border, difficult ground conditions were encountered, during the development of the mains, under shallow cover, along a narrow mountain ridge. An area having unusual density of vertical joints was encountered. Mining operations were suspended because of roof stability concerns. The ground conditions were studied, analyzed, and control measures proposed to ensure safe passage through the difficult roof area of the mine. The advancement of the mains, through this area of challenging ground conditions, was successfully completed. In this paper, ground conditions are described, with an explanation of the geologic process contributing to their formation. It is interesting to note that similar locations were predicted by the authors, to be encountered in the same mine, based upon the experience gathered from studying conditions of the case described within this paper. When approaching the predicted locations, stability problems appeared to be even more challenging than those encountered previously. Due to changes in the coal market, mine management chose to move the affected mining section to another area of the mine having better coal quality. The advancement of the mains, through the difficult ground conditions, was temporarily postponed. Based upon the analysis of a probable kinematical mechanism of blocks in the mine roof, ground control measures were proposed. The actual application of these principles proved to be appropriate, ensuring safe passage through the difficult ground conditions encountered in the mine.

Jan 1, 2006

By E. C. Westman

Anomalous geologic or stress-induced conditions can create a hazardous working environment for underground miners. Seismic attenuation tomography provides a valuable tool for understanding static and dynamic conditions induced by mining. As a longwall shearer cuts coal, it generates seismic energy. As this energy travels through the rock mass it is attenuated in a manner proportional to the degree of fracturing of the rock mass. By calculating this attenuation at specific locations underground, the degree of fracturing of the rock mass, and hence the relative stress state or material properties can be inferred. The goal of this work has been to create a tool which can be easily used by the mining industry to obtain information about the distribution of stress concentrations or material properties across a broad area in advance of mining.

Jan 1, 1996

By Frank E. Chase

Clay veins, also referred to as clay elastic dikes, have been responsible for numerous underground injuries and fatalities. These hazardous structures are also responsible for increased production costs during all phases of mining. For these reasons, the Bureau of Nines has investigated the physical characteristics of and roof instability problems associated with clay veins in order to make support and other mining recommendations. In addition the occurrence and origins of clay veins were examined to determine whether or not the structures could be projected into unmined portions of the coalbed. This investigation included portions of the Arkoma, Illinois, Northern Appalachian, Southern Appalachian, and Uarrior Coal Basins. Virtually a11 clay veins observed or documented occurred in the Illinois and Northern Appalachian Basins. Observed clay veins ranged in sire from 1 inch to 16 feet In cross-section and were found under shale, siltstone, sandstone, and limestone roof rock. Individual clay veins were predominantly composed of claystone; however, limestone. siltstone. and sandstone infilled clay veins do occur. Clay vein formation has been associated with the infilling of fissures which result from compressive, tensile, or shear ground failures. These gaping fissures were propagated by compactional processes and/or tectonic stresses active during and subsequent to coal iff cation. Associated fault, fracture, and slickenside planes commonly parallel clay veins and disrupt the lateral continuity of the immediate and, sometimes, main roof. When clay veins parallel or subparallel the direction of face advance the roof is segmented into cantilever beams causing unstable conditions. Therefore, the strata on either side of the clay vein must be bolted and strapped together to form a beam.

Jan 1, 1984

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 David Bigby

In the UK, it is becoming increasingly necessary to open new longwall mining areas above old, abandoned workings. In such cases, strategic and gate roadways must often be driven and maintained across highly stressed areas and pre-existing and sometimes only recently formed subsidence troughs. Ground control at the edges of the subsidence trough can be particularly problematic, with complex combinations of stress and rock conditions being encountered depending upon the precise position in relation to underlying pillar edges. For economic recovery of these reserves, it is essential that rockbolt support is used wherever possible. It is therefore very important to the industry to develop its understanding of ground conditions and support requirements in these areas. This paper describes; ? the historical background to these problems and a series of documented UK case studies from the last 20 years. ? the experiences at UK Coal?s Riccall colliery, where a number of longwall panels were worked at a depth of around 800m, some 65m above previously worked longwalls. Actual experience is compared with pre-mining expectations and model predictions. ? design considerations and experience to date at UK Coal?s Welbeck and Thoresby collieries, which are embarking upon a series of longwalls in the Deep Soft seam, 30-40m above previously worked longwalls. The paper outlines numerical modelling techniques used to optimise such mine layouts within the complex geological and geometric constraints of a mature coalfield. These involve combinations of 2D and 3D finite difference and 3D boundary element models. It also discuss new modelling approaches which will be investigated in forthcoming EU funded research.

Jan 1, 2007

In this paper a numerical formulation is presented for determination of the axial load along a cable bolt for a prescribed distribution of rock mass displacement. Results using the program CABLE indicate that during excavation, the load distribution that develops along an untensioned fully grouted cable bolt depends on three main factors: (i) the properties of the cable itself, (ii) the shear force that develops due to bond at the cable-grout interface (i.e. bond stiffness), and (iii) the distribution of rock mass displacement along the cable bolt length. In general, the effect of low modulus rock and mining induced stress decreases in reducing bond strength as determined from short embedment length tests, is reflected in the development of axial loads significantly less than the ultimate tensile capacity even for long cable bolts. However, the load distribution is also dependent on the deformation distribution in the reinforced rock mass. Higher cable bolt loads will be developed for a rock mass that behaves as a discontinuum with deformation concentrated on a few fractures, than for one which behaves as a continuum, either due to a total lack of fractures or a very high fracture density. This result suggests that the stiffness of a fully grouted cable bolt is not simply a characteristic of the bolt and grout used, but also of the deformation behavior of the ground. In other words, the same combination of bolt and grout will be stiffer if the rock behaves as a discontinuum than if it behaves as a continuum. This paper also explains the laboratory test programme used to determine the constitutive behavior of the Garford bulb and Nutcase cables bolts. Details of the test setup as well as the obtained results are summarized and discussed.

Jan 1, 1996

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 G. K. Rogers

The portal, which is the near-horizontal, surface point of entry to an underground excavation, can often be an exceedingly difficult area in terms of ground control. Surface and subsurface failures at portals, as discerned during a study involving over 300 case histories, are unfortunately, common. To aid in the engineering evaluation of portals, the Geomechanics Classification System was appended with discontinuity orientation assessment values and support/excavation guidelines. Also, a design model utilizing RMR predicted rock loads in conjunction with half-dome theory is proposed for the most common type of portal failure. Other pertinent comments and recommendations relating to portal stability are included.

Jan 1, 1989

By Kray Luxbacher

Three-dimensional time-lapse velocity tomograms were generated to image stress redistribution around a longwall panel to produce a better understanding of the mechanisms that lead to ground failure. Mining-induced microseismic events provided passive sources for the three-dimensional velocity tomography. Surface-mounted geophones monitored microseismic activity for 18 days. Most similar studies have utilized active sources including longwall shearers, controlled explosions, and hammer blows. These active sources generally require a person to initiate and measure the location and time of the source, and are not conducive to continuous monitoring. This method utilizes mining induced microseismic events as passive sources and lends itself well to continuous monitoring. Two event location methods were compared in order to move toward quantification of tomographic results in terms of ground conditions. Additionally, LAMODEL was utilized to determine the expected stress distribution around the longwall panel and compare these images to the velocity tomograms. Eighteen tomograms were generated, one per day, and high velocity regions in the set of tomograms generated from the initial location algorithm correlated with abutment stress redistribution predicted by numerical modeling. This research is significant as it has the potential to improve safety in underground mines and is the only example of long term passive velocity tomography implementation for the purpose of inferring stress redistribution around an area of active mining. Though still immature, this technology has the potential to allow mine personnel to image areas of active mining on a regular basis and detect areas with anomalous velocity distributions indicating potential safety hazards.

Jan 1, 2007