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By Francis S. Kendorski

Underground stone mines tier construction aggregates and industrial minerals are being planned in increasing numbers throughout the United States. At the middle of the last century (1950s) underground stone mines were much more common, and characteristically had no thorough site characterization other than proving reserves and trial-and-error planning. Ground control employed was minimal. Consequently, there was a tendency to conduct minimal pre-mining studies. However, the deposits that lend themselves to little site characterization and minimal ground control are few today, with the best being mined out. Failures of a number of underground stone or similar mines from that era are more common than realized, Underground stone mines almost always use some combinations of square or rectangular pillars, sized to support the overburden load, and spaced to allow the use of larger, more-efficient equipment. Heights are controlled by the available stone quality, interbeds of waste, or technological or safety considerations. Prior to committing to a specific mine plan with subsequent significant capital investment, and prior to having available the wealth of knowledge and experience gained once mining underground, a safe and workable mine layout must he achieved. Early attention to the several different rock units involved (roof, mine level. floor) and their strength as a rock mass with the inherent discontinuities, will allow first approximations of acceptable mine geometries and sequencing.

Jan 1, 2000

By Anil K. Ray

Roof falls are one of the greatest hazards in underground coal mines worldwide. Due to stringent safety measures and extensive support system development, roof falls have been reduced significantly but they still persist in many underground coal mines. Some of the roof falls initiate with cutter roof or guttering. In the past, many factors have been identified as being responsible for the development of cutters and roof fall. These responsible factors can be broadly classified as stress related or non-stress related. In this paper the finer details of mining technique have been taken into consideration for better understanding of cutter roof and roof fall problems. Three dimensional finite difference modeling has been used to explain the mechanism of cutter development. Strain softening material behavior has been used to realistically simulate the cutter formation, and common cutting sequences as implemented in the field have been simulated. This was done with an objective to determine whether the cutting sequence has really any significant control over cutter formation or not. In addition to cutting sequences, the effect of cut length is also studied. The effect of step cutting and turning of crosscut into and away from major and minor horizontal stresses is also examined to see its influence on cutter development.

Jan 1, 2009

By Michael Alber

Resin grouted rock bolts are the major support measure in coal mining. They are rather easy and foolproof to install, take immediately load and are the least time consuming support measure for controlling gateroad deformation. The use of only resin grouted bolts allows fast gateroad development which is essential for economical resource exploitation. The design of reinforcement / support systems for coal mining typically bases on empirical methods, which work fine when encountering known or usual ground conditions. However, coal mining approaches depths where little experience exists and numerical modeling is often the only method to estimate ground behavior. This paper discusses the geotechnical properties relevant to numerical model and focuses particularly on input values for modeling resin grouted rock bolts for safe gate design.

Jan 1, 2006

By Keith D. Koper, Kris L. Pankow, Derrick J. A. Chambers, Michael K. McCarter

"In a mining environment, high quality seismic event catalogs are often useful in interpreting local stress states and identifying areas with elevated ground control risk. Installation, operation, and maintenance of local/in-mine seismic networks, however, may be prohibitively expensive, and mine operators may not anticipate the need for seismic data until unusual mining conditions arise. If regional, generally public, seismic instrumentation could be used to reproduce the results generated by local/in-mine instrumentation to an acceptable degree of fidelity, it would be a valuable and inexpensive tool for mine planners and ground control engineers.In this study we apply subspace detection—a non-traditional seismic event detection technique—to data from a sparse, regional network. The process is employed to identify mining induced seismicity (MIS) from an underground coal district. The results are compared to a seismic event catalog produced by a local seismic array operating within the permit boundaries. Additionally, we analyze the performance of two different stations of the same sparse, regional network in identifying blasts at a surface coal mine for which we have a detailed blasting log. In both cases data from stations between 10 and 60 km away from mining activity are used. We also explore the trade-offs between the number of detections and the number of false positives, which is influenced heavily by detection thresholds and the number of stations used in the process.For the surface mine we are able to identify more than 90% of the production blasts documented in the blasting log. We also find that the detected blasts from each of the four active pits produce waveforms similar to one another. The results are not dependent on source-receiver distance, and by requiring detections to occur on both stations the number of sure false positives is reduced to zero. In the underground mining district we identify between 30% and 75% of the events detected by the local array, depending on the acceptable number of false positives and the source-receiver distance. We also find many events that were missed by the local array, showing that regional subspace detection can usefully augment traditional local catalogs. Moreover, subspace detection can provide an estimate on event location and classification using data from as little as one seismic station."

Jan 1, 2015

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 Anastasia M. Suchowerska

The design of effective ground support requires a sound understanding of the pre-mining state of stress in the stratum to be mined. Irregularities in the stress field encountered during multiseam mining are governed primarily by the previous mining in the surrounding strata. The aim of this study is to quantify the effects of rock mass transverse isotropy on the vertical stress redistribution in the rock strata beneath supercritical longwall panels. Wilson?s equations for determining the stress distribution induced in and beyond a single longwall panel after it has been mined have been adopted, and a plane strain finite element analysis of an elastic medium, together with the stress changes predicted by Wilson, was used to calculate the final pre-mining stresses in the underlying strata. Values of the independent shear modulus (G¢) of the rock strata, ranging from 0.1 to 1 times the isotropic shear modulus (Giso) in the plane of the strata, and a Poisson?s ratio (?) of 0.25 have been investigated. The key finding of this study is that the predicted magnitude of vertical stress in the transversely isotropic strata beneath a mined supercritical longwall panel is larger and changes more quickly with horizontal distance than is the case for an isotropic rock mass. This finding is of significance for the design of ground support measures in multi-seam mining operations.

Jan 1, 2012

By Nic Slatalla

Acoustic emission monitoring techniques are a powerful tool for analysing deformation behaviour and fracture characteristics of rock samples under uniaxial loading. Uniaxial compression tests were carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage develop-ment. All sandstone samples revealed congruent characteristic acous-tic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages proved to be entirely different due to variations in their microstructure.

Jan 1, 2008

By Gabriel S. Esterhuizen

This paper addresses the need for a method to compare the effectiveness of different support systems when designing ground support in coal mines. At present, support design methods include empirical methods based on observations of past performance of installed support systems, analytical methods where the roof is typically simulated by elastic beams and numerical model analysis. The approach presented in this paper estimates the relative stability of a support design through geotechnical evaluation of the rock mass and numerical model analysis of the interaction between the rock mass and the support system. Models are used first to simulate the design performance at the expected rock conditions. The rock strength is then reduced until collapse is indicated in the model. The stability factor is then calculated as the ratio of the expected rock mass strength to the rock mass strength at the onset of collapse, and is similar to the well-known factor of safety used in engineering practice. The stability factor can be used to assist in developing a final support design by comparing the effectiveness of various support systems and the stability of excavations under various geological and loading conditions. Examples of the method?s application in three different geological settings are presented.

Jan 1, 2012

By Ihsan B. Tulu, David F. Gearhart, Gabriel S. Esterhuizen

"Ground deformation and support response was monitored in a longwall gateroad as part of a research project into gateroad stability under changing loading conditions. In this study, instrumentation was installed in a gateroad located between two active longwall panels. The first panel to pass was the second panel in the district, and the second panel to pass the instrumentation sites was the third panel in the district. All of the panels in the district were right-handed. All of the instrumentation was installed in the number one entry, the tailgate, the second panel to pass the instrumentation sites at a mid-pillar location, and at an outby intersection location under about 600 feet of cover. The instrumentation included roof extensometers, cable bolt load cells, hollow inclusion cells, borehole pressure cells, standing support load cells, and convergence sensors, and a multipoint borehole extensometer. The roof extensometers measured roof convergence and sag. The cable bolt load cells measured the roof load applied to the cable bolts in the secondary support array. Hollow inclusion cells measured the magnitude and direction of the principal and shear stresses. The borehole pressure cells measured the stress on a pillar in the entry nearest to the second panel to pass the instrument site. The instrumented wood cribs in the entry nearest the second panel recorded the closure and load on the cribs, and the multipoint borehole extensometer in a pillar closest to the second panel recorded the location and width of cracks that occurred and total expansion of the pillar. This paper describes the effects of the changing stress caused by the first panel as it approached and passed the locations of the instruments on the opposite side of the gateroad system and the later effect caused by the second panel as it approached and passed the mid-pillar instrument location. The first panel passing caused significant loading at the mid-pillar instrument location, but not at the intersection location, while the second panel had passed the mid-pillar location and just arrived at the intersection location when the dataloggers needed to be removed."

Jan 1, 2017

By Lorraine Kent

Excessive floor and side deformation are major problems in deep coal mine entries in Europe, and in particular in the UK where rockbolt support is used at depths of working of around 3000 ft. This has major economic and safety implications for the industry resulting in the potential for sidewall instability, disruption to mine infrastructure, (belt conveyors, floor mounted transport systems and powered supports), and the requirement to safely manage excavation of large quantities of floor material. European research aimed at improving deformation control is described. The outcomes include innovations in support systems, materials and installation practices, as well as better methods for predicting and monitoring deformation levels. As coal mines get deeper, some entry deformation is inevitable, and methods for mechanised removal of floor lift and repair of sidewalls have been developed to allow production to continue as mining clearances are re-established.

Jan 1, 2014

By Tara Bajpayee

Modern, highly productive underground mining operations need to assure the safety of their workforce by understanding where major strata fractures and roof failures could occur. Mines depend on a host of tools to assess ground instability. In-mine microseismic monitoring systems with sufficient number of sensors have also been used to provide this information. These in-mine systems must be robust in design and moved regularly to keep up with the mining advancement. System performance issues depend on maintaining ever increasing cable runs, preventing component damage from mining activities (scaling, blasting, etc.) and avoiding signal degradation due to interference from mining equipment (fans, trucks, electrical equipment, etc.). A potential solution is to use a surface based monitoring system having sensors placed in boreholes above and adjacent to the underground mine workings. These sensors must be capable of detecting and locating strata fracturing associated with rock failure events. A great advantage of a surface based monitoring system is its independence from the underground mine infrastructure. There is less concern with cable maintenance problems, signal degradation by interference from mining activity, or loss of power from the mine?s power grid. The disadvantages include the cost of drilling boreholes and challenges associated with placing sensors in the boreholes, maintaining radio communication and solar power, and public interference (vandalism, etc.). This paper describes a case study where a surface based microseismic system, using triaxial geophones in boreholes drilled from the surface, was deployed at a large limestone mine. The system was operational during a roof fall that occurred in October 2007. It detected the first rock fracture event 17 minutes before the rock fall event. The geophone array was sensitive enough to identify all large rock fracture, impact, and blast events as well as medium size rock fracture events occurring close to the geophone array.

Jan 1, 2008

By D. Su

This paper presents the details and results of a preliminary investigation into the response of borehole pressure cells (BPCs) in soft coal. Two parallel approaches, laboratory calibration and nonlinear finite element simulation, were adopted for deter- mining the BPC K-factor, which was defined as the ratio of cell pressure change to the corresponding change in external pressure. Results from both the laboratory calibration and the nonlinear finite element simulation revealed a nonlinear response of cell internal fluid pressure to external loading. The K-fact& also is shown to increase with increasing setting pressure and decreasing medium modulus. -Depending on the setting pressure, the K-factor for coal having a Young's modulus of 1.6 x lo5 psi ranges from nearly 1.2 at 2,000 psi of external pressure to 1.485 at 8,400 psi of external pressure. The improved under- standing of BPC behavior leads to the more accurate calculation of BPC K-factor in coal which significantly improves the correlation between pillar pressures measured in underground coal mines and those predicted by nonlinear finite element simulations of these pillars.

Jan 1, 1990

By Michael Amick

Due to the importance of maintaining open airways on a longwall panel, mine management decided to install cribs made of foamed cement. Foamed cement had been used at other company operations on several projects. A recent test at a sister operation had proven the crib installation technique and established that foamed cement cribs could provide better support than wooden cribs. The area to be supported was in a remote location; the limited material needs of foamed cement cribs offered a substantial advantage. In mid February 1993, 75 foamed cement cribs were installed on the tailgate of the active longwall panel. The 75 cribs resisted both horizontal and vertical stresses while successfully supporting the entry.

Jan 1, 1993

By C. Dinis da Cama

In order to reduce the complexity of mechanisms influencing rock- fragmentation by blasting a simulation approach is proposed, using the capabilities of micro-computer interactive graphics. Situations involving cratering and bench blasting are simulated, and both jointed and intact rocks can be subjected to explosive action, taking into consideration its breakage mechanisms. Size analysis curves relating rock distribution before and after the blasts are determined and compared with field data.

Jan 1, 1984

By V. V. Nazimko

Abandoned coal mines cause subsidence and structural damages. Shallow abandoned mines induce the most severe and harmful damages. In addition, they produce time-dependent subsidence which is difficult to predict. Backfilling of underground cavities with waste materials have been used to prevent the time-dependent subsidence or to mitigate its effect. However, this procedure is expensive. A new drill and blast technique has recently been proposed to prevent surface damage from time¬dependent subsidence over abandoned mines. It utilizes the principle that blasting in the caved rock and roof in the vicinity of a cavity will dilate the ground and create a self-supporting effect owing to increase in the volume. Physical modeling has demonstrated that artificial dilation of caved rock using the drill & blast technique creates positive effects due to the following sequence of events: the dilated rock creates a supporting effect for the suspended rock strata and reduces its span which decelerates 3.4 times the process of caving. The roof span is reduced by the next caved layer, which causes the hole boundaries to incline to each other. This effect converts the process of caving from a developing mode to a reduction mode. Finally, the sinkhole zone assumes a triangular shape with a negative angle of draw 64-700 in contrast to a rectangular shape of the chimney zone for natural caving. Vertical dimension of the triangular zone is 3.87 times of the room height. Final surface subsidence decreases at least 2.09 times; displacement 1.31 times, slope 2 27 times; curvature 3.42 times, horizontal strain 1 21 times in comparison to natural caving of the ground over an abandoned mine.

Jan 1, 1999

By Archie Richardson

The trona mines in the Green River Basin, commonly known as the "trona patch," present an interesting set of rock mechanics issues stemming from mining in a unique underground environment. Like other minerals occurring in tabular deposits, trona is mined using high-productivity room-and-pillar and longwall methods. However, trona does not behave quite like other evaporites and behaves far differently than coal. In this paper, the following key issues in trona patch rock mechanics are discussed: material properties, in situ stress field, water and gas pressure effects, creep characteristics, pillar behavior, roof span stability, floor stability, underground tailings storage, longwall mining, two-seam mining. and solution mining. Many of the existing trona producers are entering potentially more difficult mining conditions, and much consideration is being given to "new" technologies such as longwall mining and solution mining. In analyzing 20 years of experience with trona patch rock mechanics issues the authors have identified key comparisons and contrasts with conventional tabular deposit rock mechanics that will help illuminate what has been learned as the trona patch enters the 21" century.

Jan 1, 1999

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 K. Hanna

This manuscript describes an extensive ground control study at the Inland Steel No. 2 Mine near McLeansboro, IL, conducted by the U.S. Department of the Interior's Bureau of Mines (USEM) in cooperation with Inland Steel Coal Co. The mine experiences severe ground control problem primarily due to high horizontal stress, particularly at entry intersections. Mine experience has shown that reorienting mine entries, modifying the bolting pattern, and altering the mining sequence are effective in reducing the severity of roof falls and out-of-seam dilution. However, the frequency of roof falls has not been significantly reduced. This research project was designed to provide a thorough explanation of the intersection failure mechanism through combined analyses of company in-mine experience and Bureau field measurements. Stresses and deformations were monitored in a four-way intersection during all stages of development and were related to site observations and theoretical analyses. The results of the combined analyses indicate that high horizontal stress is the predominant factor contributing to intersection roof falls in the Inland Steel No. 2 Mine. The horizontal stress induces shear zones to form along the outby 1 ribs, causing the immediate roof layer to detach 1 and behave as a cantilever beam oriented diagonally across the intersection and parallel to i the maximum horizontal principal stress. As the shear zone progresses upward, roof layers successively detach and fail, forming a dome-type roof fall.

Jan 1, 1986

By James Pile

Long, high capacity, highly tensioned, cable bolts (strand bolts), stiff enough to be installed with resin like a solid bolt, yet flexible enough to be used in any lengths, have been widely used in Australian coal mines since the early 1990?s. A brief history and update on recent developments of this technology is provided. The first major US trial of this Australian technology, has recently taken place at BHP Billiton?s San Juan longwall coal mine in New Mexico. Thirty-five foot long, high capacity, highly tensioned, post-grouted cable bolts are being used to provide active secondary support of intersections in areas where operator safety and life of mine considerations are paramount, and standing support cannot be installed for reasons of equipment access. The cable bolts are resin anchored into overlying sandstone channels, tensioned, and post-grouted. The installation procedure and support results for these highly tensioned cable bolts are given.

Jan 1, 2007

By Hamid Maleki

In this paper, the authors present a review of data gathering programs and stress analyses for improving mine layout designs at BRL mines located near Paonia, Colorado. BRL and other companies have used both longwall and continuous miner methods for the extraction of the Upper D, Upper B, Lower B and C seams from the multiple-seam reserves in the North Fork Valley. Historic mining in the B and C seams has created stress footprints in the neighboring unmined seams compounded by stress transfer from recent longwall extraction in the Upper D Seam. Thus optimum positioning of workings in the remaining reserves requires careful collection of pertinent data and consideration of load transfer from previous workings through remnant abutment pillars. Based on long-term subsidence monitoring, underground mapping and stress analyses, we have confirmed initial load transfer analyses finding mild interactions between the UD and LB workings, and the mining conditions have proven to be good and manageable during the development stage of the LB Seam. This has been achieved through prudent mine planning, prudent positioning of the gateroads in multiple seam workings and support allocations.

Jan 1, 2007