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By Andre Vervoort

Falls of ground, such as falls of roof and sidewall collapses, are a major cause of fatalities and injuries in South African collieries. Based on information obtained from the accident reports of the various Chief Inspectors of nines, a statistical analysis was conducted covering the period 1970 to 1988. This study shows that the volume of rock in a fatal fall of ground is often quite small. In intersections, fatal falls are on average larger than in roadways and at the face. It was also found that in 36 % of all fatal falls of ground no temporary or permanent support was installed at the site of the accident.

Jan 1, 1990

By A. G. Pasamehmetoglu

This paper describes the conver¬gence and load measurements carried out in inclined seams at five longwall faces, two of which were supported by hydraulic props and the other three by conventional timber supports in Karadon-Gelik mine. The minimum support capacities determined in this study are compared with the results obtained from two empirical approaches suggested by Wilson (1975) and Jacobi (1981). The characteristics of hydraulic props to be used and support patterns in the longwalls are found, conclu¬sions are drawn and suggestions are made. Design nomograms are prepared for field engineers, in order to be able to approach the problem more confidently and realistically.

Jan 1, 1992

By Peter Cain

Longwall extraction in the Phalen Scam in the Sydney Coalfield of Nova Scotia is carried out under flooded abandoned workings in the Harbour Seam 140 m above, and at depth of up to 700 m beneath the Atlantic Ocean. Careful planning of panel and pillar dimensions is necessary to prevent inflows from the overlying workings and to prevent the development of excessive subsidence strains at the seabed. This paper briefly summarises the research undertaken over the past five years by CBDC and CBCRL into the problems associated with mining under water, and then describes the development and application of a design/planning code of practice developed jointly by CBDC, CBCRL and mine regulatory agencies. The design method employed in the Code of Practise is based on empirical data from the coalfield and parameters developed from subsidence modelling and previous water inflow experience. It includes precautionary measures to be adopted during development and mining, and monitoring procedures to be followed. It is flexible enough to allow future developments in the understanding of inflow mechanisms to be incoporated without major modifications, and represents a major step forward in reducing the potential hazards associated with coal extraction in the coalfield.

Jan 1, 1996

By Victor V. Nazimko

Long term stability of the pillars that have been remained after R&P extraction causes postponed subsidence. To prevent unforeseen subsidence and to diminish postponed subsidence 25-50% new method for its control has been developed by the author. It supposes to use drilling and blasting technology and other original approaches. New method has been fulfilled through the holes being drilled from the surface or from the active underground openings. Explosives had been charged into the holes and were blasted. This had increased bulking factor dramatically and prevented natural caving of the strata. Physical scale modeling corroborated efficiency of the new approach. The bulking factor has grown from 0.0623 to 0.1185 and surface subsidence has been diminished from 1.1 m to 0.2 m or 5.5 times.

Jan 1, 1996

By C. Fairhurst

Most of the papers designated as being within the topic of underground storage at this Symposium are concerned with aspects of radioactive waste isolation. Although this is, unquestionably, an important international application of underground storage, and one involving a considerable number of research investigations, the subject of underground storage is indeed much broader. Several major international conferences have been devoted exclusively to it. Applications include storage, under a range of temperatures and pressures, of oil and liquefied petroleum products, compressed air, heat, foodstuffs of various kinds, hazardous and 'nuisance' industrial by-products, and a variety of special applications, both civilian and military - some in use, others proposed. Storage may be in caverns excavated specifically for the purpose, exhausted mines may be adapted for storage or, in some cases, the material to be stored may be injected under pressure into the pore spaces of porous-permeable formations at depth. A variety of rock types and geological environments are used, e.g. excavations in salted or domal salt, where long-term creep closure of the openings is a concern; massive granite, where fluid loss through fissures may be important. Cavities may have spans of several tens of meters, and there is interest in achieving still larger excavations. As would be expected, this breadth of underground storage applications introduces a similarly broad range of questions in rock mechanics and rock engineering. The report will review these questions and recent developments in rock mechanics research, computational procedures, and design applications - and will also discuss the opportunities for a still greater range of applications of underground storage to current industrial and social problems.

Jan 1, 1989

By F. Ziaie

Line electrical resistivity method using physically infinite distance between current line electrodes is proposed to determine the location of mine workings. In this method at least three line electrodes are used. One of the current electrodes is used as a sinkhole electrode while the other two are used for different activiations of line electrodes when the survey area is between one of the line electrodes and outside the other one. The resistivity profiles perpendicular to the current line electrodes for different activation of electrodes is plotted versus the distance of the midpoint of the measuring potential electrodes. The anomaly due to presence of any locally lateral resistivity variation (e.g. mining cavity) will represent itself as peak or trough depending on resistivity contrast between the source of the anomaly and surrounding medium. Physical simulation of the proposed model was conducted by using a tank model in the laboratory. The results of experiments appear to be consistent with the theoretical model. Experiments for different depths and different spacial locations of the simulated cavities appeared to be successful for locating the depth and size of the cavity.

Jan 1, 1989

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 Daniel W. Su

This paper presents the results of a detailed geotechnical study on the potential multiple seam interaction between the No. 2 Gas (upper) and the Powellton seams, which are separated by an average interburden of 45 ft. The interburden consists of 4 ft of clay shale below the No. 2 Gas seam, 38 ft of massive sandstone, and 3 ft of shale above the Powellton seam. The No. 2 Gas seam was mined about 60 years ago, and many water-pooled areas were projected to be present. The deepest overburden over the No. 2 Gas seam is about 750 ft. To evaluate the potential interaction between the two mine works and to determine the pillar safety factor, Analysis of Multiple Seam Stability (AMSS) software was initially used to determine pillar dimensions to obtain a minimum safety factor of 2.0 for areas under pooled water, as well as a minimum safety factor of 1.5 for areas not under pooled water. Furthermore, to assess the quality of the interburden rock strata, two surface core holes were drilled, and rock cores were tested in the laboratory for their strength and elastic moduli. Using the experimentally determined strengths and moduli as part of the input parameters, the RocScience Phase2 finite element program was employed to model the stress field scenarios and estimate the safety factors of the interburden rock strata before and after Powellton seam mining. The pillar safety factors derived from the Phase2 analysis were in excellent agreement with those obtained from the AMSS analysis. Phase2 results also predicted that the effect of the Powellton seam mining, first mining only, would reach a maximum of 19 ft above the Powellton seam, which led to the proposal of drilling 25-ft deep test holes to detect water, instead of the 45-ft deep test holes mandated by MSHA. The proposed test-hole height was accepted by both Mine Safety and Health Administration (MSHA) District 4 and the state of West Virginia. Mining of the Powellton coal seam commenced in February 2011 and continued for eight months. To date, no 25-ft deep hole has encountered water under the water-pooled zone, which validates the RocScience Phase2 finite element analysis. Also, inmine observation of excellent pillar stability and roof condition confirms the validity of the AMSS analysis.

Jan 1, 2012

By Gang Chen

This paper explores the possibility of using short-term time-dependent plate loading tests to estimate the elastic and viscoelastic parameters of the weak floor strata typically associated with coal seams in Illinois. Plate loading tests with incremental loads, similar to incremental creep experiments, were conducted until the floor failure occurred. For each increment, the load was held constant for a period ranging from 1-4 hours until a constant deformation rate was achieved. The deformation-time data were analyzed for viscoelastic parameters of weak floor strata assuming its behavior to be adequately represented by the standard Burgers model. The analysis required development of mathematical equations for the time-dependent deformation of the standard Burgers material under a three- dimensional state of stress. Time-dependent pillar settlement data at one mine were also analyzed for elastic and viscoelastic parameters of the weak floor strata using the developed equations. These parameters were compared with similar parameters estimated from plate loading tests.

Jan 1, 1990

By Navid Hosseini

Several empirical equations to estimate coal pillar strength have been presented in academic studies. The development processes of these equations are similar and are usually obtained by fitting the mathematical function (curve) to field data. One of the best criteria to evaluate the quality of fitting for such equations is the correlation coefficient R2, which has limited applicability. It is necessary to calculate the correlation coefficient access to the initial data for which the equation is presented; this is impossible for many coal pillar strength formulas. This paper presents a new approach based on the analysis of uncertainty amplitude to compare the coal pillar strength. This approach utilizes a combination of parameters such as Mean Squared Error (MSE), Root Mean Squared Error (RMSE), function type and degrees of freedom. The confidence level of constants is subsequently formed and the correlation coefficient becomes more comprehensive. Therefore, for an effective comparison, the efficiency and accuracy of coal pillar strength formula can be used. Keywords: Coal Pillar Strength, Correlation Coefficient, MATLAB, Uncertainty Criterion

Jan 1, 2012

By Nicholas Chlumecky

One of the oldest mine supports in use today is timber cribbing. In many mining operations material, installation and maintenance costs for timber cribbings are substantial. Their performance characteristics also add to mining costs. During the 1970'6, the U.S. Bureau of Mines recognized many of the deficiencies of timber cribbing and very successfully initiated the development of steel-fiber- reinforced (SFR) concrete blocks. Since the early 1980's this SFR concrete cribbing has been manufactured as a high quality product which has found increasing acceptance by mine operators. The rapid acceptance of the SFR concrete block cribbing has proven that its use is eliminating many of the deficiencies of timber cribbing while providing superior load carrying capability. The use of SPR concrete blocks to build the cribbing is based on the same principle as wood. That is, stacking longitudinal, rectangular blocks in layers in a crisscross manner. The blocks are positioned at right angles to the blocks of the layer below. With this situation, the possibility of misalignment exists when the concrete blocks at the base are not placed exactly parallel or when the blocks of the base are subject to unequal settlement. Such misalignments creates localized, high stress areas which reduces the load capacity of the cribbing made from SFR concrete blocks. Recognizing the problem of misalignment, the Underground Supports Division of Commercial Shearing, Inc., conceived a circular, one-piece, steel-fiber-reinforced concrete cribbing which is being marketed under the name - Donut Cribbing. This concept eliminates the possibility of misalignment and provides 100% bearing between layers of stacked cribbing as well as other advantages. The purpose of this paper is to discuss the performance characteristics, installation speed and economical factors of each of the three cribbing types -- timber cribbing, SFR concrete block cribbing and SFR concrete Donut Cribbing.

Jan 1, 1986

By Christopher Mark

Pillar recovery continues to be one of the more hazardous activities in underground coal mining. Safety requires that the roof above the intersection remain stable until after the pillar has been extracted. Artificial supports (timbers and Mobile Roof Supports) arc essential to roof stability, but so is the final remnant stump or pushout. Traditional mining practices usually called for the complete extraction of the final stump, but the recent trend (both in the U.S. and internationally) seems to be towards mining plans that leave a remnant stump. For this study, a sample of roof control plans from the Mine Health and Safety Administration (MSHA) Coal Districts were analyzed to determine current pillar recovery practices. Both full- and partial- extraction plans were included. Special attention was paid to whether plans require that a final stump be left, and whether requirements regarding the dimensions of the remnant stump are included. Foreign experience with final stumps is also summarized. It seems that the risk of major pillar falls can often be reduced by leaving final stumps that are large enough to protect the intersection, but small enough that they do not inhibit the caving of the gob. Because final stumps are often irregular in shape, a new approach for estimating their strength is described. Analyses were conducted to assess the effect of seam height and depth of cover on the potential variation in the size of remnant stumps.

Jan 1, 2001

By David F. Gearhart

The Burrell Can1 is a thin, steel, tubular shell filled with aerated concrete that is used as a roof support in coal mines. The Can height is always shorter than the mining entry, so it is capped with wooden timbers and wedges when it is installed. This combination of the Can and the wooden header is a standing support system that is designed to yield in a controlled manner and exhibit a constant yield, or elastic-plastic behavior. The Burrell Can is capable of withstanding 20 in or more of vertical convergence and 15 in or more of horizontal displacement. Because of this exceptional ability to sustain load through extended convergence and displacement, it is one of the most robust standing supports in use today. The National Institute for Occupational Safety and Health (NIOSH) conducted testing on seventeen Burrell Cans in 1995, during development of the support where the ratio of the height to the diameter, or the aspect ratio, was between 2:1 and 4:1. Based on the results of that testing, it was suggested that the standard aspect ratio of the Can be less than 5:1. This suggestion is intended to preserve the overall stability of the Can, which will help to maintain the initial stiffness and prevent load shedding. Recently, the necessity of the aspect ratio has been questioned, and this issue will be addressed in this paper. Since the initial development and introduction, more than 130 tests of the Can have been conducted at the NIOSH Mine Roof Simulator (MRS), and 25 of those specimens had aspect ratios greater than 5:1. The 18-in-diameter by 10-ft-tall specimens had the largest aspect ratio, 6.67:1.However, the aspect ratio is not the only parameter that affects the performance characteristics of the Can. Multiple factors, such as the use of different wood species, the use of different header or footer configurations, and the use of pedestals, as well as biaxial loading conditions can also decrease the stiffness and capacity of the Can. Data from full-scale tests that include these other factors are analyzed to show that the initial stiffness is primarily affected by the roof and floor contact configurations. Furthermore, load shedding is more likely to occur or begin earlier when the aspect ratio is greater than 5:1 and one or more of the other factors are present.

Jan 1, 2012

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 G. I. Gritsko

In this paper we consider the elements of a distributed information geomechanic monitoring system which is used in computerized mine planning and mining operations control. The automated system is a technical means of geomechanic monitoring, as a complex system of regulated observations, estimation and prediction of geomechanic state of rock mass and workings during underground mining of coal deposits. Geomechanic monitoring at coal mines and the automated system execute data collection, analysis and calculations of geomechanic situation in the panel during planning, development and carrying-out of mining operations for ensuring geotechnical stability of underground mining.

Jan 1, 1996

By J. Nielen van der Merwe

This investigation was done to develop a method to predict the lie of coal pillars. It has long since been known that coal pillars deteriorate over time, but there was no method to quantify the deterioration. It was found that the rate of pillar deterioration was essentially a function of two parameters, namely mining height and time. The higher the mining height, the more rapidly the pillars deteriorated. The rate of deterioration is not constant, but decreases exponentially with time. The pillar life should be viewed in conjunction with the safety factor. While the relationship between safety factor and pillar life is ambiguous, the trend is nonetheless for higher safety factor pillars to have longer lives. The safety factor can be used to quantify a failure probability. The pillar life prediction indicates when the pillars which are predicted to fail, can be expected to do so.

Jan 1, 1998

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 K. F. Unrug

In the paper a numerical modeling method is used for evaluation of a roof support system, which is commonly used in the US coal industry. The effectiveness of mechanical roof bolting with or without, header boards and steel straps are compared. This analysis takes into account the response of a shaly roof to changing environmental conditions in mines related to the seasonal changes in temperature and humidity of the intake air. Discussed are principles of roof support in coal mines and based on them a critical evaluation of header boards interaction with bolts and roof itself is carried out. Header boards create a soft inclusion within a much more stiffer system comprising bolt-rock. As an alternative to header boards, steel straps are analysed and corresponding results are presented. It is demonstrated that application of steel strap is beneficial from the stand point of stress distribution and maintaining tension on roof bolts. The laboratory investigations and modeling work previously undertaken by the authors, clearly indicate the detrimental effect of header boards on effectiveness of roof bolting. There are significant practical implications of the research considering that header boards are commonly used in mines. This practice has been probably originated from the wooden support used by mining in "pre-bolting era?, and it has stayed with coal mining based on tradition more than principles of engineering design. It is demonstrated that header board$ are detrimental and actually contribute to development of roof problems.

Jan 1, 1989

By P. M. Lin

A subsidence monitoring program over a longwall panel was established (1) to explore the impacts of dynamic subsidence on the ground surface and structures and (2) to correlate the movements between the structures and their corresponding points on the ground. The program consisted of a transverse line survey and a series of survey points around the structures on the ground surface and their corresponding survey points on the exterior walls of the structures. The vertical displacement of each point can be expressed as: where Si is subsidence at i point; Sif is the final subsidence at i point; F is face location in terms of distance from the i point and SD is seam depth. The development of subsidence can be divided into five stages. Damage to the structures starts at the beginning of the second stage, The maximum damage developed in the middle of the third stage. The relationship of the vertical displacement between ground surface and structures can be expressed as: G = aH + b where G is ground displacement, H is structures displacement, and a and b are constants. The relationship is linear. However, the strain is non-linear and more complicated because of the interaction of the ground surface and structure.

Jan 1, 1989

By Yingxin Zhou

Virtually every coal seam in Appalachia will at some time be subject to multi-seam interaction (Wu et al, 1987). Many such problems result from seams which lie in close proximity or have split and lie within thirty feet vertically of one another. Existing techniques for multi- seam design have concentrated on conditions where the entire inner burden is not the failing member and are unsuitable for ultra-close multi-seam design. Structural integrity of the inner burden must be maintained if two superimposed seams are to be mined separately and under- standing possible failure mechanisms of the inner burden is an essential first step in the design process.

Jan 1, 1989