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By Khaled Morsy

The tragic incidents at Sago and Darby coal mines in 2006 pointed out the need for stronger seals to resist the blast effects of violent gob explosions. Mine Safety and Health Administration (MSHA) has published the final rule on Sealing of Abandoned Areas in April 2008. According to the final rule, the mine seals for unmonitored gob areas should withstand an instantaneous pressure of 120 psi. Reinforced concrete (RC) seal is one of the feasible options that can be used to meet the final rule. Concrete is a heterogeneous, cohesive-frictional material and exhibits complex non-linear inelastic behavior under multi-axial state of stress. The increased use of concrete as primary structural material requires the development of sophisticated material models to have an accurate prediction of the material response to a variety of loading situations. The constitutive modeling of concrete has advanced considerably in the past three decades. Choosing a reasonable concrete material model is important in order to obtain an accurate mine seal design. This paper attempts to address the effect of different concrete material models on the response of RC seal subjected to dynamic loads from methane explosion. Three concrete models, from ABAQUS library, were selected to address the effect of non-linear behavior of concrete and damages due to tensile and compressive stresses. In addition, a typical and verified concrete material model was used for conducting 3D finite element models simulating the RC seal by ABAQUS, EXPLICIT ver. 6.6.

Jan 1, 2008

By G. J. Hasenfus

This paper presents the results of an extensive hydrological and geomechanical monitoring program which Gas conducted at a longwall coal mine in W. Va. The field program included monitoring of groundwater levels. overburden hydraulic conductivity, overburden movement, and surface subsidence relative to the passage of the longwall. Overburden geology and rock strength characteristics were evaluated prior to mining. A postmining corehole was drilled to evaluate main roof fracturing. An overburden response model is proposed based on the correlations between hydrological and geomechanical data.

Jan 1, 1988

By Vincent Scovazzo

Cost-effective underground mine configurations have been designed in highly fractured rock masses where the stone possesses a high material strength. A limestone mine in the eastern United States is used to demonstrate procedures for the geotechnical design of one such operation. The mine discussed is located in both limbs and the axis of a recumbent fold that fractured the rock mass. In a highly fractured rock mass, where the host rock has high strength, a design can be developed to alleviate intact rock failure. This is possible because in most cases as the stress levels needed for intact failures are approached, the stresses are quickly dissipated due to movement along discontinuities, which represent planes of weakness. As a result most underground failures in highly fractured rock are due to sliding or toppling. Even when intact rock fails in a highly fractured rock mass, it is the movement along an existing joint or other weakness plane that often initiates or causes this failure. It is therefore important to obtain the characteristics of weakness planes that are the discontinuities within the mass. The required information on weakness planes, geology, and strength of materials is discussed and recommendations for obtaining this information are presented. A discussion is provided on failure modes and the use of developed information for their evaluation. The impact on geotechnical design is demonstrated through pillar sizing procedures; however, this approach can also be applied to mine roof and pillar foundations.

Jan 1, 2002

In this paper, the authors attempt to apply a simplified non-homogeneous void diffusion model (NHVDM) to predict surface subsidence for longwall mining in Illinois. A generalized error function subsidence profile is derived from this model. The model validation indicates that the NHVDM approach appears to fit the observed subsidence data as well or better than the error and the trigonometric functions, particularly around the trough edge areas.

Jan 1, 1990

By Jinsheng Chen

For underground openings, many factors can cause roof failure and thus roof falls. The most common factors related to Mother Nature are weak roof lithology, geological structures, and an abnormal in-situ stress field. Theoretically, any underground opening can be successfully supported under any geological and mining conditions if the opening width is limited and the roof supporting system is well designed. However, roof control cost must be taken into account. In practice, to minimize the roof control cost, different roof control plans have to be considered to take care of the uncertainty or variation in geological and mining conditions. For most underground mines, it is unsafe or economically unrealistic to use one fixed roof control plan for the entire mine. Therefore, local, abnormal geological and mining conditions must be identified and a site- or area-specific roof control plan must be employed. This study tries to identify the major factors that have significant impact on entry roof stability by analyzing the historical roof fall data at Rofomex mine. Finally, the appropriate roof control plans or roof bolting systems for different geological and mining conditions are recommended.

Jan 1, 2011

By E. H. R. Schumann

The local stratigraphy and composition of the coal measures, including competent dolerite sill and massive sandstone lavers is shown to have a restricting influence on the development and magnitude of surface subsidence above total extraction panels in South Africa. The critical panel width necessary to induce nonviolent failure of a dolerite sill is quantified. Small subsidence due to the deflection of a 70 metre thick dolerite above subcritical panels is shown by example. Maximum surface subsidence tilt an; ground strain is compared to the NCB model. An exponential subsidence profile curve, SPC, describes a trough half-profile. A parabolic relationship between horizontal displacement and tilt, which has been discovered to exist can be substituted in the first derivative of the SPC in order to generate transverse displacement and strain profiles.

Jan 1, 1988

By Robert M. Cox

Abrupt tailgate roadway ground failures, described as floor bumps, sometimes occur in the tailgate entry outby the face during the high-speed extraction of coal from mechanized longwall panels. These floor bumps typically damage the tailgate entry support structures for a distance of from 12.2 to 24.4 m (40 to 80 ft) outby the face. The relative closure of the tailgate entry may be a key indicator of potential tailgate ground control problems. Regression analyses of tailgate entry convergence data conducted by the U.S. Bureau of Mines (USBM) indicate that entry closure may be characterized as an exponential function of the distance outby the longwall face. Assuming that the entry closure is caused by the abutment loads created by the excavation of the longwall panel, the resulting convergence equation can be integrated to locate the centroid of abutment loading. This information can be used to document zones of potential ground failure ahead of the operating face.

Jan 1, 1994

By Edward A. Barrett

A significant number of injuries resulting from fa118 of roof and rib may be avoided by modifying the reactive behavior of miners as they encounter potentially hazardous ground conditions. In 1984-85, the Bureau investigated barriers which prevent miners from recognizing and correcting groundfall hazards. One basic problem from which these barriers originate is the inability of miners to recognize dangerous roof and rib conditions. This study is a report of experimental research on the use of stereo scopic (3-D slides for presenting visual cues associated with hazardous roof and rib conditions. Findings reveal that, in comparison to conventional slides, 3-D provides for significantly greater hazard recognition among both experienced and inexperienced miners. Details such as the presence of t cracks and gaps in the surrounding rock are more accurately and realistically perceived. Acceptance by the mine training community has been affirmative and is additionally encouraged by the fact that 3-D slides stimulate trainees' overall interest.

Jan 1, 1986

By Gennaro G. Marino

As part of the development of a shopping center in West Virginia, a significant bench had to be cut into a mountainside that was over 700 ft. high. As a result, a steeply stepped, over 400-ft.- high wall was created. The high wall exposed old abandoned room-and-pillar coal workings at the toe of the cut. Because of exposure to the elements, the mine openings began to severely deteriorate with significant potential of undermining of the rock cut face and causing a large rock fall(s). Exacerbating this wall problem was a set of fairly planar sub-parallel and sub-vertical joints creating, in essence, thin rock slabs. Also present were stress relief fractures that were observed in the exposed pillars. Given the proximity of the project building to this high rock face, mitigation of the rock fall potential was deemed necessary. In order to prevent continued damage and deterioration of these exposed mine entries, the rubblized openings were filled with bulkhead grout and pressure injected with grout. The total amount of grout installed in these openings was about 3,945 CY. Also, a rock drape was attached to the entire face to mitigate small rock falls from natural causes. There also existed another level of mining below the ground surface and under an existing building at a depth of about 160 ft. Under a portion of the building, the pillars in these old works were second mined, and the building seemed to have been already affected by limited subsidence above this area. Also, calculated pillar safety factors in this high extraction section of the mine were too low. Consequently, these old works were stabilized by saturation grouting. An indication of overall roof distress was the significant fracturing encountered in the bedrock overburden during the drilling of the injection holes and the volume of grout placed in these holes. The total grout installed in the high extraction section of the lower mine was about 6,660 CY. This paper describes the investigation and the grouting techniques employed at the rock cut face and under the existing building to mitigate continued damage to avoid future subsidence problems at the face and the building premises.

Jan 1, 2013

By Steven Derycke, Stephen C. Tadolini, Anand Bhagwat

"The tensile and shear strength of intrinsic bolting support systems has always been a major concern of designers. A comprehensive laboratory testing program was designed to evaluate the tensile and shear strength of individual wires and completely wound PC-strand cables. PC-strand cables with smooth wires and the recent anchorage enhancement innovation of indentation were evaluated and compared. The testing protocol detailed in ISO Standard 15630 utilizes a mandrel system that was investigated at 3 different diameters which alters the wire to mandrel ratio from 2:1 to 9:1. The results demonstrate that the difference between smooth and indented wires is statistically insignificant when larger diameter mandrels are used, and that indentation does not adversely affect strand properties and performance. Insight into the shearing mechanism and evaluation techniques are discussed with the introduction of triaxial loading to describe the PC-strand tensile and shearing mechanisms. Another important result indicates that the shear strength of PCstrand cable bolting systems has a greater shear strength value than traditional steel bar bolting systems.INTRODUCTIONPC-strand cable bolts have proven to be a global cost effective ground control solution in metal/non-metal and coal mines. The inherent strength to weight ratio of cable supports, combined with flexibility has been used in underground entries that range from 20 to less than 1.2 m in height (65 to 4 ft). In coal mining, the systems are used as primary and secondary support installed both passively or actively with installation tensions that range from 38 to over 178 kN (8,500 to 40,000 lbs) . While cable can be specified in several diameters, the most predominant sizes in the US remain 15.2mm (0.6 in) and 18mm (0.7 in). The capacity of the systems defined in the ASTM 432-13 is 90% of the minimum strand strengths of 260 kN (58,600 lbs) and 320 kN (72,000 lbs), respectively.Cables are not only suitable to support vertical loads, but are also often used to effectively support fractured and jointed materials and can be exposed to shear forces across the wound PC-strand structure. In fact, Goris et al. reported that cable bolts placed across jointed surfaces could more than double the shear resistance. These results were duplicated for both smooth and rough jointed surfaces (Goris et al., 1996). An example of tensioned cable bolts used to support a fractured and sliding roof in a longwall tailgate entry is shown in Figure 1."

Jan 1, 2015

By Wenjun Ju

It's very important to determine the limitation of bed separation in entry roof in order to predict roofs stability and take some necessary precautions. Based on numerical modeling and monitoring in the field, the new concept of Limitation of Roof Layer Separation (LRLS) and its method of determination LRLS is given in this paper

Jan 1, 2001

By Hamid Maleki

Prudent design of coal pillars in any mining system requires an estimation of in-situ pillar strength, overburden lateral load transfer capability, and a failure criterion. Pillar strength is typically determined using one of several different empirical pillar strength formulas. For coal reserves in the U.S., a new pillar design method was proposed based on extensive U.S. field measurements in underground mines (7) which accounted for confinement effects. Field measurements in seven U.S. seams indicated highest strength for some Utah mines where the coal seam developed high confining stresses. Lower strength was measured in structurally controlled seams. This method is expanded here including the data from the weaker tertiary coal measure rocks and lower confinement for pillars with low width-to-height ratios. The determination of how much load a pillar is expected to take is partially dependent on the ability of overburden to transfer load laterally; this is useful for selection of panel width and barrier pillars size in select mining applications where long-term stability is required in conservative designs. In caving mining systems, caving of the strata is influenced by geologic, mining and stress conditions and the released energies influences how the pillars react at loads approaching pillar strengths. Overburden deformation and caving mechanisms are analyzed in this paper in multi-panel extractions to enhance the understanding of load transfer, seismicity and coal bump control in deep western U.S. operations.

Jan 1, 2008

By Mohummad Akram

Statistical analysis of multi-seam mining case studies demonstrated that geologic structure and lithological type and sequencing were predominant factors in entry failures. Finite element analysis was used to evaluate the effects of stress field changes on entry instability under differing geologic conditions. Analysis showed the roof to be particularly vulnerable to shear with the location of the failure site changing to meet loading and geologic conditions. Effects of external support on roof stability were examined. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Based on this research design, criteria were developed for roof stability evaluation under specified interactive loading conditions.

Jan 1, 1993

By Yongping Wu

Roadway deformation and stress migration occur with uncertainty and randomness because of geology, tunneling and mining influence. In order to study the surrounding rock deformation and failure mechanisms in three-dimensional stress state, an air return roadway of Yuanyang Lake mine of Shenhua Ningxia Coal Industry Group Co., Ltd. was chosen as a research object. The three-dimensional model was a designed and completed experiment with a dynamic-static coupling load. The model was loaded by hydraulic cylinder, simulating the in-situ rock stress field and the induced stress field of surrounding rock. The deformation monitoring system of the roadway model was constituted by optical total stations, borehole observation instruments, acoustic emission monitoring, and stress sensors. A wealth of data was obtained through an omni observation of surface displacement, characteristics of stress distribution, and deformation and failure of surrounding model material. By acoustic-optic-electric methods, a real-time testing on physical, mechanical, and damage parameters was realized through the experimental process. The results showed that the data of deformation and failure obtained from the deformation monitoring system of the roadway were authentic, which has great significance for warning and forecasting safety hazards of surrounding rock in coal mines.

Jan 1, 2013

By Leo Gilbride

Western U.S. longwall operators face increasing challenges with optimizing ground control and productivity as mines reach greater depths and coal bursting hazards increase. Some western U.S. mines, many known to be bump-prone, achieved a successful balance between ground control and productivity by transitioning to side-by-side longwall panel mining combined with a yield pillar gateroad system. With this design, development footages could be minimized and pillar bumping averted by controlled yielding at moderate depths, generally in the range of 450 to 600 m. Over the past four decades, the yield pillar system has won wide acceptance among western mines facing pillar bursting hazards, particularly those in the Wasatch Plateau and Book Cliff coal fields of central Utah. However, recent attempts among the deepest Utah operators to mine side¬by-side panels with yield pillars at depths in excess of 600 m have been met with mixed success and, in some circumstances, with serious difficulty. Challenges include violent face bursting and excessive tailgate convergence outby the face, which can be crippling to ventilation. The use of interpanel barriers, i.e., barriers left between longwall panels, offers one possible solution to mining under deep cover with bump-prone geology. Interpanel barriers have already been adopted by Utah's deepest longwall mine, and others are considering their use. The geomechanical implications of mining with and without interpanel barriers, and the competing tradeoffs between ground control and ventilation are discussed.

Jan 1, 2004

By Zbigniew Lubosik, Andrzej Walentek, Stanislaw Prusek, Aleksander Wrana

"The Carboniferous coal measures, mined across Europe including the major European underground coal mining countries of Poland, UK, Germany, and Czech Republic, suffer from increasing extraction depth. This results in major geomechanics problems associated with ground control and support design.With production being concentrated into ever fewer high performance longwalls, located at greater and greater depth, pressure to ensure the roadways’ functionality (required dimensions) continues to increase. Mining in deeper and more highly stressed mining environments creates several problems. The high stress and deterioration in geological and mining conditions in many cases causes problems with gateroads maintenance (due to excess convergence). That leads to costly production gaps, longwall slowing, transport problems, an increase in the methane hazard and in the general health and safety of the workforce, and deterioration in climatic conditions with major economic risk associated with interruption of the revenue stream. Time—and thus production—delays due to maintenance often result from insufficient geological and geotechnical data acquisition and misinterpretation combined with inadequate support for the type of rock mass.longwall face on the gateroad stability (described by the gateroad convergence, support load, and shape and height of the fractured zone around the longwall gateroad) are presented. The investigations were carried out in the longwall panel with roof caving located at the great depth, in conditions of the Upper Silesian Coal Basin in Poland.The measurement results showed that the gateroad fractures in roof strata began to occur at the distance of about 1000 m ahead of the face. Additionally, the load on support and gateroad convergence increased as the longwall face got closer.It is very important for coal mining research institutes and mine operators to investigate and develop practical solutions and apply innovative support technologies at a great depth (particularly for reinforcement and stabilization of roadways and other openings).Underground measurements of stability conditions around the gateroad is of great importance to mining practice, especially in planning and designing supports for longwall gateroads.The influence of the front abutment pressure results in an increase of support load, gateroad convergence, and shape of fractured rock zone."

Jan 1, 2015

By Satyendra K. Singh

If the extraction is beneath massive roof strata, the roof tends to 'bridge' across large spans without failing, the caving behaviour is complex and irregular. Understandably, these give rise to many geotechnical problems with safety implications and involving sometime huge direct and indirect costs. Only two important problems, 'feather-edging' in pillar mining and `severe periodic weighting' in longwalling, are discussed in this paper in the background of geomechanics investigations in Australia at Blue Mountain Colliery and in South Bulga Colliery respectively. `Feather edging'- a major source of injury and fatal accidents in Australia and South Africa in such situations, refers to a phenomenon where the roof, at the goat edge, falls as a thin wafer of rock often suddenly, over-riding conventional timber breakerlines. The results of the first comprehensive, monitored trial in an Australian pillar extraction coal mine, suggests how the roof falls into feather-edging caving (generally) because of multiple shear-planes (interpreted) and how it may be alleviated. Because of complex fracturing behaviour of massive roof strata, periodic weighting at a longwall face is certain to manifest itself as 'yielding supports', heavy conditions', rapid convergence and in the worst case, as a cause of face stoppage. After briefly discussing the programme of instrumentation (in a chain-pillar and over the would-be goal) and results, this paper attempts to improve mechanistic understanding of the related geotechnical issues associated with the weighting behaviour and aims at establishing a 'threshold' value of weighting severity index (suggesting intensity of periodic weighting) with an objective to optimise operational controls before mining through potential (future) weighting areas.

Jan 1, 2000

By S. Paul Singh

Rock burst is one of the most serious mining hazards, adversely affecting the safety, economy and productivity of mines. It is a sudden manifestation of the release of strain energy stored in the rock mass. At present, there is no universally accepted method for predicting the timing of the rock burst. However, if the seismically hazardous regions in mines can be identified before they are exposed to mining then precautionary measures can be planned before the work begins. A study was conducted in Sudbury Nickel Belt and an approach has been proposed to identify rock burst prone workings on the basis of (a) Rock properties and bursting indices (b) In-situ approaches (c) Geological features (d) Mining conditions (e) Observations during mining operations (f) Comparison and analogy with previous case histories of bursting. This approach will facilitate the judicious application of the burst control techniques to seismically hazardous areas.

Jan 1, 2008

By Ken Mills

A program of stress change and roadway deformation monitoring was undertaken to measure the forward abutment load distribution about a retreating longwall panel at Dartbrook Mine. The results of this monitoring allow roadway conditions observed at various stages of mining to be ranked according to the estimated vertical stress they have experienced. This ranking also provides a means to predict and visualise future roadway conditions at various stages of mining. This paper presents the results of the monitoring and describes the approach developed to predict future roadway conditions. The monitoring results show the vertical stress distribution associated with the front abutment from a longwall goal decreases exponentially away from the goaf edge. The vertical stress distribution peaks at the goaf edge at 5 to 6 MPa stress increase and decreases to 1 MPa within 50 m becoming imperceptible beyond 100 m By scaling this measured distribution to reflect the loading environment, the vertical stress can be estimated for different overburden depths and different stages of mining. Photographs of roadway conditions at particular vertical stress levels provide a way to visualise roadway conditions at various stages of mining in the future

Jan 1, 2000

By N. I. Aziz

The importance of using disc cutters for rock excavation is reflected by their wide application in hard rock cutting machines such as tunnel boring machines and raise borers. The forces involved in the cutting process are those due mainly to thrust and rolling forces and are greater than that required by cutting picks. An interesting feature of cutting with discs is the formation of surface crushing, surface and sub-surface chipping and primary crack formation. Experimental, as well as analytical, work has been carried out to determine the optimum spacing between the disc cutters to control primary crack orientation. Wedge bit indenters were used to simulate disc cutters. The paper concludes by proposing a new approach to rock excavation by hybrid cutting based on controlling the primary crack orientation.

Jan 1, 1992