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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

By Yajie Wang

High horizontal stress and its adverse effects on longwall gate entries have been witnessed in many US coal mines for the past years. It can cause cutter roofs and leads to roof falls either in the headgate or tailgate entries depending on the relative orientation of the panel to the maximum principal horizontal stress. Underground observations have shown that if the maximum principal horizontal stress is perpendicular to the panel entries, the roof is in the worst condition and if the maximum principal horizontal stress is parallel to the entries, the entry is in the best condition. However, little has been done about the high horizontal stress effects on panel entries subject to different orientations of high horizontal stress. In order to design a longwall panel and control the roof effectively, the redistribution and concentration of high horizontal stress field must be fully understood. Using 3-D finite element analysis, this paper presents the high horizontal stress redistribution and concentration for a retreat longwall face with different orientations of high horizontal stress. Based on the results, possible roof failures for different orientations of high horizontal stress are analyzed. A panel design criterion is recommended.

Jan 1, 1996

By B. G. D. Smart

An understanding of the nature of yield zones surrounding coal mine excavations is fundamental to the design of mine support systems. Various methods have been designed for investigation of underground rock masses and an extensometric technique, the Magnetic Rod Extensometer, has been developed to overcome inherent design limitations in existing equipment of this type. This paper refers to the prototype form of the extensometer and details changes designed to overcome a variety of drawbacks indicated by routine use of the equipment. The principal change has been one of manual to computerised data acquisition and presentation in the laboratory phase of the system. This has dramatically reduced the time taken to process underground measurements and produce numerical and graphical results. The revised Magnetic Rod Extensometer represents an integrated instrumentation system that combines speed and accuracy in all areas of data handling.

Jan 1, 1984

By Rajagopala Reddy Kallu

After the Sago and Darby No. 1 mine incidents, the new regulatory standards for the design of mine seals in underground coal mines of the United States require the mine seals to withstand higher explosion standards. This necessitated for the development of new design approaches and standards. For development of any new design standards or approaches a detailed understanding of effect of key influencing parameters on the stability of mine seal is a must. This paper attempts to address one of the factors, i.e. roof convergence, that play an important role in the stability of mine seal under explosion loading. Initially a couple of three dimensional numerical models were built and tested for different levels of roof convergence. These models confirmed that roof convergence has great influence on seal stability. A detailed study is undertaken, with some simple assumptions, to establish typical influence of roof convergence on the stability of mine seal under different levels of static explosion pressures. Three dimensional distinct element code 3DEC is used for this purpose.

Jan 1, 2007

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

By Dave Bigby

Hatworth Colliery, operated by RJB Mining (UK) Ltd, is successfully mining the Deep Soft Seam at a depth of 1000m using single face retreat longwalling. The seam is characterised by a weak roof generally comprising 4 to 6m of well bedded carbonaceous mudstones overlain by weak siltstones. The paper describes the layout and support design strategies which have enabled the colliery to adopt sole support by rock reinforcement techniques for its longwall gateroads. To achieve this success the colliery layout has had to be radically modified to take account of the in-situ horizontal stress field. The layout is also closely controlled by environmental considerations, the seam being one of the gassiest in the UK Economic support by strata reinforcement has been achieved using a combination of rockbolting of the roof and sides installed at the face of the heading, and systematic double birdcage cable bolting installed on drivage and supplemented ahead of the retreating panel. The layout and support strategies have been developed through a combination of detailed underground measurement of support and stress parameters backed up by numerical modelling using FLAC. The FLAC modelling has paid particular attention to careful representation of bedding properties and validation through underground stress measurement.

Jan 1, 1996

By Peter Zhang

The immediate roof at the test site consisted of laminated clay shale that is likely to be susceptible to weathering. With seasonal dry-wet cycles in the mine ventilation air, the roof in the intake air would deteriorate and lose integrity over time. In many instances, the weathering-prone roof would gradually scale off piece by piece between bolts and over the entry corners. In some long-term entries, roof weathering could develop up to 2-3 ft deep with columns of bolted roof hanging under the upper intact roof. If wire mesh is used, the roof bags down and reduces the entry height, and the weight of the bagged roof tends to pull the bolts out and poses a safety hazard. An effective measure is always sought to support this type of weak roof that is sensitive to moisture degradation. One solution is to seal the roof shortly after entry development to prevent moisture penetration so that roof integrity can be maintained for the long term. In this application, a polymer-based sealing material was tested in the Rockspring mine for weathering protection and reinforcement. The material is a one-component polymer powder for spray application onto the roof and rib. The sealant material is mixed with water in the spraying nozzle to apply to the roof and rib as a paste. The product sets within 5-10 minutes and forms a membrane as thin as about 1/8-in. The membrane will quickly develop tensile and bonding strength within minutes and completely cure within a few days. Through sealing, bonding and lateral reinforcement, the integrity of the weak roof can be maintained. The material has been tried in a long-term evaluation over several months in a set of main entries in the Rockspring mine owned and operated by Rockspring Development, Inc., affiliate of Foundation Coal. Preliminary evaluation has shown that the material is an effective and economical solution to ensuring the long-term stability of weathering-prone roof.

Jan 1, 2009

By David Newman

Multiple seam underground coal mining is ubiquitous in Appalachia. Many coal properties have from two to ten or more economically mineable coal seams. Because of the ability to penetrate 600-feet to 1,000-feet or deeper into the mountainside leaving panels of web pillars separated by barrier pillars, highwall mining is a hybrid between surface and underground mining. As a result, many of the ground control problems commonly associated with multiple seam underground mining exist with multiple seam highwall mining. These include: i) superjacent mines, subsidence and caving propagating to overlying seams and ii) mines, pooled water and the potential for inundation resulting from a iii) breakthrough between an active highwall mine and abandoned underground mines in the same or superjacent seams, the presence of overlying or underlying mine workings where iv) pillar columnization in the active mine is not possible, and v) the question of whether to mine ?top-down? or ?bottom-up.? Case histories are presented from southeast Ohio where the Middle Kittanning No. 6 and Lower Kittanning No. 5 seams are successfully highwall mined with 20-feet to 40-feet of largely shale interburden. A highwall miner was caught in a highwall collapse in the Middle Kittanning No. 6 seam when it was overmining auger workings in the Lower Kittanning No. 5 seam. Multiple seam interaction caused by the overmining transferred overburden stress onto the Lower Kittanning No. 5 seam. The overburden concentrated through web pillars initiated auger web pillar failure, resulting in roof collapse and subsidence in the Lower Kittanning No. 5 seam that propagated up to the Middle Kittanning No. 6 seam. A second case history involves multiple seam highwall mining in southeast Kentucky. The A3 Rider, A3, B2, and C1 seams are being highwall mined around abandoned underground mines in the A3 and B2 seams and augering in the A3 Rider. Because of spoil storage constraints for the surface contour mines, the highwall miner follows the active pits in each seam so a standard sequence of ?top-down? or ?bottom-up mining does not exist. The planning and engineering design considerations associated with each case history is presented along with geotechnical information. In the case of the highwall collapse a back analysis is presented of the stability and safety factors for the auger webs, highwall miner web pillars, and barrier pillars.

Jan 1, 2009

By G. J. Hasenfus

Although theoretical and conceptual knowledge regarding the impact of horizontal stress on underground coal mining excavations have existed since at least the mid-1900s, it wasn?t until the late 1970?s that the theory and practical understanding began to mesh. Since that time, particularly over the last 25 years of this Conference, increased empirical study, as well as advances in measurement technology, computer simulation, and bolt design, has significantly improved our understanding of horizontal stress impact, and provided coal mine operators with tools to reliably predict and manage it. This paper provides a retrospective review of some of the more important horizontal stress-related advancements made during the 25 years of Conference proceedings, particularly those for predicting and controlling horizontal stress impacts within underground coal mines. Relevant and significant CONSOL experiences and studies will be presented in context, with particular attention to horizontal stress considerations for longwall faces and gateroads. The significance of horizontal stress will also be placed in context with respect to local geology and structure, ground support measures, and mining configuration, method, and sequence.

Jan 1, 2006

By S. J. Jung

A fundamental study of fragment size distribution and characteristics of fracture pattern is presented for the dynamic penetration of a conical bit into coal block. In this paper, damaged zones during crater formation and fragment size distribution were studied. Bit with a sharper tip angle and a smooth body configuration performed better penetration. Results suggest that in coal cutting the trend should be toward the face cleat direction using a sharper angled tip bits which consumes less energy for an optimum range of penetration depths, thus attaining maximum cutting efficiency and reducing fine fragments.

Jan 1, 1989

By Peter Fischer, Heinz-Jiirgen Kateloe, Anton Sroka, Axel Preusse

"Since mining in Germany has a long history, there are still many locations where the surface fracture problem has to be solved. Shallow mine workings from an earlier era can affect the surface in a discontinuous way. In many cases, due to light overburden load, those mine rooms did not collapse and a downfall can suddenly happen for many reasons, e.g., in connection with a changed load entry into the subsurface or by additional influence of water. This results in ground movements, which in extreme cases, appear as surface fractures.Depending on the use of the surface, ground movements caused in such a way constitute an endangerment by law. Particularly, within the southern Ruhr district, surface fracture and subsidence endangered ranges are determined on the basis of an empirical method that resulted from a systematic evaluation of surface fractures and rock disclosures. This method indicates maximum ranges within which impacts from abandoned shallow mine workings must be assumed.With a special emphasis on a risk management approach, RWTH Aachen University adapted the model of stochastic media to the conditions of abandoned shallow mining to compute and illustrate potential ground movements due to the collapse of shallow mine workings. Further investigations were concerned with the analysis of characteristic parameters which can serve for the assessment of the surface fracture probability in a comparative view of different cases. In recent years, more than 50 surface fractures in the southern Ruhr district were analyzed in order to develop a risk assessment of surface fracture endangered ranges over shallow mine workings. The results that have been obtained so far are described in this paper.INTRODUCTIONAbandoned underground mine workings of shallow depth can pose a threat to the surface. Due to small overburden rock loads, shallow excavations often remain intact for decades after the operations have ceased and may collapse abruptly (Braemer et al., 2005; Hollmann and Nuerenberg, 1972; Preusse et al., 2007). Several approaches for prognosis of surface fracture resulting from 97 subsurface collapse deal with this topic (Loebel and Sroka, 2001; Tajdus and Sroka, 2009)."

Jan 1, 2010

By Meagan S. Boltz

From October 2000 to April 2001 a local array of fourteen seismograph stations was deployed at the Trail Mountain Mine (TMM), an underground coal mine in Emery County, Utah, to locate mining-induced seismicity (MIS). Original analysis of the data provided epicentral locations that coincided closely with the active longwall panel. The original analysis yielded poor quality hypocenters, however, preventing a useful relation from being made between mining activity and the event depths. For the original analysis, only stations at elevations above mine level were used for locating MIS. As a result, P- and S-arrival times from four of the stations in the local array were not used: an inmine station and three stations at elevations below mine level. This study compares mine workings and active mining to a revised TMM dataset that included P- and S-arrival times from the lower elevation stations that were not used in the previous study. In the revised locations, a modified master event methodology was used to calculate station delays. These station delays correct for errors in the shallow layers of the velocity model. The resulting hypocentral locations have improved depth resolution, determined by a lower root mean square (RMS) for the locations, compared to the original study. Most of the events occur in the roof of the mine and follow the longwall panel. When located using the velocity model determined in the original study, the elevations of the events increase as the longwall panel progresses. One possible explanation for the elevation increase is that it may be related to geology or the caving process. As presented here, a 10% decrease in the shallow velocity structure results in a more plausible location for the hypocenters. Consequently, it is hypothesized that progressive development of the TMM longwall panel(s) may have resulted in a progressive change in the velocity model.

Jan 1, 2012

By Khaled Morsy

Cementitious seals are made of foamed cements with strength ranging from 100 to 600 psi. These cementitious seals are usually used at locations where the seal will be subjected to some convergence, assuming that this material can absorb entry closure without compromising the structural integrity of the seal. Prior to 2006, Lake Lynn Experimental Mine (LLEM) conducted full-scale tests on various cementitious seals designed to meet or exceed the 20-psi horizontal static load requirements of the old 30 CFR1 75.335. Both hydrostatic and dynamic loading tests were conducted. The structural response (pressure and displacement readings) obtained from these tests were recorded. The stability of the cementitious seal is based on balancing the load applied to the face of the seal from the explosion against the shearing resistance at the seal/rock interface. Therefore, the properties of the seal/rock interface, such as stiffness, cohesion, friction, etc. are critical to determine the stability of a cementitious seal. These properties are usually assumed by the researchers in their models. This paper emphasizes the importance of having the correct properties of seal/rock interface in the numerical modeling. FLAC 3D models were developed to simulate Celuseal2 cementitious seals subjected to both hydrostatic and dynamic loadings. Using the hydrostatic and dynamic loading test data conducted at LLEM, the static and dynamic properties of the seal/rock interface were back calculated. The FLAC 3D models showed good agreement with the test results.

Jan 1, 2009

By Hani Mitri

Understanding the interaction between rock bolts and underground rock movement is critical for safe and cost effective underground excavation design. Although early research on this subject involved a balance of theoretical analysis and field measurement, recent work has been heavily focused on analytical and numerical studies. This paper describes technologies that have the potential to redress the balance through instrumentation of rock bolts deployed under routine operating conditions in underground mines. Two new products are presented in this paper. Firstly the U-cell coupler is a load cell device that can be routinely coupled to the threaded end of a bolt prior to installation. It measures the ?series? load at the head of the bolt. Secondly, the d-REBAR involves an array of small-diameter, long-base length displacement sensors recessed into grooves along the entire length of the bolt. Both new instruments are interfaced with on-board digital signal conditioning and telemetry. Methodologies for the deployment of the new instrumentation and guidelines are presented for the interpretation of results based on data obtained from field trials. Successful field installations demonstrate the viability and practicality of these new technologies and, moreover, provide important insight in to rock bolt/rock mass interaction.

Jan 1, 2012

By Xiangxi Wu

Longwall and continuous mining are prevalent methods employed by Chinese underground coal operations. The main ground control challenges include roof skin deformation, roof collapse, and outbursts of coal, gas, and water. Microseismic monitoring provides valuable information on rockmass behavior and fracture propagation caused by stress redistribution, active geological structures, or gas build up within the coal strata and the surrounding rockmass. Collecting and assessing seismic data has proven to be an instructive tool for engineers to better assess ground conditions and mitigate seismic hazards associated with mining. The development of intrinsically safe and explosion proof certified seismic monitoring equipment has revolutionized China?s underground coal mines. The use of this technology allows for the optimization of gas drainage at an increased rate of two at a 50% higher purity without any increase in drilling costs. Microseismic monitoring can also be used in identifying seismically active faults and shear zones. This information can be used to assess and revise mining strategies to improve safety and ground stability.

Jan 1, 2012

By Nan Hua

As a high production and high efficient mining method, top coal caving method is now widely used in China. However, as more extra-thick coal seams are mined, it becomes more and more difficult to make the top coal fully broken up, resulting in more and more top coals flowing into the gob, rather than onto the rear AFC at the working face as designed. This research is aimed to investigate the failure mechanisms of the extra-thick top coal caving mining method.

Jan 1, 2009

By Sanjay Kumar Singh

The mechanics of load transfer in bord-and-pillar mining is not well understood despite the evolution of the art of pillar extraction over the last 20 years via trial and error (empirical) or Rock Mass Rating (RMR), simple monitoring and pillar extraction experiences at neighboring mines. Variations in geologic conditions and pillar extraction methods, along with a lack of comprehensive instrumentation programs for monitoring ground behavior during pillar recovery, are among the factors that make it difficult to predict how strata will respond to pillar extraction. Efficient and safe use of roof supports depends on the interaction between the designed support and coal strata. Of particular interest in this study is the mechanics of strata deformation as influenced by geologic conditions, pillar extraction methods and optimum support with load-bearing capabilities in strata control during mass exploitation of underground coal deposits required detailed investigation,?. To provide a better understanding of the mechanics of strata deformation, as researchers, we have collected data and reviewed measurements of convergence and stress in one mine of South Eastern Coalfields Limited (S.E.C.L.), i. e., in North Chirimiri Pondary Hills (NCPH) and have completed numerical modeling with FLAC software for (Fast Lagrangian Analysis of Continua modeling, Itasca India Consulting Pvt. Ltd.) for two typical pillar extraction plans. Stress distribution in the mine roof above pairs of roof supports was calculated to demonstrate how Resin roof bolt support installed support contributed to roof block movement control. It was shown that overall stress and roof-floor convergence patterns were most influenced by the stiffness of coal measure rocks and by pillar extraction sequences and layouts. Also, resin roof bolts and other support systems play a critical role in controlling stability of both the immediate roof and the main roof for a distance of up to 18 meters (i.e., working coal seam height X three times) above the seam. Installed supports provide unique strata control advantages over other types of secondary support by significantly reducing the time between mining and installation of the secondary support. Although resin roof bolts are similar to posts in stiffness and capacity, they are more effective than posts under static loads because they can yield and still maintain loads at nearpeak capacity(Singh, et al., 2006), whereas posts fail and lose their ability to limit mine roof deformation.

Jan 1, 2012

By David Allen

Cable bolting is a relatively new strata reinforcement technique that has been used successfully in Australia and the United Kingdom. Jim Walter Resources, Inc. has examined the principles and can visualize its useful application in many areas, such as: a) Longwall pull-out areas, b) Roadway junctions, particularly those involving yield pillars, c) Rib reinforcement, and d) Possible replacement of cribbing in roadways, especially longwall tailgates. This paper describes the conditions which led to cable bolt testing, the results of tests conducted to date, and conclusions about the direction that Jim Walter Resources, Inc. is heading as far as testing and potential applications are concerned.

Jan 1, 1994

By Shuangsuo Yang

Based on field observations and physical modeling of simulated materials, the dynamic equilibrium principle on the whole interaction process between the surrounding rocks and support of large deformation roadways in coal mines has been developed. It explains the methods of forming the bolted strata and its changing load capacity with deformation. It also points out the factors and conditions for forming the bolted strata as follows: (1) many entries in coal mines are subject to large deformation. During their service life, it goes through the following process, elastic > plastic > fiacture > broken > loosening, etc. During this dynamic process, the mechanisms of supporting structure are changing and therefore the interaction between support and rocks experiences a process of dynamic equilibrium. Furthermore, the required supporting force for stabilizing the surrounding rocks shows overall decreasing trend; (2) the bolted strata is a special supporting structure consisting of bolt, rock skin protection devices and rock strata within the bolted horizon. As deformation increases, the supporting structure of the broken strata within the bolted horizon experiences a process of periodic change, from equilibrium > not equilibrium > equilibrium. Corresponding to this change, the supporting capacity of the bolted strata also fluctuates and gradually decreases; (3) under the bolted strata, the stability of a large deformation roadway is dynamic and step-wise; and (4) the formation of bolted strata, especially bolted broken strata depends on the existence of compressive force. Supporting the bolted strata is not constrained by the quality of the surrounding rocks. It has broad range of application.

Jan 1, 2005

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