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By Bruce K. Hebblewhite

This paper provides an overview of the international ?state of the art? with respect to underground coal mining methods used in seams of thickness greater than 4.5 m, through a review of a selection of high performing thick seam mining operations around the world?both Single Pass Longwall (SPL) and Longwall Top Coal Caving (LTCC). The paper discusses the recent operational experiences from these selected faces, together with the ground control issues faced, and identifies some of the major, inherent geotechnical factors that can influence the mining method selection, design criteria, and ultimate performance, from both an operational and safety perspective. It is clear that both of these methods are developing quickly? not only from an operational and equipment perspective, but also from the understanding of critical geotechnical criteria. There is an increasing role for both methods in thick seam environments, although the higher SPL faces will only be successful in the near to mid-term under benign geotechnical environments. Regardless of the method, a thorough understanding of the critical geotechnical factors is a key to successful implementation of either method.

Jan 1, 2013

By Ted Klemetti, Gabriel Esterhuizen, John L. Ellenberger

"Advanced numerical models can be used to evaluate entry support systems in coal mines. However, these methods require specialized software and specialized skills to create the models and evaluate the results. Practicing support design engineers require a method to rapidly assess a proposed support system for a given geotechnical scenario. A prediction equation that can be used to assess roof support systems has been developed from the results of hundreds of FLAC3D analyses that were conducted during recent research. These models were validated against field cases and empirical design approaches and were found to adequately predict entry stability in coal mines. The understanding developed from the model outcomes was used to develop an equation to predict entry roof stability against large roof falls. Least-squares error analysis was conducted to find appropriate parameters for the nonlinear equation. The developed equation can be solved using spreadsheet software, allowing for rapid assessment of alternative support system performance in variable geological conditions. The performance of the prediction equation is evaluated against empirical design methods and against selected case histories of support practices at currently operating mines. Examples are presented that demonstrate the performance of the equation in variable geological settings. The stability factor (SF) prediction equation can be used as an assessment tool to assist in the design of coal mine roof support systems.IntroductionGround falls represent a significant proportion of injuries and fatalities in underground coal mines in the US. During 2013, ground falls were responsible for 4 of the 14 fatalities and 16.6 % of the 1,577 reportable lost-time injuries (MSHA, 2015). In addition, each year about 400 to 500 large roof falls are reported that can extend up to or above the bolted horizon. Large roof falls and associated ground fall hazards can be mitigated through improved support design.The research presented here particularly addresses the problem of large roof falls that extend more than 90 cm (3 ft) above the roof line of an entry and may extend above the bolted horizon. Smaller roof falls that fall out between supports or extend less than 90 cm (3 ft) above the roof line are excluded. These smaller roof falls can be controlled by appropriate surface support such as roof screen or other skin control methods. Also excluded are roof falls that are associated with an individual geological structure such as a slip of fault. Structure-related instabilities can be controlled by strategically locating individual roof bolts or other supports appropriate for the local situation."

Jan 1, 2015

By Christopher Mark

The Analysis of Retreat Mining Pillar Stability (ARMPS) program has been used to evaluate room and pillar retreat mining layouts since it was first released in the mid 1990s. After the Crandall Canyon mine disaster focused attention on the importance of pillar design, ARMPS became an essential component in most Roof Control Plans developed by mine operators. In 2010, NIOSH released ARMPS version 6, which features a ?pressure arch? loading model for deep cover room and pillar mining. NIOSH has also provided general guidance for using ARMPS, including design criteria based on statistical analyses of an extensive case history data base. Many real-world retreat mining scenarios entail mining configurations that ARMPS does not directly address. Some of these situations that the MSHA Roof Control Division (RCD) has encountered in the course of its reviews of more than 100 ?complex and non-typical? plans are ? Unmined pillars are left at the mouth of a retreat panel that function as a ?composite barrier pillar? ? The floor is extracted on retreat, increasing the mining height ? More than one row of bleeder pillars is left in an adjacent, previously mined panel ? A retreat panel is located above or below a gob area in a previously mined seam ? Bleeder pillars are partially extracted on retreat RCD has developed solutions that allow these situations to be fitted into the ARMPS framework, and these are presented in this paper. Also discussed are some of the rules of thumb that have been developed for various input parameters, such as the treatment of inseam rock and slab cuts into solid coal.

Jan 1, 2011

By R. F. Valene

This paper is not available for inclusion in the Proceedings at the time of printing. but it will be included as an insert

Jan 1, 1984

By Christopher Mark

After the United States and Australia, South Africa has the largest. modern underground coal mining industry in the world. Historically, South Africa has been the cradle of many innovations in ground control, particularly in the area of pillar design. Today, South African mines are grappling with many of the same issues faced by their U.S. counterparts, including safety during pillar retreat operations, selecting the proper roof support, and maintaining long-term pillar stability. The author recently visited seven underground mines in the Mpumalanga, Free State, and KwaZulu-Natal provinces. Mining methods included longwall, room-and-pillar with continuous miners, and drill-and-blast. The mines represented a cross-section of geology and ground support practices. At each mine evaluations were made of the Coal Mine Roof Rating (CMRR), roof support practices, pillar design methods, and general ground control experience. The coal seams observed were generally thick (2-5 m), the depths of cover moderate (less than 200 m), and the roof rocks relatively strong (CMRR greater than 45). Unfortunately, the South African roof fall fatality rate is approximately three times greater than in U.S., perhaps in part because roof bolts seem to be more widely spaced in most South African mines. Roof bolting equipment was generally outdated, and the quality of bolt installation was a major concern. Pillar design, on the other hand, is quite advanced, and pillar failures are rare. Several novel partial pillaring techniques are employed, including "checkerboard" mining in which every other pillar is removed, 3-cut systems, and high-extraction mining where engineered final stumps are left by design. The paper also offers observations on ground control management techniques (including recently introduced Codes of Practice), the role of the mine inspectorate, and the status of the research community.

Jan 1, 1999

By Stephen C. Tadolini, Robert Hawker, Peter Mills, Colin Grubb, Tom Meikle

"In unfavorable strata conditions, it is often necessary to install additional long tendon support as part of a primary support cycle. Although these support systems provide the required additional stability to the roadway installation does have a significant effect on the rate of development. To overcome the slow cure time of Portland cement-type grouts, the use of two component resin systems, such as polyurethanes and urea silicates, have become favorable. However, concerns over low modulus and creep properties has led to the development of a two-component grouting medium that displays the properties of traditional cable grouting materials with the rapid reaction times of pumpable resin systems. This paper describes product testing, application, surface trials, underground trials, and full-scale applications of the Tekthix System in a dynamic underground environment. INTRODUCTION Single-component cement grouts have been used to anchor long tendon supports for many years. They range from high- to low-slump Portland cement and can be slow-setting to fast-setting grouts. However, current cement grouting systems, which includes the grout and equipment, have some disadvantages: 1. Low-slump grouts require pumps that can handle a thick, high-viscosity grout, so choice of pumps is limited. 2. Pumping distance is limited with current cable bolt grout pumps; operators often tend to add water to allow pumping long distance, which adversely impacts the quality and strength of the material. 3. Fast-setting grouts tend to cause issues with pump and line blockages due to the reduced working life of these single-component materials."

Jan 1, 2017

By Brian E. Perrine

Underground mining requires a variety of specialized equipment. With ever-changing mining conditions and methods, some mines have had to resort to purchasing existing industrial equipment and adapting it for underground use. Since this equipment is not made for underground mining or their specific task, the result is often lost time, money, and sometimes an unsafe working condition. One such application is the setting of a CAN. A CAN is a type of cylindrical secondary roof support made from a cement composite and encased in steel. The CAN has become popular because of its simple one-piece design that can handle heaving and sagging better than other available designs. ?Because of this exceptional ability to sustain load through extended convergence and displacement, the CAN is one of the most robust and stable standing supports in use today.?It also promotes better air flow due to the size and shape of the CAN. CAN size and weight have increased over time to handle more difficult mining conditions. Significant down time and inefficiencies with existing CAN setting machines at Twentymile Coal were directly due to the item in use not being designed to be task-specific. They were undersized with very limited movement. The miners had to take extra steps and precautions when setting CANs. Maintenance delays were significant. Twentymile Coal Company realized that they required a safer, more reliable and more efficient machine to handle and set CANs, and looked for a partner to develop a new design-specific machine. They worked jointly with J.H. Fletcher® engineering and sales to design and manufacture a machine specifically made for setting CANs. The objective was to produce a machine that would handle the conditions and methods that the mine required for setting CANs during longwall retreat. The resulting design had to be safe, stable, have a wide range of motion, be ergonomic, simple to use, and easy to maintain. The machine also had to improve efficiency. The resulting machine has been operating for approximately one year at Twentymile Coal and has been proven to improve safety, reliability, and efficiency. In conclusion, with the task-specific CAN Manipulator, the mine is able to better plan daily tasks with the knowledge that the machine will be available throughout a shift, and the employees will be safer because of the machine?s ergonomics, stability and safety. This has resulted in better productivity and a safer environment.

Jan 1, 2013

By Vincent A. Scovazzo

"The most significant study on the effects of longwall mining on gas and oil well integrity was completed by a consortium of coal and shale gas industry participants. A field study was conducted in 2013 and 2014 and is referred to by its location on Well Pad NV-35. The experiment site was located at the CONSOL Enlow Fork Mine in Washington County, Pennsylvania.The experiment was completed to verify John T. Boyd Company (BOYD) calculations and studies completed under contract by for the Pennsylvania Department of Environmental Protection Gas Well Pillar Study Update. The findings of the original study were presented by BOYD at the 32nd Conference, while numerical analysis of the Well Pad NV-35 field study was presented by Dr. Su at the 35th Conference.This paper addresses the findings of the experiment and how it confirmed the conclusions of the original study.INTRODUCTIONA field study was conducted in 2013 and 2014 by a consortium of coal and shale gas industry members to test the effects of longwall-induced ground movement on the stability and integrity of wells drilled through a gateroad abutment pillar. The project was initiated to test initial John T. Boyd Company (BOYD) findings of the Gas Well Pillar Study Update (Scovazzo and Moran, 2014), completed for Pennsylvania Department of Environmental Protection, Bureau of Oil and Gas Management (PDEP). Background and initial findings on this project were presented at the 32nd Conference (Scovazzo and Moran, 2012). The field work and numerical analysis were spearheaded by CONSOL Energy, Inc. (CONSOL) with participation by Chevron, EQT, Noble Energy, Range Resources, Marcellus Shale Coalition, and the Pennsylvania Coal Alliance.Dr. Wen H. Su of CONSOL was the leader of the field experiment with input from the participants. Dr. Su also completed numerical analysis of the results and model casing deformation. Analytical results were presented by Dr. Su at the 35th Conference (Su, 2017)."

Jan 1, 2018

By B. Tulu, D. Gearhart

A comprehensive monitoring program was conducted to measure the rock mass displacements, support response, and stress changes at a longwall tailgate entry in West Virginia. Monitoring was initiated a few days after development of the gateroad entries and continued during passage of the longwall panels on both sides of the entry. Monitoring included overcore stress measurements of the initial stress within the rock mass, changes in cable bolt loading, standing support pressure, roof deformation, rib deformation, stress changes in the coal pillar, and changes in the full three-dimensional stress tensor within the rock mass at six locations around the monitoring site. During the passage of the first longwall, stress measurements in the rock and coal detected minor changes in loading, while minor changes were detected in roof deformation. As a result of the relatively favorable stress and geological conditions, the support systems did not experience severe loading or rock deformation until the second panel approached within 30–50 ft of the instrumented locations. After reaching the peak loading at about two to three inches of roof sag, the cable bolts started to unload, and the load was transferred to the standing supports. The standing support system was able to maintain an adequate opening inby the shields to provide ventilation to the first crosscut inby the face, as designed. The results were used to calibrate modeled cable bolt response to field data and to validate numerical modeling procedures that have been developed to evaluate entry support systems. It is concluded that the support system was more than adequate to control the roof of the tailgate up to the longwall face location. The monitoring results have provided valuable data for the development and validation of support design strategies for longwall tailgate entries.

Jan 1, 2017

By Takashi Sasaoka, Hiroshi Takamoto, Kikuo Matsui, Hideki Shimada

"Indonesia is the second largest coal exporter to Japan. Almost all the coal is produced from open cut mines. However, a great deal of coal is left in the highwalls of the mined-out pits. The use of highwall mining systems has increasingly come into play in the U.S. and Australia, when overburden depth exceeds economical recovery in an open cut mine. However, the stability of the highwalls is always a major safety concern in such mining. This paper describes the characteristics of the highwall mining system and discusses the appropriate highwall mining system for use in Indonesia.INTRODUCTIONIndonesia produces over 200 Mt of clean coal and is the second largest coal exporter to Japan, accounting for about 32 Mt of coal annually. Over 99% of the coal produced in Indonesia comes from surface mines. More open cut mines will be developed and more coal will be mined in order to fill the great demand both in Indonesia and the rest of the world.In open cut mines in Indonesia, there are many sites where mining operations have developed long highwalls that have been abandoned due to the current economics. Mining operations have been transferred to lower stripping ratio blocks of coal or overlying seams. In some cases, coal lies buried beneath spoil heaps or is covered with mud and water. It is estimated that there is a great deal of coal beneath abandoned and working highwalls. Considering these situations, it seems to be worth introducing highwall mining systems into Indonesian open cut mines.Final highwalls of open cut mines can form the starting point for other mining methods, such as highwall or underground mining. In its basic application, highwall mining is a technique utilized after an open cut portion of a reserve has been mined, sometimes prior to the introduction of underground mining (Seib, 1993). In this system, the coal seam is mined by remotely operated equipment, such as an auger machine or a modified continuous miner incorporated into a highwall mining system. The major problems with highwall mining systems are less coal recovery due to leaving coal as the pillars and the instability of openings and highwalls due to the pillar and roof failures."

Jan 1, 2010

By Ali Naeimipour

Availability of accurate geological information about the rock mass surrounding a tunnel or mine working is the key to enhance and optimize the required ground support measures. Various approaches have been used to obtain data on ground conditions, including the possibility of characterizing the ground while drilling for roof bolts, by the Measurement While Drilling (MWD) system, via optical, sonic and acoustic televiewers, and using various types of gamma probes. While there are many different systems for characterizing the ground in longer, larger-diameter holes drilled downward, the options for using probes and logging systems in short, small-diameter holes drilled in horizontal or an upward direction are very limited. This paper will present the results of recent field tests using optical televiewer (OPTV) to analyze the joints and voids in boreholes drilled in different directions. The studies show that OPTV is a promising tool; however, getting a good quality image from the upward drill holes still need to be studied. Meanwhile, estimated in-situ strength of the intact rock is needed to complement the jointing pattern for rock mass classification. For this purpose, preliminary steps for the development of a borehole scratcher to estimate rock strength will be discussed in this paper.

Jan 1, 2014

By Bob J. MacDonald

The Cape Breton Development Corporation is a federally owned Crown corporation in Canada that operates two coal mines the Prince and Phalen Colliery in the Province of Nova Scotia. Retreat longwall mining was first introduced to the shallower workings (I 80m) of Prince Colliery in the early 1980's. Phalen Colliery started longwall retreating in 1987 and since that time has extracted twelve panels. In 1993 there had been a total of eight longwalls varying in widths from 140-230m mined in Phalen Colliery that is on the Phalen Seam. Up to this point there were only minor ground control problems on the faces except for one of the earlier longwalls that was mining a split seam. There is some interaction on Phalen workings from remnant structures in the overlying Lingan and No. 26 Collieries which worked the Harbour Seam. Induced abutments loads along pillar edges have been observed and resulted in poor face conditions when mining the Phalen Seam. The ninth panel, 6 East was the first panel to suffer a major roof fall from the First Major Weighting. This was also the first panel to undermine a paleo sandstone river channel named the Lower Sandstone Unit (LSU). For the first time in Phalen's history subsequent longwalls began to experience periodic weighting events. The 6 East panel suffered a number of periodic weightings and on two occasions had to use foam cement to fill the cavities that resulted. On the next east side panel, 7 East, there were numerous periodic weighting events, but interestingly there was no significant First Major Weighting. 7 East was the first 260m wide panel and all others were 230m or less. This longwall was 3,400m long and began producing in December of 1994 and was still cutting coal in May of 1997. There were five significant roof fall events on this panel, three of which required foam cement to fill the cavities. Of these three the interruption to production varied from four days to seven months. There were definite precursors to each of the weightings including: 1) an increase in audible noise (termed bumping) on the face, 2) supports went rapidly into yield, 3) roof convergence increased, 4) water percolated from the face roof strata, and 5) severe spalling of the coal occurred resulting in increased tip to face distance. Longwall face weightings are a result of geomechanical factors and cannot be avoided However, the loss of roof control is a different matter and highly subjective to operational procedures. The causes of weightings, their magnitude and impact on the mining at Phalen have been investigated. The following factors have been identified as relevant to these occurrences. 1. The influence of pillars in the overlying Harbour Seam workings. 2. Location of the LSU relative to the Phalen Seam. 3. Thickness of the LSU. 4. Pressure arch effect from the adjacent gobs. 5. Poor operational practices on the longwall including: a) slow rate of retreat; b) inadequate supply of pump pressure to the hydraulic supports; c) premature yielding of the yield valves; and d) poor face alignment practices. This paper will review the events that took place on the 7 East longwall and show that the Lower Sandstone Unit is the primary cause of the weightings. However, other factors contributed to the loss of roof control and the formation of the roof cavities.

Jan 1, 1997

By Keith Heasley, Christopher Newman, Zach Agioutantis

"Previously, in 2013, a laboratory version of the computer code ARMPS-LAM, introduced at the 32""d International Conference of Ground Control in Mining, successfully integrated the laminated overburden model of LaModel into an ARMPS-style pillar stability analysis. The ARMPS-LAM analysis has now been fully integrated into the ARMPS program allowing users to more accurately classify the stability of a given mine design utilizing a laminated overburden model for the determination of pillar loading within theAMZ. ,This paper introduces the new Windows-based ARMPS-LAM program. The program's ease of use, output result generation, and limitations are highlighted through the parametric analysis of three hypothetical case studies where traditional ARMPS output is compared to output generated by LaModel. As mining operations continue to produce at greater depths and in more complex geometric and geological conditions, flexible and efficient analyses of mine stability for underground room and pillar mining has become more and more essential. The ARMPS-LAM program provides the mining industry with an empirically backed, numerical design tool for the evaluation of underground room and pillar mine plans.INTRODUCTIONIn the Southern Appalachian coal fields of the United States, retreat room-and-pillar methods have allowed mining operations to maintain high rates of production in the face of adverse geologic conditions and shrinking coal reserves. Over the years, research has been dedicated to understanding and mitigating mining-related dangers due to second mining practices, such as: pillar squeezes, floor heave, roof falls, pillar bumps, etc. However, as mining operations continue at deeper depths and with more complex geometric and geologic conditions, there is an inherent industry need for more accurate, flexible, and faster evaluations of stability for underground room-and-pillar mining."

Jan 1, 2016

By Ihsan B. Tulu, Keith A. Heasley, Aanand Nandula

"The objective of this paper is to present the development of a software tool that will assist in an area calculation of the Analysis of Roof Bolting Systems (ARBS) support intensity while incorporating more detailed geology, stress, and intersection span input. In this area ARBS tool, a detailed CMRR distribution can be determined from the available geologic database at the mine. In addition, the overburden and multiple-seam stresses, as obtained from the boundary-element program LaModel, can be converted to a “pseudo-depth” which is then used as the depth input to the area ARBS calculation. Finally, the section-specific intersection spans can be determined from the mine design for input to the calculation. All this detailed area input information for ARBS is handled by the latest version of the Stability Mapping (StabMap) program, which now outputs an area ARBS suggested support density given the required input parameters. This final ARBS grid can be plotted, analyzed and overlaid on the mine map for optimum use in support design and for presentation to production personnel.INTRODUCTIONSince the inception of roof bolts in the 1940s, there has been a lack of a universally accepted roof bolting design method (Mark, 2002; Mark and Pappas, 2012). To address this situation, the National Institute of Occupational Safety and Hazard (NIOSH) developed roof support design guidelines, which were incorporated into a computer program called the Analysis of Roof Bolting Systems (ARBS). These guidelines were based on the effects of roof quality, stress, and mine geometry (Mark, Molinda and Dolinar, 2001). The roof rock quality in ARBS is represented by the Coal Mine Roof Rating (CMRR), the depth is used as a surrogate for the stress, and the intersection span serves to represent the mine geometry (Mark, Molinda and Dolinar, 2001). These three input parameters are used in the fundamental, empirically derived equation of ARBS to calculate a required support density factor, called ARBS, (Mark, Molinda and Dolinar, 2001):"

Jan 1, 2018

By G. C. Xiao

A database, incorporating historical data from British mines, to study the factors affecting inflows from proximate aquifers into longwall workings, showed that a main casual factor was the immediate roof strata lithology. When this comprised principally stronger sandstone, there was a tendency for periodic inrushes associated with face weighting. This phenomenon is explained by the collection of water in bed sepa¬ration cavities, where free water connected to a high pressure head aquifer source, may be trapped.

Jan 1, 1990

By Andre Zingano

The coal seam strength and the failure criteria that represent the behavior of the coal seam are very important for mine section and pillar design. Additionally, mine section and pillar design take into account roof and floor layers, which form the roof-pillar-floor system. Therefore, underground ground control design begins, most importantly, with rock mass characterization. Another issue is the distribution or behavior of the coal seam strength for the whole mine area. Sometimes, the coal seam strength is assumed to be the same for an entire area, but this is not true because the coal seam is heterogenic and anisotropic, and also the coal strength is affected by faults and spacing between fractures. The objective of this paper is to demonstrate the use of the methodology for geomechanics characterization to determine the strength for the Bonito coal seam, and discuss the coal seam strength distribution around the entire mine area. The intact coal strength was determined by laboratory tests that were completed during the last 17 years, including triaxial and uniaxial strength tests. It was noted that there is a strong correlation between all laboratory strength tests with 90% confidence interval. The explanation for this coal seam strength variation is the presence of the discontinuities and their variation in quantity and quality at each location in the mine area, because the coal seam rock classification and its behavior will be affected by the quantity and quality of fractures.

Jan 1, 2014

By Arka Jyoti Das, Prabhat Kumar Mandal

"Coal mining in India has a strategic importance for the energy security of the country. Presently, the majority of coal production is coming from open-cast mining. Due to the fast depletion of reserves amenable to open-cast mining, future coal production will depend primarily on underground production. In conventional bard-and-pillar/room and pillar mining as it is practiced in India, the percentage of extraction is very low. In fact, within most Indian coalfields, a large number of coal seams extract only 15- 20% of coal, but no pillar extraction is done due to the presence of surface or sub-surface structures leading to huge amount of coal being locked-up in underground. Spontaneous heating and fire, the accumulation of poisonous gases, severe stability issues, unsafe workings and environmental hazards are all major problems associated with these developed coal pillars, so there is a pressing need for technology in the mining industry to extract this huge amount of coal locked up under different constraints. In this study, we propose that the locked-up coal be extracted by replacing the developed coal pillars with artificial pillars. The paper briefly discusses the extraction methodology designed for this and its evaluation through numerical modeling for applied suitability underground.INTRODUCTIONDriven by rising populations and urbanization, an expanding economy and also a quest for improved quality of life, energy usage in India is expected to increase manifold in the coming years. As for India's energy scenario and its projected future plans, there is no doubt that coal will remain a prime source of energy, even over the next few decades.The majority of coal production in India comes from open-cast mining because of large-scale mechanization, but the coal reserves amenable to open-cast mining are gradually depleting - which may impose adverse impacts on the energy sector if underground production does not increase. Underground coal extraction, meanwhile, is going to become more complicated in near future due to the fast exhaustion of easily mineable coal reserves."

Jan 1, 2016

By Muhammad Zaka Emad, Rangzeb Goraya, Muhammad Usman Khan

"Numerical modeling has been used extensively to assess the stability of surface and underground mines. Tabular, steeply dipping ore bodies are generally mined out by open stoping methods with delayed backfill. Cemented rockfill (CRF) is employed as a backfill material for narrow vein deposits with low production rates. Sublevel stoping methods with delayed backfill are employed to mine out steeply dipping ore bodies. Backfill is exposed to blast-induced vibrations and dynamic loading due to loss of confinement, which could lead to backfill failures. Backfill failures could influence the operating cost of a mining stope and the productivity of the entire mining project. This work shows a detailed overview of the factors contributing to backfill failure. A detailed literature review is presented along with case studies of longhole stoping operations from the Canadian Shield region for this work. The backfilled stopes are analyzed with the finite difference code FLAC3D. The literature review shows that the factors primarily responsible for fill failure are the strength of backfill, stope geometry, mining sequence, and blast-induced vibrations. INTRODUCTION Sublevel stoping methods with delayed backfill are popular in Canadian hardrock and metal mines. Backfill facilitates ground control and greater recovery of ore with pillar mining. It reduces wall slough, thus decreasing ore dilution from adjacent walls. The production of cemented rockfill (CRF) is simple; rock aggregates are mixed together with cement slurry and then placed in a mined-out stope. CRF has a low water content, thus it requires no additional drainage unlike some other backfill. Low water-to-cement ratio enables faster curing rate, hence a faster mining cycle can be achieved (Annor, 1999; Hassani and Archibald, 1998; McKay and Duke, 1989). CRF is generally placed in stopes from the upper sill drift using a truck or a load haul dump machine. Some of the problems associated with CRF are the moderate to high operational cost, the difficulty to get a tight filling, and segregation. Backfill failure is a product of many factors, such as cementation, particle size distribution, placement method employed, water quality, segregation, and blast-induced vibrations. The problem of segregation in aggregates has been observed when aggregate is dumped from a backfill raise (Meech, 2012; Indiana DOT, 2013). Segregation becomes unavoidable when the aggregates are not properly sized (Yu and Counter, 1983; Stone, 2007; Yu, 1989). The exposed backfill zones will contribute to ore dilution due to backfill when subjected to dynamic loading due to fill failures. This paper presents a detailed literature review and numerical modeling results of a mine backfill, along with case studies from longhole stoping operations. The backfilled stopes are analyzed with the finite difference code FLAC3D (Itasca, 2017). The literature review shows that the primary factors responsible for fill failure are the strength of backfill, geometry of stope, sequence of mining, and the vibrations induced by blasting operations."

Jan 1, 2017

By Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The response of the surrounding rock mass to the creation of mining excavations determines the ultimate load on a pillar support system. In conditions where the ground is relatively soft and weak, the full overburden weight can be transferred to the pillar system. However, in stiffer and stronger rocks, a greater portion of the overburden load is transferred to the unmined coal barriers or abutments, and the pillar stress is reduced. This paper makes use of numerical models to examine the interaction between typical pillar systems and the surrounding rock mass for weak and strong geological conditions at various spans and depths of cover. The concepts of structural failure and functional failure of pillars are used to assess pillar performance when pillars are deformed beyond their peak resistance. The results show that the span-to-depth ratio is an important factor in determining the pillar stress and the ultimate deformation of pillars. The ultimate pillar strain appears to be closely related to the functional success of pillar systems.INTRODUCTIONThe National Institute for Occupational Safety and Health (NIOSH) was requested by Congress to study the safety of retreat pillar mining following the Crandall Canyon Mine disaster that took place near Price, Utah in August, 2007. As a part of this study, an investigation was made to better understand how the rock mass responds to the creation of mining excavations and how the loads are distributed among the pillars and the surrounding abutments or barriers. The outcome of these investigations contributed to the development of a modified loading model for the ARMPS-2010 (Mark, 2010) method ofretreat mining pillar design.Structural and Functional Failure of PillarsPillar design is typically conducted by estimating the pillar strength and stress, and then sizing the pillars so that an adequate margin of safety exists between the expected strength and stress. The pillar strength can be defined as the maximum resistance of a pillar to axial compression (Brady and Brown, 1985). If a pillar is loaded beyond its peak strength, load shedding or yielding can occur, and the pillar is considered to have failed as a structure. Structural failure generally refers to a loss of loadcarrying capacity."

Jan 1, 2010

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013