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By Thomas M. Barczak

The performance characteristics of Mobile Roof Supports (MRS's) were evaluated through full-scale testing in the Strategic Structures Testing Laboratory at the National Institute for Occupational Safety and Health, Pittsburgh Research Center. Safety issues pertaining to the operation and design of these supports for pillar extraction during retreat mining are examined. A unique load frame, called the mine roof simulator, provided a realistic simulation of mining conditions by inducing vertical, horizontal, and lateral loading on the support. The purpose of these experiments was to determine the capabilities and limitations of the supports and to investigate factors that influence the measurement of loading and loading rate. An evaluation of the support design and load conditions that can cause support failure or loss of support capacity are discussed relative to the laboratory tests. In general, lateral loading perpendicular to the longitudinal axis of the canopy is most severe, although horizontal loading in the direction of the long axis of the canopy can also produce critical loading in some cases. The experiments indicate that both setting force and leg pressure measurements are influenced by the staging of the leg cylinders. The implications of these factors on load rate measurement are evaluated. A comparison of MRS capacity and stiffness characteristics is made to conventional timber supports.

Jan 1, 1997

By Jim Sandford

The South Bulga Colliery longwall operation, owned and operated by Cyprus Coal Australia. Oakbridge Pty. Ltd., and located near Singleton, NSW, is currently operating under an average 80-ft-thick (24-nt) sandstone unit residing immediately above the coal seam, resulting in sudden and severe face weightings. To date, nine severe weightings have occurred at the property (within Panels 3, 4. and 5), three of which resulted in face closures accompanied by significant equipment damage and several weeks of downtime. The remaining six events nearly resulted in face closures, incurring single-pass vertical convergence of up to 8 in (200 mm) along the pan line. In an effort to mitigate and/or eliminate the occurrence of these severe weighting events, mine management has established an in-house program to immediately address the geotechnical and operational mechanisms underlying rapid weightings, as well as specific measures to mitigate these events on future panels. In support of this in-house effort. NSA Engineering Pty. Ltd., is assisting the engineering staff at South Bulga in developing a more aggressive face monitoring program. The aim of this program is to provide weighting occurrence predictions sufficiently ahead of time to ensure that operational controls are optimized prior to attempting to mine through potential weighting areas. This paper reviews the occurrence of weightings to date at the South Bulga Colliery as well as the latest developments in shield monitoring technologies to forecast these events in near-real time.

Jan 1, 1999

By Kanchan Mondal, Anthony (Sam) Spearing, Gopi Bylapudi, Joseph C. Hirschi

"The U.S. coal mining industry uses about 100 million rock anchors per year. ASTM F432 specifies the requirements for rock bolts on mines but does not address corrosion. Corrosion has been found to be an issue in Australian coal mines (Hebblewhite et al., 2003 and Villaescusa et al., 2007), where the problem has been researched and stress corrosion has been found to be a significant cause of rock falls. Corrosion is also a major concern for underground civil construction in the U.S. but has not been considered an issue or adequately researched in coal mines. Underground conditions are conducive to corrosion mainly because of water quality and humid conditions. There is a perception, however, that when bolts are fully grouted, adequate corrosion protection is offered to the steel. Research has shown that this is not necessarily the case, due to the formation of micro-cracks as the resin sets and with subsequent rock movement shearing the resin column. This paper outlines a method to determine the corrosion potential of bolts used in long-term excavations and suggests ways to mitigate such effects based on research conducted in the lab and on three mines.INTRODUCTIONResearch on rock anchor corrosion in mines has not been extensive enough in the past, and the effects of several factors in the mine atmosphere and waters are not clearly understood. One probable reason for this lack of research may be attributed to the time required for gathering meaningful data, which makes the study of corrosion quite challenging.Almost all the rock bolts used in coal mines are made from steel, hence the corrosion of iron needs to be understood. The corrosion of iron is an electrochemical phenomenon with the overall equation"

Jan 1, 2010

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 Jun Lu

This paper presents the evaluation, enhancement and comparison of two hydraulic fracturing techniques which were employed to mitigate the impact of a massive sandstone channel on two successive longwall faces. The hydraulic fracturing technique and Longwall Visual Analysis (LVA) software were initially employed to mitigate the impact of thick, massive sandstoneroof during mining of longwall Panel 21. Improved longwall face conditions and productivity were observed as Panel 21 mined through the sandstone channel. However, moderate sandstone fractures and falls at the face were still present in areas where hydraulic fracture influence was not present. Such adverse conditions caused two to three weeks of production delays due to large face cavities, jammed face conveyor and subsequent face gluing. Based on the mining experience in Panel 21, and site-specific roof geology in Panel 22, a modified and more effective hydraulic fracturing program was employed. Fewer well-placed holes were drilled, and larger volume of water was pumped into each hole in order to propagate the individual horizontal pancake fractures over a larger area, thereby enhancing caving and relieving face pressure caused by overhang of the massive sandstone. Longwall Visual Analysis (LVA) software was also employed at Panel 22 to predict the cavity risk index and guide the longwall face crew in double-pulling shields behind the shearer drum. This helped to reduce the tip-to-face distance at the critical spots along the longwall face, which helped mitigate the formation of the roof cavities and failure of massive sandstone onto the face conveyor. Underground observations, face advance rate and face rock delay analysis confirmed that longwall face conditions and advance rate in Panel 22were significantly improved compared to those of Panel 21. Consequently, safety and productivity were improved.

Jan 1, 2014

By Yongping Wu

The steeply dipping coal seams in this paper refer to those with the angle of inclination ranging from 35° to 55°. The reserves of the steep coal seams account for approximately 15%~20% of the total coal reserves in China. In the past 12 years, the top-coal caving mining method had been used to mine the steeply dipping thick (11.5-16.4 ft or 3.5-5.0m) coal seams. However, the mining process of top coal caving is complex with low recovery rate and nearly 30% of coal was wasted. Those problems can be solved by using the single pass large mining height (11.5-16.4 ft or 3.5-5.0m) method. But the movement of the overlying strata around the mining face area of large mining height is complex and it is hard to control the stability of ?rock-shield support? system. Therefore, field measurement, numerical simulation and physical (similarity material) simulation methods were used simultaneously to analyze the strata movement, roof structure and rock-shield support interaction characteristics. The results indicate that strata movement, which is the same as mining the steeply dipping seams with conventional mining heights, is asymmetrical about the dipping direction of the face; deformation, failures, and movement of the surrounding rocks are more intense; the roof weighting interval decreases; the weighting intensity increases significantly; and face sloughage occurs frequently. In the gob, the space for sliding and rolling of the roof increases; the broken main roof tends to form an anti-dip pile structure; the backfilling and compactness of broken roof increase at the lower portion of the face, thereby inducing significant unbalance in stress distribution between the upper and lower portions of the face. The overlying strata above the face form a multi-level step structure. The characteristics of contact and loading of roof-shield support system are more complex. The range of shield load variation is larger, and interaction between shield supports occurs frequently. Anti-toppling and anti-sliding devices at the face become more difficult to implement. The research results provide a theoretical basis for the mining practices in the steeply dipping thick seams with a large mining height. The practice in panel 25221 of 2130 coal mine indicates that the accident rates have been greatly reduced and good technical and economic benefits have bee realized.

Jan 1, 2014

By Long Fan, Shimin Liu, Guijie Sang

"Weak shale roof falls have long been the main cause for fatalities in underground coal mines. Moisture-induced swelling is one of the root causes for the degradation of roof shales. In this study, slake durability tests are conducted on shale samples collected from Illinois basin coal mines, and different levels of durability are analyzed according to Gamble slake durability classification. There are two main mechanisms influencing the shape of retained fragments after two cycles of wetting and drying: mechanical abrasion and swelling pressure. A prototype optically based shale swelling apparatus is designed and manufactured for the shale free-swelling measurement. The experimental results suggest that swelling pressure tends to deteriorate shale laminations. Moisture-induced swelling of the No. 6 roof shale from Bear Run Mine was measured under 100% relative humidity condition. The measured swelling strain normal to beddings is ~5 to 7 times greater than the swelling strain parallel to beddings. This suggests that interlayer expansion plays a key role in moisture-induced swelling. Cracks induced by swelling pressure tend to occur along the bedding plane and lead to shale deterioration. INTRODUCTION Background Ground falls have long been the cause for nearly 50% of all fatalities in underground coal mines and continue to be one of the greatest safety hazards in underground spaces. Among the various ground fall hazards, roof failure takes up 23% of total fatalities in underground coal mines, of which 13% are due to skin failure and 10% are due to massive roof falls (Mark, Pappas, and Barczak, 2011). Roof failures associated with weak shales are summarized in previous studies (Molinda and Mark, 2010; Aughenbaugh and Bruzewski, 1973; Bajpayee, Pappas, and Ellenberger, 2014; Murphy, 2016). For mine roof instability and failure, two aspects can explain the failure mechanism of weak roofs. One is the elevated stress concentration in lateral direction due to the mining-induced excavation and its stress redistribution around the mine opening (Song and Stankus, 1997; Fan and Liu, 2017; Hill, 1986). The other mechanism is the passive stress-relief due to the strength weakening induced by the ambient moisture and temperature fluctuations during a period of time. Shale is moisture sensitive and presents a lower durability if it is exposed to a highly humid environment for a long time, thus leading to roof deterioration and instability in underground coal mines (Aughenbaugh and Bruzewski, 1976; Stateham and Radcliffe, 1978; Chugh and Missavage, 1981; Klemetti, Oyler, and Molinda, 2009). It is well known that shale is a clay-bearing sedimentary rock and that clay minerals are water sensitive. The moisture uptake for shale will induce the micro-deformation of a shale matrix, thus the structure and strength of shale is a function of time and intensity of moisture exposure. All mine engineers notice moisture-induced shale deterioration; however, the underlying mechanism(s) are still not fully understood due to the complexity of geophysical-chemical water-shale interactions (Huang, Aughenbaugh, and Rockaway, 1986; Hensen and Smit, 2002; Diaz-Perez, Cortés-Monroy, and Roegiers, 2007). The coal mining industry is still in the process of finding a technically sound, cost-effective technology to prevent this unexpected and unpredictable shale roof failure. Therefore, understanding how the moisture-sensitive shale responses to various ambient moisture and what role exposure duration plays in shale roof failure is key to prevent unexpected roof incidents, which will lay the foundation for the long-term roof support design and ground control management."

Jan 1, 2017

By Bo-Hyun Kim, Steve Berry, Mark K. Larson

"When coal pillar bursts occur, they involve the sudden expulsion of coal and rock into the mine opening. These events occur when relatively high stresses in a coal pillar, which serve as support in underground workings, exceed the critical capability of the pillar. This causes the pillar to rupture without warning. A cleating is a type of naturally occurring joint system unique to coal seams. However, cleatings are pervasive throughout the coal seam and are much more tightly spaced than joints occurring in other rock types. In this study, researchers from the National Institute for Occupational Safety and Health (NIOSH) investigated 84 coal samples obtained from a Utah coal mine by conducting both unconfined and triaxial compressive tests in order to examine the response to stress as a function of both confinement and orientation between cleat and loading direction. The results of the study show that the confining pressure dictated not only the peak compressive strength but also the brittleness as a function of the major-to-minor principal stress ratio. The spalling limit of the coal estimated by the s-shaped brittle failure criterion appears to be very dependent on the angle between the cleat and the loading direction, particularly at lower confinement.INTRODUCTIONCoal pillar bursts, or bumps, remain a most critical safety concern in underground coal mining, despite significant advancements in engineering controls. As shown in Figure 1, between 1983 and 2014, there were nearly 400 cases of reportable dynamic failure accidents in coal and nonmetal mines, resulting in 20 fatalities, 155 lost-time accidents, and an estimated 48,000 lost man hours (MSHA, 2015). These types of events have been documented for well over 100 years within the American underground coal mining industry. During this time, mining practices and support technologies have evolved considerably, resulting in an overall decrease in the rate of dynamic failure-related injuries and fatalities. Although techniques and practices of mining are highly advanced, coal pillar bursts, or bumps, continue to occur (Mark and Gauna, 2015). If coal is expected to fail in a brittle manner, behavior changes associated with relatively low tensile strength, such as the transition from tensile to shear failure, have to be considered and reflected in the adopted failure criteria. Rock failure in tension takes place at low confinement around excavations due to tensile failure in heterogeneous rocks (Diederichs, 2007). The prospect of tensile-dominant brittle failure diminishes as the confinement increases away from the excavation boundary. Therefore, it is expected that the transition from tensile to shear mode in the failure mechanism occurs as the confinement level changes and conditions for tensile failure are prevented or strongly diminished. In addition, many uncertainties remain because of the highly anisotropic characteristics of coal seams associated with geologic structure and the mining-induced spatial redistribution of stress in coal pillars. Thus, to prevent fatalities in underground coal mining, continuous effort is required to better understand the catastrophic failure mechanisms in coal mines."

Jan 1, 2018

By Stephen C. Tadolini, Anand Bhagwat

"Intrinsic supports installed in mines with limited seam or mining heights have always proven to be difficult when the required support length is longer. To date, three solutions have been used to ensure that the correct length bolt is installed to achieve the required anchorage or to maintain immediate roof material: combination bolts, milled-notch bolts, and hot-notch bolts. The combination bolts use threaded sections and couples, which can be screwed together to form the required length. The primary bolt of choice, to minimize costs and achieve the desired support, has been milled- or hot-notch rebar bolts. These notches can be placed at strategic (required) locations along the bolt axis, so the bolt can be successfully bent and straightened during the installation process. However, these notches can reduce the strength of the bolting system; special reductions are detailed in ASTM F432-13 (2013). Technical advances in steel rolling and forged heading processes have led to the development of a deformed rebar bolt termed, the BORE bolt, which stands for Bendable Oval REbar. The BORE bolt can be bent at any location and straightened during the bolt installation cycle. This support exceeds the minimum designed capacities described in ASTM F432-13 and meets the yield and tensile strength requirements of a full diameter no. 6 rebar bolt. The profile and shape also reduces the resin annulus during the installation process to enhance mixing and shreds the cartridge film to help eliminate glove fingering.INTRODUCTIONThe height or depth of competent strata material, in the immediate mine roof, can be greater than the developed mine openings heights. This is especially true in coal mine openings where 50% of the minable seams in the US are less than 50 inches (132 cm). When this scenario occurs, it can be difficult to insert steel bolts in a solid single length. Figure 1 illustrates what difficulties can be experienced when a roof bolter operator is working and supporting a thin seam coal mine. There are three viable options currently available. Lengths of the bolt can be threaded and a steel coupler can be used between the sections to reach the required depth of anchorage. These systems, called “coupled bolts,” usually require a larger diameter hole to facilitate the larger coupler diameter and can be anchored with an expansion shell, resin, or a combination of both. To ensure that the coupler will develop the tensile capacity of the bolting system, required by ASTM 432-13 Standard Specifications for Roof and Rock Bolts and Accessories (2013), the threaded bars should be completely rotated and engaged into the coupler. Premature failures can occur when the threads “strip out” if not inserted completely."

Jan 1, 2018

By Filippo Vallianatos, Michael Verigakis, Vasili Saltas, Kosta Kaklis, Stelio Mavrigiannakis, Zach Agioutantis

"Development of failure in brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emissions. This paper presents results from acoustic emission measurements obtained during three-point bending tests on Nestos marble under laboratory conditions. Acoustic emission activity was monitored using piezoelectric acoustic emission sensors, and the potential for accurate prediction of rock damage based on acoustic emission data was investigated. Damage localization determined based on acoustic emissions evident in the critically stressed region in the form of scattered events or stresses below and close to the strength of the material.IntroductionNon-destructive testing (NDT) has gained popularity in recent years due to commonly available sensors, devices and software tools. Although the majority of such testing still occurs under laboratory conditions, the ultimate goal is a direct field application, especially in the case of underground mining. NDT, if properly applied, may be used to directly provide material property values and characteristics that can be used to determine rock mass or soil mass behavior, with a direct impact to mine design (Iannacchione et al., 2003).AE is generally defined as the high-frequency transient elastic waves originating from the sudden release of energy at localized points within a loaded material (Nomikos et al., 2010). The field of Acoustic Emissions (AE) due to compressive, tensile or other types of loads is a promising research area due to its wide application. The accurate identification of failure precursors stemming from AE is still an open research topic (Lei et al., 2004).Rock is a typically inhomogeneous and anisotropic material that contains several natural defects with various scales such as grain boundaries, microcracks, pores and joint inclusions (Jing, 2003; Read, 2004). Damage induced by microcracks is an essential mechanism in many brittle rocks subjected to stresses. An applied stress induces microcrack growth and damage is distributed uniformly throughout the sample. Finally, the microcracks coalesce and the damage is localized to a shear band, leading to strain softening and macro-scoping failure (Dragon et al, 2000)."

Jan 1, 2015

By Ashok Kumar, Rajendra Singh, Arun Kr. Singh

"A simple analysis of site conditions and capability of the mechanised depillaring (MD) operation found that this approach is suitable to address the strata control issues in coal pillar extraction under the weak and laminated roof strata of Pinoura mine. A continuous miner (CM) with a ram car and quad-bolter are used for single pass pillar extraction (first time in the country) to achieve a relatively faster rate of depillaring. A total of eleven panels are depillared under weak and laminated roof strata with a fair-rated rock mass rating (CMRI-RMR). It is observed that there is less influence of roof caving over the surrounding mining structures, especially those located out-bye the side of the extraction line. An attempt is made to quantify the goaf influence by field studies, which show that the maximum value of mining-induced vertical stress is 2.83 MPa, and the range of influence remains limited to around 15 m from the line of extraction. However, the problem of roof instability under the influenced zone was experienced due to the inherent nature of the weak roof strata. The chances of trapping of CM under weak and laminated roof strata of the panels is reduced by deriving a threshold value of the roof dilation, on the basis of a simple statistical analysis of the obtained field results. However, few incidences of roof collapses are observed over the CM due to the unpredictable behaviour of the geological structures below the freshly exposed strata, which also caused an obstruction in the speed of the extraction. This paper presents results of the field and laboratory studies, conducted for a successful application of the single-pass pillar extraction method, along with details of some CM trapping cases below the weak and laminated roof strata.INTRODUCTIONCoal is the major natural resource to meet the growing energy demand of India and this trend is likely to continue for the next 20 years in the country. A major part of the Indian coal belongs to Gondwana period, and only a small part belongs to Tertiary period. The estimated coal reserve in India per the exploration work done up to a maximum depth of 1200 m is 315.149 billion tonnes, including the proven coal reserve of 143.058 billion tonnes (CMPDIL, 2017). Even in presence of these huge resources of coal in India, there is a considerable gap between production and the users’ demand of coal. Most of the mines are operated by conventional semi-mechanised Bord and Pillar (B&P) methods; thus, the mining industry has gained a considerable amount of operational experience (Singh, Singh, and Dhar, 1996) and knowledge about strata behaviour (Singh and Singh, 1999; Singh et al., 2008) using this method. However, recently, the Indian coal mining industry has introduced some mechanised depillaring (MD) operations, where the level of production is relatively high. This approach of depillaring improves safety along with the production and productivity of a depillaring operation (Ram et al., 2017). The nature of the overlying strata of these depillaring operations vary greatly: from highly laminated, weak, and easily caveable to massive, strong, and difficult to cave."

Jan 1, 2018

By Michael Gauna, Christopher Mark

"Underground coal mining in the U.S. is conducted in numerous regions where previous workings exist above and/or below an actively mined seam. Miners know that overlying or underlying fully extracted coal areas, also known as gob regions, can result in abutment stresses that affect the active mining. If there was no full extraction, and the past mining consists entirely of intact pillars, the stresses on the active seam are usually minimal. However, experience has shown that in some situations there has been sufficient yielding in overlying or underlying pillar systems to cause stress transfer to the adjoining larger pillars or barriers, which in turn, transfer significant stresses onto the workings of the active seam. In other words, the overlying or underlying pillar system behaves as a ""pseudo gob."" The presence of a pseudo gob is often unexpected, and the consequences can be severe. This paper presents several case histories, summarized briefly below, that illustrate pseudo gob phenomenon:• Pillar rib degradation at a West Virginia mine at 1100-foot (335 m) depth that contributed to a rib roll fatality.• Pillar rib deterioration at a Western Kentucky mine at 570-foot (175 m) depth that required pillar size adjustment and installation of supplemental bolting.• Roof deterioration at an eastern Kentucky mine at 1300- foot ( 400 m) depth that stopped mine advance and required redirecting the section development.• Coal burst on development at an eastern Kentucky mine at 1700-foot (520 m) depth that had no nearby pillar recovery.• Coal burst on development at a West Virginia mine at the relatively shallow depth of 1100 feet (335 m) that also had no nearby pillar recovery.The paper provides guidance so that when an operation encounters a potential pseudo gob stress interaction the hazard can be mitigated based on an understanding of the mechanism encountered.INTRODUCTIONThe U.S. underground coal mining industry has conducted mining in multiple seam environments, where the active mining has underlying or overlying old workings at varying interburden distances, throughout its history. Often no serious consequences arise from the multiple seam mining. However, sometime mines have been confronted with hazards from underlying or overlying workings that include localized roof and rib failure, pillar system failures through propagating roof falls and floor heave, and also pillar bursts."

Jan 1, 2016

By Takashi Sasaoka, Pinyo Meechumna, Hiroshi Takamoto, Pipat Laowattanabandit, Akihiro Hamanaka, Kikuo Matsui, Hideki Shimada

"The EGAT Mae Moh coal mine is an open-cut coal mine in Thailand that produces about 16 million tons of lignite annually to generate 2,400 mw of electricity. The surface mine pit covers is 4 by 7 km (2 by 4 mi) with mining depths ranging up to 260 m (853 ft). A large pit slope is formed with the progression of the mining operation. However, massive slides in the pit slope often occur due to weak planes, such as faults and bedding, and the weak mechanical properties of the rocks. Hence, a 200-300 m (656-984 ft) boundaries of rock block including coal seams is left in front of faults in order to prevent slides and maintain the stability of the pit slope. As a result, there are significant coal reserves beneath the abandoned area along the pit slope.The focus of this paper is on the applicability and design of the highwall mining for recovering much of the coal left along the pit slope.INTRODUCTIONThe EGAT Mae Moh Mine is located about 630 km (186 mi) north of Bangkok, Thailand (see Figure 1). It is the largest coal mine in Thailand and produces about 16 million tons of lignite annually from open-cut mining (70 % of the total coal produced in Thailand). The coal is used to generate 2,400 mw of electricity at the Mae Moh power plant, which is situated near the mining area. The mine pit covers an area of 4 by 7 km (2 by 4 mi) at varied depths up to 260 m (583 ft) as shown in Figure 2. As the mining operation progresses, the pit becomes to be deeper and deeper. In the year 2035, the bottom of the pit is expected to reach depths of 500-600 m and the open-cut mining operation will be finished. The possibility of an underground mining operation is being investigated.A large pit slope is formed with the progression of open-cut mining. However, massive slides often occur in the pit slope along weak planes including faults and bedding planes due to the weak nature of the rock. As a result, 200 to 300 m (656 to 984 ft) boundaries of rock including coal seams are left in place in front of faults to prevent slides and to maintain the stability of pit slope. Moreover, the dip of the slope is less than that of the planned design. As a result, there significant coal reserves abandoned along the pit slope."

Jan 1, 2010

By K. Y. Haramy

This report describes the development of an equation for predicting the crushing strength of 2-inch (5.08-cm) cubes of coal from point load tests made on irregular lumps. The equation was developed from point load and cube crushing strength tests performed on coal samples from six Western U.S. coal seams. For the six coals tested, the means of the point loads [(P)] in kg at failure were approximately proportional to the deans it the distances [(V)], in cm between the platen points for lumps in the 2 to 10-cm size ramie. This finding is at variance with results of tests on other rock types reported in the literature, which tend to show that P is proportional to D2 , as is normally assumed (in theoretical grounds. [The average crushing strength of a 2-inch (5.08 cu) cane for each type of coal was plotted against the log 10 of the slope of the P vs D plots, AP/AID . A straight line was fitted to these plots. The equation of the fitted line is cr = 1.72 x 10 4 10gK -2.10 x 10 4, where Cr is re estimated 2-inch (5. 08-cu,) cube crushing strength of the coal in psi and K is the scope of the straight line fitted to a plat of P v D for the same coal.]

Jan 1, 1982

By Madan M. Singh

Currently longwall coal mining operations require at least 3 entries on both the headgate and tailgate ends because of ventilation and safety requirements. Driving of entries, however, is a slow and costly process. Often it is a bottleneck in longwall development. Single entry gateroads are permitted in Europe and commonly used there. This study entailed ascertaining the feasibility of using a single entry in the United States, without sacrificing safety. This design was conducted for an operating longwall mine in the Black Warrior coalfield in Alabama, An understanding of rock loads and consequent support requirements is prerequisite to the design and successful demonstration of the single entry system. Efforts in this regard were directed at determining center (packwall) support requirements for various single entry widths. Critical to the analysis is knowledge of the nature and extent of side abutment stresses. In collecting geological and geotechnical information relating to the stability of the single entry, Engineers International, Inc. engaged in the following: ? Examination and inspection of the single entry demonstration at the Sunnyside Mine in Utah. Severe ground conditions and unacceptable levels of entry closure were experienced there prior to the tailgate phase. Observations at the mine presented a clear perspective on possible ground control problems for future single entry designs. ? Reconnaissance inspection and classification of roof rock conditions. ? Detailed mapping of roof structure in the areas to be instrumented. ? Installation of convergence stations in the longwall headgate to monitor roof movement during mining. ? Measurement of side abutment stresses in a yield pillar in the area of the convergence measurements. ? Measurements of coal fracture distribution by the detail area method. ? Rock properties determinations of roof, coal, and floor rocks; samples obtained by core drilling. ? Determination of floor bearing capacity by in-situ testing.

Jan 1, 1982

By R. Karl Zipf

Stress analysis programs such as MULSIM/NL, LAMODEL, MinSim 2000, and EXAMINE TAB are used in the mining industry to analyze stresses and displacements in coal mines, platinum mines, gold reefs, and tabular-type deposits. These relatively simple numerical models can efficiently simulate yielding and failure of a rock mass near a mine opening and subsequent stress transfer. The main input parameters for these models are a family of nonlinear stress-strain curves for the in-seam material. For many applications of these models, such as in coal mining, extensive field measurements and observations exist from which to estimate these stress-strain curves and calibrate the numerical models; however, such measurements are lacking for other types of mines. A three-step method is presented to determine nonlinear stress¬strain curves for boundary-element (BE) programs used in many mining applications. The method requires a suite of laboratory¬scale strength tests at various confining pressures. Note that this analysis uses laboratory-measured strength and modulus properties of alluvium. The dependency of the mechanical properties of alluvium on scale has not been established, although more tests are being planned. However, it is believed that the relative comparisons performed are valuable regardless of scale effects. First, the FLAC2D computer program is used to model the laboratory tests and determine cohesion (c) and friction angle (0) for a Mohr-Coulomb material model. Second, another FLAC2D model uses the c and 0 values to calculate vertical stress and strain around a single opening in the rock mass. This model calculates the stress-strain path of points at various distances from the opening boundary. Finally, based on these stress-strain paths, the stress-strain curves needed for a BE analysis are derived. This three-step method was demonstrated in a large, flat-lying underground test facility in very weak rock. The BE analyses agreed well with observations of failure at the test facility and provided a basis for evaluating the behavior of alternative layouts.

Jan 1, 2003

By Herryal Z. Anwar

In recent years, many both mining and civil engineering projects are developed in the area which may be categorized unstable, such as fractured and jointed rocks or "water sensitive rocks". For example, in underground coal mining, in which the floor strata normally consist of weak shale, the problem that may arise in roadway is floor heaving. In this case cement injection in borehole appears to be an economically effective method in order to make roadway more stable. This method may be also applied in the projects in which the constructions are laid on fractured and jointed rock. Nevertheless, the successful cement injection is much dependent on the water-cement ratio and the injection technique. This paper discusses the application of cement grouting to improve the ground stability in longwall mining, the laboratory experimentation of cement grouting and the method of floor injection.

Jan 1, 2000

By James H. Fletcher

Much data and experience has been accumulated, especially in the last 5 or 6 years, to show that modern roof trusses, both of the Birmingham type and the bolt-and-channel type, perform well to support certain kinds of coal mine roof. For some time now, roof trusses of the Birmingham type have been effective in controlling thick shale roof, which tends to sag and fall over a period of about eight weeks. These falls leave dame-shaped cavities which are reminiscent of the pressure arch theory of roof failure. Lately there has also been significant interest in supporting some longwall panel entries with channel and-bolt systems. These systems utilize two vertical center bolts and two 45 degree end bolts through a heavy gage channel. Both types of truss systems have long bolts, which require 8 to 9 Foot holes on a 45 degree angle, and point anchor resin bolts, which utilize a 2 or 3 foot rebar grouted in a thin annulus of polyester resin. Consequently, many coal mining people have been impressed with the very good anchorage characteristics and tensionability features of these systems. However, the same people have been proportionately dismayed to find that very often the most difficult part of introducing a trussing program to a mine is deciding on the machine specifications which determine the truss installation rate. The purpose of this paper is to indicate aspects of truss bolter design and use which are not shared by roofbolter design and its vertical pattern of bolts. Of the two types of drill units conventionally accepted, arm feed units and mast units, only one 15 judged suitable. While the arm feed unit offers low collapsed height with a long drill feed length, a great attribute for low seam use, it has not lent itself to being rolled over to a 45 degree angle. One reason for this inability is that many items which are conventionally mounted on the arm feed unit cannot be allowed to rotate with the unit. These items include drilling controls, tool Cray, lights, canopy or safety post, panic bar, and stab jack(s). If these items were independent of the arm feed drilling unit, they would have to be out of the way of the boom arms, links, and feed cylinder as the unit moves. To ease this problem, the arm feed unit can be made very short and even mounted across the machine. However, the resulting short feed length is undesirable. Several short drill steel extensions are required to drill a relatively long hole, making this procedure time-consuming as well as increasing the likelihood of drilling a crooked hole. Another reason for abandonment of the arm feed unit is that a very stiff drill guide (or "centralizer") is required to start the hole. Using the mast tilt control, the roof bolter operator will force the bit against the roof and will rotate the bit without feeding to cut sideways into the roof to form a hole face. Omission of this step often results in the skidding of the bit toward the rib. To date, no such sturdy drill guide has been designed for an arm feed drill unit. The mast style unit has been the most successful design for angle drilling. It is easily rolled over to a 45 degree angle or to 90 degrees and, on single head models, often has been made to roll through 180 degrees. The mast style is compact, in plain view and is easily fitted with heavy duty manual or hydraulically operated drill guides. It can be mounted directly on a machine chassis, or onto a swinging boom, or onto a slider arrangement in a broad variety of configurations which will accommodate controls, tool trays, stab and roof support jacks, lights, canopies, and panic bars. The only significant difficulty with mast drilling units occurs in lower mining heights when desirable low mast height is accompanied by undesirable short feed length. While the lead extension in the drill string can be as large as the mining height will allow, a short feed length will permit only the use of shorter middle extensions and drivers, and perhaps will. require the use of two different length wrenches. The associated problem is the relatively short distance from the drillhead to the drillguide. This distance is further shortened when raising the drillhead to get the bit to the roof. One might keep longer lead extensions on the machine; drill a vertical pocket for the bit, starting the angle hole at this pocket; or modify the mast. If the mast is made so that the distance from the drillhead to the drillguide remains constant while the mast extends, after which the drillhead travels up the mast, the shorter mast can be made to start the hole more easily. However, short teed length will still result in longer drilling time. It is obvious from these points that a proper choice of mast is simply the highest one which will be moved easily in the given mining height. This, of course, goes against the grain of most experienced coal mining people who realize that a relatively small decrease in mining height can create a problem correctable only with the purchase and installation of shorter masts. If the mining

Jan 1, 1982

By Dennis R. Dolinar

Roof falls, even of supported roof, still constitute a major hazard in underground mines. However, associated with any fall or instability is a pattern of roof movement. Therefore, the National Institute for Occupational Safety and Health, Pittsburgh Research Center, has been conducting research into the development of practical extensometer systems that can be used to measure this displacement and to determine if a roof fall may occur, thus increasing miner safety. For extensometers to be used in the detection of potential roof falls in coal mines, an understanding of the movement and displacement rates that will lead to roof failure is required. Because roof falls are unpredictable, it is often very difficult to obtain this information. Therefore, in a coal mine in the lower Kittanning seam in Pennsylvania, a room widening experiment was conducted to intentionally cause a roof fall while roof movement data was collected. In this experiment, two adjacent rooms 5.5 m (18 ft) wide supported with 1.5 m (5 ft) resin-grouted bolts and instrumented with extensometers were widened to 9 m (30 ft). In one of the rooms, 3.6 m (12 ft) cable bolts were installed as supplemental support. In the room with no additional support, a roof fall occurred 72 h after the room was widened. Much of the 3 cm (1.2 in) of movement in this room resulted from the instantaneous and transient response to mining with displacement rates up to 7.6 cm/d (3 in/d) being measured. This was followed by a steady state displacement phase with a rate of 0.43 cm/d (0.17 in/d). Just 12 h before the fall, movement deeper in the roof caused the rate to accelerate to 1.1 cm/d (0.44 in/d). The roof in the room with the supplemental cable support did not fall even though the total roof deformation was over 7.6 cm (3 in). Significant roof movements and strains in the cabled room were detected to a depth of 3 m (10 ft) while maximum cable loads were over 180 kN (40,000 lbf). Essentially, the roof was in postfailure with the cables maintaining the material, while roof stability was indicated by a steady state displacement rate of only 0.025 cm/d (0.01 in/d) at the end of the test.

Jan 1, 1997

By Kirk W. McCabe

RokLok binder is a two-component polyurethane system consisting of a polymeric isocyanate (Component A) and a polyol resin (Component B). The two chemicals are mixed and injected into the mine rock under pressure. The initial low viscosity enables the RokLok material to permeate many of the smallest fractures. About 3-5 minutes after injection, the reacting material begins to expand in the fractures; the reaction is completed after 1-2 hours, and the material sets to a mechanically strong binder which adheres tightly to the rock. The consolidated rack provides resistance to any further f movement w thin the strata. The RokLok binder injection system is fast becoming an important engineering tool for coping with adverse underground geological conditions. Two case histories discussed in this paper, a longwall mining problem and an entry-roof problem, demonstrate the effectiveness of using the RokLok binder system to help solve complex ground-control problems in a cost-effective manner.

Jan 1, 1981