Search Documents

By D. C. Oldroyd

Western Coalfield?s Tandsi Mine, has difficult support conditions and a history of roof falls, resulting in mining roadway widths and depths being restricted for safety reasons. Rock Mechanics Technology Ltd was contracted to improve the support system to allow the introduction of mechanised room and pillar mining. The introduction of a ?world best practice? bolting system resulted in major improvement in roof stability, allowing mining restrictions to be lifted and Tandsi to become one of the first two Indian mines to introduce a mechanised continuous miner system. The first year?s production was some six times previous levels. Because of the improvement in the support system, RMT was also able to design a partial de-pillaring system suitable for the Tandsi continuous miner districts together with the introduction of monitoring procedures and guidelines to maintain safe roof support conditions. In situ monitoring of pillar and roof behaviour was undertaken as the first pillars were extracted to confirm the design. De-pillaring operations have also proved highly productive, and to date there has been no associated support related major injury or dangerous occurrence. The success of this project demonstrates the potential for mechanised room and pillar mining in India.

Jan 1, 2006

By Jeff Whyatt

Deformation of underground salt, trona and potash mines is generally time dependent, providing for gradual adjustment of strata to mining induced stresses. Time dependence can allow for higher extraction ratios provided eventual failure can be tolerated. However, this eventual failure can be violent if creep deformation can shift stress and potential energy to strong, brittle geologic units. The mine failure case studies reviewed here illustrate this process. Yield pillars and defects in bridging strata figure prominently in these cases. Yield pillars provide local and temporary support to the roof, temporarily delaying the cave; and allowing extraction ratios and overburden spans to increase beyond the long term capacity of overlying strata. Defects (faults, voids, thinning) of strong overburden strata reduce the critical span, sometimes to less than panel width. Analyses of many of these cases have focused on a cascading pillar failure mechanism, but recent work and this review point to failure of strong overburden strata as the essential element. The suddenness of failure and attendant seismic events pose hazards to miners and, in some cases, to those on the surface. Characterizing these failures is a first step towards recognizing and managing the risk of catastrophic collapse in underground mines.

Jan 1, 2008

By John C. Stankus

The 3 Right pre-developed longwall recovery entry at Signal Peak Energy?s Bull Mountain No. 1 mine was designed to prevent the abnormal roof conditions encountered in its firsttwo recovery entries. Unique geological conditions, such as the low overburden of approximately 200 feet and prevalence of roof joints in the sandstone overburden, contribute significantly to abnormal conditions during longwall recovery. The 3 Right recovery entry was 42 ft wide and developed in two stages, 21 ft wide in each stage. The entry roof was reinforced with steel wire mesh/non-metallic recovery mesh, high-capacity primary bolts and cables. After all the support was installed, the entry was completely backfilled with specially designed, cuttable, low-density cement. The roof bolting and backfilldesign were based on current practices in this mine and successful practices in other mines in the U.S. and Australia. Extensive instrumentation was installed in the recovery entry for continuous monitoring of load transfer as the face approached and cut into the recovery entries. The support design was completely successful with a smooth cut-through on January 29 and 30, 2014, significantly reducing the shield recovery time and meanwhile protecting the safety of miners. Based on actual observance and practices of the 3 Right recovery entry, recommendations on improvement and modificationsof roof support for 4 Right recovery room are presented.

Jan 1, 2014

By Stanislaw Prusek

In this paper, results of underground measurements conducted in gateroad 3, seam 615, at the Bobrek-Centrum Colliery are presented. The Bobrek-Centrum Colliery is owned by Kompania Weglowa, the largest hard coal producer in the European Union. Yearly production at the colliery, including the longwall system, is about 2 million tonnes. Gateroad 3 was used simultaneously for two retreating longwall faces, No. 1 and No. 2. This working was also employed for the conveyor belt for longwall No. 1 and served as a transport road for longwall No. 2. Approximately 100 meters of gateroad 3 was maintained behind the face of longwall No.1 and then, with the progress of longwall No. 2, it was liquidated. In this paper, we present the results of underground measurements of vertical and horizontal convergence in gateroad 3. We also present the results of numerical modeling for deformation of the working. Finally, we analyse the accuracy of the numerical calculations by comparing them to the actual deformation of gateroad 3.

Jan 1, 2014

By Hongwei Wang, Hao Zhang, Panshi Xie, Xicai Gao, Youfu Zeng, Yongping Wu, Juan Dou

"The reserves and production of steep coal seams account for approximately 15–20% of the total coal reserves in China and 5–10% of the total annual coal production. More than 50% of the coal is high-quality coking coal and anthracite, based on reviews of steeply dipping seam mining at home and abroad. Fully mechanized mining is considered to be the only safe and efficient way to mine steeply dipping seams. To achieve fully mechanized longwall mining, four issues must be addressed: ground control, equipment stability control, development of complete sets of equipment, and safety. Engineers must focus on many factors of steeply dipping seams, such as the following: stope, roof asymmetrical structure, stability control, “roof-support-floor” system dynamics control, multi-region big-range ground control technology, adjustment false-inclined method, nonlinear layout method, control technology, safety, and development of complete sets of equipment. Engineering practices in China introduce these concepts systematically: fully mechanized mining of medium-thickness seams, fully mechanized high mining of thick seams, fully mechanized caving of extremely thick coal seams, fully mechanized mining of coal seams, roadway support, and so on. A steeply dipping seam is one with a dipping angle in the range of 35°–55°. This accounts for about 15%–20% of the total coal reserves in China. Because of the environment, more than 50% of the coal is high-quality coking coal and anthracite. Mining steeply dipping coal seams is known to be difficult in the mining industry. After the study conducted by Germany, Poland, and the former Soviet Union in the1970s–1980s, the fully mechanized mining basic theory, core technology, and key equipment research have not been a breakthrough. Being fully mechanized is a technological “restricted area” at home and abroad. A lot of steeply dipping seams are excavated by non-mechanized mining, leading to harsh job environments, labor-intensive work, low productivity, frequent accidents, and significant security hazards. The death rate per million tons of coal reaches above 4.0 and is far higher than slow inclined coal seam mining. Making steeply dipping seam mining safe and efficient is a global problem that urgently needs to be solved. Fully mechanized mining is the only way to overcome these problems (Wu, 2003; Wu, Yun, and Zhou, 2000). The realization of fully mechanized coal seam mining must solve four problems. (1) Examine the disaster mechanisms induced by overburden “key strata” mutation under steeply dipping seams as a basis for ground control methods and technical problems. (2) Work to solve the dynamic stability control technical problems of thousands of tons of complete sets of equipment under the steeply dipping seams. (3) Complete sets of equipment must be developed to adapt to the conditions of steeply dipping seams. (4) Overcome the steeply dipping face safety protection technical problems. In the last two decades, a number of universities, research institutes, and enterprises have realized the importance of safety and efficiency of steeply dipping seams in China. This is especially true of Professor Wu Yongping’s “hard coal layer safe and efficient mining” team at Xi’an University of Science and Technology. After several generations’ research and technical work, the team puts forward China’s longwall mechanized coal mining of steeply dipping seams strata control basic theory. This paper puts forth a series of key technologies of fully mechanized mining, develops and manufactures complete sets of equipment by independent innovation, and establishes a perfect face security system, forming a theory and technology system for fully mechanized longwall mining of steeply dipping seams."

Jan 1, 2017

By Gregory M. Molinda

Historically, miners have known that roof rocks originally composed largely of mud were prone to slaking and deterioration when exposed to water and humidity. Mine Safety and Health Administration (MSHA) records show that roof falls are more prevalent in summer and late fall. Field data shows higher bolt loads and increased room convergence in summer months. Laboratory data shows that very high internal pressures can be generated by significant strain in clay-rich rocks. In summer months humidities ranging from 70-100% can cause rock strains up to 7%, and rock pressures of 14,000 psi, more than enough to break apart weak shales and fireclays. The National Institute for Occupational Safety and Health (NIOSH) conducted wet/dry immersion cycling of clay-rich rocks. Results show that the rocks most prone to moisture deterioration are fireclays, claystones, and sandy shales, all of which have disrupted bedding. Of the 7 mudrocks with a weatherability index above 40%, none showed consistent fissile bedding. Black shales were surprisingly resistant to wet/dry water immersion cycling. Generally, mixed rocks (stackrock), typically susceptible to horizontal stress, are more resistant to moisture-deterioration. Mine roof rocks tested with moisture sensitivity ratings above 50% (moisture sensitivity index ranges from 0-100 with 100 being the most sensitive) all came from mines which had roof skin problems and used supplemental support to prevent rock fall injuries. Some roof rocks showed lateral variability of moisture sensitivity values, suggesting the need for more samples in highly varying roof rocks. Swelling strains of mudrocks showed that almost all of the swelling occurred in the first 24 hours after immersion. The wet/dry immersion cycling test used provides an accurate, quick and inexpensive method for estimating the moisture sensitivity of mudrocks. With this information mine operators can begin to anticipate hazardous ground and prepare appropriate roof support.

Jan 1, 2006

By James D. Pile

Following a methane ignition in the active panel of a longwall coal mine, the affected panel was successfully isolated from the rest of the mine. This allowed for the remainder of the mine to return to regular operation, and the conventional practice of having to seal the portals for a defined period of time was avoided. After the affected panel had remained isolated for a regulatorily mandated period of time, it was reopened in preparation for longwall face recovery. As it was not possible to move the shields from their existing positions to facilitate the installation of recovery mesh prior to shield recovery, alternative methods had to be considered to stabilise the gob and minimise flushing, thereby improving operator safety. The objective of the paper is to show how stabilisation of the gob was achieved during shield recovery, thereby minimising flushing, and improving operator safety. Phenolic foam was chosen and injected from between the shields into the gob behind them. This is a technique that has been used to great effect in Germany, to a lesser extent in Australia, but not knowingly to date in the United States. To ensure nothing was left to chance and thereby guarantee the project?s success, the procedure used in Germany was employed, down to the application of equivalent hardware, which was sourced through the North American supplier.

Jan 1, 2013

By Xinzhi Du

Coal strength is essential to analyze the stability of excavation in coal. It is well known that the strength of coal tested in the laboratory is size-dependent, and different size specimens will produce different strength. Many researchers have studied the uniaxial compressive strength of coal and developed a number of empirical formulae to describe the strength variation with specimen size. According to a comprehensive data base that includes most of the laboratory testing results in the last century in U.S. and South Africa, the size effect and shape effect were reviewed and analyzed. The results show that different groups of coal specimens have different size effect on the compressive strength. The critical size for the size effect from the data analysis is about 40 in. The shape (W/H) of the specimens has a large effect on the compressive strength. The pillar strength formulae obtained from the data regression have a larger strength than all other existing formulae. Most pillar strength formulae have their own place in coal pillar design field. In order to analyze the difference of those formulae, twelve design formulae have been selected to compare using the Pittsburgh coal seam. The intention is not to determine the preferred formulae, but only to compare them, and see how do the W/H ratio, width, length, angle, cross-section area and effective width affect different pillar strength formulae?

Jan 1, 2008

By Khaled Morsy

In the past two decades, high horizontal stress has been attributed to many ground control failures. Most believe that entries parallel to the high horizontal stress is most stable and least stable when perpendicular to it. Entry stability decreases when the angle between entry and horizontal stress increases. The concept of 2-D stress shadow has been proposed for selection of proper panel orientation, often with mixed results. In this paper 3-D finite element modeling was used to analyze the stress distribution at the headgate of a longwall panel subject to high horizontal stress at various angles. It was found that the directions of induced principal stresses at the longwall face and headgate are independent of the panel orientation. The induced principal stresses concentrate or relief at headgate based on the panel orientation with respect to the directions of the induced principal stresses. Accordingly, only 3-D modeling can fully account for the effects of high horizontal stress on panel orientation. The paper illustrates in details the changes in orientation and magnitude of all three principal stresses around the longwall face and T-junction area as panel orientation changes.

Jan 1, 2006

By Dae S. Young

A new probabilistic approach is introduced for slope stability analysis including adverse impact of blasting vibrations. The material strength within a slope area is randomized in this bi-variate joint probability analysis. To be a completely general engineering method, the new probabilistic approach employs the random variable transformation technique; the Hermite model of the Gaussian transformation function, which transforms the experimental histogram of strength values to the standard Gaussian variables. Because bi-normal joint probability is analyzed on the true probability region projected on the Gaussian transformed variable plane, it is an exact solution of slope stability based on the available sample data. Details of the probabilistic approach are given through a case study on an iron ore mine. The case study demonstrated that the probabilistic approach is effective and simple enough to be a routine engineering practice for slope analysis.

Jan 1, 1984

By Stephen Tadolini

The annulus thickness, for bolts anchored with resin, effects the systems final performance. Recent developments in the manufacturing process of these support systems, coupled with improved resin characteristics, have resulted in design changes to minimize hole drilling requirements and reduce resin usage. The amount of resin to properly anchor a 7/8-SRD (Special Rolled Dimensions) bolt in a 25.4 mm (1-in) diameter hole has been reduced to about one-third of the resin required to anchor a No. 7 rebar bolt (22 mm (7/8-in) diameter) traditionally installed in a 35 mm (1-3/8-in) diameter borehole. More importantly, laboratory and field tests, indicate that higher bond strengths and anchorage capacities can be realized with reduced annulus systems. These factors have a direct bearing on reinforcement efficiency and subsequent roof support performance.

Jan 1, 1998

By Nikzad Oraee, Parham Khajehpour, Kazam Oraee, Arash Goodarzi

"Rock fall related accidents continue to occur in coal mines, although artificial support mechanisms have been used extensively [1]. Roof stability is primarily determined in many underground mines by a limited number of methods that often resort to subjective criteria. It is argued in this paper that stability conditions of mine roof strata, as a key factor in coal mines, must be determined by a survey which proactively investigates fundamental aspects.Failure of rock around the opening happens as a result of both high rock stress conditions and the presence of structural discontinuities. The properties of such discontinuities affect the engineering behavior of rock masses causing wedges or blocks to fall from the roof or slide out of the walls [2]. A practical rule based approach to assess the risk of a roof fall is proposed in the paper. The method is based on the analysis of structural data and the geometry and stability of wedges in underground coal mines. In this regard, an accident causing a huge collapse in a coal mine leading to four fatalities is illustrated by way of a case study. A comprehensive investigation of the hanging wall has been gathered through a systematic collection of evidence. The investigation results are then analyzed, and an interpretation of the evidence gathered is provided.For this purpose, horizontal and vertical profiles are prepared by geophysical methods to define the falling zone and its boundaries. The collapse is then modeled by the use of sophisticated computer programs in order to identify the causes of the accident and hence recommend corrective actions to prevent or minimize reoccurrence probability of similar accidents. The corrective measures are placed into a rule-based framework for the benefit of the mine operators.INTRODUCTIONRoof rock failure has generally been the most dangerous hazard for underground coal mining in the world. Failure of rock around shallow coal mine openings often results from loosening of blocks of rock on the weakened planes under the influence of gravity. This condition is not typically seen in large deep coal mines. There are a number of differences between large mechanized mines and small mines related to ground control [3]."

Jan 1, 2015

By Peter Cain

The Phalen Colliery in the Sydney Coalfield is extracting the 2.6 m thick Phalen Seam by the panel and pillar longwall retreat method. Abandoned workings occur in the Harbour Seam 140 m above and overlie the Phalen Colliery. Parts of these workings are flooded, and pose a number of questions relating to the safety of operations in the Phalen Colliery. In order to answer some of these questions, the Cape Breton Development Corporation re-entered an abandoned non-flooded roadway in the Harbour Seam directly above a retreat panel in the Phalen Seam to allow the deployment of instrumentation to monitor interaction effects. Fifteen monitoring sites, each 4.5 m apart, were installed in the 5 East Top Level after it had been re-entered and made safe. Each monitoring station included a vibrating wire strain gauge piezometer and a potentiometric extensometer. The piezometers were attached to rock bolts anchored into the floor and were connected by a flexible standpipe to a constant head apparatus. Variations in hydraulic head due to subsidence were monitored to determine subsidence. The extensometer units were mounted on rock bolts anchored into the floor of the roadway beneath the ribside and monitored ground strains. The instrumentation was capable of remote reading in the event that access to the site was restricted after installation. This paper describes the instrumentation, the installation programme, and some preliminary results.

Jan 1, 1993

By Brian Clifford

A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980's. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support. A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional 'gun barrel' and shear tests to provide full assessment of bolt and tendon performance. This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.

Jan 1, 2001

By Morgan M. Sears

In recent years, one large mine collapse and numerous smaller bump events have resulted in a renewed interest in coal bump research. With numerical modeling, the Energy Release Rate (ERR) calculation quantifies the ?release? of the gravitational potential energy of the rock mass into the environment as mining progresses. This release of energy can occur passively in the form of heat and sound or dynamically in the form of pillar bumps or rock bursts. From the initial application of a calculated energy release rate in the deep hard-rock mines of South Africa, the ERR was found to have a significant correlation with the risk or potential of damaging coal bumps or rock bursts. In this research, an ERR calculation is incorporated into the modern LaModel program to facilitate the analysis for potential coal bumps. Initially the ERR calculations in LaModel are verified using a case study of cut sequences originally modeled using the MULSIM/NL program. Then, the ERR calculations are applied to a bump-prone mine in Southern Appalachia were a number of different pillar recovery cut sequences were used in order to minimize the risk of bumps.

Jan 1, 2009

By Ashok Kumar, Rajendra Singh, Petr Waclawik

High-grade coal is locked in the shaft protective pillars at 900 m below surface in the Czech part of Upper Silesian Coal Basin. To optimise recovery of coal from the larger size shaft protective pillar, room and pillar (called bord and pillar in India) is trialled to further develop these on smaller pillars. This development followed a constant gallery width of 5.2 m and 3.5 m height. The monitored pillar size in the first developed panel varied from 860 m2 to 1225 m2 of area. The stability of these pillars (i.e., natural supports) at this depth of cover is observed to be a vital issue to control ground movement and safety of the surface structures. A simple calculation of safety factor revealed that application of an empirical method for estimating size and shape of the pillar is inappropriate due to complex geo-mining conditions of the site. Accordingly, geo-technical instruments are installed in the panel for monitoring the stability of the diamond shaped pillar at different stages of the development. Extensive monitoring of the pillar behaviour by these instruments provided an important set of data for such optimum recovery. An established process of simulation-based pillar strength estimation is often used by the mining industry, but calibration is generally done with the help of empirical formulation. However, for the deep-seated pillar, the strength suggested by the existing empirical formulations (except the CMRI formula) is found to be much different from the instrument observations. Using a study conducted at Czech hard coal mines, an attempt is made to develop a strain-softening-based numerical modelling approach to assess pillar performance at deeper cover. The recorded value and nature of the pillar spalling in the field along with the CMRI formula are used to calibrate the developed simulation approach. An assessment of the stability of the developed pillars using simulated models did not indicate any collapse of the natural supports. This paper briefly presents difficulties in pillar design at great depth of cover and field and simulation results of pillar strength estimation in Czech hard coal mines.

Jan 1, 2018

By Andre Vervoort

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

Jan 1, 1990

By 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 Thomas M. Barczak

This report examines practical considerations in longwall support selection that have evolved from Bureau of Mines ground control research. The scope of discussion includes trends in support selection, analysis of shield designs, performance testing, and an examination of support and strata interaction theories and their relevance to in situ support loading. Application recommendations are made for two-leg shields. four-leg shields, and chock-shields. Two-leg shields account for 67 pct of the current active longwall installations and are used in a variety of applications. Automated control systems are likely to continue to grow in utilization. The next major change in shield design is likely to be the application of wider supports. The most effective method of performance testing longwall shields is by controlled displacement of the canopy relative to the base. Controlled force application does not control the load transfer through the shield and may not produce maximum loading in all support components. Theories of estimating support capacity based upon a detached roof block that must be maintained in equilibrium by the powered support are being challenged by concepts based upon the equivalent stiffness of the total ground supporting system.

Jan 1, 1990

By Zhang Hongzhen

The fast convergence of boundary is a major problem of probability integral method, especially for the prediction of mining under thick alluvium, abandoned mine goaf, etc. To solve this problem and improve the accuracy of probability integral method, a modified model is constructed for surface movement and deformation based on theoretical analysis. In which, three weighted values i.e. 1, -1/t1, 1/t2 are set for the mining movement and deformation of three different major influence radius i.e. r1, R1, R2, respectively, then the parameters are selected as following: (1) Parameters are inversed based on the observation data, in which the subsidence factor q, deviation of inflection point s, displacement factor b are considered as generic parameters, the tangent of major influence angle tanß is modified as tanß2, that is the major influence radius r is modified as r2. (2) Inversion parameters of tangent of major influenceangle tanß or major influence radius r, based on the observation station edge data which beyond major influenc eradius distance from work-face mining boundary. The inversed tanß and r will be used as tangent of major influence angle tanß1 and major influence radius r1, respectively. (3) Other relevant parameters i.e. t1, t2, R1, R2, are calculated with the algorithm of this paper. A comparison experiment between the traditional and modified models has been conducted using the observation data from a observation station, the results show that in its central area, 58.6 percent of observation points accuracy is improved and 36.6 percent is decreased, while in its edge area, 61.5 percent is improved and 15.4 percent is decreased, totally, 68.6 percent is improved and 30 percent is decreased. Therefore, we can draw the conclusion that our modified model can improve the prediction predication accuracy, in particular, the boundary region.

Jan 1, 2014