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By Larry Stolarczyk

A clear vision of the future coal mining industry was developed into a hierarchy of technology needs by the National Mining Association (NMA) and the Chief Executive Officers of the mining industry. Although improvements in safety and environmental factors were the main drivers in the technology roadmapping workshop sponsored by the Department of Energy, the agile mining machine of the future must efficiently extract coal from deeper, thinner, and more geologically complex coal beds. One of the challenges will be to develop sensors and software that enable clean coal cutting in a changing geologic environment. The roadmap identifies imaging ahead of raining and horizon control within the top ten needed technologies. For these technologies to be adopted by the mining industry, they must be proven to be effective while driving down mining cost and risk. This paper describes how three-dimensional imaging ahead of mining and horizon sensors enable improved ground control and safety in the mining of underground coal.

Jan 1, 2003

By C. Thomas Blevins

This paper is intended to be a hands on experience account about ground control in a Southern Illinois coal mine. Its aim is to show how a combination of real world mining practices and constraints along with practical engineering theory mesh together to formulate a ground control program in adverse conditions. These conditions - your typical localized slips, splits, rolls, clay intrusions, water, changing types and thicknesses of materials, etc. - being amplified by a very strong horizontal stress field. A review will be made about the formation of the overall ground control plan and to what degree it has been successful. A presentation of the various roof control systems and their component parts, how these systems were developed, and the theory behind them will be made. The pointing out of some of the flaws that we found in the components, installations, and/or theory will be part of this presentation. A brief look at some of the efforts in the following items will be made: quality control of the roof control products; using the computer to help keep track of roof control information and soliciting help from various universities, governmental agencies and consultants to give different perspectives to the problem.

Jan 1, 1984

By R. D. Lama

The concept of rigid or yielding bolts is discussed based upon support requirements for excavations of equivalent geomechanical behaviour. The concept of equivalent geomechanical behaviour is introduced for comparison purpose where the rock mass properties are varied for a given depth (stress) so that the deformation of the roof without any support remains constant = 300mm. A simple experimental theoretical analysis for a uniform stress field acting on an axi-symmetric excavation is conducted to establish roof bolt requirements under different conditions such as width of a given excavation, time of setting of support etc. Results are presented in graphical forms.

Jan 1, 1992

By John Stankus

Seam interaction in multiple seam mining has significant effect on entry stability. The remaining pillar in an old working usually creates a high stress concentration zone while the gob creates a stress release zone. A good panel layout should take advantage of the stress release effect created by old workings. This paper presents two case studies of multiple seam mining. The Jennmar Finite Element Analysis System (JFEA) is utilized to analyze the seam interaction effect and identify the mess concentration and relief zones Panel layouts are designed bad on the results of the finite element analysis Appropriate roof support system are also designed for different seam interaction areas.

Jan 1, 1999

By David Conover

Knowledge of the magnitude and direction of the horizontal secondary principal stresses is a critical factor in designing the layout and mining sequence of underground openings. Typically, horizontal stress measurement has been accomplished using overcoring or hydraulic fracturing. Hydraulic fracturing can be accomplished from surface boreholes prior to development, but the technique relies on numerous assumptions regarding the rock mass and the stress field, and interpretation of results is somewhat subjective. Traditional overcoring provides a more precise measurement, but requires underground access. To allow for precise measurement of the horizontal stress field from surface boreholes, the In-situ Stress Tool (1ST) was developed by SIGRA Pty of Brisbane, Australia. Used in conjunction with HQ wireline coring, the 1ST contains a battery-powered data logger that records tool orientation and monitors six diametral deformations as the tool is overcored. The IST has been used successfully for several years in Australia, but has only recently been introduced in the United States This paper discusses how the IST was used to measure the pre¬mining horizontal stresses at a western U.S. coal mine. Seven successful measurements were obtained in two holes at depths ranging from 250 to 800 ft. Results were consistent, showing moderately biaxial stresses greater than those expected from gravity loading, but relatively low compared to rock strength. In addition to describing the overcoring equipment and procedures, various problems encountered during testing and the accuracy of the results will be discussed.

Jan 1, 2004

By Thomas M. Barczak

The initial Support Technology Optimization Program (STOP), Version 1.0, was released at the 19th Gmund Control Conference. This original program has since been updated to Version 2.3 which was released in May, 2001. This paper describes the additional features of Version 2.3, focusing primarily on the following three aspects: (I) including uncontrolled convergence into the design requirements for standing roof supports, (21 the addition of design procedures for cable bolts as an alternative secondary roof support, and (3) the addition of new standing roof support technologies. Another new feature which should facilitate the development of design criteria for standing roof support is the capabilily to define ground reaction curves through convergence measurements alone wthout having to make support loading measurements. There are several other new features provided in Version 2.3 which are only briefly addressed in the paper. These include: (I) additional graphics capabilities to facilitate rapid assessment of support comparisons. (2) an East-West designation for support selection to more accurately provide cost and support availability data, (3) additional safety measures including a safety factor to identify support design near the peak support capacity, checks to see if the support dimensions comply with aspect ratio requirements, and measure of roof coverage for the design layout of the support system These new features of STOP provide more capabilities to design and analyze secondary roof support systems for conditions whlch could not be fully analyzed in the original version of the program.

Jan 1, 2001

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By Antoni Kidybinski

Coal mining both with longwall and room-and-pillar method often encounters severe operational difficulties due to had roof conditions. Rooffalls at entry cross-cuts or along the rib line in longwalls as welt as dynamic and excessive loads on supports due to overhanging of strong roof layers in a goab - are well known phenomena to the mine operators. Also, a number of practical examples may be quoted of dramatic improvement of working conditions after changing of coal winning technology or supports type. It is therefore a question to be answered how far we are able to recognize roof strata properties before mining starts and adjust both coal extraction technology and supports of entries to rock stability foreseen. Furthermore, engineering principles and criteria for decision making should be established. In recent years several in situ rock strength and discontinuities detection testing techniques have been developed both indirect (geophysical) and direct thus making possible closer roof-strata examination. In this paper critical roof span criterion is discussed as a basis for selection of proper mining system in terms of desirable stability of mine opening .

Jan 1, 1982

By Eric Westman

Ground control problems caused by multiple-seam interaction frequently sterilize coal reserves. Interaction mechanisms differ for overmining and undermining and are controlled by mining, spatial. and geologic conditions at each site. A multiple-seam interaction analysis software program, UGLY (Uppersearn Gateroad Longwall Stability), has been used to evaluate sterilization of coal reserves due to overmining at three locations. Four case histories were reviewed to determine the effectiveness of improved ground control in mitigating interaction effects.

Jan 1, 1997

By Brian E. Travis

Recent Developments in the utilization of polymer grid structures for Longwall Shield Recovery operations are discussed. Specifically: various Longwall screen designs and the numerous methods employed during installation. Also discussed is the role of polymer grid structures in head and tailgate mf and rib control and their role in controlling soft bottom often associated in shield recovery operations.

Jan 1, 1993

By Xianxin Shi

In surface electrical exploration the high resolution earth resistivity method (HRRM) is a very effective method for detecting abandoned drift mines workings. When the abandoned mines are more than 500 ft (150 m) deep, its detection capability reduces greatly and requires more effort to implement. In Yan Quan coal mine, Shanxi province, we tried to adapt this method for underground application. Two survey lines were designed with the spacing of current and potential poles 20 m and 10 m, respectively and measuring points at 2m. The measurement radius of I line and I1 line were 140 m and 60 m, respectively, and the infinitely far pole was 1200~2000 m from the survey line. (Note the I and II lines are located on the north and south ribs of main tunnel, respectively). The survey results showed that abandoned workings were located at 25-70m, Survey Line I and at 80-1 10m, Survey line 11. Based on this finding, the longwall panel gateroads and set up entry were properly located thereby providing safe mining of the No. 15 Coal seam. Key Words: Abandoned coal mine, High resolution earth resistivity, Underground electrical exploration

Jan 1, 2005

By Johann van Wjk

Mandatory Codes of Practice to Combat Rock Fall Accidents came into force on South African Collieries in 1997. These were drawn up in accordance with guidelines issued by the Chief Inspector of Mines, The Codes of Practice (CoP's) are legal documents in terms of the South African Mine Health and Safety Act 1996. One of the main results Of the introduction of the COP'S from a practitioner's point of view is that they formalized much of what would have constituted a typical visit to an underground section previously. Other important requirements were as Billows: Support rules and Strata Control Standard Procedures were to be drawn tip. Identification of Special Areas (areas requiring a higher level of rock engineering input and with, typically, higher order of support and restrictions on mining geometries), Changing conditions underground to he identified timeously. Implementation of the CoP to he monitored. To comply with these requirements on Ingwe Collieries (a division of BHPO Billiton Energy Coal) it was decided to produce too separate forms to be used in conjunction with each other. The first of these, the "Underground Section Physical Risk Rating" measures the physical conditions in each section whereas the second form, the "Section Perfomance Rating" determines how well the section personnel are Coping with the conditions. This form also pleasures compliance with the Support Rules and Strata Control Standards. Both forms are essentially risk matrices but with various scenarios described, each with a certain weighting. The "Risk and Performance Form" system for bord and pillar operations has by now been used for five years in the Ingwe and Eyesizwe Mining Collieries and has achieved a great deal of acceptance. This system has enabled the quantification of previously subjective observations and has improved the strata control awareness and management of underground mining sections. Amongst other initiatives it has assisted ill reducing the severity and frequency of fall of-ground accidents.

Jan 1, 2002

By D. W. H. Su

Following a roof fall in the B12 headgate of a CONSOL Pennsylvania Coal Company (CCPC) underground coal mine in February 1998, CONSOL R&D and Exploration initiated an underground roof geology reconnaissance program along the B13 and B14 developments to define potential roof instability areas and to recommend appropriate supplemental headgate support. This reconnaissance program revealed the presence of a thick, massive Pittsburgh sandstone channel overlying the B13 and B14 panels. Surface and underground core holes confirmed the close proximity and the very massive, nonlaminated nature of the sandstone. To mitigate anticipated adverse conditions at the B 13 and B 14 longwall faces due to delayed caving of the massive sandstone behind the shields, a task force consisting of Operations, Exploration, and R&D personnel was formed in May 1998. Several operational adjustments were made by mine management, including repairing damaged lemniscate links, maintaining immediate forward shield support, and implementing measures recommended by the task force intended to boost the shield set load density at the face. After reviewing available geotcchnical information, the task force also recommended fracing of the massive sandstone over the B13 and B14 panels to enhance caving characteristics and further reduce the bending-induced stresses in the sandstone near the face. Horizontal fracs were created at a horizon about 25 feet above the Pittsburgh seam main bench over the two panels in June and November 1998, respectively. To evaluate the effect of sandstone fracing and operational adjustments on the bending-induced stresses in the sandstone, three finite element models were constructed and analyzed. Results from the finite element analyses indicate that one horizontal frac, 25 feet above the Pittsburgh seam, would reduce the bending-induced stresses in the sandstone by approximately 50 percent, maintaining them at a level below the ultimate strength of the sandstone. Operational adjustments to increase the support set load density and support stiffness could reduce the bending-induced stresses in the sandstone by a maximum of an additional 10 percent. Shield pressure monitoring at the B14 midface confirmed the presence of a lower roof pressure zone inby and outby the 1314 frac holes. This lower face pressure significantly contributed to improving the B13 and B14 longwall productivity and safety. The total production downtime due to bad top at the B 12 face within the interval of channel influence was 30,600 minutes compared to about 3,500 minutes each for the B13 and B14 faces. The average longwall advance per machine shift for the B13 and B14 faces within the channel was about 70 percent higher than that for the B 12 face.

Jan 1, 2001

By R. W. Hill

Multiseam mining has been carried out at many collieries in South Africa. It the seams are In close proximity difficult and dangerous mining conditions may arise. In the Witbank area multiseam bord and pillar mining has been carried out, while in Natal where the seams are thinner and of a higher quality, total extraction methods are common. Several combinations of multiseam mining have been carried out. The paper highlights a number of case studies and the strata control problems that can arise.

Jan 1, 1995

By Jialin Xu

Bed separations position, size, development characteristics and their influence factors have been analyzed in-depth by experimental and theoretical methods. The results show that the key strata determine the creation, development and space-time distribution of the overburden bed separations and fractures. Based on the dynamic growth principle during the two-stage development of bed separations, the theory of section-grouting for the overburden bed separation space and the O-ring theory for the pressure-relieved methane drainage are proposed. The technique has been applied in some coal mines in China to reduce surface subsidence and for methane drainage. Keywords: rock strata movements; key stratum; bed separation; bed separation grouting; pressure-relieved methane drainage

Jan 1, 2003

By Dion Pastars

Kestrel Coal has undertaken a surface-to-seam consolidation program of faulted ground prior to longwall retreat in the 304 panel. Micro fine cement is used to improve the rock mass quality and therefore reduce the risk to personnel and equipment whilst increasing previously assumed sterilised reserves. The program is based on information attained from previously successful grouting campaigns in the Queensland coal area and follows the decision making process to ensure there is sufficient spatial permeation of cement across the faulted area. This data is used to assess the residual risk to the business in terms of health, safety and production

Jan 1, 2007

By D. W. H. Su

This paper presents the results of ill situ horizontal stress measurements using a Minifrac system in several northern Appalachian coal mines. The effect of stress magnitude and orientation on longwall gate entry stability was analyzed using a series of three-dimensional finite element analyses. The best headgate stability is achieved if the maximum horizontal stress is aligned with the direction of longwall retreat or if the panel is oriented such that the maximum stress relaxes over the headgate due to the presence of the newly formed gob. The degree of horizontal stress damage to longwall gate entries depends largely on the stress magnitude and roof geology. Accurate assessments of support or design techniques which may mitigate longwall headgate or tailgate instability in areas of thinly laminated roof and high horizontal stresses must account for the in situ horizontal stress state and site specific roof geology. The ability to measure stresses and map geology is thus essential to prevent ground control failures at the longwall headgate.

Jan 1, 1995

By Boqiang Wu, Ganggang Xu, Xiaodong Wang, Hai Wang, Shibin Zhu

"Using a paste-like slurry as a filling from the ground surface to the goaf, artificial pillars can be built in the cavity goaf to support the goaf roof to prevent roof collapse and ground subsidence. If the accumulation laws of paste-like slurry are known well, the filling technology can be optimized. The laws can establish the foundation for finding out the best methods for building artificial pillars and filling technology. In order to understand the accumulation laws of paste-like slurry in the cavity goaf, Aeolian sand with wide distribution in northwestern China was selected as aggregate, and Portland cement was used as cementitious material to prepare a paste-like slurry. A test model of the cavity goaf was built to simulate the filling process by pump from the ground surface to the goaf to study accumulation laws of the paste-like slurry. The results show that the whole process of expansion and accumulation of the paste-like slurry could be divided into stages: free expansion, fill tightening expansion, and stable expansion. The angle of repose of the accumulation body of the paste-like slurry is constant in the stage of stable expansion, and it is the final angle of repose of the accumulation body. The angle of repose of the accumulation body is highly relevant to the slump of the paste-like slurry, and it is unrelated to the distance between the floor and roof. Under certain conditions of the cavity goaf, reducing the slump of paste-like slurry is one of the effective ways to obtain cost-effective pillars.INTRODUCTIONA large number of cavity goafs of coal mines were left in northwestern China; they were formed in the mining process by room-pillar or strip mining. Safety coal pillars were used to support the roof to keep long-term stability in the goaf, so the ground subsidence was not obvious (Fu, Deng, and Zhang, 2011; Xie, 2014). With gradual weathering, destruction of coal pillars, or additional load from ground surface buildings and structures, there could be a risk of instantaneous collapse of the roof in the goaf. This would affect the safety of existing structures on the ground and adjacent mines (Qu et al., 2017; Li, Xiang, and Jia, 2011; Tan, Guo, and Zhao, 2016; Tan et al., 2017). For disaster prevention of the falling cavity goaf, the current common practice is full filling (Hu et al., 2008; Liu, Zhu, and Huang, 2015; Pan and Wu, 2010). However, for buildings and structures with lower deformation requirements, full filling is excessive. For example, it is acceptable for goaf treatment under general roads and industrial sites to control the maximum deformation and keep the deformation continuous (Liu, 2011; Liu, 2012)."

Jan 1, 2018

By Jaco J. van Vuuren

Modikwa Platinum Mine, in the Eastern Bushveld of South Africa's Limpopo Province, is a new operation employing a hybrid mining method where mechanised methods are used for the main developments and the narrow, 60cm reef is conventionally stoped using a down-dip conventional mining method. Throughout the mine, the hanging wall immediately above the reef comprises 3 jointed and laminated layers of varying thickness, commonly known as the triplets, below a non-cohesive leuconorite parting plane. At North shaft, using pre-stressed standing support and mechanically anchored rockbolts, it was found difficult and hazardous to hold all three triplets in the hanging wall, however bringing them down caused a significant increase in dilution and therefore cost of mining. A novel solution was conceived whereby two of the triplets are kept up using short, stiff resin anchored rockbolts installed systematically within 0.5 m of the face. This resulted in both safety and productivity benefits. The rockbolts create a stable and safe hanging wall and allow the mine to extract UG2 reef more economically. New drilling equipment, developed locally from available drilling technology, can be operated from a safe position, and in situ tests and measurements are made to confirm effective support. The technology and underlying concepts were successfully tested in a trial stope prior to rolling out the new support system across all the stopes at North Shaft. This has involved an extensive training programme to ensure that the rockbolt system is installed correctly and that the conditions are appropriate for this technology. The paper describes the motivation behind the project, the underlying technology and its application to Modikwa, and the benefits now being realised.

Jan 1, 2004

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