Search Documents

By Sun Henghu

This paper analyses problems associated with cement based technique in metal mines, and provides a new mining technique of packing the tailing sand and water into solidified material. In the new technique, the cementing agent is a solidifying material containing a high percentage of water and tailing sand. The sand and water solidify into the packed body at the same time. The slurry of the packed body varies widely in concentration and suitability for different mining conditions. It consists of two liquid materials, each composed of a cementing agent, tailing sand and water. The thick liquids are transported to the packing place at the same time. Their pumping property is good; they do not solidify or block up the pipe during transportation. At the packing place, the two liquids are evenly mixed. The mixed liquid flattens out well and its solidifying speed is so fast that it solidifies into a packed body within a very short period of time. The compressed strength of the packed body can reach 1.2-5MPa in one day, which satisfies the requirements of different mining conditions. By using the new mining technique, all the tailing sand can be used again in the packed body and it is low in cost and investment. The mechanisation is easy and thus the working efficiency is increased and the use of labour decreased. Contamination problems caused by excess water drainage under the ground are resolved.

Jan 1, 1992

By Ke Yang

"Engineering practices have shown that the skip-fault longwall mining is frequently used when faults with offsets larger than 1 Om are present. Since the skip-fault longwall mining not only reduces the recovery rate of coal reserves, but also increases the cost due to developing another setup room on the opposite side of the fault and moves the longwall into it. The coal pillar so created induced residual stress in and thus affects the safe mining of adjacent coal seams. In order to investigate the influence on panel layout and retreat by a l 6.5m offset normal fault, the similar material simulation model was constructed based on the geological and engineering conditions of No.13 coal seam in the western district of Liuzhuang coal mine employing the down-dip longwall mining. The characteristics of deformation, movement and failure of the overlying strata and abutment pressure when the face cut through the hangingwall and footwall of a normal fault were obtained. There were significant differences between the hangingwall and footwall as conformed by the equilibrium structure model. The results showed that superposition of the tectonic stress from a normal fault of large offset and that induced by mining is one of key factors that lead the severe pressure difficult to control during panel retreating from the hangingwall to footwall. Accordingly, the technical solutions for a fully mechanized mining face directly cutting through the fault are presented and optimized, and the applications of the technical solution in Panels 171301and171303 of Liuzhuang mine enabled them to complete the mining safely and efficiently. INTRODUCTIONFaults have been one of the important technical factors in underground coal mining l1-2i. Engineering practice has shown that the methods of mining through the faults depend on the seam thickness, seam dip angle, offset of fault throw, equipment height, rock strength, and advancing direction l3-5l. In the fully-mechanized longwall mining, the face normally proceeds to cut through the fault directly if the offset of the fault less than 5m, and to skip Uump) the fault if the offset is larger than 5m. In the former case, the most important problem is to efficiently control or avoid roof fall accidents (i.e., crushing, falling, pushover, and rock burst). In the latter case, the most important problem is very expensive to drive a new setup room and move the face equipment to it. The skipped coal pillar may poses problems for mining in adjacent seams."

Jan 1, 2016

By Yanfa Gao

Clusters of underground openings were developed around the shaft bottom of Tangshan Mine. They were deep and located in soft rock strata. After development, those openings have been subjected to considerable deformation and damages. A combined supporting method including grouting and bolting, shotcrete lining and brickwork was used. This paper analyzing the theories on which this supporting method was based. A new theory called "Disturbed Creep" was developed for deformation prediction of openings surrounded by strata subjected to stresses close to limiting or failure strength. Keywords ground pressure, deep mine, soft rock, stability control

Jan 1, 2002

By Liu Changyou

This paper analyzes experimentally the stress-strain relationship and the characteristics and variation trends of acoustic emission signals of the medium- and high-burst-prone coal/rock fracture process. The precursor and sensitive characteristics of the acoustic emissions when the burst-prone coal/rock fracture occurs were obtained. The characteristic index and the feasibility for predicting rock bursts using acoustic emissions technology were proposed. Keywords: burst-prone coal/rock, acoustic emissions, energy, prediction index

Jan 1, 2003

By Song Guo

This paper will present the roof bolting finite element model developed at Jennmat Corporation used to study the control mechanism of the tensioned bolt system in the laminated roof with a large horizontal stress. In addition, it will discuss general support concepts pertaining to the tensioned bolt system which are derived 6om a series of studies in conjunction with the authors' working experience and extensive research efforts over the years.

Jan 1, 1997

By Raymond M. Stateham

The Bureau of Mines has developed a portable, lightweight device for determining the integrity of resin-grouted roof bolts. The instrument functions by sending a known pulse of ulstrasonic energy into the bolt and comparing the amount of energy reflected hack with the original measured pulse. Where the bolt is properly bonded or coupled with the rock, the energy passes through the bolt-resin-rock interface and dissipates in the rock. Where the bolt is not properly coupled, the energy is reflected back along the bolt to the instrument. The ratio of reflected energy to the pulse approximates the fraction of the bolt's surface that is improperly bonded. The instrument is easy to use. The operator holds the sensor head of the device firmly over the head of the bolt to be tested, causing the pulse of ultrasonic energy to be imparted to the bolt. The device then listens for reflected energy. This cycle is repeated about 200 times per second until an evaluation of the bolt installation is made and indicated by colored lights in the display window. Bolts having 50% or less of their surface bonded are considered had and cause a red light to he illuminated. Bolts with 50% to 75% of their surface bonded cause a yellow light to be lit and are considered marginal. Bolts having more than 75% of their surface bonded are considered good and cause a green light to be lit. The indicator lamp remains on for about 4 seconds and then goes off, indicating that the instrument is ready for another test.

Jan 1, 1982

By Michael J. Peacock

The Powhatan No. 4 Mine is located in southeastern Ohio along the Ohio River in Monroe County. The mine produces approximately 3.2 million tons of steam coal annually from the Pittsburgh No. 8 seam. The Pittsburgh No. 8 seam in this area dips to the southeast in varying amounts from 10-feet to 40-feet per mile. The seam is made up of a main bench coal (4' to 5f' ) containing several thin and variable partings and a rider ("roof") coal (0" to 18") which is separated from the main bench by a parting (0" to 18") that is reasonably consistent in occurrence and position. Strata overlying the seam varies from a bedded to nonbedded calcareous mudstone-claystone (6' to 12') in the north and east regions of the mine to a sandstone (Pittsburgh Sandstone C10' to 40'1 in the south and west regions. The Redstone Limestone (10' to 18' ) overlying the mudstone-claystone in the north and east thins and rises toward the area of sandstone deposition in the south and west. From its opening in April of 1971 until late 1981, primary roof support in the long-1 ived entries at Quarto Mining Company's Powhatan No. 4 Mine had been achieved with 8-foot and 10-foot. 518-inch mechanical roof bolts using a Single expansion shell. These bolts were installed so that the expansion shell achieved a competent anchorage in the limestone. However, as the mine developed westward, the limestone began to trend upward becoming inaccessible as an anchorage medium at the 8-foot and 10-foot horizons. Anchorage achieved in the underlying mudstone-claystone strata was failure-prone due to weathering (after exposure to air and moisture) and inconsistencies in the anchorage horizon, leading to an increase in the incidence of reportable roof falls at the mine. Responding to the threat to safety and productivity posed by the increase in roof falls. Mine Management through its Safety and Engineering Departments, sought solutions to the problem. In seeking a solution to the problem, the safest and en engineering groups examined the reportable roof falls to determine the cause of those failures. Their examination pinpointed two items which were found to be a common link in the majority of those falls; namely, anchor integrity and geologic trends. Analysis of the roof falls indicated that due to effects of air and moisture on the anchoraae zones in the mudstones and clay- itones, the integrity of the anchorage over time could not be maintained. Geologic trends noted included the upward trend of the -main limestone as mining progressed westward, inconsistencies in the amount of roof coal and draw slate in the immediate mine roof and increasing evidence of heavily slickensided immediate mine roof. In the absence of the protective barrier provided by the roof coal, the immediate strata and particularly the draw slate, is susceptible to deterioration due to the weathering effects of air and moisture and thus is prone to "eat out" around the roof bolts and cause roof falls. The presence of heavily slickensided roof creates a need for increased contact surface such as that provided by plank and header blocks to aid in holding up the roof and tends to make the behavior of such roof very unpredictable. Having determined the causes of the roof falls. Mine Management began to identify the kind of roof support which would best solve these problems, while at the same time providing the mine with a safe, cost effective and efficient roof support system. This paper will provide an analysis of how this decision was reached, the background that was researched to aid in forming the decision, the considerations that played key roles in the formulation of the plan, the mechanics of following through and implementing the new roof support system, and an assessment of the resultant gains in safety, productivity and improved costs derived from the implementation of the new roof support system.

Jan 1, 1986

By Winton Gale

"Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar.INTRODUCTIONA coal burst is defined as a rapid expulsion of coal (and potentially gas) from the boundary of the roadway. The volume of a burst can be variable, but volumes above 10-50m3 are noted and cause significant disruption to operations.Rock and coal fracturing is common about roadways, particularly under the influence of elevated stresses, either tectonic or related to mining abutment stress. Shear fracture is very common; however, it is very uncommon that the coal is propelled from the face or ribs as a result of fracture of the material. The nature of rock fracture about roadways under elevated stress is presented in Figure 1.A burst requires energy to propel the coal or rock from the rib or face of the roadway. The depth of material impacted may vary from 0.5m to greater than 3m inside the roadway boundary. The strength of the ground in this zone is variable depending on the nature of the coal seam (or surrounding material). Typically, this material has significant residual strength as confinement increases into the ribside.The energy required to propel this material is typically in addition to that which is associated with normal fracturing about the roadway."

Jan 1, 2018

By Christopher Newman, Zach Agioutantis, Gabriel S. Esterhuizen

"As conventional underground deposits are continuing to be depleted, mining operations have been forced to produce at greater depths and in more geologically and geometrically challenging conditions. Over the past decade, there has been a global increase in the application of cemented paste backfill (CPB) material as a means of ground support in open stope mining operations. Although CPB has been extensively used within the industry, there are many fundamental factors affecting the design of safe and economical fill structures that are still not well understood. A critical design issue with respect to backfilled stopes is determining the stress state within the fill material as well as the surrounding rock.This paper details the development of a reliable numerical model for the simulation of stress distributions around the excavated stope area, as well as through the backfill material. Through discussions on modeling parameters and output results, one is provided with insights into the mechanisms of stress redistribution allowing for further accuracies in simulating the behavior of the CPB material as well as its interaction with the surrounding rockmass.INTRODUCTION AND BACKGROUNDModern technological innovations with respect to material processing, cementitious composition and chemical additives, and material transportation have provided the mining industry with a ground support material that greatly reduces mine waste costs, while, when used appropriately, increases reserve production and mine stability. As mining operations continue to produce in more complex geologic and geometric conditions, cemented paste backfill (CPB) has become commonly employed within primary-secondary excavation-support sequencing allowing for total extraction of the mining reserve. As shown in Figure 1, primary stopes are initially excavated and then backfilled in a two-staged pour: plug and final pours. The plug (initial pour), is initially placed at the bottom of the stope with a CPB material of heightened cementitious material. While the plug provides a strong foundation on which to build the rest of the CPB structure, it also provides protection against barricade breakthroughs (Li and Aubertin, 2009), as well as providing operations a means of underhand stope-and-fill mining through the use of an artificial sill pillar (Hughes, et al., 2011). Following placement, the plug is allowed to cure to a designated design strength. Upon completion of the plug, a less cementitious “final” pour material is used to backfill the rest of the excavated area."

Jan 1, 2018

By Russell Frith

Probably the most expensive but least well understood roof falls in underground coal mining, are those that occur ahead of the powered supports in the centre of longwall face. Even small roof cavities result in the dilution of the ROM product and large cavities can stop the face for days if not weeks and the incurred cash flow losses can be catastrophic. Expensive and time-consuming remedial measures (e.g. PUR, cavity filling, removal of stone from the AFC by hand) are required and equipment wear and damage can be significant when the need to blast and convey stone is also considered. Through four major research projects within the Australian coal industry and numerous consulting assignments, the author has spent the best part of fifteen years attempting to identify and defme the various geotechnical and operational factors that determine whether a major roof cavity occurs ahead of the powered supports or a normal goaf fall occurs just behind them, the difference in operational outcome being enormous. The paper presents a condensed summary of the overall technical findings covering such issues as roof fall mechanisms, the measured influence of cover depth, periodic weighting of massive strata units and its quantification, the role of cut-throughs in chain pillars, face width and height, the role of the powered support and the influence (both positive and negative) of mine site operations. The big picture outcome is the view that high production longwall extraction is well-suited to certain geotechnical environments, is marginal in others and under certain conditions is fatally flawed and beyond the control of mine operators, although fortunately the latter category is a relatively rare situation. Various risk-based research findings in this regard will be given as a basis for future discussion.

Jan 1, 2005

By D. Bigby

The current British Standard on rock reinforcement components used in coal mining (BS7861:1996) employs the axial double embedment tensile PET) test to determine the main system load transfer performance criteria, in terms of bond strength and stiffbess. This test involves measurement of load/displacement characteristics of a tendon fully grouted into internally threaded thick-walled steel tubes. Part 1 of the Standard covers rock bolting systems and Part 2 covers birdcaged cable bolting systems. Both parts are highly prescriptive, defining a specific design of bar (AT) and cable bolt (birdcaged). In 2000, it was agreed by the UK Rockbolt Research Liaison Committee that Part 1 of the Standard should be revised to allow further industry innovation of bolts and resins. Amongst other aspects, it was recognised that the double embedment test was limited by its inability to examine load transfer between the bolt and the host rock and the UK Health and Safety Executive commissioned an RMT research project to develop a new test to replace the DET test. The new test, the Laboratory Short Encapsulation Pull (LSEP) test has now been incorporated into a draft revision of BS7861 Part1 which has been issued for industry wide consultation. In 2003, it was agreed to begin revision of Part 2 of the Standard to encompass the many types of long tendon reinforcement which were, by then, being used in the UK coal mining industry. These included flexible bolts, deformed strands and tensionable systems encapsulated with resin, cementitious grout and combinations of the two. A second RMT research project was funded by the HSE to examine adapting the Laboratory Short Encapsulation Pull Test for the revised long tendon Standard. This project is nearing completion and a revised Part 2 of the Standard is being drafted. This paper presents some of the results of the two research projects, describes the laboratory tests and acceptance criteria developed and discusses the implications of the research on improving the understanding of reinforcement performance and its characterisation. In particular it shows how the new test can reveal very different load transfer characteristics to the double embedment test, as it takes the load transfer at the holefrock wall into account.

Jan 1, 2005

By Gabriel S. Esterhuizen

"Cable bolts are sometimes used in low-seam coal mines to provide support in difficult ground conditions. This paper describes cable bolting solutions at two low-seam coal mines in similar ground conditions. Both mines used support systems incorporating cable bolts as part of the primary support system. Two original cable bolt based support systems as well as two modified systems are evaluated to estimate their ability to prevent large roof falls. One of the support systems incorporated passive cable bolts, while the other used pre-tensioned cable bolts. The results and experience at the mines showed that the modified systems provided improved stability over the original support systems. The presence of the cable bolts is the most important contribution to stability against large roof falls, rather than the details of the support pattern. It was also found that a heavy steel channel can improve the safety of the system because of the ‘sling’ action it provides. Additionally, the analysis showed that fully-grouted rebar bolts load much earlier than the cable bolts, and pre-tensioning of the cable bolts can result in a more uniform distribution of loading in the roof.INTRODUCTIONCable bolting is sometimes used as primary support in coal mines experiencing difficult roof conditions. In low-seam mines the flexibility of the cable bolts allows greater length supports to be installed near the advancing face without the use of couplers. When used as primary support, the cables are typically installed in the same row as fully grouted bolts, replacing two or more of the bolts in each support row. A heavy steel channel may be used as a strap to spread the support load over a greater portion of the roof. Historically, the Mine Safety and Health Administration (MSHA) has not allowed widespread use of partially grouted un-tensioned bolts (e.g., passive cable bolts) for primary support; however, pretensioned cable bolts have been accepted.Various solutions using cable bolts as primary support were attempted at two low-seam coal mines in Western Pennsylvania that were experiencing difficult roof conditions. Both mines originally used fully grouted rebar bolts as primary support and cable bolts as supplementary support. It was found that when a large roof fall occurred, the cable bolts may be contained within the dome of fallen rock. As problematic roof conditions continued to exist, both mines decided to use cable bolts as part of the primary support system. The cable bolts were located near the ribs of the entry, to increase the likelihood that they would be anchored outside the dome of potentially unstable roof. The cable and rebar bolts were installed on a heavy steel channel that acts as a ‘sling’ to distribute the load across the width of the entry. At the first mine, Mine A, pre-tensioned cable bolts were used while at Mine B, un-tensioned cables were used. The two support systems consist of essentially the same support components installed in different patterns and with varying degrees of pre-tension."

Jan 1, 2015

By Andrew Hyett

Over the last 30 years, a transition away from end-anchored bolts to fully grouted rock bolt systems has occurred in many underground mines. It seems geomechanically intuitive that the success of such bolts, which interfere with rock movements around underground excavations through a variety of complex interactions, is precisely why they remain challenging to instrument and monitor. Whereas measuring the load profile for an end-anchored mechanical bolt is trivial, for a fully grouted system, the load may vary dramatically due to discrete rock movement at cracks and joints. Despite the challenges, measuring the strain (i.e., load) along fully grouted rock bolts remains very appealing, and a lot of valuable research has been conducted using foil resistance strain gauges, particularly in US coal mines (Signer and Jones, 1990). Despite good intentions, more widespread adoption of this instrumentation remains unlikely for the following reasons: (i) The technology is expensive, partly because Intrinsically Safe (IS) requirements limit the number of suppliers and solutions. (ii) The technology is perceived as challenging to successfully implement except by those who are skilled in the art. (iii) The reliability of the technology is questionable in harsh environments. (iv) The technology relies on discrete pairs of strain gauges that may not capture the complete strain distribution along the bolt. No existing alternative technology can offer a solution to all of these limitations. However, in this paper, we introduce results from a new Distributed Optical Sensing technology that promises significant potential. Since the technology is capable of distributed sensing, our results suggest that this new technology can, for the first time, reveal the detailed strain profile along fully grouted rock bolts.

Jan 1, 2013

By Guy Reed, Russell Frith

"Current coal pillar design is the epitome of suspension design. A defined weight of potentially unstable overburden material is estimated, and the dimensions of the pillars left behind are based on holding up that material to a prescribed or user-defined Factor of Safety. In principle, this is seemingly no different to early roadway roof support design. However, for the most part, roadway roof stabilisation has progressed to reinforcement, whereby the roof strata is assisted in supporting itself. This is now the mainstay of efficient and effective underground coal production. Suspension and reinforcement are fundamentally different in their roadway roof stabilisation approach and, importantly, lead to substantially different requirements in terms of roof support hardware characteristics and their application. In suspension design, the primary focus is the total load-bearing capacity of the installed support to ensure that it is securely anchored outside of the potentially unstable roof mass. In contrast, reinforcement recognises that roof de-stabilisation is a gradational process with an ever-increasing roof displacement magnitude leading to ever-reducing stability. In a reinforcing situation, key roof support characteristics relate such design issues as system stiffness, the location and pattern of support elements within the roadway, and mobilising a defined thickness of the immediate roof to create (or build) some form of stabilising strata beam. The objective is to ensure that horizontal stress acts across the roof of the roadway and is maintained at a level that prevents mass roof collapse. This paper presents a prototype coal pillar and overburden system representation where reinforcement, rather than suspension, of the overburden is the stabilising mechanism via the action of in situ horizontal stresses within the overburden, the suspension problem potentially being an exception rather than the rule, as is also the case in roadway roof stability. Established principles relating to roadway roof reinforcement can potentially be applied to coal pillar design under this representation. The merit of this assertion is evaluated according to documented failed pillar cases in a range of mining applications and industries found in a series of published databases. Based on the various findings, a series of coal pillar system design considerations and suggestions for bord and pillar type mine workings are provided. This potentially allows a more flexible and informed approach to coal pillar sizing within workable mining layouts, as compared to common industry practices of a single design Factor of Safety (FoS) under defined overburden dead-loading to the exclusion of other potentially relevant overburden stabilising influences."

Jan 1, 2017

By C. A. Vining, J. R. Nopola

"Evaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. Evaporites are subject to continual creep and no amount of ground support can successfully arrest this process. Common ground-support techniques that are useful for most rock types may reduce the effective life of bolts in evaporite mines and can create unseen hazards. A different approach to ground support must be applied to provide long-term support in areas where high creep rates are present. The goal of effective ground-support designs in these situations is not to contain the rock and prevent it from moving; rather, the ground-support design must be able to move along with the rock and contain any fully detached slabs until additional remediation measures can be undertaken. The most appropriate ground-support designs will consider both the expected movement of the rock and the duration that the area needs to remain accessible.INTRODUCTIONEvaporites, such as salt and potash, pose unique problems to ground-support designs not encountered in other rock types. The factors that must be considered in design stem from the deformation behavior of salt, which can fail in a brittle manner, akin to most rocks, but can also deform in a slow, ductile manner, resembling the stretching and elongation of steel. The deformation behavior of salt has been well established in technical circles where it has been studied for decades (Hardy and Langer, 1984). Much of the research in salt mechanics is related to repository science, specifically at the Waste Isolation Pilot Plant (WIPP) in southwest New Mexico. RESPEC has been continually involved in the WIPP from the initial design considerations (Gnirk, Grams, and Zeller, 1977) to the current day (Keffeler, 2016) and has been able to leverage cutting-edge research of salt mechanics into practical solutions for mining operators. While a detailed description of salt mechanics is not provided in this document, Fossum and Fredrich (2002) include a fine overview of salt mechanics. The following list paraphrases their chapter on basic facts about salt to provide some context for the reader unfamiliar with salt mechanics:"

Jan 1, 2018

By Naj Aziz

n Australia, the common method of laboratory testing of bolt for load transfer capacity determination is by short encapsulation push testing. Some concerns are raised about the validity of the test methodology, as the method does not reflect on the actual load transfer characteristics of bolt in real field situation. Thus, laboratory testes were carried out to examine the load transfer mechanisms of bolts in both the push and pull conditions. Tests were conducted by shearing a short resin encapsulated bolt out of a cylindrical steel sleeve. Three types of bolts with different surface profile configurations were tested. The study was complemented with numerical simulation of the test methods. Irrespective of bolt type the average shear load and shear stress values were found to be greater in push test, and the displacement at peak shear load was greater in pull test. The average shear stifhess values were greater in push test. The numerical simulation of the bolts provided a clear understanding of the stresses and strains generated by different bolt profiles during both the pull and pull testing process, thus allowing a better appreciation of the load transfer mechanism process.

Jan 1, 2005

By S. S. Peng

To acquire knowledge on the general trend of primary and supplemental supports used in tailgates of US longwall mines a survey of 76 mines with 86 longwall panels was carried out in the summer of 1993. This paper attempts to provide some insight on most commonly used support types as primary and supplemental supports in tailgates. Analysis of the surveyed data show that fully grouted resin bolts and cribs are the most commonly used primary and supplemental supports in tailgates in longwall mines of USA.

Jan 1, 1994

By Jesse Puller, Ken Mills

"This paper describes the Australia, New Zealand Inflatable (ANZI) strain cell, its operation, and recent development for overcoring in exploration boreholes. The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three-dimensional, in-situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects, but technical advances over the last decade or so have allowed the system to be deployed in surface exploration boreholes to greater depths than was previously possible. Recent development of a downhole electronic data logger, a wireline-enabled drilling system, and an instrument deployment system has simplified the process of obtaining three-dimensional overcore measurements at depths approaching 1km to a single shift operation.The capability to deploy ANZI strain cells in surface exploration boreholes represents a significant breakthrough for the design of underground mines and underground excavations generally. High-confidence characterisation of the in-situ stresses at the design stage provides the opportunity to design key infrastructure and mining systems to take advantage of the in-situ stress field from the outset before mining begins. Understanding the three-dimensional, in-situ stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it also provides insight into the mechanics of all the various processes driving ground deformations, including which geological fault structures are at limiting equilibrium.INTRODUCTIONThe overcoring method of stress relief is one of the most direct methods for determining three-dimensional, in-situ stresses in rock. The Australia, New Zealand Inflatable (ANZI) strain cell combines overcoring with several other techniques to provide determinations of the three-dimensional, in-situ stress field and the properties of the rock and, importantly, provides an indication of how much confidence can be placed in the measurement. This paper describes the advances that have enabled overcoring of the ANZI strain cell in deep surface exploration boreholes and the development of various complementary techniques that build additional confidence in the determination of the in-situ stress."

Jan 1, 2018

By D. Newman

The objective of this is paper is to review the current state of knowledge and practice in highwall mining (HWM). HWM has become a standard method in surface mining, commonly used alone or in conjunction with contour or slot mining. It provides 800-feet to 1,200-feet of additional recovery when the economic stripping ratio is reached in contour mining or in slot mining when surface access to a reserve is limited. A significant attribute of the highwall miner is its versatility. HWM has been used successfully to mine abandoned pre-reclamation law highwalls, points or ridges uneconomic to mine by underground or other surface methods, outcrop barriers left adjacent to underground mines, separate benches of the same seam where the parting thickness or quality differences between benches render complete extraction uneconomic, previously augered areas containing otherwise inaccessible additional reserves and close or widely spaced multiple seams. The theory and design methods to assess roof; pillar, and floor stability are presented followed by three case histories. Simple design charts for sizing HWM web and barrier pillars are also presented. A recommended web pillar width may be obtained from the design charts given the overburden depth, the HWM cut width, and the mining height. Given the depth and panel width for a set of HWM cuts, another set of charts gives a suggested barrier pillar width. The case histories, hm Northern and Southern Appalachia are used to illustrate the application of rock mechanics to quantify the stability of the highwall, roof, web pillars, and floor. The case histories involve 1) mining through a previously augered highwall, 2) mining under back-stacked spoil and 3) selective mining of closely spaced benches of the same seam. Because each site is unique, the appropriate pre-mining geotechnical analyses range from the calculation of roof, web pillar, and floor bearing capacity stability factors to detailed numerical modeling of the auger and underground mine workings. When operating in the vicinity of existing underground mine or auger workings, the determination of ground deformation and strains resulting from highwall mining is a necessary facet of a ground control investigation.

Jan 1, 2005

By Kevin W. Harris

Multiple-seam coal mining is a major issue in underground coal mining that adds additional complexity to the practice of ground control. As reserve exploitation is driven by economics, it is now difficult to mine coal without the existence of previous workings above and/or below the active seam; this is particularly true in the Appalachian coalfields. The frequency in which problematic multiple-seam interactions are now encountered has resulted in significant research regarding the underlying mechanisms of the problems and methods of mitigation. In the last few decades, a great deal has been learned about these interactions and the ability to minimize problems associated with these hazards has greatly advanced, resulting in better economics and improved safety. Minimizing the adverse effects of stress interactions often requires a thorough understanding of site-specific geotechnical parameters and previous mining geometry. This is especially evident in bump-prone ground, where pillar behavior is highly dependent on local conditions. This paper presents a case study in a deep cover, Central Appalachian coal mine where three bump events occurred. Initially, empirical design software was used in an effort to analyze the situation and prevent further instability. The empirical study proved ineffective. A detailed investigation of the bump events using the boundary element software LaModel 3.0 was then performed. This study resulted in a site-specific calibration standard that will be applied in numerical pillar design for the remaining reserve.

Jan 1, 2014