Search Documents

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

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

Jan 1, 2010

In this paper a numerical formulation is presented for determination of the axial load along a cable bolt for a prescribed distribution of rock mass displacement. Results using the program CABLE indicate that during excavation, the load distribution that develops along an untensioned fully grouted cable bolt depends on three main factors: (i) the properties of the cable itself, (ii) the shear force that develops due to bond at the cable-grout interface (i.e. bond stiffness), and (iii) the distribution of rock mass displacement along the cable bolt length. In general, the effect of low modulus rock and mining induced stress decreases in reducing bond strength as determined from short embedment length tests, is reflected in the development of axial loads significantly less than the ultimate tensile capacity even for long cable bolts. However, the load distribution is also dependent on the deformation distribution in the reinforced rock mass. Higher cable bolt loads will be developed for a rock mass that behaves as a discontinuum with deformation concentrated on a few fractures, than for one which behaves as a continuum, either due to a total lack of fractures or a very high fracture density. This result suggests that the stiffness of a fully grouted cable bolt is not simply a characteristic of the bolt and grout used, but also of the deformation behavior of the ground. In other words, the same combination of bolt and grout will be stiffer if the rock behaves as a discontinuum than if it behaves as a continuum. This paper also explains the laboratory test programme used to determine the constitutive behavior of the Garford bulb and Nutcase cables bolts. Details of the test setup as well as the obtained results are summarized and discussed.

Jan 1, 1996

By Lewis Martin

Spokane Research Center personnel are currently conducting research on cable bolts to assess their material and support properties, to provide design criteria for roof control systems under various types of underground mining conditions, and to provide background data for computer models of cable bolt supports. Laboratory studies of cable bolts made of conventional steel cables with Garford bulbs and collars resulted in recommendations on characteristics such as embedment length, hole diameter, cable bolt diameter, and configuration of cables. A completed series of laboratory tests has determined the minimum embedment length of resin required for resin-grouted, cable bolt ground support.

Jan 1, 1996

By Tingkan Lu

To understand the reinforcing mechanisms of the fully grouted resin bolt and clarify its performance in weak roof strata, the field monitoring investigation using instrumented bolts has been conducted in a coal mine. The total time of monitoring was around ten months. In the first 8 month period, the rock bolt performance relates to the initial development of the gateroad and subsequent time dependent strata response. After 8 months, the rock bolt performance relates to the effect of the approaching longwall mining operation, up to the point where the measuring station was destroyed due to the mining operation.

Jan 1, 1998

Characterizing coal mine roof rock is extremely important for hazard assessment, reinforcement, and entry design. The Coal Mine Roof Rating (CMRR) is an innovative rock mass classification that has found acceptance in the coal mining industry. A disadvantage of the original CMRR is that it cannot be determined in advance of mining, because it requires underground observations. An entirely new set of procedures have now been developed to determine the CMRR from exploratory drill core, as described in this paper. The new procedures employ Point Load Tests (PLT) to determine strength parameters that account for approximately 60% of the rating. Both diametral (parallel to bedding) and axial (perpendicular to bedding) PLT are conducted. The diametral tests allow estimates of bedding plane cohesion and rock anisotropy, both of which are critical to estimating susceptibility to horizontal stress. Traditional core logging procedures are used to determine discontinuity spacing and roughness. To ensure compatibility with the original CMRR, the new rating scales were verified by comparing drill core results with nearby underground exposures. To assist mine planners in using the new CMRR procedures, a large database of strength ratings of roof rocks was assembled through extensive point load testing and logging. Over 2000 point load tests (both axial and diametral) have been made on 30 common coal measure rock types from mines representing most U.S. coalfields. This database has been merged with the popular core rating system of J.C. Ferm into a field guide for geologists and engineers. By using the pictorial classification system of Ferm to identify the rock, typical CMRR can be compared with values calculated using the new CMRR procedures.

Jan 1, 1996

By J. Willan

This paper addresses the problem of designing a destress blasting procedure to reduce the incidence of rockbursting in hard rock mines. A review is made of the developed theory and practice of destress blasting in rockburst-prone ground. Specific reference is made to a recent survey of past and current practice with particular reference to Ontario mining operations. The need is identified for a more scientific basis for destress blast design and monitoring. Reference to developments in blasting technology as well as rock mechanics are then made to support a proposed, integrated design procedure and research strategy.

Jan 1, 1984

By Douglas Tesarik, Mark K. Larson, Habte Abraham, Heather E. Lawson

Dynamic failures, or "bumps", remain an imperative safety concern in underground coal mining, despite significant advancements in engineering controls. While many factors have been empirically linked to the occurrence of dynamic failure events, identifying a consistent, repeatable set of criteria within a field setting has proven elusive; conditions generally associated with dynamic failure might produce an event at one site, but not another. Conversely and more troubling, dynamic failure could occur where relatively few of these factors exist. The presence of spatially discrete, stiff roof units, such as paleo channels, are one such feature that has been linked to the occurrence of dynamic failure events. However, an empirical stratigraphic review investigating the relative frequency of discrete units in bumping versus non-bumping deposits indicates that no significant difference exists based on this criterion alone, and that instead an apparent relationship exists between reportable bump history and the overall character of the host rock with respect to stiffness. Due to the complexity of the bump problem, however, these results are not conclusive, as they do not take into account any variable other than the presence or absence of stiff members in the roof lithology; To weight the relative impact of changes in a single variable, such as the thickness or location of sandstone members, it must be examined in isolation-i.e. in a setting where all other variables are held constant. Numerical modelling provides this setting, and the effects of variability in a stiff discrete member in a hypothetical longwall mining scenario are investigated within the context of three stratigraphic "types'', as determined by the ratio of stiff to compliant stratigraphic members; Compliant, Intermediate and Stiff A modelling experiment examines changes in rupture potential in stiff roof units for each stratigraphic type as discrete unit thickness and location are manipulated through a range of values. Results suggest that the stiff-to-compliant ratio of the host rock has an impact on the relative stress-inducing effects of discrete stiff members. In other words, it is necessary to consider both the thickness and the distance to the seam, within the context of the host rock, to accurately anticipate areas of elevated rupture induced hazard; acknowledging the presence of a discrete unit within the overburden in general terms is an insufficient indicator of risk. Through modelling of anticipated changes in the placement and dimensions of discrete units within their stratigraphic setting, elevated rupture-induced bump hazard can be anticipated on a case by case basis. Were similar modelling studies conducted at mine sites in tandem with tracking of problematic discrete stiff units, areas of elevated rupture risk could be anticipated in advance of mining. Developing this predictive capability beyond identifying rupture potential in discrete roof members is essential to the eventual elimination of dynamic failure related worker injuries and fatalities. As stress is a necessary component in the occurrence of dynamic failure events, this finding helps to refine our understanding of the role of individual stiff, strong roof members in bumping phenomena, and suggests that a more holistic view of overburden lithology, combined with site-specific numerical modelling, may be necessary to achieve greater miner safety.

Jan 1, 2016

By Alan J. Zeni

Drainage of water from abandoned underground mines has become important in recent years because of safety concerns when mining near abandoned works or reentering abandoned mines for further development. Seepage from inundated mines causes environmental damage and may threaten homes and communities. Where topography permits, horizontal drilling can provide a quick and cost effective method for solving trapped water problems. The mechanics of the horizontal drilling operation as applied to water drainage are discussed in this paper.

Jan 1, 1990

By James Pile

Long, high capacity, highly tensioned, cable bolts (strand bolts), stiff enough to be installed with resin like a solid bolt, yet flexible enough to be used in any lengths, have been widely used in Australian coal mines since the early 1990?s. A brief history and update on recent developments of this technology is provided. The first major US trial of this Australian technology, has recently taken place at BHP Billiton?s San Juan longwall coal mine in New Mexico. Thirty-five foot long, high capacity, highly tensioned, post-grouted cable bolts are being used to provide active secondary support of intersections in areas where operator safety and life of mine considerations are paramount, and standing support cannot be installed for reasons of equipment access. The cable bolts are resin anchored into overlying sandstone channels, tensioned, and post-grouted. The installation procedure and support results for these highly tensioned cable bolts are given.

Jan 1, 2007

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 Khaled Morsy

The tragic incidents at Sago and Darby coal mines in 2006 pointed out the need for stronger seals to resist the blast effects of violent gob explosions. Mine Safety and Health Administration (MSHA) has published the final rule on Sealing of Abandoned Areas in April 2008. According to the final rule, the mine seals for unmonitored gob areas should withstand an instantaneous pressure of 120 psi. Reinforced concrete (RC) seal is one of the feasible options that can be used to meet the final rule. Concrete is a heterogeneous, cohesive-frictional material and exhibits complex non-linear inelastic behavior under multi-axial state of stress. The increased use of concrete as primary structural material requires the development of sophisticated material models to have an accurate prediction of the material response to a variety of loading situations. The constitutive modeling of concrete has advanced considerably in the past three decades. Choosing a reasonable concrete material model is important in order to obtain an accurate mine seal design. This paper attempts to address the effect of different concrete material models on the response of RC seal subjected to dynamic loads from methane explosion. Three concrete models, from ABAQUS library, were selected to address the effect of non-linear behavior of concrete and damages due to tensile and compressive stresses. In addition, a typical and verified concrete material model was used for conducting 3D finite element models simulating the RC seal by ABAQUS, EXPLICIT ver. 6.6.

Jan 1, 2008

By V. Bondarenko

The forecasting of floating earth inrush into the working is to a great extent connected with the investigation of the rock mass stressed-deformed state around the working. The aim of such investigations is to estimate the carrier ability of the protective layer (which is stabilized by the physico-chemical mode of fixing) and to determine it's thickness, which is to be sufficient to present the floating earth inrush into the working. We consider the protective layer to have exhausted it's carrier ability if the rocks of this layer reached the overlimit phase of deformation and it's greater part (section), which adjoins the working. is destroyed up to the limit of the remaining strength. So, the aim of investigations is to determine the size of the rocks inelastic deformation zone in the roof of the working. It is necessary to take into account the presence of water saturated seam in the roof and the overlimit characteristics of the rocks, their ability to lower the carrier capacity up to the level of the remaining strength.

Jan 1, 1996

By Joe McQuerrey, William Kendall, Gene Wilson

"Roof bolting is the standard for ground control in underground coal mining. Rib and truss bolting is just as important when conditions are less than desirable. As a manufacturer of roof drills, J.H. Fletcher & Co. (""Fletcher"") promotes a walkthrough design machine to protect the machine operators from rib hazards.When installing rib and brow bolts in high mined heights, the operator's natural desire is to step from under the canopy onto the drill feed mechanism. This is done to allow a better reach to the drilled hole in the rib or brow. This, of course, is not an approved or acceptable practice.The effort to improve the operators' ability to reach the rib and brow has resulted in a new boom design. The design was first shown in September 2008 at the MINExpo in Las Vegas, NV. The design incorporates a step out area towards the rib side of the machine. The step provides improved access to the mine rib and brow.Having a solid concept does not guarantee an accepted design. Operator acceptance is the goal. When the machine was first tried in the underground environment, the machine operators did exactly as requested: run the machine as supplied and suggest improvements.In the fast pace job of roof bolting, any ergonomic improvements are welcome. These improvements usually increase the operator's productivity and safety. Based on operator feedback and final evaluation with Fletcher, machine revisions were generated. When the machine revisions were completed, Fletcher again asked the operators for comments, giving the boom design an evolving path into our industry.INTRODUCTIONIt's human nature to find the most comfortable position in which to do a task, especially for repetitive tasks. Designing a machine that will withstand the rigors of underground mining is a challenge in itself. With stringent safety requirements and limited real estate, there is a constant evolution in design to converge safety with productivity. With the ongoing development of the ""Rib Access Boom,"" one more step in the evolution process has been taken."

Jan 1, 2010

By Peter Cain

In response to the appearance of excessive convergence of bolted gateroads in the front abutment zone of retreating coal faces at Phalen Colliery, staff from the CANMET Cape Breton Coal Research Laboratory in Sydney, N.S. initiated a programme of roof profile measurements in the front abutment zone of longwall headgates. The results of these measurements allowed a characteristic deformation curve to be determined for each headgate, and the early identification of areas showing an unstable deformation trend. These areas were then reinforced with supplementary support. The monitoring programme was adopted and enlarged by the rock mechanics staff of the Cape Breton Development Corporation, and has been continuing for a period of approximately two years. A sufficient quantity of data now exists to be able to make an assessment of the effects of a number of mining parameters on the performance of the gateroad in the front abutment zone. The following paper describes the method of measurement, illustrated by typical results, and goes on to relate variations in rate and patterns of movement to geology, interaction, method of support and pre-mining stresses.

Jan 1, 1996

By Peter Cain

The paper presents previously unpublished results derived from an unique interaction moitoring program conducted in the Sydney Coalfield of Nova Scotia. An abandoned, sealed longwall gateroad was re-entered and instrumented prior to extracting a longwall face 140 m below it. Instrumentation included piezometers connected to a constant head tank to monitor subsidence, and horizontally mounted extensometers to measure strains associated with subsidence. Survey levelling data was also collected. The monitoring results have allowed a number of significant conclusions to be drawn, all of which were either unanticipated or contradict current mainstream thinking. Firstly, interaction subsidence was less than anticipated. Secondly, the effects of rate of face advance on the development of the subsidence profile are evident. Thirdly, compressive strains are shown to remain in the subsided strata. Finally, strata response to undermining was almost immediate. The impact of these findings on the extraction of longwall panels beneath flooded workings in the Sydney Coalfield are discussed.

Jan 1, 1996

By Tim Ross

Several shear failures were observed in Shaft #1 at the Mountain Coal Company, LLC, West Elk Mine, after mining longwall Panel 23, 1,100 ft to the east of Shaft #1. It was speculated that this shear damage could be related to differential ground movement caused by in situ stress relief from the ?stress shadow? of the caved zone above longwall Panel 23. A numerical study was conducted to assess the possibility of the shaft shear damage being caused by in situ stress relief and the potential for additional damage to Shaft #1 and two other nearby shafts, due to mining nearby longwall Panel 24. Three-dimensional (3D) models were built in FLAC3D to simulate past and future mining near shafts, the estimated local anisotropic and directional horizontal stresses, and the overlying variable surface topography. The numerical analyses indicated that stress relief due to mining Panel 23 caused the shear damage to Shaft #1and that additional damage to Shaft #1 and the other two shafts, would likely result from mining longwall Panel 24. Additional shear damage was documented in Shaft #1 when longwall Panel 24 was mined, confirming the results of the numerical analyses.

Jan 1, 2006

By Sanjay Kumar Singh

The mechanics of load transfer in bord-and-pillar mining is not well understood despite the evolution of the art of pillar extraction over the last 20 years via trial and error (empirical) or Rock Mass Rating (RMR), simple monitoring and pillar extraction experiences at neighboring mines. Variations in geologic conditions and pillar extraction methods, along with a lack of comprehensive instrumentation programs for monitoring ground behavior during pillar recovery, are among the factors that make it difficult to predict how strata will respond to pillar extraction. Efficient and safe use of roof supports depends on the interaction between the designed support and coal strata. Of particular interest in this study is the mechanics of strata deformation as influenced by geologic conditions, pillar extraction methods and optimum support with load-bearing capabilities in strata control during mass exploitation of underground coal deposits required detailed investigation,?. To provide a better understanding of the mechanics of strata deformation, as researchers, we have collected data and reviewed measurements of convergence and stress in one mine of South Eastern Coalfields Limited (S.E.C.L.), i. e., in North Chirimiri Pondary Hills (NCPH) and have completed numerical modeling with FLAC software for (Fast Lagrangian Analysis of Continua modeling, Itasca India Consulting Pvt. Ltd.) for two typical pillar extraction plans. Stress distribution in the mine roof above pairs of roof supports was calculated to demonstrate how Resin roof bolt support installed support contributed to roof block movement control. It was shown that overall stress and roof-floor convergence patterns were most influenced by the stiffness of coal measure rocks and by pillar extraction sequences and layouts. Also, resin roof bolts and other support systems play a critical role in controlling stability of both the immediate roof and the main roof for a distance of up to 18 meters (i.e., working coal seam height X three times) above the seam. Installed supports provide unique strata control advantages over other types of secondary support by significantly reducing the time between mining and installation of the secondary support. Although resin roof bolts are similar to posts in stiffness and capacity, they are more effective than posts under static loads because they can yield and still maintain loads at nearpeak capacity(Singh, et al., 2006), whereas posts fail and lose their ability to limit mine roof deformation.

Jan 1, 2012

By Ismet Canbulat, John Naylor, Jason Emery, Peter Craig, Andy Sykes

"Longwall mining is by far the most common method of underground coal extraction in Australia. The industry trends and expectations are placing increasing emphasis on the reliability and productivity of these operations. The longwalls are becoming wider and longer while retreat rates are continuously increasing due to significant improvements achieved in longwall equipment reliability and automation. This increased longwall productivity is placing significant emphasis on the reliability of longwall panel development.Although there have been significant improvements in the reliability of continuous miners and hydraulic drill rigs, the traditional resin encapsulated bolt installation is still the principal method used in all major coal-producing countries around the world. Anglo American realised an opportunity existed to introduce an alternative roof bolt installation technique called “spin to stall” in Australia. Spin to stall was first introduced in Anglo American Coal in South Africa where it has been successfully used for over a decade, though implementation of South African spin to stall resin in Australia has proven to be near impossible due to a significant difference in geotechnical conditions, mining method, and, subsequently, roof bolting equipment. Therefore, a new spin to stall development project was initiated between Anglo American Coal and Jennmar Australia (SPIN2STALL®). This paper summarises the journey of this project, the results, and the successful implementation of spin to stall achieved at Grasstree Mine.INTRODUCTIONThe Anglo American Coal business unit operates Grasstree and Moranbah North underground longwall mines in the Bowen Basin of Central Queensland, with a third longwall mine, Grosvenor, currently being developed, and two more (Aquila and Moranbah South) in the project pipeline. The vision for Anglo American is to achieve “Zero Harm” through the effective management of safety at all operations, while for the coal business unit, at the same time to achieve industry-leading operational performance. Currently, both Grasstree and Moranbah North longwalls are operating at industry best practice rates. This is placing increased pressure on the development process to allow such high productivity levels to be sustainable."

Jan 1, 2015

By C. Fairhurst

Most of the papers designated as being within the topic of underground storage at this Symposium are concerned with aspects of radioactive waste isolation. Although this is, unquestionably, an important international application of underground storage, and one involving a considerable number of research investigations, the subject of underground storage is indeed much broader. Several major international conferences have been devoted exclusively to it. Applications include storage, under a range of temperatures and pressures, of oil and liquefied petroleum products, compressed air, heat, foodstuffs of various kinds, hazardous and 'nuisance' industrial by-products, and a variety of special applications, both civilian and military - some in use, others proposed. Storage may be in caverns excavated specifically for the purpose, exhausted mines may be adapted for storage or, in some cases, the material to be stored may be injected under pressure into the pore spaces of porous-permeable formations at depth. A variety of rock types and geological environments are used, e.g. excavations in salted or domal salt, where long-term creep closure of the openings is a concern; massive granite, where fluid loss through fissures may be important. Cavities may have spans of several tens of meters, and there is interest in achieving still larger excavations. As would be expected, this breadth of underground storage applications introduces a similarly broad range of questions in rock mechanics and rock engineering. The report will review these questions and recent developments in rock mechanics research, computational procedures, and design applications - and will also discuss the opportunities for a still greater range of applications of underground storage to current industrial and social problems.

Jan 1, 1989

By David Forrester

Subsidence beneath the land surface has traditionally been monitored using the most accurate surveying methods of the day. Over the several hundred years of subsidence investigation, these techniques have improved, and thus the accuracy of subsidence measurements has improved. In circumstances where the subsiding surface is masked by water or rock, novel methods must be developed. This method describes the recent successes of subsidence monitoring using advanced bathymeric techniques in the Sydney Coalfield. In order to obtain subsidence profiles above submarine coal extractions, seafloor bathymetry surveys, run before and after mining, have shown that subsidence profiles can be detected, despite the primitive nature of some of the trials. Very recently, there have been considerable advances in this field, associated with the availablity of high-powered personal computers to control data collection and analyse the results. The recent surveys described in this paper have demonstrated the considerable potential for such methods. This paper describes bathymetric subsidence monitoring developed in the Sydney Coalfield, illustrated with some typical results, and summarizes the findings of the research leading to the development of subsidence design criteria specific to the Sydney Coalfield.

Jan 1, 1996