Search Documents

By Mike Amick

As virgin coal reserves dwindle, more and more mines are being opened in areas with previous mining. As addressed in this conference before, there are solutions available for designing mines extracting coal from reserves with known underground workings above or below the target seam. Often times, however, mining begins with limited knowledge of the underground workings. Even with careful planning, the proposed design has limited success because the ?facts? as presented during planning turn out not to be as accurate as one would like. At National Coal?s Mine #14, mining the Walnut Mountain seam, after driving the main entries in some 34 crosscuts, it was found during routine mine examinations that the pillars had separated from the roof in a few areas of the mains. Immediate roof conditions proved to be sound but questions arose concerning the stability of the main roof and floor. Core tests were done to better determine the rock strengths and characteristics. The paper presents the ideology used to tackle the effects of mining over old works in an operating coal mine. This paper discusses the analysis of the symptoms observed and the steps taken to ensure that this pillar separation was caused by the underlying old workings and that the separation was expected and controllable.

Jan 1, 2009

By Anthony Iannacchione

Coal mine bursts have represented a major hazard for U.S. mining operations for more than 90 years. During this time, many prevention controls have been developed and tested. This paper reviews 11 prevention control techniques. Although coal mine bursts are not common events in most underground coal mines, their occurrence almost always requires a change in mining practice. Over the many years of dealing with these hazards, specialized requirements for layouts and novel extraction sequences have been developed on a site specific basis. The keys to mitigating risks are to properly assess the coal burst hazards and to possess the knowledge and skills to prevent or remediate their occurrence.

Jan 1, 2008

By M. J. Beus

This paper will describe the research approach being initiated by the Spokane Research Center to assess hazards and design criteria to prevent ground and muck falls in and around ore passes in hard-rock mines. The major goal of this research is to compile detailed information about ore pass processes and hazards, identification and testing of prevention strategies, and development of computer-based measurement and design methods. Recent ore pass failures have underlined the lack of OE pass design capabilities and standards available to both mine engineers and MSHA personnel. Design criteria and ore pass hang-up prevention and remediation strategies will include the effects of internal ore pass loads, blasting to relieve hang-ups, and rock mass stress redistribution and structural interaction from nearby mining. New visualization and three- dimensional analysis methods will be presented as an extension of existing ore pass design and analysis techniques. A sophisticated particle-flow analysis program will be used to simulate the response of various ore pass designs to a wide range of loading conditions driven by rock mass deformation, muck flow, ore pass blockage, and blasting in both nearby stopes and the ore pass. Analysis techniques will be calibrated and design criteria validated at the Spokane ore pass test facility. These tools will serve to reduce research costs by limiting the number of experimental field tests required.

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 Rob Thomas

Adequate load transfer is considered essential for optimum ground support and is increasingly important in weak roof and/or high stress environments. If adequate load transfer is achieved not only can this have a positive impact on the resulting roof behavior, but it may also allow the use of lower and hence more economic densities of roof support. A wide variety of post-groutable cable bolts are currently in use in the Australian coal mining industry, each of varying capacity, design, and grouting methodology. Though all of the cables are supplied with the appropriate steel specification, strength, and modulus data, the Australian coal mining industry does not have access to a comprehensive and comparative database on the corresponding load transfer performance, which is a critical problem. On the basis of the above, this paper describes the methodology and results of a laboratory pull testing program that aims to assess the impact of cable bolt design on load transfer, and specifically, the impact on the capacity and stiffness of the anchorage. As part of the testing program, 19 cables were tested including (1) plain-strand cables installed in holes drilled with 28 to 55 mm diameter drill bits and (2) bulbed and nutcaged cables installed in holes drilled with 42 to 64 mm diameter drill bits. Other parameters assessed included the significance of smooth and profiled wire, wire packing, and expanded geometry bulbs. The main results of the testing indicate that in comparison to plain-strand cables, if the cable is bulbed or nutcaged, the capacity of the anchor can increase by up to 400%, and the peak stiffness by between one and two orders of magnitude. The results also indicate that increasing hole diameter may have a positive impact on load transfer in the case of bulbed or nutcaged cables and a negative impact on plain-strand cables. No conclusive results were obtained with regard to the impact of grout strength on load transfer.

Jan 1, 2012

By Ian Misich

A research programme has been undertaken to evaluate and predict the effect of total extraction of coal by underground methods on the superimposed strata, aquifer systems and ground surface in the Collie Basin. Western Australia. The research work has integrated comprehensive field monitoring of surface and subsurface subsidence with empirical, analytical and physical modelling in an effort to develop an effective predictive tool which can be used in all mining scenarios. Early results indicate that although each method has certain limitations, this approach has enabled the researchers to develop a very good understanding of the subsidence mechanisms from the mine panel through to the surface in the weak, water saturated sediments in the Collie Basin. Given that both subsurface and surface subsidence characteristics have tied in well, greater confidence can be held for predictions made by representative models.

Jan 1, 1993

By Hamid Maleki

Roof falls are one of the leading ground control problems that adversely influence productivity and worker safety in underground mines, construction sites, and nuclear waste repositories. To control roof stability problems, the operator needs to have practical methods for measuring changes in strata conditions and criteria for defining the timing of secondary support installation. The objective of this study was to develop measurement techniques and criteria for assessing roof conditions at the mining face. An integrated rock deformation and tomographic imaging method was developed and applied at two sites in a western U.S. trona mine. It was shown that rock damage occurred at a deformation of 30 to 60 mm (1.2 to 2.4 in) (2 to 4 pct strain) and was associated with a significant (16 pct) reduction in wave velocity. Thus, supplementary support may be installed when there is a measured change in wave velocity of at least 16 pct. Tomographic measurements can be obtained in rocks rapidly, thus providing a powerful tool for analyzing growth of failure and for installing supplementary support systems in a timely manner.

Jan 1, 1996

By Jiulong Cheng

Underground coal mining causes overburden deformation and failure, one major form of which is bed separation in the overburden. Surface subsidence can be controlled by injecting grout into the bed separations. For a long time, the grouting results of bed separation can not be validated. This is the problem in the application of this technique. This paper analyzes the conditions leading to bed separation, develops mathematical models for seismic wave fields generated before and after bed separation, and bed separations grouted with different materials, and computes the seismic wave field characteristics using the finite difference method. The results show that the existence of bed separation causes changes in the characteristics of seismic wave field, and that different grouting materials generate different seismic wave field characteristics. Intact strata, bed separation, and bed separation filled with air or water all display distinctive seismic wave field in amplitude and frequency spectra. After mining, the original location that provides strong seismic reflection wave no long exists. It is replaced by a weak and syncline-shaped reflection wave. The area, where the seam wave disappears, is larger than that of the mined area. The location, where a bed separation occurs, generates a set of reflection waves stronger than the normal one. The energy of the reflection waves varies with the materials filling the separations. Bed separations filled with air generate stronger reflection waves than those filled with water. The major frequencies for those filled with air are lower than those filled with water. Therefore, it is possible to determine the property of filling materials by analyzing and comparing the amplitude and frequency spectra of the reflection waves. Case studies of application of seismic method for exploring bed separations and grouting were analyzed. The results show that it is feasible to use seismic technique for exploring bed separations and grouting.

Jan 1, 2006

By Floyd D. Varley

Outbursts, the sudden and violent out- pouring of material into mine openings, is a ground control problem increasingly encountered in the United States as deeper seams are developed. Outbursts, caused by several mechanisms, are the result of the loading of ground support systems be- yond their designed capacity to failure, manifested in a violent release of stored energy. There are several types of outbursts typical to deep underground coal mines. - Outbursts of the coal face due to the presence of pressure abutments in close proximity to the face. These outbursts are usually accompanied by liberation of large amounts of methane. - Instantaneous convergence of the ribs and floor due to overloading and failure of ribs which lengthens the beam of floor strata sometimes resulting in a chain reaction failure of the mine floor. - violent failure of the confined core of a pillar due to rapid loading beyond the yield point of the pillar.

Jan 1, 1986

By Xiangxi Wu

Longwall and continuous mining are prevalent methods employed by Chinese underground coal operations. The main ground control challenges include roof skin deformation, roof collapse, and outbursts of coal, gas, and water. Microseismic monitoring provides valuable information on rockmass behavior and fracture propagation caused by stress redistribution, active geological structures, or gas build up within the coal strata and the surrounding rockmass. Collecting and assessing seismic data has proven to be an instructive tool for engineers to better assess ground conditions and mitigate seismic hazards associated with mining. The development of intrinsically safe and explosion proof certified seismic monitoring equipment has revolutionized China?s underground coal mines. The use of this technology allows for the optimization of gas drainage at an increased rate of two at a 50% higher purity without any increase in drilling costs. Microseismic monitoring can also be used in identifying seismically active faults and shear zones. This information can be used to assess and revise mining strategies to improve safety and ground stability.

Jan 1, 2012

By Thomas Barczak

A cooperative study was conducted with Emerald Coal Resources, L.P., an affiliate of Foundation Coal Corporation, and the National Institute for Occupational Safety and Health (NIOSH) to evaluate the effectiveness of (partially) pre-driven longwall recovery rooms supported with pumpable roof supports. This paper evaluates the load transfer mechanics associated with the advancement of the longwall face into the pre-driven recovery room and the widening of the room to recover the longwall shields. The results show that the yielding of the panel fender produced uncontrollable convergence that caused yielding of the shields and stiff pumpable roof supports when the face was approximately 10 ft from the recovery room. The shields continued to yield until the face advanced into the recovery room. Convergence in the pre-driven recovery room typically reached 6-8 in once the pumpable supports yielded and shed load through a declining residual load capacity prior to being cut out by the longwall shearer. Roof deformations occurred beyond the 32-ft horizon with total deformation ranging from 2 to 4 in, indicating that standing support was necessary to help control the span as the longwall face advanced into the pre-driven recovery room. Stability improved as the pumpable supports were cut out, thereby reducing the span between the shields and the outby pillars. In the end, the shields were successfully recovered under stable ground conditions. However, a disconcerting discovery was the load shedding of the outby pillars as the recovery room was mined into. It is postulated that this may be due to the progression of the rear ?abutment? moving toward the recovery room as the panel fender and standing support in the recovery room yield and shed load. This behavior was not expected with a relatively narrow room that is fully supported with standing support. If this mechanism is indeed occurring, the width of the room and performance of the standing support becomes even more critical. Premature failure of the support can lead to excessive convergence on the shield line that cannot be controlled even with modern shields, resulting in roof instabilities that can lead to catastrophic results.

Jan 1, 2007

By N. I. Aziz

The importance of using disc cutters for rock excavation is reflected by their wide application in hard rock cutting machines such as tunnel boring machines and raise borers. The forces involved in the cutting process are those due mainly to thrust and rolling forces and are greater than that required by cutting picks. An interesting feature of cutting with discs is the formation of surface crushing, surface and sub-surface chipping and primary crack formation. Experimental, as well as analytical, work has been carried out to determine the optimum spacing between the disc cutters to control primary crack orientation. Wedge bit indenters were used to simulate disc cutters. The paper concludes by proposing a new approach to rock excavation by hybrid cutting based on controlling the primary crack orientation.

Jan 1, 1992

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 Michael Bayerl

The HILTI OneStep rockbolt was implemented for roadway development in German deep coal mines in 2004. The development by HILTI Company is based on a technical joint venture with RAG Deutsche Steinkohle AG. The technical development of a One-Step bolting system was required for weak strata conditions at the roadway development face. Standard resin bolting has reached the limit of performance when boreholes are squeezed and cannot be filled with capsules. In some cases the heading face is brittle and requires additional support for installation of the bolts and a person working immediately next to the borehole. The Hilti One-Step system allows automated and safe processing at the heading front and provides the key technique to avoid these problems.

Jan 1, 2009

By Hamid Maleki

In this paper, the authors present a review of data gathering programs and stress analyses for improving mine layout designs at BRL mines located near Paonia, Colorado. BRL and other companies have used both longwall and continuous miner methods for the extraction of the Upper D, Upper B, Lower B and C seams from the multiple-seam reserves in the North Fork Valley. Historic mining in the B and C seams has created stress footprints in the neighboring unmined seams compounded by stress transfer from recent longwall extraction in the Upper D Seam. Thus optimum positioning of workings in the remaining reserves requires careful collection of pertinent data and consideration of load transfer from previous workings through remnant abutment pillars. Based on long-term subsidence monitoring, underground mapping and stress analyses, we have confirmed initial load transfer analyses finding mild interactions between the UD and LB workings, and the mining conditions have proven to be good and manageable during the development stage of the LB Seam. This has been achieved through prudent mine planning, prudent positioning of the gateroads in multiple seam workings and support allocations.

Jan 1, 2007

By Stephen C. Tadolini

The International Ground Control Conference in Mining has always provided an open forum for the publication, presentation, discussion, and often heated debate on roof bolting systems mostly with attention to how, when, and why they work. During the last 24 Conferences at least 170 papers have been presented by International Experts on these ?simple? devices that range in length from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof bolts are primarily mechanically anchored, glued, cemented, or driven. Bolts are placed vertically, angled, or tied together with special fixtures (trusses and slings). They have been termed active, passive, stick-slip, and several other descriptive acronyms or mining slang expressions. This retrospective paper will present the changes in bolt types and usage, point out some of the biggest myths and hail the most significant advances (according to the authors? opinions of course).

Jan 1, 2006

By P. Oliver

Rockburst conditions were first encountered at Inco's Sudbury area operations in the mid 1930's when mining advanced to below the 610 m (2000 ft.) horizon. Now, mining is being carried out at depths down to 2130 m (7000 ft.) at Inco'a Creighton Mine. While rockburst potentials have increased with depth. improvements in mine planning, methods, support systems and developments in destressing have alleviated the condition to where it is less of a problem now than it was in the early years. Hang of the improvements were developed, of necessity. through observation and reasoning because these were the only tools available at the time. Now, we are making use of stress - measurements, numerical modelling and microseismic source location technology to - help further reduce rockburat hazards.

Jan 1, 1987

By I. V. Baklashov

Geomechanical conditions of mining operation are determined by varying in time and space mechanical properties and initial state of the rock mass, by geological and technological factors. We define adaptive mining technology as a technology capable of adjusting to changing geomechanical conditions of mining operation. The ongoing research at Moscow State Mining University consisting of laboratory, theoretical, and field investigations aims at the development of theoretical basis and methodology of the system of geomechanical support of adaptive mining technology for the bedded salt deposit in the Urals, mined by room-and-pillar method. This system of geomechanical support is capable of - dynamically assessing mechanical properties and state of the rock mass varying within the panel; - mapping mechanical properties of the rock mass within the panel; - assessing and predicting mechanical condition of load-bearing elements of the rock mass and its changes due to the adjustment of the parameters of mining operation considering the in-situ geomechanical conditions.

Jan 1, 1996

By Y. L. Sun

The feasibility of using the ultrasonic wave attenuation method for measuring in-situ stress is examined theoretically and experimentally. The theoretical analyses show that for a given rock type the relation between stress and attenuation coefficient ratio can be approximated closely by second-order polynomials. Combining the elastic and statistical theories, the magnitude and direction of the existing principal stresses can be predicted by using the relations between stress and attenuation coefficient ratio obtained in the calibration of core specimens. Several experiments are conducted in the laboratory on two types of rock. The experimental study consists of two parts: the attenuation-coefficient measurements in core specimens under uniaxial stress and the attenuation coefficient measurements in boreholes drilled through blocks of sandstone and limestone subjected to uniaxial compressional stress. Compressional waves are used exclusively in the measurements.

Jan 1, 1989

By B. Macdonald

In 1991, roofbolts were introduced as a primary support for inseam retreat longwall entries at CBDC's Phalen Colliery. Increasing mine depth (+550 m) necessitated replacing conventional inseam steel sets in these entries because they were being severely damaged during wall retreat and subsequently causing gate end problems. Roofbolts had been used in the Sydney coalfield between 1949 and 1963 in room and pillar working, however, some concerns arose with using roofbolts as a primary support in long (2.2 - 3.4 km) single entry roadways. In order to alleviate these concerns, a phased approach was adopted whereby conventional steel densities remained along with roofbolts. The roofbolt design was such that the roofbolts would be the primary support and the steel sets would experience little or no loading. A detailed geotechnical monitoring program was developed by both CBDC and the Cape Breton Coal Research Laboratories technical staffs. The results of this monitoring were used to build-up confidence in roofbolts and subsequently reduce the steel set densities until eventually only roofbolts were used as a primary support. This paper will discuss briefly the phased introduction of rootbolts at Phalen Colliery and more specifically the development of geotechnical data collection and interpretation.

Jan 1, 1994