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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

By Peter Fischer

Reducing mining costs has become such an important factor in the German coal industry that operators are forced to work only those areas where production costs can be kept low. These areas will normally be located close to the mine shafts because they are easily accessible. Even the shaft pillar is now being considered as part of the winning zone. Before working coal in shaft pillar zones it is essential to assess the impact this will have on the shaft lining. There is a huge difference between sliding and strata-anchored shaft systems. However, if the overburden comprises lightweight soil both types of shaft will have a tendency to protrude from the surrounding ground. This paper presents the geomechanical reasons underlying this reaction and the methods used for predicting and measuring it.

Jan 1, 2007

By Mingju Liu

China has been suffering from coal and gas outbursts since 1950s. Nowadays, the situation has become worse than ever with the need to mine deeper coal deposits at higher production rates in a more complex underground mining environment. In the year 2006 alone, 37 outbursts occurred in underground coal mines with a death toll of 232. This paper describes a case study of an outburst event, which occurred in July 2006 during the shaft development of the Mengjin Coal Mine, Henan Province, China at a depth of 768m below surface. Detailed analysis of the possible causes that led to the occurrence of the outburst is reported. These include the study of complex geological features, gas storage conditions, possible improper technical management practice as well as incomplete prevention and control measures. Based on the case study and the observation of outburst occurrence in China in the year 2007, suggestions have been put forward to help prevent and control future coal and gas outbursts.

Jan 1, 2007

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 Nic Slatalla

Acoustic emission monitoring techniques are a powerful tool for analysing deformation behaviour and fracture characteristics of rock samples under uniaxial loading. Uniaxial compression tests were carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage develop-ment. All sandstone samples revealed congruent characteristic acous-tic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages proved to be entirely different due to variations in their microstructure.

Jan 1, 2008

By Francis S. Kendorski

Over the past 25 years, the present author has written with several co-authors a series of papers beginning with the landmark paper in the 1st International Conference on Ground Control in Mining in 1981, in turn, based upon U. S. Bureau of Mines (USBM) funded contract research managed by the author with a final report issued in 1979, all concerning this paper?s title subject, Effect of Full-Extraction Underground Mining on Ground and Surface Waters. Initially the work was a summary of British, Russian, and Hungarian experience tailored to United States strata conditions, but has evolved into a consistent and well-documented model of the behavior of strata influenced by full-extraction underground mining such as longwall coal mining. The several strata zones recognized in these works, Surface Fracture Zone, Constrained or Aquiclude Zone, Dilated Zone, Fractured Zone, and Caved Zone, have been observed by several workers in the field. The concepts presented initially 25 years ago have been adopted, rightly or wrongly, by state regulators, ground and surface water researchers, and mining practitioners. The development and utility of these concepts and recent findings will be summarized.

Jan 1, 2006

By Paul Singh

Blasting is an integral part of mining activity and is an inherently destructive process. The quality of the rock mass in which an excavation is being driven has a strong influence on the level of dynamic forces to which the rock can be subjected and sustain damage. Though each specific rock mass occurrence is unique, yet there are common rock mass features which influence the outcome of controlled blasting in a similar way. A field study was conducted which involved assessment of blast damage at several operating mines and blasting of small scale models. The effects of block size and strength, joint parameters, ground water, weathering of rocks and rock mass rating on the results of controlled blasting have been investigated and discussed in this study. The influence of number and thickness of weak layers on overbreak control has been discussed in the light of practical data. Critical particle velocity ranges for damage to rock masses with different qualities have been outlined. An approach for the judicious design of perimeter hole pattern and charge concentration has been proposed.

Jan 1, 2007

By David H. Tang

Massive pillar failures were observed at two underground coal mines. The results of stability analysis using traditional pillar design formulae did not agree with the underground observations. Finite element models were used to analyze the causes and mechanisms of the pillar failures. The results were confirmed by the underground observations. Finally, the improved mine plan for each case was derived to ensure the stability of the underground structure.

Jan 1, 1990

By Bruce Hebblewhite

This paper recognises the 25th year of the Morgantown International Ground Control in Mining Conference, by offering a twenty five year review of ground control in the Australian mining industry ? primarily in underground coal. The paper provides an overview of the developments in ground control design methodologies, practices and products. It not only summarises what has been done, but attempts to highlight at least some of the major issues, significant developments, controversies and areas of contention, plus some of the continuing gaps in our understanding or capabilities as we go forward into the next 25 years. Some of the issues included are in relation to: the development of the high strength bolt and cable reinforcement practices widely adopted today and the contrast between stiff and soft systems; a focus on local detail, at the expense of understanding regional stability concerns; dramatic improvements in the use of, and systems available for underground geotechnical instrumentation and monitoring; some of the ground control management trends, including occupational health and safety concerns, availability of suitably trained staff and legislative requirements; the opposing approaches of the empiricists versus the numerical modellers ? is there some middle ground?

Jan 1, 2006

By A. G. Pasamehmetoglu

This paper describes the conver¬gence and load measurements carried out in inclined seams at five longwall faces, two of which were supported by hydraulic props and the other three by conventional timber supports in Karadon-Gelik mine. The minimum support capacities determined in this study are compared with the results obtained from two empirical approaches suggested by Wilson (1975) and Jacobi (1981). The characteristics of hydraulic props to be used and support patterns in the longwalls are found, conclu¬sions are drawn and suggestions are made. Design nomograms are prepared for field engineers, in order to be able to approach the problem more confidently and realistically.

Jan 1, 1992

By Holger Witthaus

Beside the traditional geomechanical evaluation methods the geotechnical parameters are recognized as important facts of the planning work for multiple seam mining. Standards for applied monitoring and investigation methods for geotechnical determination of drill cores, laboratory rock-tests and observation of roadheadings are the basics for rock mechanics classification and support design. The geomechanical classification is in this context an essential element. The planning procedure, as it is becoming more and more detailed, beginning with the panel layout up to the technical instructions of roadway development and design of roadheading machinery, nowadays uses a lot of geotechnical information that also are expanding to include quantitative determination of more parameters. The current monitoring system includes the description of separation planes using drill cores and roadheading observation as well as laboratory tests of rock-strength and geophysical borehole examinations. This information is analyzed within the geomechanical standard-classification. The re-engineering process of comparison the predicted support success to the actual monitoring results provides the opportunity for continuous optimization of the planning work und support design. This way of empirical analysis is used for basic planning work in Germany for decades. But using the technical standard of information technology and a centralized database now it is possible to reach quick-win effects in implementing research results on the one hand and actual experiences on the other hand.

Jan 1, 2006

By Kot F. Unrug

Strength of coal is one of the governing factors in size determination of mine pillars, and therefore affects the percentage of extraction and thus, utilization of reserves. Coal strength is also important from the subsidence control standpoint, where mine pillars with appropriate safety factors should be designed for long term stability. Determination of coal seam strength has been addressed through the extrapolation of the coal strength obtained from a small number of samples on the whole section of the seam. This procedure often gives unreliable results. Presented in this paper is a method of in-situ strength testing of coal, allowing for a realistic determination of seam strength. Results of a field case study dealing with the coal seam, where samples for the laboratory strength tests could not be obtained due to the dense cleat, are presented. However, in these conditions, the in-situ testing was successful. A comparison of both conventional and the in-situ penetrometer method was carried out. It was found that the penetrometer data not only improve the accuracy of strength determination of the entire seam, but also prove to be much more cost effective than the conventional method on a per test basis.

Jan 1, 1995

By Kot Unrug

Mine seals are necessary in nearly every mine to isolate mined-out areas from the ventilation network. There is a large number of seals in active mines and that number continually increases as the development spreads across the reserves. The recent accident caused by the failure of seals in the Sago mine resulted in new regulations. These regulations have been set based on a principle of eliminating the risk of failure, rather than the sound engineering and understanding of the conditions that may cause seal failure during an explosion of methane in sealed areas of the mine including the probability of such occurrences. This paper discusses the following subjects related to mine seals: The potential causes of methane explosions and the dynamic effects of pressure wave propagation. Seal materials and methods of construction in view of the interaction of the seal with the host rock environment as well as requirements for gathering geotechnical data. The design of seal dimensions is based on loading conditions, therefore the probable ranges of such loading is reviewed. There is also discussion of possible methods for shielding of seals from pressure pulses following an explosion. Particular seal?s types are examined in reference to the conditions at the seals location. A novel design is presented that will be a low- cost solution for the sealing of mined out areas. Recommendations are given for further research leading to a rational engineering system for design of new seals in coal mines.

Jan 1, 2008

By S. Paul Singh

Rock burst is one of the most serious mining hazards, adversely affecting the safety, economy and productivity of mines. It is a sudden manifestation of the release of strain energy stored in the rock mass. At present, there is no universally accepted method for predicting the timing of the rock burst. However, if the seismically hazardous regions in mines can be identified before they are exposed to mining then precautionary measures can be planned before the work begins. A study was conducted in Sudbury Nickel Belt and an approach has been proposed to identify rock burst prone workings on the basis of (a) Rock properties and bursting indices (b) In-situ approaches (c) Geological features (d) Mining conditions (e) Observations during mining operations (f) Comparison and analogy with previous case histories of bursting. This approach will facilitate the judicious application of the burst control techniques to seismically hazardous areas.

Jan 1, 2008

By John E. Feddock

Retreat mining plays an important role in the coal production within Kentucky and the Appalachian Coal Basin. In Kentucky, the majority of retreat mining is performed in the Eastern coal field. However, since September 2004, Kentucky has suffered four mining fatalities during retreat mining operations, and the coal industry has focused attention on the safety concerns associated with retreat mining practices. The Kentucky Department of Natural Resources (KYDNR) engaged Marshall Miller & Associates, Inc. (MM&A) to conduct a study to ensure that retreat mining is utilizing the best technology currently available and that the current available employee training programs afford maximum safety to miners. MM&A studied 34 coal mines that were conducting active retreat mining operations and represent about one third of all the mines with approved retreat mining plans in Kentucky. This paper summarizes the major research work conducted (Feddock and Ma, 2006) and recommends the practical methods for improving safety during retreat mining operations.

Jan 1, 2006

By Kieran Black

The immediate roof lithology at United Colliery is extremely variable and provides a challenge for the design of secondary support. Support design in recent longwall installation roads at the mine has been carried out using strata management techniques. These techniques allow a robust yet economical secondary roof support design to be achieved. Analysis of roof lithology and roof displacement during first pass driveage are used as a basis for the optimisation of the secondary support design prior to roadway widening. This paper will demonstrate the application of practical strata management techniques in the optimisation of secondary roof support in six installation roadways at United Colliery. The experience gained from the installation roadways for LWs 1 to 5 will be briefy discussed. The experience from LW 6 will be discussed in greater detail. The paper will describe the strata management system that was used, including mapping, borescoping and extensometry, as well as the development and application of a Trigger Action Response Plan (TARP).

Jan 1, 2007

By Guo Junfeng

The strength of coal mass saturated with gas is an essential parameter in studying the mechanism of gas and coal outburst, and its prediction method. Up to now, there has not been a method for determining the strength of coal mass saturated with gas. This paper describes a new method whereby the strength of in-situ coal mass can be determined and stored continuously and automatically. The simulation results obtained in the laboratory indicate that the new method allows a fast, reliable, inexpensive and convenient determination of the strength of coal mass.

Jan 1, 1992

By Luis Giraldo

The installation procedure of fully grouted rebar bolts requires that the bolt be pushed through the resin grout until it is fully inserted in the borehole. Resin grout is a highly viscous fluid that is forced to flow through the annulus between the bolt and the borehole as the bolt advances. Variables such as bolt insertion speed, bolt vs. borehole diameter and grout viscosity determine the magnitude of the pressure generated ahead of the bolt tip. Under weak rock conditions, the magnitude of the pressure of bolt insertion is often sufficient to initiate hydraulic fracturing along joints or bedding planes that allow injection of grout into the strata leading to significant loss of bolt encapsulation. The grout pressure front has also been linked to gloving, a bolt installation problem that reduces bolt effectiveness. A series of tests were conducted to characterize the pressure of bolt insertion during bolt installation under various conditions. Analysis of the data shows three distinct stages of pressure build up during bolt installation. Understanding of the mechanisms involved may allow development of strategies for controlling the pressure buildup and its negative impact during roof bolting under weak rock conditions.

Jan 1, 2006

By Jeffrey Whyatt

Mining coal in deep, gassy strata is difficult, particularly in deep mines of the west. Dynamic failure (bumps, bounces, etc.) is commonplace, particularly where strong sandstone strata are encountered in the overburden. The disaster potential posed by dynamic failure in these mines is examined and found to be significant. Recent trends in MSHA reportable bumping of deep western coal mines have been increasing. Recent clusters of bumps are explored, a number of which have ended badly. The mechanisms of these bumps and their interactions with geologic structures are explored. Significant progress in controlling these hazards depends on a full understanding of their interaction with geologic conditions. It is apparent that not all MSHA reportable bumps have the same mechanism. These reports describe bumping of pillars, rolling of ribs, outbursts of coal, heaving of floor, shaking-induced roof falls, etc. Progress depends on understanding the underlying mechanisms of these events, and then determining which are potentially active at a particular location. Designs and other protective measures can then be adapted and deployed appropriately. Explicit consideration of mechanisms also supports extrapolation beyond experience as design extends into new mine geometries, geologies, mining methods and increasing depth. Finally, the goal should not be only to design hazard out of mines, but also to provide assurance that this, in fact, has occurred. Pursuit of this goal has been taken up by a major NIOSH research project. The centerpiece of this project is development of a Dynamic Failure Control Program to monitor evolution of dynamic failure hazards with changing geologic conditions to assure that control and protective measures are appropriately deployed.

Jan 1, 2008

By Wenquan Zhang

Based on the mining and geological conditions of Wadong Mine, Longkou City, Shandong Province, China, a viscoelastic model with proper creep parameters was established. The 3-D ANSYS was used to simulate surface subsidence, analyze overburden subsidence characteristics, and propose the rational mining and pillar widths for mining in weak rocks and under special mining conditions, and provide the policy makers scientific data for decision making on mining activities.

Jan 1, 2006