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By G. C. Sun

Rock mechanics properties, i.e. uniaxial and triaxial compressive strength, indirect tensile strength shear strength, Young's modulus, and poisson's ratio were collected for coal and related strata from 50 coal seams in 90 mines. The methodology of data organization and retrieval is described in details. The values and .ranges of those properties are also discussed.

Jan 1, 1993

By Mike Watts

Since the first longwall panel development started in the new mining, (Southern) area, a number of roof falls have occurred in L1 and L2 gate-entries. The longwall was delayed for a total of two months due to roof falls in the gate-entries. To prevent roof falls in future gate-entries, an extensive ground control analysis has been conducted which incorporated regional horizontal stress and panel orientation, tectonic stress, roof strata, surface stream valleys, etc. Results of the analysis indicate that a combination of these factors caused the roof falls. Therefore, a Roof Instability Rating (RIR) system was constructed to predict the roof condition and identify areas where supplemental support should be implemented. The L3 gate-entries, supported by the prediction using the RIR system, have not experienced any roof problems.

Jan 1, 2000

By Youzhen Zhang

The drilling boom is the key component of a roof bolter, and it has significant impact on the performance, safety, and reliability of the overall working device. In this paper, the experimental modal parameters of a drilling boom at low frequencies are identified with the Laplace wavelet correlation filtering method. Based on the obtained experimental parameters, a finite element model of the drilling boom is revised using an optimization algorithm to improve its accuracy. The dynamics of the drilling boom are analyzed through its modal and harmonic response. As a result, an improvement for the drilling boom is proposed to fix the weak links and enhance the dynamic performance.

Jan 1, 2012

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By Bill Stormes

This paper will discuss implementation of the Hilti OneStep rock anchor technology to improve face bolting operations during the May 2008 longwall move; increase rock anchor installation capacity; and improve safety by reducing employee exposure periods by utilizing the Hilti OneStep rock anchors.

Jan 1, 2008

By Thomas Barczak

Roof support systems are designed for roof control to prevent unplanned falls. It sounds logical and has been the conventional thinking in support design since supports were first installed. In order to determine which support system should be used or which would be the most effective, the degree of control provided by the support system must be known. This question embodies the concept of a ground response curve, which is a measure of support control by assessment of the convergence in the mine entry as a function of the support capacity. In this National Institute for Occupational Safety and Health (NIOSH) study to optimize standing roof support design, ground response curves were developed for longwall tailgate conditions from numerical models of Pittsburgh Coal Seam geology. The models were calibrated against tailgate convergence measurements that were made in two Pittsburgh Coal Seam mines as the depth of cover varied during the panel extraction. Ground response curves were developed for four loading conditions: (1) development, (2) side abutment, (3) front abutment near the longwall face, and (4) full extraction inby the face. In general, the tailgate convergence and required support capacity increase through each of these loading stages. It was concluded that prior to failure of the rock mass, the support system regardless of its capacity has relatively little impact on tailgate convergence. Recognizing that the support cannot prevent much of the (pre-failure) convergence that is occurring is an important part of support design. Supports that exhibit high loading stiffness but cannot sustain that loading through a displacement compatible with the ground response curve will not provide adequate roof control because they can fail prematurely. The required capacity depends on the loading plus yielding characteristics of the support and the amount of rock failure that has occurred. The loading plus yielding character establishes where the support loading intersects the ground response curve. Design criterion for support capacity based on the identifying the onset of strain-soften rock response leading to ?damaged roof? is proposed as a foundation for assessing support design requirements. The capacity without accounting for the loading and yielding character of one support should not be used to assess the capacity requirement of an alternative support design, particularly when the loading and yielding characteristics are significantly different. A sensitivity study was made to evaluate the impact of mining height and overburden depth. As expected, the results show that convergence increases with increase in mining height and increase in overburden depth. Some specific support examples are also analyzed. Although this is a first step and the two-dimensional numerical models have limitations, these initial studies provide valuable insight into the control provided by the roof support system and will ultimately lead to optimizing support design based on specific mine site conditions.

Jan 1, 2008

By Jan A. Nemcik

Steel mesh has been used successfully for many years to control friable roof conditions and prevent loose roof and rib material from caving into the roadway. Despite its extensive use, the installation of steel mesh is difficult to automate and many other products have been evaluated as an alternative for skin control. Ideally, the properties of these new products should be similar or better than those of the steel mesh. In particular, development of a strong and resistant shell that minimizes movement along the fractured rock and coal surfaces between the bolt anchors is recommended. A strong surface adhesion and the strength of a reinforced polymer skin can provide the necessary toughening mechanism required to enhance roadway surface support by forming a reinforced polymer/rock surface layer. Most of the Thin Spray-on Liners (TSL) evaluated in the mines are weak with slow curing times, and the plastic mesh currently used to support the coal ribs is relatively weak. Therefore, neither material can seriously compete with steel mesh. Currently, a new polymer product that cures in seconds and forms an instantaneous strata binder that surpasses the properties of steel mesh is under development at the University of Wollongong.

Jan 1, 2009

By S. L. Huang

The location and shape of potential failure surfaces in soil slopes were directly determined based on the stresses induced in the soil mass and the strength of the soil itself. Stress calculations were carried out by the finite element method with the Drucker-Prager nonlinear material model. The Mohr-Coulomb failure criterion and Mohr's circle stress analysis were then applied to the calculated stresses to determine the characteristics of the failure surface.

Jan 1, 1989

By Greg Hendon

Various methods have been used to evaluate the effectiveness of shields in place. The most common method used is static recording of leg pressures by visually examining pressure gauges. In an effort to more closely examine the shield cycle and therefore the shield effectiveness, Jim Walter Resources has implemented a program of monitoring utilizing a polycorder system which is capable of recording convergence and pressure measurements on a continuous basis. This paper will discuss two longwall panels at Jim Walter Resources which have been monitored.

Jan 1, 1990

By Kourosh Shahriar

The occurrence of rock falls in underground coal mines entails detrimental effects as fatal or non-fatal injuries on workers, stoppages in mining operations and breakdown of equipment. In this paper, a risk assessment approach on the basis of a decision analysis trend is employed in order to assess the possibility of and manage roof falls. Risk is then assessed by determination of likelihood of occurrence and the cost of consequences (outcomes). In this regard, collected real roof fall data from Tabas and Kerman coal regions comprising of several underground coal mines are used. It is concluded that the annual accidents due to the roof falls occurrence in the all investigated mines are so high that it is economically feasible to improve the support systems and to implement a suitable educational program as well as an accurate supervision and other elements of safety management.

Jan 1, 2009

By K. Unkug

It is commonly accepted that the tension of roof bolts can be used as a measure of the effectiveness of roof support performance in coal bearing strata. The conceptual base for the assumption originates from beam theory. Roof bolt tension acting across the bedding and other weakness planes increases friction, preventing potential separation and slippage within the roof strata and therefore, increases the stiffness of the roof beam between the open face and the rock-mass enclosed within the anchorage. Thus, it is important to monitor roof bolt tension, especially for research purposes, and consequently many measuring systems were developed based on numerous working principles. Research planned by the Department of Mining Engineering at the University of Kentucky required a large number of tension measuring devices of high precision. These were intended to operate reliably in corrosive mine environment over long periods of time. After evaluating the existing systems, it was decided to develop a new type of roof bolt dynamometer which would fulfill the above requirements at minimal Cost. Nearly one hundred such dynamometers were constructed, calibrated and installed in two underground mines. They remained in service for a period of one Year, producing a valuable database for the Project. In this paper the working principle of this new type of dynamometer will be described and technical considerations and samples of the collected data will be given.

Jan 1, 1986

By Stephen Iverson

The National Institute for Occupational Safety and Health (NIOSH) is currently performing research to help mine operators minimize the amount of loose or damaged rock surrounding a blasted opening. Improperly designed blast rounds can result in excessive wall rock damage that may reduce the rock mass competency and increase ground support requirements. Unnecessary blast damage can cause many safety and operational concerns including an increased potential for injury from rock falls and other hazards resulting from sub-optimal blast designs. This paper focuses on the ground control and safety implications of blast damage in underground mines. A reduction in rock quality due to blasting can increase the amount of ground support required. Case study data collected by NIOSH in underground metal mines during standard drill and blast production rounds included blast vibration monitoring, detailed three-dimensional laser scanning of the as-built excavation, and geotechnical mapping of development headings.

Jan 1, 2007

By Francis Kendorski

The mining engineering design professional has limited practical and reliable tools for planning successful room-and-pillar stone mines using readily-available and collectible information. Three techniques are in common use today: the hard rock CANMET method of Hedley and Grant, the hard rock method of Stacey and Page, and the oil shale method of Hardy and Agapito. Other methods have been proposed, such as the USBM method of Obert and Duvall, the CSIR/Penn State method of Bieniawski, and the soft rock confined core method of Abel, Wilson, and Ashwin. However, the latter have practical shortcomings when applied to room-and-pillar stone mines such as developed for construction aggregate production. Ideally, the use of multiple techniques resulting in the same acceptable and reliable answer is the goal. Recently, in several underground stone mines areas of undersized pillars were examined. These undersized pillars apparently resulted from non-adherence to a mine plan. These undersized pillars now exhibit strong evidence of incipient failure such as slabbing, opening of through-going fractures, and hour-glassing. This situation allows examination of pillar designs at a ?safety factor? of essentially one. This rare opportunity allowed the examination of the suitability and adjustment of the first three design methods discussed, resulting in greater overall confidence in the methodologies.

Jan 1, 2007

By Christopher Mark

Knowledge of in situ stresses is fundamental to many studies in earth sciences, and coal mine ground control is no exception. During the past 20 years, it has become clear that horizontal stress is a critical factor affecting roof stability in underground coal mines. The theory of plate tectonics and the World Stress Map (WSM) project has been extremely helpful in explaining the sources and the orientations of the horizontal stresses observed underground. Recently, WSM geophysicists studying deep-seated stresses have developed a model of how stress magnitudes vary with depth in the crust. They have devoted relatively little attention to near-surface stresses, however. This paper explores the relationships between deep-seated and shallow in situ stresses in several of the world?s coalfields, using a data base of more than 350 stress measurements from underground coal mines. The analysis indicates that distinct regional trends exist, corresponding roughly to the regional stress fields identified by the WSM. The paper presents equations for estimating stress magnitudes that were developed by treating depth and elastic modulus as independent variables in regression analysis. The magnitude of the horizontal stress increases with depth, at rates that range from 0.8 to 2.0 times the vertical stress, just as the WSM ?critically stressed crust? model predicts. Overall, it seems that the stress regimes encountered in underground coal mines are closely linked to those that exist deep in the earth?s crust.

Jan 1, 2008

By P. Trueman

A project is underway to convert a validated methodology to a system that will permit the near real time interpretation of loading data from longwall powered supports, allowing rapid adaptation of mining strategies, maintenance scheduling and set pressures for optimal roof control. Such a system has the potential to significantly reduce roof control problems and is a pre-requisite to successfully fully automating a longwall face. The development depended upon a robust methodology for interpreting the pressure sensor data coming from the face in the context of support strata interaction using a novel characteristic load cycle analysis concept. New software was developed capable of handling the large quantity of data emanating from a modern longwall with the current level of signal corruption and loss, accurately delineating load cycles and extracting the critical load cycle features essential for interpreting how the support and strata are interacting. Existing software being used to access the pressure data were found either to be incapable of delineating load cycles at all or, where they could, the accuracy was low, making it impractical to use them in combination with the new advances in load cycle analysis. The software is currently being extended from ?off line? to ?on line? in order to provide a real time response to changing strata conditions.

Jan 1, 2006

By Anthony T. Iannacchione

The Roof Fall Risk Index (RFRI) is a new method introduced by the National Institute for Occupational Safety and Health (NIOSH) to assist the underground stone mine operator in 1) assessing defects the mine strata and 2) rating the relative roof fall risk these defects pose. The RFRI utilizes observational techniques to identify defects in the roof strata caused by local geologic, stress, and mining conditions. Assessment values for ten defined defect categories provides the foundation for estimating a range of risk conditions between 0 and 100 that define the potential for a roof fall. This paper examines how the defect information is collected using two field verification sites and proposes methods to analyze the RFRI data. These examples provide information on how well the method replicates observed conditions and how it might be applied in practice.

Jan 1, 2006

By Michael Alber

Coal mining by the longwall method typically causes seismic events. This paper describes the research on mining induced seismicity around two specific longwalls at 1100m depth under several old longwalls. A local network featuring 21 geophones has been installed, underground core drilling and an extensive rock testing program were executed. A rock mechanical model featuring a multi-layer strata was created for use with numerical analyses. Seismic events were localized and three different event types are described. Focal analyses yielded distinct modes of failure. Those types are explained by different failure mechanisms, namely fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses allows evaluating the findings from seismology approaches. Conclusions are drawn that the orientation of longwall with respect to the major horizontal stress orientation plays an important role in mine induced seismicity.

Jan 1, 2008

By Ke Yang

In Chinese coal mines, extraction of deep, highly gassy soft coal seams sandwiched between soft floor and soft roof in complicated geology is commonly encountered. How to safely and efficiently excavate such seams is a very significant and important problem. It has been a focused research area in recent years. By combining the characteristics of fully mechanized longwall mining methods such as large cutting height and mining along the dip, an integrated method and technologies for the Zhangji Coal Mine were developed. This technology includes the design of a complicated geological exploration system, optimization of mining method and working face parameters, control of rock pressure and high levels of gas, reliability model of the production system, and selection and compatibility of equipment. Engineering practice shows that the integrated technology was successful in mining these complicated seams. This study not only enriches the mining method, but also improves the coal mining practice, especially ensuring both safety and productivity under complicated geological conditions.

Jan 1, 2009

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 Khaled Morsy

The studied mine extracts the Sewickley seam using room-and-pillar method. The overburden at study site ranges from 400 to 800 ft. Eight years earlier, the Pittsburgh seam was mined 80 ft underneath the Sewickley seam. Parts of the mains at the study site experienced problems of rib spalling associated with floor heave. Numerical modeling was used to explain the mechanism of ground control problems encountered at the mains of the upper seam. The modeling results show that the ground control problems encountered were caused by multiple seam interactions with the previously mined longwall panel of the lower seam. Based on the modeling results, a criterion to predict the probability of multiple seam interactions was proposed.

Jan 1, 2006