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By Mietek Rataj

Installation of pre-tensioned Strand Bolts i.e. resin grouted cable bolts is very common now in Australian coal mines. A short historical development of this type of rock reinforcement is outlined Various systems have been developed to apply 20 to 25 tonnes of pre-load to these bolts but there are still some technical problems which have to be overcome. One of the major problems encountered is bleed off tension which is highly undesirable as required "over loading" rocks the immediate roof thus inducing fractures iii weak strata. The major reason for the loss of tension apart from the design of the tensioning jack, is the design of the conventionally used barrels and wedges. Due to a roof plate tilting during tensioning the cable moves out of the barrel centre which impedes proper positioning of the wedges and subsequent excessive return of the cable once load of the jack is released. This problem has been largely resolved by the design of a special barrel having a centralizing rim allowing for significant reduction of the toss of tension. KEYWORDS: Pre-tensioned Strand Bolts, Cable Bolts, Bleed off Tension, Tensioning Jack, Loss of Tension

Jan 1, 2002

By Kot Unrug

For the purposes of underground mine design, twelve cores of the Springfield (V) Coal (Pennsylvanian) roof and floor were retrieved. Each core was photographed, described fresh in the barrel, and the rock quality designation (RQD) was measured. Cores were wrapped in plastic and delivered to the rock mechanics lab. At the lab RQD was remeasured and new photographs were taken before running geotechnical tests including weatherability. Significant differences in barrel RQD's vs. lab RQD's have been noticed in spite of careful handling. The drill site RQD is generally used to calculate geotechnical results, but in some types of coal measure rocks, further fragmentation of core takes place with slight changes of moisture content, which occur even in well wrapped core. This creates a dilemma which RQD should be used for support design. Rocks around excavations are exposed to the significant changes in moisture content due to the seasonal weather changes and related fluctuation of humidity of air ventilating a mine. Therefore, it appears to be more realistic using laboratory RQD which corresponds closer to the actual rock condition around the excavation. Weatherability test, developed for characterization of time dependent behavior of rock exposed to fluctuations in moisture content, is thought to be related to the above-mentioned changes of RQD. In weak shales, mudstones and claystones which weather rapidly, it is essential to characterize their future behavior in the pre-mining stage. This can allow for a realistic design of primary, and particularly secondary support in coal mines, especially in coal fields where such geologic conditions exist.

Jan 1, 2003

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By Bob Thomas

The roof drill operator productivity can be enhanced by the introduction of control technology The operators control station can be improved by considering human factors such as handle forces and control location. The smaller electrical controls can be located much easier than the traditional hands on valves. The operators control can he improved by using processors with programmable subroutines that assist the operator in the decision making process. For example to increase feed rate - to slow rotation - to avoid clogged steels - etc. The overall affect is to improve drilling and bolting consistency. The control system uses a dedicated processor packaged as an intrinsically safe control station. The roof drill operator is assisted in the drilling; arid bolting by a closed loop feedback system. The processor monitors the drilling parameters such as feed force, feed rate, rotation torque, RPM, drill position, and vacuum condition Based on the drilling parameters the processor executes pre-determined condition statements intended to enhance drilling The drill operator has an active display to alert him to the real time machine status. For example if the feed were to slow or delay due to a high vacuum level the display would show "high vacuum". The dedicated processor is available on the arm feed style machines, The prototype machines used programmable logic controllers to develop the feedback relations. The ability to record drilling parameters was extremely helpful in this development project. The extended use of the recording feature will aid operators in mapping the mine roofs Interpreting the recorded drilling parameters can reveal a void in the top or hard competent drilling The prototype machines also showed the importance of vacuum monitoring This feature is extremely valuable in softer materials, The benefits of the control system range from efficient use of the consumable bolting materials to uniform operator performance.

Jan 1, 1999

By A. H. PH. D Wilson

Compared to the rocks encountered in, say, metalliferous mining or tunnel drivage in igneous and metamorphic rocks, the strata associated with the coal measures are relatively soft. If excavations are made at depths greater than one or two hundred metres, zones of fractured or yielded rock develop around the openings. The stress which can be taken by yielded rock at the boundary of an opening is low, but because of the friction present between the fragments or particles, the sustainable stress rises rapidly with distance from the opening until sufficient confinement has been developed to prevent failure of the rock in the first place. Thereafter the laws of elasticity will apply and increasing distance will reduce the stress field until conditions consistent with the cover load are reached. For rocks which maintain their elastic condition right to the edge of an opening, finite element analysis and similar techniques have proved successful in determining the distribution of stress around the opening. This approach, however, is not successful when a yield zone is introduced between the elastic zone and the opening. Nevertheless, some estimate is required if help is to be given in the siting of roadways and in the determination of adequate pillar sizes. This matter is of particular importance in the planning of new mines especially if retreat faces are involved, as extensive roadway drivage must be planned and executed before experience of the effects of face extraction is gained. In 1977, the author published [I] a method based on 'stress balance'. The total aggregate downward load over a large area will remain that of the cover load even after part of the coal seam has been removed. Any rise in stress in ribsides and pillars must be offset by an equivalent stress reduction across roadways and other areas of extraction, and vice versa. Knowledge of one can lead to an estimate of the other, provided the general form of the stress distribution can be postulated. It is the object of this paper to revue the method in more detail and to give further examples of its successful application.

Jan 1, 1981

By Alexander Kattner

Surface fractures in urbanized areas show the risk potential of near-surface mining. This kind of mining subsidence damage occurs mainly in areas where hard coal was mined near the surface with very little overlying rock strata. In North Rhine-Westphalia (NRW), the mining authorities assess areas that are at risk for caveins by means of envelopes constructed according to Hollmann and Nürenberg (1972). Drafting abandoned underground structures depends on the assessment of historical documents such as mine plans, mine titles, and operation plans of prior mines in this area. A research and development project establishing a comprehensive Geo Information System (GIS) for hazard prevention was undertaken in cooperation with the mining authorities of NRW. It aims to display the areas endangered by abandoned, shallow underground mines for the purpose of hazard prevention. A tool was programmed using ArcGIS as a result of this project. It is used to digitize underground workings from preserved mine plans, calculate outcrop lines of coal seams and display hazardous areas. In the future, a database of shallow mine workings will be established. The tool will be used for a systematical mapping of shallow workings and assessment of endangered areas.

Jan 1, 2011

By Richard R. Wilding

The United States Coal Mining Indus- try had the opportunity to greatly improve their roof control techniques during the nineteen seventies by the use of fully grouted resin roof bolts. This new tool was first approved for underground coal mines by M.S.H.A. ten years ago and during the next decade has been perfected to meet many problems previously thought to be unsolveable. The initial approved cartridge was very similar to the ones used in Europe in the nineteen sixties and since then has undergone a great deal of modification for our mines. These refinements in cartridge construction have been obtained by the close cooperation of resin manufacturers with mining personnel in perfecting pro- ducts customized to particular needs. This type of continuing program naturally necessitates a responsibility upon the resin anchor manufacturer to maintain a competant technical department. These roof control technicians by constant exposure to many of the most demanding underground problems, have developed through the years into leaders in successful applications. Today, Celtite has over twenty technically trained underground technicians with an accumulation of over one hundred years experience in resin anchors. Our documented technical reports are in excess of five thousand, involving over six hundred mines representing every major coal mining area in most mineable seams. This department has been the implement used to work with our research and development people in perfecting a continual line of improved resin anchors. This program has been a significant factor in leading the resin anchor industry into a position where approximately one third of the 1980 under- ground coal production used the fully grouted system. However, as gratifying as this accomplishment is, our experience has lead us to believe that there is an additional demand for a roof control system capable of combining the mechanical bolts ability to apply tension with resins proven superiority in anchoring. A significant number mines visited during the last decade had problems that we believed could be better answered by applying tension rather than the fully grouted approach. In most in- stances, their current mechanical installations were doing an adequate job until anchor slippage occured. Fully grouted systems solved this bleed-off problem. However, an acceptable number of falls still occured. In these some- what unique areas which we believe may amount to as much as fifteen percent of the total bolt installations, the need for improved anchors was a necessity. In developing this program we listed the ultimate requirements necessary for both the bolt and the anchor. These are for the bolt: A. Have one end manufactured so it can be grouted to the mine roof with polyester resin, obtaining a variable length from 12" to 36". B. Have a mechanism which allows the operator to tension the roof from zero to 10,000 psi. This mechanism should indicate if the roof is in compression and the bolt in tension. C. Have a mechanism which allows the operator to thoroughly mix the resin. D. The head of the bolt should not extend below the roof line any more than a normal mechanical bolt. E. Not require any extra tools for installation. F. Be bendable for use in low seam coal. G. Be durable and inexpensive. The ultimate point anchor resin should have the following characteristics: A. Have a mix and gel cycle which would compete with a mechanical bolt for speed of installation. B. Have a pull out strength in excess of a normal 5/8" mechanical bolt.

Jan 1, 1981

By Shengli Yang

Solid backfill mining, as a green mining technology, has the advantages of a higher resource recovery, less damage to the environment and the ability to dispose of waste underground. It has been applied successfully in the Xingtai and Xinwen mining areas in China. Strata behavior and the movement of overburden strata in this mining method are very different from fully mechanized longwall mining, which results in a natural caving, because the gob is filled with waste rock, fly ash and other backfill material. The results of field measurements showed that the periodic weighting interval of the main roof is longer; the magnitude of periodic weighting is smaller; the peak and zone of influence of the front abutment pressure are smaller than those in fully mechanized mining; that major deformation of mine roadways occurs within the influence zones of the abutment pressures; that the falling space of immediate roof is smaller, with the lower portion falling more sufficiently than the upper; that the main roof subsides slowly; and that the falling height of the overlying strata is greatly reduced.

Jan 1, 2011

By Aarao de Andrade Lima

Mechanical properties typical of coal are used in the comparisons of pillar strengths based on finite elements and the empirical equations of Holland-Gaddy, Salamon-Munro, Obert¬Duvall, and Bieniawski-PSU. A correlation among the strength parameters which are employed in the empirical equations, in Mohr-Coulomb and Hoek-Brown failure criteria is established. Emphasis is given to the important distinction between the mechanical properties of small rock specimens and those related to the rock mass. An elastoplastic version of Hoek-Brown criteria has been developed and implemented in a finite elements code. The strengths calculated using the empirical equations showed a good agreement when the ratio width by height of the pillars varied from I to 8, except for Holland-Gaddy equation which exhibited lower values. Under the hypothesis of associative perfect elastoplasticity used, pillar strength calculated using Mohr¬Coulomb yield criterion became excessively high for all the models having a pillar width by height ratio above 2. The finite elements solution based on Hoek-Brown criterion predicted strengths around fifty percent larger than Bieniawski-PSU equation when the pillars width by height ration varied from 3 to 8, showing good agreement below that range. It has been concluded that, under the hypothesis of percfect plasticity, stress analysis techniques may overestimate the strength of rock structures subjected to high confining stresses.

Jan 1, 1997

By Stanislaw Prusek

The present state of Czech and Polish coal mining industry is presented at the beginning of the article. This paper will inform the reader of the general data concerning hard coal reserves? number of mines, number of miners, output, number of longwalls, productivity, depth of extraction, and extraction methods?in both countries. The main part of the paper describes the methods of gateroad development as well as gateroad support characteristics (support types, gateroad shape and dimensions, distance between support sets, types of reinforcements). The development prospects of gateroad support in the Czech and Polish coal mining industry are also presented.

Jan 1, 2011

By Maynard O. Serbousek

The mechanics of load transfer of fully-grouted roof bolts in coal mines has been investigated in laboratory and analytical studies. The purpose of this research is to increase the understanding and better interpret data from pull tests which are routinely performed in the field. Laboratory work includes variation of bolt length, hole diameter and grout type in pull tests. Four-foot bolts were tested in holes of 1- and 1-3/8-inch diameters. Both polyester resins and inorganic grouts were used as the anchorage medium. The bolts were strain gauged at six locations to observe the rate of load transfer from the bolt to the rock mass when a force was applied at the bolt end using standard Jack pull testing procedures and equipment. Analytical studies using closed form solutions and finite-element techniques were compared to experimental results. The models are for the elastic regime only but do include slip elements. A detailed analysis of the load transfer mechanisms from the bolts through the grout to the rock mass is presented.

Jan 1, 1984

By William Gray

It has been ten years since the Mine Safety and Health Administration (MSHA) promulgated a safety standard requiring the use of Automated Temporary Roof Support (ATRS) systems in underground coal mines. The use of ATRS systems with roof bolting machines and continuous miners equipped with integral bolters has all but eliminated fatal roof fall accidents associated with manually setting temporary roof support. The last ten years have seen an evolution of the ATRS systems themselves and of MSHA's policy regarding ATRS systems. These changes are a response to operational and safety issues regarding ATRS systems often brought about by the ever-changing coal mining environment. As a result, MSHA's Technical Support personnel receive numerous inquiries from all segments of the coal mining industry on various ATRS topics. These requests for technical assistance range from an interpretation of a facet of the ATRS standard to an in-depth evaluation of a prototype ATRS system. This paper will address the aforementioned evolution of ATRS systems and will also summarize the factors MSHA Technical Support uses to evaluate these systems in the field. In addition, some of the more common misconceptions regarding the design, use, and regulatory requirements regarding ATRS systems will be covered.

Jan 1, 1998

By Dieter Wittenberg

The successful extension of the applications in which rockbolts can be used in the German coalmining industry is, because of the very great raining depth combined with a high degree of stress in the native rock, only possible on the basis of a comprehensive quality management system. The quality assurance system consists of material tests on the individual component parts of a rockbolt, of test rig trials of installed rockbolts, of suitability tests underground and of measures for quality assurance during roadway drivage. Only on the basis of exact knowledge of the way in which installed rockbolts function in the underground and possible sources of error and faults can quality assurance be guaranteed during roadway drivage.

Jan 1, 2000

By Anton Sroka

The German hard coal production predominantly derives from underground multiseam mining activities. Beside common surface damages, ?inner mine damages? on underground infrastructures (i.e. main roads, cross cuts and shafts) due to mining excavations occur to an increasing extent. As these "inner mine damages" dramatically increased because of the development towards high performance longwall mining technologies, this matter raised to be a severe problem concerning mining economics. To protect durable and long lasting underground infrastructures, precautionary measures such as suitable mine layouts, adapted planning of mining operations and moderate face advances have to be considered. Some examples of "inner mine damages" recorded by means of mine surveying methods are given in this paper. An empirical-mathematical model based on the principles of mining subsidence engineering methods will be introduced. This model allows to minimize ?inner mine damages? and helps to support and optimize mining processes.

Jan 1, 1999

A procedure fm the estimation of pilla1 strength is presented where the design of underground coal pillars is emphasized. An analytical approach has been developed, proposing that the effect of end constraints on the pillar strength and the ductile behavior of coal material should be incorporated in the computation of the stability of coal pillars. Bad on this approach, it has been found that end constraint has a significant influence on the pillar strength. The study indicates that when the end constraint is zero the computed strength of the pillar is the same as the unconfiied compressive strength of the coal mass and will not increase with the increase in the width to height ratio. However. when the end constraint is greater than zero the pillar strength will increase exponentially as the width to height ratio increases.

Jan 1, 1990

By Ian W. Farmer

A description of the factors causing outbursts and rockbursts in coal mines is given. The mechanics of both outbursts and rockbursts can be described in terms of conversion of stored strain energy in the coal and surrounding rocks into kinetic energy through failure. The major difference between the two phenomena is that gases adsorbed onto the coal structure and desorbed during deformation contribute to the energy available during outbursts and may also affect the loading and unloading characteristics associated with failure. The amount of energy involved and the type of phenomenon are related principally to gas content depth and rock behavior.

Jan 1, 1988

By Anthony T. Iannachione

The location and time of 2,007 microseismic emissions from a limestone mine in southwestern Pennsylvania were compared with the development of mine faces and the characteristics of the mine layout. Based on analyses of these results, the occurrence of roof failure zones appears to be associated with certain characteristics of the mine plan. It was determined that significant relationships exist between the intensity of the microseismic activity and the scale of the roof failures. Microseismic activity associated with these roof falls occurs in distinct episodes, with the final failure event occurring during approximately a 12-hour period. Each roof fall episode appears to be composed of dozens of distinct roof beam failures. As each beam fails in shear and tension, tens to hundreds of audible noises representing rock fracturing or bedding plane separation can occur. While every roof fall can be viewed as unique, certain mechanistic similarities can be realized through careful observation and monitoring of these complex systems. Understanding these similarities in characteristics allows mine personnel to design the most effective and efficient control technique.

Jan 1, 2001

By Madan M. Singh

Currently longwall coal mining operations require at least 3 entries on both the headgate and tailgate ends because of ventilation and safety requirements. Driving of entries, however, is a slow and costly process. Often it is a bottleneck in longwall development. Single entry gateroads are permitted in Europe and commonly used there. This study entailed ascertaining the feasibility of using a single entry in the United States, without sacrificing safety. This design was conducted for an operating longwall mine in the Black Warrior coalfield in Alabama, An understanding of rock loads and consequent support requirements is prerequisite to the design and successful demonstration of the single entry system. Efforts in this regard were directed at determining center (packwall) support requirements for various single entry widths. Critical to the analysis is knowledge of the nature and extent of side abutment stresses. In collecting geological and geotechnical information relating to the stability of the single entry, Engineers International, Inc. engaged in the following: ? Examination and inspection of the single entry demonstration at the Sunnyside Mine in Utah. Severe ground conditions and unacceptable levels of entry closure were experienced there prior to the tailgate phase. Observations at the mine presented a clear perspective on possible ground control problems for future single entry designs. ? Reconnaissance inspection and classification of roof rock conditions. ? Detailed mapping of roof structure in the areas to be instrumented. ? Installation of convergence stations in the longwall headgate to monitor roof movement during mining. ? Measurement of side abutment stresses in a yield pillar in the area of the convergence measurements. ? Measurements of coal fracture distribution by the detail area method. ? Rock properties determinations of roof, coal, and floor rocks; samples obtained by core drilling. ? Determination of floor bearing capacity by in-situ testing.

Jan 1, 1982

By Robert C. Speck

Floor heave or deformation of the mine floor into the mine opening is a problem which has plagued coal mines in this country and others. In mining areas where room and pillar mining is practiced, floor heave generally occurs when the stress applied to the floor material by a coal pillar exceeds the bearing capacity of the floor strata. When the floor strata beneath the coal pillar fail, the pillar moves downward and displaces the material under it. This material, in turn, moves outward and upward into the mine opening (Figure 1). As the underlying materials fail, the load supported by the coal pillar may be transferred to adjacent pillars. If these are marginally stable, they too may become overloaded and push into the floor. Thus, floor heave can spread through a large portion of the mine in a progressive manner. As the pillars push into the floor, differential movement and flexure of the roof strata may cause them to weaken and fail, resulting in subsidence of the ground surface above the mine. This paper describes the problem as it was manifested in the Zeigler Coal Company No. 5 and Murdock room and pillar coal mines near Murdock, Illinois, and reports the results of a U. S. Bureau of Mines- sponsored study conducted by the University of Missouri-Rolla. During the course of the study, floor heave in the two mines hindered the transportation of men, machines, and coal to and from the working mine-face, interrupted ventilation plans, drastically increased roadway maintenance costs, intensified safety and strata control problems, destroyed large pieces of equipment, and, several times, pre- vented access to otherwise minable coal resources. The ultimate objective of the study was to develop one or more simple procedures to allow detection of a potential floor heave problem during the exploration phase of mine design--before initiation of production mining.

Jan 1, 1981

By Chao-jiong Hou

The necessity of using tensible rockbolt in the roadway with soft rock, high in-situ stresses, mining influence and other complex conditions was discussed, the structures of three kinds of tonsibte rockbolt and design principle were introduced in this paper. The tests of analogue material simulation under plane strain condition and underground industrial tests indicated that the effect of controlling deformation surrounding roadway by use of tensible rockbolt was good.

Jan 1, 1989