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By C. J. H. Brest van Kempen

The techniques developed should provide a useful tool, not only during the initial formulation of a suitable bolting plan for a new section, but also for periodic check¬ing on the utilization factor of hardware installed in the roof. Indications are that with present practice, utilization seldom exceeds 507.. Retro-fittable bolter equipment now available (FCBS?) allows doubling of the utilization factor in many cases. We are also introducing a computer pro¬gram based on the material outlined in this paper. This program can save a great deal of time in examining the real effect of implementing alternate bolting plans or using different installation techniques. It allows fine-tuning of bolt lengths, spacing and installation tension to mini¬mize roof support cost, while maximizing safe life of the mine opening. We have developed a procedure which allows easy, site-specific quantification of the need for reinforcement of a mine roof. The concept is based on the forma¬tion of a direct link between theory and empirical data to custom-fit each mine section. In its simplest form, the proce¬dure requires only recording of roof bolt torques in a sample working place (16 to 25 bolts) several times during the first week or so after initial bolting. The way in which the torque pattern changes during this time can reveal whether or not the bolts carry enough tension to maintain stability in that specific roof. From the same data, the nature of the anchorage horizon is derived. Specifically, shale or limestone can be distinguished from sandstone. If the procedure is repeated a few times (within the same mine section) with different bolt installation tensions, we can numerically characterize the roof in terms of one single value: an "effective angle of internal friction". This number can then serve to calculate trade-offs in bolting plan design, such as bolt length, bolt spacing and optimum bolt tension. Additional refinement is possible if a sagmeter is installed in the sample work¬ing place being monitored and if bolt anchorage capacity is determined. The lat¬ter can be done automatically on each bolt as it is being installed, using a system we developed and patented. Alternately, a number of pull tests could be performed. Using the sagmeter, bolt torque and anchor¬age data, it is possible to project the safe life of the supported opening. The details of the procedures described apply specifically to tensioned roof bolting, but a brief discussion paints the way for a similar analysis of full-column resin bolting also.

Jan 1, 1986

By Zuoliang Xiu

Experience indicates that rockbolting is the most efficient and cost effective means of supporting gateroads of retreating longwalls. However, gateroads supported solely by rockboltiug currently accounts for less than 16% of the 4500km of annual gatemad drivage in China According to a stated target of the Ministry of Coal Industry, this percentage should be at least 50% by the year 2000. Since the learning stage of the technology which is indicated by low drivage rates and frequent roof falls in rockbolted gateroads must be overcome , strategies for the application of rockbolting to gateroads have become the focus of discussions on this topic. This pap presents the recent development on longwall gateroad rockbolting technology in China The development work has concentrated on understanding the mechanism of rockbolting; establishing new design procedures of bolting parameters fix gateroad support upgrading the consumable; optimizing bolt installation procedures; adopting better roofbolting machines; and developing adequate secondary reinforcement technique. The authors believe that this development work together with the on-going wok in other aspects of gaterosd rockbolting, as discussed in this paper 9 represent the rational strategies for the application of rockbolting technology to a wider range of longwall gateroads in Chinese d mines .

Jan 1, 1997

By John Feddock

This study deals with underground mines and ground control and the prediction of seats strength characteristics based upon ultrasonic logging. Exploration drill holes, both rotary and core holes are used for proving coal reserves and determining mining parameters. Drill hole cores can be subjected to geologic study and geotechnical evaluation. In addition, the use of geophysical logging techniques is well established in determining certain strata characteristics and ground parameters. Since the characterization of the rock mass in mines remains insufficient in most cases, the idea of possibly characterizing the strength parameters without expensive core retrieval and laboratory testing is very attractive. The elastic properties of the rock can be deduced from measurement of the sonic velocity if the density is known. Through the use of in-hole geophysical density logging combined with an ultrasonic logging tool, the estimation of strata properties is possible. An investigation was conducted at two Indiana underground coal mines using geologic logging, geophysical logging, ultrasonic logging and laboratory strength testing to establish the strength properties of the roof rock. A specialized ultrasonic logging tool with one transmitter and two receivers enables nearly continuous readings along the length of the hole. The results of correlating portions of the data are presented. It is anticipated that the ultrasonic logging will be a supplemental tool for extending laterally the strata characteristics obtained from the existing program to surrounding areas. This may be obtained from the relatively inexpensive logging data obtained by ultrasonic logging.

Jan 1, 2003

By Ross Seedsman

There are many models for how roof bolts behave, one of which is prevention of shear along bedding. The basis of this model is that a roof beam will not delaminate if bedding parallel shear can be prevented. A key point in such a model is that bedding discontinuities are present naturally in the roof and that mining induced stresses do not generate new surfaces. The presence of these discontinuities means that the rock mass must be modelled as a transversely isotropic material if continuum codes are used. A limit-equilibrium type design method for specifying roof bolts is being developed using voussoir beam and excess shear force concepts. The method incorporates much of what operators know intrinsically about roof reinforcement, and allows mining engineers to focus on the key uncertainty - the frequency and location of bedding surfaces in the immediate roof. Areas where more research is required are highlighted.

Jan 1, 2000

By Thomas Barczak

A new generation of hydraulic mine support prestressing devices has been developed. These thin-walled steel shells are machine-welded and can be inflated with water or any liquid to provide prestressing for a wide variety of roof support products ranging from Can supports to props and cable bolts. With an expansion capability of several inches, depending on the geometry and size of the unit, they can also establish roof contact eliminating the need for wooden wedges or crib blocks that are commonly employed with several standing roof support products. Capable of withstanding pressures from 1,000 to 6,000 psi, these cells are able to transfer roof loading into the support structure without rupturing. A recent development has been the addition of an inexpensive yield valve that provides control of the maximum pressure and load development on the support. This paper examines the performance capabilities of these inflatable prestressing units and the impact they have on the performance of various support systems, including an evaluation of the overall stiffness of the support system and the load control during yielding of the prestressing unit. Recommendations are made regarding the design requirements of the prestressing unit to optimize the support performance. A summary of mine applications using this technology is provided with general comments regarding their impact on ground control.

Jan 1, 2004

By Jiacheng Wang

The current state of thick seam coal mining methods is illustrated in three typical cases of thick seam longwall mining, each pointing out the outstanding achievements in top-coal caving mining and large cutting height mining in China. At the same time, based on current thick seam mining practices in China, some problems urgently need to be solved. These include improving the recovery ratio of coal reserves, determination of overburden strata pressure, shield support selection, supporting methods for large cross-section in-seam roadways, top coal bulk flow theory, and gas flow and control technology. In addition, the rational production capacity should be studied from both economic and technological perspectives in order to optimize coal recovery and efficiency.

Jan 1, 2011

By Vasundhara

Underground coal mining in soft floor environments has often led to strata control problems ranging from small scale issues such as floor heave which disrupts traffic and ventilation, rib spalling and induced fractures in pillars, through to large scale instability and long term subsidence. Past investigations of the problems encountered in Newcastle Coalfields of New South Wales, Australia, have revealed that the soft floor unit comprises of claystone which is very sensitive to moisture. It is prone to swelling, reduction in stiffness and loss of strength in the presence of moisture. Long term stability of mine panels in such an environment is governed by the extent of deterioration of the floor strata. The paper reports the field investigations conducted at Cooranbong Colliery to assess the extent of softened floor zone in the roadway and under a pillar.

Jan 1, 2001

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

Jan 1, 1993

By William Kendall

INTRODUCTION Stillwater Mining Company, with its two underground, hard rock mines in South Central Montana, is the largest primary producer of palladium and platinum in the Western Hemisphere. Platinum Group Metals (PGMs) are mined from a specific zone within the Stillwater Complex, a layered mafic/ultramafic igneous intrusion, named as the J-M Reef. Over much of the J-M Reef, the economically minable, mineralized horizon is contained at typical horizontal widths of 6.5 ft to 8 ft wide. This feature of narrow mining widths has made it challenging to safely and effectively mine these areas of the J-M Reef using mechanized cut and fill and sublevel sill development mining methods. Mechanization of most or all of the steps of the mining process is important to Stillwater in the continuous pursuit of high standards in safety and health. Mechanized bolting within the J-M Reef ore production headings is an important piece of reaching these objectives. In general, metal/non-metal mining requires varied mining techniques, as compared to more traditional bedded mineral/resource deposits. Narrow vein mining is a selective mining technique that generates a minimal amount of waste material while obtaining the more valuable ore deposits. Drilling and bolting in narrow vein mining has traditionally been performed with air-powered, hand-held ?jackleg? equipment because mechanized equipment is, typically, too large for the selective narrow vein techniques. In addition, traditional hand-held pneumatic equipment provides significant flexibility for small-opening, narrow-vein developments, reducing waste rock handling and improving ore recovery management. The hand-held equipment is also relatively inexpensive to purchase. Due to its simplicity, it is also relatively easy to train operators to use hand-held equipment. Hand-held equipment has been routinely used to safely install ground support and to develop production faces in hard rock mines for many years. Although it is effective, the hand-held equipment requires operator exposure to certain inherent hazardous conditions in the active face/development mine areas. These include falling rock, tripping hazards, bending/twisting/lifting hazards, high noise levels, and chemical fumes from the hammer exhaust. Based on health and safety statistics from the Stillwater mines, operation of the rotary-percussive hand-held equipment adversely affects the long-term health of the operator?continual jackleg operation frequently results in long term damage to hearing, shoulders, arm and wrist joints, and back injuries. This reduces the quality of life for the operator and also results in higher personnel turnover rates for the mining company.

Jan 1, 2014

By Yundong Ma

In this paper the dynamic process of rock deformation and failure in the roadways of the #9 coal seam, Wuhushan Coal Mine by was simulated using the software "Rock Failure Process Analysis System (REPA)" [I]. Based on the simulation results, this paper analyzes the failure process, characteristics and weighting of the overburden as the longwall retreat mining proceeds. The roadway's behavior including deformation characteristics, failure mechanisms, stress distribution in the surrounding rocks and the effects of various support systems on roadway stability were analyzed and studied. While simulating the rock failure process with REPA, the model stresses can be obtained through two approaches: (1) the stress is reflected by the grayscale of pictures in the model failure process and (2) the model stress can be obtained by muti-element information, which is one of the commands in RFPA system. Data files of stress can automatically be created, and curves of stress and displacement of each element can be displayed in each step of computation. Keywords: Rock Failure Process Analysis System (RFPA); roof control; roof to floor convergence; combined support

Jan 1, 2005

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 Duk-Won Park

A vast majority of the operating longwall sections use shield-type face supports to provide ground control in the United States. As a co-operative research program between the University of Alabama (UA) and Jim Walter Resources, Inc.(JWR), a joint research program is underway to study interaction of shields with surrounding rock masses. A series of analyses on the response of shields are performed using monitored data at a longwall face in a deep coal mine. In this paper, various effects of parameters, such as leg pressure, setting load, developing load, maximum loading rate, pressure in capsule, and convergence between canopy and base, are discussed and recommended as stability indices. A microcomputer program is being developed for instantaneous interpretation of the shield monitored data, and for giving warnings of potential problems. The preliminary results define the effects of the setting load, developing load, and rate of developing load or leg closure rate. The objective of the study is to understand the shield characteristics and their interaction with surrounding strata in real time, then develop a reliable program for maintaining longwall stability.

Jan 1, 1992

By N. P. Kripakov

This paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines located in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8 m (32 ft)-wide, yield-pillar design. A U.S. Bureau of Mines field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site-specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study conducted at another mine located in the same district, but subjected to different geologic conditions, documents the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Use of pillar widths ranging between 9.1 m (30 ft) and 27.4 m (90 ft) always resulted in violent bumps in the tailgate pillars. The pillars were either too large to yield, or too small to support peak operational loads. Hypothetical gateroad systems were evaluated using both analytical and empirical approaches for configurations comprised of two rows of conventional pillars, and systems incorporating both yield and abutment pillars. Analysis concluded that yield pillars less than 6.1 m (20 ft) wide and abutment pillars ranging between 30.5 m (100 ft) and 39.6 m (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6 m (120 ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining. Without the benefit of a demonstrated in-mine success, it is not clear which design would ensure elimination of gateroad bumps.

Jan 1, 1992

By Gregory M. Molinda

Difficult mining conditions in deep western U.S. Iongwalls have necessitated the use of novel tailgate support systems. Extreme ground movement in these yielding gate systems has caused operators to utilize secondary support systems which match yield and strength characteristics with expected ground reaction. Other benefits include ventilation and safety gains, as well as lower costs. Recently, the use of novel support has been tried at longwall mines in other U.S. coal basins. One of the main considerations is the capability of the support to maintain an adequate airway for gob ventilation in gassy eastern coal mines. In one large Pennsylvania mine working the Pittsburgh seam, a number of support configurations were monitored over a 2,300 ft tailgate test area. The test included 4-pt wood cribs, Link-N-Lock cribs, Propsetter yieldable posts, cable trusses, and vertical cable bolts. Ground reaction, and thus support performance, was documented by roof sag and convergence instruments, ventilation measurements, and photographs. Various combinations of standing support and cable truss and vertical cable bolts provided adequate support for ventilation and safe escape. A cribless configuration consisting of only cable trusses and vertical cable bolts was inadequate to provide inby gob ventilation to the next open crosscut. Instrument ground movement data and visual documentation are presented.

Jan 1, 1997

By David Bartsch

The Ohio Valley Coal Company (Ohio Valley) has operated the Powhatan No. 6 Mine in Belmont County, Ohio since 1972. Ohio University owns a 455-acre research farm over a portion of the Ohio Valley reserve containing 51 acres of old-growth forest known as Dysart Woods. In 1988, when Ohio Valley applied for a new mining area for its first longwall located several miles away from Dysart Woods, Ohio University claimed that mining at that distance would lower the regional water table and kill the old trees. In response, Ohio Valley began to study potential impacts from longwall mining on hydrology, soil moisture, and tree growth. Early studies examined the impacts of longwall mining on hydrology, soil moisture, and tree growth directly over mined areas compared to woodlots that were not undermined. A subsequent study examined tree-ring growth for a five-year period before and after longwall mining for several woodlots, some directly over panels and others at distances of up to 2000 ft away. No significant differences in growth rate associated with longwall mining were observed. New research was begun that compared longwall-mining impacts on groundwater hydrology, available soil moisture, and tree growth in two stands of mature trees located near Dysart Woods in order to confirm the earlier results. Using a test technique known as "Before-and-After-Controlled-Impact-Pairs," data was collected from one woodlot (test) that was undermined and from another (control) that was not undermined. Data collection began several years prior to longwall mining and has continued for several years after mining. The data for the test site shows that although the shallow and deep groundwater levels were affected by longwall mining, the soil moisture, vigor rating, or the growth rate of trees did not decline significantly compared to the control site where there were hydrologic impacts. The study shows that there are no known significant impacts to trees from longwall mining. The results of these studies were used by Ohio Valley to obtain State approval to longwall to within 300 ft of Dysart Woods

Jan 1, 2005

By Ross Seedaman

The specification of roof support needs to consider rock strength, imposed ground stresses, and more importantly the bedding and joint structure of the immediate roof rocks. A database structure has been developed that allows these considerations to be incorporated into modern mine planning software so that better predictions of mining rates can be made. Rock strength is estimated from core and/or geophysical logs (especially the sonic velocity log) and ground stresses are estimated from depth of cover and knowledge of the regional stress field. The database is unique in the way it addresses bedding structure. Only 10 roof lithologies are allowed and these are allocated a numerical value that reflects an increase in the average spacing of bedding partings. These are referred to as 'Geotechnical Ferm Units' (GFU). The locations of all bedding discontinuities logged in the core are entered into the database and referenced to the GFUs. It has been found that, within each GFU, bedding spacing follows a negative exponential distribution and so it is a simple matter to predict the probability of the occurrence of any roof beam thickness. Output from the database can include plans of roof strength, the onset of overstressing, and the statistics of the distribution of beam thickness in the immediate roof.

Jan 1, 2003

By Bruce K. Hebblewhite

This paper provides an overview of the international ?state of the art? with respect to underground coal mining methods used in seams of thickness greater than 4.5 m, through a review of a selection of high performing thick seam mining operations around the world?both Single Pass Longwall (SPL) and Longwall Top Coal Caving (LTCC). The paper discusses the recent operational experiences from these selected faces, together with the ground control issues faced, and identifies some of the major, inherent geotechnical factors that can influence the mining method selection, design criteria, and ultimate performance, from both an operational and safety perspective. It is clear that both of these methods are developing quickly? not only from an operational and equipment perspective, but also from the understanding of critical geotechnical criteria. There is an increasing role for both methods in thick seam environments, although the higher SPL faces will only be successful in the near to mid-term under benign geotechnical environments. Regardless of the method, a thorough understanding of the critical geotechnical factors is a key to successful implementation of either method.

Jan 1, 2013

A two-dimensional time-dependent analysis of a overburden-coal pillar-weak floor strata interaction problem is presented as a beam model consisting of a composite roof beam resting on multiple elastic foundations (pillars) underlain by a composite rock mass representing immediate floor strata. Several different material models may be considered for the immediate floor strata. The analysis can include all openings and pillars in a panel and permits bed separations in the roof composite beam. The model has enabled identification of the relative significance of different geometric and mechanical behavior parameters which govern the system. The paper presents the theoretical background for the model as well as its application to a mine in Illinois where surface and underground geotechnical observations have been conducted over the last two years.

Jan 1, 1988

By Peter Altounyan

Goedehoop Colliery in the Witbank Coalfield of South Africa produces some 8 million tonnes of coal from fully mechanised room and pillar mining methods, using resin anchored bolts as roof support. Investigation of the quality of the installed bolts resulted in the finding that it was possible to pull out fully encapsulated roofbolts. This led to a drive to improve the quality of installation by making it easier to install a roofbolt correctly and more difficult to install incorrectly, whilst at the same time achieving high bond strength. The method of installation which was investigated, developed and implemented is known as 'spin to stall.' The roofbolt is pushed through the resin capsule to the top of the hole and then spun to mix the resin, until the increasing resistance to rotation exceeds the breakout limit on the bolt torque nut. The nut then runs up the bolt thread and is tightened to the roof as the drill stalls. This procedure is simple and avoids human factors such as non adherence to 'spin' and 'wait' times which contribute to poor installation with conventional methods. However spin to stall installation has been avoided in the past because of concerns over damage to the resin bond. This paper reports on the efforts made to investigate and understand the factors involved, and the progress made in overcoming the problem of resin damage so that the advantages of this installation method can be realised.

Jan 1, 2003

By Craig Collins

The objective was to develop a Real Time Drilling Display System for Rotary Roof Bolting using Analyzing Software to display mapped holes as they are being drilled in real time. The Analyzing Software was developed to analyze and interpret drill sensor data produced during a rotary drilled roof bolt hole by using algorithms and drill sensor data trends to find and locate roof separations or voids. After this software was validated through lab and field testing, its application needed to be tested in a production environment. However, the current method used to record the drill sensor data proved to be cumbersome for the mine to use on a continuous production basis. With different more practical mediums being explored and considered, a Drilling Display System was developed that will both map holes in real time and record the data for later review. With the Analyzing Software's current level of accuracy and with the convenience offered by the real time Drilling Display, the Drill Display System was implemented along with a test mine's current roof control plan and tested as a roof separation "Early Warning System". After installing the Drilling Display System onto a machine, input and suggestions from the mine environment were considered and design changes were made. The real world application provided a design direction that made the information produced by the Analyzing Software both convenient and significant for the test mine. Also, using the Drilling Display System increased the number of drilled holes that were both mapped and recorded. The results from studying these recorded data files lead to refinements in the current algorithms used by the Analyzing Software which increased the overall accuracy and capability of the system A fairly accurate representation of void or separation locations in the mine roof can be determined fromsensor data recorded during production bolting cycle. The immediate feedback on the local roof condition that the Drilling Display System provides, amounts to a quicker and safer alternative to the scratch hole and the video bore-scope examinations that arc performed after the bolting is completed. This project has shown that making the alternative method of examining the roof condition practical for the mine to use is just as important as the accuracy of its results. Based on the results of this field test, this or other methods of reviewing the analyzed drill hole data will be explored.

Jan 1, 2004