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By Archie Richardson

Periodic weighting of longwall supports occurs during retreat ofthe face under certain geologic conditions. Strong stratain the immediate and main roof tend to cantilever over the gob, weighting the supports periodically. The distance between weighting peaks, as well as the intensity ofperiodic weighting, is determined by the suength and thickness of roof members, their location relative to the seam, the frequency of joinling, and characteristics of the gob. In this paper the effects of varying several of these variables ate investigated using block modeling. The models are calibrated against a published case history of periodic weighting in the Pittsburgh Seam. It is concluded that block modeling is a useful tool for correlating measured support pressure histories with expected modes of strata behavior in the roof, with potential for predicting shield behavior in new geologic enviromnents.

Jan 1, 1998

By Rao Pothini

Measurement of pressure distribution under the base plate of the Hemscheidt 2-leg 800-ton shield was performed underground for two shifts using 12 pressure cells installed in two rows, one each under each split base plate. Pressure distribution was uneven depending on contact condition. In general the highest base pressure occurred at 1/3 of the base plate contact length from the rear edge. The resultant was near vertical, and the pressure under the tail side split base plate was always higher than the head side. For comparison, pressure distribution on the canopy and under the base plate was also measured under controlled conditions at the USBM's Mine Simulator Testing Facilities in Pittsburgh, PA. A total of 45 tests were conducted under several simulated contact conditions. The results showed that the actual contact area is highly dependent on the compliance of the immediate roof and floor due to the sloped canopy tip and a base lifting device under the base tip. The loading distribution tends to change in magnitude and profile as the leg pressure increases. The base lifting device exaggerates the high toe loading on the base plate characteristics of this type of shield support. This paper describes in detail the testing methods, data analysis results obtained, and recommended changes.

Jan 1, 1992

By Vladimir I. Klishin

Complex method of ruling of rock pressure is considered in conditions of rapid roof subsidence. The purpose of this method is elaboration of emergency devices into hydraulic props. Also method of unloading of solid roofs is worked out. An analysis of means of protecting hydraulic props of powered supports 6om dynamic loads made it possible to divide them into four classes according to principle of operation: emergency, operating hm an increase of pressure; emergency damping; emergency, operating according to the "plastic element" principle; inertial emergency, operating hm dynarmc loads. It was established that the overwhelming 'majority of designs of the first three classes of emergency devices (EDs) are based on !mditional approaches, which exhausted their possibiiities of high-speed response. Delay on their opening is caused mainly by inertia of the moving locking parts. In our opinion, the most prospective are EDs using the inertia of the moving locking parts for depressurizing the piston cavity of the hydraulic prop with a minimum delay (inertial EDs). The essence of the designs of EDs having an inertial principle of action is that into the hydraulic prop containing movable and immovable elements is additionally introduced a spring-operated contracting support, which is installed on the movable element. Under dynamic loading of such a hydraulic prop hm the roof, the support moves relative to the movable element, performing the role of an inertial mass, which leads to instantaneous depresswization of the piston cavity of the hydraulic prop. Technological measures for unloading of solid roofs consists in creating of long oriented cracks in rock massif. Description of method for creating cracks, demanding equipment and means for cone01 the progress of splits are included. Also technological schemes for realization of the method of saving underground mines and rock mass are included. The results of experimental realization of this method on the Kuzbass and Poland coal mines are also included.

Jan 1, 1997

By B. T. Wells

This work concerns the stress fields encountered around mining tunnels. A critical review is undertaken of the various analytical assessments of the support requirements for mining tunnels. Comments are made regarding degree and type of support together with method and timing of installation. The results have been used to indicate the type of support required in different mining and geological conditions.

Jan 1, 1981

By Xun Luo

In this paper we report on the use of microseismic monitoring techniques to investigate the fracturing process associated with longwall mining. The study was carried out at three mines in Australia and concentrated on some major concerns of the mines, such as failure patterns and growth rate associated with mining progress, their implications to water ingress and gas emissions, caving processes in the goaf and precursors of roof falls. The results have shown that a zone of shear fracturing has occurred further ahead of the longwall face than predicted using conventional longwall geomechanics theory. Failure patterns in the roof and floor vary from mine to mine and this appears to be dominated by geological structures. The reaction of faults, the formation of fractures which allow gas emissions and precursors roof falls have also observed. Results from this work have opened up new understanding of longwall caving processes and proved that microseismic monitoring is a very useful technique for studies of longwall geomechanics and hazard controls.

Jan 1, 1998

By S. I. Al-Ameen

Monolithic packing systems have gained in popularity as a gate side packing method, However, there has been no detailed investigation to assess their underground strength behaviour. This work includes the laboratory and in situ investigation for strength behaviour determined on various monolithic packing materials namely Tekpak-XX, Hydropak-20, Hydropak-30, Astrapak-3R, Flashpak, Thyssen pack and Aquablends. A simple technique for assessing the pack strength was developed at the laboratory by using a penetrometer, of which penetration resistance can be directly correlated with the uniaxial strength of the pack. The in situ strength investigations were made on core samples obtained from three pipes inserted inside the pack, while the penetration test were made on the pack surface. The in situ results indicated that the packing material develops an inconsistent strength on various part of the pack which was mainly related to an inadequate mixing of grouts inside the pack.

Jan 1, 1992

By J. D. Dizon

Conventional strata control methods (i.e. roof bolts. cable trusses) are not always effective for ground control of very weak coal strata. An alternative method, termed presupport. is being investigated by the U.S. Bureau of Mines. In this paper structural modeling of presupport conceuts and field teatinn of a DresuDuort application is presented. Pinite-elem&; modeis indicate that forepoles significantly lower beam stresses in the roof strata overlying a coal seam. but they cannot he expected to prevent or reduce I roof strata failure at the coal face due to the presence of a high stress concentration at that location. However, their presence near the coal face is needed to sustain the fall of roof if it does occur. Because of an ever present severe stress concentration at the coal face, it would appear that stresses could exceed material strengths, causing local structural damage, especially if the rock is very weak. Therefore previous coal face positions along an entry may have a higher than normal probability of roof fall. In the field investigation. a loorcost air injection test has thus far shown promise as a tool for evaluating that effects of forepoling on the roof strata. Observations of coal mine en- tries in which forepoles have been installed indicate forepoling is effective in controlling very weak roof strata.

Jan 1, 1990

By Wei Wu

A new bench blast design method and its realization with the assistance of the computer is introduced. The basic design principle is to define a rhomb cell, called fully-fractured zone, through a series of full scale bench crater tests and apply this geometric pattern in multi-hole, multi-row blast design. To illustrate the design method, the optimization process and assessment of blast performance are presented in the context of a case study.

Jan 1, 1992

By Linsheng Xu

When coal seam is under the integral thick sandstone (or conglomerate), the structure of thick sandstone is integral and its strength is rather high. After coal is extracted, the roof is hanging and caving is difficult. When the method of full caving is adopted in the longwall working face to control this kind of hard and difficult caving roof, the effect of roof-controlling affects directly the production and safety. In China Datong Coal Mines the extracting coal deposit is Datong Jurassic period. Most of the roof is sandstone and conglomerate. It is hard and difficult to cave. Having used special hydraulic power supports and other special ways to deal with the roof, the hard and difficult caving roof has been controlled.

Jan 1, 1986

By Dieter Wittenberg

The use of thc ultrasound technique for monitoring rockbolt support has at last provided support engineers with a measunng system capable of pinpointing fracture failuns in fully-grouted rockbolts. The measurement data are based on the operatmg characteristics of the bolt type, which are determined both under test-bench conditions (DIN 21521) as well as in underground trials. Practical experience acquired to date suggests that this new measuring system is both effective and reliable.

Jan 1, 2001

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 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 John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By James J. Scott

Over the past two years research, development ad field tests have been conducted on a new product called the "Scott Cable Sling". This work resulted in the awarding of U.S. Patent No. 4265571 on May 5, 1981, to the inventor. Work on this' project was done by the staff of Scott Mine Technical Services, Inc., and is in keeping with its philosophy of advancing mine grad control technology. Recent acceptance by industry has resulted in volume purchases of the product and indications are that it will gain wide acceptance by the mining industry. The system features flexibility in its application and is useful in both roof and rib control. This paper describes the product, discusses its range of applications and cites specific field examples of its use.

Jan 1, 1981

By John Ellenberger

The nature of competition in the coal market tends to deplete the most favorable coal reserves first, and forces subsequent development of mines in more extreme ground conditions such as those associated with multiple-seam mining. In fact, 70% of the United States coal reserves are in multiple-seam situations. The National Institute for Occupational Safety and Health (NIOSH) is conducting research to develop mine design algorithms that will result in safer multiple seam mines. This paper presents an overview of multiple seam issues in the central Appalachian coalfields. To date, more than 50 case histories have been analyzed from 20 operating mines in Eastern KY, Western VA, and Southern WV. Each case history is classified according to the degree of multiple-seam interaction, ranging from no apparent interaction to severe interaction with direct impacts on mine safety and resource recovery. The case histories are also examined with regard to amount and geologic characteristics of interburden and overburden, and design stability factors. The sequence of mining has also been found to be critical when the lower seam is fully-extracted. This paper compares current case histories to traditional rules of thumb. In addition two situations were modeled using LaModel software. LaModel has been upgraded to import pillar grids and topographic data from AutoCAD files. The cases presented demonstrate vertical stress capabilities of LaModel.

Jan 1, 2003

By Martin Bolesta

The use of strata injection as a means of stabilizing brittle and heavily stressed ground as well as faulted zones is now state-of-the art technology. Users are obliged to select the right combination from a wide range of injection products and equipment in order to ensure that the operation will be a success. Laboratory tests and a fundamental understanding of the technical aspects of injection products not only help in drawing up the correct injection procedure but have in fact become indispensable for planning the most reliable and cost-effective solution.

Jan 1, 2001

By David C. Johnson

Coal mining in Illinois has resulted in many structures being exposed to subsidence damage. Damage mitigation techniques (DMT) applied in the foundation design/construction have proven successful in minimizing damage in several test cases. To determine the practicality of these techniques, the Costs of their application are investigated. The AMT costs are compared to foundation and total house costs for various classes and sizes of houses. An economical and effective subsidence resistant foundation construction is proposed.

Jan 1, 1993

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992

By Brian G. D. Smart

A scheme is presented whereby the complexities of design and operation of a hydraulically powered coalmine support are modelled using the finite element method. This enables predictions to be made of the interaction of the support with its environment in a variety of operating conditions. The model is a two dimensional representation. The modelling scheme was applied in two case studies. In the first, the performance of a four leg support was uprated by increasing the front leg force. The finite element model enabled changes in overall support performance to be verified. In the second case study, the change in lateral force developed by a two leg support was assessed with changes in support operating height and floor foundation stiffness. The modelling scheme is capable of further development with the possibility of extending the model into three dimensions and the addition of control elements that would enable an actual mining sequence to be represented.

Jan 1, 1992

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995