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By George Mishra

In mid-1979, J&L Steel Corporation experienced rapid failure of pillars after only two weeks' operation with a combination of full retreat and partial mining in the 154 Road Section of its Nemacolin Mine, Greene County. The rate of advance of the "squeeze line" was very rapid for the first 500 feet, but it slowed down considerably as it approached some large coal blocks and gas well barrier pillars. After moving all section equipment out tc a safe distance, all accessible headings and crosscuts were filled with cribs and tri-set posts to prevent further spread of the squeeze. At one time, there was great apprehension by mine management that without implementation of some immediate corrective measures the safety of #3 airshaft and the only haulage and access road to the 8 Road mining area would be in jeopardy. In September, 1979, the U.S. Bureau of Mines established convergence monitoring stations outby the squeeze area. Readings were taken at weekly intervals until January, 1980, and monthly thereafter. Mathematical calculations, plot- ting of contour maps, and graphing of total convergence at different stations were used to follow and analyze the progress of the squeeze and provide guidelines for mining plans in the adjoining areas.

Jan 1, 1981

By Lu Jiam Zhang

Coal and gas outburst is one of the serious natural disasters in the coal mines of China. Since 1950, more than 10,000 coal or gas outbursts have occurred in more than 200 underground coal mines of different types. The outburst prone coal mines are widely spread out. It occurs in different types of coal -ma and is very serious in some areas. Most outbursts occurred at the time of roadway driving in coal seam, serious outbursts occurred mainly when entering the coal seam during the cross-cut driving; of the outbursts, coal and gas outbursts We UP major proportion, while pressing out and pouring out ah account for a large proportion; outburst. occur under different operating conditions, but mostly occur during shotfiring. This paper introduces the prevention and control measures as practiced in China and illustrates the three types of outburst cla8sification. Both technology and management are equally important in outburst prevention and control. Measurement of the volume of drilling cuttings and the initial gas outflow rate fm- boreholes have proven to be effective. A special team and organization must be set up to deal with the problems.

Jan 1, 1987

By Gerald I. Finfinger

Identifying the properties of overlying rocks in underground mining operations is important to ensure the appropriate roof support design is used to maintain stability of the mine entries. Recently J. H. Fletcher & Co. developed a monitoring and control system for roof bolters for the underground mining industry. The system records the drilling parameters used during roof bolt drilling and the information can provide insight into the physical properties of the roof strata. The parameters include thrust, rotational speed, torque and velocity and the measurements are collected every 0.1 second during the operation. The drilling parameters were analyzed to determine the application of identifying the strength of rocks being drilled from the measurements. The data was converted into the specific energy of drilling which is a measure of the amount of energy required for removing a given unit of rock during a drilling operation. The laboratory studies completed to date indicate a fairly high correlation between the specific energy of drilling and the unconfined compressive strength of the rocks that were drilled. Additionally, the drilling parameters were shown to be effective for identifying the presence of fractures or bed separations between rock ]avers. The thrust, torque and specific energy of drilling were all good indicators for identifying the fractures or separations. Regardless of the drilling parameters used during the drilling experiments, the location of the fractures were identified, In order to determine the application of the drilling parameters for identifying rout rock properties, two series of experiments were conducted. The first series of experiments used three "manufactured" roof layers that had various rock samples embedded in concrete blocks. The rock samples included three types of sandstone, marble. and argillite. Another concrete block was poured with foam inserts to simulate large bedding separations (2 to 8-in). Two other manufactured blocks were constructed using high-strength concrete with cardboard layers embedded to simulate smaller fractures or bedding separations. The size of the cardboard layers varied from 1/8- to 1-in thick. One of the blocks had the cardboard layers embedded at an inclined angle to determine the effect of the orientation or, the drilling parameters. A series of experiments was conducted with the rotational speed and the penetration rate held constant and the thrust and torque allowed to vary. The information collected from the experiments is used to determine the application of the drilling parameter measurements for identifying rock properties, fractures and bedding separations.

Jan 1, 2000

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982

By Douglas C. Peters

Part of the ground control research at the U.S. Bureau of Mines is involved with improving ground control data collection and analysis methods. A geographic information system (GIs) is being used within this research effort to standardize the data collected and provide a common graphical and tabular format and analysis system for researchers to access information on underground coal mines. In addition, GIS's can perform relational analyses on multiple map and tabular data sets which are not possible through map-creation or CAD packages or through data bases alone. In addition to standardizing ground control analyses, there is an increasing need for rapid analysis of ground control data in connection with increasing production rates at longwall mines. The examination of a large number of interrelated parameters and their impact on short-term mine safety and productivity under the time constraints imposed by rapid production requires computer-aided techniques. A GIs provides the means to quickly, quantitatively, and reproducibly evaluate mine-wide conditions. This overview of GI8 techniques for mine data integration and analysis, using a mine in Colorado as an example site, will cover the input of data into a GIs, including mine-wide monitoring system data and general data such as roof control plans and geology. The methods to geographically reference these data sets and retrieve and combine them also will be covered. Automation of such processes briefly will be reviewed to allow future users to automate their mine data analyses.

Jan 1, 1990

By Anil Mahyera

Under contract to the Bureau of Mines, a roofbolting machine has been developed which will: control torque on the bolthead to within 4 percent of a set point, control thrust to a low level (150 lbf), and implement a tightening method to minimize variability due to bolt tension decay of a group of bolts caused by differing anchor creep rates. This bolter was used to determine bolthead and thread friction factors, and to determine the effect of adding a hardened washer to the bolt assembly on the uniformity of bolt tension in the laboratory and field. Recommendations for torquing speed, bolt lubrication, use of hardened washers, and thrust level on the bolthead are made. The relationship of this system to Improved roof control is discussed, along with additional work that remains to be performed. Patent Notice Patents are pending which protect the hardware and methods of controlling tension decay as discussed in this paper.

Jan 1, 1981

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 Liang Yin-Huai

In Fuxin Coal Mines, there are more than 20 million tons of coal buried under seasonal streams. The coal seams are mostly shallow. We have mined more then 50 mining blocks distributed in 5 coal mines. Their natural conditions are different, and the consequences are also more or less different. However, no stream-water-inrush accidents under regular mining conditions are the same. There were 3 exceptions. They were caused by unexpected roof caving which made valleys or alluvia subside as funnel-like cavities and let in inrush of stream flow. The key to our success in mining shallow under-stream coal seams, according to our analysis, is that valleys or alluvia subside gently and basically evenly with contractible tension fissures and that fissures both in valleys or alluvia and in rock seams are blocked up or sealed by cohesive deposits either from alluvia or rock. This paper treats: (1) some typical illustrations of mining shallow coal seams of different occurrences under streams of different sizes, flowing capacities and valley structures, (2) observations of the height of caved and fissured zones and subsidence of valleys or alluvia, (3) observations of underground water flow before, during, and after mining and the fluctuation with respect to stream flow, (4) observations and analyses of blocking up of fissures caused by mining, and (5) feasibility of mining shallow under-river coal seams.

Jan 1, 1990

By Thomas Barczak

Hydraulic cylinders are used in several roof support systems, such as longwall shields and mobile roof supports, to provide critical ground control Many state-of-the-art hydraulic support system operate at pressures as high as 7,000 psi to provide the necessary support capacities Pressure in the upper stages of multi-stage cylinders can be intensified by a factor of 2 or 3 providing pressures of 20,000 psi or greater. A proper understanding of the operation of these high pressure systems is necessary to safely maintain and repair these support systems. Likewise, the performmce characteristics need to be fully understood to evaluate the impact of design parameters on the capability of the support to maintain effective ground control This paper examines the basic operating principles of state-of-the- art hydraulic cylinders and discusses relative issues pertaining to. (1) setting loads, (2) support stiffness, (3) yielding behavior, (4) errors in assessing support loading, and (5) hydraulic failure mechanisms and how to detect them

Jan 1, 1998

By Ding Xu Chu

There are many methods developed for measuring the crustal stress in China. Among them only two methods have been adopted in practice, i.e., overcoring method (for which the sensors include: Piezo-magnetic stress gauge, steel-ring strain cell, 3-dimensional hollow-inclusion stress ,gauge, and Leeman's-like stress gauge) and hydraulic fracturing method. For the former method, one of those sensors is installed in a borehole, 36 mm in diameter, then prestressed, and finally overcored with a drill bit of 130 mm in diameter. After that the deformation of the borehole wall is measured, from which the in-situ stresses are determined. In order to avoid the errors resulting from the difference in elastic modulus for various kinds of rocks as well as from the installment of the cells, we have developed a technique to calibrate the sensor on-spot using a confining pressure calibrator and produced a very good result. For the later method, the measurements are achieved by isolating a certain section of a borehole and then injecting water to pressurize it till a tensile break is induced. It is identical to the typical technology used internationally. For many years, through field observations and a large number of laboratory experiments, we believe that the most efficient way of stress measurement for engineering purposes is the overcoring method using Piezo-magnetic stress gauge. It is characterized by stable and reliable measurement values, high accuracy and high adaptability. Taking the measurements in the intact rock mass as an example, the measured stress magnitudes are within accuracies of 8% and the errors in orientation are about 5 degrees. It can even be used perfectly in deep water. Up to now, we have performed in-situ stress measurements in more than 70 sites and obtain more than one thousand sets of data. The deepest depth used in the overcoring method is 80 m in vertical borehole and 780 m in tunnels.

Jan 1, 1987

By Hamid Maleki

This study presents a historic overview of the role of mobile roof support (MRS) technologies in improving stability and worker safety and presents the results of recent field evaluations of the MRS load rate monitoring device and other remote deformation-monitoring techniques. Field studies were implemented at two sites in cooperation among researchers from the National Institute for Occupational Safety and Health (NIOSH), Maleki Technologies, Inc., and J. H. Fletcher & Co. The objective of the field programs were to (l) study the interaction between MRS's and coal mine strata and (2) develop and test suitable monitoring systems for assessing roof and pillar stability. An MRS consists of a roof canopy, four hydraulic cylinders, a caving shield canopy, and associated electromechanical systems mounted on crawler tracks. The machines are controlled by radio from a remote location and operate on self-contained power units. Typically, MRS's have capacities of 5,340 and 7,120 kN (600 and 800 tons). In comparison to posts, an MRS is capable of maintaining the yield load after significant amounts of roof-floor deformation. Because the mining cycle is accelerated, MRS's help reduce the potential for time-dependent roof falls. MRS performance has been monitored in the laboratory under controlled static loading conditions and in the field under deep, two-scam mining conditions. Laboratory studies have quantified support capacity and system stiffness as a function of machine height. Field investigations have focused on determination of optimum operating conditions and development of warning systems that indicate excessive load on the machine and/or impending roof-pillar stability problems. Analyses of field data show that roof instabilities are influenced by (1) pillar failure, (2) pillar yielding, (3) mine seismicity, (4) geologic structures, and (5) panel layout designs and mining practice. Pillar yielding and failure (unloading) and seismicity can be conveniently monitored by the load rate monitoring device, but for consistent detection of roof falls, additional deformation measurements directly within the cuts ate needed.

Jan 1, 2001

By B. G. D. Smart

The development of a softwood alternative to the hardwood chockpiece used to build the waste-edge breakers commonly incorporated in gateroad packs is described. The results of laboratory tests on softwood prototypes and two underground trials are presented. The results of the last trial suggest that a viable alternative chockpiece has been designed, offering advantages over availability of wood, weight and given reduced wood content, price.

Jan 1, 1992

By Xiaolin Wu

Most coal bump occurrences in the U.S. are directly related to mining operations under strong roofs. An understanding of the behavior of bridging or cantilevering strong roofs over working longwall faces is a key to a safe and productive longwall operation. In this paper, an attempt has been made to study the behavior of strong roof beds using the elastic beam theories. Four structural models are formulated to simulate the locations of strong roofs and their distinct weighting stages. For each model, analytical solutions for the deflection, bending moment, stress, and the strain energy stored in both the coal scam and the strong roof are obtained. Solutions for the critical spans, which are controlled by the caveability of the strong roof beds, are developed. The effects of rocks' differing elastic moduli, under tension and compression, on the actual flexural rigidity, and stress redistribution are analyzed.

Jan 1, 1993

By Erdal Unal

In order to define the support parameters, by use of empirical approaches, it is particularly important to analyze the rock-load height that should be controlled by supports. The controlling mechanism however, depends on the type of support used there- fore, the rock loads should be treated differently. In this paper, an empirical equation is introduced which relates the rock-load height (ht) to a quantitative rock quality index (REIR), and roof span (B) as fallows:

Jan 1, 1986

By Stanley J. Michalek

Good planning is critical to operating a safe and efficient mining operation. As the shallow coal scams become depleted, attention turns toward deeper mineable seams or remnant coal reserves. It is becoming more common to place a mine in a scam that is below or adjacent to existing mine workings. Significant research has been conducted regarding the ground control issues of multiple scam mining. However, safety aspects of the operation may not be fully considered. Safety issues include working below or adjacent to a flooded mine and controlling gas Communication between the active mine and the nearby mine. This paper will discuss the safety issues a mine operator should consider when working under or adjacent to mines that may be flooded with water.

Jan 1, 2000

By Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

By Edward A. Barrett

A significant number of injuries resulting from fa118 of roof and rib may be avoided by modifying the reactive behavior of miners as they encounter potentially hazardous ground conditions. In 1984-85, the Bureau investigated barriers which prevent miners from recognizing and correcting groundfall hazards. One basic problem from which these barriers originate is the inability of miners to recognize dangerous roof and rib conditions. This study is a report of experimental research on the use of stereo scopic (3-D slides for presenting visual cues associated with hazardous roof and rib conditions. Findings reveal that, in comparison to conventional slides, 3-D provides for significantly greater hazard recognition among both experienced and inexperienced miners. Details such as the presence of t cracks and gaps in the surrounding rock are more accurately and realistically perceived. Acceptance by the mine training community has been affirmative and is additionally encouraged by the fact that 3-D slides stimulate trainees' overall interest.

Jan 1, 1986

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By H. Maleki

Gob pressure measurements were made in a Western U. S. coal mine as part of a long-term program to evaluate cave progress and to determine the influence of geological discontinuities on caving conditions, load transfer, and resulting instabilities. Gloetzl cells were selected for the measurements due to their simple, robust construction and a history of being able to monitor pressures in a broken medium. A methodology was developed for the successful installation and protection of the cells and hydraulic lines in the gob as the face retreated. The measurement indicated a cave progress controlled by the frequency of, major faults. The pressure-mining progress profile was compared to those observed in other parts of the world. It was concluded that the significant differences in profiles was caused by the thick-bedded strata, the existence of high lateral stresses and the spacing between faults. Recommendations are given for future applications of Gloetzl cells for gob pressure measurements.

Jan 1, 1984

By Michael Alber

Coal resource extraction reaches higher depth of overburden and rockbursts as well as associated seismic events appear to become an additional problem to he addressed by ground control engineers. For example, German coal mines approach a depth of overburden of some 1400 m (4200 ft). Certain strata at those depths and associated high stresses may react upon resource extraction and subsequent deformation by violent release of strain energy, termed rodcbursts. Rock mechanics tests on numerous rock types have been conducted and three types of brittle rock behavior were classified. This paper describes different types of rock behavior in uniaxial compression and discusses a possible classification of rock with respect to their likelihood of violent response upon loading.

Jan 1, 2004