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By H. N. Maleki

The results of five years of geotechnical investigations are presented to develop productive and stable longwall layouts for the Foidel Creek Mine. The program was initiated during the pre-mining stage, and has continuously provided the data required for mine design. From several stress determinations at the mine, the relationship between geologic structure and the stress field/depth was established. The results were used for orienting the entries for improved stability. Cave conditions and load transfer to panel boundaries were evaluated from closely monitored full extraction mining and computer analysis. Back-analyses of ground movements, as compared with the actual measurements, indicated that cave conditions were favorable in spite of the presence of competent, thick-bedded sandstones in the mine roof. Gate pillar design using a yield-rigid concept was developed through computer analyses and underground instrumentation, resulting in 35 and 75 ft wide pillars. The influence of a slickensided bedding plane at the roof contact on pillar yield zones, core confinement, and pillar behavior was evaluated.

Jan 1, 1988

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 C. Tom Blevins

The purpose of this paper is to discuss production and roof control problems associated with directional lateral stresses in an in situ stress field and the approach that Inland Steel Coal Company made in trying to cope with them. Before proceeding, a description of the mine location, product, geology, restraints, and mine history is in order. The mine is located in Hamilton County in Southeastern Illinois. It produces a high volatile metalurgical coal and is slated to produce 2.1 MTY. Product ion is in the Illinois No. 5 seam at a depth of 930 feet, approximately 490 feet below sea level. The scam height averages from 5.5 to 6.0 feet throughout the reserves with height increasing in the eastern reserve area to approximately feet and a 5 Foot average in the West. The scam tends to dip to the North, North East, but is locally very undulating. The Galatia channel washout outlines the eastern boundary of the reserves and there is a split coal area toward the central part of the property than runs south-southeast. The top above the #5 seam is laminated medium gray shale called Dykersburg. Individual shale bedding planes vary locally From ½ to 1" in thickness to a more normal thickness of 1 to 1 ½ with some areas being interlayed by thin carboncous material. The top normally must be considered as materially competent. Even though the mine is still relatively small in area with only 1.3 million tons of total production to (late, we have encountered dense zones of slicker sides and premining, stress cracks (joints). There are also areas of siltstone, that vary from a few inches to several feet thick, and areas of rider coal, clay instrusions, and rolls. We have crossed lineament concentrations that show strong evidence of corelation to localise top problems. These are being monitored closely to see their predictability on our present and future mining conditions. The face cleat runs N 55° E and butt cleat runs S. 25° E. On a regional basis, there are several anticlinal belts that trend in a northerly direction. The one major restraint is that Inland does not own surface subsidence rights. Therefore, a partial extraction mining plan is being followed. The Holland-Gaddy formula for pillar strength design is used to calculate theortical pillar sides. When production began in March of 1979, it was noted that ground control problems occurred as we drove north entries. East-west cross cuts tended to have better working conditions. This phenomia occurred from a couple hundred feet, but seemed to dispate as we got away from the men and material shafts. Once the production shaft was connected and enough entries extended for installation of belts, ground control problems were again experienced in north-south headings. It was also observed that when mining north-south headings with east-west crosscuts, if the crosscut was driven through ahead and supported, the intersection was stable upon entry connection. This held true even where fairly extreme ground control, problems were encountered. The reverse held true. when driving east-west headings with north-south crosscuts. At this time, we started searching in earnest for a working theory to explain this roof phenomia. It also became the first step of what eventually was a four step sequence to derive a workable solu¬tion to the problem. The combination of the many geological changes and the fact that north-south condition was at times intermitent, helped to disguise the stress field. The mine also being in a fairly virgin area with no mines in approximately 25 milt, radius, meant we didn't have a reliable case history to examine. Various theories were researched before it was concluded that our primary ground control problem was a directional lateral stress Field, with the major stress axis in an east-west direction. The second stop of the procedure was then to test. mining methods and determine if these stresses could be modified to a

Jan 1, 1982

By I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

By Eric R. Bauer

Skin failures of roof and rib in underground coal mines continue to be a significant safety hazard for mine workers. Skin failures do not usually involve failure of the support systems but result from rock or coal spalling from between the support elements. For instance in 1997, over 800 miners were injured by roof and rib falls, of which 98 pet were the result of skin failures. Also, nearly 80 pct of the roof and rib failure injuries occurred at or near the working faces in development sections. The face area is a zone where the potential for skin failure accidents and injuries and roof and rib failures, in general, is high because of mining activity, ground readjustment due to changing stress conditions, and the higher exposure of mine workers. In addition, failures occur where the roof and rib are unsupported. This paper features a review of the roof and rib accident statistics resulting from skin failure, highlighting the incidences by average days lost, in-mine location, state, MSHA District, and worker activity at the time of injury. A detailed analysis of recent fatalities caused by skin failure is included. Also discussed are the causes of roof and rib skin failures, current methods and support materials for skin surface control, as well as a historical literature review of skin failures and control methods.

Jan 1, 1999

By Klaus-Dieter Beck

RAG American Coal Holding, Inc. affiliates operate two longwall mines in the Pittsburg seam in Pennsylvania, the Cumberland mine and the Emerald mine. A unique situation in longwall operation occurred at these mines as the mining activities reached the property boundary between the two mines. The retreat direction in the longwall panel in Cumberland mine is West-to-East while in Emerald mine, it is East-to-West. This unique condition required an adequate design for barrier pillar between the two mines to minimizing the mutual effects of longwall retreat when both longwall faces met and an adequate roof control plan for gateroads. Field studies that involved mainly roof-to-floor convergence monitoring were conducted to assess the effectiveness of the employed barrier pillar in isolating the mutual effect of longwall retreat. Three-dimensional finite element numerical modeling was used to evaluate the gateroad roof and pillar stability. The modeling was conducted such that it duplicated the actual mining process as realistically as possible. The modeling results predicted that the employed barrier pillar and the roof control plan would ensure stable gateroad systems in both mines during and after the two longwall faces met. The prediction was confirmed because mining operations in the two longwall panels were completed without any noticeable ground control problems.

Jan 1, 2004

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

By Tim Ross

Slurry injection into mined-out panels is an attractive alternative for disposal of coal preparation plant fines. however, in areas where surface subsidence cannot be tolerated, either by law or necessity to protect surface structures or aquifers, special consideration trust he given to the effect of slurry injection on panel stability. Slurry injection can lead to panel failure and unplanned subsidence by degrading roof, pillar, and floor strength. Water degradation is of particular concern where clay is present in the floor. Recently. unplanned subsidence related to slurry injection occurred at an Indiana room-and-pillar coal mine subject to weak floor conditions. This paper discusses the efforts of the mine operator to determine the causes of the panel failure and to design panels and injection procedures to allow for future economic slurry injection without subsidence. The numerical and empirical/analytical techniques applied give the operator a rational approach to pillar panel design for future injection panels under various cover depths and weak floor conditions.

Jan 1, 1998

By Nielen van der Merwe

The data base of failed pillar cases as used by Salamon and Munro (1967) in their original analysis to determine the strength of coal pillars empirically, has been updated for this study. It is shown that the Vaal Basin and Klip River coal fields exhibit similar behaviour, i.e. failure at high safety factors within a short period of time after their creation. Those collapse cases have been omitted from the new data base. New failures that occurred after 1967 have been added, except for those that occurred in the Vaal Basin and Klip River coal fields. (1967) a technique to minimise the area of overlap between the populations of failed and intact pillar cases, the new data base was re-analysed in conjunction with the original Salamon and Munro (1967) data base of intact pillar cases. It was found that a linear formula reduced the overlap area by 22%. The new formula predicts higher strength for pillars with a width-to-height ratio greater than approximately 2S and a lower strength for smaller pillars. For most current South African coal mining conditions, using the new formula for pillar strength will result in leaving smaller pillars without sacrificing stability. The reason for this is that the Salamon and Munro (1967) formula underestimated the strength of larger pillars and overestimated the strength of smaller pillars. The potential benefit for the South African coal mining industry amounts to saving reserves equal to the total production of two large mines per year.

Jan 1, 2002

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 Jinsheng Chen

The major functions of longwall headgate entries are to provide access ways to the outby submains or mains for coal transportation and to the longwall face for fresh air and for transporting men and material during longwall operations. Any roof control problems such as cutter roofs or roof falls in the outby headgate entries could result in significant production delays to the longwall face and may be a threat to miners' safety Under a thinly laminated immediate roof and a high horizontal stress field, even a longwall headgate entry that has an angle of less than 45 degree to the maximum horizontal stress may experience roof deterioration in the outby headgate if the longwall panel mining sequence and retreat direction are not properly considered. This paper attempts to analyze the causes of the roof control problems experienced in the longwall gate entries at our Cumberland and Emerald mines in Pennsylvania and to determine the optimal longwall panel mining sequence and retreat direction for the future mining area at Cumberland

Jan 1, 1998

By Francis S. Kendorski

Underground stone mines tier construction aggregates and industrial minerals are being planned in increasing numbers throughout the United States. At the middle of the last century (1950s) underground stone mines were much more common, and characteristically had no thorough site characterization other than proving reserves and trial-and-error planning. Ground control employed was minimal. Consequently, there was a tendency to conduct minimal pre-mining studies. However, the deposits that lend themselves to little site characterization and minimal ground control are few today, with the best being mined out. Failures of a number of underground stone or similar mines from that era are more common than realized, Underground stone mines almost always use some combinations of square or rectangular pillars, sized to support the overburden load, and spaced to allow the use of larger, more-efficient equipment. Heights are controlled by the available stone quality, interbeds of waste, or technological or safety considerations. Prior to committing to a specific mine plan with subsequent significant capital investment, and prior to having available the wealth of knowledge and experience gained once mining underground, a safe and workable mine layout must he achieved. Early attention to the several different rock units involved (roof, mine level. floor) and their strength as a rock mass with the inherent discontinuities, will allow first approximations of acceptable mine geometries and sequencing.

Jan 1, 2000

By Lisa Burke

In underground coal mines, concrete block stoppings are widely used to control mine ventilation. Although stoppings are not intended as roof support, they are subjected to roof to floor convergence, and may fail as a result of this loading. Premature failure of stoppings can significantly affect mine ventilation, limiting the amount of fresh air reaching the working faces and increasing the risk for methane explosions. Softening materials are often used in stoppings to reduce the damaging effects of roof to floor convergence. However, to date, research has not focused on the behavior of stopping construction materials subjected to roof or floor loading, leaving the design process to trial and error. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The first series of physical models were constructed using standard CMU (concrete masonry unit) blocks and tested in a load frame. The behavior of these walls was simulated with a distinct element model. A key finding was that very small variations in block size and irregularities in block shape significantly affect wall performance. These variations, which are inherent to the type of blocks typically used underground, create stress concentrations that initiate the failure of stopping walls. A second series of models employed concrete stoppings that incorporated woos planks, a shredded wood material, and foam planks like those commonly used underground. Numerical models were again able to match the physical test data. Analysis of the softening layers used in stopping construction indicated that the strength and stiffness of the material relative to that of the wall is crucial in determining the amount of roof to floor convergence the wall can withstand prior to failure. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment. Testing was done in a laboratory setting with walls constructed on flat, parallel surfaces and subjected to uniform loading at a constant rate. Parametric studies with the model indicate that in this ideal environment it is possible to control the amount of vertical displacement the stopping can withstand by adjusting the number of softening layers built into the wall and the thickness of the layers. Additionally, under these conditions, the position of softening layers within the wall appears to have little effect on the amount of displacement walls can withstand prior to failure.

Jan 1, 2004

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 Khaled Morsy

Longwall mining is one of the most widely practiced underground coal mining method. The behavior of the longwall gob is very critical in the understanding of the complex ground response to longwall mining. In the limited data available, the values of gob moduli used in numerical modeling ranged from 1,000 psi to over 300,000 psi. Such a wide variation in moduli would greatly affect the results of the numerical analysis. A better understanding of the behavior of the gob will allow numerical models to be more accurate for simulating longwall mining conditions. In this study, a gob model based on the Terzaghi's model was incorporated into the comprehensive finite element package, ABAQUS, to simulate the loading behavior of longwali gob material. A case study was used to verify the proposed gob model.

Jan 1, 2002

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

By Bret E. Leisemann

The geomechanical environment in which highwall mining must operate represents a hybrid between surface and underground operations. Mine design must take into consideration the complexities associated with unsupported spans, pillars with extreme aspect ratios and the instability related to the interaction between coal extraction and the highwall stability. The rock mass adjacent to the highwall, and for a significant distance in, is affected by lateral stress relief and blast damage. This paper describes a rigorous site investigation program aimed at providing some of the input parameters for the design of highwall mining layouts at the Moura Mine and focuses upon defining the nature and extent of the relaxed zone behind the highwall. The combination of borehole imaging and C factor analysis allows delineation of blast damage and relaxation of the rock mass immediately adjacent to the highwall excavation, as well as the more subtle relaxation effects deeper into the rock mass. This has led to the formulation of a proposed geomechanical model for the relaxed zone for direct input into highwall instability analysis and mine design.

Jan 1, 1993

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

By Chaojun Fan, Du Wenzhang, Sheng Li, Hong-Hong Shi

"Coal and gas outburst is an extremely complex dynamic disaster in coal mine production process which will damage casualties, equipment facilities and disorder the ventilation system by suddenly ejecting a great amount of coal and gas into roadway or working face. This paper analyzed the interaction among the three essential elements of coal and gas outburst dynamic system. A stress-seepage-damage coupling model was established which can be used to simulate the evolution of the dynamical system, and then the size scale of coal and gas outburst dynamical system was investigated. Results show that the dynamical system is consisted of three essential elements, coal-gas medium (material basis), geology dynamic environment (internal motivation) and mining disturbance (external motivation). On the case of C13 coal seam in Panyi mine, the dynamical system exists in the range of 8-12 m in front of advancing face. The size scale will be larger where there are large geologic structures. This research plays an important guiding role for developing measures of coal and gas outburst prediction and prevention. INTRODUCTIONCoal and gas outbursts are a complicated mine gas dynamic phenomenon that pose a serious threat to coal mine safety production. Since the first recorded coal and gas outburst occurring in Isaac coal mine of the Lule coal field in 1843, more than 40,000 outbursts have occurred in the world. China ranks first in outburst frequency and intensity. In recent 20 years, China on average increases 37 pairs of coal mines and more than 280 times outburst accidents every year (Yan, 2013). The coal and gas outburst occurred on Oct 20, 2004 in Daping Coal Mine, Zhengzhou Coal Group, caused serious gas explosions and ejected 1,894 tons of coal into the face, causing 148 fatalities. In recent years, despite implementation of numerous control measures, coal and gas outburst still occur frequently ash mining depth and mining intensity increase. The outburst mechanisms can be summarized in the following four aspects: the gas leading role hypothesis, the geostress leading role hypothesis, the chemical effect hypothesis, and combination hypothesis. The last hypothesis has won the acceptance of most scholars who consider that an outburst is the result of combined effect of stress, gas and coal (Yu, 1992).Numerous experimental measurements and numerical simulations were conducted to investigate the permeability and damage development process in coal seam. Sobczyk, et al (2011) carried out a laboratory research of the influence of sorption processes on gas stresses leading to coal and gas outbursts. Wang, et al (2013, 2015) conducted laboratory experiments to investigate the rapid decompression and desorption induced energetic failure in coal using a shock tube apparatus. Xu, et al (2006) developed a coupled gas flow and solid deformation numerical model and applied to simulate the coal and gas outbursts in underground coal mines using RFPA2D-Gasflow. An, et al (2013) and Xue (2014) analyzed major parameters and the effect of gas desorption on outburst initiation and established a model of gas migration and mechanical processes during excavation."

Jan 1, 2016

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