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By Rob G. Jeffrey, Ken W. Mills, Jesse W. Puller

"A coal mine in New South Wales is longwall mining 300m wide panels at a depth of 160-180m directly below a 16-20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage potential windblast and periodic caving hazards associated with this conglomerate strata, the in situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. This paper presents the results of in situ stress measurements and stress change monitoring undertaken as part of the hydraulic fracturing program.Overcore stress measurements have indicated the vertical stress is the lowest principal stress so that hydraulic fractures placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of the full-scale hydraulic fractures has confirmed the hydraulic fractures grow horizontally.Stress changes monitored during hydraulic fracturing formed part of a broader program to investigate fracture geometry and growth rates to confirm the characteristics of the hydraulic fractures placed. This monitoring showed that vertical stress in the conglomerate strata was being increased by up to 1 MPa in close proximity to the hydraulic fracture and had the effect of tending to promote asymmetrical growth of subsequent fractures about the injection borehole.Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about 7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face consistent with the observations of face conditions underground adjacent to both measurement locations. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden strata overhanging the goaf indicated by subsidence monitoring."

Jan 1, 2015

By Feiya Xu, Syd S. Peng

"As a highly efficient and new surveying and mapping technique, 3D laser scanning technology has been gradually applied to surface subsidence monitoring in mining areas. 3D laser scanning technology was used to monitor the mining subsidence in a coal mining area in this paper. Based on the 3D laser scanning data obtained in March, May, and December 2016 in the studying mining area, the surface subsidence basin and points deformation were acquired. The 3D laser scanning data were compared with that of the conventional surface movement surveying method. The results show that the 3D laser scanning technology could obtain the data of the subsidence basin more efficiently and reflect the true shape of surface subsidence caused by mining. The maximum surface subsidence obtained by 3D laser scanning technology was 3.60m, while the conventional surface movement surveying was 3.56m; the relative observation error between 3D laser scanner and the conventional surface movement surveying was less than 10%. Therefore, it is feasible to apply 3D laser scanning technology to monitor surface subsidence in coal mines.INTRODUCTIONWith the over-exploitation of coal resources, the problem of surface subsidence has become the focus of attention (Peng, 2011; Guo Wenbing, 2012). Setting up observation stations for surface movement was the traditional method to study the characteristic of surface subsidence and its parameters. At present, hundreds of observation stations have been set up in longwall mining, and a large number of measured data have been obtained (Ministry, 2017). However, due to the length of time surveying requires, great workload in the field, the high cost of observation, and difficult protection for monitoring points, conventional observation stations for surface movement have not met the research on surface subsidence. Therefore, as a new surveying and mapping technique, 3D laser scanning technology has been gradually used for surface subsidence monitoring in mining areas (Ma Liguang, 2005). Li Qiu, Qin Yongzhi, and Li Hong (2006) first proposed 3D laser scanning technology in the application of monitoring surface subsidence in 2006. It was showed that the 3D laser scanning technology had obvious advantages in efficiency and accuracy of data acquisition than other methods of deformation monitoring technology of surface subsidence (such as total station, precision level technique, GPS, etc.). Subsequently, Zhang Shu, Wu Kan, and Wang Xiang (2008) and Chen Ranli and Wu Kan (2012) analyzed the feasibility and precision of 3D laser scanning in subsidence monitoring, concluding that the 3D laser scanning can be used to obtain the subsidence basin and the monitoring precision can reach millimeter. Hu Dahe, Wu Kan, and Chen Ranli (2013) introduced the data processing method of 3D laser scanning for surface subsidence in detail. On the basis of a case study, the subsidence basin in subsidence area was obtained, which reflected the shape of mining subsidence basin intuitively and comprehensively. Wang Qisheng, Wu Kan, and Zheng Ruyu (2010) carried out the research on prediction parameters of mining subsidence by using 3D laser scanning data and got the parameters of major influence angle and the offset distance of inflection point. Meng Wanli, Cai Lailiang, and Yang Wangshan. (2017) made a profile analysis for the 3D view of the surface subsidence using 3D laser scanning data and put forward a “Geomagic” method of surface subsidence prediction."

Jan 1, 2018

By Adam Mackenzie, Dan Payne, Yass-Marie Rutty

"Historically, the Crinum mine has experienced significant falls of ground when longwall production slowed to prepare for recovery in weak roof areas. These conditions continue through the recovery process and result in both safety concerns and delays. When mine plans and exploration revealed that most of the mine's future longwall recoveries were located in weak roof areas, a decision was made to try pre-driven recovery roads as a solution to the problem. After completing eight pre-driven recovery roads with varying degrees of success and numerous lessons, the Crinum North mine now utilizes a modified Pre-driven Recovery Roadway (PDRR) to improve the longwall take-off process in weak roof areas. During mine development, a standard roadway is driven where the final recovery location of each longwall is planned. After the installation of secondary support, the PDRR is backfilled with a cement-flyash mix to provide support to the roof, and confinement to the ribs and floor of the roadway. The method has been refined over the last four years to provide greater strata stability and improved operational and safety performance compared with conventional take-offs at Crinum, and has resulted in a site record of longwall relocation in 11.5 days (pull mesh to picks in coal). This paper describes the evolution of the PDRRs from Crinum East to Crinum North, including lessons from initial attempts and changes to the secondary support regime, the operational approach during the final stages of retreat, the backfill strategy and plans for the future.INTRODUCTIONThe Crinum mine is the underground component of BHP Billiton Mitsubishi Alliance's (BMA) Gregory Crinum mine, located northeast of Emerald, Queensland (Figure 1).The Crinum mine has a history of roof control problems coming into and during longwall recoveries. Weak roof, combined with the slow mining, which is a consequence of preparing for equipment recovery, pulling mesh (which also creates tip-to-face issues), and the lengthy bolt-up process, often results in loss of immediate roof and subsequent roof falls. In fact, approximately 50% of the first 13 longwall recoveries (which were at depths of 180 to 200m) experienced roof fall delays, the longest of which took four months to mine the last pillar and recover the longwall. This recurring problem prompted the mine to investigate options to reduce or eliminate the problem."

Jan 1, 2016

By Yuanfei Chen, Jianfeng Zha

"China is the largest producer and consumer of coal in the world. The mining-induced surface subsidence has bad influences on the surface and structures above the surface. In order to minimize disasters, removal, reinforcement, or maintenance structures are often adopted. In the application of maintenance methods, it is necessary to carry out dynamic surface subsidence prediction to choose maintenance methods, optimize maintenance time, and evaluate the amounts of work. Time series methods and influence function methods are common for dynamic surface subsidence prediction, but the former is difficult to consider with underground mining conditions, and the latter has low prediction precision. Therefore, this paper develops a dynamic subsidence prediction model aided by field-measured data and realizes the model programming. The model could achieve high-precision prediction of dynamic surface deformation through optimizing the parameters in time by using field-measured data of surface subsidence. The research results of engineering cases show that, with the method of dynamic prediction, the standard error can be controlled in the range of 5%, greatly improving the accuracy of dynamic prediction of mining subsidence, which provides strong technical support for the maintenance and management of structures. INTRODUCTION With the rapid economic development and continued industrialization process, the demand for energy is growing in China. Today, China is one of the largest coal, oil, and gas consumers in the world. Coal consumption accounts for 47.7% of the total consumption in the world (Tian, 2016). In order to meet the demand for coal resources in the process of modernization, more and more coal mines carry out coal mining under villages, buildings, highways, and bodies of water. The mining-induced overlying strata and surface subsidence have bad effects on surface and on various structures (such as buildings, railways, highways, high-tension transmission lines) and could even cause various disasters (Williams et al., 1998; Bell et al., 2000; Doyle et al., 2002). Generally, methods, such as removal, reinforcement, or maintenance are taken to eliminate the adverse impacts (Kraatz, 1984; Bell and Genske, 2001; Wang, 1989). When reinforcement or maintenance is selected to remove bad effects, the proper maintenance methods must be chosen. In addition, maintenance time, as well as an evaluation of time it will take to do the work are necessary to predict the surface subsidence with high precision."

Jan 1, 2017

By Liang Dong, Jinxuan Liu, Biao Qiu, Jing Chai

"The Cenozoic unconsolidated strata in the east area of China are composed of clay, sandy clay, fine sand, hardpan, etc. Numerous shafts of the coal mines in these areas have been damaged in various degrees since 1987. The main reason for this disaster is the vertical additional stress on shaft caused by the subsidence of the thick unconsolidated strata. In order to monitor the subsidence deformation of the unconsolidated strata, a sensing network system with multiplexed Fiber Bragg Grating (FBG) for monitoring settlement deformation of the unconsolidated strata in deep borehole is proposed. The field monitoring results show that, soil layers settle or expand with decrease or increase in ground water levels. The thick hardpan generates the main compressive deformation. This FBG strain monitoring method can take place of traditional ground water measuring method. This system is stable and reliable, and the monitoring results can show the subsidence deformation of different layers, which has significance for the predictive maintenance of the shafts.INTRODUCTIONSince 1980s, a specific geologic hazard has occurred frequently on the coal mine shafts in eastern China: non-mining-induced shaft deformation, involving up to 100 shafts located in Jiangsu, Shandong, Anhui and Herran province, distributed in Xuzhou, Datun, Huainan, Huaibei, Yanzhou and Yongxia coal fields. This kind of geologic hazard can lead to shaft wall deformation, water infiltration, cage stuck, water-sand burst or shutting down of coal mines, which seriously risks the coal mines. Zhangshuanglou coal mine, Haizi coal mine and Linhuan coal mine shut down for 2-8 months due to this hazard (Chen, 1997; Cui, 1998). The main reason for this geologic hazard is the subsidence of the super thick unconsolidated strata (Liu et al., 2007; Xu et al., 2006).An innovative structure health ·monitoring and damage estimation method has been developed in recent years, i.e., optical fiber smart material and structure (Nobuo, 2002; Wang et al., 2000). A Fiber Bragg grating (FBG) sensor is a device that can transfer the change of the measured strain or temperature to the change of Bragg wavelength. One of the most important applications of FBG sensors is to monitor the internal stress and strain distribution of material and structure (Philipp et al., 2000; Ansari et al., 1998). Compared to traditional electrical sensors, FBG sensors have many advantages; they are adaptable to harsh environments, abrasion resistant, immune to electrical magnetism (EM) interference, safe and reliable. They also offer integrated sensing and transmission on a single fiber, have simple structure and are convenient to use, which satisfies the requirements of long distance, distributional and long term use in monitoring mining and geotechnical engineering structure. Due to such advantages, the FBG sensor has already been applied in petroleum engineering (Kersey, 2000), rock mechanics testing (Cornelia et al., 2003; Yang et al., 2007; Wei et al., 2008), physical simulation experiments (Chai et al., 2007; Chai et al., 2006; Zhu et al., 2008; Li et al., 2008; Chang et al., 2008), strain sensor (Li et al., 2007; Chai et al., 2008) and construction structure damage diagnosis (Dong et al., 2004; Kerrouche et al., 2008; Whitten et al., 2001)."

Jan 1, 2010

By Vincent A. Scovazzo

The Pennsylvania Joint Coal and Gas Committee Gas Well Pillar Study of 1956 is the bases for well protection pillar design in the US and many other countries. An update was necessary because the well construction common to the gas and coal industries as well as mine configurations, mine depth, and extraction thickness have changed considerably since 1956. As initially envisioned the update would involve a similar empirical study that formed the basis of the initial investigation. However, there are no records of any gas or oil well failures related to coal mining since 1956 when mines followed the initial recommendation, making an empirical study problematic. Thus, the study had to be more analytical, which in return required at least some fundamental research into the behavior of the interaction of coal mining and oil and gas wells. A call to industry about research they have undertaken resulted in obtaining excellent research on the subject from three companies; Alpha Natural Resources, Inc., CONSOL Energy Inc., and Range Resources. These research projects were initially undertaken for internal use and not previously published. John T. Boyd Company reanalyzed these studies and determined how their results would affect protective pillar designs. These research projects pointed to the importance of horizontal movement and shear caused by subsidence and the strength of pillars and their foundations. The importance of geology is important in understanding horizontal shear, ground movement, and floor strength. The failure modes of casing and pipe needed to be understood so the threshold of these movements and stresses could be determined.

Jan 1, 2013

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013

By Jessica M. Wempen

Differential Interferometric Synthetic Aperture Radar (DInSAR), a satellite-based remote sensing technique, has been demonstrated as a potentially practical method for measuring mining induced surface subsidence. Unlike traditional methods of subsidence monitoring, DInSAR has the capacity to generate subsidence data on a regional mining scale with a relatively high data density. Using DInSAR to evaluate subsidence over large regions with relatively long time scales has the potential to help quantify the impact of subsidence in the mining region, and provide a means for validating whole-mine geotechnical models. In this study, nine pairs of interferometric data from the Japanese Advance Land Observing Satellite (ALOS) were processed using DInSAR to generate displacement maps for a group of trona mines in southwest Wyoming. The data cover a time span from December 2007 to March 2011. The maximum subsidence for one of the mines totaled 1.3 meters for this period. Cumulative DInSAR displacement maps show the development of subsidence over time. In this mining region, surface subsidence is generated primarily by extraction of trona using retreating longwalls. Additionally, trona is extracted by solution mining, which also generates measurable surface subsidence.

Jan 1, 2014

By Kevin J. Ma, John Stankus, Dakota Faulkner

"Expandable rock bolts are widely used in hard rock mines as an efficient ground control product. However, capacity and service life can be significantly reduced if the metallic body is subjected to corrosion. In some hard rock mines in the U.S., highly corrosive ground conditions exist, and it has been reported that inflatable rock bolts have corroded within a few months after installation. To provide mining industry a cost-effective inflatable bolt and combat the corrosion issues, Jennmar Corporation, Inc., and its subsidiary Keystone Mining Services, LLC (KMS), analyzed corroded bolt samples, identified root causes, evaluated properties of various coating materials, and developed a new inflatable rock bolt, Python M3TM, that is protected with an innovative PyFlexU2TM coating. The new generation Python M3 features improved steel chemistry for reliable performance, modified profile for better inflation, and surface preparation and coating application. The PyFlexU2 is impervious to liquid and air, durable, and UV resistant. With a flexible adhesive, and highly corrosion-resistant undercoating, and a very hard sacrificial surface coating, the PyFlexU2 coating system provides the Python M3 superior protection against chemical corrosion and physical scratch damage. The under-coating has exceptional flexibility and adhesion to prevent coating micro-cracks or fractures after bolt inflation and possesses excellent corrosion resistance to acids (pH <3), alkalis (pH >11), fuels, and salt solvents. The corrosion and scratch resistant PyFlexU2 coating offers very effective bolt protection for extra longevity in highly corrosive environments. The Python M3 coated with PyFlexU2 has been tested in the most challenging conditions, including laboratory corrosion tests in extreme acidic and basic solvents, rock slurry, and borehole scratch insertion tests. With demonstrated corrosion and scratch resistance, the product has been greatly welcomed by hard rock mines in the West and is currently installed in large scale. This paper identifies the root causes of the bolt corrosion, discusses the analysis process, and details laboratory and underground tests carried out on the Python M3 coated with PyFlexU2. The Python M3TM and PyFlexU2TM are subjects covered by pending U.S. Patent Applications assigned to FCI Holdings Delaware, LLC."

Jan 1, 2017

By Kresho Galovich

Quinsam Coal Mine located in Vancouver Island, BC, Canada is mining the No. 1 coal seam at the 2 North/3 North mine and the No. 3 coal seam at 4 South Mine. This paper describes its geology and structure, mining and processing, development and depillaring practices as well as methods of roof supports under various geological conditions.

Jan 1, 2005

By P. Palmer

Using 3D numerical modeling as a tool to predict rock behavior during mining extraction is still an uncommon practice at many underground operations. This paper will discuss how 3D numerical models were used to identify potential underground instability problems in the later stages of mining. Detailed modeling was then used to make modifications to the mine design in order to minimize the impact of the potential instability problems. This paper will also discusses how the back analysis of a previous underground collapse event at the same mine in a room and pillar area was used to identify the instability problems for a new orebody being developed deeper in the mine. A 3D numerical model was first created in the MAP3D software to determine the conditions of the pillars and the magnitude of the vertical displacement above the mining zone prior to the underground collapse. The results from the ground collapse model were then used as a threshold limit for predicting potential instabilities and developing a model for the new mining zone. sing the results from this analysis, mine management has been able to implement a much higher percentage recovery of the reserve in a new, high-grade area of the orebody while minimizing potential instabilities in the future.

Jan 1, 2005

By T. L. Dobecki

As a means of demonstrating the effectiveness of reflection seismology in determining continuity of coal seams, three U. S. field projects are reviewed. The three projects involve coals of varied thickness (2-14m) and age (Pennsylvanian to Eocene) from three coal areas of the U. S. (Pennsylvania, Wyoming, and Washington). Each project also employed its own particular seismic technique, recording system, and seismic energy source although all are considered state-of-the-art, high resolution, digital seismic surveys. Project 1 (thin, Pennsylvania coal) sought detection of sand channels using dynamite and standard in-line (2-D) seismic technique. Project 2 (thick, Wyoming underground coal gasification) involved a gas-explosion (Dinoseis) source with areal (3-D) acquisition methods. Project 3 (thick, Washington underground coal gasification) employed a shotgun-type source and standard in- line methods. Data processing was handled by different contractors for each project. Each project was successful in accomplishing its own particular objective; however, data quality and interpretation seem to be more a function of thickness of the target seam, complexity of the overburden, and processing contractor than a seismic source, acquisition scheme (2-D versus 3-D), or recording instrumentation.

Jan 1, 1981

By David A. Newman

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

Jan 1, 2014

By James H. Fletcher

Much data and experience has been accumulated, especially in the last 5 or 6 years, to show that modern roof trusses, both of the Birmingham type and the bolt-and-channel type, perform well to support certain kinds of coal mine roof. For some time now, roof trusses of the Birmingham type have been effective in controlling thick shale roof, which tends to sag and fall over a period of about eight weeks. These falls leave dame-shaped cavities which are reminiscent of the pressure arch theory of roof failure. Lately there has also been significant interest in supporting some longwall panel entries with channel and-bolt systems. These systems utilize two vertical center bolts and two 45 degree end bolts through a heavy gage channel. Both types of truss systems have long bolts, which require 8 to 9 Foot holes on a 45 degree angle, and point anchor resin bolts, which utilize a 2 or 3 foot rebar grouted in a thin annulus of polyester resin. Consequently, many coal mining people have been impressed with the very good anchorage characteristics and tensionability features of these systems. However, the same people have been proportionately dismayed to find that very often the most difficult part of introducing a trussing program to a mine is deciding on the machine specifications which determine the truss installation rate. The purpose of this paper is to indicate aspects of truss bolter design and use which are not shared by roofbolter design and its vertical pattern of bolts. Of the two types of drill units conventionally accepted, arm feed units and mast units, only one 15 judged suitable. While the arm feed unit offers low collapsed height with a long drill feed length, a great attribute for low seam use, it has not lent itself to being rolled over to a 45 degree angle. One reason for this inability is that many items which are conventionally mounted on the arm feed unit cannot be allowed to rotate with the unit. These items include drilling controls, tool Cray, lights, canopy or safety post, panic bar, and stab jack(s). If these items were independent of the arm feed drilling unit, they would have to be out of the way of the boom arms, links, and feed cylinder as the unit moves. To ease this problem, the arm feed unit can be made very short and even mounted across the machine. However, the resulting short feed length is undesirable. Several short drill steel extensions are required to drill a relatively long hole, making this procedure time-consuming as well as increasing the likelihood of drilling a crooked hole. Another reason for abandonment of the arm feed unit is that a very stiff drill guide (or "centralizer") is required to start the hole. Using the mast tilt control, the roof bolter operator will force the bit against the roof and will rotate the bit without feeding to cut sideways into the roof to form a hole face. Omission of this step often results in the skidding of the bit toward the rib. To date, no such sturdy drill guide has been designed for an arm feed drill unit. The mast style unit has been the most successful design for angle drilling. It is easily rolled over to a 45 degree angle or to 90 degrees and, on single head models, often has been made to roll through 180 degrees. The mast style is compact, in plain view and is easily fitted with heavy duty manual or hydraulically operated drill guides. It can be mounted directly on a machine chassis, or onto a swinging boom, or onto a slider arrangement in a broad variety of configurations which will accommodate controls, tool trays, stab and roof support jacks, lights, canopies, and panic bars. The only significant difficulty with mast drilling units occurs in lower mining heights when desirable low mast height is accompanied by undesirable short feed length. While the lead extension in the drill string can be as large as the mining height will allow, a short feed length will permit only the use of shorter middle extensions and drivers, and perhaps will. require the use of two different length wrenches. The associated problem is the relatively short distance from the drillhead to the drillguide. This distance is further shortened when raising the drillhead to get the bit to the roof. One might keep longer lead extensions on the machine; drill a vertical pocket for the bit, starting the angle hole at this pocket; or modify the mast. If the mast is made so that the distance from the drillhead to the drillguide remains constant while the mast extends, after which the drillhead travels up the mast, the shorter mast can be made to start the hole more easily. However, short teed length will still result in longer drilling time. It is obvious from these points that a proper choice of mast is simply the highest one which will be moved easily in the given mining height. This, of course, goes against the grain of most experienced coal mining people who realize that a relatively small decrease in mining height can create a problem correctable only with the purchase and installation of shorter masts. If the mining

Jan 1, 1982

By Denise Y. Dixon

The objectives of this study were to (1) document the hydrologic impacts of longwall mining on ground water, (2) identify the factors which affect the extent of dewatering, and (3) develop empirical trends to predict the extent of dewatering and recovery in advance of mining. The study was confined to three mine sites in north-central West Virginia. At each site, available ground water supplies were identified and monitored for dewatering effects and recovery. Mine A, located in Upshur County, West Virginia, has relatively thin overburden (approximately 715-280 feet). At this site domestic water supplies and specially constructed monitoring wells were monitored, continuing with an earlier study done by Cifelli arid Rauch. Most such supplies were completely dewatered over the longwall panel. Some of these wells have shown significant recovery, especially near a stream. Extensive monitoring of two nested piezometer wells revealed that dewatering generally started in advance of undermining in the lowermost monitored overburden zones first and then progressed upwards. Most dewatered wells completed in bedrock have shown no recovery, except. for valley drilled wells located within 100 feet of a Stream. Affected dug wells completed in valley alluvium rend located within 200 feet. of the nearest stream have shown significant recovery. Some wells have partially collapsed following mine subsidence. Mine B, having relatively thick overburden (approximately 585 to 900 feet), is located in Monongalia County, West Virginia. Ground water supply surveys have been conducted there in an attempt to study past dewatering effects. Owners reported Chat several wells were dewatered by longwall mining arid vertical air shafts that preceded longwall mining. Dewatered wells over longwall panels hove shown some recovery within 3 ½ years of mining. Mine C also has relatively thick overburden (approximately 585-900 feet), and is located mostly in Greene County, Pennsylvania. In addition to a survey of domestic water supplies at this site, five specially constructed monitoring wells were tested. The shallow monitoring wells suffered drops in water level due to mining but totally recovered within one day to three weeks after dewatering occurred. However, they did occasionally exhibit partial collapse during mine subsidence. Subsidence fracturing was beneficial in increasing the lateral hydraulic conductivity by a factor of about S to 22 for rock strata located within 124 feet of the surface, indicating enhanced shallow aquifer potential. One deep well suffered both partial dewatering and structural damage, with the state of water recovery bring uncertain. Also, two shallow domestic supplies out of five located over the mine were reportedly dewatered, but this claim could not, be substantiated.

Jan 1, 1988

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 P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993

By T. P. Mucho

Mine roof failure due to excessive horizontal stress has been recognized as a major cause of hazardous roof conditions in some mines. Stress measurements gathered by the U.S. Bureau of Mines (USBM) at 24 different eastern mines show horizontal stress to be typically 2 to 3 times as great as vertical stress. The focus of current Bureau work is to develop detailed design parameters to assess and control the effects of horizontal stress. To facilitate the recognition of horizontal stress effects and to easily determine principal stress direction without resorting to cumbersome, expensive, and time consuming field measurements, the Bureau has developed a stress mapping methodology. Stress recognition and direction determination are required to successfully employ a number of control strategies such as mine layout, mining sequences, and support optimization to control the effects of horizontal stress, thereby increasing the safety of U.S. coal mines. This approach has been used in other major coal producing countries, most notably Australia and the UK, and has greatly enhanced the safety of their mines (1). This paper discusses horizontal stress, directional based control techniques, and provides guidelines for the use of the stress mapping technique. A case study of a mine where the technique has been used is also presented.

Jan 1, 1994

By Frederick Carr

1. The application of retreat longwall mining to the North American coal mining conditions is be¬coming more advantageous as more experience is gained. The productivity results being achieved and potential possible illustrates that this system can be the keystone of future underground coal production. 2. In the American coal fields multi-entry is normally practiced as this allows profitable longwall development with existing equipment and systems. Difficulties are being experienced however in the longwall retreat phase by the col¬lapse of multi-entry roadways and excessive use of supplemental support in the following panel tailgates. In some instances the headgate of an operating face can also be in difficulty. 3. Jim Walter Resources Mining Division is developed on a mining plan that calls for three retreat longwalls at each of its four deep mines which have varying depths of cover and immediate roof and floor conditions. The #3 Mine has experienced longwall entry difficulty and there were differing viewpoints with respect to the best development systems to adopt particularly regarding the number of entries and pillar sizes. In an attempt to reconcile these different points of view J.W.R. asked Dr. Wilson to examine the problem and this paper outlines the new approach developed.

Jan 1, 1982

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