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By Raymond M. Stateham

The Bureau of Mines has developed a portable, lightweight device for determining the integrity of resin-grouted roof bolts. The instrument functions by sending a known pulse of ulstrasonic energy into the bolt and comparing the amount of energy reflected hack with the original measured pulse. Where the bolt is properly bonded or coupled with the rock, the energy passes through the bolt-resin-rock interface and dissipates in the rock. Where the bolt is not properly coupled, the energy is reflected back along the bolt to the instrument. The ratio of reflected energy to the pulse approximates the fraction of the bolt's surface that is improperly bonded. The instrument is easy to use. The operator holds the sensor head of the device firmly over the head of the bolt to be tested, causing the pulse of ultrasonic energy to be imparted to the bolt. The device then listens for reflected energy. This cycle is repeated about 200 times per second until an evaluation of the bolt installation is made and indicated by colored lights in the display window. Bolts having 50% or less of their surface bonded are considered had and cause a red light to he illuminated. Bolts with 50% to 75% of their surface bonded cause a yellow light to be lit and are considered marginal. Bolts having more than 75% of their surface bonded are considered good and cause a green light to be lit. The indicator lamp remains on for about 4 seconds and then goes off, indicating that the instrument is ready for another test.

Jan 1, 1982

By Gift Makusha

Pre-feasibility work at Goedehoop Colliery indicated that stooping of the undersized pillars could be undertaken safely and economically. In light of the findings a decision was taken in may 2003 to undertake the project in the 600 area where three panels (628, 630 and 632) were identified for pilot test sites. The findings of the pre-feasibility studies were presented in a paper to the 22nd International Conference on Ground Control in Mining held at Morgantown in 2003 (1). This paper is a progress report for the project 12 months down the line. Work commenced to prepare the sites for mining including, acquisition of equipment, equipping the panels, selecting and training people to undertake the project. Three monitoring sites were set up distributed evenly along panel 628. Each site comprised of a surface extensometer station in the overburden strata adjacent to a pillar underground containing hard inclusion vibrating wire extensometer cells. This combination would be used to record pillar loading ahead of stooping as well as the depth of caving and strata deformation in the goal zone behind the stooping face. In July 2003, support work began to secure the roof and rib sides. The support was required to be at least three-pillar center distances ahead of the mining front to ensure that the effective stooping zone is secured at all times. The NEVID method of pillar extraction developed at Sasol and described in detail by Ismet and Moolman (2) in a Coaltech 20-20 report was adopted for the pilot project. As part of training the selected stooping team was send for a week to Sasol's Bosmanskrans Colliery to familiarize with the NEVID method of stooping. Further training sessions were carried out on site especially on the use of Mobile Roof supports. The first cut was taken on 20 August 2003 by implementing a form of chequerbord mining taking every second row of pillars. Three rows were mined this way which period was considered training to get workers familiar with the new equipment and mining method. On the fourth row the full design span equivalent to mining 4 pillars in a row was implemented taking out all the pillars in successive rows. Production figures gradually increased from 300 to more than 1,000 tons run of mine coal per shift. Panel 628 was mined out at the end of December 2003 and the second of three panels (630) is being mined. Substantial data from the monitoring set up has been recorded since the start of the project. Monitoring will continue to the end of the pilot project and the data will be analysed and fed back into the design tools to improve the design in future undertakings. The pilot project has been a success so far and other old workings are being evaluated for stooping.

Jan 1, 2004

By Jun Lu, Greg Hasenfus

"In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh seam are introduced. The longwall headgate T-junction experienced very high face convergence (up to 2 feet), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded.Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter, and Borehole Pressure Cell (BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment, were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.INTRODUCTIONIt is well known that high horizontal stress will affect the stability of roof, floor, and ribs within underground mines. In general, the headgate entries will experience stress concentration for the right-handed longwall face. On the contrary, the stress concentration will be in the tailgate entries for the left-handed longwall face (Hasenfus and Su, 2006). Without the stress shadow, the stress concentration in the headgate for the first longwall panel may be more significant (Figure 1).A mine in the CONSOL Pennsylvania Operation was extracting the first right-handed longwall panel in the Pittsburgh seam, which created high horizontal stress concentrations at the longwall headgate. Because of concern over soft wet floor within the syncline, floor coal had been left in place on development. Early on, upon retreat, the longwall headgate T-junction experienced very high face convergence, accompanied by roof sag, floor heave, and rib loading. The floor heave was exacerbated by the presence of floor coal, which buckled upward near the center of the beltline. Roof movement was occasionally aggravated by laminated roof conditions, but was kept in check by aggressive supplemental support including cable bolts, yielding props, cribbing, and roof mesh. Rib bolting and rib mesh helped to further maintain the entryway stability by keeping sloughage at a minimum. Nevertheless, despite the heavy support, headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded."

Jan 1, 2018

By Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The response of the surrounding rock mass to the creation of mining excavations determines the ultimate load on a pillar support system. In conditions where the ground is relatively soft and weak, the full overburden weight can be transferred to the pillar system. However, in stiffer and stronger rocks, a greater portion of the overburden load is transferred to the unmined coal barriers or abutments, and the pillar stress is reduced. This paper makes use of numerical models to examine the interaction between typical pillar systems and the surrounding rock mass for weak and strong geological conditions at various spans and depths of cover. The concepts of structural failure and functional failure of pillars are used to assess pillar performance when pillars are deformed beyond their peak resistance. The results show that the span-to-depth ratio is an important factor in determining the pillar stress and the ultimate deformation of pillars. The ultimate pillar strain appears to be closely related to the functional success of pillar systems.INTRODUCTIONThe National Institute for Occupational Safety and Health (NIOSH) was requested by Congress to study the safety of retreat pillar mining following the Crandall Canyon Mine disaster that took place near Price, Utah in August, 2007. As a part of this study, an investigation was made to better understand how the rock mass responds to the creation of mining excavations and how the loads are distributed among the pillars and the surrounding abutments or barriers. The outcome of these investigations contributed to the development of a modified loading model for the ARMPS-2010 (Mark, 2010) method ofretreat mining pillar design.Structural and Functional Failure of PillarsPillar design is typically conducted by estimating the pillar strength and stress, and then sizing the pillars so that an adequate margin of safety exists between the expected strength and stress. The pillar strength can be defined as the maximum resistance of a pillar to axial compression (Brady and Brown, 1985). If a pillar is loaded beyond its peak strength, load shedding or yielding can occur, and the pillar is considered to have failed as a structure. Structural failure generally refers to a loss of loadcarrying capacity."

Jan 1, 2010

By James M. Tennant

In the mid 1970's, American Electric Power significantly expanded its internal coal production. This involved opening several large mines in seams that had never been mined on a large commercial scale before. One of the mines opened was the Martinka No. 1 Mine which is located approximately five miles southeast of Fairmont. The mine was originally designed for room and pillar mining with continuous miners but was later modified to permit the utilization of longwalls and continuous miners for entry development. The coal seam to be mined was the Lower Kittanning which averages fifty-six inches. In order to successfully mine this seam, it required that the mine design recognize the effect caused by the dip of the seam which averages four percent to the Northwest with a maximum of sixteen percent for short distances and a relatively low cover. In fact, seventy-four percent of the reserves have cover of less than four hundred feet. Also, the core hole data indicated that the immediate roof varied from zero to eighteen feet of shale with two to forty-five feet of sandstone overlying the shale.

Jan 1, 1982

By Katya V. Babenko, Victor V. Nazimko

We address the problem of the stochastic nature of ground pressure manifestation. Stress distribution in a rock mass and ground irreversible movement are governed with a set of random factors that affect the over-peak strength of the rocks, making them extremely sensitive to any perturbation of the external environment and to internal fluctuations of the thermodynamic state of the rock. We investigate the fluctuation evolution during a longwall face movement to find out whether the fluctuation dissipates or builds up. Five adjacent powered support failures simulate the ground pressure fluctuation. The main roof has been presented as a plate that reposed on the rigid base. The stiffness of the base depends on the thickness of the extracted coal seam and on the dilation of the caving roof. Maximum stiffness occurs in situ, and minimum stiffness is in the gob when the roof is highly flexible and subsides elastically, without caving. The more stress acts in the roof, the higher the cavity that occurs and the larger increment of the base stiffness. The wave of the fluctuation has extended both to the direction of the longwall advance and along the face from the point of the fluctuation agitation. There was an incubation period when the fluctuation was negligible, and the process of the main roof caving remained unaffected. The distance of the longwall advance during the incubation period was twice the longwall length when the fluctuation magnitude was 1% relative to the maximum possible disturbance and 0.76 of the longwall length for the magnitude of 24%. However, the pattern of the roof caving has dramatically changed during further movement of the longwall. All caving parameters have changed: step of the caving, ground pressure distribution, and the manifestation in the longwall and entries. A histogram of the fluctuation magnitude is symmetric in accordance with normal distribution. However, the distribution has abnormal excess. These findings are the first step toward answering the question whether it is possible to forecast the process of main roof caving and weighting consistently. Our preliminary conclusion is that the space and time intervals where this prediction can be made with certain reliability are limited, so further investigation is needed.

Jan 1, 2017

By Yang Li

For the study of overburden movement in longwall mining of shallow coal seams, most scholars merely consider the thickness of bedrock and top soils. In this paper, the overburden movement in longwall mining of shallow coal seams with thin top soils and key strata group was studied. By using numerical simulation, the development of fractures in the overburden was analyzed for different ratios of panel width to depth (W/D) and depth to mining height. The results show that the overburden movement changed with the variation of mining height, panel width and depth under the specific geological conditions. It also explain how different overburden disturbance structures, such as ?three zones? (caved, fractured, and continuous deformation zones), ?two zones?(caved and fractured zones) or ?steps-like collapse? are formed under different mining conditions. The results broaden the vision for research on overburden movement in longwall mining of shallow coal seams.

Jan 1, 2014

By Kevin W. Harris

Multiple-seam coal mining is a major issue in underground coal mining that adds additional complexity to the practice of ground control. As reserve exploitation is driven by economics, it is now difficult to mine coal without the existence of previous workings above and/or below the active seam; this is particularly true in the Appalachian coalfields. The frequency in which problematic multiple-seam interactions are now encountered has resulted in significant research regarding the underlying mechanisms of the problems and methods of mitigation. In the last few decades, a great deal has been learned about these interactions and the ability to minimize problems associated with these hazards has greatly advanced, resulting in better economics and improved safety. Minimizing the adverse effects of stress interactions often requires a thorough understanding of site-specific geotechnical parameters and previous mining geometry. This is especially evident in bump-prone ground, where pillar behavior is highly dependent on local conditions. This paper presents a case study in a deep cover, Central Appalachian coal mine where three bump events occurred. Initially, empirical design software was used in an effort to analyze the situation and prevent further instability. The empirical study proved ineffective. A detailed investigation of the bump events using the boundary element software LaModel 3.0 was then performed. This study resulted in a site-specific calibration standard that will be applied in numerical pillar design for the remaining reserve.

Jan 1, 2014

By Gabriel Walton, Sankhaneel Sinha

"Rib failures are a major hazard in underground mining. When pillars are subjected to loading, extensile cracks are generated parallel to the excavation boundary, which can manifest as rib falls. A common mitigation measure is to install supports, especially bolts, normal to the rib surface. The type and spatial distribution of these supports along the pillar is often based on experience rather than comprehensive analysis. Since conducting field experiments is often impractical and expensive, a convenient alternative is to perform 3D numerical modeling. In this study, FLAC3D has been used to capture the interaction of rib bolts and the failure processes governing the stability of pillars. An associated goal is to determine whether, at all, a continuum software is capable of modeling the effect of supports. The modeling involved two important steps: selection of a proper constitutive model, and, explicit simulation of rock bolts. Based on precursory work in the last decade, an S-shaped strength criterion was chosen that can describe the tensile fracture mechanism of rock at low confinement and shear failure mechanism of rock at high confinement. The built-in pile structural element in FLAC3D is a simplified representation of the complete bolt system. Hence, the different components of a rock bolt (i.e., steel, grout, steel-grout interface, and grout-rock interface) have been explicitly modeled within the rock mass and then tested for its ability to replicate the ground-support interaction behavior. INTRODUCTION Rib failure-related hazards and injuries have plagued the mining industry for decades (Mohamed et al., 2016; Iannacchione and Prosser, 1998). In highly stressed environments, excavation parallel cracks are the primary initiators of skin instability. In absence of any support, this failure can indirectly destabilize the excavation by increasing the exposed span of intersections and roadways. Over the last two decades, there have been numerous injuries (fatal and non-fatal) due to rib failures in underground coal/non-coal mines."

Jan 1, 2017

By David Gearhart, Peter Zhang, Gabriel Esterhuizen

"Researchers from the National Institute for Safety and Health (NIOSH) installed instruments in the track entry and in the pillars of a northern West Virginia longwall coal mine between two planned panels for the purpose of monitoring the behavior of the ground. The instruments included four cable bolt load cells to measure the change in gateroad loading, six borehole pressure cells to measure the loading in the pillars, three six-channel roof extensometers to measure roof sag, four convergence measurements on the cribs to measure the crib loading, and a multipoint borehole extensometer to measure the pillar expansion. The instrumentation was all connected through two multiplexers to a single Campbell Scientific permissible datalogger located about 350 feet outby the instruments. The goal of the project was to record all of the measurements as both longwall faces passed the site of the instrumentation and to use the data to calibrate numerical models.This report describes the recording of all of the measurements taken as the first panel approached and then passed the instrumentation site. This study provided valuable data related to the ground response and support performance that will be used to improve safety in longwall gateroads.INTRODUCTIONA longwall mine in northern West Virginia was selected as a second study site for an ongoing research project into gateroad layout and support design. The area was affected by a transition from limestone to sandstone in the roof. The entries were 16 feet wide, and the height was approximately 80 inches. The layout for the longwall panels was 1,170 feet wide by 4,000 feet long. The panel configuration was right-handed, and a three-entry gateroad was used between panels. The pillars next to the headgate were 137.5 feet by 115 feet center-to-center, and the pillars next to the tailgate were 275 feet by 100 feet center-to-center, with the crosscuts at 60 degrees. The width of the gateroad was a total of 215 feet."

Jan 1, 2018

By Russell C. Frith

Risk-based roadway roof support design is now a critical part of the Australian Coal Industry. Safe and efficient mining demands that roof support be tailored to the prevailing geotechnical conditions and coal mining legislation in New South Wales, Queensland, and Tasmania is clear in requiring that formalized roof support design be undertaken. The assumption of dead-load suspension of an otherwise unstable roof has been used in roadway roof support design for many years. However longevity does not necessarily translate into either a best or even reasonable practice in the current industry, particularly when pro-active strata management systems are now routinely used and reinforcing support design methods are available. The paper discusses why the assumption of dead-load suspension is fundamentally incorrect in almost all instances when pro-active or reinforcing roof support is applied and how it can easily result in misleading and potentially under-designed roof support systems under various circumstances. The commonly used design methodology of balancing the installed axial capacity of long tendons with the assumed ?weight? of a future roof fall is summarized, and several fundamental flaws are identified. The critical area of selecting the design Factor of Safety is also discussed in detail. Accepting that under certain circumstances the roof of a roadway will require it to be ?suspended?, suggestions are given for how a robust support system can be developed, designed and applied.

Jan 1, 2011

By J. G. Stewart

Polyurethane injection for the consolidation and control of broken strata is a common practice in the deep mines in Europe. The technology has gained wide acceptance in the coal mines of America since the introduction of RokLok polyurethane binder by Mobay Chemical Corporation in 1977. A popular and growing use for polyurethane consolidation grouting is to control the roof and prevent gob flushing during recovery of longwall face equipment. The technique provides many time and performance-related benefits compared to more conventional methods using wire mesh. Kitt Energy Corporation installed the nation's first 1000-feat longwall face in April and May of 1984. The face completed the first panel in October, 1984, and recovery was begun. Roof Strata consolidation for the recovery was accomplished by a RokLok polyurethane binder infection program designed and supervised by Halbach & Braun Industries. The application provided the most rigorous and demanding test to date of this method's performance and cost-effectiveness relative to bolts and wire screen.

Jan 1, 1984

By Yi Luo

The responses of a section of railroad to ground subsidence process was monitored as it was undermined by a longwall panel. The subsidence data collected and the observations made through this monitoring program could help us understand betteer the ground subsidence process and its impact on linear structures such as railroads. The data analysis techniques used and the assessment methods presented could be useful to researchers practitioners in the field of mining subsidence.

Jan 1, 1994

By Anthony Iannacchione

As the amount of new fractured surfaces or "damaged rock layers" within roof rock increases, the stability of the rock mass decreases. While direct measurements of this phenomenon are not easily made, there is good circumstantial evidence to support this hypothesis. For example, it is common to observe increased cracks or fractures in the immediate mine roof rock before a roof fall. Likewise, roof drill holes placed in areas that later fail often reveal increased numbers and/or separations of fractures within the rock column through time. And finally, the frequency of microseismic activity, representative of rock fracturing, increases before a roof fall. For this study, more than 700 microseismic emissions were collected from two underground limestone mine roof fall areas in southwestern Pennsylvania. Microseismic events were located and magnitudes determined using the moment magnitude technique. Moment magnitude is based on the event seismic moment, which is a measure of the seismic deformation. The amount of new fracture surface length was calculated based on the stored strain energy within the rock prior to fracture. In the two case studies presented, a significant amount of microseismic activity was observed as much as two days before the first signs of failure in the roof fall areas. Additionally, results from this analysis reveal much about the behavior of strata prone to failure and allows for the construction of hazard maps based on microseismic emissions. The potential use of this technique as a means of anticipating roof falls is analyzed and discussed.

Jan 1, 2004

By Liu Xiao, Lin Xiaoying, Ma Geng, Tao Yunqi

"The soft coal in China has several issues, low permeability, high gas content and prone to gas outburst, making it difficult to drill and improve its permeability. Using the rock mass loading test, the relationship of stress-strain-permeability-time of sandstone was investigated. The fracture network that communicates with coal seams to provide the pathways for gas migration can be established by drilling, hydraulic fracturing or loose blasting in the roof and floor about 5m from coal seams top and bottom. The gas migration can be summed up as ""desorption-diffusion-two levels of seepage"" for gas drainage of the quasi-protection-layer in hard coal seams, and ""desorption-two levels of diffusion-seepage"" in soft coal seams. In the quasi-protection-layer, different gas drainage techniques were used during different stages of mining. Gas drainage of the quasi-protection-layers was demonstrated at Hebi Zhongtai Mining Co., Ltd. The results showed that the technique promotes the gas seepage flow, and improves the efficiency of drainage. The influence zone of the quasi-protection-layer was 1601 m2 and the average amount of gas drainage was 328m3/d, with the maximum being 661m3/d. INTRODUCTIONGas is one of the main factors that cause disasters and influence safety production in underground coal mines (SACMS, 2009). The coal-bearing formations in China have been subjected to multiphase tectonic stress field during the period of geological evolution. Coal body was often crushed into fragmented coals and mylonitic coals under the tectonic stresses (Su and Lin, 2009). The gas permeability of coal is generally poor. So in underground, drilling and hydro-fracturing technology have been used to improve the permeability of coal, such as intensive drilling, hydraulic punching, hydraulic cutting, hydraulic extrusion and deep hole blasting. But the gas drainage rate was low, only 23% (Liu, Li, and Li, 2006; Li and Wei, 2006; Henan Polytechnic University, 2008; Ma, et al., 2014; Lin and Wu, 2009; Zhou, et al., 2011) that was much lower than the rates achieved in the main coal-producing countries such as America and Australia whose average rate of gas drainage was 50%. The protection layer mining technology has been used widely as an effective technique to improve the permeability of coal in the mining of multiple coal seams (Cheng and Yu, 2003; Diaz and Gonzalez, 2007; Karacan, et al., 2007; Palchik, 2003; Latham, et al., 2013; Yuan, et al., 2013; Shi and Liu, 2007; Wang, et al., 2008, Guo, et al., 2015; Lu, et al., 2015; Slazak, Obracaj, and Swolkien, 2014 ). In the single low permeability coal seam, there is no protection layer. This kind of coal seam has low permeability, high gas content, soft coal body, and is prone to serious gas outbursts. During drilling, the holes very often spray, collapse, stuck and wedged the drilling tool. The hole length drilled is difficult to reach the required minimum, the service life of holes for gas drainage is short, the construction cost is high, and the holes are difficult to maintain. The gas drainage efficiency of single low permeability coal seam, especially soft coal seams, cannot be improved even though hydro-fracture technology has been implemented, because its permeability is not improved."

Jan 1, 2016

By C. P. Mangelsdorf

The success of the Birmingham roof truss (Figure 1) in supporting some difficult roof conditions, particularly in the Illinois coal basin, has given impetus to the development of a number of alternate schemes. These schemes attempt to alleviate two major problems with the Birming-ham truss, namely, the time required for installation and the cost of material. Because of the difficulty of drilling angled holes and the number of different pieces of hardware which must be handled manually, most operators have been unable to install Birmingham trusses in the cycle and have had to schedule truss crews during the maintenance shift or whenever they would not impede production. Due to this delay the truss has traditionally been viewed as a supplemental form of roof support. In fact, the truss might qualify as primary support and replace at least two bolts if it could he installed in the cycle (and MSHA would accept it as an alternate in the roof plan.) [ ] Figure 1. The Birmingham truss. Two attempts to deal with the first difficulty have sought to streamline the truss hardware while preserving the essential character of the Birmingham truss. One of these, by Peabody Coal, is reported elsewhere in these proceedings by C. Bollier. Another is a contract issued by the Bureau of Mines to Bendix Corporation for the development of a machine, particularly suited for use in low coal, which will automate many of the operations currently executed manually. This machine is expected to be ready for field testing by fall of 1982. Once in place, a truss installed by either of these schemes will interact with the roof in the same way as a conventional Birmingham truss. An alternate approach has been proposed by several different manufacturers independently. Although the hardware details differ. the essential idea consists of two angle bolts, the heads of which are connected by a horizontal chord tightened to some pre-determined tension. Figure 2 shows the generalized concept and does not depict any one of the four systems proposed. The advantage of this scheme is that the angle bolts can be installed and tightened in the cycle, and as bolts, immediately satisfy the roof control plan. The horizontal chord can be added later to achieve almost any initial desired roof loading, the limits of which are defined below this scheme is referred to here as the angle-bolt truss. Because of a fundamental difference in the way in which load is transferred between chords, this design does not interact with the roof under subsequent loading in the same manner as a Birmingham truss. The purpose of this paper is to examine this fundamental difference and its consequences. [ ]

Jan 1, 1982

By Keith Heasley

In this paper, a state-of-the-art, three-dimensional, full waveform, microseismic system was used to analyze the rock failure around a deep (> 750 in (2500 ft) of cover) bump-prone longwall panel. The microseismic system consisted of both underground and surface geophones coupled through radio telemetry and a fiber-optic network to produce pseudo real-time detection and location of seismic events surrounding the coal seam. This was the first microseismic installation of this scope at a U.S. coal mine, and the system was intended to help determine the exact strata mechanics associated with the redistribution of stress and the associated gob formation at one of the U.S.'s deepest longwall mines. Overall, 5,000 calibrated seismic events were recorded during the mining of one panel, including a Richter magnitude 4.2 event which occurred inside of the array. Analysts of these events provides a number of notable insights into the rock mass behavior. For instance, the longwall panel was widened during the retreat process, and the mining-induced seismicity shows a distinctly different behavior between the start of the panel, the mining of the narrow part of the panel and the mining of the wider part of the panel. Also, in itself, the acquisition of the ML 4.2 event was a major milestone in geomechanical and bump research. This is the first time that such an event has been recorded with this detail and accuracy at a bump-prone coal mine in the U.S. `The analysis of this single event has set distinct new limits on the relationship between mining seismicity and coal bumps.

Jan 1, 2001

By Thomas L. Vandergrift

The type and severity of coal mine entry failures are affected by the strength and stiffness properties of the roof, floor, end coal. Knowledge of the mine-wide trends of these properties Is valuable in developing effective ground control plans. Within the context of a larger effort to Improve entry stability In high horizontal stress fields, the U.S. Department of the Interior, Bureau of Mines, has evaluated three portable, on-site test devices to determine their suitability for identifying mine-wide physical property trends. The devices evaluated were the borehole shear tester, the Schmidt harmer, and the point load tester. Despite the lack of standardized data Interpretation procedures, each device provided useful on-site physical property data. The Schmidt homer was preferred for trend identification because of Its speed and simplicity. Schmidt hammer data were collected over a large area of an underground coal mine to compare calculated roof strength trends with a known geologic feature.

Jan 1, 1990

By George J. Karabin

The Roof Control Division of the Pittsburgh Safety and Health Technology Center, MS HA, is routinely involved in the evaluation of ground conditions in underground coal mines. Assessing the stability of mined areas and the compatibility of mining plans with existing conditions are essential elements in assuring a safe working environment at a given site. Since 1985, the Roof Control Division has successfully used the Boundary Element Method of Numerical Modeling as a tool to aid in the resolution of complex ground control problems. This paper will present an overview of the modeling methodology established and details of techniques currently used to generate coal seam, rock mass, and gob backfill input data. A summary of coal and rock properties used in numerous successful evaluations throughout the country will be included and a set of Deterioration Indices that can aid in the quantification of in-mine ground conditions and verification of model accuracy will be introduced. Finally, a case study will be detailed that typifies the complexity of mining situations analyzed and illustrates various techniques that can be used to evaluate prospective design alternatives.

Jan 1, 1994

By Rocky Wu

Rock bursts are one of the greatest challenges to ground control in the mining industry. With increasing mining depth and mining scale, there are more and more industry requirements on yielding rock support. Since 2008, Jennmar has been conducting large scale research and development works to develop a technically reliable and cost-effective yielding rock bolt. This paper introduces a new yielding rock support: the Yield-Lok bolt. The bolt is characterized by the designed yielding ability to produce 150?250 mm (6?10 in) of deflection at 8?10 metric tons (MT) (7.9?9.8 t) loads for every 16.4kJ (12,096 ft lb) energy input. The bolt?s performance characteristics are consistent through multiple and varying amplitude of impacts. In this paper, the design criteria of the bolt and the principle of performance are described; the dynamic testing results are discussed; and the features and application of the bolt are presented.

Jan 1, 2011