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By Thomas M. Barczak

The performance characteristics of Mobile Roof Supports (MRS's) were evaluated through full-scale testing in the Strategic Structures Testing Laboratory at the National Institute for Occupational Safety and Health, Pittsburgh Research Center. Safety issues pertaining to the operation and design of these supports for pillar extraction during retreat mining are examined. A unique load frame, called the mine roof simulator, provided a realistic simulation of mining conditions by inducing vertical, horizontal, and lateral loading on the support. The purpose of these experiments was to determine the capabilities and limitations of the supports and to investigate factors that influence the measurement of loading and loading rate. An evaluation of the support design and load conditions that can cause support failure or loss of support capacity are discussed relative to the laboratory tests. In general, lateral loading perpendicular to the longitudinal axis of the canopy is most severe, although horizontal loading in the direction of the long axis of the canopy can also produce critical loading in some cases. The experiments indicate that both setting force and leg pressure measurements are influenced by the staging of the leg cylinders. The implications of these factors on load rate measurement are evaluated. A comparison of MRS capacity and stiffness characteristics is made to conventional timber supports.

Jan 1, 1997

By Soheil Bahrampour

Development of measurement while drilling (MWD) systems for ground control applications in mining applications has recently been an active research topic. The main goal is to utilize the drilling information for characterizing the ground condition. Such characterization is the key to the accurate and efficient mapping of hazards and proper planning of ground support. However, most of the existing drilling units and measurement while drilling systems are mainly developed for joints and voids detection, and much less research is done to estimate rock strength. This paper focuses on rock classification based on estimated strength from the drilling data. For this purpose, data from J.H. Fletcher roof bolters instrumented with the vibration and acoustic sensors as well as torque, thrust, penetration rate, RPM and position signals is used. All the available data was utilized for rock classification using pattern recognition algorithms. The algorithms were developed based on data from drilling, and into three different rock types. The classification is performed using a simple feature extraction algorithm with a well-known pattern recognition algorithm. The results demonstrate the suitability of the proposed algorithms in identifying the rock types based on their strength properties, which can be adopted in measurement while drilling systems.

Jan 1, 2014

By Andy L. Shaffer

A novel liquid settlement system (LSS) was installed in advance of mining at a western United States (U.S.) longwall mine to measure total roof sag from the time of cutting to days after mining during gate road development. The system was used to compare differences in roof sag between conventional place-change mining using a Joy continuous miner and a Fletcher roof bolter and mining using a miner-bolter. The LSS was installed with an intrinsically safe, miniature data acquisition system (MIDAS) to record roof response at four locations in the same gate road. The system utilized pressure transducers to measure roof sag by the principle of falling head (fluid pressure) in fluid-filletubes installed in the immediate roof. Details of the LSS design, installation, data collection, post-processing, and future applications are discussed.

Jan 1, 2014

By Michael Gauna, Christopher Mark

"Coal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. Despite decades of research, the sources and mechanics of these events are not well understood, and therefore they are difficult to predict and control. Experience has shown, however, that certain geologic and mining factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present, and then identifying appropriate control measures to mitigate the hazard. This paper summarizes the U.S. and international experience with coal bursts, and describes the known risk factors in detail. It includes a framework that can be used to guide the risk assessment process.BACKGROUNDCoal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. During the years 2012-2014, serious coal bursts occurred at three different U.S. room and pillar mines. These events resulted in three fatalities and two permanently disabling injuries (MSHA, 2014; MSHA, 2013). In all three instances, the events occurred during pillar recovery at depths exceeding 1,000 feet (300 m). None of these three mines had previously reported a burst to MSHA. Coal bursts also occurred at three longwall mines during this same time period.Despite decades of research, the sources and mechanics of bursts are not well understood, and therefore these events are difficult to predict and control. Experience has shown, however, that certain risk factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present. In addition, some control techniques are effective in reducing the likelihood of an event or protecting miners from their effects."

Jan 1, 2015

By Kanchan Mondal, Anthony (Sam) Spearing, Gopi Bylapudi, Joseph C. Hirschi

"The U.S. coal mining industry uses about 100 million rock anchors per year. ASTM F432 specifies the requirements for rock bolts on mines but does not address corrosion. Corrosion has been found to be an issue in Australian coal mines (Hebblewhite et al., 2003 and Villaescusa et al., 2007), where the problem has been researched and stress corrosion has been found to be a significant cause of rock falls. Corrosion is also a major concern for underground civil construction in the U.S. but has not been considered an issue or adequately researched in coal mines. Underground conditions are conducive to corrosion mainly because of water quality and humid conditions. There is a perception, however, that when bolts are fully grouted, adequate corrosion protection is offered to the steel. Research has shown that this is not necessarily the case, due to the formation of micro-cracks as the resin sets and with subsequent rock movement shearing the resin column. This paper outlines a method to determine the corrosion potential of bolts used in long-term excavations and suggests ways to mitigate such effects based on research conducted in the lab and on three mines.INTRODUCTIONResearch on rock anchor corrosion in mines has not been extensive enough in the past, and the effects of several factors in the mine atmosphere and waters are not clearly understood. One probable reason for this lack of research may be attributed to the time required for gathering meaningful data, which makes the study of corrosion quite challenging.Almost all the rock bolts used in coal mines are made from steel, hence the corrosion of iron needs to be understood. The corrosion of iron is an electrochemical phenomenon with the overall equation"

Jan 1, 2010

By James D. Pile

Following a methane ignition in the active panel of a longwall coal mine, the affected panel was successfully isolated from the rest of the mine. This allowed for the remainder of the mine to return to regular operation, and the conventional practice of having to seal the portals for a defined period of time was avoided. After the affected panel had remained isolated for a regulatorily mandated period of time, it was reopened in preparation for longwall face recovery. As it was not possible to move the shields from their existing positions to facilitate the installation of recovery mesh prior to shield recovery, alternative methods had to be considered to stabilise the gob and minimise flushing, thereby improving operator safety. The objective of the paper is to show how stabilisation of the gob was achieved during shield recovery, thereby minimising flushing, and improving operator safety. Phenolic foam was chosen and injected from between the shields into the gob behind them. This is a technique that has been used to great effect in Germany, to a lesser extent in Australia, but not knowingly to date in the United States. To ensure nothing was left to chance and thereby guarantee the project?s success, the procedure used in Germany was employed, down to the application of equivalent hardware, which was sourced through the North American supplier.

Jan 1, 2013

By Pengfei Wang, Guorui Feng

With increase of cover depth, dynamic incidents, such as large gateroad convergences and coal bursts, are frequently encountered when mining deep longwall panels. It would be more adverse due to large inclination of the coal bed. Because of the shearer and armored face conveyor (AFC) sliding downhill, toppling of shields, disconnection of AFC to the stage loader are related to it. Tangshan coal mine is a typical deep, inclined coal mine situated in the east of China. The split-level longwall panel layout is being adopted for mitigating problems above, where tailgate entries of longwall panels are driven below gob edge and along the floor, while headgate entries are driven along the roof. Wire mesh and steel sets have been used empirically for support for tailgates for about a decade, which reduces the tunneling speed. In view of this fact, the paper serves to investigate the proper adjustment of the tailgate locations and to analyze the resulting stress and yield conditions according to support strategy. First, field measurements and observations were carried out. Stress data distributed within surrounding rock mass were obtained, especially within the roof since the roof consists of only a coal sheet with a thickness of 2–4 m. Roof pressure of the tailgate represents the pressure at the gob edge since it is below the gob edge. The convergence data were also collected. Numerical modelling using FLAC3D™ was conducted and validated by side abutment stress data. The double-yield constitutive model was used to simulate the gob. The finalized tailgate layout and design were determined through comparison of three scenarios. The research in this paper can help engineers choose the proper tailgate layout and support strategies for coal mines that are going to adopt a split-level longwall panel layout for mining deep, inclined longwall panels.

Jan 1, 2018

By Peter Zhang, Dan Neilans, Scott Peterson, Scott Wade, Joe Pugh, Ryan Mcgrady

"A coal outburst is a severe safety hazard in room-and-pillar mining under deep cover. It is more likely to occur during pillar retreating. Multi-seam mining dramatically increases the risk of coal outburst within the influence zones created by remnant pillars and gob-solid boundaries. Though coal outburst is generally associated with heavy loading of coal pillars, its occurrence is difficult to predict. Risk management provides a proactive tool to minimize coal outburst in room-and-pillar mining under deep cover. Risk assessment is the first step in identifying and quantifying outburst risk factors. The primary risk factors for coal outburst are overburden depth, roof and floor strength, geological anomalies, mining type, multi-seam mining, and panel width. A risk assessment chart can be used to proactively screen out mining sections with high risk of coal outburst for further analysis. Gob-solid boundaries and remnant pillars are critical factors in evaluation of the coal outburst risk of multi-seam mining. Risk identification, risk assessment, geologic influence mapping, geotechnical evaluation, risk analysis, risk mitigation, and monitoring are essential elements of coal outburst risk management process. Training is an integral part of risk management for risk identification and communication between all the stakeholders including management, technical and safety personnel, and miners.INTRODUCTIONA coal outburst is a sudden violent burst of coal from a pillar with broken coal or blocks of coal forcibly ejected into open entries. A coal outburst is a severe safety hazard as the mining crew is highly exposed at the site when the event occurs. Though deep cover and strong roof and floor are underlying geologic conditions of a potential burst incident, its real occurrence is also the result of additional mining factors. In room-and-pillar mining, a coal outburst could occur during both development and pillar retreating, but the latter greatly increases the risk of outburst. The other risk factors of coal outburst also include mining layout, multi-seam mining, presence of adjacent gob, cutting sequence, and local abnormal geologic conditions. Over the years, the cases of coal outbursts have been studied by many researchers and mining practitioners(Peng, 2008; Newman, 2008; Iannacchione and Tadolini, 2008; Gauna and Phillipson, 2008; Hoelle, 2008; and Newman, 2002). It is commonly believed that the coal outburst is the result of a sudden release of elastic strain energy stored in coal pillars and is highly associated with cutting into heavily loaded coal pillars, but its occurrence is a rare event and is difficult to predict. A number of engineering controls have been recommended to mitigate outburst potential. For room-and-pillar mining, sufficient pillar sizes have been the primary control for coal outburst prevention. The pillar design tools such as ARMPS and AMSS developed by NIOSH have played an important role in the design of stable pillars to prevent pillar collapse and squeezing as well as coal outbursts. In fact, with the implementation of pillar design using proper stability factors, pillar collapse and squeezing have been almost eliminated, and the number of coal outbursts has been greatly reduced in the US over the past decade. However, after a few outbursts occurred during pillar retreating in the US over the past a few years (MSHA, 1996; MSHA, 2014; MSHA, 2013), it has been realized that sufficient pillar size is still not enough to prevent coal outbursts. The investigations of the incidents showed that other factors such as multi-seam mining, panel layout, cutting sequence, and local geologic factors also seemed critical in causing the events. Therefore, it has become imperative that additional proactive measures beyond proper pillar design be implemented to prevent coal outbursts."

Jan 1, 2015

By Gang Ren, Behrooz Ghabraie

"Observational data suggest that the ground surface subsidence profile from multiple-seam longwall mining is rather different from that of single seam. Despite the increase of coal production from multiple-seam mining, available predictive methods are mostly unable to predict a reliable subsidence profile for multiple-seam extractions. In this paper, a case study of multiple-seam subsidence is modelled using physical modelling techniques. The subsidence mechanism as revealed in the physical model is explained and the differences between multiple and single-seam subsidence are highlighted. On the same case study, a commonly used influence function method (IFM) and Generalised Influence Function Method (GIFM) are utilised to predict the subsidence profile. The results are compared with predictions achieved by a newly developed predictive method based on the understanding of mechanism of the multiple-seam subsidence. Results show a significantly improved correlation between the newly developed method in comparison with the conventional IFM and GIFM. INTRODUCTIONField observations and subsidence monitoring data from multiple-seam coal mining cases show a different subsidence profile as compared with single-seam ones (Sheorey et al., 2000; Li, et al, 2007; Luo and Qiu, 2012; Unlu et al, 2013). The multiple-seam subsidence curve is generally deeper and steeper than of single-seam cases, and varies from one mining configuration to another. Various researchers had made efforts to understand the mechanism behind this phenomenon and the interaction between the two mining horizons (Li et al., 2010, Sui et al., 2014; Zhang et al., 2014). However, the mechanism of the multiple-seam subsidence is yet to be comprehensively understood (Suchowerska, 2014; Ghabraie et al., 2015b).In addition, differences between the single and multiple-seam subsidence profiles make it difficult to adapt single-seam subsidence prediction tools for predicting the subsidence over multiple-seam cases, unless significant modifications are applied This requires understanding the mechanism of the multiple-seam subsidence as the first step to achieve reliable predictive results.The mechanism of the subsidence due to a multiple-seam case study can be investigated via physical modelling. The physical model allows observing the substrata movement as well as the interaction of the two mining activities. An understanding of the mechanism of the multiple-seam subsidence can be developed based on these observations, which would provide basis for developing a subsidence prediction tool."

Jan 1, 2016

By Filippo Vallianatos, Michael Verigakis, Vasili Saltas, Kosta Kaklis, Stelio Mavrigiannakis, Zach Agioutantis

"Development of failure in brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emissions. This paper presents results from acoustic emission measurements obtained during three-point bending tests on Nestos marble under laboratory conditions. Acoustic emission activity was monitored using piezoelectric acoustic emission sensors, and the potential for accurate prediction of rock damage based on acoustic emission data was investigated. Damage localization determined based on acoustic emissions evident in the critically stressed region in the form of scattered events or stresses below and close to the strength of the material.IntroductionNon-destructive testing (NDT) has gained popularity in recent years due to commonly available sensors, devices and software tools. Although the majority of such testing still occurs under laboratory conditions, the ultimate goal is a direct field application, especially in the case of underground mining. NDT, if properly applied, may be used to directly provide material property values and characteristics that can be used to determine rock mass or soil mass behavior, with a direct impact to mine design (Iannacchione et al., 2003).AE is generally defined as the high-frequency transient elastic waves originating from the sudden release of energy at localized points within a loaded material (Nomikos et al., 2010). The field of Acoustic Emissions (AE) due to compressive, tensile or other types of loads is a promising research area due to its wide application. The accurate identification of failure precursors stemming from AE is still an open research topic (Lei et al., 2004).Rock is a typically inhomogeneous and anisotropic material that contains several natural defects with various scales such as grain boundaries, microcracks, pores and joint inclusions (Jing, 2003; Read, 2004). Damage induced by microcracks is an essential mechanism in many brittle rocks subjected to stresses. An applied stress induces microcrack growth and damage is distributed uniformly throughout the sample. Finally, the microcracks coalesce and the damage is localized to a shear band, leading to strain softening and macro-scoping failure (Dragon et al, 2000)."

Jan 1, 2015

By Hongwei Wang, Hao Zhang, Panshi Xie, Xicai Gao, Youfu Zeng, Yongping Wu, Juan Dou

"The reserves and production of steep coal seams account for approximately 15–20% of the total coal reserves in China and 5–10% of the total annual coal production. More than 50% of the coal is high-quality coking coal and anthracite, based on reviews of steeply dipping seam mining at home and abroad. Fully mechanized mining is considered to be the only safe and efficient way to mine steeply dipping seams. To achieve fully mechanized longwall mining, four issues must be addressed: ground control, equipment stability control, development of complete sets of equipment, and safety. Engineers must focus on many factors of steeply dipping seams, such as the following: stope, roof asymmetrical structure, stability control, “roof-support-floor” system dynamics control, multi-region big-range ground control technology, adjustment false-inclined method, nonlinear layout method, control technology, safety, and development of complete sets of equipment. Engineering practices in China introduce these concepts systematically: fully mechanized mining of medium-thickness seams, fully mechanized high mining of thick seams, fully mechanized caving of extremely thick coal seams, fully mechanized mining of coal seams, roadway support, and so on. A steeply dipping seam is one with a dipping angle in the range of 35°–55°. This accounts for about 15%–20% of the total coal reserves in China. Because of the environment, more than 50% of the coal is high-quality coking coal and anthracite. Mining steeply dipping coal seams is known to be difficult in the mining industry. After the study conducted by Germany, Poland, and the former Soviet Union in the1970s–1980s, the fully mechanized mining basic theory, core technology, and key equipment research have not been a breakthrough. Being fully mechanized is a technological “restricted area” at home and abroad. A lot of steeply dipping seams are excavated by non-mechanized mining, leading to harsh job environments, labor-intensive work, low productivity, frequent accidents, and significant security hazards. The death rate per million tons of coal reaches above 4.0 and is far higher than slow inclined coal seam mining. Making steeply dipping seam mining safe and efficient is a global problem that urgently needs to be solved. Fully mechanized mining is the only way to overcome these problems (Wu, 2003; Wu, Yun, and Zhou, 2000). The realization of fully mechanized coal seam mining must solve four problems. (1) Examine the disaster mechanisms induced by overburden “key strata” mutation under steeply dipping seams as a basis for ground control methods and technical problems. (2) Work to solve the dynamic stability control technical problems of thousands of tons of complete sets of equipment under the steeply dipping seams. (3) Complete sets of equipment must be developed to adapt to the conditions of steeply dipping seams. (4) Overcome the steeply dipping face safety protection technical problems. In the last two decades, a number of universities, research institutes, and enterprises have realized the importance of safety and efficiency of steeply dipping seams in China. This is especially true of Professor Wu Yongping’s “hard coal layer safe and efficient mining” team at Xi’an University of Science and Technology. After several generations’ research and technical work, the team puts forward China’s longwall mechanized coal mining of steeply dipping seams strata control basic theory. This paper puts forth a series of key technologies of fully mechanized mining, develops and manufactures complete sets of equipment by independent innovation, and establishes a perfect face security system, forming a theory and technology system for fully mechanized longwall mining of steeply dipping seams."

Jan 1, 2017

By William Blake

Mining induced rock bursts have been a severe operational hazard associated with the deep silver mines of the Coeur d'Alene mining district for over 30 years. Recent studies during the past fm years have led to a better understanding of the different types of rook bursts and their causative mechanisms. This has resulted in ground control measures that more effectively minimize burst occurrences as well as contain burst damage. Case histories of burst incidences and examples of control measures are presented.

Jan 1, 1987

By Bo-Hyun Kim, Mark K. Larson

"While faults are commonly simulated as a single planar or non-planar interface for a safety or stability analysis in underground mining excavation, the real 3D structure of a fault is often very complex with different branches that reactivate at different times. Furthermore, these branches are zones of non-zero thickness where material continuously undergoes damage even during interseismic periods. In this study, the initiation and the initial evolution of a strike-slip fault was modeled using the FLAC3D™ software program. The initial and boundary conditions are simplified and mimic the Riedel shear experiment and the constitutive model in the literature. The FLAC3D model successfully replicates and creates the 3D fault zone as a strike-slip type structure in the entire thickness of the model. The strike-slip fault structure and normal displacement result in the formation of valleys in the model. Three panels of a longwall excavation are virtually placed and excavated beneath a main valley. The characteristics of stored and dissipated energy associated with the panel excavations are examined and observed at different stages of shear strain in the fault to evaluate bump potential. Depending on the shear strain in the fault, the energy characteristics adjacent to the longwall panels present different degrees of bump potential, which is not possible to capture by conventional fault simulation using an interface.INTRODUCTIONThis paper is part of an effort by the National Institute for Occupational Safety and Health (NIOSH) to identify risk factors associated with bump-prone potential in highly stressed ground conditions. More specifically, this paper reports an exploratory effort with an unconventional approach using a numerical tool to try to better understand a possible scenario where bump risk might be increased—that is, a scenario involving a strike-slip fault. The objective is to assess whether a numerical tool might be useful or practical in forecasting bump potential associated with a strike-slip fault for a typical, structural geologic setting."

Jan 1, 2018

By David W. Evans

"Induced hydraulic fracture of strata during roof bolt installation is a potentially prevalent, but masked phenomenon within the underground coal industry. Previously reported resin testing programs (McTyer et al., 2014) examined the relationship between resin mixing effectiveness and varying bore hole diameter. The methodology employed within this earlier test program facilitated a further critical area of research – the measurement of back pressures generated within the bore hole during standard rock bolt installation practices. Experimental data has indicated that fluid resin can be pressurised to levels where it exceeds the compressive strength of the strata, inducing hydraulic fracture within the immediate area of the bolting horizon. The routine cycle of roof bolting serves to propagate this effect, progressively fracturing and delaminating the roof during mine advancement. This masked phenomenon can lead to a perception of difficult ground conditions - mining efficiencies and costs are therefore affected, with increased needs for additional support subsequently required to restabilise the inadvertently damaged roof.Further analysis of the parameters associated with resin bolt installation has now been conducted, assisting in the development of an empirical relationship between bore hole pressure, bore hole diameter and bolt insertion times. This relationship has been analysed for 15:1 ratio resins and 2:1 ratioresins, within 28mm and 30mm boreholes. Further to this, load transfer performance has been comparatively assessed for both 28mm and 30mm boreholes, suggesting that for 2:1 resins, acceptable resin mixing and load transfer can be obtained within a 30mm bore hole. The combination of 2:1 resins, utilised within a 30mm bore hole, may well provide the optimal solution to reduce the risk of hydraulic fracture in weaker strata during resin bolt installation.INTRODUCTIONA growing number of industry papers have previously investigated areas of concern associated with the performance of cartridge style resins in roof bolting, predominantly focussing on the effects of plastic film gloving, inadequate resin mixing and the pressurisation of resin within the bore hole. These three effects have a critical influence on the load transfer of the steel bolt element, through the cured resin and into the surrounding strata.Gloving occurs due to the plastic film of the resin cartridge partially encasing or wrapping around the steel roof bolt element – a known phenomenon over many years (Pettibone, 1987). The plastic film creates regions of discontinuity between the bolt, cured resin and borehole, reducing effective load transfer into the strata. Experimentation conducted into the effects of bolt ends fully encased by plastic film (Pastars and MacGregor, 2005) revealed that load transfer can be reduced by 85 to 90% in worstcase occurrences. It is also known that the aggregate filler size used within the resin can assist in shredding and breaking up the plastic film – this effect was observed in a previous study on resin mixing effectiveness (McTyer et al., 2014)."

Jan 1, 2015

By Madan M. Singh

Currently longwall coal mining operations require at least 3 entries on both the headgate and tailgate ends because of ventilation and safety requirements. Driving of entries, however, is a slow and costly process. Often it is a bottleneck in longwall development. Single entry gateroads are permitted in Europe and commonly used there. This study entailed ascertaining the feasibility of using a single entry in the United States, without sacrificing safety. This design was conducted for an operating longwall mine in the Black Warrior coalfield in Alabama, An understanding of rock loads and consequent support requirements is prerequisite to the design and successful demonstration of the single entry system. Efforts in this regard were directed at determining center (packwall) support requirements for various single entry widths. Critical to the analysis is knowledge of the nature and extent of side abutment stresses. In collecting geological and geotechnical information relating to the stability of the single entry, Engineers International, Inc. engaged in the following: ? Examination and inspection of the single entry demonstration at the Sunnyside Mine in Utah. Severe ground conditions and unacceptable levels of entry closure were experienced there prior to the tailgate phase. Observations at the mine presented a clear perspective on possible ground control problems for future single entry designs. ? Reconnaissance inspection and classification of roof rock conditions. ? Detailed mapping of roof structure in the areas to be instrumented. ? Installation of convergence stations in the longwall headgate to monitor roof movement during mining. ? Measurement of side abutment stresses in a yield pillar in the area of the convergence measurements. ? Measurements of coal fracture distribution by the detail area method. ? Rock properties determinations of roof, coal, and floor rocks; samples obtained by core drilling. ? Determination of floor bearing capacity by in-situ testing.

Jan 1, 1982

By Tom Blevins, Roland Walker, Jinsheng Chen

"The Dywidag-Systems International DSI Tandem Anchor bolt, with an improved expansion shell-washer design, can substantially reduce potential entry roof falls caused by twisted or deformed expansion shells of the traditional mechanical anchor bolts during installation. In addition, the improved anchorage system can also ensure that a consistent and high installed load is achieved, which is critical for effective beam building under thinly laminated or layered roof conditions. This paper discusses the genesis and anchorage performance of the Tandem Anchor bolt, and analyzes the tension/torque relationship of different mechanical shell anchors using data obtained from both lab and field tests. It also discusses roof bolting mechanisms, roof bolting systems, and factors influencing effective beam building. Finally, numerical modeling was used to analyze the effect of bolt pretension magnitude on entry roof stability.INTRODUCTIONThroughout the history of underground mining, keeping the roof from uncontrollable collapse has been the greatest concern of the miner. Tremendous strides have been made in better understanding, predicting, and supporting the mine roof. This conference has spent the last twenty-eight years sharing information and distributing it to the mining community. A paraphrased statement from former Secretary of Defense Donald Rumsfeld adequately describes the challenges to the mining engineer as it pertains to roof control: it is a study of known knowns, known unknowns, and unknown unknowns. It is the hope of the authors to address some of these unknowns.The paper discusses the development of bolt anchorage systems from dry shells to shells with resin-assisted anchorage. It looks at some of the issues faced with the anchorage shells being used when introducing the insertion pressures caused by resin cartridges. The rationale behind the shell design changes was to compensate for this resin induced stresses.An improved expansion shell assembly (a slotted expansion shell base, a tapered cramming plug, and a novel support nut with tapered top) for mine roof bolts makes up the Tandem Anchor (U.S. Pat. No. 6742966); this minimizes the possibility of twisting seen with traditional mechanical shells and thus prevents springy bolts and shell leaves from breaking. A fully grouted Tandem Anchor bolting system has the advantages of both the pure mechanical point-anchor and the non-pretention fully grouted bolting systems. Once installed, the installed load or pretension will be locked in and never bleed off. Compared with the fully-grouted (2-speed resin) tension rebar bolting system, the fully-grouted Tandem Anchor bolting system can (a) reduce bolting cycle time (about 15 seconds per bolt), boosting coal production and productivity; (b) provide a greater installed load or pretention; and (c) achieve a consistent tension/torque ratio, which gives better bolting quality control. Therefore, a fully grouted Tandem Anchor bolting system is the most effective means to reinforce a weak rock mass intrinsically by the theory of beam building. For thinly laminated or layered roof conditions, a higher installed load or pretention (50% or more of the bolt's yield strength) can make a roof bolting system much stiffer, which results in less roof movement. Numerical modeling results show that the magnitude of the roof's tensile stress and relaxation zone can be substantially reduced under a higher installed load or pretention."

Jan 1, 2010

By Xingkai Wang, Wenbing Xie

"To control the large deformation of soft rock around the roadway subjected to high stress, it is important to improve the stability of support structure, in addition to increasing support strength. In this study, the causes for the destabilization of U-steel frame and its loading characteristics were analyzed by theoretical analysis, laboratory experiments and field tests. The technology of constant support resistance, the principle of support structure compensation and the compensation technique are proposed. The results indicated that the main reasons for frame structure destabilization are caused by low strength of connector, irrational structure of connector, poor interaction between the surrounding rocks and support frame and structure, and unstable support structure. Thus, the corresponding technological measures, such as connector with high strength and rational structure are adopted. The bearing capacity of frame is unstable after slipping. In order to maintain the high support resistance when the frame is performing the support function, the frame connector should be retightened after slipping. Though the working resistance may be improved greatly by using the backfilling or back grouting of U-steel, the frame remains unstable because of the unstable frame structure. Therefore, according to the deformation characteristics of rocks surrounding the roadway and the structure instability of U- steel frame, the stable structure device is used to carry out the structural compensation of U-steel frame, and the whole bearing capacity of the frame and its structural stability are improved greatly.INTRODUCTIONThroughout the development of the roadway supports for coal mines in China, it can be seen that the supporting strength and supporting resistance increase continuously with increase in mining depth and supporting difficulties [1]. With respect to the ground control mechanism and supporting technology of rocks surrounding the soft rock roadways subjected to high stress, considerable research has been done both at home and abroad. The backfilling technology behind the high-resistance and yieldable U-steel wall, the new type of grout/bolt support technology and the bolting with high-strength mesh support technology were developed, and have been used widely in coal mines with notable economic benefits [2-7]. However, a large number of engineering practices show that in many cases the existing rock control technology of roadway may not control the large deformation of the soft rock roadways subjected to high stress. The reason that the large deformation of soft rock roadway occurs is mainly due to poor structure stability of existing frame bearing structure, not insufficient support strength."

Jan 1, 2015

By Gheorghe Oncioiu

The phenomena developed on the influence of thick coal seam no.3 mining, by horizontal slices and rocks caving roof control, in the Jiu Valley coal basin, are very complex. By analyzing the influence volume of underground faces and defining the stress and strain zones, around the faces, it is possible to evaluate the suitable powered support performances. Also, Jiu Valley thick coal seams involve about 50% of reserve balance in the main safety pillars of social, industrial and natural objectives, situated on the mining surface perimeters. For a correct design and drawing of main safety pillars complex research was done with the aim of establishing the subsidence parameters. Upper face rocks medium could be assimilated as a granular medium, which is in a permanent movement. For describing the rocks movement, respectively the ground surface displacement, was taken into account the stochastic mechanics principles. The theoretical solutions are in accordance within situ measurements.

Jan 1, 1999

By Herryal Z. Anwar

In recent years, many both mining and civil engineering projects are developed in the area which may be categorized unstable, such as fractured and jointed rocks or "water sensitive rocks". For example, in underground coal mining, in which the floor strata normally consist of weak shale, the problem that may arise in roadway is floor heaving. In this case cement injection in borehole appears to be an economically effective method in order to make roadway more stable. This method may be also applied in the projects in which the constructions are laid on fractured and jointed rock. Nevertheless, the successful cement injection is much dependent on the water-cement ratio and the injection technique. This paper discusses the application of cement grouting to improve the ground stability in longwall mining, the laboratory experimentation of cement grouting and the method of floor injection.

Jan 1, 2000

By Michael Karmis

During the last 25 years, technological advancement and subsidence research have resulted in more accurate and diverse prediction capabilities. The work presented in this paper focuses on the development of integrated prediction capabilities for dynamic deformation indices and for strains over sloping terrain. In order to assist subsidence engineers with accurate prediction methodologies, the research also addresses the development and validation of techniques that can enable model calibration using alternative measured subsidence parameters such as horizontal or ground strain instead of the traditional vertical subsidence values. This paper presents the basic concepts and validation of the above mentioned enhanced prediction and control methodologies, which are necessary for effective assessment and control of mining-induced subsidence, using examples and case studies. In addition, a risk assessment approach for evaluating landscape stability over mined areas is presented. The enhanced prediction methodologies have been incorporated into the Surface Deformation Prediction Software (SDPS) package.

Jan 1, 2008