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By Tim Ross

Several shear failures were observed in Shaft #1 at the Mountain Coal Company, LLC, West Elk Mine, after mining longwall Panel 23, 1,100 ft to the east of Shaft #1. It was speculated that this shear damage could be related to differential ground movement caused by in situ stress relief from the ?stress shadow? of the caved zone above longwall Panel 23. A numerical study was conducted to assess the possibility of the shaft shear damage being caused by in situ stress relief and the potential for additional damage to Shaft #1 and two other nearby shafts, due to mining nearby longwall Panel 24. Three-dimensional (3D) models were built in FLAC3D to simulate past and future mining near shafts, the estimated local anisotropic and directional horizontal stresses, and the overlying variable surface topography. The numerical analyses indicated that stress relief due to mining Panel 23 caused the shear damage to Shaft #1and that additional damage to Shaft #1 and the other two shafts, would likely result from mining longwall Panel 24. Additional shear damage was documented in Shaft #1 when longwall Panel 24 was mined, confirming the results of the numerical analyses.

Jan 1, 2006

By Stephen C. Tadolini

The International Ground Control Conference in Mining has always provided an open forum for the publication, presentation, discussion, and often heated debate on roof bolting systems mostly with attention to how, when, and why they work. During the last 24 Conferences at least 170 papers have been presented by International Experts on these ?simple? devices that range in length from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof bolts are primarily mechanically anchored, glued, cemented, or driven. Bolts are placed vertically, angled, or tied together with special fixtures (trusses and slings). They have been termed active, passive, stick-slip, and several other descriptive acronyms or mining slang expressions. This retrospective paper will present the changes in bolt types and usage, point out some of the biggest myths and hail the most significant advances (according to the authors? opinions of course).

Jan 1, 2006

By Rob Thomas

Main gate roof control during longwall retreat is subject to a significant increase in horizontal stress and risk. The implementation and management of an appropriate roof support design is as a result, an imperative requirement for any longwall mine and must in addition to the likely consequences of a roof fall, also consider the variables of the geotechnical environment, associated support costs and practicalities. This paper will therefore summarise a number of recent advances in main gate roof control in the Australian coal industry. Points considered include; (i) the ?Longwall Acceleration? concept, (ii) macro-scale horizontal stress notching, (iii) micro-scale horizontal stress notching, (iv) bedded versus laminated roof types and (v) the type of cable reinforcement used.

Jan 1, 2006

By Peter B. Lilly

Thank you, Syd, for the kind introduction and for inviting me to give the Keynote Address at this very prestigious gathering of some of the finest intellect and talent in the domestic and international coal industry. CONSOL Energy has been a part of this conference for its entire 24-year existence. We have presented 27 papers over that time. CONSOL employees have also been members of the planning committee for the conference, and we have been pleased to support the event both financially and with many participants. So I am honored to be here with all of you this morning, especially during such a positive period for coal. You can just feel the enthusiasm in this room for our industry, and for this conference. I can tell you, there?s also a lot of enthusiasm and excitement in CONSOL Energy?s boardroom. And our enthusiasm is mirrored on Wall Street. But there remains an ironic twist to all of this attention. Even when times are good, the coal industry seems to go largely unnoticed by the mainstream. Outside of West Virginia and other coal-producing regions, the industry still seems to lack influence and support, at least here in America. As an example, last month, an ABC News crew visited our Blacksville 2 Mine, near here, to do a feature story on increased employment in the coal industry.

Jan 1, 2006

By Gregory M. Molinda

Historically, miners have known that roof rocks originally composed largely of mud were prone to slaking and deterioration when exposed to water and humidity. Mine Safety and Health Administration (MSHA) records show that roof falls are more prevalent in summer and late fall. Field data shows higher bolt loads and increased room convergence in summer months. Laboratory data shows that very high internal pressures can be generated by significant strain in clay-rich rocks. In summer months humidities ranging from 70-100% can cause rock strains up to 7%, and rock pressures of 14,000 psi, more than enough to break apart weak shales and fireclays. The National Institute for Occupational Safety and Health (NIOSH) conducted wet/dry immersion cycling of clay-rich rocks. Results show that the rocks most prone to moisture deterioration are fireclays, claystones, and sandy shales, all of which have disrupted bedding. Of the 7 mudrocks with a weatherability index above 40%, none showed consistent fissile bedding. Black shales were surprisingly resistant to wet/dry water immersion cycling. Generally, mixed rocks (stackrock), typically susceptible to horizontal stress, are more resistant to moisture-deterioration. Mine roof rocks tested with moisture sensitivity ratings above 50% (moisture sensitivity index ranges from 0-100 with 100 being the most sensitive) all came from mines which had roof skin problems and used supplemental support to prevent rock fall injuries. Some roof rocks showed lateral variability of moisture sensitivity values, suggesting the need for more samples in highly varying roof rocks. Swelling strains of mudrocks showed that almost all of the swelling occurred in the first 24 hours after immersion. The wet/dry immersion cycling test used provides an accurate, quick and inexpensive method for estimating the moisture sensitivity of mudrocks. With this information mine operators can begin to anticipate hazardous ground and prepare appropriate roof support.

Jan 1, 2006

By Kot F. v. Unrug

The present combination of coal market prices and reserves, situated in the Central Appalachian Coal Field, is such that mining of smaller coal reserve blocks has become economically attractive. In above drainage deposits, and particularly under mountain ridges, overburden depth can be changing within a wide range. Mines operating in these deposits are usually developing mains along the center of the ridge, with panels projecting from the mains towards the outcrop. Due to changing topography, proximity of the outcrop, and related cover depth, the panels have various lengths. In this reported case study, a mine in Pike County, Kentucky, along the Kentucky ?West Virginia border, difficult ground conditions were encountered, during the development of the mains, under shallow cover, along a narrow mountain ridge. An area having unusual density of vertical joints was encountered. Mining operations were suspended because of roof stability concerns. The ground conditions were studied, analyzed, and control measures proposed to ensure safe passage through the difficult roof area of the mine. The advancement of the mains, through this area of challenging ground conditions, was successfully completed. In this paper, ground conditions are described, with an explanation of the geologic process contributing to their formation. It is interesting to note that similar locations were predicted by the authors, to be encountered in the same mine, based upon the experience gathered from studying conditions of the case described within this paper. When approaching the predicted locations, stability problems appeared to be even more challenging than those encountered previously. Due to changes in the coal market, mine management chose to move the affected mining section to another area of the mine having better coal quality. The advancement of the mains, through the difficult ground conditions, was temporarily postponed. Based upon the analysis of a probable kinematical mechanism of blocks in the mine roof, ground control measures were proposed. The actual application of these principles proved to be appropriate, ensuring safe passage through the difficult ground conditions encountered in the mine.

Jan 1, 2006

By Dr. Daniel W. H. Su

t is both an honor and privilege for me to nominate Dr. Daniel W. H. Su for the inaugural Syd S. Peng Ground Control in Mining Award. I can think of no better candidate than Daniel that is deserving of this award ? an award to recognize technical excellence in advancing ground control technologies [and] approaches. Daniel?s education and career spans over three decades. Having graduated from National Cheng Kung University in Taiwan in 1972 with a B.S. degree in Mining and Petroleum Engineering, he spent the next four years in the Taiwanese military and mining industry, before moving to the United States, where he enrolled in West Virginia University?s Mining Engineering Department. He received his M.S degree in Mining Engineering from WVU in 1978; then in 1982 became the first person to ever receive a Ph.D. in Mining from WVU. His thesis was titled, ?Development of Ultrasonic Techniques for the Measurement of In Situ Stresses?.

Jan 1, 2006

By Yingda Zhai

The range of interburden thickness between coal seams in Datong Mining District of China is large. From mining operation point of view, the closer is between the two seams, the greater is the impact from the upper seam when the lower seam is mined. This impact is most notable when the two seams are ultra-close. In this paper the definition of ultra-close seams and its practical method of determination are presented. Based on the difference in strata thickness in the interburden and the concept of "yield depth ratio" which is defined as the yield zone depth in the interburden to the total thickness of interburden, the roofs of the ultra-close multiple seams in the Datong District of China were divided into 3 types: (1) false roof or rock partings in coal seam. This type of roof does not require special handling except mining them simultaneously as one single seam. The roof thickness is slightly less than 0.5 m; (2) completely broken roof. In this type of roof, the yield depth ratio is larger than (or equal to) 1. When the roof thickness is larger than 0.5 m but less than 2.0 m, the yield depth ratio must be smaller than 1; and (3) fractured roof. In this type of roof, the yield depth ratio is less than 1 while the roof thickness is larger than 2.0 m. In retreat longwall mining, if the gateroads of the lower seam are located under the gob of upper seam, roof bolting can be used in the 3rd type of roof. But if the 2nd type roof exists, powered supports must be used.

Jan 1, 2005

By C. Srinivasan

Microseismic emissions are generated by deformation and cracking of rock around an opening inside a mine. The rock tends to achieve equilibrium in a new geometry under changing stress conditions was found to provide useful clues to the impending roof falls and ground instability. The typical characteristics of these microseismic event patterns is shown to be of diagnostic value in sensing in advance rook fall that invariably follow such events. In most of the longwall faces, the main problem faced was due to strata behavior in the presence of massive sandstone bed, which is generally difficult to cave. Microseismic monitoring system was used to monitor strata behavior during longwall mining operation at Rajendra underground coal mine of South Eastern Coal Fields limited (SECL). More than 44,000 microseismic events with local magnitude between -3.4 and 0.5 were recorded during the monitoring period from September, 2001 to March 2003. Analysis of these events was correlated with respect to roof falls. The time series of range of source parameters such as Radiated energy, seismic moment, Energy Index, Seismic Schmidt number, time of occurrence and location of fractures before and during roof fall exhibit qualitatively distinct patterns about six hours before roof falls. &ong the several seismic parameters investigated the microseismic event rate as precursor was found to be most significant indicator of an impending roof fall. The results are discussed as case study in this paper.

Jan 1, 2005

By Yi Luo

The efforts to mitigate longwall subsidence influences on various residential and farming structures by many coal mining companies have been successful in the recent years (Luo, et al, 2003). The successes were largely dependent on the accurate subsidence predictions, correct assessments of potential influences, appropriate design and implementation of the mitigation measures. However, such efforts for protecting large industrial structures are still very rare. In this paper, the successful assessment and mitigation of the subsidence influences associated with a very intensive dynamic longwall subsidence process on a large workshop are presented.

Jan 1, 2005

By Shugang Li

The layout and efficiency of the degasification system is heavily dependent on the distribution of and the methane migrating mechanism in the mining induced fractures in the overburden strata In this paper, physical modeling and numerical simulation are employed to study the mining-induced fracture zones and the stress distribution in these zones. The physical models show that both the fracture and compaction zones form a parabolic shape in the over- burden strata along the panel transverse direction. When this is generalized to the panel longitudinal direction, the shape of the mining induced fracture zone over the longwall gob can be represented by a 3-D elliptic paraboloid function. Through the observation of bed separation, fracture density and distribution pattern in the physical models, it is believed that the development of the parabolic fracture zone can be divided into three distinguishable phases. The expansion characteristics and sizes of the fracture zones can be estimated. The numerical stimulation studies can be used to estimate the extent of the stress release zones. The stress release zones can be also divided into a high compressive stress - fracture closure zone, stress relieved - fracture expansion zone, and intact zone. The methane desorption model is then used to analyze the methane migrating pattern in the fracture zone. The developed model has been verified by the results of a longwall gob degasification tests at a coal mine in Yangquan, China with a fair agreement. Such studies show that the model can provide a reasonable accurate tool for designing the gob-well degasification system for longwall mining operations.

Jan 1, 2005

By Terry Medhurst

Geophysical logging is routinely undertaken as part of most Australian coal mining exploration programs. Typically, the main application is the determination of coal seam depth and the qualitative estimation of coal quality, lithology and rock strength. For rock strength estimation, sonic transit times relate to the elastic properties of the rock and provide a general correlation with uniaxial compressive strength. However, experience has shown that site-specific relationships are necessary to account for different geological environments. In this paper we describe results from a series of recent studies that have provided a more comprehensive method of geotechnical strata characterisation using detailed lithological interpretation and rock property estimation from borehole logs. This has resulted in an improved method of estimating rock strength via the determination of the proportions of quartz and clay plus the porosity. We also propose a rock classification scheme based on geophysical log analysis. This builds on the existing acceptance of the sonic log as an indicator of rock strength and adds scores according to the moisture, cohesion, porosity and quartz content inferred from the geophysical logs. The new scheme can be related to existing mapping based schemes such as CMRR and the Q-system.

Jan 1, 2005

By Charlie Li

A cut-and-fill mine stope, located at a depth of about 1000 m in a lead-and-zinc mine, was closed down because of large roof convergence and fractures appearing on the hanging wall. The instability problem was owing to the relatively low strength of the rock subjected to high in-situ stresses. Extra reinforcement was required to secure the stope so that the remaining ores in the stope could be mined. Assessment of the fracture mode in the country rock surrounding the stope was carried out and then the reinforcement de- sign was worked out. It was proposed that the unstable section of the stope would be reinforced by bolt-shotcrete ribs as well as selective long bolts. The philosophy of the design was to hold up the fractured country rock by bolts and shotcrete so that a load-bearing arch could be formed in the roof and in the hanging wall. Six shot- Crete ribs were set up in the unstable section of the stope. Displacement measurements showed that the roof convergence reduced from about 2 &day to the ordinary creeping level, about 0.25 &day, immediately after the reinforcement operation. Six month later, the hanging wall collapsed into the stope in a place just outside the reinforced section, which indirectly proved the effectiveness of the bolt-shotcrete ribs in reinforcing the fractured country rock. This case showed that an appropriate rock mechanics assessment would form a solid base for reinforcement/support design. It also provided an evidence for the philosophy of the reinforcement by bolt-shotcrete ribs - establish a load-bearing arch in the rock.

Jan 1, 2005

By John Magyar

Special foamed cements were used to stabilize and secure caving ground in a newly-reamed 12.5-ft diameter underground ventilation raise. Two zones of very poor Rock Mass Rating (RMR) material were encountered during reaming of a ventilation raise at Placer Dome's Turquoise Ridge Mime in Northern Nevada These zones caved to large cavities, continued failing, and could not be safely secured using standard rock bolting, wire mesh, and shotcrete procedures. In stable sections of the raise, the ground control plan called for the contractor to install 8-ft and 124 length resin grouted No. 7 Dywidag bolts, chain-link fence mesh and shotcrete. This work was conducted on three decks installed on the raise bore reamer. The first of the caved areas was approximately 1,500 cubic yards in volume and extended out to 40 ft beyond the wall of the raise. Placer Dome considered several options for structural fill to repair the area including pumped shotcrete, concrete, and placing and grouting aggregate. A safe, cost effective, and expedient method of stabilizing the raise with foaming grout and bolted liner plate was selected and successfully executed. The special fast-setting, foaming cement grouts used were Tekfoam and Tekseal® manufactured by Minova. The raise was quickly placed in service. Factors considered in selecting the foaming grout are discussed.

Jan 1, 2005

By Shuangsuo Yang

Based on field observations and physical modeling of simulated materials, the dynamic equilibrium principle on the whole interaction process between the surrounding rocks and support of large deformation roadways in coal mines has been developed. It explains the methods of forming the bolted strata and its changing load capacity with deformation. It also points out the factors and conditions for forming the bolted strata as follows: (1) many entries in coal mines are subject to large deformation. During their service life, it goes through the following process, elastic > plastic > fiacture > broken > loosening, etc. During this dynamic process, the mechanisms of supporting structure are changing and therefore the interaction between support and rocks experiences a process of dynamic equilibrium. Furthermore, the required supporting force for stabilizing the surrounding rocks shows overall decreasing trend; (2) the bolted strata is a special supporting structure consisting of bolt, rock skin protection devices and rock strata within the bolted horizon. As deformation increases, the supporting structure of the broken strata within the bolted horizon experiences a process of periodic change, from equilibrium > not equilibrium > equilibrium. Corresponding to this change, the supporting capacity of the bolted strata also fluctuates and gradually decreases; (3) under the bolted strata, the stability of a large deformation roadway is dynamic and step-wise; and (4) the formation of bolted strata, especially bolted broken strata depends on the existence of compressive force. Supporting the bolted strata is not constrained by the quality of the surrounding rocks. It has broad range of application.

Jan 1, 2005

By André C. Zingano

The domino effect in a violent pillar collapse occurs due to the redistribution of stress in the remaining pillars near the collapse, and this stress transfer occurs in the immediate and main roofs. The remaining pillars do not have enough load capacity to support this overstress and also fail, and the domino effect propagates further to the entire panel. The domino effect does not occur only due to low load capacity of the pillars, but also the roof rock mass quality, because the stress is transferred by the roof. In high competent roof without discontinuity (fault), like massive sandstone seam, the deformation of the roof is the cause of pillar collapse extension and propagation. Depending on how close it is approaching the collapse limit; the immediate and main roofs behave differently. When it is close to the collapse limit roof convergence and pillars compression occur. Conversely when it is far from the collapse limit divergence and pillars decompression occur. This is called the arc-effect. This paper studies this behavior of the roof (immediate and main roofs) using numerical modeling to simulate the roof deformation and stress distribution in the roof and pillars. This simulation represents the progression of pillar collapse and shows the zone of convergence and divergence of the roof, and also the effect of this behavior on pillar confinement. Two-dimensional plain strain model was applied to simulate the arc-effect. The simulation results were compared to those of the instrumentation that was installed before the collapse. Keywords: pillar collapse, room and pillar, numerical modeling

Jan 1, 2005

By Russell Frith

Probably the most expensive but least well understood roof falls in underground coal mining, are those that occur ahead of the powered supports in the centre of longwall face. Even small roof cavities result in the dilution of the ROM product and large cavities can stop the face for days if not weeks and the incurred cash flow losses can be catastrophic. Expensive and time-consuming remedial measures (e.g. PUR, cavity filling, removal of stone from the AFC by hand) are required and equipment wear and damage can be significant when the need to blast and convey stone is also considered. Through four major research projects within the Australian coal industry and numerous consulting assignments, the author has spent the best part of fifteen years attempting to identify and defme the various geotechnical and operational factors that determine whether a major roof cavity occurs ahead of the powered supports or a normal goaf fall occurs just behind them, the difference in operational outcome being enormous. The paper presents a condensed summary of the overall technical findings covering such issues as roof fall mechanisms, the measured influence of cover depth, periodic weighting of massive strata units and its quantification, the role of cut-throughs in chain pillars, face width and height, the role of the powered support and the influence (both positive and negative) of mine site operations. The big picture outcome is the view that high production longwall extraction is well-suited to certain geotechnical environments, is marginal in others and under certain conditions is fatally flawed and beyond the control of mine operators, although fortunately the latter category is a relatively rare situation. Various risk-based research findings in this regard will be given as a basis for future discussion.

Jan 1, 2005

By Luis Giraldo

Rock bolt installation characteristics near roof falls have been identified as contributing to failure. One documented and regularly occurring failure mechanism is loss of anchorage between the grout and the rock wall of the bolt hole. Key contributors to the integrity of the grout interlocking with the rock mass are the diameter of the hole relative to the diameter of the bolt, resin versus cement type grouts, rock type, and condition of the hole. Smooth bolt holes consistently exhibit less rock bolt load bearing capacity than rough walled holes. The goal of this study was to quantify the benefits of borehole conditioning and develop a systematic approach that would allow improvement of roof bolt performance. A new rock drilling technology, the Helical Drag Bit (HDB), was used in the early stages of this project to condition the borehole. The HDB cuts a prescribed helical groove in the borehole wall. When a bolt is grouted into a hole that has been conditioned using the HDB, the grout fills the helical groove and provides a stronger, more reliable mechanical lock between the rock and the grout. This paper presents results of a series of tests conducted in the laboratory and in the field. Short encapsulation pull out tests have demonstrated considerable increase in pull out strength and more consistent bolt performance when hole geometry is modified using the HDB. The results have also shown that greater load bearing capacity improvements are obtained in mines with weak roof rock, where it is most needed. Additionally, bolts recovered for examination suggest that hole conditioning during roof bolting reduces the effects of finger gloving. Therefore, mines with stronger roof rock could also benefit from hole conditioning. A new roof bolt that uses the HDB principle has been designed and tested. The new system works with existing bolting equipment requiring minimal modifications to the traditional procedure used for installation of fully grouted rebar bolts. The use of these higher anchorage capacity bolts will allow mines to reduce severe injuries and fatalities resulting from ground failures while maintaining or increasing productivity.

Jan 1, 2005

By David Bartsch

The Ohio Valley Coal Company (Ohio Valley) has operated the Powhatan No. 6 Mine in Belmont County, Ohio since 1972. Ohio University owns a 455-acre research farm over a portion of the Ohio Valley reserve containing 51 acres of old-growth forest known as Dysart Woods. In 1988, when Ohio Valley applied for a new mining area for its first longwall located several miles away from Dysart Woods, Ohio University claimed that mining at that distance would lower the regional water table and kill the old trees. In response, Ohio Valley began to study potential impacts from longwall mining on hydrology, soil moisture, and tree growth. Early studies examined the impacts of longwall mining on hydrology, soil moisture, and tree growth directly over mined areas compared to woodlots that were not undermined. A subsequent study examined tree-ring growth for a five-year period before and after longwall mining for several woodlots, some directly over panels and others at distances of up to 2000 ft away. No significant differences in growth rate associated with longwall mining were observed. New research was begun that compared longwall-mining impacts on groundwater hydrology, available soil moisture, and tree growth in two stands of mature trees located near Dysart Woods in order to confirm the earlier results. Using a test technique known as "Before-and-After-Controlled-Impact-Pairs," data was collected from one woodlot (test) that was undermined and from another (control) that was not undermined. Data collection began several years prior to longwall mining and has continued for several years after mining. The data for the test site shows that although the shallow and deep groundwater levels were affected by longwall mining, the soil moisture, vigor rating, or the growth rate of trees did not decline significantly compared to the control site where there were hydrologic impacts. The study shows that there are no known significant impacts to trees from longwall mining. The results of these studies were used by Ohio Valley to obtain State approval to longwall to within 300 ft of Dysart Woods

Jan 1, 2005

By A. Wilkinson

An investigation into the performance of support systems that are currently used in South African coal mines was conducted. Five most critical components of a support system have been identified. These components are resin, bolt, hole, machinery/equipment and the rock type. All five of these components are equally important as failure in any of these components will result in an inadequate support system. The performance of resin used in South Africa was investigated through in situ short encapsulated pull tests. The results indicated that in the majority of pull tests, failure took place at the rock-resin interface, indicating that the rock failed before the resin shear strength had been reached. It is therefore concluded that the strength of resin currently used in South Africa is adequate. It is found that although the maximum bond strengths achieved form both resin suppliers were similar, there was a considerable difference in the stiffhess of the resins. Roof bolt diameters and rib heights of bolts from three major manufacturers were evaluated (approximately 80 roof bolts from each manufacturer). The results showed there is a significant variation in bolt diameters and rib heights in the same batch in roof bolts supplied by certain manufacturers. The results also showed that on average there are insignificant differences between the parameters that determine the bolt profile (rib angle, spacing between the ribs and rib thickness). The influence of these parameters on bolt performance was very small and could not be established in situ. Roof bolts from four major suppliers were also evaluated through in situ short encapsulated pull tests. The results showed that the reinforcing system using roof bolts from all four manufacturers performed almost identically in sandstone, but somewhat differently in other rock types. The effect of bit types was evaluated. Both the stresses and the maximum loads obtained from the two-prong bits were greater than those obtained from a spade bit. Borehole annulus was found to be another important parameter that determined the support performance, and was investigated. The results from these tests showed that an annulus between 2.8 mm 4.5 mm resulted in the highest bond strengths. The results also showed that as the annulus drops below 2 mm, it appears to have a negative effect on the bond strength. The effect of wet and dry drilling on support performance was investigated. The results showed that wet drilling can-provide relatively greater system stiffnesses and slightly greater bond strengths. Short encapsulated pull test results showed very distinct differences between bolt system performance in different rock types and that sandstone produces significantly better results than shale and coal. From the results it is concluded that rock type is probably the primary factor influencing support system performance. A detailed investigation into the specifications of roofbolters that are currently being used indicated that the quality of installation of a support system is directly related to the performance of the equipment that is used to install the bolts. The important parameters, torque, thrust and speed of bolting equipment were investigated. The results showed that there are significant variations in these parameters in South African roofbolters.

Jan 1, 2005