Search Documents

By G. J. Hasenfus

Although theoretical and conceptual knowledge regarding the impact of horizontal stress on underground coal mining excavations have existed since at least the mid-1900s, it wasn?t until the late 1970?s that the theory and practical understanding began to mesh. Since that time, particularly over the last 25 years of this Conference, increased empirical study, as well as advances in measurement technology, computer simulation, and bolt design, has significantly improved our understanding of horizontal stress impact, and provided coal mine operators with tools to reliably predict and manage it. This paper provides a retrospective review of some of the more important horizontal stress-related advancements made during the 25 years of Conference proceedings, particularly those for predicting and controlling horizontal stress impacts within underground coal mines. Relevant and significant CONSOL experiences and studies will be presented in context, with particular attention to horizontal stress considerations for longwall faces and gateroads. The significance of horizontal stress will also be placed in context with respect to local geology and structure, ground support measures, and mining configuration, method, and sequence.

Jan 1, 2006

By David Oyler

Sonic travel time logging of exploration boreholes is routinely used in Australia to obtain estimates of coal mine roof rock strength. Because sonic velocity logs are relatively inexpensive and easy to obtain during exploration, the technique has provided Australian underground coal mines with an abundance of strength data for use in all aspects of ground control design. However, the technique depends upon reliable correlations between the unconfined compressive strength (UCS) and the sonic velocity. This paper describes research recently conducted by the National Institute for Occupational Safety and Health (NIOSH) aimed at developing a correlation for use by the U.S. mining industry. At three coreholes in Illinois, Pennsylvania, and southern West Virginia, sonic velocity logs were compared with point load tests for a broad range of coal measure rock types. For the entire data set, the relationship between UCS and sonic travel time is expressed by the following equation, where UCS is in psi and t is the travel time of the P-wave in microsec/ft. UCS = 468,000 x e-0.054t (1) The r-squared value for this equation is 0.87, indicating that a strong correlation between sonic travel time and UCS can be achieved with this technique. The paper also addresses the steps that are necessary to ensure that high-quality sonic logs are obtained for use in estimating UCS.

Jan 1, 2008

By S. J. Mitchell

An analysis of bolting reinforcement of several large [approximately 18 m (60 ft) cube] pillars was performed. The many overcoring stress profiles in pillars at the mine were used to produce generalized stress distributions at various stages in pre- and post-failure. A complete stress-deformation curve was estimated from this data. The effect of pillar bolting on the load-deformation behavior was extrapolated from the observed unbolted behavior. Bolting can increase pillar strength by up to 10 percent. The effect of bolting on general mine stability was examined with a shoW computer analysis. The computer model was calibrated against measurements performed at the mine, and produced good agreement between measured and modeled stresses. The analysis indicated that bolting increases the bolted pillar safety factors in the event of additional pillar failure by a small amount (approximately 5 percent). Progressive pillar failure is unlikely, because arching transfers most load from yielding pillars to large unmined abutments rather than to the smaller panel pillars.

Jan 1, 1984

By Yunqing Zhang

Roof failure occurs much more frequently at intersections than entries and crosscuts. Using a commercial finite element package, four-way and three-way intersections reinforced with the tensioned bolts are modeled. The 3-D model considers bolt pre-tension, bolt installation procedure, bedding planes and in-situ horizontal stresses. The modeling results from both four-way and three-way intersections are analyzed with regard to the intersection stability. The mechanisms of tensioned bolting at the intersections are investigated and some tensioned bolting guidelines for intersections are given. Based on this study, a better way to enhance intersection stability is to use longer bolts than entries or crosscuts and increase the bolt density near the intersection corners.

Jan 1, 2003

By Alan Bugden

Goedehoop colliery in South Africa has experienced a number of substantial roof falls in roadways and intersections. Many of these have been associated with geological slips and/or horizontal compressive stresses. This paper describes a numerical modelling study conducted after one such fall with the objective of clarifying the roof failure mechanism associated with these falls. The fall was located at an intersection and occurred as the last cut was being made to complete the intersection. One bounding surface of the fall was formed by the geological slip; the others were formed by fresh failure surfaces. The fall took place despite the slip being identified as a hazard and additional bolts being placed by the workforce. In conjunction with the modelling work tests were conducted on samples from the roof in which the fall occurred, these gave a U.C.S. of 80 -100MPa. Several in-situ stress tests had already been conducted at the mine; a further test was conducted close to the fall site. The tests show that the largest stresses are horizontal. However, the maximum stresses of 6-12MPa are small compared to the compressive strength of the root. Computer modelling highlighted a mechanism arising from the conjunction of a geological slip and horizontal compressive stresses that could result in tensile failure of the roof leading to the fall. Sensitivity studies were conducted to identify risk factors associated with this mechanism and the modelling results used to assist in formulating a support strategy to combat falls of this nature.

Jan 1, 2003

By Thomas M. Barczak

The engineering of a support system is driven largely by economics. The goal is to provide effective support at minimal cost. For this reason, wood has been extensively used as a support material for centuries. The cost of timber for mining applications has risen by more than 50% in the past 5 years and is likely to continue to increase as the supply diminishes and further environmental restrictions are placed on timber harvesting. The increased cost and lack of high strength wood in the Western United States has provided incentives to improve the utilization of wood and to develop alternative support materials. This U.S. Bureau of Mines' (USBM) report discusses the basic engineering requirements for the application of various materials for supplemental roof support construction and provides an overview of support systems that use materials other than wood. A comparison of support performance and cost for various material constructions is made to assist the mine operator in support selection. The scope of materials discussed in the report include: (1) conventional mine timber loaded perpendicular to the grain, (2) parallel to the grain timber loading, (3) steel fiber reinforced and air entrained concrete, (4) steel, (5) granular fill materials, and (6) combinations of these as composite support structures. The range of support systems analyzed include wood and concrete cribs, concrete columns and props, steel beams, friction props, and several novel support concepts.

Jan 1, 1994

By Dezhong Kong, Jaichen Wang, Shengli Yang

"The stability control of longwall coal face is the key technology of large-cutting-height mining method. Therefore, a systematic study of the factors that affect coal face stability and its control technology is required in the development of large-cutting-height mining method in China. After the practical field observation and years of study, it was found that the more than 95% of failure in coal face are shear failure. The shear failure analysis model of coal face has been established, that can perform systematic study among factors such as mining height, coal mass strength, roof load, support resistance, and face flipper protecting plate horizontal force. Meanwhile, sensitivity analysis of factors influencing coal face stability showed that improving support capacity, cohesion of coal mass and decreasing roof load of coal face are the key to improve coal face stability. Numerical simulation of the factors affecting coal face stability has been performed using UDEC software and the results are consistent with the theoretical analysis. The coal face reinforcement technology of largecutting- height mining method using the grouting combined with coir rope is presented. Laboratory tests have been carried out to verify its reinforcement effect and practical application has been implemented in several coal mines with good results. It has now become the main technology to reduce longwall coal face failure of large-cutting-height mining method.INTRODUCTIONLarge-cutting-height mining method of more than 3.5m mining height, is one of the main methods for thick-seam mining in China. Currently, the largest mining height is 7.3m and the support capacity is 1,800mt. Shendong and Jincheng mining area have the best use of large-cutting-height mining technology (Wang, 2009). Generally, the mining height ranges from 6.5m to 7m and the support capacity ranges from 1,000mt to 2,000mt, annual output of the panel is between 10 and 12 million tons and the highest are 15 million tons. The main goal of large-cutting-height mining technology is to achieve high yield and high efficiency by increasing the advancing speed of longwall face (Yuan, 2011- 2012). However, in certain geologic conditions, rib spalling and roof falls in the unsupported area often occur. Because of the large tonnage of large-cutting-height mining equipment, once the rib spalling and roof falls cause the equipment to be out of alignment, the face advancing speed could be affected seriously that in turn affects the output (Yang, 2012). Therefore, studying the failure mechanism and control technique of coal face in large-cuttingheight mining method is of great significance."

Jan 1, 2015

By Syd S. Peng

A total of 209 cases of subsidence data over longwall panels in 16 US coal seems have been collected and built into a subsidence database. The database is developed under MS Windows environment. It uses a very user-friendly menu driven system. The empirical formulae for a number of commonly used subsidence parameters have been derived from those collected subsidence data.

Jan 1, 1995

By Jeffrey M. Listak

This report describes a study, conducted jointly by BethEnergy Mines Inc. and the Bureau of Mines, to assess the effects of longwall front abutment loading on various supplemental support materials utilized in a predriven recovery room. The purpose of the predriven recovery room is to reduce longwall equipment transfer time by eliminating the premove preparation associated with conventional recovery methods. In addition, the room provides a large area in which face salvage equipment can maneuver. Results of four recovery-room study areas show the progression and influence of the front abutment on the longwall panel and supports and their subsequent behavior. The best performance (i. e. the support that most facilitated recovery) of support material was achieved through the use of a concrete mixture which had a sand content of 35 percent by volume. The longwall transfer time was reduced by 30 percent which translates to a production advantage of 37,000 raw tons of coal per panel.

Jan 1, 1989

By Stephen C. Tadolini, Peter A. Gray, Robert Hawker, Andrew Sykes

"Self-drilling rock bolts (SDRBs) that use pumpable cement grout to anchor bolts have been used successfully in the tunnelling and civil engineering industries for many years. Pumpable cement grout offers a considerable advantage in that the SDRB can be fully encapsulated right down to the bearing plate, including filling cracks and cavities in the borehole. However, cement grout cannot provide the immediate rock support required for the coal industry. This paper describes a new SDRB system that uses pumpable resin to anchor the SDRB in a borehole, which not only provides all the advantages of a pumpable grout anchoring system, but also provides immediate roof and rib support as required in the coal industry. This paper further outlines the latest developments in SDRB technology including (1) extendable self-drilling rock bolts, which can almost double in length during the installation process and are particularly suited to low seam heights and rib support and (2) pre-tensioned SDRBs, where a pre-tensioned load can be applied during the installation process. These new SDRB systems offer both considerable advantages in rock support and improvements in mining productivity.INTRODUCTIONThe mining industry is continually striving to develop more efficient methods to support the roof and walls of roadways and tunnels, and rock bolting has proven to be one of the most significant improvements in ground support over the last 50 years. Rock bolting is now almost universally used in the underground mining industry worldwide, and there are many different rock bolts available.Rock bolts are traditionally made from steel or fiberglass and are anchored into a borehole with a friction-anchoring device, such as an expansion shell mechanism; cement grout; or chemical resin. Friction-anchored bolts are widely used in the Australian metal mining industry, predominantly as short-term support. They are used in conjunction with resin-anchored bolts where there is a requirement for longer life. Resin-anchored bolts are used in the coal mining industry.Friction-anchored bolts provide immediate rock support, but their load transfer capacity is poor compared to resin or cement anchored bolts. In addition, friction-anchored bolts are more susceptible to corrosion than resin or cement anchored bolts."

Jan 1, 2010

By Jim Galvin

4 and 6 point timber chock constructions are used extensively on both a systematic and spot basis by Australian longwall operators to support tailgate roadways. In 1996, the School of Mining Engineering at UNSW published design and performance criteria for timber chocks constructed from Australian hardwoods. The research was funded by the Australian Coal Association Research Program (ACARP). In 1997, ACARP awarded funding for a stage 2 testing program. This testing focussed on examining the effect of using thinner timber elements in chock constructions to improve OH&S and comparing the full-scale performance of traditional 4 point chocks to Link-n-Lock constructions. It was established that chocks constructed from thinner elements have failure loads much less than thicker elements. However up to the failure loads of the thinner elements there was no significant difference in the response to convergence of chocks constructed from 25, 50 or 100mm thick elements. Chocks constructed from 150mm thick elements have a similar ultimate strength to the 100mm thick elements. However, they have a significantly lower resistance to load, thereby permitting more convergence to occur. There is more potential for chocks comprised of thin elements to be constructed 'out of plumb' and so suffer eccentric loading. The full scale testing program established that at any given level of convergence up to 10% strain, the timber in a Link-n-Lock chock generates twice the support resistance of the same quantity of the same timber in an equivalent size 4 pointer chock. On the basis of support resistance per cubic metre of timber, there is only a marginal difference in the performance of 1.0m Link-n-Lock chocks and 1.2m Link-n-Lock chocks constructed from select Blue Gum. The cost per tonne of support provided by a Link-n-Lock chock is effectively half of that provided by a 4 pointer chock of equivalent dimension and timber. Up to a strain level of 4%. Link-n-Lock chocks constructed from 1.2m long elements are slightly more cost effective than the same chocks constructed from 1.0m long elements. Thereafter, there is no significant difference in cost per tonne of support resistance. Beyond a strain level of about 4%, Link-n¬Lock chocks constructed from landscape grade Blue Gum are just as cost effective as Link-n-Lock chocks constructed from structural grade 4 Blue Gum.

Jan 1, 2000

By Anthony (Sam) J. S. Spearing

Backfilling using coal waste is not a novel means for waste disposal throughout the world; yet, backfilling using a high density (paste) backfill is relatively new, except in Germany and Poland. One of the main benefits of a high density backfill, as opposed to a low density (slurry) backfill, is that, if placed at a low permeability and low moisture content, the risk of water table contamination can be relatively low. In addition, if paste backfill can be placed cost effectively, environmental benefits are huge because the surface impoundment footprint can be greatly reduced or even eliminated. This is especially true in room-and-pillar applications. In its current state, backfilling is considerably more costly than surface disposal. This is mainly because paste, as opposed to slurry, significantly increases backfill placement costs; however, this could be offset by increasing the achievable extraction rate. Overall, such a system could potentially improve the public?s image of coal mining in the US for a myriad of reasons. This paper discusses a proposed backfill system for a typical room-and-pillar coal mine in the Illinois Basin of the US and clearly shows the technical advantages, although costs are still higher with fill. The cost differential will likely reduce over time, in favor of the backfill option, as costs of traditional surface disposal increase, due mainly to ever-restrictive disposal regulations.

Jan 1, 2013

By David C. Oyler

The Bureau of Mines conducted research at the 9-Right Section of the Olga Mine to determine the cause of bumps and aid in the development of appropriate control techniques. The purpose of this paper is to identify the factors which affect the occurrence of bumps at the Olga Mine. These factors have been broken down into two categories, in situ and mining related. The in situ factors include those conditions which exist prior to mining, such as variations in lithology and overburden, presence of jointing, the composition and the physical properties of the rock, and the original stress field existing in the coalbed and adjacent strata. The mining factors include the mining sequence, changes in stress in the coalbed and adjacent strata during mining, rates of convergence, and the pattern of failure of the coal and adjacent strata. The factors which appeared to have the greatest effect on bumping were the overburden, the lithology of the roof and floor rock and the physical properties of the rock. The roof rock is generally a massive, stiff sandstone which does not break until coal pillar extraction is complete. The immediate floor is a strong shale which generally grades into siltstone and then into sandstone. The overburden at 9-Right ranged from 990 to 1,600 ft. Secondary factors appeared to be the in situ (pre-mining) stress and the mining sequence. When barrier pillars were split., pressures measured in hydraulic cells (Borehole platen flatjacks) located in those barriers showed large pressure increases, and the incidence of bumps increased, especially when the barriers were near old gob areas. The pillar extraction system used at Olga, where three rows of pillars are mined concurrently, appears to reduce pillar loads at the face areas, thereby reducing the exposure of men and equipment to the risks of bumps during mining operations. The horizontal components of in situ stresses at Olga were found to be nearly the same as those at 5 other mines in southern WV, where humps were not common, so it appears that horizontal in situ stress alone is not a major influence on bumping at the Olga Mine.

Jan 1, 1987

By Greg Hendon

Various methods have been used to evaluate the effectiveness of shields in place. The most common method used is static recording of leg pressures by visually examining pressure gauges. In an effort to more closely examine the shield cycle and therefore the shield effectiveness, Jim Walter Resources has implemented a program of monitoring utilizing a polycorder system which is capable of recording convergence and pressure measurements on a continuous basis. This paper will discuss two longwall panels at Jim Walter Resources which have been monitored.

Jan 1, 1990

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 Kray Luxbacher

Three-dimensional time-lapse velocity tomograms were generated to image stress redistribution around a longwall panel to produce a better understanding of the mechanisms that lead to ground failure. Mining-induced microseismic events provided passive sources for the three-dimensional velocity tomography. Surface-mounted geophones monitored microseismic activity for 18 days. Most similar studies have utilized active sources including longwall shearers, controlled explosions, and hammer blows. These active sources generally require a person to initiate and measure the location and time of the source, and are not conducive to continuous monitoring. This method utilizes mining induced microseismic events as passive sources and lends itself well to continuous monitoring. Two event location methods were compared in order to move toward quantification of tomographic results in terms of ground conditions. Additionally, LAMODEL was utilized to determine the expected stress distribution around the longwall panel and compare these images to the velocity tomograms. Eighteen tomograms were generated, one per day, and high velocity regions in the set of tomograms generated from the initial location algorithm correlated with abutment stress redistribution predicted by numerical modeling. This research is significant as it has the potential to improve safety in underground mines and is the only example of long term passive velocity tomography implementation for the purpose of inferring stress redistribution around an area of active mining. Though still immature, this technology has the potential to allow mine personnel to image areas of active mining on a regular basis and detect areas with anomalous velocity distributions indicating potential safety hazards.

Jan 1, 2007

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 Mark K. Larson, Douglas R. Tesarik, Heather E. Lawson

"Load transfer distance (LTD) is simple in concept and usually evident in underground coal mines. Many people in the mining industry recognize LTD as the distance from the panel to where the mine ribs and pillars show evidence of renewed spalling, floor heave, or other effects of increased weight due to longwall mining or pillaring. Barrier pillars, in particular, are designed to shield mains and other relatively permanent facilities from this loading. LTD is also important in determining the extent and degree to which mining increases the load on pillars and ribs in the gate roads. Generally, a long LTD reduces loads in gate roads, but may require larger barrier pillar widths to protect mains or adjacent panels from excessive loads. Therefore, a long LTD may make it easy or difficult to meet various objectives of mine layout design . LTD reflects overburden geology in two ways: First, overburden that is weak and readily caves will limit both the amount and distance of load transfer. Second, stronger, stiffer, sandier, and more massive overburden increases both the amount and distance of load transfer. As such, LTD is central to coal mine layout design and calibration of numerical models that seek to anticipate these transfers of load during mining. However, the term does not have a consistent definition in the literature, and various instruments and observational techniques with varying sensitivities and thresholds have been used to indicate the onset of mining-induced stress increase and, subsequently, to determine an LTD. Consistency is important for meaningful comparison of ground response between mining sites and for accurate calibration of models.This paper adds to the body of observed and reported LTDs by presenting cases with relatively sandy and massive overburden at two western coal mines. These two cases include observation and measurement of LTD with multiple methods and provide insight into how distances reported relative to different criteria and instrumentation could be adjusted for comparison against a common base. This base is the criteria used by Peng and Chiang (1984) to determine their empirical equation. In one of these two cases, LTD was recently measured at a western U.S. coal mine (Mine A) to be approximately four times that calculated using the Peng and Chiang empirical equation. LTD measurements were examined using various instruments at Mine A and at adjacent Mine B. The measurements at Mine B showed that LTD was significantly greater than that calculated by the Peng and Chiang equation. Difficulties with BPC installation and fittings at Mine B permitted only ranges of LTD to be determined from some BPCs. Even so, the LTDs and ranges of LTDs determined were consistent with the LTDs determined at Mine A."

Jan 1, 2015

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982

By Alan Campoli

The Eclipse System was developed by Minova USA Inc. formerly Fosroc Inc. to improve the performance of 5/8-inch deformed rebar fully grouted into a one inch borehole. An 1/8-inch offset bolt head combined with specially formulated Eclipse Lokset resin cartridge results in reduced gloving and increased mixing efficiency. The benefits of the Eclipse System have been demonstrated, under the supervision of MSHA, in Kentucky, West Virginia and Illinois coal mines in partnership with Excel Mining Systems, a leading roof bolt manufacturer. Over 100,000 units have been installed to date. Eclipse system bolts exposed during overcast construction at the Panther Mine of Cumberland Resources displayed an extremely well mixed, uniform resin anchorage over the entire bolt length.

Jan 1, 2003