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By Wm. Mark Hart

Under a specific geological condition, roof supporting method, and pillar-entry system, an entry convergence concept may be the most effective means that can be used to effectively indicate the longwall entry stability (i.e. pillar bumps, roof falls, or floor beaver This concept may also be used to verify the effectiveness of the current pillar-entry system and roof supporting method. Furthermore, convergence measured under different mining phases may lead to a better understanding of the longwall mining-induced abutment loads and may be used to estimate stress change in pillars and thus pillar stability. In this paper, a remote entry activity monitoring system was designed; a comprehensive entry convergence monitoring strategy was proposed; thee abutment loading phases in longwall mining were defined; the relationship between roof deflection and stress change in a pillar was derived, and finally the extent and magnitude of the mining-induced abutment loads as well as their impact on entry convergence were discussed using the field data obtained so far.

Jan 1, 1995

By Nikolaos Polysos

For the planning and driving of gateroads supported by roof bolts a practical concept has been developed in order to determine the geological and rock mechanical parameters derived from exploration drillholes. Based on this, an assessment scheme for the design of roof bolting has been established. The new methods of investigation comprise of geotechnical logging of drill cores for the determination of structural-geological rock characteristics and complementary geophysical well-logging which enables the evaluation of petro-physical and rock mechanical properties. The tools and their measurements will be briefly presented. The relationship between the geophysical log readings and the geotechnical parameters will be illustrated. The aim of the data acquisition is to improve roof bolting design which is based on the assessment scheme. This will be adjusted to the prevailing rock mechanical conditions and stands up to the required stability of the gate roads. In order to ensure consistent data acquisition, a method of core logging has been standardized for the respective demands. To achieve this it was necessary to introduce specific parameters and new methods. For the recording of linear and planar structural elements at the drill core, a special system for the documentation of the angles in relation to each other has been set up. The rock mechanical parameters and their acquisition will be explained. In order to carry out an objective analysis as fast as possible, software has been developed for the acquisition, management and presentation of the data. These programs and their functions will also be presented. The software is characterized by being user-friendly and the flexibility of its interfaces, Different options for the combined presentation of input and evaluated data give a more objective basis for the planning of roof bolt design. Examples will illustrate the operational implementation of the results during the planning and driving phases.

Jan 1, 2001

The formation of tension cracks in slopes is well known. Analytical studies require the determination of a critical slip surface for any slope and, in doing so, it is necessary to consider an appropriate tension crack. A general yet simple approach is proposed for identifying the critical slope failure surfaces associated with critical tension cracks. The minimum factor of safety consequently reflects both safety against shear sliding and tensile failure. In general, the critical slope failure surface for a slope determined in this way differs from that which is obtained without optimizing the position and depth of the tension crack. In particular situations, the difference is significant.

Jan 1, 1989

By K. Y. Haramy

Strong roof helps minimize roof fall problems in coal mine entries. However, the Inability of strong roof to cave readily may contribute to major ground control problems In Iongwall and retreat mining operations. The concern expressed by mine operators prompted this Bureau of Mines study to analyze the effects of strong roof members on ground stability around Iongwall openings. The problem was approached from three directions: development of a theory to analyze the strain energy conditions In the coal and surrounding strata, modeling the effects of different thicknesses and strengths of Coal and roof strata, and field Instrumentation of powered supports In two Iongwall mines. Results from the theoretical and modeling analysis confirm the accepted belief that a strong roof does, In fact, play a major role in high stress concentration problems In and around the Iongwall panel. Analysis of the effect of different roof overhang dimensions and properties on relative strain energy buildup In the coal and roof Is presented. Shield pressure data Indicate possible methods for detecting noncaving roof behind the shields. Strategies to monitor roof overhang and control dangerous conditions caused by the noncaving roof are also presented.

Jan 1, 1988

By Ismet Canbulat

The cut out distance in the continuous miner sections in South African collieries is limited to 12 m. However, this distance can be extended to 24 m by the permission of the Department of Mineral and Energy (DME). This paper investigates the risk of roof Calls associated with the extended cut out distances permitted by the DME. Extensive underground experiments have been conducted using sonic probe extensometer under various geological conditions to identify the effect of cutting distance and tinge on the stability of the workings and the performance of the support. The results showed that that the most critical parameter with respect to ground control aspects of extended cut out distance is the bord width, which determines the amount of deformation that will take place in the roof. This was confirmed by the numerical modelling. The results also showed that a significant portion of the maximum deformation in the roof takes place before the ratio of cut out distance to bord width exceeds two. This indicates that once the face is extended more than twice the bord width, the roof will stabilise, assuming that no geological discontinuity is present and that there will be no further geometrical changes in the area as a result of for example the development of an intersection.

Jan 1, 2000

By William Bookshar

The development of increasingly more sophisticated and powerful retreat longwall equipment has led to ever wider and longer panels. To maximize gateroad development efficiency, provide for effective face recovery, andlor reduce ventilation pressures, entries are cut across longwall panels in advance of longwall retreat mining. These entries must be supported to prevent falls of immediate roof in advance of the face. Recently, cut through entries and predriven recovery room entries have been supported with various combinations of roof bolts and passive support. The cost and relative success of these support systems has varied considerably. Eighty-Four Mining Company, Fosroc, and Sub-Technical have developed and implemented an efficient, yielding, multiple component, cement crib to support longwall panel cut through entries and pre-driven recovery rooms. The cutable crib consists of an inexpensive flyash cement pillar poured in a bag, within a temporary metal form. The form is moved to a subsequent cutable crib location after the initial hardening of the flyash pillar. The remaining entry height between the mine roof and the top of the flyash cement pillar is filled with a specially designed bag filled with Fosroc's Tekset cement formulation. The diameter of the pillar and thickness of the Tekset bag are designed to accommodate the anticipated front abutment loading and entry convergence.

Jan 1, 1998

By John C. Stankus

ldentifcation of potential roof problems in coal mines has long been a complex issue due to the wide variety of mining and geological conditions. It is well known that mine roof falls are related to many different factors such as roof strata properties, sandstone channels, regional and localized horizontal stress, vertical stress, and tectonic stress. Many times, a combination of these factors causes roof problems. Through years of ground control experiences, Keystone Mining Services LLC, (KMS) has developed a Roof Instability Rating (TUR) system to predict potential roof problems. The first RIR analysis was presented at the 19th Ground Control Conference (Watts, et al., 2000). The RIR system includes: quantifying each individual factor, weighting each factor, combining individual factors, finalizing the RIR, and setting the criteria for the areas where special support is needed. In this paper, the application of the Roof Instability Rating (RIR) system at Enlow Fork mine will be presented.

Jan 1, 2001

By Rao Pothini

Measurement of pressure distribution under the base plate of the Hemscheidt 2-leg 800-ton shield was performed underground for two shifts using 12 pressure cells installed in two rows, one each under each split base plate. Pressure distribution was uneven depending on contact condition. In general the highest base pressure occurred at 1/3 of the base plate contact length from the rear edge. The resultant was near vertical, and the pressure under the tail side split base plate was always higher than the head side. For comparison, pressure distribution on the canopy and under the base plate was also measured under controlled conditions at the USBM's Mine Simulator Testing Facilities in Pittsburgh, PA. A total of 45 tests were conducted under several simulated contact conditions. The results showed that the actual contact area is highly dependent on the compliance of the immediate roof and floor due to the sloped canopy tip and a base lifting device under the base tip. The loading distribution tends to change in magnitude and profile as the leg pressure increases. The base lifting device exaggerates the high toe loading on the base plate characteristics of this type of shield support. This paper describes in detail the testing methods, data analysis results obtained, and recommended changes.

Jan 1, 1992

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By Kikuo Matsui

Standing supports such as steel arches or square work supports along with the necessary struts and lagging have been traditionally employed in the longwall gateroads of the Japanese underground coal mines. Miike Colliery has newly introduced the interpanel-pillar mining system and the strata bolting system in gateroads to raise productivity levels and reduce operating costs. The Department of Mining Engineering, Kyushu University, in cooperation with the Miike Colliery and the Coal Mining Research Center, Japan (CMRCJ), has been conducting research into the optimum design of strata bolting as a primary support system in longwall gateroads. This paper presents a brief overview of the current state of strata bolting technique being applied in longwall gateroads at the Miike Colliery and discusses some problems in employing bolting.

Jan 1, 1993

By J. D. Dizon

Conventional strata control methods (i.e. roof bolts. cable trusses) are not always effective for ground control of very weak coal strata. An alternative method, termed presupport. is being investigated by the U.S. Bureau of Mines. In this paper structural modeling of presupport conceuts and field teatinn of a DresuDuort application is presented. Pinite-elem&; modeis indicate that forepoles significantly lower beam stresses in the roof strata overlying a coal seam. but they cannot he expected to prevent or reduce I roof strata failure at the coal face due to the presence of a high stress concentration at that location. However, their presence near the coal face is needed to sustain the fall of roof if it does occur. Because of an ever present severe stress concentration at the coal face, it would appear that stresses could exceed material strengths, causing local structural damage, especially if the rock is very weak. Therefore previous coal face positions along an entry may have a higher than normal probability of roof fall. In the field investigation. a loorcost air injection test has thus far shown promise as a tool for evaluating that effects of forepoling on the roof strata. Observations of coal mine en- tries in which forepoles have been installed indicate forepoling is effective in controlling very weak roof strata.

Jan 1, 1990

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By David Bigby

A remote reading dual height telltale system has been developed to provide mine management with early warning of impending roof failure. This system is a logical development from the visual dual height and triple height telltales which are increasingly being used worldwide for routine monitoring of mine roof. The paper describes the principles and history of telltales from their origins in France, their refinement and routine use in the UK and their subsequent spread around the world. They are now being used in various forms in USA, Canada, Germany, Spitzbergen (Norway), Poland, Ukraine, Russia, Australia, South Africa, Mexico, India, Egypt and China. The remote reading telltale system has coal mine intrinsic safety approval in Europe and the United States (MSHA) and the first commercial system was installed in a German colliery at the beginning of the year. Up to 100 dual height telltales can be connected, in a simple "daisy chain" configuration, to each of 4 underground interrogation units. These are connected through a twisted pair link to a surface computer which displays the readings, updates the database, generates warnings and provides the user interface. The paper describes the features and principles of operation of the system which differs considerably from conventional mine-wide monitoring systems. A European Community funded research programme is currently underway with the aim of developing improved alarm generating algorithms including evaluation of interfacing a neural network to the system. The paper concludes by exploring other potential applications of the system for ground control monitoring currently in development, including extensometry, stope closure monitoring and support load measurement.

Jan 1, 2001

By A. Bortolussi

The paper discusses the state of the progress reached in the development of the diamond wire tool for cutting hard rocks. To the advances of this relatively new technology a substantial contribution has been provided by the research being carried out at the University of Cagliari on samples of granite rocks from different Sardinian stone quarries. On the basis of field experiments and laboratory test data, the mechanism of diamond wire cutting has been assessed. In particular the influence of the rock features and operational variables an tool performance (cutting rate and bead wear) has been elucidated.

Jan 1, 1992

By Andre Zingano

It is known that the backfilling confinement increases the pillar strength and also reduces the rib lateral displacement. With backfilling, it is possible to increase the room-and-pillar mining recovery. This paper aims to study the behavior of coal pillars confinement using rock fill as backfilling material. Laboratory tests were carried out to determine the behavior of backfilling materials. Constitutive models were developed for the tested backfilling materials. Numerical simulations for the effect of backfilling materials on the ground stability in room-and-pillar coal mine entry were performed. The backfill method is a good strategy to (i) improve the ground stability in room-and-pillar coal mines for long term, especially for weak coal seams, (ii) increase the coal mining recovery and (iii) reduce the quantity of waste material on waste piles at the surface.

Jan 1, 2011

By Peter Zhang

As simpler and easier reserves are exploited to depletion, underground coal mining continues in more complex geologic settings and conditions. Roof bolting for long-term entry stability is always a challenge. For long-term entries of a mine such as belt, track entries and return airways, roof could be under-supported if one uses the same bolting plan as in short-term entries or even seem over-supported with increasing use of longer, stronger bolts along with mesh, pans, straps or trusses. In most underground coal mines, roof deteriorates over time with cutters developing deeper in entry corners; roof sagging and popping out between bolts; entry width increasing due to rib sloughage; and separations developing deeper up into the roof. In deteriorated long-term entries, roof falls could occur at certain locations and re-bolting could be costly or sometimes unfeasible. Therefore, it is important to choose an appropriate bolting plan during development based on mining and geologic conditions if the entries will be used for the long-term. In both short-term and long-term entries, primary roof bolting is critical in building a strong and effective beam. But in long-term entries, supplementary bolting is necessary in certain cases to ensure roof stability can still be maintained even if a roof failure develops beyond the primary bolted horizon. This paper deals with roof bolting for long-term entries based on studying the progressive roof failure and roof falls under different geologic conditions. The paper presents bolting considerations in terms of bolt type, length, diameter and spacing as well as bolting strategy to minimize the risk of roof falls in long-term entries.

Jan 1, 2011

By Dave Forrester

Mining subsidence over shallow abandoned coal mine workings is unfortunately not an uncommon experience in the coalfields of North America and there is a significant knowledge and literature base addressing the phenomenon and related issues. From time to time, however, a case history arises which is outside the more routine occurrence. This paper addresses one such case and presents an overview of a subsidence Event above abandoned coal workings, which badly damaged a two-story High School building. The subsidence at the Cape Breton & Victoria Regional School Board (CBVRSB)'s MacDonald High School in Dominion, N.S., Canada on October 30, 2002 led to a detailed investigation. It was concluded that the ground movement and associated building damage had been caused by mining subsidence in underlying old underground coal mine workings of the 1890s, only 150 ft deep at the School. The Event was not isolated and a similar but smaller event happened nearby a few months later. The paper describes the events and summarizes the subsequent investigations and findings. The 17-year old High School building was eventually demolished.

Jan 1, 2004

By Asmaa Yassien

Finite element models were developed to study the effect of tensioned and fully-grouted bolts on the stability of the mine opening in a three-entry development system using a typical geological column for the Pittsburgh Coal Seam. The models were used to study the effect of each bolt type on the stress distribution in the immediate roof, yield zone development and the vertical deformations. Comparison of all of those features show that the effect of a tensioned bolt on the stress distribution in the immediate roof is restricted to limited areas, whereas the fully-grouted bolt has a much wider effect on the stress distribution. Both types of bolts can reduce the yielding and deformation in the immediate roof to different degrees. The fully-grouted bolt reduces the sliding along the bedding plane more than the tensioned bolt. Both types of bolts were able to maintain the bedding planes without separation. The induced forces in both types of bolts are analyzed. A rapid increase in the shear force and bending moment occurred in the fully¬grouted bolt due to the relative motion of bedding planes.

Jan 1, 2002

By Rob Robinson

Following a massive rib wall failure along the reclaim belt of the large underground surge bunker at CONSOL?s Bailey Mine, a multifaceted, cleanup and rehabilitation effort was initiated to rapidly stabilize the bunker rib walls and place the bunker back into production. Rehabilitation of the bunker included infilling and re-mining of the failed excavation area, installing a system of horizontal tie-back cable bolts, constructing a large knee wall at the base of the rib, reducing water inflow, and providing drainage for rib water. As a result of the coordinated effort, the bunker was safely placed back into production within 38 days. The 7500-ton surge bunker, an engineering feat in itself, was constructed in 2003 to accommodate production surges from two longwalls. In 2005, 14.8 million raw tons passed through the bunker. The 22-foot wide by 75-foot high surge bunker excavation extends as much a 52 feet below the base of the Pittsburgh seam. The bunker excavation measures 325 feet in length, with feeder and reclaim belt excavations extending the bunker excavation by approximately 500 feet. The massive rib roll, which occurred near the bottom of the reclaim belt slope, measured as much as 45 feet high by 60 feet long. Failure of the rib was attributed to progressive failure of a thick claystone unit present near the slope bottom area. Saturation of the claystone by water inflow, high insitu horizontal stress, and pre-existing rock joints were considered contributing factors. Immediately after rib failure, mine production was temporarily rerouted to a parallel belt. Two bulkheads were then constructed 175 feet apart along the reclaim belt slope, and a customized fly ash-cement mixture was pumped from the surface into the unstable reclaim slope area. A tunnel was then mined through the backfill along the original reclaim slope contour; the new excavation supported by a system of customized steel sets. To assure stability of the adjacent bunker rib wall, a pattern of 70-foot long cable bolts were installed downward from the adjacent entries to effectively tie back the rib. Drainage holes and a 12-foot high knee wall were installed. Adjacent gas wells were also plugged to help reduce inflow. Periodic inspections and continuous monitoring are being used to help assure bunker stability.

Jan 1, 2007

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999