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By S. M. Hsiung

In late September 1984, a longwall panel in West Virginia lost 60 powered supports when the face had advanced for 250 ft. from the setup entry. An in-mine investigation showed that it was a first weighting failure resulting from inadequate Support capacity. This paper presents the results of finite element modeling, which have led to a better understanding of the first weighting failure mechanism. The dynamic impact of the first weighting failure on the powered supports is analyzed. The optimum capacity for the powered supports in order to control the roof at face area under the geological condition is discussed.

Jan 1, 1984

By M. Afsari Nejad

A Transversely Elasto-Plastic Model was configured in FLAC (Fact Lagrangian Analysis of Continua) as a more realistic simulation of stratified and inclined strata behaviour examining surface subsidence above the inclined longwall panels (I ). The failure criterion employed was a modified Mohr-Coulomb failure criterion that assumed a variable cohesion, which changed with respect to the orientation of the plane of anisotropy . Using the proposed numerical modelling technique several longwall panels with different configurations were simulated. The results of both surface subsidence and horizontal displacements were validated against UK data using the Subsidence Engineer's Handbook (S.E.H) subsidence prediction method and the Influence Function Method (2). It was found from the research that the incite properties as well as laboratory to field reduction factors arc depth dependent. A set of relationships for calculation of anicotropic stiffness and strength parameters have been derived. The proposed modelling approach has been successfully applied to surface subsidence prediction for 3 collieries in the UK allowing the results of the simulation to he validated directly against measured data.

Jan 1, 1998

By Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002

By Klaus Opolony

The world's most popular method of longwall mining requires multiple entry systems for the panels. In contrast to this mine layout the roadways in German coal mining are used for advanced mining from the rock mechanic point of view or are even re-used by a second panel. This procedure of roadway use is presented within this paper using a 3D visualisation software tool. In the previous conferences in Morgantown several papas had addressed the basics of rock mechanic background, planning tools and variants. This paper gives a practical case study for reuse of a roadway in a German coal mine. The heading and the use of various alternatives are compared under the aspect of efficiency and costs. Within the comparison the specific requirements and benchmarks of German coal mines are taken into consideration. The costs for a multiple entry system and first of all the development rates that can be achieved with common road heading systems are compared with international layout alternatives. A prospect includes a discussion of pro and con for various development methods. Devices are given due to this prospect if and how multiple entry systems are applicable to German coal mines. The paper gives an initial point for international comparison and is a base for further discussion.

Jan 1, 2003

By Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001

By Brian E. Travis

Lightweight polymer grids have recently been introduced to the mining industry. They are currently being used for roof and rib control, longwall shield recovery, safety guarding or barricade, highwall screening, internal reinforcement of gunnite/shotcrete applications, sub-base reinforcement, construction of steepened slopes, retaining walls, mine reclamation (i.e. erosion control, turf reinforcement), and in numerous mining waste related projects. These polymer mining grids utilize a strong, lightweight polymer, usually special grades of polypropylene or polyethylene. High tensile strengths are achieved by molecular orientation of these polymers in the manufacturing process. The first step in the manufacturing process is to extrude the polymers into thick continuous sheets. These sheets are then punched, and drawn into open grid structures. Aperture size and configuration are controlled by the geometry of the punched hole and the degree to which the grids are stretched. The drawing process is the key to the molecular orientation required for the tensile strengths. These long chain polymer molecules in their natural state have a random orientation. When stretched, the molecules are pulled into a parallel alignment, thus creating a lightweight product with tensile strength comparable to steel. The finished polymer mining grid product is a viable alternative to conventional supplemental roof control meshes such as chain link fence and welded wire. Polymer mining grids are a fraction of the weight, have comparable load support qualities, and will not rust or corrode in even acidic or alkaline conditions. Specific flame retardant additives can also be incorporated to make the polymer grid self extinguishing for use in underground "gassy" environments.

Jan 1, 1992

By Nikolaos Polysos

Besides considering the given geomechanical conditions, the geological/geotechnical logging of drill cores and heading faces during the driving of parallel entries and the resulting rock classification form the basis of the assessment of the proneness to caving of the roof strata in the actual mining phase. Both types of separation planes, parallel to the stratification as well as intersecting the bedding, will be defined and assessed within the immediate roof strata depending on the lithological structure. These data may hold the potential to plan the winning and design its operational organization in a way that - to a major extent - future damage of the workings by increased caving or cavities can be predicted in advance. Following the general description of the separation plane structure, the combined influence of the petrographic and structural geological roof properties on the operating cycle of the planned working face will be presented and illustrated.

Jan 1, 2003

By Chris Haycocks

The relevant literature concerning ground control, mine planning and reserve conservation in multi- seam mining is reviewed. Factors affecting interaction, interaction mechanisms, and design guidelines are presented in detail. The impact of multi-seam mining on future mining, measures for ameliorating negative interaction effects, and future research needs are also discussed.

Jan 1, 1990

By Thomas M. Barczak

Wood cribs are used extensively to stabilize mine openings by providing resistance to deflections of the immediate roof and floor and by supporting the weight of unstable rock masses. While the unit costs of these supports are relatively low, their extensive use results in significant costs to coal mining. The Bureau of Mines has developed a model that computes the load capacity of wood cribs as a function of the displacement of the crib structure induced by mine roof and floor convergence. This permits comparison of the loading characteristics of various crib constructions and enables systems to be designed with consideration of the load conditions imposed by the mine environment. The design method matches the stiffness, strength, and stability of the crib structure with expected rock mass behavior to determine a crib design and employment spacing that will provide the lowest cost support.

Jan 1, 1993

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 J. Nielen van der Merwe

The effects of horizontal stress in coal mine roofs in South Africa did not receive the attention it deserved for several years, but now there is a danger that it may be over emphasized. A number of phenomena that are ascribed to elevated levels of horizontal stress, are shown to he explained by fundamental considerations. The stress concentration caused by the creation of an opening in rock and the beam effects in a stratified roof are briefly reviewed It is shown that guttering can be present without high horizontal stress, depending on the nature of the roof material. Three examples from practice are presented where horizontal stress was initially erroneously believed to be responsible for roof collapses A comprehensive program of roof displacement monitoring in South Africa yielded results that are compatible with gravity loading on roof beams without the assistance of horizontal stress. It is also shown that one of the diagrams that are used to explain guttering as a result of horizontal stress is misleading The over emphasis on horizontal stress may obscure the real causes of roof instability, resulting in ineffective remedial action to be taken by mine operators.

Jan 1, 2000

By Vladimir I. Klishin

Complex method of ruling of rock pressure is considered in conditions of rapid roof subsidence. The purpose of this method is elaboration of emergency devices into hydraulic props. Also method of unloading of solid roofs is worked out. An analysis of means of protecting hydraulic props of powered supports 6om dynamic loads made it possible to divide them into four classes according to principle of operation: emergency, operating hm an increase of pressure; emergency damping; emergency, operating according to the "plastic element" principle; inertial emergency, operating hm dynarmc loads. It was established that the overwhelming 'majority of designs of the first three classes of emergency devices (EDs) are based on !mditional approaches, which exhausted their possibiiities of high-speed response. Delay on their opening is caused mainly by inertia of the moving locking parts. In our opinion, the most prospective are EDs using the inertia of the moving locking parts for depressurizing the piston cavity of the hydraulic prop with a minimum delay (inertial EDs). The essence of the designs of EDs having an inertial principle of action is that into the hydraulic prop containing movable and immovable elements is additionally introduced a spring-operated contracting support, which is installed on the movable element. Under dynamic loading of such a hydraulic prop hm the roof, the support moves relative to the movable element, performing the role of an inertial mass, which leads to instantaneous depresswization of the piston cavity of the hydraulic prop. Technological measures for unloading of solid roofs consists in creating of long oriented cracks in rock massif. Description of method for creating cracks, demanding equipment and means for cone01 the progress of splits are included. Also technological schemes for realization of the method of saving underground mines and rock mass are included. The results of experimental realization of this method on the Kuzbass and Poland coal mines are also included.

Jan 1, 1997

By David H. Y. Tang

The reinforcement mechanism of suspension effect is analyzed based on beam-column theory. The equations of the maximum bending stress, deflection and transferred bolt load for the bolted roof strata are derived. In the analysis, the bolt load is assumed to be point load and the horizontal stress is uniformly applied to each stratum. The reinforcement mechanism of friction effect is also analyzed. The major function of roof bolting in this case is to create the friction¬al resistance by tensioning the roof bolts, thereby the individual thin strata are "welded" into one single thick stratum. Based on the results, equal spacing of roof bolts is used in the suspension effect while for friction effect, the bolts should be spaced based on equal shear force concept. The design nomographs for the proper bolting pattern and bolt ten¬sion of mechanical roof bolting systems are presented for practical applications.

Jan 1, 1984

By David Oyler

Unusual circumstances may require that a longwall retreat into or through a previously driven room. The operation can be completed successfully, but there have been a number of spectacular failures which exposed miners to serious roof fall hazards. To help determine what factors contribute to such failures, the National Institute for Occupational Safety and Health (NIOSH) compiled a comprehensive international database of 130 case histories. The cases include five failures where major rock falls occurred in front of the shields, and six even more serious failures involving major overburden weighting. This suggests two room failure mechanisms. The first is a roof fall type failure caused by loading of the immediate roof at the face as the fender narrows. The second is a weighting type failure caused by the inability of the roof to bridge the recovery room and face area, and affecting rock well above the immediate roof. The data indicate that the roof fall type of failure is less likely when intensive roof reinforcement (bolts, cables, and trusses) is employed together with higher-capacity shields. The overburden weighting failures, in contrast, occurred when the roof was weak and little standing support was used.

Jan 1, 1998

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

By Kot Unrug

The selection of tensioned versus non-tensioned roof bolts, for primary coal mine roof support, has been debated for well over 25 years. The wide spread use of fully grouted rebar marked the beginning of the discussion of the relative importance of bolt tension and the resultant immediate roof compression. The focus is on roof conditions encountered in coal mines however the discussion may be extended to hard rock mines as well. The theoretical bases for both systems are presented, with the analysis focused on the bolt's ability to actually fulfill the theoretical assumptions. Of specific concern are assumptions about bolt tension upon installation and the likelihood of maintaining useful bolt tension over time. The usefulness of tension roof bolting is limited by the mechanical interaction of roof bolt assembly and the rock, which may be valid at the time of installation only. Ambient mine atmosphere and fluctuation in moisture content changes the physical condition of rock, which may have a significant impact on the effectiveness and longevity of bolt tension. Changes in the material properties of the surface of the immediate roof can invalidate the assumptions, which were made in the design stage of roof support system. This means that the roof support in the long term excavations may be inadequate requiring costly rehabilitation. The presented arguments are illustrated with the examples of some relevant experimental data. Tension primary roof bolting is recommended where the immediate roof is: ? Stiff with a high modulus of elasticity ? Surface layers are continuous and fairly thin ? Not easily weathered

Jan 1, 2004

By I. Geldmacher

In-Seam Seismic (ISS) methods have been used extensively worldwide for the past 10 years to assist mine planning, espe¬cially longwall mining in coal seams. The technique is only starting to gain wide acceptance in North America where it is being applied in a much wider variety of applications than was initially considered. Surveys elsewhere have concentrated on the detection of faults (structures) in coal mines prior to longwall mining. The majority of surveys in North America has been the location of abandoned workings which were not adequately surveyed prior to closure. Modeling data based upon the lithological logs, geophysical logs and interpreted sections show whether the method will be successful prior to carrying out the survey. The surveys have been performed to locate abandoned wor¬kings from the surface and underground using both seismic reflection and transmission techniques. Data from coal seams in both Illinois and West Virginia will be presented. A second method where ISS surveys are being applied to detect old workings will be presented. The ISS tomography produces an image of the P-wave and Channel Wave velocities which is representative of the material through which the signal traveled thus identifying the location of the old workings. Data from Texas will serve as an example.

Jan 1, 1990

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 Z. C. Song

[THIS PAPER WAS TRANSLATED FROM CHINESE TO ENGLISH BY S. S. PENG, WEST VIRGINIA UNIVERSITY] Immediately after coal extraction, the pressure that causes the movement of the surrounding strata is called mine pressure. Through the strata movement, certain features such as supports taking load arise. This is called "manifestation of mine pressure". The existence of mine pressure is ab¬solute whereas its manifestation is relative and conditional. The visible movement of the surrounding strata occurs only when the applied pressure exceeds the plastic breaking strength. Me support begins to take load only when it offers resistance to the strata movement. The key to investigating mine pressure and its control hinges on understanding the rules and conditions for the manifestation of mine pressure. Since the face moves continuously, the mine pressure and its manifestations are also constantly changing. 'Therefore, it should not only emphasize to obtain its instantaneous magnitude, but more importantly the rules of development changes and their relations to the movement of overlying strata. Once this is resolved, one can estimate the movement of the overlying strata through mani¬festation of mine pressure and predict the time and magnitude of incoming roof weighting. Consequently the location and timing of rib excavation can be correctly selected and thus solve several major problems in the design of mining operation.

Jan 1, 1982

By Graham Daws

State of the Art Room and Pillar mining techniques have been employed at two coal mines in the UK in the last two years. This paper describes the design of pillars and rockbolted support systems for Room and Pillar mining operations in these collieres. Within the last year, rock mass classification systems have been used in the design process of a room and pillar mining layout at one of these collieries. In particular, the roofbolt support system was detailed using these techniques and the support system was linked with operational aspects such as depth of cut. The depth of cut was initially estimated using rock mass classification systems and then verified by a stand up trial which was monitored remotely. The stand up time has a direct influence on the operational aspects of the mine and this is described. The design was verified by performance monitoring and the results are discussed. The importance of understanding the role of rockbolting is emphasised along with some examples of accurately determining in-situ bolt performance. The paper also recommends some modifications to rock mass classification systems when applied to underground coal mining operations.

Jan 1, 1998