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By Frank E. Chase

Clay veins, also referred to as clay elastic dikes, have been responsible for numerous underground injuries and fatalities. These hazardous structures are also responsible for increased production costs during all phases of mining. For these reasons, the Bureau of Nines has investigated the physical characteristics of and roof instability problems associated with clay veins in order to make support and other mining recommendations. In addition the occurrence and origins of clay veins were examined to determine whether or not the structures could be projected into unmined portions of the coalbed. This investigation included portions of the Arkoma, Illinois, Northern Appalachian, Southern Appalachian, and Uarrior Coal Basins. Virtually a11 clay veins observed or documented occurred in the Illinois and Northern Appalachian Basins. Observed clay veins ranged in sire from 1 inch to 16 feet In cross-section and were found under shale, siltstone, sandstone, and limestone roof rock. Individual clay veins were predominantly composed of claystone; however, limestone. siltstone. and sandstone infilled clay veins do occur. Clay vein formation has been associated with the infilling of fissures which result from compressive, tensile, or shear ground failures. These gaping fissures were propagated by compactional processes and/or tectonic stresses active during and subsequent to coal iff cation. Associated fault, fracture, and slickenside planes commonly parallel clay veins and disrupt the lateral continuity of the immediate and, sometimes, main roof. When clay veins parallel or subparallel the direction of face advance the roof is segmented into cantilever beams causing unstable conditions. Therefore, the strata on either side of the clay vein must be bolted and strapped together to form a beam.

Jan 1, 1984

By L. J. Terrill

Lateral displacement in the immediate roof beam at the Waste Isolation Pilot Plant (WIPP) is a significant factor in assessment of excavation performance for the design of ground control systems. Information on roof beam lateral displacement, expansion, fracture formation, as well as excavation convergence, is gathered using a variety of manually and remotely read instruments. Visual observations are also used when possible. This paper describes the methods used to measure lateral displacement, or offset, at the WIPP. Offset magnitudes are determined by the degree of occlusion in drillholes that intersect the offset plane. The Borehole Lateral Displacement Sensor (BLDS) was developed for installation at WIPP to monitor offset at a high degree of accuracy at a short reading frequency. Offset measurements have historically been obtained by visual estimation of borehole occlusion. Use of the BLDS will enable relationships between time dependent roof beam lateral displacement and expansion to be established in much shorter periods than is possible using visual observations. The instrument will also allow remote monitoring of roof beam displacement in areas where visual estimations are not possible. Continued monitoring of roof beam displacement, convergence, and expansion, is integral to timely and pertinent assessments of WIPP excavation performance.

Jan 1, 1996

By G. M. Molinda

Ground Penetrating Radar (GPR) is being investigated for the potential to determine roof hazards in underground mines. GPR surveys were conducted at four field sites with accompanying ground truth in order to determine the value of GPR for roof hazard detection. The resolution of the current system allows detection of gross roof fractures (>63 cm (s1/4 in) zone) or rider beds in coal measure roof. Data quality is not yet sufficient to detect small bed separations or subtle lithologic changes in the roof. Differences in data quality are discussed, as well as suggestions for collecting improved data.

Jan 1, 1996

By Yajie Wang

High horizontal stress and its adverse effects on longwall gate entries have been witnessed in many US coal mines for the past years. It can cause cutter roofs and leads to roof falls either in the headgate or tailgate entries depending on the relative orientation of the panel to the maximum principal horizontal stress. Underground observations have shown that if the maximum principal horizontal stress is perpendicular to the panel entries, the roof is in the worst condition and if the maximum principal horizontal stress is parallel to the entries, the entry is in the best condition. However, little has been done about the high horizontal stress effects on panel entries subject to different orientations of high horizontal stress. In order to design a longwall panel and control the roof effectively, the redistribution and concentration of high horizontal stress field must be fully understood. Using 3-D finite element analysis, this paper presents the high horizontal stress redistribution and concentration for a retreat longwall face with different orientations of high horizontal stress. Based on the results, possible roof failures for different orientations of high horizontal stress are analyzed. A panel design criterion is recommended.

Jan 1, 1996

By Y. L. Sun

The feasibility of using the ultrasonic wave attenuation method for measuring in-situ stress is examined theoretically and experimentally. The theoretical analyses show that for a given rock type the relation between stress and attenuation coefficient ratio can be approximated closely by second-order polynomials. Combining the elastic and statistical theories, the magnitude and direction of the existing principal stresses can be predicted by using the relations between stress and attenuation coefficient ratio obtained in the calibration of core specimens. Several experiments are conducted in the laboratory on two types of rock. The experimental study consists of two parts: the attenuation-coefficient measurements in core specimens under uniaxial stress and the attenuation coefficient measurements in boreholes drilled through blocks of sandstone and limestone subjected to uniaxial compressional stress. Compressional waves are used exclusively in the measurements.

Jan 1, 1989

By V. Eu. Anufriev

Despite of significant cost advantages and the positive expierence roof bolting is limited up to 15-20% of all workings at the Cuzbass mines. The low level of application of the technology is the result of specific mining and geological conditions and defects of organization and technical support. When analysing mining and geological conditions of development working maintenance and geomechanical aspects the necessity of changing of technological schemes of preparation and extraction of mine sections is obvious. The prospects of large-scaled transferring from frame support to roof bolting are connected with introduction the technology of support with the help of non- expanding bolt classes including bolts with mineral filler fixation. The design of bolters with mineral filler fixation is given here. The results of testing of bolt load characteristics in artificial holes and in situ are proved the technology efficient. For increasing of drilling works efficiency it is necessary to master the production of oilless compressor and heading machines with the manipulator. For bolt dimension forecast it is recommended to use the relations of measured parameters of "dome" to roof shifts. Effective monitoring of rock pressure in the junctions is connected with the use of support of conventional bolts together with bolts of deep installation with simultaneous supporting of face ends with special mechanical sets, which compensate face length change.

Jan 1, 1996

Recent improvements are described in several techniques for ground control in underground mining. Promising methods for analysis and mitigation of rockbursts are discussed. Methods of assessing damage to underground excavations from repetitive seismic loading are reviewed. Analysis of mining-induced surface subsidence is discussed.

Jan 1, 1989

By Shosei Serata

The Stress Control Method improves productivity and safety in underground coal mining. The method stabilizes the roofs and floors of mine openings in both shallow and deep coal beds, regardless of whether or not there are ground stability problems. This paper describes the means by which a tapered pillar experiment is used to collect in situ data, which is then used as input for a computer model. The resulting computer model is then used to optimize Stress Control design for the mine under study. The behavior of any underground coal mine is not elastic, as is generally conceived in the conventional view of rock mechanics, but rather is highly time-dependent and site-specific. Unfortunately, conventional rock mechanics methods based on the elastic principle are not able to capture the highly non-elastic nature of ground behavior which actually occurs. To capture actual behavior, a new method of study is required. In this regard, geomechanical study of an underground coal mine may be compared to studying the behavior of a living fish. Its true behavior zannot be understood by examining it "dead in the laboratory. It Is the "live" nature of the coal mine that is required for design optimization of the Stress Control Method in the given mine. The Stress Control Method is based on a time-dependent theory of the global behavior of the mine ground, which generally contradicts the conventional elastic theory of ground behavior. Therefore, site-specific adaptation of the Stress Control Method required a comprehensive understanding of the global behavior of the mine as a function of time and site-specific ground conditions.

Jan 1, 1986

By Daniel W. H. Su

The importance of incorporating fundamental principles of rock material response and failure mechanics into a pillar strength design model has been demonstrated. To accurately assess pillar strength, a model should account not only for the characteristics of the coal, but also for those of the surrounding strata. The interactions between the pillar and the surrounding roof and floor dictate ultimate pillar strength. Confinement, or the lack thereof, provided by roof and floor frictional end-constraint is one of the most significant factors in the strength of very wide pillars. The best conditions for end-constraint occur with ideally strong roof and floor. In this paper, it will be shown that weak roof and floor may decrease the ultimate pillar strength by as much as 30% compared to ideal conditions. Field observations confirm this pillar strength reduction in the presence of weak roof and floor. Furthermore, rock partings within the coal have a variable effect on pillar strength, depending on the parting strength. A competent shale parting within the coal reduces the effective pillar height, thus increasing the ultimate pillar strength; a weak claystone parting slightly decreases the strength. Imposed load distribution also has a significant effect on pillar strength, such that asymmetric pillar loading, similar to that observed in pillars adjacent to high extraction mining, reduces the ultimate pillar strength below that of equivalent pillars subject to uniform load distributions.

Jan 1, 1997

By A. W. Khair

Laboratory studies indicate that the specimen size influences the compressive strength of coal significantly. In the past, the diameter/height ratio of coal and rock specimens with different geometry and its influence on failure characteristics was discussed (Khair, 1993; Khair, 1994; Khair and Xu, 1994). In the current attempt, tests are designed and carried out under uniaxial condition in the laboratory to investigate the influence of specimen size. Cubical specimens of coal with edge dimension ranging from 5 cm (2 in.) through 20 cm (8 in.) were prepared according to the standards and tested uniaxially. Results show that compressive strength of coal is dependent on specimen size until certain level and later the change in specimen size would not yield significant change in compressive strength. Cubical specimens were chosen for testing because they are not only in close resemblance to underground pillars, but they are also the most stable shape under loading. The failure characteristics of these specimens were analyzed and their applicability to pillar design aspects in coal mines is studied. A detailed analysis of the physical and mechanical properties of the tested coal seam is also presented.

Jan 1, 1996

By K. Unkug

It is commonly accepted that the tension of roof bolts can be used as a measure of the effectiveness of roof support performance in coal bearing strata. The conceptual base for the assumption originates from beam theory. Roof bolt tension acting across the bedding and other weakness planes increases friction, preventing potential separation and slippage within the roof strata and therefore, increases the stiffness of the roof beam between the open face and the rock-mass enclosed within the anchorage. Thus, it is important to monitor roof bolt tension, especially for research purposes, and consequently many measuring systems were developed based on numerous working principles. Research planned by the Department of Mining Engineering at the University of Kentucky required a large number of tension measuring devices of high precision. These were intended to operate reliably in corrosive mine environment over long periods of time. After evaluating the existing systems, it was decided to develop a new type of roof bolt dynamometer which would fulfill the above requirements at minimal Cost. Nearly one hundred such dynamometers were constructed, calibrated and installed in two underground mines. They remained in service for a period of one Year, producing a valuable database for the Project. In this paper the working principle of this new type of dynamometer will be described and technical considerations and samples of the collected data will be given.

Jan 1, 1986

By I. V. Baklashov

Geomechanical conditions of mining operation are determined by varying in time and space mechanical properties and initial state of the rock mass, by geological and technological factors. We define adaptive mining technology as a technology capable of adjusting to changing geomechanical conditions of mining operation. The ongoing research at Moscow State Mining University consisting of laboratory, theoretical, and field investigations aims at the development of theoretical basis and methodology of the system of geomechanical support of adaptive mining technology for the bedded salt deposit in the Urals, mined by room-and-pillar method. This system of geomechanical support is capable of - dynamically assessing mechanical properties and state of the rock mass varying within the panel; - mapping mechanical properties of the rock mass within the panel; - assessing and predicting mechanical condition of load-bearing elements of the rock mass and its changes due to the adjustment of the parameters of mining operation considering the in-situ geomechanical conditions.

Jan 1, 1996

By Najdat I. Aziz

There are different schools of thoughts about the influence of gas pressure on coal strength. Researchers have reached contradictory results with regard to the effect of confining gas pressure on tensile strength of coal. Accordingly, this paper re-examines the influence of gas pressure on the strength of coal. A specially designed precision drill capable of drilling in a pressurised chamber was used to drill coal samples under both atmospheric and increased gas pressure environment. The drill cuttings were examined for particle size distribution using a laser controlled Malvern Mastersizer S particle size analyser. Petrographic analysis of the sample examined the effect of gas pressures on the maceral structure and the composition of coal. The paper demonstrates that there is a definite variation in the range of particle size distribution with particles obtained from drilling coal samples not subjected to gas pressures, in comparison with those obtained from samples drilled under confined gas pressure. Also observed were the changes in the rate of drilling in soft coal obtained from geologically-disturbed zones compared with other coal samples. Some variation on the maceral compostion of coals were also observed in relation to increased gas pressures and gas type.

Jan 1, 1996

By M. J. Beus

This paper will describe the research approach being initiated by the Spokane Research Center to assess hazards and design criteria to prevent ground and muck falls in and around ore passes in hard-rock mines. The major goal of this research is to compile detailed information about ore pass processes and hazards, identification and testing of prevention strategies, and development of computer-based measurement and design methods. Recent ore pass failures have underlined the lack of OE pass design capabilities and standards available to both mine engineers and MSHA personnel. Design criteria and ore pass hang-up prevention and remediation strategies will include the effects of internal ore pass loads, blasting to relieve hang-ups, and rock mass stress redistribution and structural interaction from nearby mining. New visualization and three- dimensional analysis methods will be presented as an extension of existing ore pass design and analysis techniques. A sophisticated particle-flow analysis program will be used to simulate the response of various ore pass designs to a wide range of loading conditions driven by rock mass deformation, muck flow, ore pass blockage, and blasting in both nearby stopes and the ore pass. Analysis techniques will be calibrated and design criteria validated at the Spokane ore pass test facility. These tools will serve to reduce research costs by limiting the number of experimental field tests required.

Jan 1, 1996

By Floyd Varley

In deep and highly-stressed coal mining, advanced design practices are used to minimize the potential for damaging events to occur. Despite these design efforts, the hazard of coal mine bumps can not be completely eliminated. This paper will review current and historic work practices used internationally to minimize the hazard to miners from mining in conditions which could produce a bump. These site specific work practices have a common base in monitoring for conditions which could result in a bump and application of controls to prevent the occurrence or minimize the severity of an event where miners are working. Monitoring practices range from purely observational to semi quantitative. Control methods attempt to initiate events in a controlled manner, absorb the energy of a potential event or reduce the potential for or magnitude of an event by altering the stress conditions of the mine workings. Building from the experiences of other coal fields, an empirical framework to manage the potential hazard of coal mine bumps can be developed to enhance the level of safety provided by prudent mine design.

Jan 1, 2008

By Dave Bigby

Hatworth Colliery, operated by RJB Mining (UK) Ltd, is successfully mining the Deep Soft Seam at a depth of 1000m using single face retreat longwalling. The seam is characterised by a weak roof generally comprising 4 to 6m of well bedded carbonaceous mudstones overlain by weak siltstones. The paper describes the layout and support design strategies which have enabled the colliery to adopt sole support by rock reinforcement techniques for its longwall gateroads. To achieve this success the colliery layout has had to be radically modified to take account of the in-situ horizontal stress field. The layout is also closely controlled by environmental considerations, the seam being one of the gassiest in the UK Economic support by strata reinforcement has been achieved using a combination of rockbolting of the roof and sides installed at the face of the heading, and systematic double birdcage cable bolting installed on drivage and supplemented ahead of the retreating panel. The layout and support strategies have been developed through a combination of detailed underground measurement of support and stress parameters backed up by numerical modelling using FLAC. The FLAC modelling has paid particular attention to careful representation of bedding properties and validation through underground stress measurement.

Jan 1, 1996

By Peter (Yunqing) Zhang

Longwall recovery under weak roof conditions could have considerable delay if the immediate roof is weak and deteriorates during screening. A pre-driven longwall recovery room is a feasible solution to minimize the impact of longwall move on production. In this longwall recovery project, a 16-ft recovery room was pre-driven to cover screening, and the shearer cut another 16 ft outby to reach the final recovery line. The pre-driven recovery room was carefully monitored as the face was approaching it. The longwall face operated very well as it cut into the recovery room. The recovery process took about 13 days, and considerable time was saved compared with the previous panel. The support in pre-driven recovery room was designed to handle both front abutment pressure and weak immediate roof conditions. Pumpable standing support was used to support the cantilevered roof as the fender pillar narrowed and collapsed when the face cut into the recovery room. To ensure the stability of the recovery room, 3-D computer modeling was used to evaluate the support system. Abutment pressure, critical fender pillar width, bolting system, pumpable cribs and shield support were considered in the model. The recovery room was carefully monitored during longwall recovery. Roof-to-floor convergence and crib convergence were recorded every shift as the longwall face was approaching the recovery room. Roof, floor and pillar activities and crib performance were also observed as the face cut into the recovery room. This paper presents the support design for the recovery room, evaluation of the support system through computer modeling, and details of the monitoring results.

Jan 1, 2006

Historically, the selection of mining sequence for multiple seam conditions has been based on many factors other than ground control considerations and minimization of interaction effects. Today, numerous operations are finding themselves mining above existing or abandoned operations, some of which exhibit large scale caving, pressure arches and subsidence troughs. Development of design criteria for the upper seam that would alleviate damage due to lower seam interaction effects can contribute substantially to the safety and productivity of many mining operations. The primary objective of this re search has been to identify methods of improving ground control conditions in the upper seam when mining through either passive or active subsidence waves. These methods concentrate on improved ground control practices for the upper seam, rather than reducing the amount of subsidence in the upper seams by improved practices in the lower seam. Design factors such as pillar size, pillar and opening orientation relative to the subsidence trough, offsetting intersections, and different support methods have been studied for the upper seam in a variety of typical geologic environments. Analysis has demonstrated criteria for ameliorating negative interaction effects and facillitating design in the superincumbent seam.

Jan 1, 1986

By Abouzar Vakili

Longwall Top Coal Caving (LTCC), as the most attractive method for thick coal seams, has been suffering from insufficient geomechanical understanding for the past few years. Cavability without doubt is the most critical geomechanical issue associated with the top coal caving method. A discontinuum method has been used for comprehensive modeling of LTCC operation by using the Universal Distinct Element Code (UDEC). In addition Particle Flow Code (PFC) and Phase2 were used as support for the caving mechanics analysis. Although the previous researches of gravity flow in caving operations neglected the effect of continuous span expansion, it has been observed in this study that face advancement has a great impact on roof symmetrical behavior. The amount of symmetry fluctuates between different face distances. Horizontal displacement seems to be the best indication for roof symmetrical evaluations. Two critical caving conditions have been modeled and parameters such as horizontal displacement, caving angle and recovery rate have been compared thoroughly. Results show that unsymmetrical behavior on the roof is closely related to face stability.

Jan 1, 2007

By Lewis Martin

Spokane Research Center personnel are currently conducting research on cable bolts to assess their material and support properties, to provide design criteria for roof control systems under various types of underground mining conditions, and to provide background data for computer models of cable bolt supports. Laboratory studies of cable bolts made of conventional steel cables with Garford bulbs and collars resulted in recommendations on characteristics such as embedment length, hole diameter, cable bolt diameter, and configuration of cables. A completed series of laboratory tests has determined the minimum embedment length of resin required for resin-grouted, cable bolt ground support.

Jan 1, 1996