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By Dennis R. Dolinar

A room and pillar coal operation in central Pennsylvania was experiencing roof cutters and long running roof falls caused by high horizontal stresses. The roof conditions created hazards for the miners, and caused several production panels to he abandoned prematurely. The mine requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in applying the "advance and relieve" mining to reduce the affects of the horizontal stress during panel development. The "advance and relieve" plan that was developed called for removing a pillar on one side of the panel as it was being advanced, thus creating a cave. The caving then relieved a portion of the horizontal stress across the panel. Because the horizontal stress direction is key to the success of this method, stress mapping as well as mining experience was used to establish the direction of the horizontal stress. Subsequent field measurements provided an estimate of the stress magnitude. Three panels were mined using this technique, and good roof conditions were achieved in all these panels. Instrumentation was used to monitor the stress changes in the roof created by the cave. Stress relief of over 1.000 psi was measured 120 ft from the cave and to depths of 20 ft into the roof. The lateral extent of the stress relief zone across the panels appears to be about 400 to 500 ft. This case history has provided much insight into the practical application of the "advance and relieve" method discussed in this paper.

Jan 1, 2000

By Steve Signer

The San Juan Mine and the National Institute for Occupational Safety and Health conducted a study to measure how development mining effected bolt loads. Twelve fully grouted, instrumented roof bolts were installed at two three-way intersections as part of the standard bolting pattern. Newly developed miniature data acquisition systems (MIDAS) were used to measure bolt load changes during initial face advance and cross-cut breakthrough. The effects of cut placement and depth on roof bolt loads were studied This test showed how bolt loads increased at five positions along the bolt length during initial mining. Both entry advance and cross-cut breakthrough produced a similar percentage of increase in bolt loads. Geologic differences between the test sites were probably responsible for the differences in amounts of bolt loading. The test site with more top coal and a higher rock quality designation (RQD) had lower bolt loads.

Jan 1, 2004

By Q. F. Wang

Spontaneous combustion hazard of loose coal in the top-coal caving region of entry affects the safety of the entry and top- caving longwall face seriously. In this paper, by combining on- site temperature tests with lab tests and numerical simulation, the characteristics of microcirculation close to the top-coal caving region are analyzed. A model for the temperature fields of the microcirculation in loose coal is presented. Based on this model, the temperature distribution and possible self-ignition region can be predicted. Further, the heat and mass transfers in microcirculation are studied. Finally, the mechanism of oxygenation during the spontaneous combustion of microcirculation in loose coal of the top-coal caving region of entries is put forward. The research results in this papa may provide ideas for the prevention and putting out of the fire caused by spontaneous combustion in the caving zone.

Jan 1, 2004

By S. I. Al-Ameen

Monolithic packing systems have gained in popularity as a gate side packing method, However, there has been no detailed investigation to assess their underground strength behaviour. This work includes the laboratory and in situ investigation for strength behaviour determined on various monolithic packing materials namely Tekpak-XX, Hydropak-20, Hydropak-30, Astrapak-3R, Flashpak, Thyssen pack and Aquablends. A simple technique for assessing the pack strength was developed at the laboratory by using a penetrometer, of which penetration resistance can be directly correlated with the uniaxial strength of the pack. The in situ strength investigations were made on core samples obtained from three pipes inserted inside the pack, while the penetration test were made on the pack surface. The in situ results indicated that the packing material develops an inconsistent strength on various part of the pack which was mainly related to an inadequate mixing of grouts inside the pack.

Jan 1, 1992

By Lawrence R. Artler

The North American Coal Corporation, its subsidiaries and other coal companies, have been mining the Pittsburgh #8 seam in Eastern Ohio for several decades. Underground mining by the "room and pillar" method, and more recently by longwall methods, have been practiced. This presentation deals with ground control problems and subsequent solutions to those problems for both mining methods. But first, let me briefly discuss some pertinent points about the geologic conditions of the coal seam near the southwest edge of the Pittsburgh Coal Basin. The Eastern Ohio region of the Pittsburgh Coal Basin is situated along the western edge of the unglaciated Allegheny Plateau. The local terrain is quite steep, with topographic relief of 430-500 feet being common. The coal thickness in the region ranges from 36"to 120", with the average being 60" in the areas being currently mined. Regional dip of the seam is toward the southeast in an amount of 10 to 40 feet per mile. A cross-section of the mineable seam generally consists of the main bench (60"), a rider coal (6"-12"), and a claystone parting (6"-12") for a total excavated height of 6-7 feet. Greater excavated height occurs where the fireclay floor is so soft and friable that movement of mechanical equipment breaks up the first 4-8 inches. Generally, the main bench and rider coal thin and become more variable toward the southwestern portion of the mining area. The rider coal also grades laterally into a coaly carbonaceous shale. In those areas where the roof coal can no longer be counted on to help support the immediate top, some of the main seam (6") is left to protect the overlying 8'-12' of mudstone/claystone strata. These strata are virtually non-bedded, highly slickensided units and are "hung in suspension" by roof bolts from a more competent limestone stratum above. These mudstone/claystone units are very susceptible to moisture and thus to the weathering cycles. The rider coal or 6" of the main bench is left in place to "seal" the mudstone/claystone units in Mains and Submains development. For many years, it was thought that the strength of the limestone member was detrimental in attempts to fully extract the seam by conventional means; however, longwall methods that have been more recently employed have broken the "myth" that the Redstone Limestone prevents full extraction of pillars.

Jan 1, 1984

By Murali Gadde

Numerical simulation of cutter roof failure is a daunting coal mine excavation design issue. The complex progressive failure mechanisms associated with cutter failures are extremely difficult to replicate in computer models. However, this paper shows that with a proper selection of constitute behavior and implementation of cutting sequence, it is possible to realistically reproduce cutter failure using continuum models. Strain-softening material behavior and a systematic step-by-step model solving for a four-entry-and- three-crosscut model produced results that are similar to cutter patterns noticed in field. These results show that traditional modeling with geometries created in one model-step has limited applicability and may even provide misleading conclusions under some circumstances. Most importantly, the proposed modeling methodology provided some insights into the reasons for the inconsistent spatial distribution of cutters often noticed in coal mines. The modeling also revealed the usefulness of cutter mapping to determine the directions of in situ horizontal stresses.

Jan 1, 2005

By Jian Yong Chen

The results of a laboratory investigation of pullout tests conducted on grouted, plain cable bolts are reported. The main purpose of the study is to determine experimentally the shear bond stiffiess of cement grouted and resin grouted cable bolt systems, which is an important input parameter for numerical modeling of cable bolt support. Three nominal embedment lengths of 152mm (6 inch), 203mm (8 inch) and 254mm (10 inch) were used and two types of grout namely cement and Anchortite resin were used. Two water- to-cement ratios of 0.3 and 0.4 were employed for the cement grout. The concepts of effective shear stress and effective shear stiffiess are explained and adopted to calculate the shear bond strength and the elastic shear bond stiffiess per unit length. A comparison of results with previous research is presented to reveal the effect of water-to-cement ratio and embedment length on the shear bond characteristics of grouted cable bolts.

Jan 1, 2005

By Keith A. Heasley

Pillar recovery in deep coal mines with competent roof and floor can concentrate stresses and generate hazardous bumps. Actual calculation of the geologic strain energy released in association with these coal bumps has been rare, although energy release values have been utilized successfully for indicating burst potential in numerous hard- rock mines. This paper advances the current knowledge of energy release calculations as applied to coal bumps through an extensive analysis of an actual bump occurrence. The U.S.B.M, displacement- discontinuity code, MULSIM/NL, is used to produce a numerical model which generates stress, displacements and energy values associated with a pillar retreat section. This model is calibrated with actual field data, and then dissipated energy values from the model are examined as to their suit- ability for indicating bump potential. Ultimately, it is determined that prudent application of dissipated energy calculations can be used as a tool to investigate the coal bump potential of a mining plan or cut sequence.

Jan 1, 1990

By Laxminarayan Holla

There exist many formulae for designing coal pillars. However, when applied to a given set of mining parameters, they lead to different pillar sizes and factors of safety. From the subsidence point of view, the pillars designed as protection pillars not only have to be stable, but also have to limit subsidence over them to a level acceptable to the structure being protected. The subsidence over a pillar of given width to height ratio depends upon the mining depth. A pillar may be stable, but the subsidence over it may be quite large and unacceptable for the structure. In many cases even though pillars may yield, they support the undermined strata and reduce surface subsidence. In addition, the yielding pillars eliminate peaks and troughs in the surface subsidence profile and reduce ground strains. In a given situation, whether a pillar yields or not depends primarily upon its width in relation to mining depth.

Jan 1, 1992

By Daniel W. H. Su

Surface subsidence induced by multiple-panel coal extraction was calculated with finite element stress analysis. The use of nonlinear material behavior and GAP elements, which provide a realistic representation of shearing along existing planes of weakness, allows the finite element program to accurately reproduce observed subsidence profiles. A reduction factor of 1/6 is applied to the measured rock strength and modulus for use in the finite element model. The models satisfactorily predict a significant change in maximum subsidence and subsidence profile associated with an increase from subcritical to near critical panel width. Modeled shearing along planes of weakness is in good agreement with available time domain reflectometry field observations and is demon¬strated to have a very profound effect on the maximum subsidence and the shape of the subsidence profile. When both material nonlinearity and planes of weakness are incorporated, the finite element model appears to possess great potential for accurately predicting surface subsidence and subsurface strata deformation in environments where little subsidence information is available.

Jan 1, 1990

By S. M. Hsiung

A model (support selection model) for the selection of suitable types of powered supports under various geological conditions is proposed. This model is developed by evaluating the support, performance under different roof conditions at longwall faces in terms of five controlling parameters: (1) roof Falls, (2) front abutment pressure, (3) Face convergences, (4) gob materials entering into working area, and (5) roof caving line. This model suggests that more than one type of powered support. may be used under the same roof condition and the best selection should he based on the magnitude of suitability index of each support- type. The development and application of suitability index arcs discussed. Fourteen panels in seven coal seams are discussed in the paper to illustrate the applicability and effectiveness of this model. A roof classification system at longwall faces developed specifically for the construction of the selection model is also discussed. A simple example fur determining roof classification index is given.

Jan 1, 1988

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By P. M. Lin

The problem of abandoned mine land (AML) subsidence is getting more severe because abandoned mine lands which used to be located in the remote areas have been gradually developed into suburbs or even urban areas. Abandoned mine subsidence is also becoming more common than before because the time-dependent effect of exposed underground materials. These materials will deteriorate gradually and cause subsidence as time passes. Subsurface conditions of abandoned mines are seldom known to the public. Mine maps are not always available. If the mine mans are available, their accuracy are very - often questionable. In addition, the most common damages to surface structure and ground surface due to mine subsidence can also be created by other non-mining related geological hazards. These factors make the study of abandoned mine subsidence significantly complicated, tedious, and inconclusive. For instance, the cause, time, and amount of surface subsidence can be predicted with relative accuracy during active mining. However, it will be a considerable challenge to do so on abandoned mine subsidence. If abandoned mine subsidence can be predicted in advance, then the damages can be held to a minimum. However, this prediction technique is not yet well developed. Therefore, remedial measures require more attention. In order to have effective remedial measures, the real cause of the damage should be determined first. This paper describes the approach which was employed for the determination of abandoned mine subsidence. A site investigation was made first. According to the information obtained from the site investigation, a hypothesis was set up. Then the instrumentation was used to prove the hypothesis. By going through the procedures for several cases, a guideline which can be used by the inspectors to inspect abandoned mine subsidence will be established.

Jan 1, 1987

By Dennis Dolinar

"Tailgate entries in U.S. longwalls can be subjected to high loads and deformations. Therefore, tailgate entries are generally supported with varying types and amounts of standing support. However, even with standing support, roof falls and blockages can occur in the tailgate when the support system is not properly designed. Safe and efficient operation of a longwall face requires the selection of the appropriate type and amount of standing support. Because of the importance of maintaining a safe and functional tailgate entry, the National Institute for Occupational Safety and Health (NIOSH) is currently developing a standing support design methodology based on the ground reaction curve. Using the ground reaction curve for design requires that the ground and support interaction be quantified. This can be accomplished by the installation and analysis of data from instrumented field sites. Although necessary, the field results cannot develop the complete ground reaction curve. Numerical models, calibrated to the field results, are used to establish the complete ground reaction curve. In the present study, data was obtained and evaluated to establish points for calibration of the ground reaction curve for two western longwall operations.One test site, Mine A, was located at a depth of 1,550 ft. At this site, the floor consisted of siltstone, while the roof was an interbedded siltstone and sandstone. Because of the depth, a double row of22-in diameter CANs1 were installed to support the tailgate. Both the first and second longwall panels have mined past the instrumented site.The second site in Mine B is at a depth of only 760 ft. The floor at this mine consists of a weak shale/mudstone and the roof consists of thick, top coal overlain by weak shales. At this site, a double row of pumpable cementitious cribs was installed. The second test site has only been subjected to side abutment loading from first panel longwall mining.The measurements indicate that the support at both mines was heavily loaded when subjected to the side abutment loading from first panel mining. At the deeper site, the support began to yield when the face was more than 300 ft outby during second panel mining. In comparison to results from previous test sites in the Pittsburgh Seam and Illinois Basin, the amount of support loading and roof-to-floor convergence was an order of magnitude higher at the western sites. This has resulted in a significant difference in the position and shape of the ground reaction curves."

Jan 1, 2010

By Behdeen Oraee-Mirzamani

Tunnel stability has an important role in the production process of an underground coal mine. There are various methods for analysing tunnel stability, such as numerical methods and analytical methods. In this paper, numerical methods (Phase2 software) have been used to determine tunnel wall displacement in a mining tunnel of the Parvade underground coal mine. The Ground Reaction Curve has also been drawn using analytical methods to determine tunnel wall displacement. The comparison of results from the numerical method and the analytical method show a noteworthy difference in the tunnel wall displacement. The displacement calculated by the numerical method shows a lower value than that of the analytical method because the numerical method is more suited to modeling the various coal and rock layers, as well as the shape of the excavations found in coal mines. The methodology used in this paper together with the results obtained from both methods can serve as useful tools for the coal mine design engineer when determining the ground support requirements.

Jan 1, 2013

By Baisheng Zhang

The definition of Ultra-close multiple seams as defined by the floor disturbance depth, h, due to mining in the upper seam is proposed: it refers to those coal seams that are spaced very closely, i.e. h,, the distance between seams, is small or equal to h, , and mining of any one of the seams will significantly affect the others. Those seams are called the ultra-close multiple seam group. Theoretical formula for defining it and practical examples are given in this paper. Underground measurements and numerical analysis show that in ultra-close multiple seam mining, the characteristics of ground pressure due to lower seam mining is different from that of single seam mining. The major differences is that the roof, as a result of damaged caused by the upper seam mining, is full of factures; the roof broke up in the belt entry and fell down easily; support loading at the face was small without clear periodic weighting phenomenon. Based on these results the roof structure model and roof control theory for ultra-close multiple-seam are established.

Jan 1, 2005

At any given lime, the tidal force subjects massive regions to a spatially uniform load. The deformation of any large structure due to these forces is proportional to the elastic properties of the structure. In this aspect, the tidal force applies a load in a manner similar to that of a large-scale testing machine. Any anomalies in the measured tidal response must reflect alterations in the average elastic response of the massive structure. In the present work, tidal tilt measurements obtained in the wall of a large dam are used to infer changes in the response of the dam to applied loads. In particular, it is found that the dam response has changed by approximately 13 percent, on average, for two monitoring periods separated by 5 years. It is possible that these changes were produced by the different volumes of water in the dam for the monitoring periods. The tidal measurement technique can also be applied to monitor changes in the average structural response of large underground and surface masses relevant to coal and metal mines.

Jan 1, 1992

Roof conditions at Southern Colliery, the second Iongwall mine to be established at German Creek, were found to be much stronger and more massive than at the earlier longwall mine on the lease. Investigations were made to ascertain the level of face support required for longwall extraction under the changed geological conditions. Geotechnical information was provided from drillholes and highwall exposures. Downhole geophysical logs, notably the sonic velocity log, were used to obtain a distribution of the major lithological units over the mine area. A representative type section of the strata and geomechanical properties was compiled to form the basis for numerical modelling. Two-dimensional finite element methods were used to predict strata movements and chock loads under cantilevered roof conditions. The model incorporated a 6.3 m thick roof beam either bonded or detached, with either a 2 m or 5 m overhang. The solid rib, immediate roof, chock support and goaf were modelled as distinct parts, in addition a front abutment stress profile, derived from three-dimensional displacement-discontinuity analysis, was superimposed on the model.

Jan 1, 1992

By T. N. Singh

With a view to producing increased target output every year, Indian coal mining industry is fast moving towards mechanisation but the present approach of mechanisation suffers from lack of standardisation, mainly due to nonavailability of the cutting data for various machine/rock combinations. The coal seams of the country are relatively hard in nature and the coaling machines are reported to be affecting production and productivity of the coal mine and the very trend of mechanisation. This paper presents the result of extensive study performed to visualise the cutting characteristics of coal seams of compressive strength ranging from 20 MPa to 65 MPa. In a specially designed drag bit type coal plough rig, continuous recording of cutting forces was done with the help of a microprocessor based data logger and the specific energy of cuttability was evaluated at different depth of cut and speed of cutting. The geomechanical properties such as compressive strength, orientation and frequency of cleats etc. and different indices of coal seams were measured in the laboratory and field and relationship was developed for easy determination of specific energy.

Jan 1, 1992

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995