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Case study evaluation of geological influences impacting mining conditions at a West Virginia longwall mine
International Journal of Coal Geology 41 Ž1999. 51–71
Case study evaluation of geological influences impacting mining conditions at a West Virginia longwall mine K. Scott Keim ) , Marshall S. Miller Marshall Miller & Associates, P.O. Box 848, Bluefield, VA 24605, USA Received 15 May 1998; accepted 1 December 1998
Abstract Longwall development in a West Virginia coal mine was terminated due to severe geologic conditions. The combination of transition-type roof Žcomprised of interbedded sandstone and shale., excessive in-situ stresses, and low overburden zones caused massive roof falls. Additionally, both reduced seam thickness, caused by sandstone channels, and relatively weak floor impeded seam development. These factors were mapped using both exploration data and information obtained during mine inspections. Historical mine data was also analyzed to determine if factors such as thickness of overburden and mining orientation had an influence on mining conditions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: mining; roof fall; stress
1. Introduction This paper presents a case study of a geological analysis of multiple parameters impacting development of a northern West Virginia longwall coal mine. This study includes analysis of existing mining conditions and available geological data for the purpose of determining preferred areas for future development and to improve overall mining productivity. Critical factors analyzed include overburden thickness, orientation of roof falls, a digital geological model, and the influence of horizontal stress.
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0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 9 . 0 0 0 1 1 - 7
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The mine analyzed in this case study is located in northern West Virginia and had an established history of longwall development. The mine is located in the Lower Kittanning seam of the Allegheny Formation. The Lower Kittanning seam has an average height of 4 to 5 ft Ž1.2 to 1.5 m. on the subject property. A first phase of mining commenced in the mid-1970s and was primarily limited to the western portion of the property where reserves were nearing depletion. Typically, the mine produced 2.5 to 3.0 million tons per year. A second phase of mine development within the eastern portion of the property consisted of initial main entry and gateroad development for the first longwall panels. During this initial development in the second phase area, unstable roof conditions were encountered in the No. 1, No. 2, and No. 3 Right Entries. Relatively weak roof rock, a sandstone paleochannel, and excessive horizontal stresses all contributed to poor mining conditions. Historical data show the areas with sandstone immediate and main roof are typically several hundred feet Ž; 100 m. in width. The No. 3 Right Entry experienced a severe roof fall. The roof fall extended 800 ft Ž; 245 m. outby the face on the belt entry. For purposes of presentation, the overall mine area has been divided into two areas—western and eastern map areas. The western map area includes the previous initial deep mine and provides a historical perspective of geological mining conditions. The second phase eastern map area has undergone initial development and represents the future of the mining operation.
2. Western map area— (first-phase mining) First-phase mining was initiated in the western map area during the mid-1970s. Continuous miner sections were used to develop panels for the longwall unit. Longwall panels approached a width of 900 ft Ž; 275 m. and varied in length between 5000 and 12,000 ft Ž1525 and 3660 m.. The extensive mining in the western map area provided the basis for relating historical data and geological variances to assist with the preparation of a predictive geological model. Following is an analysis of recorded roof falls within the western map area. The roof falls have been examined with respect to overburden thickness, orientation and possible influence of in-situ stresses, and variations in roof lithologies. 2.1. OÕerburden thickness and frequency of roof falls Overburden thicknesses within the western map area vary from 0 to 650 ft Ž0 to 200 m.. Outcrop is limited to the extreme southern portion of the area where the seam was accessed ŽFig. 1.. Throughout the remainder of the reserve area, the seam is below drainage. Areas where overburden thickness varies between 150 and 250 ft Ž45 and 75 m. coincide with stream valleys and are common throughout the mined area. Historically, overburden thickness has been correlated with roof fall frequency. Approximately 183 roof falls were identified on mine maps, of which 60% are located in areas with less than 250 ft Ž76 m. of overburden ŽFig. 1.. The significance of this correlation is
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Fig. 1. Overburden map and roof fall locations.
strengthened by the consideration that only 28% of the mine is located in areas with less than 250 ft Ž76 m. of overburden. Thus, 28% of the mine Žless than 250 ft w76 mx of overburden. accounts for 60% of the roof falls ŽFigs. 1–4.. There is also a correlation between length of roof falls and overburden thickness. Approximately 20 roof falls in the western map area exceeded 100 ft Ž30 m. in length.
Fig. 2. Roof falls by overburden thickness.
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Fig. 3. Roof falls by overburden thickness and mined area.
Fig. 4. Roof falls per acre categorized by overburden thickness.
Fig. 5. Falls greater than 100 ft Ž30 m. in length categorized by overburden thickness.
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Thirteen falls of more than 100 ft Ž30 m. were located in areas delineated by less than 250 ft Ž76 m. of overburden ŽFig. 5.. 2.2. Orientation of roof falls Roof fall direction was analyzed for an indication of horizontal stress fields and maximum horizontal stress orientation. Typically, horizontal stress fields are biaxial with a maximum and minimum horizontal stress ŽAggson, 1979; Mucho and Mark, 1994.. The maximum horizontal stress Ž s H. can be much greater than the minimum horizontal stress Ž s h.. Consequently, entries oriented parallel or nearly parallel to the minimum horizontal stress Ž s h. are subject to more stress-induced damage than entries oriented parallel to the maximum horizontal stress Ž s H.. The orientation of 183 roof falls obtained from mine maps were tabulated and input into a spreadsheet. Of this total, 99 falls were oriented N358E, 54 were oriented N558W, one was oriented north–south, and the orientation of 29 falls could not be determined ŽFig. 6.. In the western map area, approximately 60% of the mine entries, including mains, submains and gate entries, are oriented in a N558W direction; whereas, 30% of the entries are oriented N358E. Roof falls were tabulated by both entry and fall orientation to determine if a correlation existed between mining direction and roof fall frequency. In the N558W heading, a nearly equal number of roof falls were observed to be parallel and perpendicular to mining direction. In the N358E heading, 80% of the roof falls parallel the entry directions ŽFig. 7A and B.. This data indicates roof falls have had a greater tendency to occur in both N358E entries and crosscuts as compared to a N558W direction. This directional control, in turn, coincides with regional horizontal stress fields indicating a maximum horizontal stress of east to west.
Fig. 6. Orientation of roof falls.
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Fig. 7. ŽA. Falls in the N558E heading. ŽB. Falls in the N358E heading.
The influence of regional horizontal stresses is well documented in the general project area. Conversations with U.S. Bureau of Mines ŽUSBM. personnel confirmed that severe horizontal stress influenced an adjacent mine. Overcore measurements conducted at the adjacent mine documented maximum horizontal stress oriented N668E while detailed mapping of approximately 6 miles of roof cutters by the USBM defined a stress field oriented approximately east–west. The nearly east–west maximum horizontal stress field caused damage to entries approaching a north–south orientation. Experience has shown entry alignment parallel or close to the major principal stress direction Ž s H. results in better or more favorable conditions compared to entry alignment close to the minor principal stress Ž s h. direction ŽMark, 1991.. Based on published data, the major principal stress direction Ž s H. is most likely east–west ŽMark and Mucho, 1994.. This is confirmed by the orientation of roof falls at the case study mine. The majority of falls at the case study mine occur in the N358E headings, which begin to approach the apparent north–south minimum stress direction Ž s h.. Roof falls in the N558W headings are less common. The N558W headings are rotated toward a more favorable mining direction Žcloser to the maximum principal stress..
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Fig. 8. Total roof falls per acre categorized by roof type.
2.3. Lithologic Õariations Drill hole data was sufficient in approximately 50% of the western map area to delineate roof lithologies and trends. Three categories, defined by variations in roof lithologies, have been mapped and are summarized as follows: Ø Sandstone-influenced roof consists of sandstone situated 0 to 12 ft Ž0 to 3.7 m. above the top of the seam. The sandstone has commonly been described as interbedded with shale. Ø Shale immediate roof consists of at least 12 ft Ž3.7 m. of shale, which may be overlain by sandstone. The shale may contain occasional thin, lenticular sandstone interbeds. Ø Shale-dominated roof is typically 18 to 24 ft Ž5.5 to 7.3 m. thick with no significant sandstone bodies within 24 ft Ž7.3 m. of the coal seam.
Fig. 9. Roof falls per acre categorized by roof type in areas with less than 250 ft Ž76 m. of overburden.
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Fig. 10. Roof falls per acre categorized by roof type in areas with greater than 250 ft Ž76 m. of overburden.
Core descriptions in the western map area were general and obtained from drillers’ logs. Consequently, geotechnical parameters and detailed geologic descriptions were not available to assist in roof mapping and roof rock characterization. When the frequency of documented roof falls by roof type are mapped, the sandstone-influenced roof areas have the highest frequency of roof falls. This is most likely attributable to shale interbeds and separation or delamination of the roof rock at the sandstone and shale contacts. The frequency of roof falls per acre decreases as shale becomes the dominant roof lithology ŽFig. 8.. The frequency of roof falls categorized by both roof type and overburden depth exhibits similar trends ŽFigs. 8–10.. 2.4. Western map area summary There is a correlation in the western map area between roof fall frequency and overburden thickness of less than 250 ft Ž76 m., when mining in a direction approaching north–south, and when coarser-grained roof lithologies are encountered. The correlation of increased roof fall frequency and the presence of sandstone in the immediate roof are somewhat unexpected but most likely attributable to relatively thin bedding and shale interbeds. Laminated rock Žalso referred to as stack rock. is susceptible to horizontal stress influences. Additionally, the area of sandstone immediate roof Žsituated within 2 ft w0.6 mx of the top of the mined seam. is relatively limited. Adjacent to this zone, the sandstone is separating from the seam resulting in several ft Ž; 0.8 m. of immediate shale roof overlain by sandstone. These areas commonly contain slickensided or fractured immediate roof caused by differential compaction rates between sandstone and shale during lithification. 3. Eastern map area— (second-phase mining) Relatively limited mine development has occurred in the eastern map area. An East Mains and No. 1, No. 2, and No. 3 Right Gate Entries were being developed ŽFig. 11..
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Fig. 11. Mine development east of the zone of disturbance.
3.1. Mine inspection Marshall Miller & Associates ŽMM & A. geologists inspected portions of the mine within the eastern map area. During these inspections, MM & A geologists observed weak roof rock associated with a paleochannel system and horizontal stress-induced damage, causing unstable roof conditions within No. 1, No. 2, and No. 3 Right Entries. A sandstone paleochannel approximately 200 to 400 ft Ž60 to 120 m. wide was observed in both No. 1 and No. 2 Right. Seam thickness was reduced to less than 1.0 foot Ž0.3 m. in the central portion of the channel area. Weak roof rock, resulting from interlayering of sandstone and shale Žoverbank. deposits, and differential compaction features Žslickensides. were observed adjacent to the channel area. The relatively weak ‘stack rock’ roof consists of thinly laminated, weak shale and distinct, hard sandstone and siltstone interbeds. Sandstone and siltstone interbeds are thicker and more abundant in the direction of the sandstone channel. Slickensides were commonly observed in shale roof underlying the channel. The interbedded shale and sandstone strata tend to separate parallel to bedding planes. Moreover, the first 2 to 4 ft Ž0.6 to 1.2 m. of immediate shale roof exhibited fracture planes parallel to and at the roof-pillar intersection near and adjacent to the sandstone channel area. Cutter roof and roof ‘potting’ Ži.e., oval-shaped roof falls or breaks of roof less than the roof bolt height as defined by Mucho and Mark Ž1994.. were observed at several locations. These features appear more severe within an area 250 ft Ž76 m. on either side of paleochannel influence ŽFigs. 12 and 13.. In No. 1 Right, the seam was displaced in a downward direction approximately the thickness of the coal seam ŽFig. 13.. In No. 3 Right belt entry, severe roof falls extended approximately 800 ft Ž245 m. outby the face. Fall propagation was both north and south from a point approximately 400 ft Ž122 m. outby the face Žbetween No. 12 and No. 13 cross-cuts.. Laminated sandstone was observed and became more prevalent southward. Trends based on both drill hole and mine data indicate only the north side of the paleochannel area of influence was penetrated by the mine entries. Several hundred feet Ž100 m. of poten-
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Fig. 12. Geologic and mine conditions in East Mains and No. 1, No. 2, and No. 3 right.
tially difficult conditions may remain as entries develop southward before the zone of paleochannel influence is transected ŽFig. 12.. Approximately one month after the 800-ft Ž245-m. roof fall in the No. 3 Right occurred, floor heave began in the middle entry of No. 3 Right ŽFig. 12.. Floor heaverfracturing is another indication of excessive horizontal stress. Although the roof fall resulted in stress relief of the roof rock, the floor remained susceptible to stress damage.
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Fig. 13. Cross sectional view of the channel area.
USBM engineers mapped portions of the East Mains and No. 2 Right. Portions of the USBM report ŽMucho, 1995. are summarized as follows: During our underground visit, we meandered through the openings in 2 RT and to some extent the associated East Mains doing cursory stress mapping. This technique is described in detail in the stress mapping paper. Stress mapping, by identifying and determining the direction of certain horizontal stress features, permits the existence of horizontal stress damage to be verified and determines the major principal stress direction of the horizontal stress field. Our results were that we saw moderate to fairly severe damage in areas of 2 RT and near the 2 RT area of the East Mains. The presence of these features and the directionality of failures indicated that horizontal stress was relatively high related to the lateral strength of the roof rock, especially in the areas associated with stream valleys. The general, directionality of failures, Žentries in 2 RT and crosscuts in East Mains. and the compass measurement of failures Žespecially the roof potting features. were approximately N208E ŽS208W. indicating a stress field direction of S708E ŽN708W. ŽFig. 14.. Implications of the above determination and known control techniques were discussed underground and at a follow-up meeting with management, and Marshall Miller
Fig. 14. Maximum horizontal stress determined by USBM engineers by stress mapping techniques.
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& Associates representatives were present. Horizontal stress control techniques discussed included: 1. Orientation of opening: Openings aligned, or close to, the direction of the major principal stress Žapproximately E–W. will have better conditions than those openings aligned perpendicular to the stress field Žapproximately N–S.. A change in orientation is a viable and effective control technique, especially for the long-term. Conditions observed in 2 RT, as well as, cursory examinations of topography over the property Žstress valleys-especially N–S trending. and geology Žsusceptible to horizontal stress damage. seemed to indicate that horizontal stress damage would be a periodic event and severe enough to justify reorientation considerations.. 2. Roof rock: While no specific measurements or testing was done of the immediate roof, the exposures and the general conditions were examined for abnormalities, lithology, etc. The USBM’s CMRR wCoal Mine Roof Ratingx was not done but it was apparent that the rating would be fairly low Žhigh 30’s or in the 40’s. due to the laminations of the shale and the interbedding of the shale and sandstone making it susceptible to horizontal stress. USBM research has noted that immediate roof rock that is especially weak in the lateral direction Žusually due to weak bedding andror interfaces. is susceptible to the effects of horizontal stress. The geology seems to be unusually variable caused, in part, by the channel sandstone meandering across this section of the property. The reader is referred to Molinda and Mark Ž1994. for information regarding the Coal Mine Roof Rating ŽCMRR. system. Recent research shows the worst mining conditions during entry development occur when the angle between the maximum horizontal stress and the direction of entry development is about 758 ŽChen and Peng, 1998.. As shown in Fig. 14, initial entry development was oriented northrsouth, which approaches the potential worst case 758 angle from the maximum stress direction. Four roof boreholes were examined with a flexible fiber-optic stratoscope by a roof bolt manufacturer. The immediate roof consisted of 3 ft Ž0.9 m. of laminated black shale overlain by 3 to 4 ft Ž0.9 to 1.2 m. of fine-grained laminated sandstone. Above these two layers, the strata were comprised of alternating layers of laminated shale, sandstone, and sandy shale. A distinct pattern of east–west lateral strata movement of 1r8 to 1r4 in. Ž0.3 to 0.6 cm. in each stratascope borehole was observed. Additionally, it was reported that some previously drilled 1-in. Ž2.5-cm. holes were completely closed. Roof layers will move in the direction of maximum horizontal stress Ž s H.. Movement occurs along failed laminations. The stratascope observations are indicative of a maximum east–west horizontal stress direction. In summary, mine inspections indicate that mine development in the eastern reserve area has been impeded by the interaction of the following geologic conditions: Ø A sandstone-filled paleochannel eroded a portion of the seam. Ø Relatively weak roof rock resulting from interlayering sandstone and shale is associated with the paleochannel. Ø The relatively weak roof rock consists of thinly laminated, weak shale and distinct, hard sandstone and siltstone interbeds. This laminated rock, referred to as stack rock by miners, is susceptible to the influences of excessive horizontal stress.
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Ø Slickensides are common adjacent to the margins of the sandstone paleochannel intrusion into the coal seam. These features result from differential compaction, which weakens the roof. Ø Horizontal stresses cause compressional-type roof failures Žcutter roof.. The cutter roof caused roof falls approaching 800 ft Ž245 m. in length. Ø The orientation of maximum stress Ž s H. appears to be N708W. Damage in entries oriented nearly perpendicular to the maximum stress is severe. Ø Mining direction is north–south, which is nearly perpendicular to the maximum stress. The preferred mining orientation is rotated towards an east–west direction Žparallel to the maximum stress.. Ø Severe roof conditions are known to occur under stream valleys at shallow overburden thicknesses. Ø Floor heave resulted from excessive horizontal stresses.
4. Future reserve area Future reserve potential is limited to the eastern map area. Access to this area is by the East Mains, which are located east of the zone of disturbance. An estimated 20 to 30 million tons of coal have been estimated as recoverable in this area. 4.1. Seam thickness The mineable seam thickness is regionally consistent throughout the reserve area scheduled for future development. Seam thickness varies between 4 and 5 ft Ž1.2 and 1.5 m., with the exception of the sandstone channel area in No. 1 and No. 2 Right and a zone of reduced thickness. These areas of seam disturbance appear to be limited in extent and are difficult to detect by exploration drilling. 4.2. Roof lithologies Three different roof types have been delineated in the future reserve area, based on both exploration data and mine observation. These three roof types are summarized as follows. Where data is not sufficient to predict the location of channel areas accurately, the width of the channel is increased on the maps to show general trends. 4.2.1. Sandstone immediate and main roof Historical mine data shows these areas are typically several hundred feet Ž; 100 m. in width. In the central portion of this zone, the sandstone appears to be medium-grained and massive and may erode portions of the coal seam. However, the sandstone appears to change laterally into thinly bedded sandstone with numerous shale interbeds ŽFig. 15.. The projections of the sandstone channel areas ŽFig. 16. are difficult to define accurately due to their limited width and limited density of exploration drilling.
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Fig. 15. Massive sandstone typical of the central portion of the channel area Žhole M95-10..
4.2.2. Transition zones These zones contain areas where finer-grained rocks Žshale and siltstone. varying between 2 and 12 ft Ž0.6 and 3.7 m. thick are overlain by sandstone ŽFig. 17.. Historically, these areas impede development. The shale and siltstone are typically slickensided and fractured, and the overlying sandstone is thinly bedded and commonly contains thin shale interbeds ŽFig. 18..
Fig. 16. Projection of sandstone channel areas.
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Fig. 17. Projection of transition roof area.
The combined effect of the laminated nature of the transition roof areas and possible slickensiderfracture development make these areas highly susceptible to the influence of horizontal stresses. These areas will most likely continue to exhibit very poor roof conditions, especially if mining parallels the minimum stress direction Žnorth–south..
Fig. 18. Transition-type roof and shale immediate roof Žhole M95-16..
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Fig. 19. Shale roof.
4.2.3. Shale roof Thick accumulation of shale with occasional to sparse sandstone interbeds and laminations dominate shale roof areas ŽFig. 19.. The overall extent of the shale roof area is defined by somewhat sparse data ŽFig. 20.. Additional exploration may alter the projection of shale roof Žespecially in the eastern portion of the mapped areas..
Fig. 20. Projection of shale roof areas.
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Historical data from the western map area indicates that shale roof areas had the most favorable conditions for development. However, if the shale roof is thinly laminated, horizontal stress could adversely influence conditions. Laminated shale roof has been documented as being very susceptible to horizontal stress damage. If the shales are laminated, the horizontal rock strength is reduced by low-cohesion bedding planes. The bedding characteristics of the shale roof areas should be carefully documented in any follow-up exploration program. The spacing of discontinuities Žnaturally occurring fractures. in the rock core from 14 drill holes were recorded. The fracture spacing and associated rock quality designation ŽRQD. measurements show sandstone core contains the least amount of fractures; whereas, core from transition areas contains the highest frequency of fractures. This data coincides with in-mine observations. Transition roof areas have historically proven to be difficult to mine and are more susceptible to horizontal stress influences. The conditions in sandstone roof areas should be favorable except where the sandstone is laminated andror interbedded with shale. Available data indicate the relatively competent sandstone roof zones result from sandstone channels that are several hundred feet Ž; 100 m. in width and locally erode a portion of the underlying seam. Adjacent to the central portion of the channel, the sandstone is interbedded with shale and quickly grades into transition-type roof. 4.3. OÕerburden Overburden thickness varies from less than 200 ft Ž60 m. to in excess of 600 ft Ž180 m.. Areas with less than 250 ft Ž76 m. have proven to be troublesome in past development. Regionally, difficult mining conditions have been associated with thinner overburden zones in north–south trending stream valleys. The north–south valleys are oriented at a large angle to the maximum horizontal stress direction and are subject to high shear stresses that can lead to compressive failure. The majority of the low overburden zones Žless than 250 ft w76 mx. are rotated towards a north–south direction. 4.4. Regional seam structure Regional seam structure contour trends show the axis of the Chestnut Ridge Anticline coincides with the east area reserve boundary. West of the Chestnut Ridge Anticline axis, the strike angle rotates to an east–west direction. The Ligonier Syncline is located south and east of the project area. The rotation of strike influences the north–south mining configuration east of the Chestnut Ridge Anticline. Panel development in a south-to-north direction will keep water away from the longwall face. However, as discussed in the previous sections, the preferred mining direction is east–west. The rotation of strike direction and presence of anticlines and synclines adjacent to the mining area may have an influence on horizontal stresses. USBM researchers have indicated there may be a relationship with enhanced horizontal stress influences near the axis of synclinal features. However, the current stress fields do not appear to be affected by folding on a regional scale.
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4.5. Horizontal stress Horizontal stress is a factor within the mine area. Compressional-type roof failure Žcutter roof. caused difficult to severe conditions in both No. 2 and No. 3 Right Gate Entries. Additionally, floor heave was observed in No. 3 Right and was interpreted to result from excessive horizontal stress. It is well established that horizontal stresses are typically biaxial ŽMucho and Mark, 1994.. It is also generally understood that ground damage will be minimized when the direction of drivage approaches the major principal horizontal stress direction. Recent research shows that during entry development, the entry system will be in the worst condition if the angle between the maximum and minimum stress and the direction of entry development is about 758 ŽChen and Peng, 1998.. Researchers have determined the maximum stress magnitude is typically 1.5 to 2 times the minimum stress magnitude ŽMucho and Mark, 1994.. The USBM compiled underground coal mine in-situ stress measurements for 25 eastern mines and determined the average maximum-to-minimum stress ratio to equal 1.95:1 for these mines ŽMucho and Mark, 1994.. USBM publications have documented that the direction of compressional-type failure is in the direction of the minor principal horizontal stress Ž908 to the major principal stress. ŽMark and Mucho, 1994.. In other words, where permitted, major features Žcutters and floor heave. will try to be aligned in the direction of the minor principal stress, which is perpendicular to the major principal stress. The USBM reports that roof potting and shear failure will exhibit this trend at all times. Researchers have determined a fairly uniform east-northeast regional horizontal stress field, although local flexures are common ŽMucho and Mark, 1994.. Near-surface and coal mine measurements have confirmed the regional directionality of horizontal stress fields.
Fig. 21. Maximum and minimum stress direction.
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During in-mine stress mapping, USBM personnel observed the location and orientation of cutter roof and features such as roof potting. The orientation of these features was north–south with an apparent migration to N208E. This observation, based on limited area mapping, indicates a maximum stress orientation of approximately N708W ŽFig. 21. ŽMucho, 1995.. As previously stated, compressional-type failures will align to the minimum stress orientation Ž s h., which appears to be N208E. Consequently, development in this direction will be more susceptible to horizontal stress influences. Correspondingly, mining parallel to the maximum stress Ž s H. direction will minimize the impact of horizontal stress. 4.6. Composite roof mapping Fig. 22 shows composite roof conditions, including the presence of sandstone channels in the immediate roof, transition zones, and areas with less than 250 ft Ž76 m. of overburden. Difficult roof conditions are projected where sandstone is proximal to the mineable seam. The combined impact of paleochannels, laminated sandstone, excessive
Fig. 22. Hazard projection map.
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horizontal stresses, and low overburden zones in north–south valleys severely impeded development in No. 1 through No. 3 Right Gate Entries. Similar conditions are defined by available exploration data south and east of existing mining. The projected sandstone channel and associated transition zones are currently defined by somewhat sparse exploration data Žespecially in the eastern portion of the map area.. Available data indicate these features exhibit a north–south trend. Historical information and the orientation of horizontal stresses indicate mining in a north–south direction is subject to compressional-type roof failure in the project area. Relatively weak roof rock and thinly bedded roof common in transition zones are more susceptible to stress-induced damage. 4.7. Floor Drill hole data shows the Lower Kittanning seam floor is typically comprised of relatively soft shale with claystone interbeds. The claystone interbeds may exceed 4 ft Ž1.2 m. in thickness. Areas where the claystone is thick Žgreater than 4 ft w1.2 mx. have been susceptible to floor heaving. Floor heaving is generally associated with relatively weak floor members that are dominated by clay lithologies. Plastic flow or heaving is also caused by high in-situ horizontal stress. In past development, compressive forces generated by horizontal stress caused the floor to fail.
5. Summary After several years of successful mining during an initial phase of mining and production, a second phase of mining that introduced a considerable contrast in mining direction and orientation commenced. This second phase encountered severe roof, floor, and seam conditions. The combination of transition-type roof, comprised of interbedded sandstone and shale, and excessive horizontal stresses caused massive roof falls in the second phase of mining. Mining in the second phase occurred in a north–south orientation, perpendicular to the maximum horizontal stress direction. Compressionaltype roof failure Žcutter roof. typically occurs perpendicular to the maximum stress direction. Historically, low overburden zones, transition roof, and mining in a direction approaching perpendicular to maximum stress have caused poor mining conditions. Additionally, both reduced seam thickness caused by sandstone channels and floor heaving associated with relatively weak floor and excessive horizontal stress have impeded development. Historical mining conditions, a digital geological model, and maps exhibiting the orientation of horizontal stresses were reviewed to determine preferred areas of future development. Mining in a north–south direction has proven to be very difficult due to the impact of transitional-type roof and excessive east-to-west horizontal stress. Future development should be directed towards the shale-dominated roof areas and oriented towards the maximum stress direction.
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References Aggson, J.R., 1979. Stress-Induced Failures in Mine Roof. United States Department of the Interior, U.S. Bureau of Mines, Report of Investigations 8338. Chen, H.J., Peng, S.S., 1998. Mechanism of Roof Failure in Longwall Entry System Under High Horizontal Stress Conditions. Society for Mining, Metallurgy, and Exploration, presented at the SME-AIME Annual Meeting, Orlando, FL, March 9–11, 1998, Preprint No. 98–163. Mark, C., 1991. Horizontal Stress and its Effects on Longwall Ground Control. Mining Engineering, November 1991, pp. 1356–1360. Mark, C., Mucho, T., 1994. Longwall Mine Design for Control of Horizontal Stress. USBM Special Publication 01–94, New Technology for Longwall Ground Control, pp. 53–76. Molinda, G.M., Mark, C., 1994. The Coal Mine Roof Rating ŽCMRR.: A Practical Rock Mass Classification for Coal Mines, 12th Conference on Ground Control in Mining, IC9387. Mucho, T.P., 1995. U.S. Bureau of Mines Internal Report. ŽAll parts of the report are not quoted.. Mucho, T.P., Mark, C., 1994. Determining Horizontal Stress Direction Using the Stress Mapping Technique, 13th Conference on Ground Control in Mining.
Case study evaluation of geological influences impacting mining conditions at a West Virginia longwall mine K. Scott Keim ) , Marshall S. Miller Marshall Miller & Associates, P.O. Box 848, Bluefield, VA 24605, USA Received 15 May 1998; accepted 1 December 1998
Abstract Longwall development in a West Virginia coal mine was terminated due to severe geologic conditions. The combination of transition-type roof Žcomprised of interbedded sandstone and shale., excessive in-situ stresses, and low overburden zones caused massive roof falls. Additionally, both reduced seam thickness, caused by sandstone channels, and relatively weak floor impeded seam development. These factors were mapped using both exploration data and information obtained during mine inspections. Historical mine data was also analyzed to determine if factors such as thickness of overburden and mining orientation had an influence on mining conditions. q 1999 Elsevier Science B.V. All rights reserved. Keywords: mining; roof fall; stress
1. Introduction This paper presents a case study of a geological analysis of multiple parameters impacting development of a northern West Virginia longwall coal mine. This study includes analysis of existing mining conditions and available geological data for the purpose of determining preferred areas for future development and to improve overall mining productivity. Critical factors analyzed include overburden thickness, orientation of roof falls, a digital geological model, and the influence of horizontal stress.
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Corresponding author. Tel.: q1-540-322-5467; Fax: q1-540-322-5460
0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 9 . 0 0 0 1 1 - 7
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The mine analyzed in this case study is located in northern West Virginia and had an established history of longwall development. The mine is located in the Lower Kittanning seam of the Allegheny Formation. The Lower Kittanning seam has an average height of 4 to 5 ft Ž1.2 to 1.5 m. on the subject property. A first phase of mining commenced in the mid-1970s and was primarily limited to the western portion of the property where reserves were nearing depletion. Typically, the mine produced 2.5 to 3.0 million tons per year. A second phase of mine development within the eastern portion of the property consisted of initial main entry and gateroad development for the first longwall panels. During this initial development in the second phase area, unstable roof conditions were encountered in the No. 1, No. 2, and No. 3 Right Entries. Relatively weak roof rock, a sandstone paleochannel, and excessive horizontal stresses all contributed to poor mining conditions. Historical data show the areas with sandstone immediate and main roof are typically several hundred feet Ž; 100 m. in width. The No. 3 Right Entry experienced a severe roof fall. The roof fall extended 800 ft Ž; 245 m. outby the face on the belt entry. For purposes of presentation, the overall mine area has been divided into two areas—western and eastern map areas. The western map area includes the previous initial deep mine and provides a historical perspective of geological mining conditions. The second phase eastern map area has undergone initial development and represents the future of the mining operation.
2. Western map area— (first-phase mining) First-phase mining was initiated in the western map area during the mid-1970s. Continuous miner sections were used to develop panels for the longwall unit. Longwall panels approached a width of 900 ft Ž; 275 m. and varied in length between 5000 and 12,000 ft Ž1525 and 3660 m.. The extensive mining in the western map area provided the basis for relating historical data and geological variances to assist with the preparation of a predictive geological model. Following is an analysis of recorded roof falls within the western map area. The roof falls have been examined with respect to overburden thickness, orientation and possible influence of in-situ stresses, and variations in roof lithologies. 2.1. OÕerburden thickness and frequency of roof falls Overburden thicknesses within the western map area vary from 0 to 650 ft Ž0 to 200 m.. Outcrop is limited to the extreme southern portion of the area where the seam was accessed ŽFig. 1.. Throughout the remainder of the reserve area, the seam is below drainage. Areas where overburden thickness varies between 150 and 250 ft Ž45 and 75 m. coincide with stream valleys and are common throughout the mined area. Historically, overburden thickness has been correlated with roof fall frequency. Approximately 183 roof falls were identified on mine maps, of which 60% are located in areas with less than 250 ft Ž76 m. of overburden ŽFig. 1.. The significance of this correlation is
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Fig. 1. Overburden map and roof fall locations.
strengthened by the consideration that only 28% of the mine is located in areas with less than 250 ft Ž76 m. of overburden. Thus, 28% of the mine Žless than 250 ft w76 mx of overburden. accounts for 60% of the roof falls ŽFigs. 1–4.. There is also a correlation between length of roof falls and overburden thickness. Approximately 20 roof falls in the western map area exceeded 100 ft Ž30 m. in length.
Fig. 2. Roof falls by overburden thickness.
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Fig. 3. Roof falls by overburden thickness and mined area.
Fig. 4. Roof falls per acre categorized by overburden thickness.
Fig. 5. Falls greater than 100 ft Ž30 m. in length categorized by overburden thickness.
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Thirteen falls of more than 100 ft Ž30 m. were located in areas delineated by less than 250 ft Ž76 m. of overburden ŽFig. 5.. 2.2. Orientation of roof falls Roof fall direction was analyzed for an indication of horizontal stress fields and maximum horizontal stress orientation. Typically, horizontal stress fields are biaxial with a maximum and minimum horizontal stress ŽAggson, 1979; Mucho and Mark, 1994.. The maximum horizontal stress Ž s H. can be much greater than the minimum horizontal stress Ž s h.. Consequently, entries oriented parallel or nearly parallel to the minimum horizontal stress Ž s h. are subject to more stress-induced damage than entries oriented parallel to the maximum horizontal stress Ž s H.. The orientation of 183 roof falls obtained from mine maps were tabulated and input into a spreadsheet. Of this total, 99 falls were oriented N358E, 54 were oriented N558W, one was oriented north–south, and the orientation of 29 falls could not be determined ŽFig. 6.. In the western map area, approximately 60% of the mine entries, including mains, submains and gate entries, are oriented in a N558W direction; whereas, 30% of the entries are oriented N358E. Roof falls were tabulated by both entry and fall orientation to determine if a correlation existed between mining direction and roof fall frequency. In the N558W heading, a nearly equal number of roof falls were observed to be parallel and perpendicular to mining direction. In the N358E heading, 80% of the roof falls parallel the entry directions ŽFig. 7A and B.. This data indicates roof falls have had a greater tendency to occur in both N358E entries and crosscuts as compared to a N558W direction. This directional control, in turn, coincides with regional horizontal stress fields indicating a maximum horizontal stress of east to west.
Fig. 6. Orientation of roof falls.
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Fig. 7. ŽA. Falls in the N558E heading. ŽB. Falls in the N358E heading.
The influence of regional horizontal stresses is well documented in the general project area. Conversations with U.S. Bureau of Mines ŽUSBM. personnel confirmed that severe horizontal stress influenced an adjacent mine. Overcore measurements conducted at the adjacent mine documented maximum horizontal stress oriented N668E while detailed mapping of approximately 6 miles of roof cutters by the USBM defined a stress field oriented approximately east–west. The nearly east–west maximum horizontal stress field caused damage to entries approaching a north–south orientation. Experience has shown entry alignment parallel or close to the major principal stress direction Ž s H. results in better or more favorable conditions compared to entry alignment close to the minor principal stress Ž s h. direction ŽMark, 1991.. Based on published data, the major principal stress direction Ž s H. is most likely east–west ŽMark and Mucho, 1994.. This is confirmed by the orientation of roof falls at the case study mine. The majority of falls at the case study mine occur in the N358E headings, which begin to approach the apparent north–south minimum stress direction Ž s h.. Roof falls in the N558W headings are less common. The N558W headings are rotated toward a more favorable mining direction Žcloser to the maximum principal stress..
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Fig. 8. Total roof falls per acre categorized by roof type.
2.3. Lithologic Õariations Drill hole data was sufficient in approximately 50% of the western map area to delineate roof lithologies and trends. Three categories, defined by variations in roof lithologies, have been mapped and are summarized as follows: Ø Sandstone-influenced roof consists of sandstone situated 0 to 12 ft Ž0 to 3.7 m. above the top of the seam. The sandstone has commonly been described as interbedded with shale. Ø Shale immediate roof consists of at least 12 ft Ž3.7 m. of shale, which may be overlain by sandstone. The shale may contain occasional thin, lenticular sandstone interbeds. Ø Shale-dominated roof is typically 18 to 24 ft Ž5.5 to 7.3 m. thick with no significant sandstone bodies within 24 ft Ž7.3 m. of the coal seam.
Fig. 9. Roof falls per acre categorized by roof type in areas with less than 250 ft Ž76 m. of overburden.
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Fig. 10. Roof falls per acre categorized by roof type in areas with greater than 250 ft Ž76 m. of overburden.
Core descriptions in the western map area were general and obtained from drillers’ logs. Consequently, geotechnical parameters and detailed geologic descriptions were not available to assist in roof mapping and roof rock characterization. When the frequency of documented roof falls by roof type are mapped, the sandstone-influenced roof areas have the highest frequency of roof falls. This is most likely attributable to shale interbeds and separation or delamination of the roof rock at the sandstone and shale contacts. The frequency of roof falls per acre decreases as shale becomes the dominant roof lithology ŽFig. 8.. The frequency of roof falls categorized by both roof type and overburden depth exhibits similar trends ŽFigs. 8–10.. 2.4. Western map area summary There is a correlation in the western map area between roof fall frequency and overburden thickness of less than 250 ft Ž76 m., when mining in a direction approaching north–south, and when coarser-grained roof lithologies are encountered. The correlation of increased roof fall frequency and the presence of sandstone in the immediate roof are somewhat unexpected but most likely attributable to relatively thin bedding and shale interbeds. Laminated rock Žalso referred to as stack rock. is susceptible to horizontal stress influences. Additionally, the area of sandstone immediate roof Žsituated within 2 ft w0.6 mx of the top of the mined seam. is relatively limited. Adjacent to this zone, the sandstone is separating from the seam resulting in several ft Ž; 0.8 m. of immediate shale roof overlain by sandstone. These areas commonly contain slickensided or fractured immediate roof caused by differential compaction rates between sandstone and shale during lithification. 3. Eastern map area— (second-phase mining) Relatively limited mine development has occurred in the eastern map area. An East Mains and No. 1, No. 2, and No. 3 Right Gate Entries were being developed ŽFig. 11..
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Fig. 11. Mine development east of the zone of disturbance.
3.1. Mine inspection Marshall Miller & Associates ŽMM & A. geologists inspected portions of the mine within the eastern map area. During these inspections, MM & A geologists observed weak roof rock associated with a paleochannel system and horizontal stress-induced damage, causing unstable roof conditions within No. 1, No. 2, and No. 3 Right Entries. A sandstone paleochannel approximately 200 to 400 ft Ž60 to 120 m. wide was observed in both No. 1 and No. 2 Right. Seam thickness was reduced to less than 1.0 foot Ž0.3 m. in the central portion of the channel area. Weak roof rock, resulting from interlayering of sandstone and shale Žoverbank. deposits, and differential compaction features Žslickensides. were observed adjacent to the channel area. The relatively weak ‘stack rock’ roof consists of thinly laminated, weak shale and distinct, hard sandstone and siltstone interbeds. Sandstone and siltstone interbeds are thicker and more abundant in the direction of the sandstone channel. Slickensides were commonly observed in shale roof underlying the channel. The interbedded shale and sandstone strata tend to separate parallel to bedding planes. Moreover, the first 2 to 4 ft Ž0.6 to 1.2 m. of immediate shale roof exhibited fracture planes parallel to and at the roof-pillar intersection near and adjacent to the sandstone channel area. Cutter roof and roof ‘potting’ Ži.e., oval-shaped roof falls or breaks of roof less than the roof bolt height as defined by Mucho and Mark Ž1994.. were observed at several locations. These features appear more severe within an area 250 ft Ž76 m. on either side of paleochannel influence ŽFigs. 12 and 13.. In No. 1 Right, the seam was displaced in a downward direction approximately the thickness of the coal seam ŽFig. 13.. In No. 3 Right belt entry, severe roof falls extended approximately 800 ft Ž245 m. outby the face. Fall propagation was both north and south from a point approximately 400 ft Ž122 m. outby the face Žbetween No. 12 and No. 13 cross-cuts.. Laminated sandstone was observed and became more prevalent southward. Trends based on both drill hole and mine data indicate only the north side of the paleochannel area of influence was penetrated by the mine entries. Several hundred feet Ž100 m. of poten-
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Fig. 12. Geologic and mine conditions in East Mains and No. 1, No. 2, and No. 3 right.
tially difficult conditions may remain as entries develop southward before the zone of paleochannel influence is transected ŽFig. 12.. Approximately one month after the 800-ft Ž245-m. roof fall in the No. 3 Right occurred, floor heave began in the middle entry of No. 3 Right ŽFig. 12.. Floor heaverfracturing is another indication of excessive horizontal stress. Although the roof fall resulted in stress relief of the roof rock, the floor remained susceptible to stress damage.
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Fig. 13. Cross sectional view of the channel area.
USBM engineers mapped portions of the East Mains and No. 2 Right. Portions of the USBM report ŽMucho, 1995. are summarized as follows: During our underground visit, we meandered through the openings in 2 RT and to some extent the associated East Mains doing cursory stress mapping. This technique is described in detail in the stress mapping paper. Stress mapping, by identifying and determining the direction of certain horizontal stress features, permits the existence of horizontal stress damage to be verified and determines the major principal stress direction of the horizontal stress field. Our results were that we saw moderate to fairly severe damage in areas of 2 RT and near the 2 RT area of the East Mains. The presence of these features and the directionality of failures indicated that horizontal stress was relatively high related to the lateral strength of the roof rock, especially in the areas associated with stream valleys. The general, directionality of failures, Žentries in 2 RT and crosscuts in East Mains. and the compass measurement of failures Žespecially the roof potting features. were approximately N208E ŽS208W. indicating a stress field direction of S708E ŽN708W. ŽFig. 14.. Implications of the above determination and known control techniques were discussed underground and at a follow-up meeting with management, and Marshall Miller
Fig. 14. Maximum horizontal stress determined by USBM engineers by stress mapping techniques.
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& Associates representatives were present. Horizontal stress control techniques discussed included: 1. Orientation of opening: Openings aligned, or close to, the direction of the major principal stress Žapproximately E–W. will have better conditions than those openings aligned perpendicular to the stress field Žapproximately N–S.. A change in orientation is a viable and effective control technique, especially for the long-term. Conditions observed in 2 RT, as well as, cursory examinations of topography over the property Žstress valleys-especially N–S trending. and geology Žsusceptible to horizontal stress damage. seemed to indicate that horizontal stress damage would be a periodic event and severe enough to justify reorientation considerations.. 2. Roof rock: While no specific measurements or testing was done of the immediate roof, the exposures and the general conditions were examined for abnormalities, lithology, etc. The USBM’s CMRR wCoal Mine Roof Ratingx was not done but it was apparent that the rating would be fairly low Žhigh 30’s or in the 40’s. due to the laminations of the shale and the interbedding of the shale and sandstone making it susceptible to horizontal stress. USBM research has noted that immediate roof rock that is especially weak in the lateral direction Žusually due to weak bedding andror interfaces. is susceptible to the effects of horizontal stress. The geology seems to be unusually variable caused, in part, by the channel sandstone meandering across this section of the property. The reader is referred to Molinda and Mark Ž1994. for information regarding the Coal Mine Roof Rating ŽCMRR. system. Recent research shows the worst mining conditions during entry development occur when the angle between the maximum horizontal stress and the direction of entry development is about 758 ŽChen and Peng, 1998.. As shown in Fig. 14, initial entry development was oriented northrsouth, which approaches the potential worst case 758 angle from the maximum stress direction. Four roof boreholes were examined with a flexible fiber-optic stratoscope by a roof bolt manufacturer. The immediate roof consisted of 3 ft Ž0.9 m. of laminated black shale overlain by 3 to 4 ft Ž0.9 to 1.2 m. of fine-grained laminated sandstone. Above these two layers, the strata were comprised of alternating layers of laminated shale, sandstone, and sandy shale. A distinct pattern of east–west lateral strata movement of 1r8 to 1r4 in. Ž0.3 to 0.6 cm. in each stratascope borehole was observed. Additionally, it was reported that some previously drilled 1-in. Ž2.5-cm. holes were completely closed. Roof layers will move in the direction of maximum horizontal stress Ž s H.. Movement occurs along failed laminations. The stratascope observations are indicative of a maximum east–west horizontal stress direction. In summary, mine inspections indicate that mine development in the eastern reserve area has been impeded by the interaction of the following geologic conditions: Ø A sandstone-filled paleochannel eroded a portion of the seam. Ø Relatively weak roof rock resulting from interlayering sandstone and shale is associated with the paleochannel. Ø The relatively weak roof rock consists of thinly laminated, weak shale and distinct, hard sandstone and siltstone interbeds. This laminated rock, referred to as stack rock by miners, is susceptible to the influences of excessive horizontal stress.
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Ø Slickensides are common adjacent to the margins of the sandstone paleochannel intrusion into the coal seam. These features result from differential compaction, which weakens the roof. Ø Horizontal stresses cause compressional-type roof failures Žcutter roof.. The cutter roof caused roof falls approaching 800 ft Ž245 m. in length. Ø The orientation of maximum stress Ž s H. appears to be N708W. Damage in entries oriented nearly perpendicular to the maximum stress is severe. Ø Mining direction is north–south, which is nearly perpendicular to the maximum stress. The preferred mining orientation is rotated towards an east–west direction Žparallel to the maximum stress.. Ø Severe roof conditions are known to occur under stream valleys at shallow overburden thicknesses. Ø Floor heave resulted from excessive horizontal stresses.
4. Future reserve area Future reserve potential is limited to the eastern map area. Access to this area is by the East Mains, which are located east of the zone of disturbance. An estimated 20 to 30 million tons of coal have been estimated as recoverable in this area. 4.1. Seam thickness The mineable seam thickness is regionally consistent throughout the reserve area scheduled for future development. Seam thickness varies between 4 and 5 ft Ž1.2 and 1.5 m., with the exception of the sandstone channel area in No. 1 and No. 2 Right and a zone of reduced thickness. These areas of seam disturbance appear to be limited in extent and are difficult to detect by exploration drilling. 4.2. Roof lithologies Three different roof types have been delineated in the future reserve area, based on both exploration data and mine observation. These three roof types are summarized as follows. Where data is not sufficient to predict the location of channel areas accurately, the width of the channel is increased on the maps to show general trends. 4.2.1. Sandstone immediate and main roof Historical mine data shows these areas are typically several hundred feet Ž; 100 m. in width. In the central portion of this zone, the sandstone appears to be medium-grained and massive and may erode portions of the coal seam. However, the sandstone appears to change laterally into thinly bedded sandstone with numerous shale interbeds ŽFig. 15.. The projections of the sandstone channel areas ŽFig. 16. are difficult to define accurately due to their limited width and limited density of exploration drilling.
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Fig. 15. Massive sandstone typical of the central portion of the channel area Žhole M95-10..
4.2.2. Transition zones These zones contain areas where finer-grained rocks Žshale and siltstone. varying between 2 and 12 ft Ž0.6 and 3.7 m. thick are overlain by sandstone ŽFig. 17.. Historically, these areas impede development. The shale and siltstone are typically slickensided and fractured, and the overlying sandstone is thinly bedded and commonly contains thin shale interbeds ŽFig. 18..
Fig. 16. Projection of sandstone channel areas.
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Fig. 17. Projection of transition roof area.
The combined effect of the laminated nature of the transition roof areas and possible slickensiderfracture development make these areas highly susceptible to the influence of horizontal stresses. These areas will most likely continue to exhibit very poor roof conditions, especially if mining parallels the minimum stress direction Žnorth–south..
Fig. 18. Transition-type roof and shale immediate roof Žhole M95-16..
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Fig. 19. Shale roof.
4.2.3. Shale roof Thick accumulation of shale with occasional to sparse sandstone interbeds and laminations dominate shale roof areas ŽFig. 19.. The overall extent of the shale roof area is defined by somewhat sparse data ŽFig. 20.. Additional exploration may alter the projection of shale roof Žespecially in the eastern portion of the mapped areas..
Fig. 20. Projection of shale roof areas.
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Historical data from the western map area indicates that shale roof areas had the most favorable conditions for development. However, if the shale roof is thinly laminated, horizontal stress could adversely influence conditions. Laminated shale roof has been documented as being very susceptible to horizontal stress damage. If the shales are laminated, the horizontal rock strength is reduced by low-cohesion bedding planes. The bedding characteristics of the shale roof areas should be carefully documented in any follow-up exploration program. The spacing of discontinuities Žnaturally occurring fractures. in the rock core from 14 drill holes were recorded. The fracture spacing and associated rock quality designation ŽRQD. measurements show sandstone core contains the least amount of fractures; whereas, core from transition areas contains the highest frequency of fractures. This data coincides with in-mine observations. Transition roof areas have historically proven to be difficult to mine and are more susceptible to horizontal stress influences. The conditions in sandstone roof areas should be favorable except where the sandstone is laminated andror interbedded with shale. Available data indicate the relatively competent sandstone roof zones result from sandstone channels that are several hundred feet Ž; 100 m. in width and locally erode a portion of the underlying seam. Adjacent to the central portion of the channel, the sandstone is interbedded with shale and quickly grades into transition-type roof. 4.3. OÕerburden Overburden thickness varies from less than 200 ft Ž60 m. to in excess of 600 ft Ž180 m.. Areas with less than 250 ft Ž76 m. have proven to be troublesome in past development. Regionally, difficult mining conditions have been associated with thinner overburden zones in north–south trending stream valleys. The north–south valleys are oriented at a large angle to the maximum horizontal stress direction and are subject to high shear stresses that can lead to compressive failure. The majority of the low overburden zones Žless than 250 ft w76 mx. are rotated towards a north–south direction. 4.4. Regional seam structure Regional seam structure contour trends show the axis of the Chestnut Ridge Anticline coincides with the east area reserve boundary. West of the Chestnut Ridge Anticline axis, the strike angle rotates to an east–west direction. The Ligonier Syncline is located south and east of the project area. The rotation of strike influences the north–south mining configuration east of the Chestnut Ridge Anticline. Panel development in a south-to-north direction will keep water away from the longwall face. However, as discussed in the previous sections, the preferred mining direction is east–west. The rotation of strike direction and presence of anticlines and synclines adjacent to the mining area may have an influence on horizontal stresses. USBM researchers have indicated there may be a relationship with enhanced horizontal stress influences near the axis of synclinal features. However, the current stress fields do not appear to be affected by folding on a regional scale.
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4.5. Horizontal stress Horizontal stress is a factor within the mine area. Compressional-type roof failure Žcutter roof. caused difficult to severe conditions in both No. 2 and No. 3 Right Gate Entries. Additionally, floor heave was observed in No. 3 Right and was interpreted to result from excessive horizontal stress. It is well established that horizontal stresses are typically biaxial ŽMucho and Mark, 1994.. It is also generally understood that ground damage will be minimized when the direction of drivage approaches the major principal horizontal stress direction. Recent research shows that during entry development, the entry system will be in the worst condition if the angle between the maximum and minimum stress and the direction of entry development is about 758 ŽChen and Peng, 1998.. Researchers have determined the maximum stress magnitude is typically 1.5 to 2 times the minimum stress magnitude ŽMucho and Mark, 1994.. The USBM compiled underground coal mine in-situ stress measurements for 25 eastern mines and determined the average maximum-to-minimum stress ratio to equal 1.95:1 for these mines ŽMucho and Mark, 1994.. USBM publications have documented that the direction of compressional-type failure is in the direction of the minor principal horizontal stress Ž908 to the major principal stress. ŽMark and Mucho, 1994.. In other words, where permitted, major features Žcutters and floor heave. will try to be aligned in the direction of the minor principal stress, which is perpendicular to the major principal stress. The USBM reports that roof potting and shear failure will exhibit this trend at all times. Researchers have determined a fairly uniform east-northeast regional horizontal stress field, although local flexures are common ŽMucho and Mark, 1994.. Near-surface and coal mine measurements have confirmed the regional directionality of horizontal stress fields.
Fig. 21. Maximum and minimum stress direction.
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During in-mine stress mapping, USBM personnel observed the location and orientation of cutter roof and features such as roof potting. The orientation of these features was north–south with an apparent migration to N208E. This observation, based on limited area mapping, indicates a maximum stress orientation of approximately N708W ŽFig. 21. ŽMucho, 1995.. As previously stated, compressional-type failures will align to the minimum stress orientation Ž s h., which appears to be N208E. Consequently, development in this direction will be more susceptible to horizontal stress influences. Correspondingly, mining parallel to the maximum stress Ž s H. direction will minimize the impact of horizontal stress. 4.6. Composite roof mapping Fig. 22 shows composite roof conditions, including the presence of sandstone channels in the immediate roof, transition zones, and areas with less than 250 ft Ž76 m. of overburden. Difficult roof conditions are projected where sandstone is proximal to the mineable seam. The combined impact of paleochannels, laminated sandstone, excessive
Fig. 22. Hazard projection map.
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horizontal stresses, and low overburden zones in north–south valleys severely impeded development in No. 1 through No. 3 Right Gate Entries. Similar conditions are defined by available exploration data south and east of existing mining. The projected sandstone channel and associated transition zones are currently defined by somewhat sparse exploration data Žespecially in the eastern portion of the map area.. Available data indicate these features exhibit a north–south trend. Historical information and the orientation of horizontal stresses indicate mining in a north–south direction is subject to compressional-type roof failure in the project area. Relatively weak roof rock and thinly bedded roof common in transition zones are more susceptible to stress-induced damage. 4.7. Floor Drill hole data shows the Lower Kittanning seam floor is typically comprised of relatively soft shale with claystone interbeds. The claystone interbeds may exceed 4 ft Ž1.2 m. in thickness. Areas where the claystone is thick Žgreater than 4 ft w1.2 mx. have been susceptible to floor heaving. Floor heaving is generally associated with relatively weak floor members that are dominated by clay lithologies. Plastic flow or heaving is also caused by high in-situ horizontal stress. In past development, compressive forces generated by horizontal stress caused the floor to fail.
5. Summary After several years of successful mining during an initial phase of mining and production, a second phase of mining that introduced a considerable contrast in mining direction and orientation commenced. This second phase encountered severe roof, floor, and seam conditions. The combination of transition-type roof, comprised of interbedded sandstone and shale, and excessive horizontal stresses caused massive roof falls in the second phase of mining. Mining in the second phase occurred in a north–south orientation, perpendicular to the maximum horizontal stress direction. Compressionaltype roof failure Žcutter roof. typically occurs perpendicular to the maximum stress direction. Historically, low overburden zones, transition roof, and mining in a direction approaching perpendicular to maximum stress have caused poor mining conditions. Additionally, both reduced seam thickness caused by sandstone channels and floor heaving associated with relatively weak floor and excessive horizontal stress have impeded development. Historical mining conditions, a digital geological model, and maps exhibiting the orientation of horizontal stresses were reviewed to determine preferred areas of future development. Mining in a north–south direction has proven to be very difficult due to the impact of transitional-type roof and excessive east-to-west horizontal stress. Future development should be directed towards the shale-dominated roof areas and oriented towards the maximum stress direction.
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