APPCOR13326 - Laserfiche WebLink

Home Browse Search

Memo to Tony Waldroh Bowie No. 2 Geotechnical Adequacy page 7 pillars, due to a combination of weak oxidized coal and an insufficient barrier pillar width. The implications of this experience are significant for the near subcrop portions of the Bowie No. 2 mine. 1.7 ZONES ALONG OUTCROP BARRIER PILLAR The experience within the 1st West and 2nd West panels of the Bowie #1 mine suggest it is appropriate to be concerned about the delineation of an adequate outcrop barrier pillar for the Bowie #2 mine. Further, observations by Dunrud throughout the book cliffs region of Utah and Colorado suggest that the concern is generally warranted. The preparer is ambiguous in this important section of Exhibit 15. It is first stated; "The 300 feet width of the outcrop barrier pillar which is assumed for the future mine appears to be conservative if most of the pillar is located in sound coal." Next it is stated; "When, during development of the panels near the outcrop, low quality coal is encountered, one row of development pillars would be a sufficient outcrop barrier for the Bowie #2 mine. This outcrop barrier 1 10 feet wide, should prevent the potential crushing, sliding, or other failure near the outcrop." If this statement is intended to commit to an additional outcrop barrier width of 110 feet, resulting in an outcrop barrier pillar 410 feet thick, I might concur. The applicant should clarify this proposal. Further, additional information should be provided to stipulate whether the extended pillar will be unmined, or developed. Obviously, at least one development room will have been extracted in order to encounter the low quality coal. The application should expand on this proposal to clarify the approach to be implemented. Chimney Collapse Prediction This evaluation applies the general roof fall bulking principles for determining the limit of chimney collapse propagation. I agree with this method for areas of uniform overburden conditions. However, as in the case of general trough subsidence propagation, chimneying may also be significantly influenced by the existence of discontinuities, such as faults. Differential settlement and stoping (chimney) collapse can also be more pervasive along discrete discontinuities. Therefore, specific accommodation should be made for thin overburden, near outcrop situations adjacent to known or mapped faults, such as panels 32 and 35.