APPCOR11596 - Laserfiche WebLink

Home Browse Search

~~~ I~~~~~~~~~~~~~~~ STATE OFCOLORADO RiC HARDD LAM M. GO~e moi 999 DEPARTMENT OF NATURAL RESOURCES D. Monte Pascoe, Executive Director MINED LAND RECLAMATION 423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. (303) 866-3567 David C. Shelton Director December 29, 1981 TO: Dan Mathews FROM: Jim Pendleton ~. RE: Review of "S1 e St Zity Analysis, West Portal Area - Bear Mine" ( e 2 of 2 - Bear Coal Company Permit Application) Pursuant to your request, 1 have reviewed the supplemental submittal by Bear Coal Company, entitled "Slope Stability Analysis, West Portal Area - Bear Mine". Z forward the following comments for your consideration. The western portal set of the Bear mine exists within an ancient landslide complex. Slope morphology of this type is more the rule than the exception for the valley of the North Fork of the Gunnison. The stability analysis performed by Rocky Mountain Geotechnical is a sensitivity-type analysis which considered a range of material strengths and alternative backfiZl and grading configurations. Material strength prop- erties were determined for both highwall materials and existing bench fill materials. The analysis also assumed that a limited perched water table might develop within the toe of the existing fill bench, but that no true phreatic condition would develop, because of steep slope grades and relatively impermeable backfill materials. This is probably a valid assumption if drainage is precluded from the portals into the backfilled bench. I recommend that you request Roy .Cox review the hydrology aspects of their abandonment plan to determine whether or not drainage will, indeed, occur. Briefly summarized, the stability analysis projects that a backfilled static slope safety factor of 1.3 or greater can be achieved in two manners: (11 The existing portal highwall could be partially backfilled (approximately half its existing height would be covered); or (2) The existing portal highwall could be totally backfilled if a supporting buttress toe berm was constructed throughout the length of the existing bench, approximately 20 feet high by 30 feet wide. The partially backfilled configuration achieves a static slope safety factor of Z.3 only if we assume an internal angle of friction of 41° or higher for the highwall materials. This value is higher than the laboratory test determined value of 33.7° for effective peak stress and should, therefore, involve monitor- ing of the final backfilled configuration to assure stability.