APPCOR12507

• Permit No: C1981033
• IBM Index Class Name: Application Correspondence
• Doc Date: 12/29/1981
• Doc Name: REVIEW OF THE REVISED BEAR MINE PERMIT APPLICATION
• From: MLR
• To: DAN MATHEWS
• Media Type: D
• Archive: No

STATE OF COLORADO R~CNeRDD ISM M. Governor
DEPART MENT OF NATURAL RESOURCES
D. MOnle Pascoe. Executive Director
MINED LAND RECLAMATION
423 Centennial Budding, 1313 Sherman Street
Denver, Colorado 80203 Tel. [phone]
Davld C. Shelton
Director
December 29, 1982
TO: Dan Mathews
FROM: Jim Pendleton `_ "
RE: Review of "Slp e St Ii ty Analysis, West Portal Area -
Bear Mine" (~ e 2 of 2 - Dear COa1 Company Permit Application)
Pursuant to your request, I have reviewed the supplemental submittal by Bear
Coal Company, entitled "Slope Stability Analysis, [Jest Portal Area - Bear
Mine". I forward the following comments for your consideration.
The western portal set of the Dear mine exists within an ancient landslide
compler.. Slope morpholoyy of this type is more the rule than the exception
for the valley of the North Fork of tJ~e Gunnison,
The stability analysis performed by Rocky Mountain Geotechnical is a
sensitivity-type anal~ISis which considered a range of material strengths
and alternative backfill and grading configurations, Aaterial strength prop-
erties were determined for both highwall materials and existing bench fill
materials. The analysis also assumed that a lirrdCed 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 re vi eta 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:
(1) 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
1.3 only if we assume an internal angle of friction of 4I° 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.