REP04232

• Permit No: C1981022
• IBM Index Class Name: Report
• Doc Date: 9/19/1987
• Doc Name: Report of tour of subsidence and landslide
• To: MLR DIV DNR
• Permit Index Doc Type: SUBSIDENCE REPORT
• Media Type: D
• Archive: No

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999
Sept. 19, 1987
Dr. James A. Pendleton
Mined Land Reclamation Div., DNR
1313 Sherman St., Room 423
Denver, CO 80203
Dear Jim:
The following is a brief summary of the results of our tour of
the subsidence and landslide areas in the Somerset, Colo. area in
Gunnison County during September 9 and 10, 1987.
Subsidence
The three areas visited in Bear Creek, located above the Somerset
Mine, showed little or no evidence of the the cracks that were
mapped 10 to 15 years ago (see attached map). The cracks were
filled in with soil and other surficial material to the point that
they were no longer recognizable as cracks. Apparently the
cracks were filled with surficial material by mass wasting, slope
wash, and sloughage. Little or no erosion occurred along the
cracks, possibly because the material was carried downward into
these areas of increased permeability, rather than laterally
channeling along the crack. The filled-in cracks now either
look like trails or are covered with grass and other vegetation.
The linear crack sequences in areas 1 and 2 (see attached map),
located above the barrier pillars away from landslide areas, which
initially ranged in width from 1 to 14 in. and from about 20 feet
to as much as 700 ft in length, were filled in with surficial
material and were unrecognizable, except for one hole about 6 ft
long, 3 ft wide and 3 ft deep in area 2, where the crack had not
healed. This hole may be filled in a year or so, based on how the
other nearby cracks filled in.
The arcuate to linear cracks mapped in a landslide area located
on the east side of Bear Creek, about 0.4 mi. north of the fan to
the Somerset Mine, also were filled in and either looked like game
trails or were vegetated within a ten-year period. Evidently the
landslide was stable after mining and subsidence in 1977.
Based on the evidence seen in the Somerset Mine area, subsidence
cracks in bedrock and surficial material, as much as 14 inches
wide, fill in by mass wasting, slope wash, and sloughage
within a period of 10 to 15 years. Cracks in bedrock will, of
course, remain open for much longer periods where either there is
no source of surficial material to fill the cracks or where the
cracks are wide and very deep (as can be the case where block
movement occurs), and the material continues to be transported
down the cracks.