2021-02-05_PERMIT FILE - C1980007A ‎(5)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 2/5/2021
• Doc Name: SUBSIDENCE EVALUATION
• Section_Exhibit Name: Exhibit 60E Subsidence Evaluation for the South of Divide and Dry Fork Mining Areas
• Media Type: D
• Archive: Yes

APPENDIX A Bob Munz
ARCH Mine Engineer
WEST ELK [e-mail]
Pi e: 970.929.2265
Mountain Coal Company,LLC
A subsidiary of Arch Resources,Inc.
West Elk Mine
5174 Highway 133
TECHNICAL MEMO Somerset,CO 81434
WEST ELK MINE
POTENTIAL FOR SURFACE SUBSIDENCE DUE TO DEVELOPMENT MINING AT
WEST ELK MINE UNDER PERENNIAL STREAMS
By Bob Munz, Mine Engineer December 2020
INTRODUCTION
The purpose of this Technical Memo was to research and summarize the published
engineering and scientific literature regarding the potential for surface subsidence due
to development mining under perennial streams. The minimum depth of cover required
for development mining in the West Elk Mine was then assessed to avoid the potential
for surface subsidence should a roof fall/collapse occur.
GENERAL DATA
Development mining produces entries or roadways underground within the mine with a
nominal roadway height of 11 feet. Roadway widths range between 18 and 20 feet and
the maximum entry-crosscut intersection diagonal spans 34 feet.
Based on drill hole data, the overburden typically consists of 25 to 50 feet of
alluvium/colluvium and/or weathered rock near the surface, although alluvium may not
be present in some areas. Below this, a variable lithology of sandstones, siltstones,
mudstones and shales exist.
DISCUSSION
To ascertain the likelihood that surface subsidence will occur due to development
mining, two mechanisms were addressed. One was that a roof fall occurs which would
extend directly up to the surface. The second was the potential associated extension of
the roof fall resulting in a subsidence basin on the surface.
With competent strata typically present in the immediate mine roof up to at least a
height of between 30 and 40 feet, it was assessed that it is practically impossible for a
roof fall in an underground roadway or intersection to extend up to the surface and
cause subsidence. In a roof fall, the strata tends to form a natural arch (often termed a
"failure arch") until it reaches a rock unit that is competent enough to span and in effect
"cap" the arch.