2008-03-21_PERMIT FILE - C1980007 ‎(5)‎

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 3/21/2008
• Section_Exhibit Name: Exhibit 78 Dry Fork Federal Lease-By-Application COC-67232 Final Enviro Impact Statement
• Media Type: D
• Archive: No

Chapter 3
longwall panels and the associated rigid (non-
yielding) gateroad pillars. A similar slope change of
up to 1.5 percent in areas of preferential subsidence
may also be expected along the Dry Fork of
Minnesota Creek channel.

•
Surface fracturing could also occur in the alluvium
and underlying bedrock in the Dry Fork of
Minnesota Creek drainage, especially where the
drainage crosses longwall panel gateroads. The
effects of this fracturing would be similar to that
described for Deep Creek, except that because the
Dry Fork of Minnesota Creek is an intermittent
drainage and is a lower energy steam system,
fracture healing may take longer to occur. If
fractures develop in the Dry Fork of Minnesota
Creek that divert water, it could reduce the amount
of water delivered through the Minnesota Canal and
Reservoir Company system if these effects occurred
during the irrigation season. These impacts are
expected to be of limited duration while the land
surface reaches equilibrium after subsidence.
Subsidence resulting from longwall mining can be
expected anywhere above or within the angle of
draw of fully extracted longwall panels. As
discussed in Section 3.2.4.1, measurable subsidence
effects for the Dry Fork LBA are expected to
attenuate within 250 feet from the edge of the
longwall panels for shallow overburden cover
(greater than 1,000 feet) and within 650 feet under
the deepest cover (greater than 2,000 feet).
Anticipated magnitudes of subsidence for E Seam
mining are 2 to 4 feet over chain pillars and 5 to 7
feet over longwall panel centers. Subsidence
magnitudes for both E Seam and B Seam extraction
are 4 feet for chain pillars and 14 feet for longwall
panels, respectively. The RFMP states that E and B
Seam extraction is limited to the northwest portion
of the Dry Fork tract. The maximum combined
height of the caved zone and fracture zone that will
develop during longwall panel retreat is expected to
extend up to 380 feet above top of the E Seam.
Based on the E Seam Depth of Cover Map (Agapito
2005, see Appendix B) and the calculated
maximum height, the fracture zone is not expected
to intercept the base of either surface water drainage
(475 to 875 feet below the base of the Deep Creek
drainage, and 550 to 875 feet below the base of Dry
Fork). The maximum expected surface crack depth
(tension crack) is 50 to 100 feet for a single seam.
Dry Fork Lease-By-Application FEIS
Affected Environment and Environmental Consequences
Where both E and B Seams are mined, this estimate
could double. In order to analyze whether the
combined effects of subsidence-induced fracture
zones over longwall panels and the development of
surface tension fractures within the angle of draw
could intercept surface water flow in the Dry Fork
LBA, one must combine the induced effects with
the depth of cover and the approximate thickness of
alluvium/colluvium (20 feel). Using a double seam
extraction assumption for the northwest corner of
the LBA, there would be 255 feet of separation
between the base of the surface tension fractures
(neutral surface) and the top of the fracture zone
below Deep Creek.
This analysis of potential subsidence impacts to
Deep Creek also draws upon experience in the
immediate and surrounding; areas where perennial
streams or intermittent/ephemeral drainages have
been undermined by longwall panels. Based on
subsidence studies and observations from the West
Elk Mine area made available in the Dry Fork LBA
Subsidence Evaluation (Agapito 2005, see
Appendix B), measured subsidence values in the
North Fork Valley are typically less than predicted
subsidence magnitudes. Adverse impacts on surface
water drainages, ponds, springs, or seeps were not
found in the three mines reviewed. Relevant
information from these observations is discussed
below.
It is the Forest Service experience that site-specific
conditions of each drainage, thickness of
overburden, amount of alluvial material present are
important factors to consider in evaluating
subsidence effects on perennial streams. Each
situation needs to be evaluated on a case-specific
basis.
MCC LLC-West Elk Mine
A longwall panel from the West Elk Mine
undermined and subsided a 900-foot reach of Deep
Creek in March 2003. This segment occurs at the
north end of the proposed Dry Fork tract. The
Forest Service analyzed subsidence in this portion
of Deep Creek in a 2001 EA (USDA Forest Service
2001 a). The EA made the following findings:
"Given the coal extraction height of 8 to 12 feet and
the 800 to 1,200 feet of overburden present, Deep
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