PERMFILE41947

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 4/8/2005
• Section_Exhibit Name: 2.05 Operation and Reclamation Part 2
• Media Type: D
• Archive: Yes

• were noticeably greater beneath the ridges than they were beneath more uniform overburden of
similaz thickness, because there is little or no lateral constraint to distribute the weight of the
isolated load of the ridge.
The rugged topography on the north, west, and south flanks of West Flatiron may cause stresses to
be concentrated beneath isolated ridges. Overburden thickness will increase by 500 to 1,000 feet in
horizontal distances of 1,500 feet similar to the isolated ridge north of the first east-trending side
canyon of Sylvester Gulch.
Fracture-Controlled Drainages
Based on mapping by Mr. Dunrud in the Somerset area and on recent field work, Mr. Dunrud
believes that there is reasonably good, but certainly not conclusive, evidence that the drainages
(Sylvester Gulch, an unnamed drainage west of Box Canyon, and Box Canyon) in the Box Canyon
permit revision area are controlled by north-trending fractures and/or joints. These fractures have
been caused in part by stresses generated by the West Elk Mountain intrusive bodies -particularly
Mt. Gunnison. Section 2.04.6 (Geology Description) includes additional discussion and references
relating to the nature and continuity of fractures.
The conservative approach may be to assume that the drainage system is fracture controlled. But
even if fractures control the present drainage system, they may notextend downwazd as continuous
joints of fractures to the B-Seam located several hundreds of feet below. Even if the fractures were
• present in the more brittle sandstone units, it would be very unlikely that these fractures would
occur in the softer siltstone and shale units. Even under the ,conservative approach that the
drainages in the Box Canyon permit revision azea are fracture cohtrolled, it is extremely unlikely
that they extend downward to the B-Seam through multiple shale and siltstone units. Using this
conservative evaluation, it is now important to evaluate the poterifial impact that subsidence may
have on any pre-mining fractures.
Evaluation of subsidence due to downwarping of laterally-constrained strata shows that rock strata
with different deformation and strength characteristics deform as discrete units. For example, strata
of shale and siltsone behave as units discrete from sandstone. Above the fractured zone and within
the continuous deformation zone these units undergo continuous flexure (Figure 1, enlazgement 2 of
Exhibit 60). Above the neutral surfaces, in zones of convex-upwazd curvature, the material is in
tension and below them, the material is in compression.
Consequently, stresses change across neutral surfaces from tension to compression with each
successive rock unit that deform§ as a plate. Fractures already present would thus tend to open
more in the zones of tension and close more in the zones of compression than would be the case for
these fractures prior to mining and subsidence. The Box Canyori and unnamed drainage west of
Box Canyon channels are above the maximum estimated height of jthe fracture zone (i.e., 20t or 240
feet fora 12-foot coal-extraction thickness).
After longwall mining is completed in the area and static conditions are attained, the zones of
• tension and compression commonly cease, and any fractures present will likely resume the pre-
mining condition. Therefore, the impacts on surface flow in the drainage of the Box Canyon
2.05-126
March 2005PR71
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