REP18833

• Permit No: C1981022
• IBM Index Class Name: Report
• Doc Date: 9/2/1997
• Doc Name: PREDICTED LONGWALL SUBSIDENCE FOR THE SANBORN CRK MINE OXBOW CARBON MINERALS INC SOMERSET CO
• Permit Index Doc Type: SUBSIDENCE REPORT
• Media Type: D
• Archive: No

Sanborn Creek Subsidence Page 11 September 2, 1997
CHIMNEY SUBSIDENCE POTENTIAL
Chimney, or roof collapse, subsidence does not present any
hazard to either the ground surface, or the planned B Seam longwall
panels, or the abandoned mine workings overlying the planned
longwall panels because of the depth of the panels and thickness of
the interburden. Gray, Bruhn and Turka (1977} physically
documented the height of collapse chimney subsidence above the
Pittsburgh Seam for 127 cases. Their compilation of the height of
collapse chimneys was doubtless conservative because all their
cases breached the ground surface. Surface weathering no doubt
weakened the rock through which a portion of these chimneys
developed. Gray, et. al. reported chimney subsidence height
without respect to either the mining height or the lithology. One
of the 127 cases breached the ground surface 200 feet above the
Pittsburgh Seam. Figure 3 presents the relative cumulative
frequency distribution for their data. Their Pittsburgh Seam based
cumulative probability of an uncontrolled chimney penetrating
through the average 238 feet of interburden between the B Seam and
Oliver No. 1 Mine workings in the D Seam is less than 1$. The
cumulative probability of an uncontrolled chimney penetrating
through the average 369 feet of interburden between the B Seam and
overlying Hawksnest Mine workings in the E Seam is less than 1/2~.
Piggott and Eynon (1977) presented a mathematical method of
predicting the theoretical worst-case height of chimney development
based on the mining height and the percent swell of the collapsing
rock, Figure 4. Table 3 presents percent swell for various rocks.
If twelve feet of coal is extracted as planned from the "B" Seam
and the interburden is entirely sandstone, 67 percent free swell,
the maximum height of potential conical, worst-case, chimney
collapse is approximately 54 feet. with the same 12 feet of "B"
Seam coal extracted but with the entire interburden shale, 33
percent swell, the maximum height of potential conical, worst-case,
chimney collapse is approximately 110 feet. Theoretically, a
collapse chimney should not penetrate through the interburden rock
between the "B" Seam and the "D" Seam 238-ft overhead. Roof
Collapse following an advancing longwall face has a rectangular
collapse geometry, as shown on Figure 9. The height of rectangular
collapse is one-third the potential collapse height for worst-case
conical collapse, i.e. 18-ft for sandstone interburden and 36-ft
for shale interburden.
Therefore, there is no possibility of a mining induced chimney
collapse connection between the B Seam workings and either the
overlying Oliver No. 1 Mine or the Hawksnest Mine. Any direct
connection between the Sanborn Creek Mine would have to be through
a fault zone or structure. No faults or fault zones have been
encountered during mining in the Sanborn Creek Mine. Longwall
gateroad development should disclose any faults or fault zones
crossing a panel before longwall mining of the panel begins.
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