10
difference in the longwall abutment loading in these two
cases. Previous NIOSH works [16, 17] showed that higher
abutment loading in a deep cover site reduces the aperture
of bedding panes and subvertical fracture. However, similar
work on the shallow-cover case showed that the proximity
to the mining level was the dominating factor due to shal-
low overburden.
CONCLUSION
For the two cases evaluated in this study, the range of per-
meability values is wide. Permeability under shallow cover
is at least two orders of magnitude higher than those under
deep cover. In the deep-cover case, the depths of cover is the
dominating mechanism for controlling post-mining per-
meability so that under deep cover, the permeability values
return to pre-mine-by values within about 305 m (1,000 ft)
of the LW panel mine-by. This is reassuring since deep
cover conditions are a common occurrence in southwestern
Pennsylvania. Under shallow cover, the caving process is the
dominating factor in controlling post-mining permeability
so that the permeabilities do not reduce down to the pre-
mining values.
The information from these studies can help under-
stand potential gas transport through ground strata and the
numerical prediction of potential inflow amounts. The next
steps include the continued monitoring of the long-term
permeability values under deep cover for potential changes.
Also, an intermediate or medium cover site will be moni-
tored for permeabilities before, during, and after a first and
second LW panel mine-by.
LIMITATIONS
In this research, the collected data was limited to two sites
targeting shallow and deep cover. Although the site loca-
tions were targeting the same coal horizons, the geology,
surface topography, site conditions, groundwater table,
Figure 13. Graphical comparison of measured permeabilities at a shallow and a deep cover site after a second
LW panel mine-by
Table 3. Comparison of permeability values under shallow and deep cover pre-, during, and post- LW mine-by
Comparison of Permeability Data
Pre-
Mine-By
Average
(mD)
During 1st LW
Mine-By (mD)*
Post-
Mine-By
Average
(mD)
During 2nd LW
Mine-By (mD)* Post-Mine-
By Average
(mD) Cover
Depth
(ft)
Borehole
ID Horizon Min Max Min Max
Shallow 417 FEB‑1 Sewickley 432 411 1,360 1,220 2,260 5,080 3,630
Shallow 250 FEB‑2 Uniontown 169 22 245 193 16 386 187
Shallow 135 FEB‑3 Top of
fracture zone
3,250 2,730 32,900 27,500 38,500 132,000 94,500
Deep 1,076 VEP-S Sewickley 0.8 0.5 17.8 1.2 1.4 8.8 2.2
Deep 955 VEP-U Uniontown 0.2 0.2 5.4 0.5 0.6 6.6 0.5
*”During mine-by” defined as the LW panel being within ± 1.2 times the Pittsburgh seam depth of the monitoring boreholes
difference in the longwall abutment loading in these two
cases. Previous NIOSH works [16, 17] showed that higher
abutment loading in a deep cover site reduces the aperture
of bedding panes and subvertical fracture. However, similar
work on the shallow-cover case showed that the proximity
to the mining level was the dominating factor due to shal-
low overburden.
CONCLUSION
For the two cases evaluated in this study, the range of per-
meability values is wide. Permeability under shallow cover
is at least two orders of magnitude higher than those under
deep cover. In the deep-cover case, the depths of cover is the
dominating mechanism for controlling post-mining per-
meability so that under deep cover, the permeability values
return to pre-mine-by values within about 305 m (1,000 ft)
of the LW panel mine-by. This is reassuring since deep
cover conditions are a common occurrence in southwestern
Pennsylvania. Under shallow cover, the caving process is the
dominating factor in controlling post-mining permeability
so that the permeabilities do not reduce down to the pre-
mining values.
The information from these studies can help under-
stand potential gas transport through ground strata and the
numerical prediction of potential inflow amounts. The next
steps include the continued monitoring of the long-term
permeability values under deep cover for potential changes.
Also, an intermediate or medium cover site will be moni-
tored for permeabilities before, during, and after a first and
second LW panel mine-by.
LIMITATIONS
In this research, the collected data was limited to two sites
targeting shallow and deep cover. Although the site loca-
tions were targeting the same coal horizons, the geology,
surface topography, site conditions, groundwater table,
Figure 13. Graphical comparison of measured permeabilities at a shallow and a deep cover site after a second
LW panel mine-by
Table 3. Comparison of permeability values under shallow and deep cover pre-, during, and post- LW mine-by
Comparison of Permeability Data
Pre-
Mine-By
Average
(mD)
During 1st LW
Mine-By (mD)*
Post-
Mine-By
Average
(mD)
During 2nd LW
Mine-By (mD)* Post-Mine-
By Average
(mD) Cover
Depth
(ft)
Borehole
ID Horizon Min Max Min Max
Shallow 417 FEB‑1 Sewickley 432 411 1,360 1,220 2,260 5,080 3,630
Shallow 250 FEB‑2 Uniontown 169 22 245 193 16 386 187
Shallow 135 FEB‑3 Top of
fracture zone
3,250 2,730 32,900 27,500 38,500 132,000 94,500
Deep 1,076 VEP-S Sewickley 0.8 0.5 17.8 1.2 1.4 8.8 2.2
Deep 955 VEP-U Uniontown 0.2 0.2 5.4 0.5 0.6 6.6 0.5
*”During mine-by” defined as the LW panel being within ± 1.2 times the Pittsburgh seam depth of the monitoring boreholes