8
orientation which typically saw more instabilities and roof
failure, and the N35W orientation which is aligned parallel
to the principal horizontal stress direction and typically saw
less instabilities and roof failure. The second indicator is
overburden, which was estimated as problematic at greater
than 140 ft. The third indicator used was distant to stream
beds. A 200 ft buffer was utilized, which is typically used in
coal to account for the influence of a stream bed on mining
conditions. Researchers also implemented a 100 ft buffer
around joints and clay veins and use caprock thickness data
tracked from test holes approximately every other crosscut
within the mine.
Caprock thicknesses less than 3 ft were categorized as
hazardous.
The AutoCAD version of the mine map that included
the mine outline and the indicators listed above was
imported into ArcGIS where a hazard map was produced
(Figure 13). This map was produced without previous
knowledge of the location of roof failure or the massive
roof collapse. However, the map was able to indicate a
potential for ground failure right over the massive roof col-
lapse region. This may indicate that heading orientation,
overburden, stream beds, joints, clay veins, and caprock
thickness might be leading factors in the cause of this mas-
sive roof collapse. The hazard map was also able to correctly
map a hazardous area in the southwestern part of the mine,
where a large roof fall occurred. This area had crisscross-
ing clay veins in the overlaying shale with several fractures
in the roof and a constant flow of water prior to the fall.
Cutter roof failures and rock flour were present during the
failure as well indicating the shear failures as discussed in
Figure 4. According to the operator, the Lower Kittanning
Coal Seam may have also rolled down towards the top of
the Vanport in this area.
SUMMARY
The massive roof collapse at Subtropolis Mine is still
expanding slowly over time. Geologic mapping and 3D
LiDAR scanning showed that instabilities are expanding
relatively quickly perpendicular to the principal horizontal
stress direction and expanding relatively slowly parallel to
the principal horizontal stress direction (Figure 14). Strata
failures around the margins of the collapse area can reiniti-
ate depending on the proximity of the mining front.
Researchers were able to utilize detailed geologic map-
ping provided by the operator to map areas of potential
ground failure in ArcGIS. Results indicated that heading Figure 13. Indicators of Potential Ground Failure Map of
the Subtropolis Mine
Figure 14. Ariel view of expected principal horizontal stress
directional changes and concentrations around the massive
roof collapse
orientation which typically saw more instabilities and roof
failure, and the N35W orientation which is aligned parallel
to the principal horizontal stress direction and typically saw
less instabilities and roof failure. The second indicator is
overburden, which was estimated as problematic at greater
than 140 ft. The third indicator used was distant to stream
beds. A 200 ft buffer was utilized, which is typically used in
coal to account for the influence of a stream bed on mining
conditions. Researchers also implemented a 100 ft buffer
around joints and clay veins and use caprock thickness data
tracked from test holes approximately every other crosscut
within the mine.
Caprock thicknesses less than 3 ft were categorized as
hazardous.
The AutoCAD version of the mine map that included
the mine outline and the indicators listed above was
imported into ArcGIS where a hazard map was produced
(Figure 13). This map was produced without previous
knowledge of the location of roof failure or the massive
roof collapse. However, the map was able to indicate a
potential for ground failure right over the massive roof col-
lapse region. This may indicate that heading orientation,
overburden, stream beds, joints, clay veins, and caprock
thickness might be leading factors in the cause of this mas-
sive roof collapse. The hazard map was also able to correctly
map a hazardous area in the southwestern part of the mine,
where a large roof fall occurred. This area had crisscross-
ing clay veins in the overlaying shale with several fractures
in the roof and a constant flow of water prior to the fall.
Cutter roof failures and rock flour were present during the
failure as well indicating the shear failures as discussed in
Figure 4. According to the operator, the Lower Kittanning
Coal Seam may have also rolled down towards the top of
the Vanport in this area.
SUMMARY
The massive roof collapse at Subtropolis Mine is still
expanding slowly over time. Geologic mapping and 3D
LiDAR scanning showed that instabilities are expanding
relatively quickly perpendicular to the principal horizontal
stress direction and expanding relatively slowly parallel to
the principal horizontal stress direction (Figure 14). Strata
failures around the margins of the collapse area can reiniti-
ate depending on the proximity of the mining front.
Researchers were able to utilize detailed geologic map-
ping provided by the operator to map areas of potential
ground failure in ArcGIS. Results indicated that heading Figure 13. Indicators of Potential Ground Failure Map of
the Subtropolis Mine
Figure 14. Ariel view of expected principal horizontal stress
directional changes and concentrations around the massive
roof collapse