1
24-074
Principal Horizontal Stress Contributing to Massive Roof
Collapse at the Subtropolis Mine
Nicole Evanek
NIOSH, Pittsburgh PA
Gamal Rashed
NIOSH, Pittsburgh PA
Tim Miller
East Fairfield Stone, North Lima, OH
Allegra Yeley
NIOSH, Spokane WA
Ted Klemetti
NIOSH, Pittsburgh PA
ABSTRACT
In 2015, the Subtropolis Mine in Petersburg, OH, expe-
rienced a massive ground collapse that continues to grow
over time. Horizontal stress effects have affected the roof
at this operation in the past and may have played a large
role in this collapse. Horizontal stress can impact the roof
and floor stability and endanger the health and safety of
underground stone miners. Mines experiencing damaging
effects from horizontal stress typically find that the princi-
pal stress direction is the same throughout the mine and the
region. At the Subtropolis Mine the principal horizontal
stress direction appears to change as conditions change and
differs from the regional horizontal stress, particularly near
the collapse or other roof fall areas. The growth over time
of this collapse and other roof falls have been captured and
analyzed using geologic mapping, 3D LiDAR scanning,
numerical modeling and geographic information systems
like ArcGIS. Utilizing these tools to better understand site-
specific conditions can be critical to reducing the potential
of massive ground collapses in the underground limestone
industry.
INTRODUCTION
Since 2015 there have been several massive ground collapses
in the underground limestone industry. A massive ground
collapse, as defined in this study, consists of broken strata
partially filling multiple entries and rendering them unsafe
to enter. Miners working in areas of unstable ground are
at risk of serious falls of roof and rib injuries. In the most
severe cases, escapeways can be blocked and a subsidence
basin could occur on the surface. These massive ground
collapses can occur or grow unexpectedly, and they repre-
sent a high- consequence health and safety hazard with the
potential to produce serious injuries to underground mine
workers.
There have been several massive ground collapses how-
ever, the Subtropolis is one of very few where horizontal
stress may have played a role. The Subtropolis Mine extracts
the Vanport Limestone, which is part of the Allegheny
Formation, Pennsylvanian System, and ranges in thickness
from 16 to 22 ft with a mining height of approximately
16 ft and a roof span of approximately 40 ft (Iannacchione
et al., 2019). Generally, the Vanport Limestone is overlain
by a closely laminated, slightly weathered to weathered silt-
stone- sandy-shale immediate roof (Newman, 2019). The
standard production pillars at this mine are from 40 ft to
60 ft in length by 30 ft wide. At the Subtropolis Mine, the
Vanport Limestone includes several weak bedding planes
in the roof. In order to create an adequate beam, the mine
implements a primary bolting strategy of five 5-ft tensioned
fully grouted resin bolts per row with 8-ft spacing and a
secondary bolting strategy (typically in the crosscuts or
24-074
Principal Horizontal Stress Contributing to Massive Roof
Collapse at the Subtropolis Mine
Nicole Evanek
NIOSH, Pittsburgh PA
Gamal Rashed
NIOSH, Pittsburgh PA
Tim Miller
East Fairfield Stone, North Lima, OH
Allegra Yeley
NIOSH, Spokane WA
Ted Klemetti
NIOSH, Pittsburgh PA
ABSTRACT
In 2015, the Subtropolis Mine in Petersburg, OH, expe-
rienced a massive ground collapse that continues to grow
over time. Horizontal stress effects have affected the roof
at this operation in the past and may have played a large
role in this collapse. Horizontal stress can impact the roof
and floor stability and endanger the health and safety of
underground stone miners. Mines experiencing damaging
effects from horizontal stress typically find that the princi-
pal stress direction is the same throughout the mine and the
region. At the Subtropolis Mine the principal horizontal
stress direction appears to change as conditions change and
differs from the regional horizontal stress, particularly near
the collapse or other roof fall areas. The growth over time
of this collapse and other roof falls have been captured and
analyzed using geologic mapping, 3D LiDAR scanning,
numerical modeling and geographic information systems
like ArcGIS. Utilizing these tools to better understand site-
specific conditions can be critical to reducing the potential
of massive ground collapses in the underground limestone
industry.
INTRODUCTION
Since 2015 there have been several massive ground collapses
in the underground limestone industry. A massive ground
collapse, as defined in this study, consists of broken strata
partially filling multiple entries and rendering them unsafe
to enter. Miners working in areas of unstable ground are
at risk of serious falls of roof and rib injuries. In the most
severe cases, escapeways can be blocked and a subsidence
basin could occur on the surface. These massive ground
collapses can occur or grow unexpectedly, and they repre-
sent a high- consequence health and safety hazard with the
potential to produce serious injuries to underground mine
workers.
There have been several massive ground collapses how-
ever, the Subtropolis is one of very few where horizontal
stress may have played a role. The Subtropolis Mine extracts
the Vanport Limestone, which is part of the Allegheny
Formation, Pennsylvanian System, and ranges in thickness
from 16 to 22 ft with a mining height of approximately
16 ft and a roof span of approximately 40 ft (Iannacchione
et al., 2019). Generally, the Vanport Limestone is overlain
by a closely laminated, slightly weathered to weathered silt-
stone- sandy-shale immediate roof (Newman, 2019). The
standard production pillars at this mine are from 40 ft to
60 ft in length by 30 ft wide. At the Subtropolis Mine, the
Vanport Limestone includes several weak bedding planes
in the roof. In order to create an adequate beam, the mine
implements a primary bolting strategy of five 5-ft tensioned
fully grouted resin bolts per row with 8-ft spacing and a
secondary bolting strategy (typically in the crosscuts or