2
headings adjacent to crosscuts) of 8-ft cable bolts between
the fully grouted bolts. Typically, the caprock left in the
roof is approximately 4 ft. The Subtropolis mine has expe-
rienced damage from horizontal stress since its opening in
2006 (Iannacchione et al., 2019).
MASSIVE GROUND COLLAPSES
Massive ground collapses have been defined as multiple
entries filled with broken strata, rendering the entries unsafe
to enter (Evanek et al., 2022). There are two types of mas-
sive ground collapses: massive pillar collapses and massive
roof collapses. Massive pillar collapses can include zones of
multiple pillars that failed and are no longer capable of sup-
porting the overlying strata. This type of collapse can initi-
ate with the failure of a single pillar that triggers cascading
pillar failure over a wide area, causing a secondary failure of
the roof as well. Massive roof failures occur when the over-
burden rock is not sufficiently strong to prevent unstable
conditions (Evanek et al., 2022). Massive roof collapses can
occur over long periods of intermittent growth.
From 2008 until 2021 there have been eight massive
ground collapses with one event resulting in three injuries,
none resulting in fatalities, and several were near misses. Six
of these massive ground collapses were determined to be as
a result of pillar failure and therefore have been defined as
massive pillar collapses. One of these six is the Petersburg
massive ground collapse, which occurred at a mine a few
short miles from the Subtropolis Mine. The remaining two
massive ground collapses were a result of a failure in the
overburden, one of which occurred at the Subtropolis Mine
and the other at the National Institute for Occupational
Safety and Health (NIOSH) Lake Lynn Research Facility
which occurred slowly from 1994 to 2006 (Figure 1). The
cause of the Lake Lynn massive roof collapse has been stud-
ied and several factors are suggested including weathered
joints, inadequate caprock thickness, exceedingly weak
strata above the caprock, strata dips that favor strata exten-
sion into the adjacent surface quarry, freeze-thaw action
in the entries near the highwall, and a lack of adequate
support.
The Petersburg Mine massive collapse, though located
in the Vanport Limestone and only a few miles from the
Subtropolis Mine, experienced very different ground con-
ditions. The mine did not experience as many damages
from horizontal stress as the Subtropolis Mine, so there
was no need to adopt the stress control layout however,
the mining dimensions were very similar to most of the
old workings at the Subtropolis Mine. The difference in
ground conditions at Petersburg is mainly due to its weak,
moisture-sensitive floor and weak bands within the pillars
(Murphy et al., 2016). The collapsed pillars were able to
remain stable for 10 years after mining. Adding moisture or
repeated wetting and drying may have caused further weak-
ening of the floor under these pillars, leading to the ini-
tiation of the pillars punching into the floor after 10 years
(Murphy et al., 2016). When the pillars punched into the
floor and failed, it caused an increase in loading on nearby
pillars, causing them to fail in rapid succession. Since this
was a failure of the pillars, occurred almost instantaneously,
and created a large subsidence basin at the surface with no
remnant pillar humps, this collapse is defined as a massive
pillar collapse. Massive roof collapses can be less dramatic
and occur over a larger span of time however, they can still
represent a significant ground control hazard.
UNDERSTANDING CHANGES IN
PRINCIPAL HORIZONTAL STRESS
DIRECTION
The massive ground collapse at the Subtropolis Mine
in Petersburg, OH, is also defined as a massive roof col-
lapse. The failure of the roof began in late 2015 and has
slowly developed over time and now encompasses several
entries in the north-central part of the mine (Figure 2). The
Subtropolis Mine, which is a room-and- pillar mine that
extracts the Vanport Limestone, has a long history of react-
ing to horizontal stress damage in the roof. Since the mine
opened in 2005, the operators have changed the orienta-
tion of the mine several times in order to alleviate the detri-
mental effects of high horizontal stress and to determine the
principal horizontal stress direction (Figure 3). Figure 1. Location and dates of massive roof collapse at the
Lake Lynn Laboratory from 1994 to 2007
headings adjacent to crosscuts) of 8-ft cable bolts between
the fully grouted bolts. Typically, the caprock left in the
roof is approximately 4 ft. The Subtropolis mine has expe-
rienced damage from horizontal stress since its opening in
2006 (Iannacchione et al., 2019).
MASSIVE GROUND COLLAPSES
Massive ground collapses have been defined as multiple
entries filled with broken strata, rendering the entries unsafe
to enter (Evanek et al., 2022). There are two types of mas-
sive ground collapses: massive pillar collapses and massive
roof collapses. Massive pillar collapses can include zones of
multiple pillars that failed and are no longer capable of sup-
porting the overlying strata. This type of collapse can initi-
ate with the failure of a single pillar that triggers cascading
pillar failure over a wide area, causing a secondary failure of
the roof as well. Massive roof failures occur when the over-
burden rock is not sufficiently strong to prevent unstable
conditions (Evanek et al., 2022). Massive roof collapses can
occur over long periods of intermittent growth.
From 2008 until 2021 there have been eight massive
ground collapses with one event resulting in three injuries,
none resulting in fatalities, and several were near misses. Six
of these massive ground collapses were determined to be as
a result of pillar failure and therefore have been defined as
massive pillar collapses. One of these six is the Petersburg
massive ground collapse, which occurred at a mine a few
short miles from the Subtropolis Mine. The remaining two
massive ground collapses were a result of a failure in the
overburden, one of which occurred at the Subtropolis Mine
and the other at the National Institute for Occupational
Safety and Health (NIOSH) Lake Lynn Research Facility
which occurred slowly from 1994 to 2006 (Figure 1). The
cause of the Lake Lynn massive roof collapse has been stud-
ied and several factors are suggested including weathered
joints, inadequate caprock thickness, exceedingly weak
strata above the caprock, strata dips that favor strata exten-
sion into the adjacent surface quarry, freeze-thaw action
in the entries near the highwall, and a lack of adequate
support.
The Petersburg Mine massive collapse, though located
in the Vanport Limestone and only a few miles from the
Subtropolis Mine, experienced very different ground con-
ditions. The mine did not experience as many damages
from horizontal stress as the Subtropolis Mine, so there
was no need to adopt the stress control layout however,
the mining dimensions were very similar to most of the
old workings at the Subtropolis Mine. The difference in
ground conditions at Petersburg is mainly due to its weak,
moisture-sensitive floor and weak bands within the pillars
(Murphy et al., 2016). The collapsed pillars were able to
remain stable for 10 years after mining. Adding moisture or
repeated wetting and drying may have caused further weak-
ening of the floor under these pillars, leading to the ini-
tiation of the pillars punching into the floor after 10 years
(Murphy et al., 2016). When the pillars punched into the
floor and failed, it caused an increase in loading on nearby
pillars, causing them to fail in rapid succession. Since this
was a failure of the pillars, occurred almost instantaneously,
and created a large subsidence basin at the surface with no
remnant pillar humps, this collapse is defined as a massive
pillar collapse. Massive roof collapses can be less dramatic
and occur over a larger span of time however, they can still
represent a significant ground control hazard.
UNDERSTANDING CHANGES IN
PRINCIPAL HORIZONTAL STRESS
DIRECTION
The massive ground collapse at the Subtropolis Mine
in Petersburg, OH, is also defined as a massive roof col-
lapse. The failure of the roof began in late 2015 and has
slowly developed over time and now encompasses several
entries in the north-central part of the mine (Figure 2). The
Subtropolis Mine, which is a room-and- pillar mine that
extracts the Vanport Limestone, has a long history of react-
ing to horizontal stress damage in the roof. Since the mine
opened in 2005, the operators have changed the orienta-
tion of the mine several times in order to alleviate the detri-
mental effects of high horizontal stress and to determine the
principal horizontal stress direction (Figure 3). Figure 1. Location and dates of massive roof collapse at the
Lake Lynn Laboratory from 1994 to 2007