4
to excessive lateral loads, it can buckle under compressive
stresses, failing in shear (Iannacchione et al., 2019).
Failures appear to originate in the lowest beams, or the
beams closest to the mine opening due to the concentration
of stresses in the lower beam and constraints to bending
in the upper beams (Iannacchione et al., 1998). Therefore,
failure begins with a shear in the first foot of the caprock.
Shear failures appear as low- angle ruptures in the rock
with a kind of ‘‘rock flour” along the newly created surface
(Iannacchione et al., 2019). Then, the shear propagates, and
the beam begins to dilate. Next, the beam begins to canti-
lever, causing tensile failure along the boundary between
the roof and the pillar. Once the beam fails completely, the
shear failure propagates to the next layer. In other mines
the shear failure continues to propagate upwards through
the roof layers until the fall has developed an arch shape.
However, at the Subtropolis Mine there are very weak
shales present above the caprock. Here, once the caprock
fails, the fall takes on a chimney shape and, in some cases,
extends approximately 30 ft into the roof.
Figure 5 shows 3D LiDAR scans capturing the shape
of this type of fall and its growth over time (shown in red).
In this roof fall, the failure extends approximately 30 ft into
the roof and up into the overlying coal seam. The shear
failure orientations are parallel to the long axis of the ellip-
tical-shaped cavities in this case study. All of these damages
have been captured using detailed geologic mapping and,
in some cases, 3D LiDAR scanning. For example, Figure 6
contains two photos from the Subtropolis Mine. Photo
A is an example of a dilating beam and Photo B shows
shear failure in three separate fully-grouted bolts from the
same area.
The operator noticed that these types of failures in the
N-S entries appear to concentrate at the northwest and
southeast corners of the pillars. In order to minimize the
number of these types of roof failures, the mine utilized
the stress control layout. The stress control layout utilizes
headings in the principal horizontal stress direction. This
minimizes damage to the headings but concentrates dam-
age into the crosscuts. To minimize damage to the cross-
cuts, the mine has implemented the use of rectangular
pillars. The mine also has implemented a secondary bolt-
ing strategy and included windows in every other crosscut.
According to Evanek et al. (2020), “Windows are used to
resist horizontal movements or deformation that occur
parallel to the maximum horizontal stress direction or per-
pendicular to the direction of stress failures. A window is
developed by leaving an increased thickness of roof rock
in the crosscuts, thus reducing the crosscut dimensions
vertically.” The first time the stress control layout was
implemented at the mine was in the northwestern region
of the mine in 2018, on the other side of the massive col-
lapse area. Numerical modeling was utilized in this study
to confirm damage patterns seen at the mine. The model,
as seen in Figure 7, maps the distribution of the maximum
principal horizontal stress over two different mine layouts
at Subtropolis. The principal horizontal stress in this model
is parallel to the Y-axis in the figure at N35W. The pillars
with a N-S orientation show a higher concentration at the
corners of the entry (indicated by red), which aligns with
damages mapped underground. The pillars oriented paral-
lel to the principal horizontal stress at N35W show damage
(indicated by red) being concentrated at the crosscuts and
not in the entries. This damage concentration has also been
mapped at Subtropolis and also confirms the stress control
layout is working as planned.
Figure 5. March 2018 roof fall in XC02 progression from
June 2019 to November 2019
Figure 6. A) Photo of shear failure in roof and B) Photo of
sheared bolts
to excessive lateral loads, it can buckle under compressive
stresses, failing in shear (Iannacchione et al., 2019).
Failures appear to originate in the lowest beams, or the
beams closest to the mine opening due to the concentration
of stresses in the lower beam and constraints to bending
in the upper beams (Iannacchione et al., 1998). Therefore,
failure begins with a shear in the first foot of the caprock.
Shear failures appear as low- angle ruptures in the rock
with a kind of ‘‘rock flour” along the newly created surface
(Iannacchione et al., 2019). Then, the shear propagates, and
the beam begins to dilate. Next, the beam begins to canti-
lever, causing tensile failure along the boundary between
the roof and the pillar. Once the beam fails completely, the
shear failure propagates to the next layer. In other mines
the shear failure continues to propagate upwards through
the roof layers until the fall has developed an arch shape.
However, at the Subtropolis Mine there are very weak
shales present above the caprock. Here, once the caprock
fails, the fall takes on a chimney shape and, in some cases,
extends approximately 30 ft into the roof.
Figure 5 shows 3D LiDAR scans capturing the shape
of this type of fall and its growth over time (shown in red).
In this roof fall, the failure extends approximately 30 ft into
the roof and up into the overlying coal seam. The shear
failure orientations are parallel to the long axis of the ellip-
tical-shaped cavities in this case study. All of these damages
have been captured using detailed geologic mapping and,
in some cases, 3D LiDAR scanning. For example, Figure 6
contains two photos from the Subtropolis Mine. Photo
A is an example of a dilating beam and Photo B shows
shear failure in three separate fully-grouted bolts from the
same area.
The operator noticed that these types of failures in the
N-S entries appear to concentrate at the northwest and
southeast corners of the pillars. In order to minimize the
number of these types of roof failures, the mine utilized
the stress control layout. The stress control layout utilizes
headings in the principal horizontal stress direction. This
minimizes damage to the headings but concentrates dam-
age into the crosscuts. To minimize damage to the cross-
cuts, the mine has implemented the use of rectangular
pillars. The mine also has implemented a secondary bolt-
ing strategy and included windows in every other crosscut.
According to Evanek et al. (2020), “Windows are used to
resist horizontal movements or deformation that occur
parallel to the maximum horizontal stress direction or per-
pendicular to the direction of stress failures. A window is
developed by leaving an increased thickness of roof rock
in the crosscuts, thus reducing the crosscut dimensions
vertically.” The first time the stress control layout was
implemented at the mine was in the northwestern region
of the mine in 2018, on the other side of the massive col-
lapse area. Numerical modeling was utilized in this study
to confirm damage patterns seen at the mine. The model,
as seen in Figure 7, maps the distribution of the maximum
principal horizontal stress over two different mine layouts
at Subtropolis. The principal horizontal stress in this model
is parallel to the Y-axis in the figure at N35W. The pillars
with a N-S orientation show a higher concentration at the
corners of the entry (indicated by red), which aligns with
damages mapped underground. The pillars oriented paral-
lel to the principal horizontal stress at N35W show damage
(indicated by red) being concentrated at the crosscuts and
not in the entries. This damage concentration has also been
mapped at Subtropolis and also confirms the stress control
layout is working as planned.
Figure 5. March 2018 roof fall in XC02 progression from
June 2019 to November 2019
Figure 6. A) Photo of shear failure in roof and B) Photo of
sheared bolts