4
bolts to a row, 8 ft (2.4 m) apart, with 8 ft (2.4 m) between
each row. The secondary roof support pattern includes 8 ft
(2.4 m) long fully grouted cable bolts, four bolts per row,
spaced between the primary fully grouted bolts.
The secondary roof support pattern is routinely used
in crosscuts and within the headings adjacent to crosscuts
(Evanek, et al., 2020). The floor rock at the study mine is
limestone, with a thickness ranging from 1 to 2 ft (0.3 to
0.6 m), underlain by sandstone. However, in the western
region of the mine, a layer of fireclay is present between the
limestone and the sandstone.
Limestone specimens from the study mine (Vanport
Limestone Formation) were tested under both uniaxial and
triaxial conditions. As summarized in Table 1, The tested
specimens had a length-to-diameter ratio of nearly 2.0,
with a diameter of approximately 2.0 inches (5.0 cm). The
unconfined compressive strength ranged from 18,270 to
19,720 psi (126 to 136 MPa), and the estimated friction
angle derived from the triaxial test results was about 45°.
Additionally, direct shear tests were performed on saw-cut
limestone specimens from the study mine to determine the
internal friction angle. The friction angle from the direct
shear tests varied between 21° and 30°, depending on the
choice of the inflection point where the slope of the shear
stress versus shear displacement curve changes. These mate-
rial properties were used to generate the FLAC3D models
to explore the effect of horizontal stress on roof stability.
Given the shallow overburden at the study mine, less
than 150 ft (45.7 m), the horizontal stress levels must be
sufficiently high to cause failure in such strong roof rock.
The direction of the maximum horizontal stress at the
mine was determined based on the pattern of roof cutters
and falls, with the predominant direction trending toward
N35W. However, stress mapping of roof damage shows
that the orientation of the maximum horizontal stress var-
ies across different areas of the mine. As a result, the mine
has had to repeatedly adjust the orientation of headings
to mitigate the adverse effects of high horizontal stress as
shown in Figure 4 (Evanek et al., 2024).
The mine operator has diligently experimented with
different techniques/methods to lessen the impact of the
instabilities in the outby crosscuts. The range of controls
used by the mine operator include, angled crosscuts, cross-
cut offsets, increase distance between crosscuts, arched
crosscuts, cable bolted crosscuts, altered blasting pattern,
and windows. A window is used to resist roof deformation
by leaving a strong brow of roof rock within the crosscuts
(Evanek, et al., 2020).
EFFECT OF MAXIMUM HORIZONTAL
STRESS ORIENTATION ON ROOF
STABILITY
The impact of maximum horizontal stress orientation on
roof stability was investigated using FLAC3D models.
Several models were developed to simulate two entries and
a crosscut under high horizontal stress. The entry width was
set at 40 ft (12.2 m), the crosscut at 30 ft (9.1 m), with a
mining height of 16 ft, and a cover depth of 130 ft (39.6 m).
These input parameters reflect the mining conditions at the
study mine. In all models, the maximum horizontal stress
was aligned with the Y-direction, while the mine geometry
rotated at an angle (θ), as shown in Figure 5. The angle θ
varied from 0 to 90 degrees in 15-degree increments. The
excavations were modeled in a uniform, elastic rock mass
with an interface between the caprock and the main roof.
Figure 4. Subtropolis Mine map with varying heading
orientations (after Evanek et al., 2024)
Table 1. Laboratory test results for uniaxial and triaxial tests
on limestone specimens from the Vanport formation
Specimen #
Length/
Diameter
σ3,
MPa
σ1,
MPa
Tangent
mod, MPa
1 1.94 0 136 24,500
2 1.95 0 126 45,100
3 1.95 3 141 64,800
4 1.95 6 136 57,400
5 1.94 8 171 NA
6 1.96 14 204 48,300
7 1.97 17 225 72,500
8 1.97 11 204 NA
bolts to a row, 8 ft (2.4 m) apart, with 8 ft (2.4 m) between
each row. The secondary roof support pattern includes 8 ft
(2.4 m) long fully grouted cable bolts, four bolts per row,
spaced between the primary fully grouted bolts.
The secondary roof support pattern is routinely used
in crosscuts and within the headings adjacent to crosscuts
(Evanek, et al., 2020). The floor rock at the study mine is
limestone, with a thickness ranging from 1 to 2 ft (0.3 to
0.6 m), underlain by sandstone. However, in the western
region of the mine, a layer of fireclay is present between the
limestone and the sandstone.
Limestone specimens from the study mine (Vanport
Limestone Formation) were tested under both uniaxial and
triaxial conditions. As summarized in Table 1, The tested
specimens had a length-to-diameter ratio of nearly 2.0,
with a diameter of approximately 2.0 inches (5.0 cm). The
unconfined compressive strength ranged from 18,270 to
19,720 psi (126 to 136 MPa), and the estimated friction
angle derived from the triaxial test results was about 45°.
Additionally, direct shear tests were performed on saw-cut
limestone specimens from the study mine to determine the
internal friction angle. The friction angle from the direct
shear tests varied between 21° and 30°, depending on the
choice of the inflection point where the slope of the shear
stress versus shear displacement curve changes. These mate-
rial properties were used to generate the FLAC3D models
to explore the effect of horizontal stress on roof stability.
Given the shallow overburden at the study mine, less
than 150 ft (45.7 m), the horizontal stress levels must be
sufficiently high to cause failure in such strong roof rock.
The direction of the maximum horizontal stress at the
mine was determined based on the pattern of roof cutters
and falls, with the predominant direction trending toward
N35W. However, stress mapping of roof damage shows
that the orientation of the maximum horizontal stress var-
ies across different areas of the mine. As a result, the mine
has had to repeatedly adjust the orientation of headings
to mitigate the adverse effects of high horizontal stress as
shown in Figure 4 (Evanek et al., 2024).
The mine operator has diligently experimented with
different techniques/methods to lessen the impact of the
instabilities in the outby crosscuts. The range of controls
used by the mine operator include, angled crosscuts, cross-
cut offsets, increase distance between crosscuts, arched
crosscuts, cable bolted crosscuts, altered blasting pattern,
and windows. A window is used to resist roof deformation
by leaving a strong brow of roof rock within the crosscuts
(Evanek, et al., 2020).
EFFECT OF MAXIMUM HORIZONTAL
STRESS ORIENTATION ON ROOF
STABILITY
The impact of maximum horizontal stress orientation on
roof stability was investigated using FLAC3D models.
Several models were developed to simulate two entries and
a crosscut under high horizontal stress. The entry width was
set at 40 ft (12.2 m), the crosscut at 30 ft (9.1 m), with a
mining height of 16 ft, and a cover depth of 130 ft (39.6 m).
These input parameters reflect the mining conditions at the
study mine. In all models, the maximum horizontal stress
was aligned with the Y-direction, while the mine geometry
rotated at an angle (θ), as shown in Figure 5. The angle θ
varied from 0 to 90 degrees in 15-degree increments. The
excavations were modeled in a uniform, elastic rock mass
with an interface between the caprock and the main roof.
Figure 4. Subtropolis Mine map with varying heading
orientations (after Evanek et al., 2024)
Table 1. Laboratory test results for uniaxial and triaxial tests
on limestone specimens from the Vanport formation
Specimen #
Length/
Diameter
σ3,
MPa
σ1,
MPa
Tangent
mod, MPa
1 1.94 0 136 24,500
2 1.95 0 126 45,100
3 1.95 3 141 64,800
4 1.95 6 136 57,400
5 1.94 8 171 NA
6 1.96 14 204 48,300
7 1.97 17 225 72,500
8 1.97 11 204 NA