10
approximately N35W. The figures also illustrate that the
cutter failure propagates along one side of the entry, result-
ing from uneven loading conditions due to horizontal
stress.
EFFECT OF CAPROCK THICKNESS ON
ROOF STABILITY
The caprock thickness refers to the unmined portion of the
limestone formation left in the roof during development.
This thickness plays a crucial role in roof stability in under-
ground stone mines, particularly when subjected to high
horizontal stress based on field experience. To evaluate the
interaction between the orientation of maximum horizon-
tal stress and caprock thickness on roof stability, FLAC3D
models were employed to simulate three different caprock
thicknesses— 2 ft (0.6 m), 4 ft (1.2 m), and 8 ft (2.4 m)—
across various orientations. The caprock is essential in sup-
porting the main roof at Subtropolis Mine, which primarily
consists of weak shale. If the caprock fails, it can lead to
progressive failure extending into the main roof, increasing
the risk of roof falls.
As illustrated in Figure 13, model results show that
with an 8-ft (2.4 m)thick caprock, the percentage of fail-
ure remains significantly lower, even at the most unfavor-
able orientation (90°), compared to the 2-ft (0.6 m) thick
caprock scenario. These results align with field observations
at the study mine, where thicker caprock (around 8 ft or
2.4 m) helps mitigate the impact of maximum horizontal
stress on roof stability. On the other hand, when the cap-
rock is thinner (less than 4 ft or 1.3 m), the orientation of
maximum horizontal stress becomes a critical factor, sig-
nificantly influencing roof stability.
To alleviate the effects of high horizontal stress, the
mine adopted a strategy of aligning headings parallel to the
maximum horizontal stress and creating “window” cross-
cuts. A window is created by leaving a thicker section of roof
rock in the crosscuts, which reduces the vertical dimensions
of the crosscuts. This method lowers the mining height
while maintaining the caprock thickness approximately 4 ft
(1.3 m) higher than the heading. Additionally, the opera-
tor opted to offset these windows to limit the propagation
of stress damage within the crosscuts. Evanek et al. (2020)
provide detailed information on the utilization of windows
to control crosscut damage caused by high horizontal stress.
This method reduces the mining height and maintains the
caprock thickness about 4 ft (1.2 m) higher than the head-
ing. This approach enhances stability: the headings are
more stable due to their parallel alignment with the maxi-
mum horizontal stress, while the crosscuts benefit from the
thicker caprock, which effectively redistributes stresses and
reduces shear stress concentrations in the immediate roof.
When the caprock is thin, shear stress in the roof across
the excavation is higher, as shown in Figure 14, compared
to thick caprock. This increases the likelihood of roof
falls since thin caprock cannot effectively redistribute the
induced stresses from the excavation. Instead, it concen-
trates them due to its higher stiffness relative to the main
roof. Thin caprock not only concentrates more stress in the
immediate roof but also experiences greater deformation,
making the roof less stable compared to thick caprock.
Based on the FLAC3D model under identical conditions,
Figure 13. Percentage roof failure from the FLAC3D models
for both thick and thin caprocks under high horizontal stress
Figure 14. Shear stress distribution across Entry 1 at 90°
orientation, 6 inches from the roofline, from the FLAC3D
model for thick and thin caprocks under high horizontal
stress. Refer to Figure 5 for the location of Entry 1
approximately N35W. The figures also illustrate that the
cutter failure propagates along one side of the entry, result-
ing from uneven loading conditions due to horizontal
stress.
EFFECT OF CAPROCK THICKNESS ON
ROOF STABILITY
The caprock thickness refers to the unmined portion of the
limestone formation left in the roof during development.
This thickness plays a crucial role in roof stability in under-
ground stone mines, particularly when subjected to high
horizontal stress based on field experience. To evaluate the
interaction between the orientation of maximum horizon-
tal stress and caprock thickness on roof stability, FLAC3D
models were employed to simulate three different caprock
thicknesses— 2 ft (0.6 m), 4 ft (1.2 m), and 8 ft (2.4 m)—
across various orientations. The caprock is essential in sup-
porting the main roof at Subtropolis Mine, which primarily
consists of weak shale. If the caprock fails, it can lead to
progressive failure extending into the main roof, increasing
the risk of roof falls.
As illustrated in Figure 13, model results show that
with an 8-ft (2.4 m)thick caprock, the percentage of fail-
ure remains significantly lower, even at the most unfavor-
able orientation (90°), compared to the 2-ft (0.6 m) thick
caprock scenario. These results align with field observations
at the study mine, where thicker caprock (around 8 ft or
2.4 m) helps mitigate the impact of maximum horizontal
stress on roof stability. On the other hand, when the cap-
rock is thinner (less than 4 ft or 1.3 m), the orientation of
maximum horizontal stress becomes a critical factor, sig-
nificantly influencing roof stability.
To alleviate the effects of high horizontal stress, the
mine adopted a strategy of aligning headings parallel to the
maximum horizontal stress and creating “window” cross-
cuts. A window is created by leaving a thicker section of roof
rock in the crosscuts, which reduces the vertical dimensions
of the crosscuts. This method lowers the mining height
while maintaining the caprock thickness approximately 4 ft
(1.3 m) higher than the heading. Additionally, the opera-
tor opted to offset these windows to limit the propagation
of stress damage within the crosscuts. Evanek et al. (2020)
provide detailed information on the utilization of windows
to control crosscut damage caused by high horizontal stress.
This method reduces the mining height and maintains the
caprock thickness about 4 ft (1.2 m) higher than the head-
ing. This approach enhances stability: the headings are
more stable due to their parallel alignment with the maxi-
mum horizontal stress, while the crosscuts benefit from the
thicker caprock, which effectively redistributes stresses and
reduces shear stress concentrations in the immediate roof.
When the caprock is thin, shear stress in the roof across
the excavation is higher, as shown in Figure 14, compared
to thick caprock. This increases the likelihood of roof
falls since thin caprock cannot effectively redistribute the
induced stresses from the excavation. Instead, it concen-
trates them due to its higher stiffness relative to the main
roof. Thin caprock not only concentrates more stress in the
immediate roof but also experiences greater deformation,
making the roof less stable compared to thick caprock.
Based on the FLAC3D model under identical conditions,
Figure 13. Percentage roof failure from the FLAC3D models
for both thick and thin caprocks under high horizontal stress
Figure 14. Shear stress distribution across Entry 1 at 90°
orientation, 6 inches from the roofline, from the FLAC3D
model for thick and thin caprocks under high horizontal
stress. Refer to Figure 5 for the location of Entry 1