11
the displacement magnitude at the center of the excavation
for the 2-ft (0.61 m) caprock was about five times greater
than that of the 8-ft (2.4 m) caprock.
Evanek et al. (2024) identified five key indicators to
predict potential instability zones at the study mine, the
likelihood of instability increases when multiple indicators
are present at a given location. These indicators include:
1. Heading Orientation: North-South headings
experienced more roof failures.
2. Overburden Depth: Depths exceeding 140 ft
(42.7 m) were flagged as high-risk.
3. Caprock Thickness: Thicknesses below 3 ft (0.9 m)
were considered hazardous.
4. Proximity to Stream Beds: Areas within 200 ft
(61.0 m) posed increased risk.
5. Proximity to Joints and Clay Veins: Areas within
100 ft (30.5 m) were flagged.
EFFECT OF FACE ADVANCE METHOD
ON ROOF PERFORMANCE UNDER HIGH
HORIZONTAL STRESS
Recognizing and assessing the stability of a mine roof is
a crucial practice that has been adopted by many under-
ground stone mines to prevent roof falls and ensure the
safety of workers. The Subtropolis Mine has tested various
ground control strategies to reduce stress concentrations
in the roof caused by high horizontal stress during face
advance. One key strategy involved identifying the most
effective face advance method to improve roof stability. The
roof performance and associated roof falls were evaluated
for two face advance methods—flat-front and arrowhead.
In the flat-front method, all faces advance simultaneously,
while the arrowhead method staggers them in an arrow
shape.
The FLAC3D models, utilizing a strain-softening
material model (see Table 4 for properties), were employed
to explore the impact of horizontal stress on roof stability
for flat-front and arrowhead advance methods. The non-
linear material model was used in this section through to
the end of the paper, rather than the elastic material model,
because interactions between entries can relieve induced
stresses in the roof. Using an elastic material model may
therefore produce unrealistic results. In the previous sec-
tion, however, the elastic model was appropriate because
only a single entry was excavated at various orientations.
Additionally, when an entry and a crosscut were excavated,
the interaction between them did not relieve the induced
stresses in the roof. Therefore, justifying the use of the elas-
tic model.
Figure 15 illustrates cutter failure based on field obser-
vations. The cutter failure depicted in Figure 15 highlights
the location of failure due to high horizontal stress but does
not account for the magnitude of the damage or failure.
Table 4. Summary of strain-softening material model used in
FLAC3D
Rock type Parameter Value
Immediate
roof
(limestone)
Cohesion, MPa 7.5
Poisson’s ratio 0.20
Friction angle, degree 30
Dilation angle, degree 10
Cohesion softening table: (0,7.5) (0.004, 2.0)
(0.01, 1.7)
Friciton softening table: (0,30) (0.0005, 15)
(0.01, 12)
Main roof
(shale)
Cohesion, MPa 3.5
Poisson’s ratio 0.20
Friction angle, degree 25
Dilation angle, degree 10
Cohesion softening table: (0,3.5) (0.001, 1)
(0.02, 0.2)
Friciton softening table: (0,25) (0.0005, 15)
Figure 15. Schematic representation of cutter failure for a)
flat-front mining and b) arrowhead mining in advancing
headings, where the high horizontal stress is parallel to the
driving direction. This schematic does not consider the
magnitude of damage
the displacement magnitude at the center of the excavation
for the 2-ft (0.61 m) caprock was about five times greater
than that of the 8-ft (2.4 m) caprock.
Evanek et al. (2024) identified five key indicators to
predict potential instability zones at the study mine, the
likelihood of instability increases when multiple indicators
are present at a given location. These indicators include:
1. Heading Orientation: North-South headings
experienced more roof failures.
2. Overburden Depth: Depths exceeding 140 ft
(42.7 m) were flagged as high-risk.
3. Caprock Thickness: Thicknesses below 3 ft (0.9 m)
were considered hazardous.
4. Proximity to Stream Beds: Areas within 200 ft
(61.0 m) posed increased risk.
5. Proximity to Joints and Clay Veins: Areas within
100 ft (30.5 m) were flagged.
EFFECT OF FACE ADVANCE METHOD
ON ROOF PERFORMANCE UNDER HIGH
HORIZONTAL STRESS
Recognizing and assessing the stability of a mine roof is
a crucial practice that has been adopted by many under-
ground stone mines to prevent roof falls and ensure the
safety of workers. The Subtropolis Mine has tested various
ground control strategies to reduce stress concentrations
in the roof caused by high horizontal stress during face
advance. One key strategy involved identifying the most
effective face advance method to improve roof stability. The
roof performance and associated roof falls were evaluated
for two face advance methods—flat-front and arrowhead.
In the flat-front method, all faces advance simultaneously,
while the arrowhead method staggers them in an arrow
shape.
The FLAC3D models, utilizing a strain-softening
material model (see Table 4 for properties), were employed
to explore the impact of horizontal stress on roof stability
for flat-front and arrowhead advance methods. The non-
linear material model was used in this section through to
the end of the paper, rather than the elastic material model,
because interactions between entries can relieve induced
stresses in the roof. Using an elastic material model may
therefore produce unrealistic results. In the previous sec-
tion, however, the elastic model was appropriate because
only a single entry was excavated at various orientations.
Additionally, when an entry and a crosscut were excavated,
the interaction between them did not relieve the induced
stresses in the roof. Therefore, justifying the use of the elas-
tic model.
Figure 15 illustrates cutter failure based on field obser-
vations. The cutter failure depicted in Figure 15 highlights
the location of failure due to high horizontal stress but does
not account for the magnitude of the damage or failure.
Table 4. Summary of strain-softening material model used in
FLAC3D
Rock type Parameter Value
Immediate
roof
(limestone)
Cohesion, MPa 7.5
Poisson’s ratio 0.20
Friction angle, degree 30
Dilation angle, degree 10
Cohesion softening table: (0,7.5) (0.004, 2.0)
(0.01, 1.7)
Friciton softening table: (0,30) (0.0005, 15)
(0.01, 12)
Main roof
(shale)
Cohesion, MPa 3.5
Poisson’s ratio 0.20
Friction angle, degree 25
Dilation angle, degree 10
Cohesion softening table: (0,3.5) (0.001, 1)
(0.02, 0.2)
Friciton softening table: (0,25) (0.0005, 15)
Figure 15. Schematic representation of cutter failure for a)
flat-front mining and b) arrowhead mining in advancing
headings, where the high horizontal stress is parallel to the
driving direction. This schematic does not consider the
magnitude of damage