5
for explicitly modeling fractures in rockmass since the fluid
flow through fractures vastly dominates the flow through
matrix pores. This is mainly because the cross-sectional area
of the fracture, and thus the flow rate through fractures, is
several orders of magnitude larger than that of pores within
the rock matrix.
Model Geometry
There were 105 geologic layers modeled from the core log
data, each with a thickness greater than 0.6 m (Figure 5).
The model length is extended in the horizontal direction to
a total of 8,000 m to avoid boundary effects on the mining-
induced deformations. One of the uncertainties in DFN
implementation for fractured rock is in constraining site-
specific fracture geometries and frictional properties that
best represent rockmass behaviors under stress.
Studying core samples from above abutment pillars
in longwall mines suggests propagation of subvertical frac-
tures perpendicular and parallel to the mining face (Van
Dyke at al., 2022). Therefore, the DFNs are modeled with
a constant dip direction of 90 degrees (perpendicular to the
face) and a dip angle with a mean of 90 degrees. From field
observations in Southwest Pennsylvania (Kohl, 1980), the
variation of the subvertical fracture dip angles is approxi-
mated to be 15° from 0–10 m above the mine roof, 12°
from 10–30 m, 10° from 30–60 m, and 5° degree for the
rest of the domain. Although fracture distribution can be
affected by the overall geology of the overburden, depth
of mining, panel width, and mining height, the use of val-
ues suggested by Kohl (1980) showed promising results for
permeability calculations as shown by (Khademian et al.,
2021).
The length of fractures is assumed limited to the thick-
ness of host strata with a negative power law distribution
and a scaling exponent of 1.1. The position of DFN frac-
tures is defined by a uniform distribution within each indi-
vidual lithology because pre-mining fracture systems are
believed to be the physical properties of the host rock (Feng
et al., 2018).
The density of fractures in each stratum is defined as
the P32 value, which is the cumulated fracture surface per
unit volume. Due to the lack of data for the fracture den-
sity in this site, the values are based on the previous cali-
bration study on a shallow Pittsburgh coal longwall mine
(Khademian et al., 2021). Figure 6 shows the fracture den-
sities in each stratum in the model. Fracture densities range
from 0.1 to 0.3 m2/m3 with higher values applied to weaker
zones that are identified by the logging data and field obser-
vations. The weak zones include the shallow weathered
zone, Waynesburg horizon, Uniontown horizon, Sewickley
horizon, and an interconnected fracture zone mainly caused
by the longwall mining. The interconnected fracture zone
represents a system of channels allowing flow transport to
the mine level. The combined thickness of the caved area
and the lower part of the fractured zone above it are known
as the interconnected fracture zone.
Through a set of field measurements, Palchik (2003)
suggested that the interconnected fracture zone is within
19–43 times the mining height, and Khademian et al.
(2021) showed the interconnected fracture zone around
Figure 6. DFN density applied to generate subvertical
fractures
Figure 7. Surface subsidence calculated in the 3DEC model
with a maximum subsidence of 1.47 m
for explicitly modeling fractures in rockmass since the fluid
flow through fractures vastly dominates the flow through
matrix pores. This is mainly because the cross-sectional area
of the fracture, and thus the flow rate through fractures, is
several orders of magnitude larger than that of pores within
the rock matrix.
Model Geometry
There were 105 geologic layers modeled from the core log
data, each with a thickness greater than 0.6 m (Figure 5).
The model length is extended in the horizontal direction to
a total of 8,000 m to avoid boundary effects on the mining-
induced deformations. One of the uncertainties in DFN
implementation for fractured rock is in constraining site-
specific fracture geometries and frictional properties that
best represent rockmass behaviors under stress.
Studying core samples from above abutment pillars
in longwall mines suggests propagation of subvertical frac-
tures perpendicular and parallel to the mining face (Van
Dyke at al., 2022). Therefore, the DFNs are modeled with
a constant dip direction of 90 degrees (perpendicular to the
face) and a dip angle with a mean of 90 degrees. From field
observations in Southwest Pennsylvania (Kohl, 1980), the
variation of the subvertical fracture dip angles is approxi-
mated to be 15° from 0–10 m above the mine roof, 12°
from 10–30 m, 10° from 30–60 m, and 5° degree for the
rest of the domain. Although fracture distribution can be
affected by the overall geology of the overburden, depth
of mining, panel width, and mining height, the use of val-
ues suggested by Kohl (1980) showed promising results for
permeability calculations as shown by (Khademian et al.,
2021).
The length of fractures is assumed limited to the thick-
ness of host strata with a negative power law distribution
and a scaling exponent of 1.1. The position of DFN frac-
tures is defined by a uniform distribution within each indi-
vidual lithology because pre-mining fracture systems are
believed to be the physical properties of the host rock (Feng
et al., 2018).
The density of fractures in each stratum is defined as
the P32 value, which is the cumulated fracture surface per
unit volume. Due to the lack of data for the fracture den-
sity in this site, the values are based on the previous cali-
bration study on a shallow Pittsburgh coal longwall mine
(Khademian et al., 2021). Figure 6 shows the fracture den-
sities in each stratum in the model. Fracture densities range
from 0.1 to 0.3 m2/m3 with higher values applied to weaker
zones that are identified by the logging data and field obser-
vations. The weak zones include the shallow weathered
zone, Waynesburg horizon, Uniontown horizon, Sewickley
horizon, and an interconnected fracture zone mainly caused
by the longwall mining. The interconnected fracture zone
represents a system of channels allowing flow transport to
the mine level. The combined thickness of the caved area
and the lower part of the fractured zone above it are known
as the interconnected fracture zone.
Through a set of field measurements, Palchik (2003)
suggested that the interconnected fracture zone is within
19–43 times the mining height, and Khademian et al.
(2021) showed the interconnected fracture zone around
Figure 6. DFN density applied to generate subvertical
fractures
Figure 7. Surface subsidence calculated in the 3DEC model
with a maximum subsidence of 1.47 m