5
5 m (15 ft), respectively. Grout or bentonite were used to
cement the borehole annulus outside of the casing.
EXPERIMENTAL METHOD
Researchers conducted falling-head slug tests to measure
the permeability of the slotted casing lengths. The test zones
were chosen to correspond to probable locations of high
ground movement where increases in permeability were
considered most likely, at the Uniontown and Sewickley
coal horizons. An INW PT2X (Seametrics ™) piezometer
[12] tracked the long-term equilibrium water height and
water slug height within the corresponding monitoring
hole (Figure 6). For the falling-head slug tests, a water slug
height of up to 3 m (10 ft) was added to the boreholes.
The previous NIOSH study [3] at the shallow cover
site recorded the fall of water head at 5, 30, or 60 s inter-
vals, depending on the expected drainage rate of the water.
However, during the current study the rate of loss was at
a much slower rate and, therefore, the data was recorded
every minute, and the falling rate was later determined
from the downloaded data.
Borehole slug testing at the deep cover site began when
the LW face was 273 m (897 ft) away from the first panel
and continued until after the completion of mining for both
panels. When the LW face was 364 m (1193 ft) away from
the monitoring site, the slug tests were conducted on each
of the boreholes twice a week. The frequency increased to
three times per week when the LW face was within 100 m
(328 ft) of the boreholes. When the active LW face passed
the borehole locations and was further away, the sampling
rate for slug tests was approximately once per week. Once
the LW panel was completely mined, long-term continuous
water pressure readings were recorded every eight hours.
The piezometer readings, water slug height (Hw), and
initial slug height (Ho) were converted to Hw/Ho values.
For the calculation of permeability from falling-head slug
tests, Hw/Ho values were plotted on a semi-log graph to
determine the T37 time, which is the time, in minutes, in
which the water slug drained to 37% of the initial water
slug height [3, 13]. For a 10-ft water slug, the T37 time
would be calculated as the time it took for the slug to drain
to 3.7 ft. If the T37 value did not fall within the given data
range, two points were chosen along the most linear section
of the semi-log plot of the slug test, and the slope of those
two points estimated the T37 time. Watkins et al. [3] details
the application of T37 in the Hvorslev method to deter-
mine the hydraulic conductivity. The hydraulic conductiv-
ity is dependent upon the well casing radius, the well screen
radius, and the length of the well screen. For both VEP
boreholes and the FEB boreholes, the well casing radius is
0.052 m (2 in), and the well screen radius is 0.104 m (4 in).
The well screen lengths are 4.6 m (15 ft) for VEP-U, 3.0 m
(10 ft) for VEP-S, 7.9 m (26 ft) for FEB‑1, 4.6 m (15 ft) for
FEB‑2, and 2.1 m (7 ft) for FEB‑3.
Multiple assumptions about the slug test and ground
aquifer are applied when conducting the Hvorslev method
to calculate permeability. This method assumed that the
water slug is added instantaneously, the groundwater flow
is described by Darcy’s Law, and the volume of water that
flows into the aquifer is equivalent to the change in water
volume within the well casing [3]. The aquifer is assumed
to be incompressible, lithologically homogeneous, isotro-
pic, and vertically confined by aquicludes. The injection
well is assumed to have a negligible radius in relation to the
size of aquifer, the well has a screen with a negligible head
loss, and the water slug flow travels horizontally away from
the well in all directions.
RESULTS AND DISCUSSION
Shallow Cover Results
Pre-mine-by levels at the shallow site are seen to be around
430 mD for the Sewickley horizon. During the time of the
mine-by, the permeability measurements at the Sewickley
horizon increased to more than 1,100 mD and fluctuated
between 1,100 mD and 1,400 mD while the LW panel
moved farther from the monitoring location (Figure 7).
The maximum measurement occurred when the LW panel
was 305 m (1,000 ft) from the monitoring borehole Figure 6. The sideview of the VEP-S and VEP-U monitoring
boreholes (not to scale)
5 m (15 ft), respectively. Grout or bentonite were used to
cement the borehole annulus outside of the casing.
EXPERIMENTAL METHOD
Researchers conducted falling-head slug tests to measure
the permeability of the slotted casing lengths. The test zones
were chosen to correspond to probable locations of high
ground movement where increases in permeability were
considered most likely, at the Uniontown and Sewickley
coal horizons. An INW PT2X (Seametrics ™) piezometer
[12] tracked the long-term equilibrium water height and
water slug height within the corresponding monitoring
hole (Figure 6). For the falling-head slug tests, a water slug
height of up to 3 m (10 ft) was added to the boreholes.
The previous NIOSH study [3] at the shallow cover
site recorded the fall of water head at 5, 30, or 60 s inter-
vals, depending on the expected drainage rate of the water.
However, during the current study the rate of loss was at
a much slower rate and, therefore, the data was recorded
every minute, and the falling rate was later determined
from the downloaded data.
Borehole slug testing at the deep cover site began when
the LW face was 273 m (897 ft) away from the first panel
and continued until after the completion of mining for both
panels. When the LW face was 364 m (1193 ft) away from
the monitoring site, the slug tests were conducted on each
of the boreholes twice a week. The frequency increased to
three times per week when the LW face was within 100 m
(328 ft) of the boreholes. When the active LW face passed
the borehole locations and was further away, the sampling
rate for slug tests was approximately once per week. Once
the LW panel was completely mined, long-term continuous
water pressure readings were recorded every eight hours.
The piezometer readings, water slug height (Hw), and
initial slug height (Ho) were converted to Hw/Ho values.
For the calculation of permeability from falling-head slug
tests, Hw/Ho values were plotted on a semi-log graph to
determine the T37 time, which is the time, in minutes, in
which the water slug drained to 37% of the initial water
slug height [3, 13]. For a 10-ft water slug, the T37 time
would be calculated as the time it took for the slug to drain
to 3.7 ft. If the T37 value did not fall within the given data
range, two points were chosen along the most linear section
of the semi-log plot of the slug test, and the slope of those
two points estimated the T37 time. Watkins et al. [3] details
the application of T37 in the Hvorslev method to deter-
mine the hydraulic conductivity. The hydraulic conductiv-
ity is dependent upon the well casing radius, the well screen
radius, and the length of the well screen. For both VEP
boreholes and the FEB boreholes, the well casing radius is
0.052 m (2 in), and the well screen radius is 0.104 m (4 in).
The well screen lengths are 4.6 m (15 ft) for VEP-U, 3.0 m
(10 ft) for VEP-S, 7.9 m (26 ft) for FEB‑1, 4.6 m (15 ft) for
FEB‑2, and 2.1 m (7 ft) for FEB‑3.
Multiple assumptions about the slug test and ground
aquifer are applied when conducting the Hvorslev method
to calculate permeability. This method assumed that the
water slug is added instantaneously, the groundwater flow
is described by Darcy’s Law, and the volume of water that
flows into the aquifer is equivalent to the change in water
volume within the well casing [3]. The aquifer is assumed
to be incompressible, lithologically homogeneous, isotro-
pic, and vertically confined by aquicludes. The injection
well is assumed to have a negligible radius in relation to the
size of aquifer, the well has a screen with a negligible head
loss, and the water slug flow travels horizontally away from
the well in all directions.
RESULTS AND DISCUSSION
Shallow Cover Results
Pre-mine-by levels at the shallow site are seen to be around
430 mD for the Sewickley horizon. During the time of the
mine-by, the permeability measurements at the Sewickley
horizon increased to more than 1,100 mD and fluctuated
between 1,100 mD and 1,400 mD while the LW panel
moved farther from the monitoring location (Figure 7).
The maximum measurement occurred when the LW panel
was 305 m (1,000 ft) from the monitoring borehole Figure 6. The sideview of the VEP-S and VEP-U monitoring
boreholes (not to scale)