ARTICLE IN PRESS

Hydraulic conductivity changes and influencing factors in longwall

overburden determined by slug tests in gob gas ventholes

* C. O ̈ zgen Karacan , Gerrit Goodman

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory, PO Box 18070, Pittsburgh, PA 15236, USA

* Corresponding author. Tel.: +1412 386 4008; fax: +1412 386 6595.
 

E-mail address: cok6@cdc.gov (C. O ̈ zgen Karacan).
 

abstract

This study presents the results of core-log analyses from the exploration boreholes, the analyses of face

advance rates, and the results of downhole monitoring studies performed in gob gas ventholes for

calculation of changes in hydraulic properties in the longwall overburden at a mine site in southwestern

(SW) Pennsylvania section of Northern Appalachian Basin. In the first part of the study, coal measure

rocks in overburden strata were analyzed and the locations where possible fractures and bedding plane

separations would occur were evaluated. In the second part, the hydraulic conductivities were

computed by two different slug test analyses methods using the water level changes measured in gob

gas ventholes as longwall face approached. Hydraulic conductivities were analyzed with respect to the

changes in overburden depth, the locations of the borehole, and mine face advance rates. These data

were used to interpret the potential productivities of the gob gas ventholes as a result of fracturing and

changes in hydraulic conductivities.

The general results showed that the probability of fracturing and bedding plane separations in the

overburden increase between strong and weak rock interfaces. Also, the probability of bedding plane

separations increases as the interface is close to the extracted coal seam. Evaluation of slug tests showed

that the hydraulic conductivity developments in the boreholes and their potential production

performances are affected by the underground strata and the roof materials. In situations where the

roof material is stiff and thick, the development of high permeability fractures around the borehole will

be less. Results also indicated that borehole location with respect to face position affects the fracturing

time and permeability evolution as well. Greater overburden depths generally cause earlier fracturing as

longwall face approaches, but eventually result in lower hydraulic conductivities and potentially less

effective boreholes. Increasing mining rates also resulted in generally lower hydraulic conductivities in

the overburden. The results of this study were intended to improve the interpretation of gob gas

venthole performance and to provide better siting of these boreholes.

1. Introduction

Methane inflow from overburden strata during longwall

mining is affected by the magnitude of fracturing and the time

that the fractures stay open as the panel is extracted. The reservoir

characteristics and their changes during mining also affect

production potential of the gob gas ventholes (GGVs) that are

commonly used to control the methane emissions from the

fractured zone and are drilled from the surface to a depth that

places them above the caved zone [1]. This consequently impacts

the efficiency of methane drainage and control in the mining

environment. Thus, the ability to predict changes in formation

characteristics based on various factors at a given location

increases the understanding of GGV production and provides

improved placement where they may be more productive.

Singh and Kendorski [2] and Kendorski [3] evaluated the

disturbance of rock strata resulting from mining beneath surface

water and waste impoundments. In their analysis, they describe a

caved zone that extends from the mining level to 3–6 times the

seam thickness, a fractured zone that extends from the mining

level to 30–58 times the seam thickness, an aquiclude zone where

there is no change in permeability that extends from 30 times the

seam thickness to 50 ft below ground surface, and a surface

cracking zone that is 50 ft thick (Fig. 1). Hasenfus [4] described the

hydro-geomechanics of overburden aquifer response to longwall

mining with specific relation to the Pittsburgh coal seam in West

Virginia. Based on the measurements, the overburden was divided

into four zones: a gob, a highly fractured zone, a composite beam

zone and a surface layer. Palchik [5] reported that, based on the

measurements in the Donetsk coal basin, the thickness of the
fractured zone can vary up to 100 times the height of the mined

coal bed depending on the size of the panel and the geology and

geomechanical properties of the layers.

Fig. 1. Strata behavior of surface and subsurface zones as a result of longwall mining [2].

The characteristics of fracturing and the subsidence of overburden are also revealed through predictive techniques and field

studies. Li et al. [6] investigated the distribution of overburden

fracturing, the distribution pattern of methane and methane

migration mechanisms in the mining-induced fracture zones, and

the subsidence over a longwall panel based on the results in a

physical model. Cui et al. [7] performed a prediction study for the

progressive surface subsidence above longwall mining using a

time function. They calculated differential subsidence characteristics, such as progressive slope, progressive curvature and

horizontal strain. Luo [8] predicted that the subsidence velocity

distribution along the longwall face is a normal distribution with

respect to the location of the on-going longwall face. Surface

dynamic characteristics were also linked to the longwall advance

rate. Preusse [9] discussed the surface time-dependent effects and

their relations with the face advance rates. He suggested that

regulating the advance rate in a longwall mining operation can be

an effective means for reducing the potential disturbance to

surface structures.

The change in hydraulic conductivity over and around longwall

panels as a result of fracturing and subsidence, which is of

primary importance for controlling methane from gobs using

GGVs, was initially investigated for its potential effect on surface

water resources [10]. A combined finite element model of the

deformation of the overlying strata and its influence on ground

water flow was used to define the change in local and regional

water resources. Their results showed that there would be no

predicted long term effects of longwall mining on ground water

flow. Fawcett et al. [11] described a theoretical investigation into

the zones of increased hydraulic conductivity caused by rock

failure above a longwall panel. They correlated predicted failure

heights with existing experimental values. Changes in the threedimensional hydraulic conductivity field that accompany undermining by a longwall panel were evaluated by Liu and Elsworth

[12] to define the potential for changes in flow patterns and

desaturation. The model results showed that hydraulic conductivity increases were restricted to shallow depths ahead of the

advancing face and to deep zones behind the face, particularly in

the caving and abutment shear zones. Gale [13] used computer

modeling techniques to simulate rock fracture, caving, stress

redistribution, and induced hydraulic conductivity enhancements

around longwall panels. As a result of this study, he reported that

the horizontal conductivity can be significantly enhanced along

bedding planes within and well outside the panel. This probably

will vary depending on the nature of each site. Also, Whittles et al.

[14,15] conducted studies on the effects of different geotechnical

factors on gas sources and gas flow paths for UK longwall

operations. They studied how roof geology and its interactions

with boreholes may cause the deformation and closure of the

boreholes drilled for methane control.

From a methane control point of view, hydrologic reservoir

characteristics and their variations during mining affect both the

emissions of gas into mine workings and the production potential

of the GGVs that are used to mitigate these emissions [16]. Slug

tests are widely used by hydrologists to evaluate hydraulic

conductivities of aquifer horizons. Although they are frequently

used to evaluate water conductivities of aquifers, soils and minewaste areas created in surface mining [17–20], their use in

longwall overburden is limited. The use of slug tests in active

GGVs to measure the transient changes in hydraulic conductivities

as a function of time and face location is currently non-existent in

the literature. This paper presents an integrated study of

evaluation of underground strata, their predicted potential

responses to longwall mining, and evaluations of mining rate

effects using ‘‘in-situ’’ slug tests. These tests were performed in

GGVs to characterize the changes in hydraulic conductivities of

the slotted sections of the boreholes.

2. General information on the geology and hydrology of coal

fields in Southwestern (SW) Pennsylvania—study site

The vast majority of coal in Pennsylvania was deposited during

the Pennsylvanian period [21]. The largest production of underground coal in Pennsylvania is mined from the Pittsburgh coal

seam which is the base of the Monongahela Group. The

Monongahela Group encompasses the stratigraphy from the base

of the Pittsburgh coal bed to the base of the Waynesburg coal bed,

extending to a thickness of 270–400 ft (82–122 m). In mining of

the Pittsburgh coal bed, the source of methane was found to be

mainly between the Sewickley coal bed horizon and the top

of Pittsburgh coal bed. Thus, a specific emphasis was given in

this study, as will be seen in the next sections, to the strata below

the Uniontown sandstone, including the Sewickley limestone and

coal bed.
Ground water movement in SW Pennsylvania section of

Northern Appalachian Basin is primarily due to flow through

fractures and bedding plane separations between strong–weak

interfaces of coal measure rocks. Ferguson [22] described a

pattern of near-surface fractures and bedding plane separations

that he attributed to stress-relief fracturing (Fig. 2).

Fig. 2. Block diagram of generalized geologic section showing stress-relief fracturing [23].

They are

generated when neighboring, supporting rocks are removed by

erosion. This pattern involves vertical fractures parallel to the

valleys and situated in valley floors. Subsidence caused by

longwall mining results in tension and compression of the nearsurface zone, increasing or decreasing the fracture transmissibility. Wyrick and Borchers [23] reported that groundwater flow

associated with stress-relief fractures occur in the valley bottoms

and valley sides. Stoner et al. [24] also showed that the

permeability of the undisturbed rocks overlying the Pittsburgh

coal bed decreases by a factor of ten for each 100 ft (30.3 m) of

depth in SW Pennsylvania, specifically in Green County, in SW

Pennsylvania. They also found, based on aquifer tests and

historical well drilling practice, that the local groundwater system

roughly parallels the surface at a depth of 150–175 ft. However,

these depths may show local variations according to where the

wells are drilled and according to the seasonal precipitation.

Fig. 3. A model showing the potentiometric head computed from a steady-state model for bituminous coalfields of Greene County, Pennsylvania [24].

In SW Pennsylvania, the hills constitute hydrologic islands. A

separate groundwater flow system exists within each hydrologic
island. They are hydrologically segregated from the local groundwater flow system in adjacent islands (Fig. 3). The base of the local

flow system is defined by the maximum depth at which groundwater originating within the hydrologic island will flow upward to

discharge in the adjacent stream valley. Recharge to the local

system is completely from within the hydrologic island. The local

system discharges into the adjacent system valleys with some

leakage into the deeper intermediate and regional groundwater

flow systems. In areas adjacent to larger systems and rivers, local

groundwater that leaks downward may commingle with intermediate, even regional flow, which rises to discharge within the

valley (Fig. 3). Among all the topographic positions, wells in the

valleys have higher change of success of producing high yields due

to fracturing beneath the valley bottom. This fracturing is

expected to diminish beneath the adjacent hills, thereby limiting

the effective areal extent and yield of such aquifers.

3. Downhole monitoring of GGVs

The production performance of GGVs can be closely related to

their locations, advances of the longwall face, borehole completions, wellhead designs, and operation of the exhausters. The

other important factor that affects the performance of GGVs and

the success of methane control is the presence of horizontal and

vertical fractures created underground during longwall mining

and their hydraulic conductivities for fluid flow.

For the characterization of both the fracturing of underground

strata and the resultant changes in hydraulic conductivity in the

GGVs during longwall mining, seven boreholes were instrumented with submersible pressure transducers equipped with selfcontained data loggers at a mine site in SW Pennsylvania. Two of

these were monitoring boreholes drilled as part of NIOSH’s

monitoring-borehole experimental program performed in

2006–2007. These boreholes will be called MBH-1 and -2

hereafter, throughout this paper. MBH-1 and -2 were 50 ft apart

from each other and were drilled to depths of 721 and 755 ft,

respectively, in Panel A (1450 ft wide; 11335 ft long) to monitor

two different horizons above the Pittsburgh coal bed. The bottom

of MBH-1 was 106 ft and the bottom of MBH-2 was 72 ft above the

top of the Pittsburgh coal, which was at a depth of 827 ft at that

location. This is compared to the 40–45 ft distance above top of

coal for the regular GGVs that the mine drilled. The monitoring

boreholes were completed with the regular GGV completion

standards except the bottoms were cased with 30 ft slotted pipe

for MBH-1 and with 20 ft slotted pipe for MBH-2, instead of the

conventional 200 ft slotted pipe utilized in GGVs. The experimental boreholes were instrumented to monitor static pressure

and gas concentration changes in the borehole, as well as water

level changes in the boreholes as they were fractured during

mining. Water levels were tracked using downhole transducers

and were used to calculate permeability changes in rock

formations. A similar approach was used for similar purposes in

the gob GGVs drilled in B (1450 ft wide; 11973 ft long), C (1450 ft

wide; 4609 ft long) and D panels (1450 ft wide; 11686 ft long). The

boreholes instrumented with downhole transducers on these

panels were B-3, B-4, C-1, C-2, and D-1.

Fig. 4 shows the locations of the boreholes on a 3-D surface

contour map generated from the surface elevation data.

B-3

C-2

MBH-2 MBH-1

B-Panel

A-Panel

D-Panel

Pittsburgh CB Top Elevation ~ -510 ft

Surface Elevation Contours

Mining

direction

Elevation above sea level 

D-1

-500

-600

-900

-800

-1,000

-1,100

-1,200

-1,300

-1,400

-1,500

B-4

C-1

C-Panel

Fig. 4. Location of the boreholes on surface topography over the longwall panels. Sea level is zero. Above-sea-level elevations were taken as the negative direction.

In this

graph, only the panels where the instrumented boreholes were

located are shown. The figure shows that GGVs were drilled at

different topographic locations with cold colors representing the

higher locations with respect to a datum, which was selected as

sea level in this study. As the figure shows, most of the boreholes

were drilled on either a hill side or a hill-top with two of them

(5B-3 and 6B-2) drilled on valley bottoms. The differences in

surface elevations of drill locations change the total drilling

depths of the wellbores as the top of the Pittsburgh seam was

nearly horizontal at those locations. Table 1 shows the drilling and

completion details of the boreholes monitored for this study.

The downhole transducers recorded water head data in the

boreholes as the mining face approached and undermined the
borehole location. This data were later used to calculate the

changes in hydraulic conductivity and transmissibility of overlying formations within the slotted sections of the boreholes. This

data, together with face location, face advance rate, stratigraphy,

and the completion parameters of the boreholes can be used to

predict how the fluid flow-related properties of the formation may

change and how the borehole location can be adjusted to drill the

boreholes in the most permeable location for increased capture of

released methane. Downhole instrumentation in the boreholes

also helps to see if there is any water build-up in the boreholes

after undermining; as this may adversely affect the operation of

the exhauster pump. The setting depths of the transducers in the

boreholes are given in Table 1.

Table 1

Monitored boreholes in mining areas and their completion and instrumentation parameters.

Borehole Distance from Distance to Total depth Depth of slotted Overburden to Bottom of slotted Transducer Surface

startup (ft) TG (ft) (ft) casing (ft) top of coal (ft) casing to coal (ft) depths (ft) elevation (ft)

MBH-1 4630 330 721 691 827 106 700 1377

MBH-2 4680 330 755 735 827 72 710 1364

B-3 4535 321 588 388 628 40 577 1155

B-4 6535 330 760 560 800 40 562 1325

C-1 460 370 802 602 842 40 736 1350

C-2 2369 330 627 427 667 40 531 1150

D-1 375 330 742 542 782 40 669 1296

4. Longwall face advance rates during the tests

The fractures and permeability enhancements in GGVs during

longwall mining may be closely related to dynamic deformation,

progression of subsidence, and settlement of the fragmented roof

mass. The main factors influencing the dynamic deformation

process are average mining height, overburden depth, face length,

rate of face advance, and residual effects from old mining

activities and face stoppages. Some of these have been discussed

in the previous sections. In addition to these, regulating the face

advance rate in longwall mining was proposed as an effective

means of reducing the disturbance potential to surface structures

associated with subsidence process [9]. However, there are

seemingly two opposite views about the effects of the face

advance rate on the magnitudes of dynamic deformations. In the

US, the common belief is that a faster and constant face advance

rate produces smaller dynamic deformations, whereas in

Germany, it is thought that a slow rate is more beneficial to

surface structures [9]. Thus, advance rate may be effective also on

the underground deformation and on the enhancement of

hydraulic conductivity around gob gas boreholes.

In order to be able to evaluate face advance history as a

parameter in evaluation of hydraulic conductivities of the boreholes and to gain some insights on flow potential of the boreholes,

the face advance rates reported by the mining company during

longwall operation of A–C panels as well as D panel were

analyzed. In these analyses, daily advance rates and the periods

when the boreholes were intercepted were evaluated. Also, in

order to match the downhole transducer data with the face

location and borehole location, the dates before and after borehole

interception were plotted as a function of face location from panel

start and polynomials were fit to those data. These equations were

used to match the location of the face at a given time and day with

the corresponding downhole pressure data recorded by the

transducer. Fig. 5 shows, as an example, the history of face

advance rate during mining of B panel, borehole interception

intervals and how the analyses described above were performed.

Fig. 5a shows the history of face advance rate during mining of

B panel. The total mining days, including two weeks of miner’s

vacation, was around 325 days for this panel. The daily advance

rates during mining were around 40 ft/day (Fig. 5a), except a

slower period with rates of 20 ft/day around the 250th day

of mining. However, a majority of advance rates were in the

35–65 ft/day range, where faster advances were achieved at the

beginning and end of the mining. B-3 and -4 were intercepted

between 90–95 days and 148–157 days from start of mining,

respectively (shaded areas in Fig. 5a). During these time intervals,

mining rates were generally around 30 ft/day and 50 ft/day for B-3

and -4, respectively (Figs. 5b, c).

Quadratic equations were fitted to face location versus mining

days. Equations shown in Figs. 5b and c and also given in Table 2

indicate that the data can be represented by these equations with

a regression coefficient (R2

) of more than 0.99. These equations

can also be used to find the rate of face advance at a given day by

taking the derivative of the equations. The equations obtained

using the similar approach for all other monitored boreholes are

collectively given in Table 2.

5. Estimating the location of bedding-plane fractures occurring

during mining

Horizontal fractures occur along weak–strong rock layer

interfaces in the movement of the overburden. The formation,

thickness and location of horizontal fractures influence the

hydraulic conductivity of the overburden strata, which creates

methane emission pathways and controls methane emissions into

the mine. Thus, the ability to estimate the location and

magnitudes of the fractures and their hydraulic flow properties

are important for placement of boreholes and, consequently, for

controlling methane more effectively.

Palchik [5] has shown that the extent of fractured zone induced

by mining in the Donetsk coal basin can be determined based on

the change in natural methane emission from this zone. After a

modification to the earlier tests system, he [25] was able to locate

the individual fractures at bedding plane separations. The method

was based on isolation of each methane emission zone in a

borehole by measuring the pressure and flow rate and correlating

these zones with the strata logs. Further, the presence and

absence of estimated horizontal fractures was correlated with

uniaxial compressive strength and thickness of rock layers,

distances from the extracted coal seam to the rock layer interfaces,

and the thicknesses of extracted coal seams. Observations on the

presence and absence of horizontal fractures at different rock

layer interfaces of the overburden showed that the probability

increased with increasing compressive strength difference of

neighboring rock layers as well as decreasing distance of the

layer from the mined coal bed.

In this paper, a similar approach was adapted for the rock

layers in the overburden of the Pittsburgh coal at the studied mine
site by imposing the lithology and thickness information obtained

from nearby core logs to GGV locations after a depth correction.

The depth correction procedure allowed the rock layers, the

slotted casing locations, the transducer positions, and all other

depth and borehole completion information given in Table 1 to be

compared in the same depth scale as shown in Fig. 6.

100 B-3 B-4 

80 

60 

40 

20 

0 

Miner’s vacation 

50.00 100.00 150.00 200.00 250.00 300.00 350.00 

Face Location from

Panel Setup (ft) Face Advance (ft/day) 

0.00 

Time (day) 

50 6750 

y = -0.5795x2 + 225.82x - 14455 

R2 = 0.9984 

4650 

4600 

y = -0.2917x2 + 86.554x - 964.84 

R2 = 0.9945 

86 

Time from Start of Panel (Days) 

40 

Advance Rate (ft/day)


Face Location from Panel


Setup (ft) 


6700 

Advance Rate (ft/day)

6650 

6600 4550 30 6550 

4500 6500 

20 6450

4450 6400 

10 6350 4400 

6300 

4350 0 6250 0

88 90 92 94 96 146 148 150 152 154 156 158 

Time from Start of Panel (days) 

Fig. 5. Daily face advance rates (a) and face location relations (b, c) for B-3 and -4 as the face approached to their locations. In these figures, dashed lines are the daily

advance rates, where continuous lines are the face distances from panel setup.

10 

20 

30 

40 

50 

60 

70 

Table 2

Face location versus time relations developed for monitored boreholes in panels A–D.

Boreholes Face position vs. mining days–[Eq. (y)] 1st derivative of Eq. (y)

MBH-1 y 1⁄4 -0.8541x2

+196.98x-6302.8 R2 1⁄4 0.99 dy/dx 1⁄4 -1.7082x+196.98

MBH-2 y 1⁄4 -0.8541x2

+196.98x-6302.8 R2 1⁄4 0.99 dy/dx 1⁄4 -1.7082x+196.98

B-3 y 1⁄4 -0.2917x2

+86.554x-964.8 R2 1⁄4 0.99 dy/dx 1⁄4 -0.5834x+86.55

B-4 y 1⁄4 -0.5795x2

+225.82x-1445 R2 1⁄4 0.99 dy/dx 1⁄4 -1.1590x+225.82

C-1 y 1⁄4 -1.4702x2

+76.78x 2

-401.48 R2 1⁄4 0.98 dy/dx 21⁄4 -2.9404x+76.78

C-2 y 1⁄4 -0.2995x +79.99x-1305.8 R 1⁄4 0.99 dy/dx 2

1⁄4 -0.5990x+79.99

D-1 y 1⁄4 -1.5714x2

+90.31x-902.2 R 1⁄4 0.99 dy/dx 1⁄4 -3.1420x+90.31

The derivatives of the equations (advance rate) are also presented.

Fig. 7 shows a plot of the ratio of thickness of each layer to

extracted coal bed thickness and their heights relative to the top

of the extracted coal bed. This graph also shows the strata

identifications as sandstone, limestone, and weak strata (coal,

shales of all kind and clay stones). Palchik [25] has noted, based on

the field measurements, that separations at the interfaces and

formation of fractures may be restrained by thick and strong rock

layers, called ‘‘bridge layer’’, in the overburden. The ‘‘bridge layer’’

in the study area is the thick limestone close to the top of the

overburden section shown in Fig. 6. According to Palchik [25], the

probability of bedding plane separation along the rock layer

interfaces decreases with increasing height above the coal seam

(H) and with increasing thickness of the bridge layer. According to

this concept and these observations, the probability of fracture

development and bedding plane separations along the bridge

layer (if it exists) at the study site is less, as marked on the upper

right section in Fig. 7. Furthermore, it was observed that the

probability of forming a fracture or bedding plane separation

increases as the height above the coal bed (H) and the ratio

formation thickness (h) to the coal seam thickness (m) decrease.

Therefore, in the study boreholes, it is more probable that such

deformations will occur within an interval between 40 and 145 ft

from the top of coal bed and up to h/m of 4 (Fig. 7). This interval

contains thinner limestones, sandstones, and weaker strata, such

as coal beds and shales (Fig. 6).

Fig. 8 shows the ratio of the thickness of top layer (ht) to the

bottom one (hb) for each interface, calculated in pairs, along

the stratigraphic sections shown in Fig. 6, versus the height of the

interface from the top of the Pittsburgh coal bed (H). In this figure,

the types of the interfaces were also defined with differentcolored markers. Palchik [25] has reported that when the strata

thickness ratio (ht/hb) is low (o2), there are very few horizontal
MBH-1 & 2 B-3 C-1 C-2 D-1 

-10

40

90

140

190

240

290

0

-10

40

90

140

190

240

290

0

-10

40

90

140

190

240

290

0

-10

40

90

140

190

240

290

0

-10

40

90

140

190

240

290

0

Strata

Thickness (ft)

Strata

Thickness (ft)

Strata

Thickness (ft)

Strata

Thickness (ft)

Strata

Thickness (ft)

Strata

Thickness (ft)

Height Above CB (ft)

-10

40

90

140

190

240

290

0

Bottom of

Uniontown SS Slotted interval Transducer Pittsburgh Limestone

Coalbed 

50 50 100 50 100 50 100 50 100 50 100

 Sandstone Shale 

 B-4 

Fig. 6. Stratigraphic log and thicknesses of the formations at each borehole locations after correction to sea level.

Fig. 7. Height of the layers above coal bed versus the ratio of their thicknesses

determined from data of Fig. 6. This graph shows the distribution of different rock

types and the regions where fracturing probability is higher within strata.

Fig. 8. Height of the formation interfaces from coal bed versus the thickness ratio

of sequential layers in the strata column (Fig. 6). This graph shows the types of the

interfaces and the region where fracturing and bedding plane separation will likely

to occur. Black dots are where separations are not expected due to height ratios

and interface characteristics.
fractures. In order to apply a similar concept for the strata in our

tests sites and to determine the lower limit of strata-thickness

ratio for fracturing or bedding plane separations to occur, we have

used water drop data measured in MBH-1 and -2. In all the water

tests conducted, the minimum numbers of interfaces were in

MBH-1 and -2, since they have the shortest slotted casings (30 and

20 ft, respectively) at heights of 72 and 106 ft above the coal seam.

However, we still measured water drop in these boreholes as

mining approached. This suggests the development of horizontal

fractures in these intervals. Therefore, the strata thickness ratios

in these interfaces in MBH-1 and -2 were used as the lower

boundary for horizontal fractures to occur. It was observed that

this boundary matches well with what Palchik [25] reported. As

for the upper boundaries for strata thickness ratios, the previous

discussion on bridge layers and the heights above coal bed were

used. Thus, it was estimated that the horizontal fractures in the

study area were likely to occur in limestone–shale, sandstone–

shale and shale–shale interfaces between 40oHo150, and

2.5oht/hbo42. Fig. 8 also shows the fracture region in each

borehole and the type of interface.

6. Hydraulic conductivities

6.1. Observed flow responses in the boreholes

The water–head drop and resultant pressure decline with

small data collection intervals created a highly resolved data file

in terms of time and water level. Analyzing the file as it was

uploaded from the transducer resulted in too many values of the

same flow period leading to a noisy result. Thus, in order to

eliminate these adverse effects, the data points (time and water

head pressure) at the beginning and at the end of each flow period

were determined by the change of slope of data trend. Fig. 9

shows these data as time versus normalized (with respect to

initial water head) water head for each of the boreholes.

Figs. 9a–d show that the change in the water level in the

borehole with time formed a variable flow trend until water

completely drained from the borehole during mining. These

variations indicate the changes in hydraulic conductivity and the

variation in the heterogeneity in the strata.

0

0.2

0.4

0.6

0.8

1

1.2

0

Time (sec) - From Start of Head Drop

Hw/Ho

MBH-1

MBH-2

0

0.2

0.4

0.6

0.8

1

1.2

0

Time (sec) - From Start of Head Drop

Hw/Ho

B-3

B-4

0

0.2

0.4

0.6

0.8

1

1.2

0

Time (sec) - From Start of Head Drop

Hw/Ho

C-1

C-2

0

0.2

0.4

0.6

0.8

1

1.2

0

Time (sec) - From Start of Head Drop

Hw/Ho

D-1

2000 4000 6000 8000 5000 10000 15000 20000 25000

2000 4000 6000 8000 1000 2000 3000 4000 5000 6000

Fig. 9. The change in the water level (normalized head) in the boreholes with time.

Various factors may control the hydraulic properties of a

porous medium and the calculated hydraulic conductivities. The

factors such as mechanical skin, layering and anisotropy were

investigated analytically and numerically [26,27]. In addition to

skin effects, formation heterogeneity may influence test results.

Moreno and Tsang [28] showed numerically that preferential flow

paths tend to develop in extremely heterogeneous media. Mean

hydraulic properties along such channels or fractures may be

significantly higher than the rest of the porous medium. In a more

recent paper, Maher and Donovan [20] investigated the relationship between extreme heterogeneity and hydraulic test results in

a mine-spoil aquifer created by backfilling the excavations during

surface mining. They have observed three different drawdown

response types during the tests: conventional, rapid response and

‘‘double’’ response. The ‘‘double’’ response curve was characterized by an initial rapid drawdown for the first 10% of the data,

followed by a slow period similar to a conventional behavior. The

differences in responses were interpreted as the boundary effects,

as either steepening or flattening of the response curve. Steepening was reported as a result of hydraulic boundary or heterogeneity effects near the borehole, whereas flattening was

interpreted as either leakage, recharge, or slow vertical drainage

from unconfined storage. They also conducted numerical experiments to examine the effects of different heterogeneities on the

response curve. They observed that increasing the skin porosity,

skin storage and skin conductivity produced ‘‘double-flow’’ type

response.
Fig. 9 show the observed flow responses measured in our tests

and indicate the existence of ‘‘multiple’’ flow periods in the wells.

Although the tests and the response curves are much more

complicated than the cases explained before, the observed flow

responses are characteristic of heterogeneous formations. These

double-flow data plots may be representative of initial saturation

of opening (dilating) fractures, followed by a slowed decline as the

water drains in the now saturated fracture also. The measured

variations in drawdown were most likely caused by changing

hydraulic conductivities resulting from a combination of factors

such as position of the face relative to the borehole, topographic

location of the borehole and completion parameters, strata

characteristics and variation in mining advance rate.

6.2. Hydraulic properties of the overburden during mining

6.2.1. Hydraulic conductivity and transmissibility

Hydraulic conductivity is the proportionality constant in

Darcy’s law, which relates the amount of water which will flow

through a unit cross-sectional area of aquifer under a unit

gradient of hydraulic head. The hydraulic conductivity (K) is

specific to the flow of a certain fluid. This means that, for example,

K will go up if the water in a porous medium is heated (reducing

the viscosity of the water), but k will remain constant. The two are

related as follows:

kg K 1⁄4 (1) m

where K is the hydraulic conductivity [LTr1

], k is the intrinsic

permeability of the material [L2

], g is the specific weight of water

[MLr2

Tr2], and m is the dynamic viscosity of water [MLr1Tr1].

The transmissibility, T, of an aquifer, on the other hand, is a

measure of how much water can be transmitted horizontally such

as to a pumping well

T 1⁄4 Kb (2)

Transmissibility is directly proportional to the aquifer thickness (b).

6.2.2. Methods of calculating hydraulic properties of the overburden

during mining

In order to calculate the hydraulic conductivities based on the

water drawdown data presented in Figs. 9a–d, two models were

used. These models were those of Cooper, Bredehoeft, Papadopulos [29] and Bauwer and Rice [30]. Cooper et al. [29] calculated

aquifer values by overlying the field data to a family of curves as

shown in Fig. 10.

Time (min) 

1 10 100 1000 10000 100000 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

0.01 

beta 

F (alpha, beta) 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

1 

D-1 

MBH-1 

MBH-2 

B-3 

B-4 

C-1 

C-2 

α = 1010 

α = 103 

α = 101 

α = 10-1 

α = 2*10 α = 10 -11 -10 

α = 10-11 

α = 5*10-12 

0.1 1 10 100 1000 10000 100000 

Fig. 10. Plot of the type curves generated using Cooper et al. [29] model and the experimental data obtained from boreholes. Arrow shows the direction to which

experimental data should be moved for type-curve matching.

Hw/Ho 

The field data was moved towards the type

curves in such a way that the axes were parallel to each other until

the best type curve was found which matched the field data. As in

all type curve matching procedures, the decision of the best match

is usually subjective and only a noise free, perfectly homogeneous

response data can be closely matched to theoretical curves. In

most cases, either the early or the later portion of the field data

has to be ignored to be able to match the rest of the data points to

one of the curves, making this rather undesirable. In extreme

cases of heterogeneity, as in the data measured for this study, the

matching procedure is more difficult and more prone to error. Due

to concerns of the nature of the field data and the sources of errors

explained in the previous paragraphs, this technique is not used

further in this analysis. Note that in Fig. 10, the field data are

shown at a stage before moving them towards the type curves in

order not to make this figure too crowded. In Fig. 10, Hw=Ho is the

dimensionless drawdown in the borehole, a 1⁄4 Sr2 =r2 w c is the

dimensionless storativity, and b 1⁄4 Tt=r2

c is the dimensionless

time, where S is the storativity, T 1⁄4 Kb is the transmissivity, and

rc and rw are the casing and well radii, respectively.

Alternatively, the Bauwer and Rice [30] method was used on

the field data to calculate the hydraulic conductivities between

consecutive data points, for the intervals of the response curve

that follow a similar flow behavior (points falling on the same

straight line), and for the average of all data points. This model

applies to the injection or withdrawal of a volume of water from

the well casing, where hydraulic conductivity (K) is defined as

r2

c ln K ðR=rwÞlnðHo=HwÞ 1⁄4 (3) 2ðl r dÞt

Bauwer and Rice [30] determines the radius of influence, R, for

different values of rw, in terms of (lrd), rw, Hw and m (aquifer
saturated thickness) using with an electric resistance analog

model. From the experiments, the following empirical equation

was developed for estimating R:

1:1 A þ B ln ln 1⁄2ðm - lÞ=rwl ðrw=RÞ1⁄4 þ (4) lnðr=rwÞ ðl - dÞ=rw

where A and B are dimensionless coefficients that are function of

ðl - dÞ=rw for the partial penetration case. If the well fully

penetrates the aquifer, in that case the appropriate equation for

lnðrw=RÞ is

1:1 C lnðrw=RÞ1⁄4 þ (5) lnðl=rwÞ ðl - dÞ=rw

A more complete explanation of these coefficient and solutions

for some specific cases can be found in Dawson and Istok [31].

However, one important aspect of this method should be

mentioned: the calculated K is dependent on the thickness open

to flow, (l-d). This is important in comparative studies where the

length of open-to-flow section of the boreholes varies. Also, this

method has more representative assumptions in its application to

the current study, including the leaky and unconfined aquifer.

Thus, the method of Bauwer and Rice [30] was used for the rest of

the calculations and for the interpretations of the results in this

paper.

6.3. Selection of aquifer depth for calculations

The Bauwer and Rice method [30] was used to calculate the

average hydraulic conductivities and the transmissibilities for

each borehole at different depths of the initial water table. The

variations in water table depths may depend on seasonal

variations in precipitation as well as the location of the borehole.

As it was presented in the groundwater hydrology section, the

water table, although it usually may be parallel to the ground

surface, can be at a shallower depth at the valley bottoms. Thus,

the boreholes drilled at valley bottoms may hit the water aquifer

at shallower depths than the hill-top wells. Also, the hydraulic

properties of the formations at valley bottoms may be different

due to stress-relief fracturing (Fig. 2).

This method was used to quantify hydraulic conductivities and

transmissibilities for different aquifer thicknesses at each borehole location. Calculations were conducted for cases where the

water table was 50, 200, and 400 ft below the surface. Because the

bottom of the Pittsburgh coal is considered the base of the aquifer

for local-regional flow system, increasing water table depth

decreases aquifer thicknesses (Table 3). As this table shows,

hydraulic conductivities are not affected by the thickness of the

aquifer, whereas transmissibilities decrease as the thickness of the

aquifer decreases (400 ft case). In these analyses, 200 ft was

selected as the depth of a static water table for all boreholes, this

value being in close agreement with the values of 150–175 ft

reported by Stoner [24] for depths in SW Pennsylvania. Thus, the

calculations and the discussions on the changes in hydraulic

properties of the overburden during mining in the next sections

will be based on the K and T data calculated using the Bauwer and

Rice [30] method and 200 ft as the depth of the aquifer. It should

be mentioned that the K-value estimated from slug testing is

dependent in some cases on the thickness of the part of the

aquifer in which flow occurs due to a slug input or another

external changes [17]. Since the slotted casing is the only open-toflow section of the boreholes in the overburden, their lengths

would make a difference. In fact, the shorter the slotted section,

the higher the K-value as observed in MBH-1 and -2 in Table 3.

Thus, the calculated values will be normalized to a conventional

200 ft slotted casing used in the GGVs.

Table 3

Average hydraulic conductivity (K) and transmissibility (T) values determined for different aquifer depths from surface using Bauwer and Rice [30] method.

MBH-1 MBH-2 B-3 B-4 C-1 C-2 D-1

50 50 50 50 50 50 50

K

T

2.52E-03

1.957

K

T

3.26E-03

2.529

K

T

4.40E-04

0.249

K

T

1.54E-04

0.115

K

T

5.22E-04

0.412

K

T

7.18E-04

0.443

K

T

7.05E-04

0.515

200 200 200 200 200 200 200

K

T

2.47E-03

1.547

K

T

3.20E-03

2.005

K

T

4.23E-04

0.176

K

T

1.50E-04

0.089

K

T

5.12E-04

0.328

K

T

6.95E-04

0.323

K

T

6.86E-04

0.399

400 400 400 400 400 400 400

K

T

2.37E-03 K

1.010 T

3.09E-03

1.318

K

T

4.36E-04

0.094

K

T

1.42E-04

0.056

K

T

4.89E-04

0.215

K

T

6.43E-04

0.171

K

T

6.49E-04

0.248

[K in ft/day; T in ft2/day]

6.4. Change of hydraulic properties of overburden during mining

This section describes the hydraulic conductivities calculated

from downhole instrumentation data. Fig. 11 shows the variation

in hydraulic conductivities as a function of face location with

respect to borehole location.

0 

0.0001 

0.0002 

0.0003 

0.0004 

0.0005 

0.0006 

0.0007 

0.0008 

0.0009 

Hydraulic Conductivity (ft/day)

MBH-1 MBH-2 

B-3 B-4 

C-1 C-2 

D-1 

-400 -350 -300 -250 -200 -150 -100 -50 0 50 100 

Face Location Relative to BH (Face locationBorehole location) 

Fig. 11. Change of hydraulic conductivities calculated from borehole tests as a

function of longwall face location relative to borehole location.

In this graph, negative values
represent a face location before it reaches the borehole location,

where as positive values represent the location of the face after

undermining. A value of zero represents the longwall face under

the borehole location. This figure suggests that hydraulic

conductivities change based upon the borehole monitored. At

C-1 and -2, initial hydraulic conductivities were high when the

effects of the approaching face were first felt in each borehole and

started to decrease as the face approached the boreholes. The

values started increasing as the face approached and eventually

undermined the borehole (Fig. 11). One noticeable difference is

that the hydraulic conductivity values show that the strata around

C-1 started fracturing when the face was approximately 375 ft

away. In B-3 and -4 boreholes (also in D-1 too), initial hydraulic

conductivities were low but followed an increasing trend as the

face approached the borehole locations. These behaviors suggest

that differences exist in overburden fracturing mechanisms.

When we compare the geology of C-1 with -2, and also the

locations of the transducers in these two wells, we see that they

are very similar (Fig. 6). However, C-1 was the first borehole in the

panel, was close to the setup entry, was located at a higher

elevation, and thus had a thicker overburden (Table 1) compared

to C-2. This may suggest that the first borehole in the panel is

fractured earlier than the other boreholes drilled close to the

middle or end of the panel. Also, the weight of the overburden

(hilltop versus valley bottom wells) and the differences in existing

stresses and how they interact with mining-induced stresses

under these topographical features may create differences in the

way that the boreholes fracture during undermining. Regardless,

the general observation, without taking the topography into

account, is that from setup entry to the first weighting as the face

moves, the roof experiences elastic, plastic deformation, failure

and development of vertical fractures [6]. Since the first weighting

does not occur immediately and usually not until one face width

of advance, the roof strata continues to cantilever, opening

existing fractures and creating new bedding plane separations

ahead of the face. After the first weighting as the overburden

caves, fractures develop ahead of the face. Separated vertical

fractures occur at higher levels and horizontal and vertical

fractures occur at the lower ends of fractured zone. After the face

advances to a certain distance, fractures and broken rocks are

compressed.

B-3 and -4 were located close to the middle of the panel and

gob. A comparison of their stratigraphic logs with the ones in

panel D shows that there was a 30 ft-thick sandstone layer on the

top of the immediate roof of the Pittsburgh coal bed in C panel

(Fig. 6). According to Palchik [5], the ratio between the maximum

height of the zone of interconnected fractures and the thickness of

the extracted coal seam increases with the increasing number of

rock layers and decreases with the stiffness of the immediate roof.

This may explain the differences in the changes of hydraulic

conductivities in boreholes between panels B and C. In panel C

borehole locations, the immediate roof of the Pittsburgh coal bed

was mainly thin shale layers and rider coals up to 20 ft from the

coal bed (Fig. 6). These thin and weak layers must have broken

very easily and initiated fracturing of the limestone intervals

above, thus increasing the height of the fractured zone and

increasing the hydraulic conductivity. On the other hand, in panel

B, the stiffer sandstone layers probably did not break easily,

limiting the increase in their hydraulic conductivities. As the

sandstone failed in a transient mode, hydraulic conductivities

increased slowly from low values to higher ones.

Furthermore, B-3 was located in a valley, whereas B-4 was

located at a hilltop (Fig. 4). Thus, B-3 had a higher hydraulic

conductivity as it was observed in C-2, probably due to it location.

On the other hand, B-4 started fracturing earlier but had a lower

hydraulic conductivity when the water in the borehole drained

completely. This may suggest that the location of borehole and the

overburden weight on the fractured strata did not promote high

conductivities.

The completion parameters of MBH-1 and -2 in panel A were

different from regular GGVs; as they had shorter slotted casings.

The distances to the Pittsburgh coal bed from the bottom of the

slotted casings were 106 and 72 ft for MBH-1 and -2, respectively

(Table 2). They had a thick sandstone layer as the immediate roof.

It is probably due to this stiff roof material that the fracturing of

the strata was delayed until the face was within 50–100 ft of the

borehole. The slotted casing in MBH-1 was located against a

weaker section with more strata interfaces compared to MBH-2

which was located against a more competent layer with fewer

interfaces (Fig. 6). Although MBH-1 was higher in the strata, it was

fractured more effectively at the beginning, compared to MBH-2,

which was fractured later and with hydraulic conductivities

increasing slowly.

6.5. Effects of overburden, face advance rate and distance from the

panel setup on hydraulic conductivities

In order to investigate the effects of overburden thickness, face

advance rate and the distance of the GGV from the panel setup

entry, the average values of the hydraulic conductivities presented

in the previous section were calculated. The comparisons were

based on these average values.

Fig. 12 shows the variations in average hydraulic conductivity

for different boreholes and their respective overburden thicknesses.

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

600

Overburden on CB (ft)

Hydraulic Conductivity (ft/day)

MBH-1

MBH-2

B-3

B-4

C-1

C-2

D-1

650 700 750 800 850 900

Fig. 12. Average hydraulic conductivities in each borehole as a function of

overburden thickness at the borehole locations. Hollow markers are the averages

for each borehole.

As the figure shows, there are mainly two regions of

overburden thickness in which the conductivities were clustered.

The first region contains boreholes with higher average hydraulic

conductivities with overburden thickness up to around 775 ft. The

boreholes in this interval are B-3, C-2, and D-1. These boreholes

are the ones drilled in valley bottoms (B-3 and C-2), thus with less

overburden, and the first borehole in D panel (D-1). Comparing

hydraulic conductivities with only one independent variable

(overburden) suggests that the boreholes in valley bottoms are

going to be more productive in producing methane. Also, the first

borehole in a panel can be as productive unless the overburden at

its location is too thick, as it was in the case of C-1.

Fig. 13 compares the average hydraulic conductivities developed in the boreholes with the average face advance rates during

mining of their locations. The distribution of the values as a

function of face advance rate suggests that as the rate of advance

increases, the hydraulic conductivities developed in the boreholes

tend to decrease. Keeping all other variables constant, this can be
explained by the fact that at a higher face advance rate,

subsidence occurs at a higher rate. This increases the maximum

slope, horizontal displacements, and the change of dynamic stress

from tensile to compressive, which in a fractured rock mass

accelerates the settlement [9]. Higher settlement rates and

compressive stresses decrease hydraulic conductivity in the

borehole. On the other hand, a slower advance rate could induce

a high dynamic tensile stress in front of the face before converting

to compressive stress some distance behind the face. The

prolonged tensile stress at high levels possibly creates more

fractures and bedding plane separations. This could increase the

hydraulic conductivity of the formations within the borehole

interval.

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

20

Average Face Advance Rate (ft/day)

Hydraulic Conductivity (ft/day)

MBH-1

MBH-2

B-3

B-4

C-1

C-2

D-1

30 40 50 60

Fig. 13. Average hydraulic conductivities in each borehole as a function of average

face advance rates during mining of the borehole locations. Hollow markers are the

averages for each borehole.

Fig. 14 illustrates the effects of distance of the borehole from

panel setup on hydraulic conductivity values.

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0

Distance from Setup (ft)

Hydraulic Conductivity (ft/day)

MBH-1

MBH-2

B-3

B-4

C-1

C-2

D-1

1000 2000 3000 4000 5000 6000 7000

Fig. 14. Average hydraulic conductivities in each borehole as a function of

distances of the boreholes from panel setup entry. Hollow markers are the

averages for each borehole.

This figure shows

that as the borehole distance from the panel setup entry increases,

the hydraulic conductivity is in a decreasing trend. This is possibly

due to closure of factures from compaction of gob with successive

weightings. This may explain why boreholes drilled at increasing

distances from panel setup are traditionally not very productive.

The graphs given in the above discussions show the variation

of hydraulic conductivities as a function of only one variable.

Although in some cases this type of representation may be

informative and helpful, in cases where the dependent variable is

affected by more than one independent variable, care should be

taken and the modeling should be based on either multivariable

regression or artificial neural network type approaches.

7. Summary and concluding remarks

This report presented an integrated study evaluating underground strata and their potential responses to longwall mining.

This included evaluations of mining rate effects and data obtained

from in-situ slug tests performed in gob gas ventholes to

characterize the changes in hydraulic conductivities. Hydraulic

conductivities developing in a borehole can help to predict

whether the GGV’s can produce methane more effectively.

The results indicate that the hydraulic conductivity developments in the boreholes and their potential production performances are affected by the underground strata and the roof

materials. In situations where the roof material is stiff and thick, it

cantilevers behind the shields leading to a lesser degree of

fracturing in overlying strata. This may decrease development of

high permeability fractures around the borehole.

In this study a double-flow behavior, in water drop during slug

tests, expected from heterogeneous formations was also observed.

The different flow-rate phases and different hydraulic conductivities can be related to initial opening and filling of fractures with

fluid and followed by changes in fractures now saturated with

fluid. Based on the slug tests and other data used in this study, it

was concluded that the locations of the boreholes are important

for their fracturing time and for the resultant hydraulic conductivities. Results showed that hill-top boreholes fracture earlier

than valley-bottom wells. However, the permeability of the

factures in hill-top wells is less compared to valley-bottom wells.

These wells may potentially be less productive. It was shown also

that greater overburden depths cause lower hydraulic conductivities and potentially less effective boreholes as opposed to shallow

GGVs. It was also observed that the proximity of the boreholes to

the start-up entries makes a difference in fracture development

too. First boreholes in the panels generally have higher hydraulic

conductivities, as opposed to GGVs drilled close to the end of the

panel. As the drilling distance to the panel setup entry decreases,

average hydraulic conductivities are potentially higher. This may

be one of the reasons why the first boreholes are generally more

productive. In addition to borehole location, mining advance rate

was found to be effective also on the hydraulic conductivities

measured using slug tests. It was shown that higher face advance

rates at the study mine site result in generally lower hydraulic

conductivity in the fractures. This may be due to the rapid change

of tensile stresses to compressive ones reducing the fracture

conductivity.

The bedding plane separation probabilities can be predicted

using the distances of bedding planes to the extracted coal seam

and the thicknesses of neighboring layers, as well as the thickness

of bridge layer. The prediction techniques based on these

information were applied for the coal measure rocks in this study

area. It was observed that mostly strong–weak rock interfaces are

prone to separation and the probability decreases as the distance

from coal seam increases. Also, the thickness of what is called a

‘‘bridge-layer’’ in this paper plays a role in fracture development.

References

[1] Mucho TP, Diamond WP, Garcia F, Byars JD, Cario SL. Implications of recent

NIOSH tracer gas studies on bleeder and gob gas ventilation design. In:

Proceedings of society of mining engineers annual meeting, Salt Lake City,

2000.

[2] Singh MM, Kendorski FS. Strata disturbance prediction for mining beneath

surface waters and waste impoundments. In: Proceedings of 1st international

conference on ground control in mining, Morgantown, WV, 1981.
[3] Kendorski FS. Effect of high-extraction coal mining on surface and ground

waters. In: Proceedings of 12th international conference on ground control in

mining, Morgantown, WV, 1993.

[4] Hasenfus GJ, Johnson KL, Su DWH. A hydrogeomechanical study of overburden

aquifer response to longwall mining. In: Proceedings of 7th international

conference on ground control in mining, Morgantown, WV, 1988.

[5] Palchik V. Formation of fractured zones in overburden due to longwall

mining. Environ Geol 2003;44:28–38.

[6] Li S, Lin H, Cheng X, Wang X. Studies on distribution pattern of and methane

migration mechanism in the mining-induced fracture zones in overburden

strata. In: Proceedings of 24th international conference on ground control in

mining, Morgantown, WV, 2005.

[7] Cui X, Wang J, Liu Y. Prediction of progressive surface subsidence above

longwall coal mining using a time function. Int J Rock Mech Min Sci 2001;38:

1057–63.

[8] Luo Y. An integrated computer model for predicting surface subsidence due to

underground coal mining (CISPM). PhD dissertation, Department of Mining

Engineering, West Virginia University, Morgantown, WV; 1989.

[9] Preusse A. Effect of face advance rates on the characteristics of subsidence

processes associated with US and Germany longwall mining. In: Proceedings

of 20th international conference on ground control in mining, Morgantown,

WV, 2001.

[10] Matetic RJ, Liu J, Elsworth D. Modeling the effects of longwall mining on the

ground water system. Report Invest 9561, US Bureau of Mines, Pittsburgh;

1995.

[11] Fawcett RJ, Hibberd S, Singh RN. An appraisal of mathematical models to

predict water inflows into underground coal workings. Mine Water Environ

1984;3:33–54.

[12] Liu J, Elsworth D. Three-dimensional effects of hydraulic conductivity

enhancement and desaturation around mine panels. Int J Rock Mech Min

Sci 1997;34:1139–52.

[13] Gale W. Application of computer modeling in the understanding of caving and

induced hydraulic conductivity about longwall panels. In: Proceedings on coal

2005, 6th Australasian Coal Operator’s conference, Brisbane, 2005.

[14] Whittles DN, Lowndes IS, Kingman SW, Yates C, Jobling S. Influence of

geotechnical factors on gas flow experienced in a UK longwall coal mine

panel. Int J Rock Mech Min Sci 2006;43:369–87.

[15] Whittles DN, Lowndes IS, Kingman SW, Yates C, Jobling S. The stability of

methane capture boreholes around a longwall coal panel. Int J Coal Geol

2007;71:313–28.

[16] Karacan CO, Esterhuizen GS, Schatzel SJ, Diamond WP. Reservoir simulationbased modeling for characterizing longwall methane emissions and gob gas

venthole production. Int J Coal Geol 2006;71:225–45.

[17] Tonder GJ, Vermeulen PD. The applicability of slug tests in fractured rock

formations. Water South Africa 2005;31:157–60.

[18] Lafhaj Z, Shahrour I. Use of water borehole tests for the determination of the

permeability of anisotrophic soils. Comp Geotech 2007;34:57–9.

[19] Hawkins JW, Aljoe WW. Pseudokarst groundwater hydrologic characteristics

of a mine spoil aquifer. Mine Water Environ 1992;11:37–52.

[20] Maher TP, Donovan JJ. Double-flow behavior observed in well tests

of an extremely heterogeneous mine-spoil aquifer. Eng Geol 1997;48:

83–99.

[21] Edmunds WE, Skema VW, Flint NK. Pennsylvanian. Part II: stratigraphy and

sedimentary tectonics. In: Geology of Pennsylvania; 1996.

[22] Ferguson HF. Valley stress relief in the Allegheny Plateau. AAPG Bulletin

1967;4:63–8.

[23] Wyrick GG, Borchers JW. Hydrologic effects of stress relief fracturing in an

Appalachian valley. US Geological Survey, Water Supply Paper 2177,

Washington; 1981.

[24] Stoner J, Williams DR, Buckwalter TF, Felbinger JK, Pattison KL. Water

resources and the effects of coal mining, Greene County, Pennsylvania. Water

Resource Report 63, Pennsylvania; 1987.

[25] Palchik V. Localization of mining-induced horizontal fractures along rock

layer interfaces in overburden: field measurements and prediction. Env Geol

2005;48:68–80.

[26] Faust CR, Mercer JW. Evaluation of slug tests in wells containing a finitethickness skin. Water Resour Res 1984;20:504–6.

[27] Narasimhan TN, Zhu M. Transient flow of water to a well in an unconfined

aquifer: applicability of some conceptual models. Water Resour Res

1993;29:179–91.

[28] Moreno L, Tsang C. Flow channeling in strongly heterogeneous porous media:

a numerical study. Water Resour Res 1994;30:1421–30.

[29] Cooper HH, Bredehoeft JD, Papadopulos IS. Response of a finite-diameter

well to an instantaneous charge of water. Water Resour Res 1967;3:

263–9.

[30] Bauwer H, Rice RC. A slug test for determining hydraulic conductivity of

unconfined aquifers with completely or partially penetrating wells. Water

Resour Res 1976;12:423–8.

[31] Dawson KJ, Istok JD. Aquifer testing: design and analysis of pumping and slug

tests. Chelsea, Mich: Lewis Pub; 1991.