UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9561 REPORT OF INVESTIGATIONS/1995

Modeling The Effects of Longwall Mining

on the Ground Water System

By R. J. Matetic, J. Liu, and D. Elsworth
U.S. Department of the Interior

Mission Statement

As the Nation’s principal conservation agency, the Department of the

Interior has responsibility for most of our nationally-owned public

lands and natural resources. This includes fostering sound use of our

land and water resources; protecting our fish, wildlife, and biological

diversity; preserving the environmental and cultural values of our

national parks and historical places; and providing for the enjoyment

of life through outdoor recreation. The Department assesses our

energy and mineral resources and works to ensure that their

development is in the best interests of all our people by encouraging

stewardship and citizen participation in their care. The Department

also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.

Cover: Predicted postmining effects to the groundwater system

utilizing finite-element model.
Report of Investigations 9561

Modeling The Effects of Longwall Mining

on the Ground Water System

By R. J. Matetic, J. Liu, and D. Elsworth 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ........................................................................................... 1

Introduction ........................................................................................ 2

Model approach ..................................................................................... 2

Theoretical analysis of modeling approach ............................................................... 2

Correlation of induced strain and hydraulic conductivity ................................................. 2

Determination of strain field ........................................................................ 3

Determination of postmining ground water regime ...................................................... 3

Input parameters used for modeling approach ............................................................ 4

Site description and ground water monitoring program .................................................. 4

Site description ................................................................................ 4

Ground water monitoring program ................................................................ 4

Geology ........................................................................................ 4

Modulus of elasticity and Poisson ratio values .......................................................... 5

Overburden deformation monitoring program .......................................................... 5

Subsidence monitoring program ..................................................................... 5

Model analysis ...................................................................................... 6

Finite-element mesh assemblage ..................................................................... 6

Subsidence profile ................................................................................ 6

Comparison of vertical displacements determined by model and field data ................................... 7

Correlation of postmining hydrologic regime with data collected from fieldsite ................................. 9

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

ILLUSTRATIONS

1. Sketch of study area ............................................................................. 4

2. Generalized cross section of study area and Water Well locations ........................................ 5

3. Generalized cross section of study area and MPBX locations ............................................ 6

4. Comparison of field data and model subsidence profiles ................................................ 7

5. Postmining effects of ground water system - predicted by model ......................................... 9

Comparison of water level change and face position to overburden ratio for—

6. Well 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7. Well 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8. Well 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9. Well 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

10. Well 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

11. Well 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

12. Model for topographic influence of infiltration on well elevations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

13. Postmining effects on ground water system using smaller-scale mesh around well area . . . . . . . . . . . . . . . . . . . . . . . 13

TABLES

1. Input parameters for generation of subsidence profile .................................................. 7

2. Vertical displacements for MPBX Nos. 1 and 2 ....................................................... 8

3. Vertical displacements for MPBX Nos. 3 and 4 ....................................................... 8

4. Vertical displacements for MPBX Nos. 5 and 6 ....................................................... 9

5. Input parameters for determination of postmining ground water effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Relative changes in hydraulic conductivity as applied to small-scale mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

Metric Units

cm centimeter m/s meter per square second 2

kg/m kilogram per cubic meter m/yr meter per year 3

Lpm liter per minute m /s square meter per second 2

m meter mm millimeter

m/km meter per kilometer MPa mega pascal

m/s meter per second

U.S. Customary Units

ft foot gpm gallon per minute

ft/mi foot per mile h hour

ft/s foot per second in inch

ft/s foot per square second lb/ft pound per square foot 2 2

ft/yr foot per year lb/ft pound per cubic foot 3

ft /s square foot per second psi pound (force) per square inch 2
Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 1

Research assistant, Pennsylvania State University, Department of Mineral Engineering, University Park, PA. 2

 Associate professor, Pennsylvania State University. 3

MODELING THE EFFECTS OF LONGWALL MINING ON THE GROUND

 WATER SYSTEM

By R. J. Matetic, J. Liu, and D. Elsworth 1 2 3

ABSTRACT

The objective of this U.S. Bureau of Mines hydrologic-subsidence investigation was to evaluate the effects

of longwall mining on the local ground water regime through field monitoring and numerical modeling. Field

data were obtained from multiple-position borehole extensometers (MPBX's) that were used to measure

subsurface displacements. Survey monuments were installed to measure mining-induced surface deformations.

Numerous drawdown and recovery tests were performed to characterize hydrologic properties of the overburden

strata. Coreholes were drilled above the study area to determine lithologic and strength characteristics of the

overburden strata using the rock samples collected. Electronic recorders were installed on all monitoring wells

to continuously monitor ground water levels in coordination with mining of the longwall panels. A combined

finite element model of the deformation of overlying strata, and it's influence on ground water flow was used

to define the change in local and regional water budgets. The predicted effects of the postmining ground water

system determined by the model correlated well with field data collected from the fieldsite. Without an

infiltration rate added to the model, a static decrease of 3.0 m (10 ft) in water level would occur due to mining

of both longwall panels and if an infiltration rate was inputted in the model, no predicted long-term effects would

occur to the ground water system.
' <

<

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) ' % & ),

2

INTRODUCTION

Longwall mining is a method used to extract large blocks of predict mining effects on the ground water regime. The U.S.

coal. During extraction of the block, the immediate overburden Bureau of Mines (USBM) is studying the overburden rock mass

is allowed to collapse, filling the void created by the excavation. and its response to high-extraction mining operations through

Mining-induced strains and displacements are transferred a comprehensive program of field studies. The first part of the

throughout the overburden rock mass due to this collapse and work involves the collection of mining, subsidence, overburden

the resultant stress redistribution creates changes to the ground response, and hydrological data before, during, and subsequent

surface and any water-bearing zones located above the mining to mining activity at numerous fieldsites. The second part of the

area. Previous studies have been conducted to delineate the program is examining methods of predicting the impact to local

effects of longwall mining on the local ground water system (1- ground water supplies after mining activity occurs. One of these

9). Few of these studies, however, have used actual field data, 4

in conjunction with numerical modeling, to determine and

methods is through the application of numerical modeling.

MODEL APPROACH

An intensive surface, subsurface, and ground water mon- overlying strata, the change in hydraulic conductivity that

itoring program was conducted at a minesite in southeastern results from the strain field may be determined. With the

Ohio. Data collected from this site served as input information modified conductivity field determined, the postmining

for a finite element (FE) model. The two-dimensional FE hydrologic system may subsequently be defined through

model incorporates the deformation of overlying strata and its application of a ground water flow model. Again, this ground

influence on ground water flow through applying a simple water flow model utilizes the FE method to determine the

relationship between mining-induced strains and changes in postmining hydrologic system where the position of the

hydraulic conductivity. The strain field that develops around piezometric surface indicates changes in well or aquifer yields.

a longwall panel as a result of mining is material failure and This methodology is used to evaluate the influence of mining

self-weight. From this predicted strain field and from on the local ground water regime in this study.

knowledge of the premining hydraulic properties of the

THEORETICAL ANALYSIS OF MODELING APPROACH

The following assumptions are made when operating the where g is gravitational acceleration, is kinematic viscosity,

model: (1) the rock matrix is functionally impermeable in

comparison with fractures; (2) fluid flow in fractures is defined

on the basis of the parallel plate model; (3) changes in fracture

conductivity result from changes in normal strains only; (4)

strains are partitioned between fractures and matrix as defined

by a modulus reduction factor, R ; and (5) fracture spacing, S, m

does not change after mining activity (10-11).

CORRELATION OF INDUCED STRAIN

AND HYDRAULIC CONDUCTIVITY

The equivalent porous medium conductivity, K , of a rock o

mass containing a parallel set of fractures can be defined as: 

(1)

Italic numbers in parentheses refer to items in the list of references at the 4

end of this report.

b is the fracture aperture, and S is spacing. R , the modulus m

reduction factor may be defined as:

(2)

where E is the deformation modulus of the rock mass and E is r

the deformation modulus of a rock specimen. The modulus

reduction factor, R , enables the closure across a fracture, )uj, m

to be determined from the difference between the strains in the

rock mass and rock specimen as shown below:

(3)

where ), is the strain in the direction perpendicular to the

fracture plane. ), is positive in extension and negative in

compression.
'

' % % & ),

' % % & ),

' % % & ),

, , '

3

Using the applied strain, ),, from above, the revised changes can then be evaluated through equations 6 and 7,

conductivity of equation 1 may be defined as: provided the mining-induced strain field is determined.

(4) DETERMINATION OF STRAIN FIELD

where

(5)

Directional conductivities, evaluated from initial conductivities,

K and K (conductivities in the x- and y- directions, ox oy

respectively) may then be determined from equation 4. With

two sets of orthogonal fractures oriented in the x and y

directions, the revised directional conductivities may be defined

for a two-dimensional system as:

(6)

and

(7)

where K and K ductivities in the x y are postmining con 

x-direction and the y-direction, K and K are the premining xo yo

conductivities in the x-direction and the y-direction and ),x and

), are the induced strains in the x and y directions, y

respectively. When R = 1, the mass modulus and intact m

material modulus are identical and the strain is uniformly

distributed between fractures and matrix. This results in the

smallest possible change in conductivity. When R = 0, the m

extensional strain is applied entirely to the fracture system and

precipitates the largest possible change in conductivity. These

values bound the possible ranges in the behavior of the system

in a natural and mechanistically defensible manner. This

representation of conductivities is extremely useful, since Rm

may be readily evaluated from rock mass classification systems

defining structural behavior as a function of readily observable

factors of rock structure (12). This avoids the difficulty of

defining conductivity enhancement in terms of the component

moduli of fractures and matrix, parameters that are unlikely to

be available in practice. The mining-induced conductivity

The subsidence field that develops around a longwall panel

may be determined directly from the FE model. The FE model

applies gravitational load, removes material excavated from the

panel and allows the overburden material to fail and deform

according to the mining-induced strains. The resulting

subsidence field may use R to calibrate against field data for a m

particular site. The insensitivity of the resulting subsidence

profile to the material properties of deformation modulus and

rock strength parameters, originates from the overriding influence of geometric controls on deformation (13-14).

Following mining, the panel span is sufficiently large that

closure between panel floor and roof is unavoidable.

Consequently, the resulting strain field, ,x and 

 ,y is defined

purely as a function of geometry, as:

(8)

where w, represents the width of the panel, t is the thickness of

the coalbed and h symbolizes the thickness of the overburden.

The assumption necessary in this evaluation is that strains

are uniformly distributed at the scale of a single element. These

assumptions seem reasonable where strains are moderate, but

may be questionable where significant strain localization

occurs.

DETERMINATION OF POSTMINING

GROUND WATER REGIME

With the modified conductivity distribution determined

from an evaluation of the strain field, and equations (6) and (7),

the influence on the postmining ground water regime may be

evaluated. The FE model may determine the influence of a

continuously distributed conductivity field (evaluated from the

calculated strain distribution) on the ground water budget and

water table where boundary conditions are applied to the local

system, to represent ground water and surface recharge.

Therefore, the change in elevation of the phreatic surface may

be determined for the postmining regime. This enables the

influence of mining on well yields, aquifer yields, and flow

patterns to be identified.
4

INPUT PARAMETERS USED FOR MODELING APPROACH

The primary parameters used as input to the model and the Ground Water Monitoring Program

measurements obtained from the field are (1) the initial

hydraulic conductivity distribution of the local lithology as A total of seven 22-cm (8-5/8 in) diameter monitor wells

determined through field measurements, (2) modulus of were drilled for the study. Perforated Schedule 80 (15-cm

elasticity and Poisson Ratio values for the rock mass diameter) (6-in) polyvinylchloride (PVC) casing was installed

determined from field measurements, (3) the measured to the total depth in each well to ensure an open wellbore for

subsidence profile, (4) measured vertical displacements, (5) the life of the study. The wells were strategically placed above

continuous fluid level fluctuations monitored at the site, and (6) both longwall panels as shown in figure 2. The wells were

flow rates entering the mine after excavation of the longwall located along a line perpendicular to the trend of the longwall

panels, as recorded by the operator. panels. This alignment permitted observations of effects during

SITE DESCRIPTION AND GROUND WATER at the center of panels 1 and 2, respectively. Wells 2 and 5

MONITORING PROGRAM were located at quarter-panel width. Well 3 was located above

Site Description above the edge of panel 2. Well 7, a control well, was located

The study site is located in southeastern Ohio (Vinton Data were collected from all wells before, during, and after

County). The study area overlies a portion of two contiguous mining of both longwall panels. Various hydrologic

longwall panels (Panels 1 and 2) measuring approximately 300 parameters were determined and included specific capacity,

m (900 ft) wide and 2,950 m (9,000 ft) long (figure 1). The transmissivity, hydraulic conductivity, and water level

panels were separated by a five entry, four pillar system fluctuations. Initial data collection began 3 months prior to the

approximately 120 m (350 ft) wide. The mined coalbed, had an undermining of Well 1 to establish baseline conditions.

average thickness of 140 cm (55 in) within the study area. Drawdown and recovery pumping tests were performed on all

However, the extraction thickness varied between 173 and 183 wells before and after undermining to determine hydraulic

cm (68 and 72 in). Overburden thickness was small and ranged conductivity parameters of the local, shallow geologic units.

from 65 to 85 m (214 to 280 ft). Overall, the strata were fairly Electronic recorders were also installed on all wells to

level with a regional dip of about 1E towards the southeast. continuously monitor water level fluctuations. The electronic

There were no major geologic structures and the topography data logger was programmed to record fluid positions every 4

consisted mainly of rolling hills with a maximum relief of h.

approximately 49 m (160 ft).

the mining of both longwall panels. Wells 1 and 6 were located

the gate roads between the two panels and Well 4 was located

427 m (1,400 ft) away from any mining activity.

GEOLOGY

The geological setting of the study area is typical of that

found in southeastern Ohio. The regional dip in this part of the

State is to the southeast with the strata striking in a northeastsouthwest direction. The average rate of dip is 6 m/km

(30 ft/mi). Irregularities to this rate can be experienced by

localized thinning or thickening of individual rock units (6).

The rock is predominantly interbedded sandstones, shales, thin

coal seams, and claystones. The individual units are thin (less

than 3 m (10 ft) thick) with one sandstone unit (Vanport

Limestone) having an average thickness of 14 m (45 ft) and

lying 43 m (140 ft) above the Clarion 4A Coalbed. To

characterize overburden lithology prior to drilling the

monitoring wells, six 5.1 cm (2 in) coreholes were drilled at the

study area to the Clarion 4A Coalbed. Generally, the

overburden consisted of about 30% sandstone, 30% shale, 30%

claystone, and 10% coal. A generalized cross-section of the

study area with the monitor well locations is shown in figure 2.
5

MODULUS OF ELASTICITY AND POISSON located 30 m (91 ft) from the ribline inside each panel. 

RATIO VALUES Borehole 4 was situated 3 m (10 ft) from the ribline (inside the

The coreholes were drilled above the study area to Borehole 3 was located in a pillar in the gate entries between

characterize the local lithology of the overburden and to the panels to observe the lateral extent of overburden

provide samples for determining geotechnical properties. For deformation.

input to the model, tests for determining compressive strength, Each borehole was outfitted with an eight-anchor MPBX.

Modulus of Elasticity, and Poisson Ratio were conducted on Two of the eight anchors in borehole 3 were installed inside a

the rock cores collected. Results of tests performed on the core coal pillar to monitor yielding of the pillar. The anchors are

samples showed that the unconfined compressive strength of numbered 1 to 8, with anchor 1 being the closest to the surface

the major sandstone units are between 37 and 41 MPa (5,300 and anchor 8 being the deepest. Figure 3 displays MPBX and

to 6,000 psi). Modulus of Elasticity and Poisson Ratio for these anchor locations with respect to the longwall panels.

units are between 17,700 to 19,450 MPa (2.57 to 2.82 × 106

psi) and 0.30 to 0.32, respectively. A limestone unit, averaging SUBSIDENCE MONITORING PROGRAM

2.1 m (7 ft) in thickness, is situated about 1 m (3 ft) above the

mined coalbed. Testing of core samples show that the un- To obtain the field subsidence profile, survey monuments

confined compressive strength of this unit is 172 MPa (25,000 were installed on the ground surface and were surveyed

psi), with a Modulus of Elasticity of 72,400 MPa (10.5 × 10 regularly to identify the dynamic characteristics of subsidence, 6

psi), and a Poisson Ratio of 0.28. the final subsidence profile, and to provide surface reference

OVERBURDEN DEFORMATION MONITORING 1.3 m (4 ft) rebar and were installed to the ground surface. The

PROGRAM array of monuments consisted of a baseline (along the

To observe overburden displacement, six 219.1 mm (8-5/8 perpendicular between the two baselines. The monuments were

in) boreholes were drilled along a profile line extending across spaced 15 m (45 ft) apart along the baseline. The profile line

the two longwall panels. Boreholes 1 and 6 were located in the was 375 m (1,136 ft) long with monuments spaced 7.5 m

center of each panel, where the maximum amount of (23 ft) apart.

subsidence was expected to occur. Boreholes 2 and 5 were

panel) in the expected zone of maximum horizontal tension.

data for the MPBX units. The monuments were constructed of

centerline) over each panel and a profile line trending
6

MODEL ANALYSIS

FINITE-ELEMENT MESH ASSEMBLAGE coupled through the dependence of hydraulic conductivity on

The finite-element mesh construction was assembled

utilizing figure 3. The effects of topography, geometry, and SUBSIDENCE PROFILE

lithology were incorporated in the mesh. The mesh utilizes

uniform spacing and was constructed of 2,066 nodes and 1,928 The subsidence profile (figure 4), generated by the model,

elements. The mesh assumes differing materials (overburden was determined with the input parameters as shown in table 1.

material and coal layer) for the determination of strain and The values of the modulus of elasticity for the overburden

displacement characteristics, and three materials (upper shale were based on lab results obtained from core samples collected

layer, sandstone layer, and lower shale layer) for determination from the fieldsite. To obtain the model inputs, an average of all

of the postmining flow characteristics within the system. The field samples was calculated (31,000 MPa) (4.4 × 10 psi) and

boundary conditions of the model for determining displacement ratios of 1/20 for the overburden material and 1/200 for the

and strain characteristics assume no horizontal movement coal material were applied to the average lab result. Prior

on either side of the mesh and no vertical movement on the research has shown that decreasing E values by several orders

base. Boundary conditions associated with monitoring of magnitude, results in a better representation of actual field

postmining ground water effects assume no flow on the bottom conditions and accounts for rock mass effects (12). Voight

of the mesh and constant head conditions on the lateral sides of

the mesh. The FE model is two-dimensional and determines

the strain field with two displacement degrees of freedom

applied to each node, and subsequently evaluates the revised

flow system using a single degree of freedom. The analyses are

the induced strain field, as defined in equations 6 and 7.

6

lab

(12), has also noted that lab results within the range of 6,900 to

690,000 MPa (1.0 × 10 and 1.0 × 10 psi) should be reduced 6 8

by at least one order of magnitude. A value of 4.0 was inputted

into the model for the postfailure ratio of E /E . This ratio in situ failed

is simply a curve fitting parameter to match the
7

Table 1.—Input parameters for generation of subsidence profile

Input parameter Overburden material Coal layer

E, MPa (psi) ......... 1,550 (2.2 × 10 ) 5 1.56 (2.2 × 10 )4

n .................. 0.30 0.30

D, kg/m (lb/ft ) ....... 3 3 2,400 (150) 390 (80)

E /E . . . . . . . . . . . in situ failed 4.0 4.0

n ............. postfailure 0.450 0.450

D Density. 

E Modulus of elasticity.

n Poisson's ratio.

field-measured maximum subsidence magnitude with that COMPARISON OF VERTICAL DISPLACEMENTS

derived from the model. In reality, the form of the subsidence DETERMINED BY MODEL AND FIELD DATA

profile predicted by the model is insensitive to the choice of

elastic parameters. The excavation of coal is simulated using a Strain distributions are determined in the FE model by solving

bimodulus model. Initially, this material is assigned a very small the boundary value problem with appropriate constitutive

modulus and a Poisson Ratio equal to zero, allowing the material relations. The subsidence profile is the surface manifestation of

to freely deform in the vertical direction. As the top and bottom this continuous redistribution of strain surrounding the mined

of the panel contacts each other, the modulus of elasticity value panel. Strains and displacements generated within the mesh, as

is increased to prevent interpenetration. a result of mining, may also be determined and analyzed. A

Figure 4 shows the subsidence profile determined by the above the gate roads between the two panels. Whereas, the

model and the actual subsidence profile obtained from the predicted subsidence profile shows a maximum of 0.15 m (0.5 ft)

collected field data. Although the two curves are not identical, subsidence.

the general trends of the curves are similar, with the maximum The anchors in MPBX 5 displayed larger magnitudes of

subsidence located above the two panels. This match is vertical displacements compared with those predicted by the

considered adequate for the subsequent hydrologic analyses. model. Again, this is mainly attributed to the differing shapes

comparison of vertical displacements, within the overburden, as

generated by the model and the displacements measured at the

fieldsite are shown in tables 2-4.

The predicted vertical displacement data obtained from the

model for MPBX's 1, 2, 3, and 6 correlate well with the field data

collected at the site (tables 2-4). The field displacement data for

MPBX 4 (table 3) showed minimal vertical displacement,

whereas, the predicted vertical displacements for these boreholes

showed slightly higher magnitudes of displacement (0.07 to 0.30

m) (0.22 to 0.99 ft). One contributing factor could be the

difference between the predicted subsidence profile and the

actual field data (figure 4). As displayed in the figure, the field

data profile shows minimal amounts of subsidence occurring
8

of the subsidence profiles above the vicinity of the ribline over subsidence to occur above the center of the panel. This

panel No. 2 (figure 4). The profile, developed from the model, nonclassical form of the subsidence profile is attributed to siteshows less subsidence occurring above the ribline compared with specific geological conditions that are undefined and therefore

the profile determined from the field data. As shown in the not incorporated into the homogeneously distributed material

figure, the maximum subsidence (field data) for panel No. 2 parameters used in the finite element evaluation of

occurs near the ribline, whereas, the model predicted maximum displacements.

Table 2.—V versus V (vertical displacements) for MPBX Nos. 1 and 2 field model

Borehole Anchor H Vfield Vmodel

 m ft m ft m ft 

1 ............ 1 54.56 179 1.14 3.73 1.00 3.28

2 50.60 166 1.16 3.79 1.01 3.30

3 47.00 154 1.17 3.85 1.01 3.30

4 38.10 125 1.30 4.26 1.01 3.30

5 25.91 85 1.50 4.90 1.01 3.30

6 16.15 53 F F 1.00 3.28

7 8.53 28 F F 1.00 3.25

8 6.10 20 F F 1.00 3.25

2 ............ 1 64.62 212 0.57 1.87 0.50 1.63

2 57.91 190 0.57 1.87 0.50 1.63

3 55.17 181 0.60 1.98 0.50 1.63

4 38.71 127 0.70 2.27 0.51 1.68

5 27.74 91 1.64 5.37 0.51 1.68

6 18.59 61 F F 0.52 1.71

7 9.45 31 F F 0.53 1.73

8 5.18 17 F F 0.53 1.74

F Anchor failed.

H Height above coalbed.

Table 3.—V versus V (vertical displacements) for MPBX Nos. 3 and 4 field model

Borehole Anchor H Vfield Vmodel

 m ft m ft m ft 

3 ............ 1 70.10 230 0.00 0.00 0.09 0.29

2 60.35 198 0.00 0.00 0.07 0.23

3 36.90 121 0.00 0.00 0.07 0.22

4 16.70 55 0.00 0.00 0.07 0.24

5 7.01 23 0.00 0.00 0.07 0.24

6 2.44 8 0.00 0.00 0.07 0.24

7 0.91 3 0.00 0.00 0.07 0.24

8 0.30 1 0.00 0.00 0.07 0.24

4 ............ 1 70.71 232 0.00 0.00 0.30 0.99

2 58.52 192 0.00 0.00 0.27 0.88

3 52.73 173 0.00 0.00 0.25 0.82

4 36.88 121 0.00 0.00 0.21 0.70

5 25.30 83 0.00 0.00 0.19 0.62

6 13.41 44 0.00 0.00 0.16 0.52

7 3.96 13 0.00 0.00 0.14 0.47

8 1.83 6 0.00 0.00 0.12 0.41

H Height above coalbed.
9

Table 4.—V versus V (vertical displacements) for MPBX Nos. 5 and 6 field model

Borehole Anchor H Vfield Vmodel

 m ft m ft m ft 

5 ............ 1 83.21 273 1.21 3.97 0.56 1.85

2 64.00 210 1.28 4.20 0.56 1.85

3 50.30 165 1.43 4.70 0.56 1.84

4 39.62 130 1.44 4.73 0.56 1.85

5 31.10 102 1.53 5.01 0.56 1.85

6 19.2 63 F F 0.59 1.93

7 10.36 34 F F 0.60 1.96

8 5.80 19 F F 0.61 1.99

6 ............ 1 75.60 248 1.09 3.56 1.10 3.62

2 62.79 206 F F 1.11 3.63

3 51.21 168 1.10 3.62 1.12 3.66

4 38.10 125 F F 1.12 3.66

5 27.43 90 F F 1.11 3.64

6 19.50 64 1.14 3.73 1.11 3.61

7 9.45 31 F F 1.09 3.57

8 3.96 13 F F 1.08 3.53

F Anchor failed.

H Height above coalbed.

CORRELATION OF POSTMINING HYDROLOGIC REGIME

WITH DATA COLLECTED FROM FIELDSITE

The postmining effects to the ground water system (figure model. This value was three orders of magnitude greater than

5), evaluated from the model, were determined though input of the measured conductivity value obtained for the upper

the parameters as shown in table 5. overburden material. This value of hydraulic conductivity was

Numerous drawdown and recovery pumping tests were used because it is believed that the sandstone unit provides a

performed on all the monitoring wells to determine hydraulic much higher permeability than that of a shale unit. The

conductivity values. The hydraulic conductivity value of 7.01 hydraulic conductivity for the lower overburden material

× 10 m/s (2.3 × 10 ft/s), for the upper overburden material (shale) was one order of magnitude less than that of the upper -8 -7

(shale), was determined by averaging the hydraulic material 7.01 × 10 m/s (2.3 × 10 ft/s). Prior USBM research

conductivity values measured at the site. The estimated has shown that fractures within the overburden rock mass

hydraulic conductivity value, 7.01 × 10 m/s (2.3 × 10 ft/s) commonly decrease in aperture and number with increasing -5 -4

for the sandstone material was used as input into the depth (13). Correspondingly, the rock mass at depth is

-9 -8

assumed `tighter' and consequently less conductive.

 The fracture spacing values used are assumed, since no

information regarding fracture spacings were determined at the

fieldsite. However, the selected values are consistent with those

incorporated in other successful calibration studies (9-11). As

mentioned earlier, the modulus reduction factor, R , reflects the m

partitioning of mining-induced strains between fractures and

the porous matrix. In less stiff materials, the matrix

accommodates proportionately more of the applied bulk strain

than stiffer materials, where fracture closure dominates the

mass response. Correspondingly, less competent materials,

such as shales, return higher magnitudes of the modulus reduction factor. Without field measured magnitudes, appropriate

magnitudes of the modulus reduction factor are selected,

reflecting these anticipated characteristics of behavior.
10

Table 5.—Input parameters for determination of postmining ground water effects

Input parameter Overburden upper shale Overburden sandstone Overburden lower shale

onductivity ...... K , horizontal c x

 7.01 x 10 m/s -8

(2.3 x 10 ft/s) (2.3 x 10 ft/s) (2.3 x 10 ft/s) -7

7.01 x 10 m/s -5

-4

7.01 x 10 m/s -9

-8 

K , vertical conductivity ........ y 7.01 x 10 m/s 7.01 x 10 m/s 7.01 x 10 m/s -8

(2.3 x 10 ft/s) (2.3 x 10 ft/s) (2.3 x 10 ft/s) -7 

-5 

-4 

-9

-8

S, fracture spacing ........... 0.31 m 0.91 m 0.10 m 

(1 ft) (3 ft) (0.30 ft) 

R , modulus reduction factor . . . m 0.80 0.05 0.97 

<, kinematic viscosity ......... 1.0 x 10 m /s 1.0 x 10 m /s 1.0 x 10 m /s -6 2

(1.0 x 10 ft /s) (1.0 x 10 ft /s) (1.0 x 10 ft /s) -5 2

-6 2

-5 2

-6 2

-5 2

g, acceleration of gravity ....... 9.81 m/s 9.81 m/s 9.81 m/s 2

(32.2 ft/s ) (32.2 ft/s ) (32.2 ft/s ) 2

2

2

2

2

I, infiltration rate ............. 0.23 m/yr 0.00 0.00 

(0.75 ft/yr) 

Flow rates entering the mine following the excavation of premining position of the longwall face; zero indicates when

both panels were monitored by the mining company. Mine the respective longwall face passed beneath the line of wells;

personnel estimated flow rates through the monitoring of a and a positive value indicates postmining positions of the

main sump located underground. Effects of surface area, longwall face past the line of wells. If at another site a longwall

lithology, and hydrogeology were incorporated into the face was approaching a well, but was 366 m (1,200 ft) away

analysis to determine flow rate estimates. This information and the thickness of the overburden at the site of the well was

provided a reasonable basis for the selection of several values 122 m (400 ft), the FP/OB ratio is still -3. This curve allows

for input into the model i.e., fracture spacing (S) and modulus one to compare well response at two different sites

reduction factor (R ). The flow rate provided by the mining (conceivably in the same study area) without having to make m

company was approximately 2,390 Lpm (630 gpm). Values of complicated adjustments for differing overburden thicknesses.

R , determined through matching m the flow rate, were 0.80 for Under the applied boundary conditions, the model determined

material 1, 0.05 for material 2, and 0.97 for material 3. Choice that a static decrease of 3.0 m (10 ft) in water level would occur

of these parameters was clearly nonunique, but was predicted due to the mining of both longwall panels. This decrease

on the anticipated response of the lithologic units to straining would occur without an infiltration rate inputted in the

and fracturing. The preexisting fractures in the upper shale and

the lower shale units have less effects on the postmining

conductivities than those in the sandstone layer unit. Actually,

the shale material may be treated as a porous medium which is

not sensitive to deformation. The difference between the two

values is due to the effect of depth. The value of R is chosen m

as 0.05 for the sandstone unit because a large part of the

extensional strain is applied to the fracture system and

precipitates the largest possible change in conductivity.

The coarse finite element mesh, used to evaluate the

subsidence profile and overburden displacements, was applied

to determine the influence of mining-induced permeability

changes on the ground water system. The predicted effects on

the postmining ground water system determined by the model

correlated well with the field data collected at the site as shown

in figures 6-11. Figures 6-11 show water level fluctuations

measured for Wells 1-6, respectively. In addition, curves were

added to the figures which show progression of the longwall

face as a function of the overburden thickness. These values

are expressed as the ratio of face position (FP) to overburden

thickness (OB). For example, consider a longwall face that is

moving towards a well, but is 183 m (600 ft) away and the

overburden at the site of the well is 61 m (200 ft). The FP/OB

ratio is -3. The negative value of FP/OB ratio indicates a
11
12

model. If an infiltration rate of 0.23 m/yr (0.75 ft/yr) is used,

the regional model predicts that essentially no effects to the

static phreatic surface would result. This infiltration rate is 25%

of the total precipitation occurring at the site and correlates well

with known infiltration rates of previous studies (14). Precipitation records provided by the mining company showed an

average precipitation of 0.91 m (3.0 ft) which occurred

at the site. Therefore, an infiltration rate of 0.23 m/yr

(0.75 ft/yr) was used. Figure 5 shows the model prediction

after excavation of both longwall panels.

In addition to applying the regional model, described above,

local behavior around the shallow well field (figure 12A) was

also represented by a more refined model to accommodate

more subtle water budget changes as a result of mining. The

changes in hydraulic conductivities predicted from the initial

calculations were applied to the zonation as defined in figure

12B. These zones represent average changes in hydraulic

conductivities evaluated from the subsidence modeling. The

refined mesh is capable of accurately representing local

changes in the location of the phreatic surface. This mesh

represents the region between the centerlines of both panels

containing Well 5 and 6 within Zone I, Wells 2 to 4 within

Zone II, and Well 1 within Zone III. No flow boundary

conditions were specified along the base and on the left side

(the centerline of panel No. 2) and constant head conditions

were applied on the remaining vertical side (about 76.2 m (250

ft) away from the centerline of panel No. 1). The mesh, utilized

uniform spacing, and was constructed of 326 nodes and 295

elements. Two different situations were simulated through use

of the refined mesh. First, the same infiltration rate and

premining ground water conditions were inputted, where the

small-scale influences of topographically induced flow were

accommodated. Second, the postmining hydraulic conductivity

magnitudes of table 5 were incorporated to determine the

anticipated postmining ground water levels. The magnitude of

hydraulic conductivities was evaluated directly from the spatial

distribution of strains, using equations 6 and 7. Average

magnitudes of horizontal and vertical conductivities were then

utilized in the refined model of figure 12. The changes in

hydraulic conductivities are documented in table 6. 

The resulting postmining modification in the location of the

phreatic surface is illustrated in figure 13. The subtle changes

result from applying the same infiltration rate as used in the

previous model run, however, the greater element density

of this revised model highlights the influence of even minor

topography and moderate changes in near surface hydraulic conductivities on the ground water system. The
13

Table 6.—Relative changes in hydraulic

conductivity (K /K ) postmining original

as applied to small-scale mesh

Zone Kh K v

1 . . . . . . . . . . . . . . . . . . . 100 1

2 ................... 1/100 1/5

3 . . . . . . . . . . . . . . . . . . . 1 10

4 ................... 1 1

onductivity. h Horizontal c h

K Vertical conductivity. v

water system. The mining-induced development of hydraulic conductivities is such that water levels in the

region of Wells 5 and 6 remain the same, water levels in

the region of Wells 3 and 4 rise, and water levels in the

region of Wells 1 and 2 fall, relative to premining water

levels. When the long-term recorded levels in these six

wells are corrected relative to the control well (a well

located approximately 425 m (1,400 ft) away from mining

activity), this distribution of behavior, and of this

magnitude is exactly as observed from the field

measurements performed at the site (6) and as shown in

figures 6-11.

SUMMARY

1. The subsidence profile, as determined from the model, for these boreholes showed slightly higher displacement.

compares favorably with maximum subsidence measured at the Also, MPBX 5 showed similar vertical displacements compared

fieldsite. with that predicted by the model.

2. The field data showed that minimal subsidence occurred 5. The predicted effects of the postmining ground water

above the gate roads between the two panels, while the system determined by the model correlated well with the field

predicted subsidence profile showed a maximum of 0.15 m (0.5 data collected at the site. Without infiltration added, the model

ft) subsidence at this location. determined that a static decrease of 3.0 m (10 ft) in water level

3. The predicted vertical displacement data obtained from would occur at the site due to the mining of both longwall

the model for MPBX's 1, 2, 3, and 6 correlated well with the panels 1 and 2. If an infiltration rate was input to the model, no

field data. predicted effects for the long term would occur to the ground

4. The field displacement data for MPBX 4 displayed water regime which correlates well with field observations.

minimal displacements, whereas, the predicted displacements
14

6. Where a fine mesh is used to define local changes in Minor local changes in the phreatic surface are consistent with

water budget within the well field area, the model is capable of the rolling topography of the site and the effects of miningreplicating relatively subtle changes in long-term water levels. induced changes in hydraulic conductivity. 

CONCLUSIONS

For this study, the numerical modeling results correlate field information is not available, the model routine requires

favorably with the field data collected at the site. The surface some additional assumptions. Although a favorable correlation

subsidence information from the field, provided an excellent exists between the mining effects predicted and the field data

foundation for the modeling routine. If this information is not collected, the authors feel that additional comparative studies at

available, one should obtain and examine available subsidence research sites with varying geology, longwall panel

prediction models to determine the profile information for the characteristics (thickness, width, etc.), hydrogeology, etc.

site. The assumptions made during the course of operating the should be performed to further substantiate the capabilities of

model were based on knowledge of the subject area, experience the model.

with the model and insight gained from field tests. Again, if the

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INT.BU.OF MINES,PGH.,PA 30130