Subsidence from Underground Mining: 

Environmental Analysis and 

Planning Considerations

GEOLOGICAL SURVEY CIRCULAR 876
COVER PHOTOGRAPH:

Subsidence pits, troughs, and cracks above an abandoned coal 

mine in the western Powder River Basin, Wyoming. Photograph by C.R. Dunrud.
Subsidence from Underground Mining 

Environmental Analysis and 

Planning Considerations

By F. T. Lee and J. F. Abel, Jr.

GEOLOGICAL SURVEY CIRCULAR 876

7983
United States Department of the Interior

JAMES G. WATT, Secretary

Geological Survey

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Lee, Fitzhugh T.

Subsidence from underground mining.

(Geological Survey circular ; 876)

Bibliography: p.

1. Mine subsidence Environmental aspects.

2. Mine subsidence. I. Abel, John F., Jr. II. Title.

III. Series. 

QE75.C5 no. 876 557.3s [622] 82-600123 [TD195.M48]

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page 

1

1

2

2

2

3

Abstract __...____..___...____..___...._ 

Introduction .._.___.______..____..___._._

Overview of subsidence processes ______________

Definition ...........................

Conceptual description of subsidence __...___. 

Subsidence produced by longwall coal mining . 

Subsidence produced by room-and-pillar coal 

mining _.___..__...______.____._ 4

Subsidence from mining other sedimentary 

resources __....__.._._....._.... 5

Subsidence from mining crystalline rocks .. 8 

Impacts of subsidence ...................... 8

Effects on surface structures .___..___..___ 9 

Hydrologic impacts ..................... 9

Effects on vegetation and animals .......... 12

Coal mine fires ....................._. 12

Factors governing subsidence ................ 12

Mining method ........................ 12

Multiple seam mining of coal beds ..__._..__. 12 

Depth of extraction .................... 13

Rate of advance .._.................... 13

Thickness of seam or deposit .............. 13

Lithology and structure ................. 13

In situ stresses and other geotechnical factors .. 14

Topography __.__---_-_-------------

Time _____......______---.....-.--

Subsidence analysis __----_.-------------

Data requirements .......-----.--..--

Geologic data _-----.-------------

Hydrologic data ..___------.--_---

Geotechnical data .................

Mining plans _______-_-----.-----

Analytical techniques for subsidence prediction .. 

Background and state-of-the-art __________

British National Coal Board method ....

Other empirical methods ......._....

Mathematical techniques .....-._-..-

Selection of an analytical technique ..._.-.._-

Subsidence models for in situ energy extraction

Monitoring methods ...__.-.----.--------

Surface instrumentation __-_.---.------

Spacing of monuments -------------

Subsidence control and reduction .....--_-.--

Design precautions _......-----.------

Conclusions __.__---_-__.--------------

Glossary ____.---.--------------------

References cited .-_--.__----------------

Page 

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17

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22

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24

24

25

25

25

26

ILLUSTRATIONS

FIGURE 1. Generalized and idealized sketch of deformations produced in rocks above and below extracted coal seam . 3

2. Photograph showing compression ridges produced contemporaneously with longwall mining near Raton, New

Mexico ---........................................--------.----------.-.-- 4

3. Photograph showing large open fissure associated with longwall mining near Raton, New Mexico ..... 4

4. Photograph showing rockfalls and tension cracks produced during longwall mining near Raton, New

Mexico ...................................................... 5

5. Generalized sketch illustrating subsidence over longwall mining _______________________ 6

6. Aerial photograph showing the surface effects of past and present room-and-pillar coal mining along the

Tongue River in Wyoming ___.____._____._____._______.___._-_---_-__ 7

7. Nomogram for the determination of change of length of a surface structure for various mining conditions . 10

8. Graphic representation of the British National Coal Board method of subsidence estimation ........ 19

9. Graph showing general relation between percent subsidence and panel width to mining depth ratio ... 20

10. Graph showing relation between subsidence and surface tilt _________________________ 21

11. Map showing location of surface monuments, turning points and reference points at the York Canyon mine

near Raton, New Mexico ____________________________________________ 23

ill
TABLE

Page

TABLE 1. Subsidence damage classification for horizontal ground strain and changes in length of manmade structures for

longwall mining .______._.__.....__.__....___.__.......___..___.. 11

IV
Subsidence from Underground Mining:

Environmental Analysis and

Planning Considerations

By F. T. Lee and J. F. Abel, Jr. 1

ABSTRACT

Subsidence, a universal process that occurs in response to 

the voids created by extracting solids or liquids from beneath 

the Earth's surface, is controlled by many factors including 

mining methods, depth of extraction, thickness of deposit, and 

topography, as well as the in situ properties of the rock mass 

above the deposit. The impacts of subsidence are potentially 

severe in terms of damage to surface utility lines and structures, changes in surface-water and ground-water conditions, 

and effects on vegetation and animals. Although subsidence 

cannot be eliminated, it can be reduced or controlled in areas 

where deformation of the ground surface would produce 

dangerous or costly effects.

Subsidence prediction is highly developed in Europe where 

there are comparatively uniform mining conditions and a long 

history of field measurements. Much of this mining has been 

carried out beneath crowded urban and industrial areas where 

accurate predictions have facilitated use of the surface and reduced undesirable impacts. Concerted efforts to understand 

subsidence processes in the United States are recent. Empirical methods of subsidence analysis and prediction based on 

local conditions seem better suited to the current state of 

knowledge of the varied geologic and topographic conditions in 

domestic coal mining regions than do theoretical/mathematical 

approaches. In order to develop broadly applicable subsidence 

prediction methods and models for the United States, more information is needed on magnitude and timing of ground movements and geologic properties.

INTRODUCTION

The worldwide need for energy resources requires increased production of coal and other 

fuels. A large amount of this production will eventually come from underground mining in areas 

where surface mining is impractical or uneconomi-

'Colorado School of Mines, Golden, Colorado 80401

cal. Past coal mining practice left much coal in the 

ground as pillars that are difficult to recover. Present-day emphasis is on improving extraction percentage. Because both the methods of extraction 

and the number of mines will increase overall subsidence risk, we must be able to accurately predict 

the surface and underground impacts of subsidence and, if necessary, to suggest means to lessen these impacts. It is clear from poorly controlled mining operations of the past that we no 

longer have the luxury of mining without regard 

to present and future land use.

The purpose of this circular is to give an overview of subsidence processes and their potentially 

harmful consequences, the methods of subsidence 

prediction, and methods to control and reduce 

subsidence impacts. The report is primarily intended to serve as an introduction and state-ofthe-art review for those individuals or groups concerned with assessing the potential environmental 

effects of underground mining. The major emphasis is on coal mining.

The time to plan for subsidence impacts is well 

before mining begins, not after surface effects are 

noticed. Because subsidence due to underground 

mining may be inevitable, the relevant questions 

to be asked are how much, when, and where, and 

what abatement procedures are possible and 

might be necessary. The impacts of subsidence are 

broad, affecting water supplies, transportation 

and utilities, vegetation, and farming. In addition, 

in situ extraction techniques for coal gasification 

and oil shale retorting are supported by extensive, 

costly surface facilities. The success of these oper-
ations may depend upon accurately estimating the 

extent of subsidence, both in area and strain magnitudes, so that surface plants are not damaged or 

located unnecessarily distant from the mining operations.

Recent Federal regulations implementing the 

Surface Mining Control and Reclamation Act of 

1977 (Federal Register, 1977) will encourage increased consideration of subsidence effects prior 

to mining. At present, however, very few mining 

companies in the United States make routine 

leveling surveys of the ground surface. Increased 

surface monitoring is needed in order to implement current mining regulations.

To a greater or lesser degree each mining area 

is different, and no one subsidence prediction 

method will serve equally well for all cases. For 

example, the prediction scheme used in the 

United Kingdom with excellent results for 

longwall coal mining2 does not work as well in 

other areas largely because of different geologic 

conditions. The great diversity of mining conditions in the United States partly accounts for the 

circumstance that subsidence prediction is not far 

advanced in this country. A systematic concentrated effort is needed to develop more broadly 

applicable and accurate methods of subsidence 

prediction in the United States. The more general 

and widely used analytical techniques are discussed here; the reader is cautioned that many circumstances will require specific expert advice.

The term "mining" as used in this report includes all extractive processes for recovering organic and inorganic resources. Thus, we are concerned with deformations caused by several mining methods in a variety of geologic environments. 

Most emphasis in this report, however, is placed 

on longwall and room-and-pillar mining of coal. It 

has been estimated that coal extraction is responsible for over 90 percent of worldwide mining-induced subsidence (Alien, 1978), Longwall mining 

currently accounts for less than 10 percent of the 

coal mined in the United States, although it has 

long been the most popular method in Europe. 

However, because longwall mining, which removes a complete tabular section of coal, is more 

efficient than room-and-pillar mining, it is being 

promoted increasingly in the United States. Also, 

longwall mining induces a generally uniform and 

contemporaneous surface subsidence that can be

Technical terms used herein are defined in the glossary, p. 25.

more accurately forecast than subsidence caused 

by room-and-pillar mining. The current state of 

knowledge of subsidence permits a more detailed 

discussion of longwall-induced subsidence than of 

the other forms.

OVERVIEW OF SUBSIDENCE PROCESSES

DEFINITION

Subsidence is a time-dependent process, either 

natural or man induced, in which there is a lowering of the ground surface in response to the removal of gas, liquid, or solid matter. Deformation 

of the rock mass may be by either elastic, plastic, 

or brittle processes or by any combination of these 

processes. Subsurface deformation leading to surface subsidence includes the local lateral and upward displacements of rock above unmined areas 

(near mine boundaries or barrier pillars) caused 

by the downward movement of overburden into 

mine cavities. Strains induced by mining and 

transmitted through intervening strata to the surface may be compressive or tensile and may have 

both horizontal and vertical components.

CONCEPTUAL DESCRIPTION OF SUBSIDENCE

The void created by the underground extraction 

of coal or other resources causes significant 

changes in the magnitude and orientation of the in 

situ stress field and results in deformations both 

in the remaining coal and in the surrounding 

rocks. In general, the sides of the excavated area 

move inward, the floor upward, and the roof 

downward. The initial deformations may be elastic, that is, they may disappear if the deforming 

forces are removed. Nonelastic deformations, 

however, occur with time and, as the region of 

cavity influence increases with continued extraction, rock strengths are exceeded and irreversible 

block movements take place. Large bending moments in the mine roof strata will ultimately cause 

local roof failure and collapse, and the mined-out 

area will fill with overburden materials. The 

downward movement of overlying rock will induce 

lateral movement of rock toward the cavity. These 

deformations are illustrated in figure 1. Rock outside the vertical limits of the mined area will also 

subside. The affected ground will lie typically 10°- 

35° outside the vertical limits of the mined area 

(angle of draw). Deformations eventually reach 

the ground surface and may form subsidence depressions, open fractures, pits, and troughs. Com-
Original surface 

-§ybsided surface^

 : '^s- ^ _- *<"> ; '.' /.' -. P-'^'--vy:'-A': :v:-^: :.^

rTTT

FIGURE 1. Deformations produced in rocks above and below an extracted coal seam (modified from Shadbolt, 1978).

pression features including doming and thrust 

faulting have also been reported.

Depending upon several factors including mining methods and rock properties, the changes at 

the surface may occur almost concurrently with 

mining or they may be delayed and take place 

with dramatic suddenness more than 100 years 

after mining.

SUBSIDENCE PRODUCED 

BY LONGWALL COAL MINING

The mechanical aspects of the subsidence process in flat-lying coal bearing rocks mined by 

longwall methods are reasonably well known, 

compared to room-and-pillar mining, and are 

documented abundantly in the literature. This 

knowledge may be summarized as follows. When 

an underground coalbed of a given thickness is extracted over a wide area, the immediate roof will 

collapse. Rock displacements are transferred to 

the ground surface when the ratio of the width of 

the extracted material to the depth of overburden 

(w/k) exceeds a value which varies from 0.1 to 0.5 

(Wardell and Eynon, 1968); the ratio value is controlled largely by the strength and structure of 

the rock overlying the mined-out area. Field measurements and theory support the concept that

there is a stabilizing compression (or pressure) 

arch in the solid rock above and below the minedout area. The duration of this arching effect is 

controlled by the height, width, and length of the 

mined opening, and subsidence will not begin until 

a critical void size is exceeded at which the arch 

will no longer span the excavated area. Consequently, there is often a delay between the 

onset of a change in state underground and the 

first appearance of land subsidence at the ground 

surface. The arching effect may be limited, however, by very weak overburden rocks or by poor 

mining practice that significantly weakens the 

overburden. Geologic conditions, mining depth, 

and seam thickness also affect arching behavior.

After subsidence has begun, it will develop 

progressively and continue so long as there is a 

progressive enlargement of the underground 

opening. Surface effects from longwall mining in 

mountainous terrain in New Mexico are illustrated 

in figures 2-4. These conditions occurred essentially contemporaneously with mining.

The concept of a "critical" area (width and 

length) of extraction is closely related to the ability of the strata above the excavated area to support loads across the mined openings. Assuming 

an infinite length for an extracted area (in practice
FIGURE 2. Compression ridges produced contemporaneously 

with longwall mining near Raton, New Mexico.

a length equal to or greater than the depth of mining), there is a critical width of extraction for 

which the subsidence (vertical lowering of the surface) reaches a maximum value. Subcritical widths 

of extraction produce a trough-like subsidence 

area with vertical subsidence less than the 

maximum. At supercritical widths of extraction 

the subsidence area has an essentially flat bottom 

at approximately the maximum subsidence. These 

relations are illustrated in figure 5.

The critical width of extraction is normally expressed in terms of the mining depth (fig. 5). In 

European coal fields it ranges from 1.0 h to 1.4 h, 

where h is the average depth of mining. This 

range has been attributed to differences in the 

types of overlying rock. Quantitative studies are

meager; however, the lower values of the depth 

coefficient appear to be associated with overburden containing thick, strong sandstone and limestone beds whereas the higher values pertain to 

overburden containing a large percentage of thinbedded shales, mudstones, siltstones, sandstones, 

and unconsolidated deposits. Maximum subsidence 

is also a function of the thickness of the extracted 

layer or the volume of material extracted, the 

mining methods, and several other factors discussed in the following sections.

SUBSIDENCE PRODUCED 

BY ROOM-AND-PILLAR COAL MINING

Room-and-pillar mining is the most frequently 

used mining method in United States coal mines. 

The coal is mined in entries (rooms) separated by 

pillars which may or may not be partially extracted later. Initially, a series of parallel entries

'^^Snc

FIGURE 3. Large open fissure associated with longwall mining 

in mountainous terrain near Raton, New Mexico. Such features can divert surface runoff, increasing landslide potential.
FIGURE 4. Rockfalls and tension cracks produced during longwall coal mining near Raton, New Mexico.

are driven through the seam with interconnecting 

openings (breakthroughs) driven at right angles 

through the pillars between the rooms. Such a 

checkerboard pattern of openings is advanced 

through the coal seam to the limit of the area 

planned for mining. At this point approximately 

50 percent of the coal will have been mined. The 

coal pillars between adjacent rooms may be fully 

or partially removed (robbed) during final, or retreat, mining. After full pillar removal, the rock 

above the mine collapses and the overburden 

gradually settles, creating surface fissures and 

subsidence.

The percent extraction by room-and-pillar mining depends upon several factors including the 

number and size of pillars deemed necessary to 

temporarily support the mine roof during retreat 

mining and the need to prevent or limit surface 

subsidence. In some sections of West Virginia and 

in other States where surface land is owned by 

the coal producer, nearly 100 percent of available 

coal can be mined using the room-and-pillar

method. In States such as Illinois, however, 

where farm and industrial land are extremely valuable, only about 50 percent of the coal may be 

mined to prevent surface damage from subsidence.

Because of the several stages of coal removal 

and the slow pillar deformation and deterioration 

in room-and-pillar mining, surface settlement is 

not as uniform and immediate as it is in longwall 

mining; rather, it may be erratic, intermittent, 

and long delayed. Figure 6 shows a representative 

example of delayed subsidence features in Wyoming resulting from old shallow underground roomand-pillar mining operations.

SUBSIDENCE FROM MINING 

OTHER SEDIMENTARY RESOURCES

Other sedimentary resources which are extracted from considerable depth (hundreds of meters) include salt, potash, sulfur, trona, and phosphate. These minerals occur in bedded, usually
Surface and datum

EXPLANATION 

a Angle of draw h Depth 

m Mining thickness 

S Subsidence (vertical deformation) 

^'max = Maximum subsidence

FIGURE 5. Description of subsidence over longwall mining. Face advance direction is perpendicular to plane of page. (Adapted from National Coal Board, 1975).
flat-lying deposits, and have been mined by several methods: conventional room-and-pillar, 

longwall, and solution. Pillar and roof collapse and 

resultant surface deformations are typically delayed and difficult to predict. In solution mining of 

salt, for example, neither the time nor the location 

of subsidence can be predicted with confidence because salt deforms slowly in a complex manner 

and the deformations are different from those of 

overlying shale and sandstone. Furthermore, solution extraction of salt and other soluble evaporites 

is a specialized mining technique and the resulting

subsidence causes unique problems (Marsden and 

Lucas, 1973). Drilling through aquifers and salt 

beds in search of oil, gas, and water has induced 

salt dissolution and subsequent subsidence (Fader, 

1975). As Ege (1979) points out, the construction 

of highways, dams, and reservoirs over saline or 

gypsiferous rock has caused subsidence, water 

loss, and dam failures. A comprehensive discussion of subsidence associated with solution extraction of sulfur is given by Deere (1961).

The subsidence produced by room-and-pillar and 

longwall mining of potash, phosphate, and most

FIGURE 6. Southeastward-looking aerial view showing the surface effects of past (room-and-pillar) and present (open pit) coal 

mining along the Tongue River in Wyoming (July 1977). Pits, troughs, depressions, and cracks have formed because of subsidence over the south part of the Acme mine. The mine was operated from the early 1900's until 1943. Overburden thickness 

ranges from 15 m in the middle ground to approximately 30 m in the foreground and consists of alluvium and soft interbedded 

shales, claystones, siltstones, and discontinuous sandstones. The dam in the right foreground across Hidden Water Creek ruptured because of subsidence. The water now is diverted into subsidence depressions, pits, and cracks upstream from the dam. 

Garbage from the town of Acme was dumped into the large pit at the left side of the photograph. Pits and troughs in the 

middle of the photograph are in alluvium. Note that the pits near the road and left of the draw do not occur in any noticeable 

depression; these pits are located above collapsed areas in haulageways of the Acme mine where adjacent coal pillars are strong 

enough to support the overburden. Alluvium is being removed at the Big Horn surface coal mine to extract the coal (background). Regrading is beginning near the river (right background). (From Dunrud and Osterwald, 1980.)
sedimentary rocks is grossly similar to that of 

mining coal, being dependent upon the thickness 

of the bed mined, the depth, the mining methods, 

and the properties of the overburden. The reader 

may want to consult Miller and Pierson (1958) for 

a discussion of subsidence over potash mines.

SUBSIDENCE FROM MINING 

CRYSTALLINE ROCKS

A very different type of subsidence is produced 

by the extraction of irregularly shaped metalliferous ore bodies by block caving or by leaching of 

disseminated deposits where the extracted width 

is small compared to the depth. A prominent collapse structure, as much as tens to hundreds of 

meters in depth, may develop rapidly. The subsidence mechanism in jointed crystalline rocks proceeds approximately as follows (Abel and Lee, 

1980):

1. Rock collapses progressively upward from the 

mining horizon (undercut level) as ore is 

withdrawn from below.

2. The ground surface does not begin to subside 

measurably until the collapse has so thinned 

the intact rock above the mined-out area 

that it cannot support the load of the overlying rock (arching effect). The overlying 

solid rock will then deflect downward toward the collapsed rock. Lateral movement 

of adjacent rock into this collapsed rubble 

column is resisted by the active pressure of 

the rubble.

3. Further extraction of caved ore from below results in increased subsidence of the ground 

surface above and outside the area of extraction. This initial trough subsidence is 

similar in shape to the trough subsidence 

observed over coal mines.

4. Continued extraction of ore will result in 

breaching of the surface. The initial breach 

is typically in the form of a circular depression, or chimney, that is roughly centered 

over the mining area; it may be offset a 

minor distance because of preferential collapse along geologic discontinuities.

5. The rock adjacent to the chimney either slides 

along geologic weaknesses, such as joints or 

faults, or topples into the collapsing upper 

part of the chimney.

6. The final, or ultimate, angle of draw is located 

where either the flattest geologic weakness

intersects both the ground surface and mining horizon or where the angle of repose of 

the broken rock mass is reached, whichever 

is flatter.

It is common practice to report an initial and 

final angle of draw for block-caving-induced subsidence. The initial angle of draw marks the extent 

of subsidence effects at the time the surface is 

breached. The final angle of draw occurs at the 

limit of measurable subsidence effects after mining ceases. Negative initial angles of draw, that is, 

those inside the mining area, have been reported; 

they were probably derived, however, from insufficient field measurements. Such a negative angle 

of draw refers to the angle between the vertical 

and a line connecting the side of the chimney with 

the nearest side of the mining level. Initial angles 

of draw in crystalline rocks range from 5° to 40° 

and final angles of draw between 5° and 65°.

IMPACTS OF SUBSIDENCE

Damage from subsidence over underground 

mines has been a serious problem in urban areas 

for many years and will become more widespread 

as the demand for resources, particularly coal, increases. Continuing subsidence has recently posed 

hazards in parts of Colorado and Wyoming where 

urban areas have spread onto land underlain by 

abandoned coal mines. Delayed subsidence has 

caused extensive damage in urban areas established over coal mines in the Eastern States.

Economic impacts of subsidence in rural areas 

can also be significant. Fields must be regraded to 

eliminate ponding of water, and, as in urban 

areas, roads must be regraded and homes must be 

repaired. Water wells may become dry when 

aquifers are disturbed by rock movements. Gas 

mains are especially vulnerable to subsidence and, 

if ruptured, can catch fire and explode.

Damage from surface subsidence can be caused 

by changes in surface slope, differential vertical 

displacements, and horizontal strains. Planners 

must know whether these changes are complete or 

in progress, permanent or temporary; further 

mining may restore the original slope or close tensile fractures thereby rendering some remedial 

measures unnecessary or even harmful. The magnitude of structural damage in buildings and manmade structures will depend to a large extent 

upon details of design and materials; therefore any 

damage classification must be a general one.
EFFECTS ON SURFACE STRUCTURES

The effects of subsidence on surface structures 

are controlled to a large extent by the mining 

method. The long-term, delayed nature of subsidence over room-and-pillar mines can make the 

task of repair and maintenance of surface facilities 

intermittent and not predictable. Longwall mining, however, is associated not only with greater 

recovery of coal but with increased surface stability after mining and, hence, earlier construction or 

resumption of previous surface activities.

Many accounts exist of severe surface deformations that occurred, often abruptly, long after 

mining ceased. With only a few exceptions, the 

notable delayed residual subsidence has taken 

place in room-and-pillar mined areas rather than 

in longwall mined regions. At Farmington, W. 

Va., intermittent episodes of subsidence occurred 

because 2-m to 3-m high coal pillars were gradually forced into the weak claystone mine floor 

(Gray and others, 1977). Surface deformation that 

damaged dozens of homes and buildings began 

while the mine, which was 85 m below the surface, was active. Subsidence movements continued 

intermittently for more than 4 years after mining 

stopped until the mine was injected with coal 

waste. At the Geneva coal mine in Colorado, compression features such as fractured bulges and 

small anticlines formed in massive sandstone 274 

m above a mined-out area about \Vz years after 

mining was completed. Measurements showed 

that the ground surface was shortened locally by 

as much as 0.92 m (Dunrud, 1976).

Damage to surface structures over room-andpillar mines has been particularly noteworthy in 

those Eastern States underlain by extensive coal 

deposits. Surface developments overlie room-andpillar coal mines in Pittsburgh, Pa., and Birmingham, Ala., as well as many smaller cities and 

towns in 17 States (Alien, 1978). For example, at 

Scranton, Pa., $29 million worth of property, including 2,000 homes, 50 commercial and office 

buildings, 2 hospitals, several schools, and various 

utility lines have either been damaged by or are 

being threatened by subsidence many years after 

the mines were abandoned. The total cost of surface stabilization of this area by hydraulic mine 

backfill is estimated to exceed $8 million (Dunrud, 

1976). Subsidence occurred dramatically in 1974 in 

Lafayette, Colo., above a coal mine abandoned for 

several decades when a pit 4.5 by 5.5 m wide and

7.3 m deep developed in 24 hours in a then unoccupied part of a trailer court (Ivey, 1978).

Engineers in PolancP classified structures into 

four categories, on the basis of importance and 

sensitivity to surface movement, and designated 

acceptable values of tilt and normal strain for each 

category (Hutchings and others, 1978, table 4). A 

classification of subsidence damage based on 

structural length changes and horizontal ground 

strain caused by longwall coal mining was developed by the British National Coal Board (NCB) 

and is given in table 1. The NCB has also developed a useful nomogram for estimating deformations of surface structures in the United Kingdom (fig. 7).

Figures 2-4 show surface effects produced by 

longwall mining in mountainous terrain in New 

Mexico. Because subsidence is an inevitable consequence of high-extraction mining, planners must 

be able to forecast its impact not only on surface 

activities but also on subsurface structures such as 

aquifers.

HYDROLOGIC IMPACTS

Subsidence depressions and associated fractures 

may disrupt surface and underground water flow, 

causing diminished well production, aquifer contamination, and decreased property values. In the 

dry areas that typically overlie western energy resources, the loss of springs and other surface 

water is especially critical (fig. 6). Proper planning 

and development of underground mining will lessen these detrimental effects. Few detailed 

studies exist to adequately document the effects of 

mining on the hydrologic regimen. Hydrologic 

changes may be more subtle than other mining effects; for example, subsurface aquifer disruption 

may occur because of displacement of rocks above 

mined-out areas without visible surface manifestation. Some shallow wells in Pennsylvania have experienced reduced production. In one instance, a 

mine face passed within 30 m of a 20-m-deep well 

which went dry. The well was deepened by 12 m 

and a good water supply was encountered (Sossong, 1973).

Of great importance in many areas is the potential hazard that surface waters present to mining. 

In Pennsylvania a longwall panel having 200-215 

m of overburden was successfully mined directly 

below a reservoir (Sossong, 1973). Normal mining 

conditions were encountered with 63-190 x 10 ~ 5
Seam thickness

extracted in

meters

200 180 160 140 120 100 80 60

Length of structure in meters

40 20 20 40 60 80 100 

Change in length in m

FIGURE 7. Nomogram for the estimation of change of length of a surface structure for various mining conditions. Example given (dash

30 m, depth of coal seam of 150 m, and seam thickness of 1.3 m, which results in change of 172 mm in length of structure. (Modified f
TABLE 1. Subsidence damage classification for horizontal ground strain and changes 

in length ofmanmade structures'from longwall mining

[|ic = microstrain; 1 ft= 0.305 m]

Class of damage

Change of length 

of structure

Description of 

typical damage

Very slight or negligible 

(example: 15-m long 

building extended 50 |xe).

Up to 0.03 m Hair cracks in plaster. Perhaps isolated 

slight fracture in the building, 

not visible on outside.

Slight

(example: 33-m long building 

extended 1,600 |xe).

Appreciable

(example: 27-m long building 

extended 3,700 |xe).

Severe

(example: 67-m long apartment 

house compressed 2,300 jjie).

0.03-0.06 m

0.06-0.12m

0.12-0.18m

Very severe

(example: 55-m long apartment 

house extended 6,000 jxe).

More than 0.18 m

Several slight fractures showing inside the

building. Doors and windows

stick slightly. Repairs to

decoration probably necessary. 

Slight fracture showing on outside of building (or

one main fracture). Doors and windows sticking;

service pipes may fracture.

Service pipes disrupted. Open fractures requiring 

rebonding and allowing weather into the 

structure. Window and door frames distorted; 

floors sloping noticeably; walls leaning or 

bulging noticeably. Some loss of bearing in 

beams. If compressive damage, overlapping of 

roof joints and lifting of brickwork with open 

horizontal fractures.

As above, but worse, and requiring partial or 

complete rebuilding. Roof and floor beams lose 

bearing and need shoring up. Windows broken 

from distortion. Severe slopes on floors. If 

compressive damage, severe buckling and bulging 

of the roof and walls.

'Modified from National Coal Board, 1975.

m3/s (10-30 gal/min) of water developed from the 

entire 183-m-wide panel. Maximum subsidence induced by an adjacent panel not under the reservoir was 0.65 m or 47 percent of the 1.37-m mining thickness with an average width/depth ratio of 

0.85. Numerous shallow workings have taken 

place under large bodies of water. Many of these 

workings are essentially dry, although some 

mined coal seams were less than 30 m below 

water or saturated alluvium (Orchard, 1973). In 

the United Kingdom, undersea longwall extraction 

is permitted with a minimum cover of 105 m and 

a maximum tensile strain of 0.01.

We have discussed the development of increased rock mass permeability through mininginduced fracturing and the structural and 

lithologic characteristics that may be important

determinants of subsidence. The significance of 

these factors is increased when mining under 

bodies of water or under productive aquifers; in 

such circumstances an adequate monitoring program is essential to warn of a possibly hazardous 

water inflow.

Guidelines for mining near surface and underground bodies of water have been published by 

the U.S. Bureau of Mines (Babcock and Hooker, 

1977).

In addition to conventional coal mining, the underground coal gasification process and in situ retorting of oil shale can also affect ground-water 

supplies in complex ways. When ground water 

reenters a gasified coal bed, the residual reaction 

products (coal ash, tars, and gases) may undergo 

leaching, dissolution, and hydrologic transport

11
(Mead and others, 1978). Subsidence effects such 

as extensive fracturing or mine roof collapse may 

cause contaminants to find their way into aquifers 

that lie above the mined zone. Any meaningful 

evaluation of this potential hazard must be based 

on long-term monitoring of typically slow flow 

rates. As a consequence of the sorptive properties 

of the rocks associated with coal, the contaminants 

may move more slowly than the ground water and 

they may react chemically with each other and 

with the rocks, potentially lengthy and poorly understood processes.

EFFECTS ON VEGETATION AND ANIMALS

As we have noted previously, collapse over 

deep mine cavities may induce tension fractures in 

nearby overlying beds and at the ground surface. 

Methane gas may leak out of shallow coal seams 

or mines through these fractures and kill trees 

and woody plants, leaving only grasses unaffected. Garner (1974) found that certain bacteria 

in the soil use methane to produce hydrogen sulfide and nitrous oxide. These gases disrupt the 

root transpiration of woody plants, ultimately killing them. Noxious or toxic gases may also overcome animals grazing on the surface.

Another source of damage to plant life and associated wildlife is the trough-like subsidence 

areas formed over areas of longwall mining. Unless drainage is maintained, these depressions 

may fill with water, creating swampy, tree-killing 

conditions and new types of habitats. Conversely, 

in some western areas (for example, North 

Dakota) collapse pits may be the only places wet 

enough to sustain the growth of cottonwoods and 

willows. Collapse pits and open fractures may trap 

animals or interrupt their migration patterns.

Losses of soil water or water in deeper aquifers 

through fractures created by subsidence could be 

equally harmful to plant and animal life, especially 

in semiarid areas.

COAL MINE FIRES

Coal mine fires are an indirect result of coal extraction and contribute to long-term subsidence. 

According to Dunrud and Osterwald (1980) several fires are burning in long-abandoned underground mines near Sheridan, Wyo. Many of the 

fires appear to have been started by spontaneous 

ignition when air and water were introduced 

through subsidence cracks, pits, and unsealed 

mine openings. Combustion is supported by the

drawing in of oxygen and the exhaustion of 

smoke, steam, and noxious gases through subsidence cracks and pits. As the coal burns, more 

cavities are created causing more cracking and 

collapse which allows greater access for air, 

thereby accelerating coal burning. This uncontrolled "in situ gasification" process is destroying a 

valuable resource. In addition, gaseous combustion products locally pollute the air; soil changes 

are produced, and vegetation is retarded or killed 

by near-surface fires.

FACTORS GOVERNING SUBSIDENCE

The magnitude, rate of development, and surface expression of the subsidence process are controlled by several factors, most of which are interdependent. These include mining method, depth of 

extraction, size and configuration of openings, 

rate of advance or extraction, seam thickness, topography, lithology, structure, hydrology, in situ 

stresses, and rock strength and deformational 

properties. We point out the significance of these 

factors, if only in a cursory fashion, because, 

taken collectively, they demonstrate the complexity of the subsidence process.

MINING METHOD

Although any underground void will potentially 

induce subsidence, the manner in which a resource 

is extracted exerts a large influence on surface deformations. Thus, room-and-pillar, longwall, and 

in situ extraction techniques will affect the surrounding rock differently; pillar layout, cavity 

shape, and the volume of material removed govern the timing and configuration of surface expressions. For example, longwall mining generally 

produces contemporaneous subsidence, whereas 

room-and-pillar mining may prolong or delay the 

deformation of walls and roofs for many years. 

Compulsory recording of mine workings was not 

introduced in the United Kingdom until 1872, and 

there are at least 30,000 unrecorded shallow workings, some over 400 years old, where even today 

gradual deterioration of coal pillars leads to pillar 

collapse and uneven surface subsidence 

(Littlejohn, 1979).

MULTIPLE SEAM MINING OF COALBEDS

The sequence in which individual coalbeds of a 

multiple-seam deposit are mined determines the 

stress concentrations elsewhere in the mining 

area, in beds both above and below the bed being

12
mined. According to Dunrud (1976) mining beds 

from top to bottom generally is safer and more efficient than other procedures, particularly if the 

final geometry is uniform; uniform geometry prevents stresses from being concentrated in isolated 

pillars and barriers and transmitted to underlying 

coalbeds. Several exceptions exist, however, to 

this generalization. If the stratigraphic interval is 

7.5-15.0 m between two coalbeds, for example, 

concurrent, uniform extraction of both beds may 

be the safest and most efficient mining procedure. 

Mining from bottom to top might be less hazardous and more productive in those areas where 

water or methane are present because mined 

voids in upper beds may store large amounts of 

these substances, later to become hazardous if 

tapped by subsidence fractures induced by mining 

lower beds.

DEPTH OF EXTRACTION

Because rocks are not perfectly elastic and do 

not deform as homogeneous, intact bodies but as 

a jointed or layered media, the depth of extraction 

governs subsidence development, particularly in 

room-and-pillar mines. The deeper the mining 

level, the greater the length of time required for 

rock deformations to reach the surface; thus the 

earliest surface deformation occurs above the shallowest coal mines. This phenomenon is frequently 

observed in areas of level topography underlain by 

dipping coalbeds. In the Boulder-Weld County 

coal field in Colorado, for example, subsidence 

problems began in the 1860's when room-and-pillar mining of shallow coal started. Subsidence was 

relatively complete above these shallow mines. 

Today, many years after mining ceased, the effects of deeper mining continue to create new surface hazards. However, poor information as to the 

amount of pillar support that remained after mining and the dates of mining make it difficult to accurately estimate the potential for further subsidence.

RATE OF ADVANCE

The British National Coal Board has found that 

for longwall coal mining the time taken for subsidence to occur depends primarily on the time it 

takes for a coal face to be worked through the 

critical area. This time in turn is controlled by the 

depth and the angle of draw (because these factors 

determine the critical area) as well as by the rate 

of advance. Typical longwall advance rates are

0.3-1.2 m per day, and "high-speed" longwall face 

advance in coal is about 1.5 m per day (Whetton 

and King, 1961; National Coal Board, 1952, 1963). 

The maximum longwall face advance reported was 

approximately 5.5 m per day in a bedded quartzite 

in South Africa (Cook, 1967). Several studies indicate that subsidence is transmitted rapidly from 

the workings to the surface. For instance, when 

mining ceases for holiday periods subsidence stops 

almost instantaneously; subsidence continues as 

soon as mining resumes. A smooth, consistent 

rate of advance promotes consistent, predictable 

surface settlement.

THICKNESS OF SEAM OR DEPOSIT

In some mining areas the relationship of seam 

thickness and overburden depth to vertical displacement of the ground surface is well established. A compilation of more than 150 measurements made above longwall mines in the United 

Kingdom showed that a maximum depth of the 

subsidence basin of 0.9 times the seam thickness 

is reached when the span of the void (the area of 

complete extraction) exceeds 1.4 times the depth 

of the deposit (National Coal Board, 1975). The 

depth of the subsidence basin is less for thinner 

seams or for shorter void spans. For example, the 

maximum subsidence for a void span of 0.7 times 

the mining depth is 0.45 times the seam thickness. 

These relations were derived from empirical observations gathered by the NCB, and although 

they are voluminous and have been verified repeatedly, their applicability to other geologic environments and different mining methods has not 

been demonstrated.

LITHOLOGY AND STRUCTURE

The strength and deformational properties of 

rock masses are largely controlled by rock type 

and structural features such as joints, faults, and 

bedding and foliation planes. These geologic conditions are responsible for significant variations in 

subsidence development and particularly affect 

the surface extent and timing of subsidence. 

Knowledge of geologic conditions in advance of 

mining can aid in subsidence prediction and in the 

development of a monitoring plan. For example, 

the angle of draw changes with the dip angle, and 

this effect can be readily estimated (National Coal 

Board, 1975, p. 16-18). In addition, monument 

line layout can be shifted down dip according to 

the dip magnitude.

13
A gradual lowering of the surface is associated 

with weak overburden rocks whereas violent, 

often delayed, collapse is more typical of strong 

overburden rocks. The presence of through-going 

faults or dikes may limit the lateral growth of a 

surface depression, particularly in crystalline 

rocks (Lee, 1966; Crane, 1931). Such discontinuities apparently act as barriers because of lateral contrasts of rock properties.

Several examples of the role of lithology and 

structure may be cited. In the county of Lanark 

(Lanarkshire) in Scotland, room-and-pillar mining 

had ceased 118 years before sandstone beds collapsed abruptly over workings that were only 16 

m deep. At the surface, structural damage to 

apartment buildings was so severe that the tenants were evacuated and several blocks of buildings were demolished (Thorburn and Reid, 1978). 

Orchard and Alien (1965) noted that 9 percent of 

total potential subsidence occurred during a 6- 

year period after a 166-m-deep longwall face advance stopped at Peterlee in the United Kingdom. 

A thick dolomitic limestone bed apparently retarded complete subsidence.

According to Kent (1974) roof falls in the 

Pittsburgh, Pa., room-and-pillar coal mines show 

two distinct patterns. In some mine areas where 

shale directly overlies the coal, nearly all roof falls 

occur along northeast-trending mine passageways 

parallel to the butt cleat (joint) direction in the 

coal. In other mine areas where shale overlies the 

coal but where thick sandstone lenses overlie 

nearby coal, severe and frequent roof falls may 

occur in the shale roof with no consistent orientation relative to the joints or passageways. Regional jointing appears to control the first type of roof 

fall, and sandstone channel deposits control the 

second type. The joint spacing is governed by the 

rock type: joints in the coal are well developed 

and closely spaced; joints in shale are less well developed and have a spacing of less than 0.3 m; and 

joints in sandstone are typically more than 0.3 m 

apart and are well developed. Through careful 

mine layout it may be possible to control roof falls 

and to more accurately predict the nature and timing of subsidence.

At a longwall mine in New Mexico, surface subsidence fractures indicated that the overburden 

was breaking primarily along the major east-west 

joint system and to a lesser degree along the secondary north-south joint set (Gentry and Abel, 

1978). These tension fractures started to close

when the mining face was only 20 m past the fracture.

In fractured crystalline rocks, the final angle of 

draw is typically controlled by faults and joints 

and is usually located where the flattest geologic 

wealgiess intersects the mining level and the 

ground surface.

IN SITU STRESSES 

AND OTHER GEOTECHNICAL FACTORS

High horizontal stresses, which are common in 

many shallow crustal rocks, act to inhibit the development of a surface depression by maintaining 

a strong ground arch in the immediate mine roof. 

Arch height and stability, however, are very sensitive to the ratio of vertical to horizontal in situ 

stresses. A highly stressed arch may fail violently 

as a result of progressive thinning as happened at 

the Urad mine in Colorado (Kendrick, 1973). At 

that mine, a molybdenum ore body in country rock 

of rhyolite and coarse-grained granite was mined 

by block caving. Horizontal stresses near the mine 

average 10.2 megapascals (MPa) (1,478 lb/in2) 

which is 4.6 times greater than the horizontal 

stress induced by gravity loading (Hooker and 

others, 1972). These "anomalous" stresses may 

have caused the extreme difficulty in breaking 

down the arch and initiating caving even though 

very high powder factors were used. An unsupported stable arch 100 by 150 m existed approximately 100 m below the surface. The caving occurred spontaneously and violently; an airblast 

from rock bursting penetrated 60-75 m of broken 

muck and was still strong enough to knock people 

down. The entire back came down breaking 

through to the surface and forming a "glory hole" 

150 m in diameter and 30-100 m deep. Similar violent deformations have been reported in other 

brittle, highly stressed rocks including sandstone, 

quartzite, and coal.

In many room-and-pillar mines little or no subsidence is anticipated, and surface use is planned accordingly. Most pillars deteriorate and deform 

with time, however, and depending upon bulking, 

deformation may extend to the surface. Lack of 

knowledge of the physical properties and rock 

stresses can lead to poor mining practices which in 

turn create stress problems in other parts of a 

mine, bringing about further uneven extraction 

procedures. In some mining regions of the United 

States, particularly those in the West (Dunrud, 

1976), concentrations of earth stresses cause rock

14
bursting and coal bumps which induce roof falls 

and pillar failures. These rock failures have forced 

changes in mining plans or abandonment of mine 

areas before mining is completed in a uniform 

manner, causing greater subsidence damage because of uneven mine geometry. Knowledge of pillar rock strength and creep properties is most 

critical in areas of high potential impact. In Allegheny Plateau coal mines, for example, high in 

situ horizontal stresses and the orientation of the 

maximum horizontal stress are related to roof instability and floor heave, making mine layout a 

major concern (Aggson, 1978).

TOPOGRAPHY

Several investigators have noted the complicating effects of topographic variations on subsidence 

development. In contrast to a level ground surface 

where the stresses produced by overburden on 

subsurface rocks are uniform, regions with irregular topographic relief will have irregular stress 

distributions that vary with the height of the column of rock above a particular underground point. 

Measurements made above three panels at the 

York Canyon mine in New Mexico, a longwall operation, show that maximum subsidence occurs 

under ridgetops and minimum subsidence under 

draws or topographic lows (Gentry and Abel, 

1978). Subsidence decreased from 25 to 30 percent 

of the maximum under a draw; subsidence was 

only 1.6 m or 50 percent of the seam thickness 

mined. Peak subsidence of 2.0 m occurred below a 

ridgetop. At the same mine, greater horizontal 

ground movement resulted when the direction of 

mining was in the downslope direction than when 

mining was in the upslope direction. This behavior 

disagrees with the calculations of Kapp (1973), 

who showed by geometrical considerations that 

horizontal strain would be greater when the 

ground surface rose in the direction of mining and 

less when the ground surface fell in the direction 

of mining. More field measurements should help 

resolve this discrepancy.

The natural stability of steep slopes may be affected by subsidence-induced deformations, triggering landslides. The definition of stress distributions related to topography is necessary for accurate slope-stability assessment prior to mining.

TIME

The time factor in mining-induced subsidence 

has been investigated in the past, mainly as applied to coal mining. It has long been known that 

the deeper the seam, the longer the duration of 

surface movement, although the reasons are not 

well understood.

In longwall mining, the subsidence of a point at 

the surface theoretically begins when a longwall 

face enters the "critical area" (fig. 1) and ceases 

when the face leaves the critical area. The surface 

point actually continues to subside (residual subsidence) for a variable period, perhaps months, although over 90 percent of the total subsidence occurs while the face is within the critical area. Most 

investigators point to rate of advance and depth ofmining as the factors governing the rate and timing of surface subsidence. As we have stated earlier with respect to the areal limits of subsidence, 

geological/geomechanical properties influence 

strain rates and modes of deformation. Orchard 

and Alien (1974) contend that when the face advances out of the "critical area," further ground 

movements occur because of complex time-dependent stress redistribution processes in the overlying rocks. The influence of depth of mining and 

face position on time-dependent subsidence becomes more significant in room-and-pillar mining 

primarily because of the difficulty of predicting 

deterioration of pillars.

SUBSIDENCE ANALYSIS

The objectives of subsidence analysis are to predict the occurrence, timing, and magnitude of vertical and horizontal components of surface deformation induced by underground resource extraction. The analysis should consider the likelihood of 

and the consequence of the impacts discussed in 

the previous section. Depending on present and 

future land use plans, how much subsidence is tolerable and, in light of resource conservation, how 

much of the resource should be extracted? Although it is desirable to recover the total deposit, 

this can rarely be done; recent United States underground coal mining practice extracted only approximately 57 percent of the coal (Lowrie, 1963).

The person concerned with assessing subsidence 

potential should determine whether subsidence 

has occurred previously in the area and its severity. Is future subsidence from old mining a possibility? We have mentioned some of the many factors that control subsidence, and each of them 

should be evaluated in a comprehensive analysis in 

order to determine the likelihood of significant 

harmful environmental impact.

15
DATA REQUIREMENTS

The following discussion is intended as an overview of the broad data needs for subsidence analysis rather than as an attempt to specify a complete 

suite of geotechnical properties. Considerable 

latitude is necessary to allow for site-specific 

needs. Knowledge of premming conditions is 

needed for prediction of disturbances that may be 

caused by mining.

GEOLOGIC DATA

Geologic maps, sections, and core logs are necessary to define topography, depth of mining, rock 

types and thicknesses, jointing, faults, and variations in the attitudes of beds, including folds. A 

drilling program should include at least one hole 

drilled to below the coal seam in the vicinity of the 

surface monuments and the panel. This drilling 

should be done for the first panel of a new mine 

and for one panel in an operating mine. The hole 

should be logged from the surface. Rotary drilling 

is satisfactory if cuttings are collected and identified for every 1.5 m of the drill run and if the 

hole is geophysically logged to produce an electrical resistivity (lithologic) log of the rock overlying 

the panel. The location of any methane gas in coal 

beds should be defined as well as the rank of the 

coal and its composition, including sulfur content. 

Knowledge of premming seismicity of an area is 

needed to determine the likelihood of complications such as severe bumps and roof falls.

HYDROLOGIC DATA

Studies should be done to define premining surface-water and ground-water volumes, flow rates, 

and quality. Aquifers and aquicludes should be 

identified. It is very important to identify all surface springs, streams, or bodies of water. Well 

data for the region should be systematically 

analyzed throughout the mining period. The sorptive properties of the rocks may be closely related 

to subsidence-induced ground-water changes that 

were discussed previously.

GEOTECHNICAL DATA

Two categories of rock-behavior data are 

needed for subsidence analysis. One category 

deals with the behavior of the underground mine 

and the surrounding rock and water environment. 

The other category deals with the behavior of the 

ground surface above the mine.

The first category includes rock strengths, 

cohesion, angle of internal friction, and elastic 

properties including, for example, Young's modulus and Poisson's ratio. Creep behavior data for 

the overburden materials are needed to forecast 

delayed subsidence. Knowledge of in situ stresses, 

especially the magnitude and direction of the horizontal stresses, is needed for planning a safe and 

efficient mining operation and for applying realistic limits to mine deformation and subsidence calculations. The second category includes horizontal 

and vertical strains and displacements and their 

magnitudes, locations, and duration.

MINING PLANS

A proposed mining plan is necessary for early 

input to the subsidence analysis. The plan should 

be based not only on geologic, hydrologic, and 

geotechnical data but also on mining methods, extraction location and sequence, location of panels, 

barrier pillars, and, in the case of multiple-seam 

deposits, the seams to be mined and the proposed 

sequence. The mining plan should anticipate the 

magnitude, location, and timing of surface disturbance. If the plan is revised, initial subsidence 

predictions may be altered.

ANALYTICAL TECHNIQUES 

FOR SUBSIDENCE PREDICTION

BACKGROUND AND STATE-OF-THE-ART

The accuracy of subsidence prediction varies 

from country to country, and is controlled mainly 

by knowledge of geologic and topographic conditions and the length of mining experience. In the 

United Kingdom, for example, data has been collected over a long period of time from more than 

150 coal mines in similar geologic environments. 

The results of these observations have been presented as empirical formulas and procedures for 

prediction of the nature, area! extent, and severity of subsidence and related events. Subsidenceinduced surface deformations can be predicted and 

described but only under conditions similar to 

those of the original observation. Particularly in 

Europe, new methods of extraction such as harmonic mining and stepped-face layout have been 

developed to minimize surface deformations and 

reduce damage to structures. It has been possible 

through integrated systems of prediction, mining 

techniques, and monitoring procedures to control 

the development of subsidence basins so that shal16
low mining may proceed under buildings and even 

towns.

In the United States, methods of subsidence 

control and estimation are less well developed. 

Subsidence theory developed in Europe has not 

been used extensively in the United States because room-and-pillar mining, rather than the 

longwall panel system, is the more common mining method in the United States. Although 

longwall mining is increasing in the United States, 

empirical methods such as that of the NCB have 

not found widespread application in U. S. mining 

because of dissimilar and varied geologic conditions.

We will briefly discuss the current methods of 

subsidence prediction and some of their limitations. Subsidence prediction techniques fall into 

two broad groups: empirical methods and 

mathematical models. Researchers have cited field 

and laboratory studies to support both approaches,

BRITISH NATIONAL COAL BOARD METHOD

This empirical subsidence calculation method 

was developed in the United Kingdom for the prediction of the vertical component of surface displacement or subsidence (S) and the horizontal 

component of surface strain (e) associated with 

trough-like subsidence caused by the longwall 

method of coal extraction (National Coal Board, 

1975). This relatively simple method is based on 

the subsidence data obtained from mining seams 

that dipped less than 25°, were 0.6-5.5 m thick 

(ra), and ranged in depth (h) from 30 to 792 m. 

The face or panel width (w) ranged from 30 to 457 

m and the panel width to depth ratio (wlh) ranged 

from 0.05 to 4.0. These observations were made 

where the panels contained no zones of special 

support, and the panel width was averaged if the 

sides were nonparallel. The NCB system provides 

for correction of horizontal strain estimates where 

the ground surface is sloping and where the coal 

seam is dipping.

Subsidence analysis using the NCB method proceeds as follows. The ratio of S to m is derived 

from the planned width and depth of the workings 

(fig. &A). Subsidence at various points on the predicted subsidence profile is found as a proportion 

of S based on the ratio of w to h (fig. SB). These 

points are then related to the position of the rib 

side for a given value of wlh and the predicted 

subsidence profile for the subcritical width of extraction is plotted (fig.SD). From the subsidence profile the values of subsidence for points at recommended intervals of h/20 are tabulated. Values of 

ground slope, differential ground slope (<j>), and 

the ratio of S to the distance between stations are 

calculated. Figure 8C shows the empirical relationship between panel length, ground slope, 

and horizontal strain in a curve where the strain 

(e) is read directly. The subsidence profiles and associated slopes and strains for critical and supercritical widths of extraction are plotted (figs. 8E 

and SF). NCB experience shows a range in the 

angle of draw from 25° to 35°. Probably because of 

geologic and topographic factors, the angle of 

draw varies much more when worldwide measurements in a variety of host materials are considered.

On the basis of the NCB curves, the greatest 

possible subsidence, Smax, is approximately 90 

percent of the seam thickness and occurs at values 

of wlh greater than about 1.2. At values of wlh 

less than 0.2, the maximum subsidence is less than 

10 percent of the seam thickness extracted. The 

method may be used to estimate the effect on subsidence of certain barrier pillar spacings and backfilling, both of which reduce subsidence. Further, 

the NCB method employs other empirical curves 

relating to surface horizontal strains to enable the 

computation of maximum tension, maximum compression, the extent of the tensile and compressive areas, and a complete strain profile. The 

NCB system is the most widely used prediction 

scheme, particularly to provide a general approximation of subsidence effects; it may be supplemented by other methods suited to local conditions. O'Rourke and Turner (1979) reported on 

their experience in applying the NCB method to 

longwall coal mining in Illinois. They found significant differences between longwall subsidence patterns observed in Illinois and those typical in the 

United Kingdom for similar conditions of panel 

width, depth, and excavated thickness of coal. 

Specifically, at the Old Ben No. 24 mine at 

Benton, 111., they found the following conditions:

1. The subsidence profile was relatively narrow, 

as demonstrated by angles of draw that are 

approximately 10° less than those in the 

United Kingdom.

2. The maximum curvature of the subsidence 

profile was four times greater than that 

predicted by the NCB system.

3. Maximum horizontal surface strains of nearly

17
81

(w/h) DEPTH OF TERMS IN WIDTH METER PER MILLIMETERS IN STRAIN, 

o

H

m 

O 

3

g 3 

o O 

METERS IN OPENING, OF WIDTH 

Ol £» CO W -» 

o o o o o 

o o o o o o 

IN SLOPE, (S). SUBSIDENCE 

PERCENT METERS IN 

(S), SUBSIDENCE 

METERS IN 

IN SLOPE, 

PERCENT

(S), SUBSIDENCE 

METERS IN 

IN SLOPE, 

PERCENT

DISPLACEMENT, 

METERS IN 

DISPLACEMENT, 

METERS IN 

DISPLACEMENT, 

METERS IN 

METER PER MILLIMETERS IN STRAIN, METER PER MILLIMETERS IN STRAIN, METER PER MILLIMETERS IN STRAIN, 
2 percent, which are four times larger than 

those in the United Kingdom, were measured.

Although a few other measurements indicate 

similar gross trends (Gentry and Abel, 1978), insufficient observations have been made at other 

U.S. longwall mines to confirm or deny a general 

trend.

The NCB method is not relevant to mining in 

crystalline rocks or to in situ extraction techniques.

OTHER EMPIRICAL METHODS

Another empirical prediction method is based 

on the so-called "stochastic" or random media 

theory discussed by Voight and Pariseau (1970). 

The method is seriously restricted because it does 

not incorporate knowledge of material properties 

prior to mining but rather transfers field measurements from known areas to areas of new mining 

via an empirical procedure. This approach may result in the uncertainty of results, and an exceedingly large number of studies would be required to 

demonstrate the general validity of the method. 

The same difficulty applies to the NCB system. 

Statistical methods that treat a broad collection of 

data concerning geologic controls on subsidence 

are more broadly applicable. We have found, for 

example, that as the percentage of shale in the 

rock mass decreases and the amount of sandstone 

increases, the angle of draw and the area of potential subsidence decreases (Abel and Lee, 1980).

MATHEMATICAL TECHNIQUES

The mathematical approaches to subsidence prediction attempt to define the conditions that lead 

to subsidence and, hence, to develop general predictive models. In these approaches the rock mass 

surrounding the mined opening is assumed to be 

an ideal material that deforms elastically. Wide 

applicability and improved understanding are the 

main advantages claimed for these approaches 

(Voight and Pariseau, 1970). Included in this 

group are elastic, viscoelastic, and plastic idealizations. Finite-element analysis is widely used to

manipulate elastic or viscoelastic continuum excavation models. As more realistic model properties 

and boundary conditions are defined, predictions 

should become more accurate. Voight and 

Pariseau (1970) were not able to reconcile field observations of surface displacements over British 

coal mines to subsidence profiles calculated on tne 

basis of isotropic theory. They did find, however, 

that transversely isotropic theory was in good 

agreement with field data.

In general, subsidence-prediction methods are 

most seriously deficient where subsidence deformations are controlled by structural discontinuities such as faults, joints, and folds, or by 

rock type.

SELECTION OF AN ANALYTICAL TECHNIQUE

The NCB method is based on the largest systematic analysis of subsidence data available. It is 

accurate enough to predict trough-like subsidence 

from longwall mining where the overburden rock 

is largely shale, siltstone, or marlstone, where it 

does not contain thick, strong beds of sandstone or 

limestone, and where the topography is subdued. 

Thus, the NCB method is applicable to many environmental impact analyses of underground mining.

Where coal is extracted by room-and-pillar 

methods, subsidence prediction will be very difficult, particularly where there is irregular room 

development, nonuniform barrier pillars, and poor 

definition of panels. Under such circumstances, 

frequently associated with old mining areas, it is 

usually impossible to predict the time, magnitude, 

or occurrence of subsidence.

In modern room-and-pillar coal mining, attempts are usually made to control subsidence by 

leaving a larger percentage of the coal in place, 

often 40 percent or more. In Illinois (Hunt, 1979), 

the magnitude, shape, and position of the subsidence profile is delineated on the basis of case-history comparisons in which extraction ratio, depth, 

panel width, and mining thickness are the principal factors. Where bedrock overburden is less 

than 50 m thick, smaller sinkhole features preFIGURE 8. British National Coal Board method of estimating subsidence. A, Relation between maximum subsidence, 

width, and depth. B, Lines of equal subsidence for various width/depth ratios. C, Strain prediction graph. D, Subsidence 

profiles and associated slopes and horizontal strains for subcritical width of extraction. E, Subsidence profiles and associated slopes and strains for critical width of extraction. F, Subsidence profiles and associated slopes and strains for 

supercritical width of extraction. (Modified from National Coal Board, 1975.)

19
dominate; if it is more than 50 m thick, subsidence 

is trough-like. Sinkhole development is also related to the hydraulic connection between the surface and the mined-out area. Caving of the roofs 

of shallow mines allows seepage of surface waters 

into the mine, decreasing roof and wall stability 

and promoting caving to the surface. The graphs 

in figure 9 show the relationship between percent 

subsidence and panel width for several extraction 

ratios in Illinois. These examples would be typical 

trough-like subsidence. Hunt (1979) has shown 

that where extraction is greater than 80 percent, 

the NCB method accurately predicts subsidence. 

Figure 10 indicates that surface tilting is directly 

proportional to subsidence and increases with the

amount of extraction. The findings of Hunt contrast with those of O'Rourke and Turner. Again, 

more data from longwall coal mines in the United 

States is needed in order to assess applicability of 

the NCB method.

Pennsylvania's Bituminous Mine Subsidence and 

Land Conservation Act of 1966 provides for protection of certain structures including public buildings and residences. Protection is defined in terms 

of support provided by coal left in the ground. 

From April 1966 to January 1969 only 1 percent of 

the 5,500 protected structures were damaged by 

subsidence, and in 1977 only six cases of damage 

to the protected structures were reported (Bise, 

1980). The formula by which these structures are

20 

u. 40 

O

z111

O

cc 111

CL 60

Development and low extraction

80

A

C)

A Medium extraction

O High extraction

 Retreat mining

A Longwall

O

C)

0.5 1.0' 1.5 

PANEL WIDTH/MINING DEPTH, IN METERS

2.0

FIGURE 9. General relation between percent subsidence and panel width to mining depth ratio. Curves indicate approximate 

range of control of mining method on subsidence. (From Hunt, 1979.)

20
10

o so

LJJ 

a

LJJ

O

<r

40

LJJ 

O

2

LJJ

a 

co Kn QQ 50

D 

CO

60

A Longwall mining

H Retreat mining

O Room-and-pillar mining

O

O

o o

10 20

SURFACE TILT, x 10'

30

FIGURE 10. Relation between subsidence and surface tilt (from Hunt, 1979).

protected was derived from many subsidence incidents over the Pittsburgh seam. We quote Bise 

(1980, p. 1):

Where a structure is located on terrain that is level or slopes 

less than 5%, the lateral distance (LD) of the support area 

from each side of the structure is equal to the tangent of 15 degrees (0.27), multiplied by the depth of cover (D), plus a safety 

factor of 4.6 m, or LD = (0.27) x(D)+4.6m. To find the total 

length or width of the support area, double the result found for 

LD and add the length or width of the structure.

This guideline deals only with defining the support area. Within this area, 50 percent of the coal 

must be left in place in uniformly distributed pillars which can be no smaller in plan than 6.1 by 

9.1 m. In addition, no mining may be done where 

the overburden thickness is less than 30.5 m 

under a protected structure. Barrier pillars cannot 

be extracted where the pillar width is less than 

the cover. As Bise (1980) points out, however, this 

method fails to consider the effect of mining 

methods on subsidence and does not allow for recovery above 50 percent.

Studies done in Europe have resulted in general 

schemes for controlling subsidence damage in partial extraction mining. Orchard (1964) found that if

21
both horizontal and pillar dimensions are equal to 

at least 10 percent of the mining depth, and if the 

pillars are uniformly spaced, subsidence will.be 

negligible while allowing 50-percent recovery of 

the coal. Wardell (1969) presented data to show 

that there is a consistent relationship between the 

percent of subsidence and depth, extraction ratio, 

seam thickness, and pillar width. The findings of 

these researchers are in general agreement.

At the York Canyon mine near Raton, N. Mex., 

Gentry and Abel (1978) compared vertical and 

horizontal strains resulting from longwall mining 

with values calculated from the NCB method. 

Much of the overburden there is weak shale, 

grossly similar to the British coal fields. Maximum 

compressive and tensile horizontal strains determined at the mine were more than twice the predicted values. The NCB method predicted 25 percent less vertical tensile strain than was actually 

measured. The York Canyon mine subsidence profile was similar in shape, however, to the NCB 

model. Presence of a strong 12-m-thick sandstone, 

strong jointing, and mountainous terrain may account for some of the observed differences.

SUBSIDENCE MODELS 

FOR IN SITU ENERGY EXTRACTION

The subsurface voids remaining after in situ 

processes such as coal gasification and oil shale retorting will not closely resemble the voids created 

by coal mining. The change in load-supporting 

ability of the surrounding rock mass in such "partial extraction" processes may be difficult to determine, and until field measurements are available, the prediction of subsidence effects for in 

situ extraction will be based mainly on mathematical analyses such as computer solutions of finite 

element codes. Examples of current practice in 

this area are given by Langland and Fletcher 

(1976) and Advani and Lin (1977).

The mathematical solutions have several limitations. Among them are:

1. The solutions are valid only for geometrically 

simple openings.

2. The material is assumed to be isotropic, 

homogeneous, and elastic.

3. The effects of water and geologic strucure are

not considered.

Mead and others (1978) discussed prediction and 

measurement of subsidence produced by in situ 

coal gasification. It is important that early monitoring be conducted to measure ground deformations produced by these types of mining operations so that their subsidence characteristics can 

be verified.

MONITORING METHODS

Geotechnical measurements are made in order 

to establish the validity of model predictions and 

to safeguard surface structures. The monitoring 

methods and types of instruments used must be 

carefully selected so that their data output is compatible with the data needed for subsidence calculations.

SURFACE INSTRUMENTATION

Our intent is to briefly describe current monitoring practice, rather than to present a specific 

set of guidelines. A premining survey should be 

made to identify geologic conditions or surface facilities that would warrant changes in a conventional monitoring scheme.

The most widely used and important type of 

subsidence-monitoring measurement is surface 

leveling. Monuments are installed before mining 

begins, and readings are taken until stability is 

reached after mining ceases.

A typical subsidence monitoring layout for a 

762-m by 168-m longwall panel below mountainous 

terrain is shown in figure 11. In this example the 

seam thickness is approximately 3 m and the average overburden thickness is 107 m. One row of 

monuments was placed along the centerline of the 

planned panel and another row perpendicular to 

the centerline. Two diagonal lines were established radiating from the centerline at 45°. These 

lines were added to define subsidence effects 

above the corners of the loitgwall panel.

For monitoring most coal mine panels in flat 

terrain, three lines of surface monuments are adequate, two perpendicular to the long axis of the 

mining panel and one directly above the centerline 

of the panel. The cross-panel lines should extend 

0.9 times the depth of the seam outside the panel 

on both sides and completely across the panel. The 

centerline monuments should also extend 0.9 

times the depth of the seam outside the starting 

position of the panel and the same distance past 

the planned end of the panel. If the panel is terminated because of poor roof conditions or faulting, 

for example, the centerline monuments can be 

shortened accordingly. These recommendations

22
EXPLANATION

CONTOUR LINE 50-FOOT 

INTERVAL

ENTRY

EXISTING REFERENCE POINT 

O TURNING POINT 

 MONUMENT LOCATION 

X MPBX

FIGURE 11. Location of surface monuments, turning points, and reference points for part of the York Canyon 

mine, Raton, New Mexico (from Gentry and Abel, 1978).

23
are in general agreement with current practice in 

the United States (Wade and Conroy, 1980; Gentry and Abel, 1978).

SPACING OF MONUMENTS

The spacing of monuments directly influences 

measurement precision. Monuments that are 

spaced too closely may result in measurement of 

local anomalous ground movements caused, for example, by the displacement of individual joint 

blocks in which the monuments are anchored. 

Widely spaced monuments may reduce costs but 

could fail to adequately define the subsidence profile, particularly in mountainous terrain. The NCB 

(1975) suggests a spacing of 0.05 h (where h is 

equal to the depth of the overburden) or onetwentieth of the depth of mining. Panek (1970) recommended a monument spacing of 0.05-0.1 h.

The presence of critical surface structures may 

also dictate the location and spacing of monitoring 

positions. For most situations the monument spacing should probably not exceed 0.1-0.2 h. This relatively wide spacing allows determination of vertical ground movements and permits a rough check 

of the horizontal strain measurements obtained by 

extrapolation from the relatively smooth vertical 

subsidence profile. Spacing monuments more 

closely in order to better define horizontal deformations is probably not justified. According to Orchard and Alien (1965, p. 622), for example, "It is 

a fact that although levels taken before and after 

subsidence will usually produce a smooth subsidence profile, the strain diagram obtained from 

horizontal measuring between survey stations is 

often erratic." They attribute this situation to "irregularities" in the subsoil. At the York Canyon 

mine (Gentry and Abel, 1978), horizontal movement measurements were less reliable than vertical measurements, apparently because of shifting 

and tilting of the joint blocks on which the concrete monuments were anchored; movement of the 

joint blocks affected horizontal measurements 

more than vertical measurements. Additional details of generally accepted monument layout and 

construction are given by Wade and Conroy (1980) 

and Gentry and Abel (1978).

Monuments should be surveyed at least twice 

before starting panel extraction, and any discrepancies should be resolved by an additional 

survey. The frequency of readings is determined 

by the rate of movement of the face, seam thickness extracted, and monument spacing. The centerline monuments should be surveyed when mining of the panel has advanced 1.9 times the seam 

depth. Each row of crossline monuments should 

be surveyed when mining has progressed 1.0 

times the depth past the crossline. All monuments 

should be resurveyed after panel completion and 

yearly until no further changes in monument positions are detected. Survey procedures and the 

need for long-term monitoring are addressed by 

Collins (1978). Gentry and Abel (1978) give detailed specifications for monument construction 

and monitoring equipment.

SUBSIDENCE CONTROL AND REDUCTION

An obvious conflict exists between minimizing 

subsidence and maximizing resource recovery. In 

some areas more surface settlement can be tolerated than in others. In urban areas, for example, 

less than 50 percent of the resource may be recovered in order to prevent subsidence. In many 

European coalfields where overburden thicknesses 

range from 60 to 900 m, deformation arches are 

stable within the overlying rocks, and subsidence 

does not reach the surface if the widths of minedout areas are held from one-fourth to one-half of 

the overburden thickness (Zwartendyk, 1971). 

Much coal must be left in the ground in order to 

obtain this high degree of surface stability.

In the United Kingdom, a "panel and pillar" 

method has been used with success when mining 

beneath towns, factories, railroads, and utility 

lines. The panels were mined without backfilling, 

and little subsidence damage occurred. Subsidence 

ratios were less than 20 percent of the coal thickness above mined panels whose widths were about 

one-third the average overburden depth.

In sedimentary iron ore deposits of the Lorraine 

area of France, support pillars were left in a 

checkerboard pattern; this method was abandoned, however, because of subsidence problems 

associated with severe pillar bursts. The 

geometry of the mining plan was changed to 

create sturdy barrier pillars separating panels 

whose widths were 0.42 times the overburden 

depth. These pillars sustained the compression 

arch within the overburden, minimizing subsidence and allowing safe recovery of 60 percent of 

the total reserves.

Another mining technique developed in The 

Netherlands coal mines is known as harmonic mining. This technique involves mining in such a way 

that the final vertical and horizontal surface

24
strains produced by mining in one area are cancelled by strains produced by mining in another area. 

For example, mine extraction panels might be 

offset and mined concurrently so as to produce 

negligible final horizontal strain in the overburden 

or at the surface perpendicular to the mining 

panels. Several drawbacks exist, however, to this 

approach. Damaging transient strains and compression arches can be produced at the surface. 

The method might also cause high stress concentrations in the rock between coal beds at the mine 

boundaries which could cause rock bursts and serious roof falls. Further, mining schedules must be 

very precise under this system to maintain the required geometry. Other factors that would lessen 

the effectiveness of this method are uneven topography and steeply dipping coal beds.

Backfilling of mined-out areas in room-and-pillar 

mines has been effective in preventing subsidence. 

This method is used primarily in mines below 

urban areas where surface stability is critical. 

Backfilling materials may be placed manually, 

hydraulically, or pneumatically and may consist of 

sand or larger grained mine waste. Hydraulic 

backfilling is generally most efficient, and 

maximum subsidence is frequently less than 10 

percent of the seam thickness extracted. The example given previously of backfilling with coal 

waste at Farmington, W. Va., illustrates the effectiveness of this method.

DESIGN PRECAUTIONS

Buildings and other structures can be designed 

to resist or tolerate subsidence deformations. 

Very flexible structures remain intact (and conformable) despite subsidence-caused distortion 

and, alternately, very rigid foundations have been 

developed that can be releveled by the use of 

jacks as subsidence progresses. The design of support mechanisms for facilities built over old mine 

workings involves a careful appraisal of subsurface conditions. Support by fill, caissons, or piers 

using boreholes may be feasible.

CONCLUSIONS

Although the damaging surface effects of underground mining have been widely documented, little research has been carried out to determine accurately the effect of subsidence on surface structures under specific geologic conditions in the 

United States. Further, the potentially broad disruptive effects of mining on water supplies have 

been studied at only a few domestic locations.

Some of the factors that influence the development of ground movement cannot be quantified 

precisely. The differences in rock-mass behavior 

caused by site conditions alone would indicate that 

subsidence prediction and engineering cannot be 

treated in purely mathematical terms. Although 

the NCB has developed quantitative, practical assessments of mining effects in the United Kingdom, there is no generally applicable subsidence 

model for the United States, nor are there 

adequately tested, empirical models for any of the 

major U.S. coalfields: the Appalachian, Interior, 

or Rocky Mountain. The influence of local geologic 

environments and mining methods on subsidence 

in the United States will require the collection of 

large masses of data in order to construct several 

accurate prediction models. Virtually no data base 

exists from which to forecast the surface effects of 

the various proposed in situ extraction techniques.

The behavior of rock units above longwall operations has been shown to be more accurately predictable than for room-and-pillar mining. An accumulation of quantitative data in the United 

States similar to that of the NCB for longwall 

mining would be valuable in establishing sound 

domestic empirical or mathematical subsidence 

models. The tools, techniques, and knowledge 

exist to conduct a coordinated nationwide subsidence research program. If the development of 

subsidence technology can lessen costly environmental or structural damage, the cost of this development will be repaid.

GLOSSARY

Angle of draw. The angle formed by the vertical and a line 

drawn from the edge of the underground workings (ribside) to the point of zero subsidence on the ground surface. 

Also called limit angle.

Arching effect. The mining-induced process by which the roof 

bows upward spanning the mined-out area and tending to 

prevent roof falls. The span width is controlled by many 

factors including depth, seam thickness, mining method, 

rock strength, structure, and in situ stresses.

Back. Roof of a mine or tunnel.

Backfilling. Placement of material in underground workings 

in order to retard collapse of roof and pillars and to reduce 

subsidence.

Barrier pillar. A solid block or rib of coal left unmined be25
tween two mine workings for protection against water inflow or squeezing conditions.

Bay. See panel.

Block caving. A sloping method of mining in which a thick 

block of ore is partly isolated from surrounding blocks by 

a series of drifts. The block is undercut by removing a 

slice from under the block causing it to cave under its own 

weight. The broken ore is removed from below, and as the 

caved mass moves downward, it is further broken by pressure and attrition.

Breakthrough. A passage cut through a pillar to facilitate ventilation from one room to another.

Bulking. The increase in volume, by virtue of increased void 

volume, of mined rock.

Butt cleat. The minor cleat system or jointing in a coal seam. 

Also called end cleat.

Chimney. Initial surface collapse above a mined-out area in 

crystalline rocks.

Cleat. A joint system in a seam along which the coal fractures. 

There are usually two cleat systems developed perpendicular to each other.

Critical subsidence profile. Subsidence profile drawn at the 

critical width.

Critical width. Width of extraction of coal seam at which the 

subsidence at the bottom of the trough has the maximum 

value.

Delayed subsidence. A variable amount of residual subsidence 

that occurs long after mining, usually controlled by 

lithologic properties. For example, a strong rock layer between the mine and the surface may retard the collapse 

process months or years.

Dip angle. The inclination from the horizontal of a seam or 

bed, measured perpendicular to the strike of the structure.

Extraction ratio. Ratio of mined-out area to the total planned 

mine area.

Face. A surface on which mining operations are in progress.

Face cleat. The major cleat system or jointing in a coal seam.

In situ. In the natural or original position; in place.

Leaching. The extraction of soluble metals or salts from ore 

by means of percolating water or other solutions.

Longwall. A method of mining coal or other resources in 

which the seam is removed in one operation by means of 

a long working face or wall. The workings (face) are advanced in a continuous line which may be several hundred 

meters long..

Mining Horizon. The level at which a deposit is mined.

Monument. A stake, rod, or concrete structure that is used to 

mark ground location points for mining surveys.

Overburden. The rock and (or) soil above a coal seam.

Panel. Areas of extracted coal, separated by long, solid barrier pillars.

Panel length. Dimension of a panel measured in the direction 

of face advance. Also called face length.

Panel width. Distance across a working coal face. Also called 

face width.

Pillar. Solid coal or ore left either temporarily or permanently 

to support the roof or prevent water inflow.

Powder factor. The amount of explosive used to mine a ton of 

rock or ore.

Percent recovery. The proportion of coal or other resource 

mined from a seam or deposit.

Residual subsidence. That amount of the total subsidence that

occurs after the face leaves the critical area. 

Retreat mining. A mining method by which a pillar of solid

coal or ore is left until the final mining while pulling out

of a room-and-pillar panel. 

Rib. The side of a pillar or the wall of an entry. 

Ribside. Edge of mine workings. 

Room. An excavation driven from an entry from which coal or

ore is produced. 

Room and pillar. A method of mining coal or other resources

in which the seam is mined in rooms separated by narrow

pillars and in which 50 percent or more of the resource is

removed during initial mining. 

Solution mining. The in-place dissolution of mineral salts of

an ore with a leaching solution. 

Subcritical subsidence profile. Subsidence profile drawn at a

subcritical width. 

Subcritical width. Width of extraction of coal seam at which

the bottom of the subsidence area is trough-like and has

less than the maximum value.

Subsidence. Vertical component of ground movement. 

Subsidence area. The entire surface area affected by subsidence over a high-extraction panel.

Subsidence basin. The depression at the surface above highextraction mining panels. 

Subsidence profile. A curve depicting subsidence of the

ground surface on a section drawn parallel to the direction

of advance of an underground excavation. 

Supercritical subsidence profile. Subsidence profile drawn at

a supercritical width. 

Supercritical width. Width of extraction of coal seam at which

the bottom of the subsidence trough is approximately flat

at the maximum subsidence value. 

Transition point. The point of transition between concave and

convex curvature of a subsidence profile.

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28