soME ENGINEERING . GEOLOGIC 

FACtORS CON1'ROLLING 

COAL MINE SUBSIDENCE 

U1'AH 

IN 

AND COLORADO 

FRONTISPIECE .-Northward view across the North Fork Gunnison 

River showing the geologic and topographic setting in the 

Somerset coal mining district, Gunnison County, Colo. The 

rugged topography shown is characteristic of the Somerset 

mining district. The ridge in the left foreground overlies the 

mine workings that were selected for one of the subsidence 

studies. The Mesaverde Formation comprises the area from the 

light-tan ledges and cliffs in the foreground (Rollins Sandstone) 

to the ridge crests in the middleground. The coal-bearing portion 

of the Mesaverde is portrayed by the pinkish-red coloration in 

the left foreground, caused by the burning of the coal beds, 

which, in turn, has caused extensive subsidence. The Mancos 

Shale underlies the Mesaverde Formation. The light-tan ledges 

and cliffs above the Mesaverde (background) are the rocks of the 

Ohio Creek Formation. The subdued terrain in the background, 

with its extensive landslides and green aspen groves (right background), is developed on rocks of the Wasatch Formation, which 

overlies the Ohio Creek Formation. Some Middle Tertiary 

intrusives, which form a part of the West Elk Mountains, are on 

the right skyline. 
SOME ENGINEERING GEOLOGIC FACTORS 

CONTROLLING COAL MINE SUBSIDENCE 

IN UTAH AND COLORADO 
Some Engineering Geologic Factors 

Controlling Coal Mine Subsidence 

in U tab and Colorado 

By C. RICHARD DUNRUD 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 969 

A discussion of the processes of 

subsidence and their effects on mine 

safety, coal resource management, 

and the environment in two geologic 

settings 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON :1976 
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

First printing 1976 

Second printing 1977 

Library of Congress Cataloging in Publication Data 

Dunrud, C. Richard 

Some engineering geologic factors controlling coal mine subsidence in Utah and Colorado. 

(Geological Survey Professional Paper 969) 

Bibliography: p. 

1. Coal mines and mining-Utah. 2. Coal mines and mining-Colorado. 3. Mine subsidences-Utah. 4. Mine subsidences-Colorado. 

5. Engineering geology-Utah. 6. Engineering geology-Colorado. 

I. Title. II. Series: United States Geological Survey Professional Paper 969. 

TN805.U8D86 622'.33'4 76-7452 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 

Stock Number 024-001-02864-6 
CONTENTS 

Page 

Abstract ........................................... . 1 Current studies of subsidence processes-Continued 

Introduction ........................................ . 1 Somerset mining district, Colorado-Continued 

Coal as an energy contributor ..................... . 2 Subsidence measurements ................... . 

Definition of subsidence .......................... . 2 Stress and deformation caused by subsidence ... . 

Acknowledgments .............................. . 2 Case study of the relationship between a mine and 

Previous studies .................................... . 3 the physical environment .................. . 

Current studies of subsidence processes ................ . 8 Summary ................................... ········ 

Geneva mine area, Utah ......................... . 8 Suggestions to mine planners ......................... . 

Somerset mining district, Colorado ................ . 9 Conclusions ........................................ . 

Geometry of subsidence cracks ................ . 13 References cited .................................... . 

ILLUSTRATIONS 

FRONTISPIECE . View showing the geologic and topographic setting of the Somerset coal mining district, Colorado. 

FIGURE 1. Sections showing early concepts of surface and rock mass subsidence processes above mine workings ................ . 

2. Sections showing subsidence trough above a mined coal bed .................................................... . 

3. Section showing author's interpretation of how further mining would affect deformation ofthe surface ............... . 

4. Cross section showing general subsidence yield conditions for surface and stratified bedrock after migration of 

compression arch to surface ............................................................................ . 

5. Graph showing subsidence ratios for various ratios of mine panel width to mean overburden depth .................. . 

6. Cross section perpendicular to mine panels, showing concurrent harmonic extraction of four flat-lying coal beds beneath 

overburden of constant thickness ....................................................................... . 

7. Photographs showing Upper Cretaceous Mesaverde Group underlying the Book Cliffs near the Geneva Mine, Utah .... . 

8. Cross section of the rocks of the Upper Cretaceous Mesaverde Group in the southern parts of the Book Cliffs and Geneva 

mines ............................................................................................... . 

9. Photographs showing subsidence damage in a massive sandstone ............................................... . 

10. Infrared Skylab 2 photograph of the Somerset mining district, Colorado ......................................... . 

11. Map showing plot of lineaments derived from the Sky lab 2 infrared photograph of the Somerset mining district ..... . 

12. Photograph of upper(?) Tertiary pluton that intrudes the Mesaverde and Ohio Creek Formations ................... . 

13. Photographs of the Mesaverde Formation in the Somerset mining district ........................................ . 

14. Map of the 3d South area in the Somerset mine ............................................................... . 

15. Photographs showing primary and subsidence fractures ....................................................... . 

16. Cross sectionA-A'through the 3d South area, Somerset mine ................................................. . 

17. Cross section B-B'throu_gh the 3d South area, Somerset mine .................................................. . 

18. Composite map of part of the Somerset mine workings and histograms of the daily seismic activity during 1973 and 1974, 

in the Somerset mining district ......................................................................... . 

19. Photographs showing stress effects from multiple-bed coal mining, Somerset district .............................. . 

20. Composite aerial photograph and underground map of the Oliver No.2 mine area ................................. . 

21. Photograph showing the rugged topography above the Oliver No. 2 mine and vegetation killed by escaping methane ... . 

22. Photograph and sections showing coal mine squeeze in "B" coal bed, Somerset district ............................. . 

TABLE 

TABLE 1. Relative displacement of U.S. Geological Survey subsidence bench marks located above the 3d South area of the 

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Somerset mine, Delta and Gunnison Counties, Colo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

v 

SOME ENGINEERING GEOLOGIC FACTORS CONTROLLING 

COAL MINE SUBSIDENCE IN UTAH AND COLORADO 

By C. RICHARD DUNRUD 

ABSTRACT 

Subsidence plays a major role in coal mining activities and in the 

future use of the land surface above the mine workings. Stresses 

and deformations produced in mine workings, other coal beds, 

bedrock, and at the ground surface by the processes of subsidence 

significantly affect mine safety, extraction efficiency, and the 

surface environment. Basically, the subsidence process comprises 

two different stress-and-yield conditions in response to the 

excavation of mine workings. 

First, arcuate zones of compressive stress, called compression 

arches, tend to occur above and below the mine panels and transfer 

the overburden load in coal-extraction areas to adjacent solid-coal 

boundaries or barrier pillars. Second, caving and flexure of strata, 

within the destressed zone encompassed by the arches, into the 

mine cavities tends to increase the stresses again in the mine 

workings. Flexure of strata also produces tensile and compressive 

stresses within lithologic units and shear stresses across lithologic 

boundaries. With time, and as the mine voids are widened, the 

compression arches tend to migrate higher in the overburden 

strata and eventually may reach the surface. This migration 

continues to transfer overburden stresses back into the extraction 

area from the mine boundaries or barriers. The rate of migration of 

compression arches, and, consequently, the rate of stress transfer, 

depends on thickness and strength of overburden strata, duration 

and rate of mining, mine geometry, and mining sequence. 

The ground surface, other coal beds, ground-water aquifers, 

methane zones, and economic deposits above and below the mine 

workings can be damaged by the subsidence processes. Deposits in 

the mine overburden and the surface are subjected to stresses 

produced by compression arches and by flexure of strata. As a 

result of the subsidence processes, deposits are subjected to 

unloading followed by loading beneath mine openings and, in 

addition, are subjected to loading followed by unloading beneath 

remnant coal pillars and near solid-coal mine boundaries. 

In an area underlain by several thin beds or by one thick bed, 

mining sequentially from upper to lower beds, or, in the case of a 

thick bed, mining separate "benches" or "lifts" from top to bottom 

commonly is safest and most efficient. However, unless a uniform 

extraction plan is followed, mining hazards, such as roof falls, 

bumps, and squeezes caused by stress concentrations, may force 

premature abandonment of an area of the mine, with resultant loss 

of reserves. Knowledge of geologic, topographic, and 

socioeconomic conditions in prospective mining areas is vital to 

planning safe and efficient mining activities, particularly in areas 

underlain by thick coal beds or by more than one coal bed. 

INTRODUCTION 

Subsidence damage resulting from underground 

coal mining will become a greater problem in the 

United States as our population increases along with 

our demand for more energy. The clash between 

population growth and man's demand for more energy 

is already very acute in many heavily populated 

industrial countries, such as Japan (Kaneshige, 1971), 

but also it is felt in the more heavily populated coal 

mining areas in the United States. Although lands 

underlain by coal reserves in the Western United 

States are still relatively free of industrial and urban 

development, many areas in the East support 

industry, housing, or farming. 

Many old established urban areas overlying coal 

mine workings, such as Scranton, Pa., have 

experienced severe and costly subsidence damage 

many years after the mines were abandoned. 

According to recent information (U.S. Bureau of 

Mines, written commun., Feb. 5 and April10, 1973), 

$29 million worth of property, including 2,000 homes, 

50 commercial and office buildings, 2 hospitals, and 

several schools are threatened by subsidence in the 

Hill section of Scranton-an area in which an 

estimated 10,000 people live and work. The total cost 

of surface stabilization of this area by hydraulic mine 

backfill is estimated to exceed $8 million. 

Subsidence damage to the rocks above underground 

coal mines also can reduce minable coal reserves, 

decrease mine safety, lower production efficiency, and 

possibly disrupt the hydrologic regimen. Coal deposits 

in parts of Wyoming, Colorado, and Utah locally 

comprise vertical sequences of several minable coal 

beds interbedded with thick to thin sandstones and 

mudstones. Deformation caused by mining one coal 

bed can damage other beds nearby unless the beds are 

extracted in proper sequence. Ruptures within the 

1 
2 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

rock mass and at the surface may tap and divert 

ground water or methane pockets associated with the 

coal beds or may even divert surface-water drainage 

to underground aquifers or mine workings, which in 

turn can upset established ground- and surface-water 

supplies as well as threaten the mine workings. 

Coal mine subsidence problems such as these point 

out that mining plans for new and existing coal mining 

areas must be based on all existing technology, with 

the goal of maximizing coal extraction percentages 

and minimizing the subsidence damage to the 

overburden and surface. To accomplish this we must 

know the processes of subsidence that occur both in 

the overburden and at the surface in various geologic 

environments and under various methods of mining. 

This is one of the major goals of coal mine deformation 

studies underway in the U.S. Geological Survey and is 

the central theme of this report. 

COAL AS AN ENERGY CONTRIBUTOR 

Coal is an important part of our current energy 

supply, particularly for use in generating electric 

power. The use of coal will no doubt continue to grow 

rapidly in response to increasing demands on domestic 

energy resources. In 1970, 17,000 trillion B.t.u.'s from 

various energy sources were required to produce 

some 1,260 billion kilowatt hours of electrical energy 

in the United States (Risser, 1973). Of this total, 1 

percent was supplied by nuclear power, 16 percent by 

hydroelectric power, and 83 percent by the fossil 

fuels. Coal provided 55 percent of the power 

contributed by the fossil fuels. Although our total 

energy requirements are increasing rapidly with time, 

the demand for electrical energy nearly doubles every 

10 years, and, assuming that coal continues to 

contribute about the same percentage of our electrical 

energy, the demand for coal will increase accordingly. 

The projected requirements for coal in the 1970's alone 

may equal the total amount consumed in the previous 

seven decades. 

Many coal deposits are too high in sulfur, ash, or 

other constituents to be burned in power plants under 

existing or proposed clean-air standards. However, 

various techniques of obtaining synthetic crude oil, 

natural gas, char residue, sulfur, and fly ash from coal 

are in advanced stages of study or implementation 

under the auspices of U.S. Department of the Interior 

Office of Coal Research (W. A. Bear, oral commun., 

1970, 1972). With these processes, the sulfur and ash 

are recoverable byproducts rather than pollutants. 

Another pollution-free method of producing electrical 

energy from coal involves the direct generation of 

electricity from coal and is called the magnetohydrodynamic (MHD) process; with this process, electrical 

energy is produced by passing hot gases, derived from 

coal, through a stationary magnetic field. Should these 

new techniques eventually prove economical, coal 

would be an increasingly important source of electrical 

energy, as well as a source of other forms of energy 

and material byproducts, for years to come. It will 

thus be of continuing importance for us to study the 

effects, including subsidence, of mining needed 

supplies of coal. 

DEFINITION OF SUBSIDENCE 

Coal mine subsidence is defined in this report as all 

deformation within most of the overburden and at the 

surface that is caused by underground mining. It 

includes the local upward movement of strata that 

sometimes occurs above solid-coal mine boundaries or 

large barrier pillars, which is caused by downwarping 

of overburden into mine cavities; it also includes the 

downwarping itself, the associated horizontal tensile 

and compressive strains produced by strata flexure, 

and the compressive strain induced by the 

compression arches. 

For the purposes of this report, the term 

"subsidence" applies to deformation or movement in 

the overburden two or more mine heights above the 

immediate mine roof; the term "roof fall" applies to 

the fall of roof rocks less than two mining heights 

above the mine roof. 

ACKNOWLEDGMENTS 

Officials of the U.S. Steel Corp., Bear Coal Co., and 

Western Slope Carbon, Inc., materially aided the 

studies undertaken for purposes of this report. They 

contributed maps of mine workings, offered helpful 

suggestions and information, and they allowed access 

to their properties and the use in this report of certain 

information and pictures taken in their mines. Their 

help was given in the spirit of making coal mining 

safer, more efficient, and less damaging to the 

environment. The author thanks them all for their 

help and cooperation, particularly Paul E. Watson, 

general superintendent, western district~ U.S. Steel 

Corp.; J. Boyd McKean, district mining engineer, 

U.S. Steel Corp.; R. W. Ramey, superintendent, 

Somerset mine, U.S. Steel Corp.; W. A. Bear, 

president, Bear Coal Co.; and C. L. Heiner, Western 

Slope Carbon, Inc. 

Two part-time coworkers provided basic data for 

the preparation of this report. R. Edward McKinley, 

retired mining engineer, U.S. Steel Corp., made 

monthly subsidence measurements at Somerset, 

which were extremely helpful in determining the 
PREVIOUS STUDIES 3 

processes of subsidence. Robert L. Rasmussen, mine 

engineer at the Somerset mine, who works part time 

on the U.S. Geological Survey seismic recording 

facility at Somerset, provided mine maps and helpful 

updates on mining operations. Two colleagues, R. L. 

Parker and R. B. Taylor, provided recent high-resolution aerial photographs of the Somerset district that 

proved very useful to the mine deformation studies. 

PREVIOUS STUDIES 

Subsidence studies began in Europe before the turn 

of the century. Early concepts of subsidence included 

the so-called law of the normal and law of the vertical 

(fig. 1). According to the law of the normal (fig. lA), 

investigators believed that the overburden strata 

broke and subsided above and parallel to the 

boundaries of the mined-out areas along fractures 

perpendicular to the strata. The so-called law of the 

vertical was developed a few years later. According to 

this concept the overburden broke and subsided along 

vertical fractures above the boundaries of the 

mined-out areas (fig. lB). 

During the early 1900's the concepts of limit (or 

draw) angle and break angle developed following 

further surface and underground measurements. 

Measurements in the Ruhr region of Germany and 

elsewhere in Europe and Russia revealed that 

subsidence or other deformation affected a surface 

area larger or smaller than the area mined (fig. 2A). 

The limit angle cp, which is the acute angle defining 

the limit of subsidence above mine workings, is 

positive if the surface area affected by subsidence is 

greater than the mined-out area; it is negative if the 

deformed surface area is smaller than the mined-out 

area. The break angle {3, which defines the zone of 

maximum tensile stress caused by flexure of strata 

(fig. 2B), is steeper than the limit angle but may 

approach the limit angle when it is negative. The limit 

angle is measured from either the horizontal or from 

the vertical. The angle relative to the horizontal is 

chosen in this report because it is consistent with 

geologic measurement of bedrock attitudes. Limit 

angles commonly range from 45° to 65° in European 

coal fields, 55° to 65° in Great Britain, and 45° to 60° 

in the Ruhr region; they are about 55° in northern 

France and about 60° in the U.S.S.R.; and they range 

from 45° to 55° in the Netherlands (where angles are 

referenced from the horizontal rather than from the 

vertical) (Zwartendyk, 1971, p. 142- 143). 

A troughlike subsidence geometry was observed 

within the limits of the draw in many European coal 

mining districts. This observation led Lehmann (1919, 

cited in Zwartendyk, 1971, p. 85- 86) to propose his 

EXPLANATION 

----•- COAL BED- Mined where unshaded 

r / z :a Surface subsidence 

Fracture 

I 

I 

I 

I 

I 

I 

I 

I 

I 

FIGURE 1.-Early concepts of surface and rock mass subsidence 

processes above mine workings. A, Law of the normal; B, law of 

the vertical. 

subsidence trough theory. This concept explains 

tensile and compressive strains in addition to the 

vertical and horizontal movement and tilting observed 

above coal mining areas. According to this concept, 

the rate of change of vertical settlement increases to a 

maximum from the limits of the draw inward, becomes 

constant at the point of inflection, and then decreases 

to zero in the center of the trough, provided the 

mining area is wide enough to prevent further 

differential settlement in this area (supercritical 

mining width) (fig. 2B). The mined-out area is said to 

be of critical extraction width if it is just wide enough 

to allow maximum subsidence (S max ) in the center of 
4 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

A 

I I I' I /71 

I I IY: 

!---supercritical width---! 

B 

STRAIN AND 

DISPLACEMENT CURVES 

-=:=11 COAL BED- Mined where --- Relative horizontal 

unshaded distance and slope 

LZJ Surface subsidence 

Direction of horizontal 

displacement 

--- Relative horizontal 

strain and curvature 

FIGURE 2.-Subsidence trough above a mined coal bed; vertical 

scale of subsidence trough greatly exaggerated for clarity. D, 

overburden depth. Modified from Zwartendyk (1971, p. 132), 

Brauner (1973, p. 4), Mohr (1956, p. 141), and Wardell (1971, p. 

206). A, Subsidence trough with positive limit (or draw) angle (c.p) 

and positive break angle ({3). B, Subsidence trough showing 

critical width of mining that will cause maximum surface 

subsidence (Smax), supercritical width of mining, and the 

general form of the curves depicting horizontal displacement, 

slope, strain, and curvature. 

the trough (fig. 28). According to Wardell (1971, p. 

205), this width varies, but in European coal fields it is 

between 1.0 and 1.4 times the average overburden 

depth, depending on the lithology and structure of the 

overburden and provided the panel length is more 

than 1.4 times the average overburden depth. 

Although most troughs do not precisely fit the 

model in figure 28, the figure illustrates the reasons 

for the vertical and horizontal strains that are 

commonly observed above mined-out areas. The 

ground surface is convex upward between the limits of 

the draw and the points of inflection; thus, the tensile 

strain increases to a maximum at the point of 

maximum positive curvature and then decreases to 

zero at the points of inflection, where ground tilt and 

horizontal extension are at a maximum. Between the 

points of inflection and the point of maximum 

subsidence (S max, fig. 28), the ground surface is 

concave upward, thus causing compressive strain. 

Between these points, the compressive stresses 

increase to a maximum and then decrease to zero, 

provided the mined-out area is of supercritical width. 

According to Wardell (1971, p. 209), maximum 

change in ground slope and length varies in relation to 

the maximum amount of subsidence (S max) divided by 

the average overburden depth (D), whereas the 

maximum horizontal curvature varies in relation to 

the maximum subsidence divided by the square of the 

average overburden depth (D 2). The area subjected to 

maximum subsidence has reached a condition of zero 

strain. However, even this area of maximum 

subsidence and zero strain was initially subjected to 

strain during mining. Furthermore, if the mine 

opening is widened (fig. 2B), the surface adjacent to 

the area affected by subsidence will be subjected to a 

transient wave of first tensile, then compressive, and 

finally zero strain as the ground subsides above the 

enlarging mine opening. Ground-water- and methanebearing rocks, as well as subsurface water, gas, and 

sewer mains, and unyielding buildings and 

foundations, will be subjected to all these strains, and 

the damage will, in most cases, be cumulative. 

Mining of coal adjacent to the mined-out area, as 

shown in figure 28, leaving a barrier pillar (fig. 3), can 

cause additional surface strains. Here, two adjacent 

subsidence troughs superposed over such pillars 

would interact to produce tensile strain approaching 

twice the strain produced above a solid-coal boundary. 

Both the rate and amount of surface settlement 

were studied by various foreign investigators (for 

example, Mohr, 1956, and research cited by 

Zwartendyk, 1971) in relation to various physical 

environments. Although much of this information is 

contradictory and somewhat confusing, they found 

that, in general, the amount of surface subsidence and 

deformation, although variable, depended on the 

strength and thickness of overburden, the width of the 

mined-out area, and the thickness of the coal bed. A 

wider mining area or a thicker coal bed increased the 

rate of surface settlement but, if the extraction was 
PREVIOUS STUDIES 5 

+ 

I 

IC=======~----.C==M=in=ed=-o=ut=ar=ea~ Barrier pillar 

FIGURE 3.-Author's interpretation of how further mining of the 

coal bed shown in figure 2 would affect deformation of the surface. A barrier pillar now separates two mined-out areas; strain 

and horizontal displacement curves are shown above cross 

section. Note that the superposition of subsidence profiles 

greatly increases the tensile strain above the center of the 

barrier; also, the horizontal displacement is nearly at a maximum 

over the entire barrier. Surface damage is most accute above the 

middle of the barrier pillar; the barrier pillar must be twice as 

wide as the overburden depth (D) multiplied by the cotangent of 

the limit angle ('P) before surface subsidence is nullified. 

uniform or complete and rapid, did not necessarily 

increase the amount of fracturing. A less complete or 

less uniform mining method and slower or variable 

mining speed increased overburden breakage. A 

thicker and stronger overburden generally decreased 

the rate of surface settlement for any given mining 

width and height (Briggs, 1929, p. 181- 184, cited in 

Zwartendyk, 1971, p. 91 - 92). 

Detailed subsidence studies in the Ruhr coal fields 

of West Germany shed more light on the behavior of 

the surface and over burden rocks (Mohr, 1956). The 

amount and rate of deformation of the surface and 

rock units were measured from vertical mine access 

shafts located above either solid-coal or mine barrier 

pillars (fig. 4). On the basis of these measurements, 

the positions both of the limits of the draw and of 

maximum vertical and horizontal strain, as well as the 

rate of change of these positions with time, were 

determined. In detail, the limit lines, which define the 

limits of draw, steepened in strong rocks having large 

angles of internal friction and flattened in weak rocks 

having small angles of internal friction. Within the 

limits of the draw, the shaft linings were sheared and 

offset many inches (fig. 4A) and thrown out of plumb 

as much as 6 feet (1.8 m) (fig. 4B) by flexure of 

stratified rocks. The linings also were vertically compressed by compaction of rock units-particularly 

weak rock units-in contact with the shafts. 

Maximum lateral shear strain occurred across 

lithologic boundaries and at points of maximum 

curvature, which also are the loci of points defining 

the break line or line of maximum tension. Cumulative 

deformation in strata above remnant barrier pillars 

between two mined-out areas suggests that the break 

lines curve together toward the surface above the 

barrier pillars owing to the superposition of adjacent 

subsidence troughs (section, fig. 4). This superposition 

of the troughs greatly increases the curvature, the 

tensile strain at the surface, the flexural strain within 

lithologic units, and the shear strain across lithologic 

boundaries or bedding planes (fig. 4A), compared to 

the flexural strain and shear strain above solid coal at 

mine boundaries (fig. 4B). 

The concepts of break line and break angle are not 

spelled out by Mohr, but are implicit in his data (1956, 

p. 141-142). According to Grard (1969, p. 46, cited in 

Zwartendyk, 1971, p. 188), maximum tensile strain 

exists in bedrock units at the surface when the pillar 

width is about 0.4 times the overburden depth. As the 

barrier pillar becomes narrower, an increasing 

amount of the strata above an interior barrier pillar is 

affected by superposition of tensile strain (fig. 4). 

Shear strains were observed in shafts along the limits 

of the draws above the barrier pillars and along the 

single limit line above the solid coal boundary (figs. 

4A., B). 

Mohr (1956, p. 149 -150) measured subsidence 

rates at the surface and within the overburden 

adjacent to mine access shafts. He observed a timelag 

between the beginning of mining and the onset of 

surface subsidence. This suggested that some type of 

strong, but temporary, support mechanism existed in 

the overburden. Because all rocks are weaker in 

tension than in compression, Mohr proposed that 

arcuate zones of compression, or compression arches, 

formed above the mined-out areas, and, as mine 

openings were widened, the arches progressed higher 

and higher into the overburden until they reached the 

surface. Beneath these compression arches, subsidence troughs formed in the overburden within the 

limits of the draws. Wardell (1971, p. 204) postulated 

that all subsidence ceases within 2 or 3 years after 

completion of mining. Speed of mining might, 

therefore, be an important factor governing the 

severity of deformation at the surface and within the 

rocks of the overburden. If coal could be extracted 
6 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

Horizontal displacement 

----~ / 

\ / "'/ I 

I 

I 

I 

due to surface 

flexure 

I~ / 

I ',, _/(~urface strain 

I I I 

I I I !Maximum I 

I I 

Center 

line Lateral shear strain 

Center 

line 

Boundary pillar 

Weak 

Strong 

Weak 

Shear strain along 

intersecting limit lines Lateral shear strain 

across lithologic boundaries 

Shear strain along 

A 

S 2-S 1 =vertical compression 

of rock unit 

B intersecting limit lines 

EXPLANATION 

Transient compression arch 

Tension break line 

Limit line (generalized) 

-- Limit line in detail 

Direction of horizontal surface displacement 

Compression break line 

---- Contact of strata (original position) 

FIGURE 4.-Cross section showing general subsidence 

yield conditions for surface and stratified bedrock after 

migration of compression arch to surface. General yield 

conditions interpreted from measurements made in the 

Ruhr coal fields by Mohr (1956, p. 146 -150); horizontal 

displacement and strain curves are modified from 

Zwartendyk (1971, p. 134) and Brauner (1973, p. 4). A, 

Enlargement x2 of a segment of shaft A showing vertical 

settlement (S2 -81 ), lateral shear strain across a 

Contact of subsided strata 

Coal bed-Mined where unshaded 

State of strain caused by flexure of 

strata above a narrow barrier 

pillar 

State of strain caused by flexure of 

strata above solid-coal mine boundary 

lithologic boundary caused by flexure of strata, and 

shear strain along intersecting limit lines above a narrow 

barrier pillar. B, Enlargement x2 of part of shaft Band 

surrounding rock mass showing lateral shear strain 

across a lithologic boundary caused by flexure of strata 

and shear strain along the limit line above a solid coal 

pillar. The lateral shear strain in A is greater than the 

shear strain in B. 
PREVIOUS STUDIES 7 

from an area large enough to minimize strata 

curvature and flexure before the compression arch 

migrated to the surface, damage to the surface and to 

the rocks in the overburden might be minimized. 

Wardell (1971, p. 203- 204), in general, supported 

the concept of the pressure arch without specifically 

naming it. On the basis of numerous field 

measurements, principally within the coal fields of the 

United Kingdom, he found that ground surfaces above 

active mining areas did not subside until certain 

lateral cavity dimensions were exceeded. A delay, 

therefore, occurs between mining and surface 

subsidence, but he contended that after surface 

subsidence begins it will continue at a rate 

proportional to rate of mining. He also found that 

these critical lateral cavity dimensions are greater 

beneath thick overburden than beneath thin 

overburden, and, for a given overburden thickness, 

are greater beneath predominantly strong strata, 

such as sandstones, than beneath predominantly weak 

strata, such as shales or mudstones. 

Another way to minimize surface subsidence 

damage, if rapid and complete extraction is not 

possible, is to restrict the width of the mining panel so 

that compression arches do not migrate to the surface 

but, instead, stabilize within the overburden. In the 

coal fields in parts of the United Kingdom, France, 

Poland, and the U.S.S.R., where overburden 

thickness ranges from 200 to 3,000 feet (61- 914 m), 

investigators found that compression arches normally 

were stable within the overburden if the widths of the 

mined-out areas were limited to one-fourth to one-half 

the height of the overburden, depending on the 

strength of the overburden strata, and provided that 

abutments of the arches were supported by barrier 

pillars strong enough to support the overburden load 

(Zwartendyk, 1971, p. 187 -190). 

This so-called panel-and-pillar method has been 

used successfully in the United Kingdom beneath 

towns, public utilities, and industrial plants. The 

panels were mined by either longwall or room-andpillar methods, without backfilling and without 

causing undue subsidence damage. The validity of this 

method is supported by Wardell's conclusions (1971, p. 

206), which were based on numerous measurements in 

the United Kingdom. He found subsidence ratios 

(ratio of maximum surface subsidence (Smax) to 

thickness of the coal bed or mining height (t)) of less 

than 20 percent of the bed thickness above mining 

panels that were about one-third the average 

overburden depth (fig. 5). 

In the stratified iron deposits of the Lorraine area in 

France, "safety" pillars were left unmined in a 

checkerboard pattern for support, but this pattern 

FIGURE 5.-Subsidence ratios (maximum surface subsidence 

(Smax) relative to the thickness (t) of a coal bed) for various ratios 

of mine panel width (W) to mean overburden depth (D). 

Measurements are from Wardell (1971, p. 206) for coal fields of 

the United Kingdom; all coal was extracted by the longwall 

method in coal beds dipping less than 15° beneath overburden 

strata with reasonably consistent geologic conditions; all panel 

lengths were greater than 1.4D. The data point bounded by a 

square is from maximum subsidence measurement above the 6th 

Left room-and-pillar mining panel, Somerset mine, Colorado, 

3 - 5 months after mining was completed. 

was abandoned because of subsidence problems and 

severe pillar bursts. A method to achieve panel-andpillar geometry was used in place of this mining 

procedure with good success (Zwartendyk, 1971, p. 

190). Statistics on overburden collapse showed that a 

panel width of 0.42 times the overburden depth 

sustained a compression arch within the overburden 

when panels were separated by sturdy barriers. Many 

of the panels were nearly 2,000 feet (610 m) long and 

averaged 280 feet (85 m) wide, while the width of the 

adjacent barrier pillars averaged 200 feet (61 m). 

Assuming complete extraction within the panels, the 

recovery approached 60 percent of the total reserves. 

The so-called harmonic method of mining was 

developed and first implemented by Grond (1947, p. 

240-291, cited in Zwartendyk, 1971, p. 192) in the 

Netherlands during 1934. Basically the concept 

involves mining in such a manner that the final 

vertical and horizontal surface strains produced by 

mining in one area are essentially canceled by strains 

produced by mining in another area. In multiple-bed 

mining, for example, mine extraction panels might be 

offset (fig. 6) and mined concurrently by longwall or 

room-and-pillar methods so as to produce little or no 

final horizontal strain within the overburden or at the 

surface perpendicular to the mining panels. 

However, transient strains and the presence of 
8 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

EXPLANATION 

HORIZONTAL STRAIN CURVE PRODUCED BY MININGNumber of the beds is indicated by number of dashes on 

curves 

~::::::::=:::=:= 2 COAL BEDS- Mined where unshaded. Numbers increase 

from bottom to top 

FIGURE 6.-Cross section perpendicular to mine panels, showing concurrent harmonic extraction of four flat-lying coal beds beneath overburden of constant thickness; direction of mining is perpendicular to cross section. Note that final horizontal strains along line of 

section produced by offset panel mining in any two vertically successive coal beds cancel the effects of each other. Modified from 

Kaneshige (1971, p. 181). 

compression arches within the overburden and at the 

surface could be damaging. The possible occurrence of 

tension fractures parallel to a longwall face or, if the 

room-and-pillar method were used, to a pillar line 

would be particularly damaging to the harmonic 

mining procedure. The harmonic method also requires 

more miners and mining equipment than most mining 

companies could muster. In addition, the mining 

operations need to be on a very tight schedule, 

prohibiting variances such as those caused by miners' 

vacations, strikes, and coal-haulage interruptions. 

The procedure might cause high stress concentrations 

within the strata between coal beds at the mine 

boundaries, which could cause rock bursts and 

excessive roof falls at the boundaries. Uneven 

topography or steeply inclined coal beds would further 

complicate the design of mine workings to minimize 

subsidence damage. 

CURRENT STUDIES OF 

SUBSIDENCE PROCESSES 

GENEVA MINE AREA, UTAH 

Coal mine deformation studies in central Utah and 

in western Colorado have yielded information on how 

geology, topography, mine geometry, and method of 

mining control subsidence. East of the Book Cliffs in 

the Geneva mining area of the Sunnyside mining 

district in central Utah, coal is mined from one bed or 

zone lying beneath overburden strata that vary in 

thickness from a few feet to more than 3,000 feet (914 

m) and dip 6° - 15° eastward or northward. The 

overburden rocks comprise, in ascending order, the 

Mesaverde Group (Upper Cretaceous), North Horn 

and Flagstaff Formations undifferentiated (Upper 

Cretaceous through Eocene), and Colton Formation 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 9 

(Eocene) (fig. 7). The Mesaverde Group is composed of 

thick strong sandstones alternating with weak shales 

and mudstones. The North Horn and Flagstaff 

Formations comprise claystones, shales, and 

discontinuous sandstones with thin limestone beds at 

the top. The Colton Formation consists of thick to thin 

sandstones which intertongue with, or alternate with, 

well indurated limy mudstones. 

Bedrock units are displaced by faults that trend 

north-northwest, east, and northwest. The northnorthwest-trending faults, which form a linear fault 

zone, dip steeply to vertically, whereas the easttrending faults dip moderately to steeply and form 

horst-and-graben structures. The northwest-trending 

faults, which essentially constitute one fault with local 

imbrications, exhibit a large left-lateral strike-slip 

component of movement; the other sets of faults are 

predominantly dip-slip normal. 

Joints are common in the resistant sandstone and 

siltstone beds. Their attitudes are variable, but two 

well defined, steeply dipping joint sets that are 

parallel to the strike and dip of the strata are the most 

predominant. 

While conducting detailed geologic mapping for a 

study of geologic causes of coal mine bumps, 

subsidence cracks were mapped and were studied in 

their relation to mining. A few months after the coal 

was mined a set of nearly vertical fractures, that 

trended south-southeast, was observed above a 

barrier pillar that separated the Book Cliffs and 

Geneva mines (fig. 8). The coal had been mined too 

close to the outcrop, particularly in the southern part 

of the Book Cliffs mine (Dunrud and Barnes, 1972), 

and the outcrop barrier, which was too thin to support 

the overburden weight, yielded, creating a cantilever 

above the mined portion of the Book Cliffs mine. The 

stresses produced by the cantilever caused the overburden above the property barrier to fail in tension 

along steeply dipping to vertical joints that parallel 

the general direction of the barrier pillar. Failure 

under these circumstances followed as predicted by 

the law of the normal. 

Large tension cracks, some of which are hundreds of 

feet long and range from about 0.06 inch (1.6 mm) to as 

much as 3 feet (0.9 m) in width, formed in massive 

sandstone at the top of the Mesaverde Group about 

900 feet (274 m) above the mine area (figs. 9A, 9B). 

Maximum cumulative surface extension across this 

zone of cracks, which was much as 250 feet (76 m) 

wide, was approximately 5 feet (1.52 m). Manmade 

structures would, of course, be destroyed by tensile 

deformation of this magnitude, but, fortunately, the 

surface in this region is currently inhabited only by an 

occasional horse, cow, or deer. Some of the wider 

cracks (figs. 9A, 9B) emit air from mine workings, 

indicating their continuity to the mine workings. 

These cracks divert all surface- and ground-water flow 

in this area to lower strata or to the mine workings. 

Compression features, such as fractured bulges and 

small anticlines, formed in massive sandstone 900 feet 

(274 m) above a mined-out area within the Geneva 

mine about 11

/2 years after mining was completed 

(figs. 8, 9B, 9C). Measurements of the features 

revealed that the ground surface was shortened 

locally by as much as 3 feet (0.92 m). The anticlines 

were alined pa1allel to the property barrier pillar and 

roughly perpendicular to retreat-pillar lines in the 

Geneva mine. This amount of compressive 

deformation in strong, competent sandstone indicates 

the presence of very strong horizontal compressive 

stresses in a direction perpendicular to the barrier 

pillar dividing the mine properties. The compressive 

stresses were due either to concave curvature of 

strata resulting from vertical subsidence or to the 

presence of a temporary compression arch which had 

migrated through the overburden to the surface above 

the mined-out area. 

The chronological development of cracks in the 

area, which was followed through periodic mapping, 

indicated that a compression arch caused the 

compressive stresses. However, periodic subsidence 

measurements would have been necessary to confirm 

this. The chronological sequence of crack formation 

was as follows. First, tension cracks were noted above 

the property barrier (area 1 in fig. 8) in September 

1963; no other cracks were noticed at that time. 

Second, compression bulges and anticlines first 

mapped in August 1966 (area 2 in fig. 8) appeared, on 

the basis of freshness of broken surfaces of rock and 

soil, to be less than 1 year old. Third, very recent 

tension cracks 0.06 -4 inches (0.16 -10 em) wide were 

also mapped in August 1966 (area 3 in fig. 8); local 

fresh breaks in soil indicated that these cracks were 

only days or weeks old. This sequence of cracking 

suggests compression-arch failure because the surface 

apparently subsided to a final profile only after the 

compression features were produced. 

SOMERSET MINING DISTRICT, COLORADO 

Although many of the geologic conditions in the 

Somerset mining district, Delta and Gunnison 

Counties, Colo., are different from those in the 

Geneva mine area, the overburden also is similarly 
10 ENGINEERING FACTORS. COAL MINE SUBSIDENCE. UTAH AND COLORADO 

FIGURE 7.-The Upper Cretaceous Mesaverde Group underlying the Book Cliffs near the Geneva mine, Emery County, Utah. A, Northeastward view of typical Mesaverde rocks and rugged topography. B, Closeup of the lower part of the Mesaverde Group showing the 

Sunnyside coal bed (arrow), the dark band overlying the second massive sandstone from the bottom and overlain by a thin sandstone. 

The Mancos Shale underlies the lowermost sandstone. 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 11 

WEST-SOUTHWEST 

-

-

0 1000 FEET 

0 300 METRES 

EAST-NORTH EAST ,; 

. g 

00 

E 

0 

~ N 

0 

- ?l 

N 

0 

0 

N 

N 

0 

0 

;::; 

0 

0 

0 

N 

0 

0 

O'l 

EXPLANATION 

EJ 

[] 

Sandstone, fine- to medium-grained 

. Sandstone, fine- to very fine grained . 

D Shale and mudstone 

-= Sunnyside coal bed - Mined where unshaded; mined during 

the period 1954-61 (barrier pillars removed in 1966 and 

1967) in the Geneva mine area and d uring the late 1950's 

to the middle 1960's in the Boo k Cliffs mine 

FIGURE 8.-Cross section of the rocks of the Upper Cretaceous Mesaverde Group in the southern parts of the Book Cliffs and Geneva 

mines, Utah. Major deformational features in rocks above the mined-out areas and adjacent barrier pillars are based on a map by 

Dunrud and Barnes (1972). (1) First set of tension cracks, (2) compression features probably caused by a compression arch, and (3) a 

second set of tension cracks. 

variable and locally thick because of rugged 

topography (frontispiece, figs. 10, 11). The 

overburden ranges in thickness from 0 at the coal 

outcrop to 2,500 feet (762 m) in short lateral distances 

beneath ridges. Plutons of Oligocene age (Lipman and 

others, 1969, p. D37) that constitute part of the West 

Elk Mountains, intrude coal-bearing strata within a 

few miles of the mine workings (frontispiece, fig. 10). 

Another small pluton, perhaps younger than 

Oligocene, intrudes Mesaverde rocks and much of the 

overlying Paleocene Ohio Creek Formation only 2 

miles (3.2 km) north of current mine workings (fig. 

12). 

Coal is mined from as many as four coal beds within 

a stratigraphic interval500- 600 feet (152.4 -182.9 m) 

thick near the base of the Mesaverde Formation 

(called formation rather than group in this locality) 

(figs. 13, 14). In general, the overburden is weaker 

than the overburden in the Geneva mine area. It 

comprises thicker mudstones or shales and thinner 

sandstones than those in the Geneva mine area. Most 

of the sandstones thicken, thin, and pinch out over 

short lateral distances, but a thick, persistant, 

quartzose sandstone, called the Rollins Sandstone 

Member, underlies the coal beds and forms the base of 

the Mesaverde Formation. The Mancos Shale 

underlies the Rollins Sandstone Member. 

The bedrock dips northwestward 3° to 6°; locally, 

the strata steepen or flatten in areas where 

interstratal warps, which the miners call rolls, are 

present (fig. 13B). Steeply dipping faults, which trend 

west-northwest, northeast, east-northeast, and north 

and which offset bedrock units from 5 feet (1.5 m) to 

more than 20 feet (6.1 m), are present a few miles 

north of the Somerset mine. A few steeply dipping 

discontinuous faults with small vertical displacements 
12 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

FIGURE 9.-Subsidence damage in a massive sandstone at the top of the Mesaverde Group that resulted from coal extraction about 900 

feet (274m) below. A and B, Tension fractures as much as 3 feet (0.9 m) wide resulting from cantilever failure above a coal barrier 

pillar. C and D, Compression bulges and anticlines probably caused by emergence of a compression arch at the surface above a minedout area. 

and with west-northwest and east-northeast trends 

are present locally in the Somerset mine. 

Joints are common in many of the sandstones. 

They are more variable than those present in the 

Geneva mine area; however, most dip steeply to 

vertically and trend north-northeast, north, west-
CURRENT STUDIES OF SUBSIDENCE PROCESSES 13 

northwest, or east. Most of the streams follow these 

same directions with a regularity that indicates that 

they are controlled by the joints. Field studies further 

indicate that most streams occur along linear zones 

that are more intensely jointed than the surrounding 

bedrock. Thus, ridges and valleys, which determine 

the thickness of mine overburden, are commonly 

controlled by jointing. 

Dikelike intrusive bodies, called rock spars by 

miners, which comprise very fine to silt·size quartz 

and feldspar in calcareous to dolomitic cement, are 

locally common in the mine workings (fig. 15C). 

Although the trend of the spars is less regular than 

the trend of the faults, they commonly trend westnorthwest, east-northeast, east, or north. The spars 

disrupt the continuity of roof rocks and commonly 

cause unstable conditions in the mine roof. This in turn 

can lead to uneven mining practices as miners are 

forced to abandon workings before normal extraction 

procedures are completed because of the roof-fall 

hazard. The presence of rock spars in the mine 

increases mining costs and difficulty, sometimes to a 

point where mining is not economic or is unsafe. The 

Cameo mine, Mesa County, Colo., was abandoned 

because of increased mining costs resulting from the 

presence of numerous spars (J. Paul Storrs, U.S. 

Geol. Survey, oral commun., 1974). 

A good general overview of the total bedrock 

structure in the Somerset mining district is available 

from Skylab 2 color-infrared imagery (figs. 10, 11). 

Most lineaments present on the photograph occur 

along streams, which in turn occur along linear, 

profusely jointed or faulted zones. Indeed, all but one 

of the major trends of these lineaments (vector 

diagram of lineaments in fig. 11) parallel the dominant 

trends of the faults, joints, and spars in the district. 

Current studies in the Somerset district include all 

the basic elements of coal mine deformation-subsidence, roof falls, and coal mine bumps. As part of the 

subsidence portion of these studies, an area above the 

active mining in the "B" bed of the Somerset mine was 

selected for subsidence measurement (fig. 14). The 

overburden above current and future mining in this 

area of the Somerset mine ranges in thickness from 

300 feet (92 m) to about 1,600 feet (490 m). The coal 

beds have been burned back from the outcrop in an 

irregular fashion for hundreds of feet. Hotspots, 

probably caused by residual heat from past outcrop 

fires, are locally present in mine workings near 

outcrop burn lines. Various shades of red clinker mark 

the burned beds at the outcrop (frontispiece). The void 

space created by burning caused the overburden rocks 

to subside along steeply dipping tension cracks that 

are similar to the fractures produced by cantilever 

failure above mine workings near an outcrop (fig. 

15A). A core from a drill hole that probably 

intersected one of these fractures revealed ground-up 

bedrock that looked like fault gouge. 

Hot steam and other gases locally vent through 

tension fractures formed in subsiding bedrock (fig. 

15B), indicating that the bedrock is locally still very 

hot and might be a potential source of geothermal 

energy. Hotspots, which perhaps are related to West 

Elk Mountain volcanism, also are locally present in 

mine workings at considerable distances from the 

outcrop burn lines. 

GEOMETRY OF SUBSIDENCE CRACKS 

Periodic surface mapping above the 3d South mining 

area during the last 3 years has revealed a pattern of 

crack development that is related both to the 

geometry and the chronology of mining and to joints in 

bedrock. The first cracks were observed in June 1971 

above, and parallel to, the barrier pillar near the 

eastern mine boundary. The barrier locally widens to 

as much as 100 feet (30.5 m) and is located between the 

8th and 9th Left mining panels, where the 8th Left 

panel extends about 400 feet (121.9 m) beyond the 7th 

and 9th Left panels (fig. 14). The cracks, which formed 

an estimated 2 - 4 months after extraction was 

completed in the area, are as much as 1 foot (0.3 m) 

wide. The cracks narrowed westward and were absent 

where the barrier pillar narrows to a constant 40 feet 

(12.2 m). 

New tension cracks were observed in July 1972, an 

estimated 3- 6 months after coal extraction was 

completed, beneath the ruptured surface area. These 

cracks occurred in two distinct sets: (1) a linear set 

that was parallel to the barrier pillar separating the 

8th and 9th Left extraction panels and was a continuation of the set mapped a year earlier, and (2) a set of 

cracks that paralleled and followed the room-andpillar line as it retreated westward (fig. 14). Both sets 

are about parallel to the trends of steeply dipping joint 

sets present in outcropping sandstones in the area. 

The linear set of cracks above the barrier gradually 

narrowed and disappeared at about the 450-foot (137 

m) overburden-thickness level. Their position 

indicated either that the 40-foot (12.2 m) barrier pillar 

between 8th and 9th Left was yielding enough to 

prevent tensile rupture of the claystone and shale 

bedrock and thin soil cover or that a compression arch 

was present in the strata over 8th Left above the 

500-foot (152.4 m) level (figs. 14, 17). Over the 40-foot 

(12.2 m) barrier pillar, maximum extension beyond 

the rupture limit in weathered claystone and shale 

bedrock measured about 2 inches (5 em). This is only 

one-sixth the amount of lateral extension beyond 
14 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

FIGURE 10.-Infrared Skylab 2 photograph of the Somerset mining district, Gunnison County, Colo., and surrounding 

area. The coal-bearing strata can be seen as a sinuous, light-colored band to the north of the North Fork Gunnison 

River and west of the Paonia Reservoir. See figure 11 for geographic locations. 

rupture that occured above the same barrier at a place 

where the pillar was 100 feet (30.5 m) wide (instead of 

the normal 40 feet (12.2 m)) suggesting that the 

100-foot- (30.5 m) wide portion of the continuous coal 

pillar may provide only about one-sixth the vertical 

yield of the 40-foot (12.2 m) portion. The overburden 

thickness, however, differs by about 100 feet (30 m) 

and may account for some difference in crack width. 

The other set of tension cracks that parallels the 

room-and-pillar retreat lines above 8th Left ranged in 

width from 0.25 inch (0.64 em) to as much as 1.5 feet 

(0.46 m) (figs. 14, 15D), the widest of any cracks 

mapped so far. All these cracks revealed horizontal 

extension in a direction perpendicular to the 

room-and-pillar retreat line and parallel to the barrier 

pillars, and one crack also was vertically displaced 

about 0.5 inch (1.27 em) down to the east. The crack, 

which is 1.5 feet (0.46 m) wide (fig. 15D) and is located 

515 feet (157.3 m) above the mine workings, is at the 

westward limit of all surface rupture. It defines the 
North 

Paonia 

Mt Lamborn 

VECTOR DIAGRAM 

CURRENT STUDIES OF SUBSIDENCE PROCESSES 

t '' ' 

/ 

' ~ Mt Gunnison ~ / 

I ',t f 

::-Pilot 

Knob 

\ .. ~ ~ 't,l 

' \ """'' I-~' ... ~' ... ';('- ~ 

, :; -

\ (j~ 

\\ 0"'.1 I .I 

...... - ,;' 

~ f/ ::West 

\~·/_ Beckwith Peak 

/ I 

I' 

I 

0 5 MILES 

0 5 Kl LOMETRES 

EXPLANATION 

y Lineaments 

-6000- Structure contour drawn on "B" coal bed. Datum 

is mean sea level 

15 

FIGURE 11.-Plot of lineaments derived from the Skylab 2 infrared photograph (fig. 10) of the Somerset mining district. The spatial 

plot of lineaments covers the area currently under study. Two representative structure contures of the "B" coal bed are shown to 

indicate bedrock attitude. The Somerset mine is between Elk Creek and Hubbard Creek south of the 6,000-foot structure 

contour line. 
16 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

FIGURE 12.-Iron Point, an upper(?) Tertiary pluton that intrudes the Mesaverde Formation and most of the overlying Ohio Creek 

Formation 2 miles (3.2 km) north of Somerset coal mine, Colorado. View looking westward across Hubbard Creek, sec. 27, T. 12 S., 

R. 91 w. 

limit of extension due to strata flexure and indicates 

that a compression arch, perhaps about 50 feet (15.2 

m) thick at the crest, is present in deeper strata west 

of this crack (figs. 16, 17). 

Subsidence pits were first observed during July 

1973 in soil and colluvium averaging 3.5 feet (1.07 m) 

thick above the 8th mining panel about 11/2 years after 

coal extraction was completed (figs. 14, 15E). The 

pits are roughly circular and 3-4 feet (0.9 -1.2 m) in 

diameter. A linear tension crack, perhaps 2-4 inches 

(5.08 -10.16 em) wide, probably exists at the base of 

the pit. Apparently the pits are caused by local failure 

of the soil and colluvium veneer which normally is 

cohesive enough to bridge a crack of this width 

without noticeable failure. Pit failure may have been 

initiated by some heavy animal, such as a cow or 

horse, stepping through the bridged soil and colluvium 

veneer. No surface cracks were observed above the 

coal barriers between 6th and 7th Left panels (fig. 14). 

This is either because the soil and colluvium or local 

landslide deposits have successfully bridged existing 

cracks or because the near-surface bedrock has not yet 

ruptured. In such situations, cohesive soil, colluvium, 

or landslide deposits could obscure potentially 

hazardous cracks and might cause planners to develop 

potentially untenable land-use plans for the surface 

area. 

In August 1973 another very fresh tension crack 

was observed and mapped above and parallel to a solid 

coal barrier between 5th and 6th Left. This crack, 

which formed in mudstone-sandstone overburden 

500- 600 feet (152.4 -182.9 m) above the elevation of 

the mine workings, opened only 4 months after mining 

in 6th Left was completed. This crack appeared sooner 

than all other cracks, including those present in even 

thinner overburden. This is probably because a 

300-foot- (91.4 m) wide coal pillar is still present 

between 5th Left and the mined-out 6th Left panels, 

thus causing maximum strata flexure into the 6th Left 

mined-out area; whereas the thin barrier pillars 

between 6th and 7th panels and 7th and 8th panels 

yielded to overburden load and therefore reduced 

lateral flexural stresses. 

This situation points out an important factor 

governing the rate and severity of coal mine subsidence damage: surface damage in overburden of a 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 17 

FIGURE 13.-Mesaverde Formation in Somerset mining district, 

Colorado. A , Southward view of the Somerset mine buildings. 

The basal coal-bearing part of the Mesaverde underlies the 

brush-covered slope in the background across the North Fork 

Gunnison River; the barren member forms the higher and 

steeper tree-covered slope. B, Upper part of the coal-bearing 

portion of the Mesaverde Formation, separated from the 

overlying barren member by the massive sandstone. Note the 

lenticular sandstones (arrows). Differential compaction of weak 

shales and claystone near these strong lenticular sandstones 

probably produces the undulations (called rolls by miners) noted 

in certain mining areas. About 2 miles (3.2 km) east of the town 

of Somerset. 

given type and thickness above a mine opening of a 

given geometry will occur more quickly, and can be 

more severe, where the mine opening is bounded by 

massive solid coal pillars than where the mine opening 

is surrounded by thin coal pillars. This suggests a 

strict adherence to a mining sequence whereby 

adjacent mining panels are mined nearly concurrently, 

as were the 9th, 8th, 7th, and 6th Left panels, rather 

than completing extraction in one panel before moving 

to an adjacent one, as occurred in 6th and 5th Left. 

Little is known about the surface damage caused by 

mining the 9th Left panel because it was mined 11

/z - 2 

years before the study began (fig. 14). Cracks could 

have been masked by forces of erosion in that length of 

time. Perhaps some of the cracks above the barrier 

between 8th and 9th Left panels at the eastern end 

were caused by mining in 9th Left; however, the 

chronology of crack development revealed by periodic 

mapping suggests that most of the cracking was 

caused by mining in 8th Left. Perhaps the tensile 

rupture limit was not reached or was not exceeded by 

flexure of strata into 9th Left until the coal in 8th Left 

was mined. This is an example of how subsidence in 

one area can influence subsidence in adjacent areas. 

(See discussion in section on stress and strain caused 

by subsidence.) 

SUBSIDENCE MEASUREMENTS 

Surface measurements made periodically from 

October 1972 through December 1973 have yielded 

considerable information on the process of 

deformation in the overburden strata relative to the 

position of the room-and-pillar retreat lines and to 

panel-pillar extraction geometry (table 1; figs. 15, 16, 

17). The surface measurements, together with the 

subsidence-crack geometry, yielded the following 

subsidence parameters: (1) limit angle relative to 

solid-coal boundaries, (2) break angle, (3) configurations of compression arches in this particular overburden strata, and (4) ratio of surface subsidence to 

the thickness of the coal bed or mining height versus 

the ratio of mining panel width to mean overburden 

depth (fig. 5). These parameters may change with 

time as the overburden continues to adjust, so the 

measurements must continue above mined-out areas 

in order to determine final limits for these parameters 

and the post-mining time necessary to achieve surface 

stability. However, it is important to measure subsidence parameters as mining progresses, as well as the 

final parameters, because the subsidence processes 

contribute to the state of stress within the mine 

workings and within the rocks above and beneath the 

mine workings. 

A complete profile of subsidence related to coal extraction was available for only the area above the 6th 

Left panel because the bench marks were installed 

before significant pillar extraction began in that panel. 

The A3 - B5 profile in figure 16 shows the relative 

position of the surface above the 6th Left panel in 

relation to the position of its pillar retreat line. This 

profile shows that no surface subsidence occurred 

until the fourth measurement on June 26, 1973, or 
18 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

29,000 E 

-4 

7000N --tf~~~~~i--t-f\i~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

0 

I 'I I II 

0 100 

500 

I I 

200 I 

300 METRES 

FIGURE 14.-The 3d South area in the Somerset mine, Colorado, showing mine workings, mined-out areas, major rock units near the 

coal beds, coal beds, and fractures in mine overburden caused by mining the "B" coal bed. Mine base is modified from U.S. Steel 

Corp. mine map, Somerset mine; overburden contours derived from U.S. Geological Survey Somerset and Bowie 71

/z-minute topographic quadrangle maps. 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 19 

EXPLANATION 

"Rocks spars"- Dikelike bodies comprising very fine 

to silt-sized quartz and feldspar grains in calcareous 

to dolomitic cement; at elevation of mine workings 

Tension cracks at surface - Date indicates when crack 

was first observed 

---L-- Active landslide- Slow to rapid downslope movement 

of bedrock and surficial debris; rate of movement 

governed by degree of saturation 

Surface bench mark - Installed by USGS in October 

1972 (row A . 1-3; row B, 1-5; row D. 0-3) and 

August 1973 (row A. 4-6; row B. 6-7) 

t.R Surface reference bench mark, on stable ground 

-500- Overburden thickness contour- Datum is top of "B" 

Retreat line 

5 

coal bed ; contour interval 100 feet (30.5 m) 

Room-and-pillar mine workings- Mined out where 

hachured during period shown: Posted to July 1, 

1973 IZ:ZJ ; Posted to Jan. 1, 1974 ~ 

Retreat line 7/13/73 

(see table 1; figs.16,17J 

~~~ Limit of mine workings (burn line) 

30,000 E Mine grid- Equals Colorado Grid, Central Region, by 

5000N + adding 400,000 and 1.400,000, respectively, to 

north and east coordinates 

A----,\' Line of section shown in figure 16; section 8 - 8' 

shown in figure 17 

until a mmmg width approximately equal to the 

average depth of overburden was excavated. Then, 

within a 71

/2-week interval, measurements showed 

that the surface at bench mark A3 subsided nearly 2 

feet (61 em) (table 1). Subsidence continued at 

irregular but shorter intervals through August, was 

stable from September through November 30, and 

then the ground surface settled another 0.16 foot (4.8 

em) in December. Bench mark B5 rose, then went 

down slightly during measurement period 1 through 4, 

then settled further during subsequent measurements 

through December 31, 1973. 

Although only two bench marks are located above 

the 6th Left mining, the periodic measurements 

yielded rather precise subsidence profiles and limit 

angles, particularly for profiles 4, 5, and 6 (fig. 16). 

The limit angle, measured with respect to the 

horizontal, is 69° - 70° in weak overburden that 

thickens from 600 to 900 feet (183- 27 4 m) over about 

400 feet (122m) of horizontal distance. The limit angle 

may be less steep for flat-lying surfaces in similar 

strata. The configuration of profiles 7, 8, 9, and 10 

suggests that the limit angle steepens from 75° to 

perhaps as much as 80° in moderately strong strata 

beneath steep hillsides which are more than 900 feet 

(27 4 m) above the mine workings, although the 

precision is reduced on these profiles because they are 

west of the B5 bench mark. 

Profiles parallel to bench marks in rows A and B 

show how the surface responded to mining at right 

angles to the orientation of mining panels (fig. 17). 

Horizontal movement in the north-south direction also 

was measured each time the vertical angles were 

measured. The resultant movement of the individual 

bench marks in rows A and B is plotted at the top of 

figure 17. The measurements of a diagonal row D 

(table 1, D row not shown in fig. 17) indicate that there 

was little vertical or horizontal movement beneath it 

after the bench marks were installed, suggesting that 

in overburden 300-470 feet (91.4 -143.6 m) thick 

subsidence was virtually complete 1lf2 years after 

mining was completed in the eastern part of the 8th 

and 9th Left panels. 

Profiles of the relative vertical and horizontal movement of bench marks comprising rows A and B show 

that the overburden strata generally formed a trough 

above the 6th, 7th, and 8th Left mining panels and the 

barriers between these panels. The horizontal movement was generally toward this trough (fig. 17, table 

1), although the bench marks often moved so erratically from measurement to measurement that conventional curves of horizontal displacement, horizontal 

strain, and curvature, such as those shown in figure 2, 

would be grossly oversimplified. In addition, as the 

strata subsided into the mined-out area in 6th Left 

through November 30, the strata were uplifted above 

the solid-coal boundary north of 6th Left; then, in 

December, the surface subsided slightly. 

Through November 30, 1973, the limit angle (cp9 in 

fig. 17)-measured from the horizontal-varies from 

30° to as much as 85° on row A and from 65° to 

vertical on row B, depending on how it is defined. If 

limit angle is defined as the acute angle formed by 

drawing a straight line between the limit of surface 

movement-whether upward or downward-to the 

solid-coal boundary and the horizontal, then the limit 

angle is about 30° in overburden 500 - 600 feet 

(152.4 -182.9 m) thick on the row A profiles and is 

about 62° in overburden 700- 800 feet (213.3- 243.8 

m) thick on the row B profiles. If it is defined as the 

acute angle formed by projecting a straight line from 

the limit of downward surface movement-the edge of 

the subsidence trough-to the solid-coal boundary and 

the horizontal, as usually measured, then the limit 

angle is 82° and 88° for the A and B profiles, 

respectively. This limit angle was measured only 5 

months and 2 months after the room-and-pillar retreat 
20 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

FIGURE 15.-Primary and subsidence fractures. A, Offset sandstone produced by subsidence above burned coal beds, Somerset 

district, Colorado. The sandstone is offset about 12 feet (3.6 m) 

along a tension fracture that is nearly perpendicular to the dip of 

the bedrock. Note the broken and weathered appearance of the 

sandstone to the left of the fracture as compared to the sandstone 

to the right of the fracture. B, Steam vent in a tension rift located 

in subsiding ground above burned coal beds; some vents might 

be a source of geothermal energy. C, Engineers inspect a rock 

"spar" that intrudes the "B" coal bed along fractures; contacts in 

the coal adjacent to the spar are irregular and brecciated but not 

burned. D, Subsidence cracks in weathered bedrock and thin soil 

which are located parallel to room-and-pillar retreat lines of 8th 

Left, Somerset mine. E, Subsidence pit in soil and colluvium that 

appears to be underlain by a linear tension crack. 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 21 

TABLE 1.-Relative displacement of U.S. Geological Survey subsidence bench marks located above the 3d South area of the Somerset 

mine, Delta and Gunnison Counties, Colorado 

[See figure 14 for bench mark locations; all displacements are in feet (1 ft=30.48 em); estimated accuracy of measurement,±0.05 feet (1.52 em); M, monthly change in position of 

bench marks horizontally (H) to north or south and vertically ( V); C, cumulative change in position of bench marks horizontally (H) to north or south and vertically ( V); negative 

sign(-) indicates change horizontally to the south and a decrease in elevation; plus sign(+) indicates change horizontally to the north and an increase in elevation. Leaders 

indicate no data collected; stations A4-A6 and B6-B7 were installed in September 1973) 

Measuring Period 1 2 3 4 5 6 7 8 9 10 

Date ........... 10-14-72 11-8-72 5-5-73 6-26-73 7-13-73 8-17-73 9-13-73 10-10-73 11-30-73 12-31 -73 

Station (shot Change H v H v H v H v H distance in ft): v H v H v H v H v H v 

A1 (3,330) ...... M 0.00 0.00 -0.06 -0.06 +0.02 +0.08 +0.08 +0.05 -0.02 +0.03 -0.05 -0.06 +0.18 -0.19 -0.11 +0.18 +0.02 +0.39 -0.23 -0.40 

c .00 .00 -.06 -.06 -.04 +.02 +.04 +.07 

A2(3,460) ...... M .00 .00 .00 -.47 +.32 -.37 +.25 -.20 

c .00 .00 .00 -.47 +.32 -.84 +.57 -1.04 

A3 (3,500) ...... M .00 .00 +.63 -.61 +.05 -.59 -1.98 

c .00 .00 +.63 +.02 +.05 -.57 -1.93 

A4(3,520) ..... M 

c 

A5 (3,600l ..... M 

c 

At) (3, 740) ... M 

c 

B1 (3,970) ....... M .00 .00 -.16 -.04 +.40 -.48 +.48 -.16 

c .00 .00 -.16 -.04 +.24 -.52 +.72 -.68 

~(4,130) ....... M .00 .00 -.10 -.06 +.44 -.80 +.40 -1.28 

c .00 .00 -.10 -.06 +.34 -.86 +.74 -2.14 

B3 (4,180) ....... M .00 .00 -.08 -.32 -.14 -.73 +.14 -.20 

c .00 .00 -.08 -.32 -.22 -1.05 -.08 -1.25 

84(4,240) ....... M .00 .00 -.10 -.08 +.53 -.04 -.04 -.18 

c .00 .00 -.10 -.08 +.43 -.12 +.39 -.30 

B5 (4,210) ....... M .00 .00 .00 +.14 +.49 -.28 -.14 +.26 

c .00 .00 .00 +.14 +.49 -.14 +.35 +.12 

86(4,090) ....... M 

c 

B7 (4,010). M 

c 

n3 (3,370J . M .00 .00 -.16 -.16 +.08 +.23 +.06 +.10 

c .00 .00 -.16 -.16 -.08 +.07 -.02 +.17 

D2(3,030) . M .00 .00 -.41 -.16 +.35 -.18 +.06 +.12 

c .00 .00 -.41 -.16 -.06 -.34 .00 -.22 

D1 (2,690) ...... M .00 .00 -.17 +.14 -.18 -.08 .00 +.09 

c .00 .00 -.17 +.14 -.35 +.06 -.35 +.15 

Do(2,470l ..... M .00 .00 -.73 -.02 +.13 .00 +.04 .00 

c .00 .00 -.73 -.02 -.60 -.02 -.56 .00 

lines passed beneath profiles A and B, respectively. In 

December, 6 months after mining, the surface near 

bench marks A 5 and A6, above the solid coal, subsided 

uniformly below the original surface level, thus 

eliminating a stable ground point of reference to 

measure the limit angle and thereby precluding 

further measurements of limit angle in accordance 

with either definition. 

Limit angles in strata above thin barrier pillars, 

such as those separating the 6th, 7th, 8th, and 9th Left 

mining panels, are not measurable because the 

overburden strata are downwarped on either side of 

the barrier pillars, creating interacting draws and 

making it impossible to define the draw attributable to 

one particular mining area. 

The break angle above solid-coal boundaries is 

considerably different from that above thin barrier 

pillars, as the subsidence theories previously 

+.02 +.10 -.03 +.04 +.15 -.15 +.04 +.03 +.06 +.42 -.17 +.02 

-.08 -.15 -.03 -.13 +.27 -.05 -.17 +.25 -.07 -.25 -.15 -.05 

+.49 -1.19 +.46 -1.32 +.73 -1.37 +.56 -1.12 +.49 -1.37 +.34 -1.42 

-.12 -.25 -.24 -.66 +.23 -.15 -.19 +.05 -.02 -.02 -.12 -.17 

-.69 -2.18 -.93 -2.84 -.70 -2.99 -.89 -2.94 -.91 -2.96 -1.03 -3.13 

-.17 -.07 -.03 -.02 -.22 -.19 

-.17 -.07 -.20 -.09 -.42 -.28 

-.17 +.31 +.14 -.21 -.40 -.21 

-.17 +.31 -.03 +.10 -.43 -.11 

-.11 +.25 -.04 -.12 -.27 -.33 

-.11 +.25 -.15 +.13 -.42 -.20 

-.08 -.02 +.08 -.14 +.12 -.14 +.02 -.16 +.24 -.04 +.04 +.02 

+.64 -.70 +.72 -.84 +.84 -.98 +.86 -1.14 +1.10 -1.18 +1.14 -1.16 

-.06 -.22 +.14 -.18 -.06 -.06 +.16 .00 +.24 +.22 +.12 -.36 

+.68 -2.36 +.82 -2.54 +.76 -2.60 +.92 -2.60 +1.16 -2.38 +1.28 -2.74 

+.02 -.22 +.14 -.34 +.10 -.28 +.16 .00 +.18 -.08 +.06 -.16 

-.06 -1.47 +.08 -1.81 +.18 -2.09 +.34 -2.09 +.52 -2.17 +.58 -2.33 

-.16 +.06 -.02 -.33 .00 -.49 +.04 -.29 +.16 -.12 .00 .00 

+.23 -.24 +.21 -.57 +.21 -1.06 +.25 -1.35 +.41 -1.47 +.41 -1.47 

-.06 -.22 -.04 -.37 -.10 -.31 -.18 -.39 +.04 -.14 +.10 .00 

+.29 -.10 +.25 -.47 +.15 -.78 -.03 -1.17 +.01 -1.31 +.11 -1.31 

-.16 -.24 +.06 +.16 +.06 .00 

-.16 -.24 -.22 -.08 -.16 -.08 

-.25 +.04 +.17 +.06 +.10 -.08 

-.25 +.04 -.08 +.10 +.02 +.02 

-.08 +.05 +.02 -.03 +.16 +.03 -.13 +.08 +.03 +.10 -.02 +.02 

-.10 +.22 -.08 +.19 +.08 +.22 -.05 +.30 -.02 +.40 -.04 +.42 

-.03 +.03 -.07 -.13 +.15 +.06 -.09 +.07 +.03 .00 -.08 -.06 

-.03 -.19 -.10 -.32 +.05 -.26 -.04 -.19 -.01 -.19 -.09 -.25 

+.01 -.10 -.08 +.10 +.13 -.01 -.09 -.20 +.12 +.13 -.12 +.01 

-.37 +.05 -.45 +.15 -.32 +.14 -.41 -.06 -.29 +.07 -.41 +.08 

-.04 -.04 -.08 +.04 +.13 +.06 -.06 -.13 +.03 -.04 -.05 +.10 

-.60 -.06 -.68 -.02 -.55 +.04 -.61 -.09 -.59 -.13 -.64 -.08 

discussed suggest (figs. 2, 3, 4). Four months after 

mining was completed beneath the ruptured area, the 

break angle above a 300-foot (91.4 m) barrier north of 

6th Left measured a negative 73°, whereas the break 

angle in the strata above the thin barriers between 8th 

and 9th Left panels was nearly vertical. The negative 

break angle above the solid-coal boundary may be a 

transient condition that, with time, will become 

positive. Further periodic mapping for cracks will 

reveal the final break angle; however, pillar 

extraction began in 5th Left in November 1973 (fig. 

14), and the thick boundary pillar may be mined to a 

thin one like the others before the final break angle 

can be determined. Any surface- or ground-water flow 

or methane pockets will, of course, be intersected by 

the cracks, and the water or gas might be diverted to 

lower strata or to the mine workings through cracks in 

the overburden strata. 
22 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

w 

oA 

0 

0 

E - oo 0 

0 

'<:!" 

N 

0 

0 

N 

N 

0 

0 

N 

A t-------------------- ------------------------------------------------------------------

-o 

0 

0 -c.o 0 

0-

00 

_____ : ________ :_ ___ . ---· __ :. _ _:_ ____ __.!__....---~--_____.:..- _;_ ___ _ 

--,------:----::----, 

" ........_.?-t- --:- -.?-

Mancos Shale 

EXPLANATION 

~Coal bed in lower coal-bearing part of Mesaverde Formation -

Mined where unpatterned; A, 8, C, D, and E denote nomenclature of coal bed in general use within the area. Bed 

position projected from existing drill-hole and outcrop information D Sandstone of Mesaverde Formation - Fine- to medium-grained, 

f ~ldspathic 

D Rollins Sandstone Member at base of Mesaverde FormationFine- to very fine qrained, quartzose; elongate elliptical 

iron concretions 0.5-2.5 ft (0.15-0.76 m) in long dimension 

locally common D Shale and mudstone- Upper Cretaceous Mancos Shale below 

the Rollins Sandstone Member; all others in Mesaverde Formation 

FIGURE 16.-Cross section A -A' through the 3d South area, Somerset mine, Colorado, enlarged to show stratigraphic, mining, and 

subsidence details. (See fig. 14 for cross section location.) Vertical exaggeration of subsidence profiles is x100; dates of surface 

measurements are keyed to subsidence profiles and the positions of room-and-pillar retreat lines by the use of surface-measurement 

date numbers (1-10); mining as of Jan. 1, 1974. 

The subsidence profiles, particularly for the Brow 

of bench marks, show that maximum subsidence 

through December 1973 occurred above the barrier 

between 7th and 8th Left rather than above the 

mined-out panels, as one would expect. This indicates 

that compression arches are bridging the overburden 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 23 

A'~ 0 

0 

0 

ro - E 

0 _o 

'<:!" 

N 

SURFACE 

MEASUREMENTS 

5 

I 

I 

I 

Burned coal beds 

Rollins Sarrdstone Member 1 / 

--,- - .........-?- :-- --:- -?.....;' 

Mancos Shale 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

1972 

Oct. 14 

Nov. 8 

1973 

May 5 

June 26 

July 13 

Aug. 17 

Sept. 13 

Oct. 10 

Nov. 30 

Dec. 31 

0 

-~ N 

0 - 0 

0 

r-- - 0 _0 

N 

0 _o 0 0 0 N 10-

CD 

0 -0 

0) 

0 - 0 o_ 

CD 0 -0 

ro 

0 500 FEET 

0 50 100 150 METRES 

load to the barriers above the 6th, 7th, and 8th Left 

mining panels. Compression arches, and an inner 

destressed zone, also probably extend beneath the 

mine openings; however, the force of gravity probably 

reduces the crest depth of the arches considerably as 

compared to their crest height above the mine 

workings (figs. 16, 17). Shrinkage stoping, a process 

wherein fractured and buckled strata occupy a larger 

volume than originally, could not, in the opinion of the 

author, account for the geometry of the subsidence 

profiles for bench marks A and B (particularly B) 

nearly as adequately as the presence of compression 

arches. 

The barriers, in turn, apparently yielded to this 

increased load, which produced a general troughlike 

depression over the mined-out panel-pillar complex, 

or "super panel," compared to the amount of yield in 

the massive, solid-coal pillar north of 6th Left. On the 

basis of subsidence-crack data and surface measurements (figs. 14, 16, 17), the arch above 8th Left 

appears to be about 500 feet (152 m) high and spans a 

panel about 450 feet (137 m) wide. Surface-crack 
24 

ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

B 

0 8-

N g 

0 

-I!) 

(0 

g-

(J) 

1- oz 

zw 

<(~ 

..IUJ 

<(> 

1-0 

z~ 

O...l 

N<C 

-u 0::-

Ol-

:r:O:: w 

> 

2 3 4 

Row B 

em ft 

4 3 4 

100 3 

5 6 

7 8 9 

2 

50 

2 3 4ft 

0 

0 50 

I 

I I 

I 

2 3 4 

em ft Row B 

OI (J) 1 

UJ 

:::! 50 

u.. 

100 : 

0 

0:: 

a. Row A 

w 

<.) 

z 

OI UJ 

0 

<n 1 

IXl 

::> 50 (J) 

100 : 

~9Left 8 Left 

Mined 1970-71* 

Zone of compressive 

"· 

5 6 

5 6 I 

-0 

g-L--------------------------~-------===~==~----~----~=============-=========-------------~-

FIGURE 17. -Cross section B-B' through the 3d South area, Somerset mine, Colorado, enlarged to show stratigraphie, numng, and 

B is X100; horizontal and vertical exaggeration of north-south and vertical movement of individual bench marks of rows A and B 

(0 

movement by use of surface-measurement date numbers (1-10); mining as of July 1, 1973. 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 

7 

9 10 

8 7 

NORTH 

B' +-' 

'+-

0 

0 

0 

r-- - E 

0 

0 

N 

0 _o IJ) 

~~~~A I o-

------------------------~--~--~--~8 (.0 

EXPLANATION 

Coal bed in lower coal-bearing part of Mesaverde Formation -

Mined where unpatterned; A, 8, C, 0, and E denote nomenclature of coal bed in general use within the area. Bed 

position projected from existing drill-hole and outcrop information 

Sandstone of Mesaverde Formation- Fine- to medium-grained, 

feldspathic 

Rollins Sandstone Member at base of Mesaverde Formation -

Fine- to very fine-grained, quartzose; elongate elliptical 

iron concretions 0.5-2.5 ft (0.15-0. 76 m) in long dimension 

locally common 

Shale and mudstone- Upper Cretaceous Mancos Shale below 

the Rollins Sandstone Member; all others in Mesaverde Formation 

Fracture in mine overburden - Projected from surface crack 

toward nearest barrier pillar underground 

SURFACE 

MEASUREMENTS** 

1972 

1 Oct. 14 

2 Nov. 8 

1973 

3 May 5 

4 June 26 

5 July 13 

6 Aug. 17 

7 Sept. 13 

8 Oct. 10 

9 Nov. 30 

10 Dec. 31 

*Mined before bench marks were installed 

**Subsidence bench marks installed Oct. 1972, except A4 , 

A5 , A6 , B6 , and B7 , which were installed Aug. 1973 

0 500 FEET 

0 50 100 150 METRES 

subsidence details. (See fig. 14 for cross section location.) Vertical exaggeration of subsidence profiles of rows A and 

(upper two rows) is XlOO; dates of surface measurements are keyed to the subsidence profiles and individual bench-mark 

25 
26 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

geometry indicates that the compression arch 

apparently migrated upward in the strata from a 

height of 300 feet (91 m) to 500 feet (152 m) in a year's 

time. Subsidence measurements indicate that similar 

arches probably are present above the 6th and 7th 

Left panels, at least at the level of row B, although 

surface cracks were not observed above these panels 

and benchmark control is not adequate to verify the 

true position of the arches at the level of row A. 

Surface cracks are absent above the 6th and 7th Left 

panel barriers either because the tensile rupture limit 

was not exceeded in the surficial veneer or, if the 

cracks occurred in the bedrock, because they are 

masked by the colluvium or landslide deposits that are 

prevalent in the area. 

If it is assumed that the compression arch above the 

8th Left mining panel approximates half of an ellipse 

with its major axis oriented perpendicular to the coal 

bed, then the ratio of major to minor axis would be 

approximately 450:225, or 2.0, for panel-pillar mining 

in 3d South. Mohr (1956, p. 150) suggested that the 

configuration of the upper half of the stress ellipse is 

related to Poisson's ratio for the overburden strata by 

the formula: 

where 

a b=m-1, 

a= the major axis of the ellipse, 

b =the minor axis of the ellipse, and 

m= the reciprocal of Poisson's ration ( ~A). 

For overburden strata comprising mudstones, shales, 

coal beds, and lenticular sandstones in the Somerset 

mine area a 

m= 1+;; =1+2=3, and 

1 1 ~=m=a-=0.33. 

Shoemaker (1948, p. 7), in his review of Fenner's 

analyses of stresses around mine workings, showed 

that a uniform elliptical zone of compressive stress 

occurs around a single mine entry when the axial 

ratios of the ellipse are a:b = (m-1):1, the same ratios 

as presented by Mohr. 

STRESS AND DEFORMATION CAUSED 

BY SUBSIDENCE 

Previous and current mine deformation studies 

have revealed that the vertical and horizontal 

components of stress caused by overburden load and 

by any tectonic stresses present cannot be 

transmitted through mine openings but must be 

redistributed around them. The redistribution is 

accompanied by deformation, or yield, that continues 

until equilibrium is once again attained. This can 

produce large changes in the natural state of stress. 

The strata above and below the mine openings, for 

example, are subjected to increased high- to low-angle 

compressive stresses in the elliptical zones of 

compressive stress. These zones encompass the strata 

above and below the mine openings and transfer part 

or all the overburden stresses to mine boundaries or 

barriers of the mining panels (fig. 17). The upright and 

inverted arches also encompass inner zones of much 

lower stress relative to the concentrations of 

compressive stress within the arches. 

As the mine workings become more extensive, the 

arches migrate upward and downward, and the strata 

transected by them are subjected to increased 

compression, followed by decompression and 

continued migration of the arches. Within the 

destressed zones, Mohr's measurements (1956, p. 

146-149) and mine studies by the author show that 

the strata buckle downward and upward (bottom 

heave) into the mine cavities and produce tensile and 

compressive stresses within major lithologic boundaries and shear stresses across lithologic boundaries 

due to flexure of strata, as shown in figures 4A and 

4B. This buckling or down warping also transfers some 

of the original overburden stresses from mine 

boundaries or barrier pillars back into the mine 

workings or mined-out areas. As the arches migrate 

farther upward and downward, an increasingly 

greater part of the overburden stresses is transferred 

back into the mine workings. 

In addition, as mining progresses, the strata above 

coal pillars, particularly the weaker strata above 

strong barriers between mining panels, are 

compressed vertically because the overburden load is 

concentrated above them by arching and strata 

flexure (S2S1 in fig. 4B). A similar compressing 

effect, although reduced by the force of gravity, 

occurs in the strata beneath large strong pillars. 

In the upper part of lithologic units, flexure 

produces both tensile stresses and compressive 

stresses that are oriented parallel to the bedrock 

attitude. The tensile stresses occur in zones where the 

strata are positively curved (above coal pillars), 

whereas the compressive stresses occur in zones 

where the strata are negatively curved (above mining 

cavities and near coal pillars or mine boundaries) (fig. 

4). Owing to flexural slip, lateral shear stresses and 

accompanying strains are present along major 

lithologic boundaries. The lateral flexural and shear 

stresses are greater above narrow barrier pillars (figs. 

4A, 17) than above solid-coal boundaries (figs. 4B, 17), 

provided that the narrow pillars are strong enough to 

support the concentrated overburden stresses. WithiP 

the strata above a barrier pillar, as in figure 4A, a 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 27 

above the barriers separating 6th, 7th, 8th, and 9th 

Left panels, two diagonal shear stresses should occur 

along the crossing limit lines; whereas a single 

diagonal shear stress should develop along the limit 

line above solid-coal mine boundaries, such as north of 

6th Left (fig. 17). As the 5th Left panel is mined, the 

single shear stress along the limit line should be 

transected by another limit line and accompanying 

shear stress, and the flexural and shear stresses 

should increase unless the barrier pillar is narrow 

enough to yield significantly. 

Unmined coal beds in the strata within the influence 

of migrating compression arches will be subjected to 

stress changes, including: (1) low- to high-angle 

compressive and tensile stress concentrations in the 

arch and the destressed zone, respectively; (2) 

horizontal tensile and compressive stresses within 

lithologic units and shear stresses along lithologic 

boundaries caused by flexure of strata; (3) possible 

high- to moderate-angle shear stresses at the limits of 

the draw; (4) local increased vertical compression 

above and below coal pillars, caused by concentration 

of overburden stresses; and (5) reduced vertical 

compression above and beneath mine voids. Coal beds 

subjected to these stresses probably will be much 

more hazardous to mine if recovery factors equaling 

those attainable in undisturbed beds (figs. 18, 19A, 

19B, 19C) are sought. 

In addition, should the compression arches 

surrounding destressed zones stabilize within portions 

of strata containing coal beds, subsequent mining in 

these beds could encounter any combination of these 

stresses, which could perhaps cause compressive roof 

failure and the possible loss of valuable coal reserves. 

The "E" coal beds, for example see figure 16, probably 

are within the zone of the compression arch above and 

parallel to the mined-out panels. The height of the 

compression zone in a profile parallel to mined-out 

panels (fig. 16) is controlled by the height of the arches 

spanning the mining panels (fig. 17), which in turn is 

controlled by the width of the panels. Ropski and 

Lama (1973, p. 118) found, from measurements in 

holes driven above the coal bed being mined by the 

longwall method, that the strata higher than 3 to 3.5 

times the thickness of the coal bed being mined were 

relatively undamaged by longwall mining because the 

strata flexed into the mine cavities without fracturing. 

However, their study revealed. only the aspects of 

subsidence damage resulting from mining above the 

longwall face; they did not investigate the nature and 

extent of subsidence effects above remnant-pillar 

boundaries of the longwall panels. 

The stresses encountered in mining coal beds above 

the "B" coal bed in the 3d South area (fig. 17) probably 

will vary greatly depending on whether the coal is 

extracted from above the mined-out areas or from 

above the barriers in the "B" coal bed. Mining in a 

multiple-bed mining area commonly is safer and 

extraction ratios are higher if the beds are mined from 

top to bottom, provided that uniform extraction 

procedures are followed. If barriers or isolated pillars 

are left in the extraction process, however, the 

underlying coal beds may be affected, particularly if 

the stratigraphic interval between beds is small and 

the overburden is thick (figs. 19D, 19E, 19F). Stemple 

(1956, p. 39) found that the damage to a subjacent bed 

can be of two types if water or gas is abundant in the 

area: (1) sudden intrusion of water or methane from 

overlying mine workings through subsidence cracks, 

and (2) weight manifestations (stress concentrations), due to increased loading, beneath isolated 

pillars or solid-coal mine boundaries. 

An example of stress concentrations caused by 

uneven extraction was seen by the author in a mine of 

the Somerset district where the "C" bed was mined 

prior to mining in the "B" bed. The stratigraphic 

interval between the two beds is about 50 feet (15m). 

Mine pillars 60 feet by 60 feet (18.3 x 18.3 m) that were 

left in the "C" bed caused serious local bump and 

roof-fall hazards when the underlying "B" bed was 

mined beneath them (figs. 19D, 19E). However, 

beneath areas of nearly complete extraction in the "C" 

bed (fig. 19F), overburden stresses in the "B" bed 

were relieved, and recovery was excellent, with no 

hazard to miners. 

Recent mining in the "C" coal bed in parts of the 

Somerset mine 50-60 feet (15.2 -18.3 m) above old 

mine workings that were driven in the "B" bed during 

the early to middle 1930's encountered unstable roof 

conditions and stress concentrations that were 

related to this prior mining. Although most of the rock 

strata above the "C" bed are unstable owing to the 

presence of numerous slickensided fractures, certain 

areas of the roof are particularly unstable (figs. 19B, 

19C). These areas commonly occur above, and aline 

with, old mine entries in the "B" bed. Bumps and 

sudden roof falls were a constant hazard to men and 

equipment when workings were driven above coal 

barriers in the "B" bed. These conditions indicate that 

stresses are still concentrated above the barrier 40 or 

more years after "B" bed mining. 

Elevated stresses on mine workings and resulting 

bumps and roof falls in the "C" Slope and 1 Right mine 

workings commonly were reflected in increased local 

seismic activity, which was recorded by a seismic 

station located about 1,000 feet (305 m) north of the 

area (fig. 18). Peaks of seismic activity in 

mid-January, early February, late February, and 
28 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

en 

a: 

0 

:! 

w 

a: 

ILL > 500 0<( 

a:C 

Wa: 

~~ :::> 

z 

...J 

<( 

1-

0 

1-

1973 

0 

I 

0 

200 400 

I I 

I 

100 

FIGURE lB.-Composite map of part of the Somerset mine 

workings and histograms of the daily seismic activity during 1973 

and 1974 in the Somerset mining district, Colorado. The mine 

workings in gray are in the "B" coal bed, which was mined in the 

early to middle 1930's; the workings in black are in the "C" coal 

bed, 50- 60 feet (15.2 -18.3 m) above the "B" bed, which was 

mined in 1973 and 197 4. Mining ·progress in the "C" coal bed is 

700 

~ Seismometer station was moved -+ 

2,300 ft (700 m) north of previous 500 

position 

1974 

600 800 1000 FEET 

f I I 

I I 200 300 METRES 

shown by lines bracketing the month and locations in which mining took place. The lower histogram is the totPJ daily seismic 

activity recorded; the upper histogram is the seismic activity between an estimated 1 and 2 on the Richter scale. Note that the 

seismic activity commonly increased when the mine workings in 

the "C" coal bed were driven above large coal pillars left in the 

"B" coal bed. 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 29 

early March 1973 coincide with mining in the "C" Slope 

above barrier pillars or isolated blocks of coal in the 

"B" bed north of 4 West and 5 West. Peaks in early 

May and late August 1973 correspond with periods of 

severe roof-fall hazard in crosscuts of 1 Right. The 

first peak above 400 tremors per day in 1973, which 

occurred in mid -October, coincided with mining in 1 

Right above a remnant barrier pillar in the "B" bed. 

The very large peaks of seismic activity in early and 

late December 1973 and late January 197 4 correspond 

with periods of unstable roof conditions during 

development and extraction in areas of 1 Right above 

old mine workings in the "B" bed. Miners were unable 

to remove all pillars because of severe roof-fall 

dangers and, as a result, stresses increased and 

bumps were common on the unmined pillars. The next 

high peak in mid-May 1974 coincides with a period 

when the "C" Slope entries were driven above a 

barrier pillar in the "B" bed. As the slope entries were 

driven northward during the period from late 

February to early May 197 4, the overburden thickness 

increased from 500 to 1,000 feet (152 -305m) (fig. 18). 

Although records show a reduction in overall seismic 

activity because the station was moved 2,300 feet (700 

m) farther north of the mine, the peaks of seismic 

activity during late July and August, mid-September, 

and mid-October 1974 again coincided with periods of 

mining above and around a large block of coal in the 

"B" bed when bumps and roof falls were very intense. 

In fact, the hazard was so severe that mining was 

stopped for most of the month of August, and mining 

was permanently terminated in two entries above the 

large block of coal (fig. 18). 

An inverse stress effect-the transmission of 

stresses from the "C" to the old "B" mine 

workings-was noted recently in one of the old mine 

entries in the "B" bed that is used for air ventilation in 

the "B" mine. Numerous roof falls occurred beneath a 

coal barrier adjacent to the "C" workings and 

threatened mine ventilation in the "B" bed. Abutment 

stresses in this barrier, caused by recent mining in the 

"C" bed, are apparently being transmitted to the 

underlying "B" mine workings where they are causing 

roof falls. This interaction of stresses between mine 

levels suggests that beds separated by a thin (perhaps 

50 feet (15.2 m) or less) stratigraphic interval perhaps 

should be mined simultaneously by uniform 

procedures, having retreat lines in the upper bed 

slightly ahead of those in the lower bed and making 

sure that all pillars in one bed are vertically alined 

with pillars in the other bed. 

High concentrations of compressive stress, such as 

those present beneath coal barriers, could be 

transmitted downward, perhaps 100 feet (30.5 m) or 

more, even if the coal beds were mined in sequence 

from top to bottom. Ideally, therefore, the barriers 

between panels, if necessary for reasons of mine 

safety, should be thin enough-perhaps 20 -25 feet 

(6.1-7.6 m) or less-to yield, and thereby reduce the 

stress concentration and ensuing subjacent or superjacent transmission of stresses to a safe level. The 

other alternative is to design for uniform partial 

recovery and thereby diffuse rather than concentrate 

overburden stresses. Stemple (1956, p. 45- 50), in his 

review of multiple-bed coal mining in Virginia, West 

Virginia, and neighboring States, found that roof falls 

(fig. 19B), bumps, and squeezes (fig. 22) often forced 

abandonment of coal reserves when mine planners 

attempted greater recovery than could be achieved 

under a uniform partial-extraction plan. 

Bumps in the Kenilworth and Castle Gate mines in 

central Utah were eliminated when a uniform mining 

pattern was substituted for pillar extraction, resulting 

in greater coal recovery with greater mining safety 

according to J. Paul Storrs (oral commun., 1974). 

Many problems faced in multiple-bed coal mining, 

therefore, are related to the processes of subsidence. 

The best remedy for these problems is to design for 

uniform extraction. If geologic conditions or thick and 

variable overburden preclude complete coal extraction, sequential partial uniform extraction from top to 

bottom, with columnized support pillars, might yield 

the maximum of a given coal reserve with a maximum 

safety factor. However, Stemple (1956, p. 51) 

reported that, according to mining men he 

interviewed, in areas where two coal beds are 

separated by a very thin stratigraphic interval 

(generally 25-50 feet (7 .6 -15.2 m), depending on 

strength of strata), concurrent partial uniform 

extraction was thought to be most successful. 

Processes of subsidence can be affected both 

directly and indirectly by the presence of active 

natural stresses. The geometry of compression arches 

might be altered by natural stresses if they add to or 

subtract from the stress field of the arches. For 

example, an eastward-trending stress field, disclosed 

by mine-deformation and seismic studies, is locally 

active in the Geneva mine area. This stress field has 

the effect of prestressing the strata in that direction 

and could reduce the height-to-width ratio of the 

eastward-trending compression arch shown in figure 

8. The height-to-width ratio seems small compared to 

that measured in the Somerset area. The greater 

strength of the strata in the Geneva mine area would, 

of course, account for some of the difference in 

geometry. 
30 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

Subsidence can be indirectly influenced by active 

natural stresses if the stresses cause numerous roof 

falls and bumps. These stresses could force 

abandonment of mine areas before mining is 

completed in a uniform manner, which in turn could 

produce greater subsidence damage because of 

uneven mine geometry. Preliminary studies indicate 

that stresses are still active in the Somerset district 

near channel sandstones that are locally common. The 

stresses and attendant fractures apparently are 

caused by flexure of strata above and below the 

channel sandstones as a result of differential 

compaction. The roof-fall hazard normally is greater 

near these channel deposits and can lead to premature 

abandonment of a mine area before mining is 

completed, resulting in uneven mine geometry and 

high concentration of stresses produced by subsidence 

processes. 

The processes of subsidence, roof falls, and bumps 

are thus interlocked in an often irreversible set of 

causes and effects. If roof falls or bumps are prevalent 

in certain areas of a mine because of physical 

conditions or poor mining practices, uneven extraction 

procedures often follow. This produces greater 

subsidence damage in the nearby strata, which, in 

turn, can create anomalous stress problems in other 

coal beds or can produce stress problems in adjacent 

mine areas, causing premature abandonment of an 

area that otherwise might have been uniformly and 

efficiently mined with minimal hazard to life and 

property. A thorough knowledge of geologic 

conditions during the mine-planning stage, derived 

from detailed drilling studies, surface mapping, and 

perhaps selected test mine entries, will help identify 

problem areas before mining starts so that these areas 

can be mined more safely and efficiently and with 

minimal adverse effects on the environment, on 

nearby coal beds or other deposits, or on the bed to be 

mined. 

CASE STUDY OF THE RELATIONSIDP BETWEEN A MINE 

AND THE PHYSICAL ENVIRONMENT 

Subsidence often sets in motion a chain of events 

that can significantly affect the mining activity as well 

as the physical environment. Not only can coal 

production and mine safety be threatened, but also 

adjudicated surface and underground water rights can 

be affected. Of course, these problems are multiplied 

and are magnified if the surface area above the coal 

mines supports dense housing or heavy industry. 

An example of how underground coal mining can 

disrupt the surface and subsurface environment is 

recorded in the history of the Oliver No. 2 mine, which 

FIGURE 19. (facing page)-Stress effects from multiple-bed coal C> 

mining, Somerset district, Colorado. In A through C, the lower 

bed of coal was mined first; and in D through F, the upper bed 

was mined first. 

A, Damage to roof and rib in the "E" coal bed caused by mining 

"D" coal bed, 150 feet (45.8 m) below "E" bed, 25 years prior to 

mining "E" bed. Roof bolts and landing mats provided 

adequate support until the limit of mining in underlying "D" 

coal bed was traversed; timbering (background) was then 

required to prevent roof falls and possible coal bumps. 

B, Sites of roof falls and generally unstable roof conditions in a 

crosscut in the "C" bed that is located 45 feet (13.7 m) above 

workings in the "B" bed that were mined about 40 years before 

mining the "C" bed. 

C, Unstable roof conditions in the "C" bed along a slope haulageway above workings in the "B" bed that were mined about 40 

years before mining the "C" bed. 

D, Damage to right rib and roof in the "B" coal bed beneath about 

1,200 feet (366 m) of overburden caused by local stresses 

beneath an isolated pillar 60 feet by 60 feet (18.3X18.3 m) left in 

the "C" coal bed about 50 feet (15m) above and collimated with 

the pillars in the "B" bed. Note the dramatic difference between the rib and roof conditions on the right side of the mine 

opening beneath the isolated pillar in the "C" bed and those of 

the left rib where the coal was mined out in the "C" bed before 

"B" bed was mined. 

E, High stress concentrations in an isolated pillar 60 feet by 60 

feet (18.3X18.3 m) in the "B" coal bed beneath about 1,300 feet 

(397 m) of overburden. The pillar ribs in the left and right foreground show no signs of stress because the coal was 

completely mined out in "C" bed before "B" bed was mined, 

whereas the isolated pillar is yielding to stresses from an 

isolated pillar in "C" bed directly above the yielding pillar in 

"B" bed, even with massive props surrounding it. The author 

monitored about 100 coal bumps during a 3-hour period, as the 

yielding pillar was split by a mining machine. 

F, View of ideal rib and roof conditions in the "B" coal bed beneath about 1,300 feet (397m) of overburden where the coal 

was mined out in the overlying "C" bed before "B" bed was 

mined. The overburden stress was reduced to essentially the 

50 feet (15 m) of strata separating the two coal beds. 

is located in the "D" coal beds south of the North Fork 

Gunnison River, about 2 miles (3.2 km) east of the 

Somerset mine (fig. 20). The mine comprises a system 

both of raises on a 5- to 7 -percent grade that roughly 

parallels the direction of the dip of the coal bed and of 

entries that parallel the strike of the coal bed. The 

mine portals are located at the outcrop of a "D" coal 

bed overlooking the North Fork Gunnison River. The 

surface area near the mine is sparsely settled and 

supports summer grazing for cattle and horses. 

The Oliver No. 2 mine, which was begun in the 

1930's and provided coal for the Oliver power plant, 

was closed in October 1953 after methane gas and 

water were encountered in quantities that were too 

costly to control. A four-entry raise was driven 

southward beneath overburden which increased in 

thickness from a few tens of feet near the outcrop to 

1,250 feet (381 m) beneath a high ridge and then 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 31 

decreased within a distance of about 1,500 feet (457 m) 

to 325 feet (99 m) beneath an east-trending, joint-controlled side canyon of Sylvester Gulch (fig. 20). At this 

point a four-entry system was driven eastward (7 

East) directly beneath, and parallel to, the east side 

canyon. The 7 East panel was driven about 300 feet 

(91.4 m), then work began on driving 6 East. Suddenly 

large volumes of water and methane began to gush 

from the floor of the top entry of 6 East, forcing the 

evacuation and closure of the mine. 

The mine was sealed and has remained closed. The 

Oliver electric power plant also was subsequently 

closed, although other sources of coal supplied it for a 
32 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

FIGURE 20.-Composite aerial photograph and underground map 

of the Oliver No. 2 mine area, Colorado. This mine was closed in 

October 1953 because of a sudden intrusion of water and methane 

gas. The scrub oak and other woody plants that originally grew 

in the bare spot above the exhaust portal were killed when the 

few years. After the mine was sealed, the methane 

leaked out of the mine to the surface through fractures 

in the overburden in sufficient quantity to be detected 

with a miner's lamp (C. L. Heiner, oral commun., 

1974). The methane killed the scrub oak and all other 

woody plants in the area (fig. 21), leaving only the 

grasses unaffected. According to Garner (1974), the 

methane leaked through fractures to the surface after the portals 

were sealed. The beaver pond apparently dried up when 

subsidence fractures above 7 East tapped the ponds and the 

nearby spring source and diverted the water underground. 

Photograph by R. B. Taylor, U.S. Geological Survey, 1973. 

presence of methane in soil provides an environment 

in which certain bacteria utilize the methane and 

produce hydrogen sulfide and nitrous oxide in their 

life processes; the presence of the hydrogen sulfide 

and perhaps nitrous oxide disrupts root transpiration 

of woody plants and ultimately may kill the plants. 

Shortly after the mine was closed, the water in the 
CURRENT STUDIES OF SUBSIDENCE PROCESSES 33 

FIGURE 21.-Southward view of the rugged topography above the Oliver No.2 mine, Colorado. The exhaust portal (P, in left foreground) 

was sealed inthe late 1950's. The scrub oak and other woody plants that originally grew in bare spot above exhaust portal apparently 

were killed when methane gas leaked through fractures from the mine to the surface. 

east-side canyon, which was fed by springs above the 

7 East mine workings, ceased to flow (W. A. Bear, 

oral commun., 1972). This water, to which the Bear 

family had the rights, has not flowed with any 

regularity since the mine was closed. Investigation by 

the author revealed the presence of large dry beaver 

ponds, with dams as much as 6 feet (1.8 m) high, above 

the 7 East entries (fig. 20). These dams indicate that a 

perennial water supply was once available from 

springs nearby but that the springs have been dry or 

only flowed intermittently for many years. Although 

no surface cracks were positively identified in 1972, 
34 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

they could have been covered by erosion-deposition, 

mass wasting, and revegetation in only a few years. 

Local scarps in soil and colluvium noted in the area 

might be erosional remnants of subsidence scarps. 

Field evidence indicates that, after the mine was 

closed, subsidence fractures formed in the strata 

above the 7 East entries and faces of the raise and 

eventually migrated to the surface and drained the 

beaver ponds in the east-side canyon (fig. 20). 

Although coal pillars were not extracted in most of the 

Oliver No. 2 mine, the vertical and downdip 

component of stress produced by the weight of the 

high ridge above the mine workings, together with the 

reduction of frictional resistance at the top of the coal 

bed owing to mine development, apparently was 

sufficient to produce tension fractures along local 

joints in the thin overburden beneath, and parallel to, 

the side canyon. The fractures tapped the beaver 

ponds and nearby springs and diverted the surface 

flow underground. The mining reports indicate that 

the water- and methane-filled fracture encountered in 

6 East might be a tension fracture produced by 

decollement-type movement of the strata above a "D" 

coal bed prior to mining. 

The "B" and "C" coal beds are present beneath these 

mine workings, and at least one "E" bed of minable 

thickness occurs above the mine workings. Subsidence 

may have significantly reduced the minable reserves 

represented by these beds because mines in the "B" 

and "C" beds may be threatened by intrusion of 

methane and water from the old "D" workings 

through subsidence cracks and the "E" bed(s) may be 

locally transected by subsidence fractures and stress 

concentrations above solid-coal boundaries. In 

summary, then, not only were wildlife, vegetation, 

and surface-water rights affected by surface-water 

diversion and methane leakage via subsidence 

fractures, but potential production from subjacent and 

superjacent coal reserves probably also was reduced 

considerably. 

SUMMARY 

Subsidence studies in Utah and Colorado show that 

the mode of subsidence depends upon: (1) the 

geometry of mine workings, (2) the lithology, 

structure, and thickness of the overburden, (3) 

direction of dip of the coal bed relative to its outcrop, 

and (4) proximity of mine workings to coal outcrop, 

unless an adequate coal barrier is left to support the 

overburden strata. In multiple-bed coal mining, the 

mining activities in one bed can cause stress problems 

in another bed during current or subsequent mining. 

Subsidence parameters, such as break line, limit 

angle, configuration of compression arch, and ratio of 

surface subsidence to coal extraction thickness, and 

how they are controlled by mine geometry and 

geology in two areas of Colorado and Utah are 

described in the following list: 

1. A nearly vertical break line is caused by cantilever 

failure in strong overburden above a wide 

property barrier in the Geneva mine area and 

illustrates operation of the law of the normal or 

the law of the vertical in gently dipping strata 

that underlie overburden comprising strong, 

thick, jointed sandstones and interbedded 

mudstones (fig. 8). 

2. Nearly vertical break lines also occurred in weak 

strata above a thin coal barrier between the 8th 

and 9th Left mining panels in 3d South, 

Somerset mine and likewise illustrate the law of 

the normal for tensile failure produced by positive flexure of strata above thin barriers separating adjacent mine cavities (figs. 3, 4, 17). 

3. A break line, with a negative angle of 78° ( (3 = 

-78°), was observed in weak overburden above 

the solid-coal barrier north of 6th Left, Somerset 

mine only 4 months after mining was completed 

beneath the ruptured area. Strength of overburden appears to govern the rate of fracturing-a -73° (fi= -73°) break line was inferred 

in strong overburden above the Geneva mine 

workings 6- 12 years after mining was completed (figs. 8, 17); in this case, however, pillars 

were not completely removed. 

4. The limit angle in weak to moderately strong overburden strata, which is 650-900 feet (198- 274 

m) thick (fig. 16), measured 69° -70° relative to 

the position of the room-and-pillar retreat line in 

the 6th Left mining panel in the Somerset mine. 

The limit angle appears to steepen to 75° or 

more in moderately strong overburden 900 -

1,000 feet (274- 305 m) thick, although bench 

mark control is not good beyond the 900-foot 

(274m) overburden level (fig. 16). 

5. The overburden above a 300-foot- (91.4 m) wide 

coal barrier north of 6th Left in the Someset 

mine was uplifted slightly through November 

30, 1973, probably in response to overburden 

subsidence into the 6th Left mine void (fig. 17). 

If the uplifted area is included as part of the surface area affected by subsidence, the limit angle 

ranges from 35° to 65° in weak overburden 

550 -700 feet (168- 214 m) thick; if the limit 

angle is measured relative to the subsiding zone, 

it ranges from 82° to 88°. A slight uniform subsidence occurred above the barrier in December, 
SUMMARY 35 

thus precluding further determination of limit 

angle and suggesting that the limit angle is only 

a transient parameter above pillars between two 

mining panels. 

6. Maximum subsidence to date (Jan. 1, 1974) in the 

B row of subsidence benchmarks above the 3d 

South area was measured above the barrier between 7th and 8th Left rather than above a 

mined-out area (fig. 17). This, in addition to subsidence-crack data, indicates that compression 

arches, perhaps about 50 feet (15 m) thick at the 

crest, are bridging the strata to the barriers and 

that the subsidence measured above roughly the 

500-foot (152.4 m) overburden-thickness level 

actually results from yield or collapse of barriers 

pillars rather than from subsidence of strata into 

mine voids. The depth of the inverted arches 

that are believed to occur beneath the mine 

voids is not known but should be considerably 

less than that of the arches above the voids. The 

height-to-width ratio for the arches in the overburden appears to be about 1: 1. The height of 

the arch parallel to the long direction of mining 

panels (fig. 16) is controlled by the position of 

the arch spanning the barrier pillars oriented 

perpendicular to the long direction of the panels 

(fig. 17). This produces abutment stresses on the 

pillar retreat line as well as on the barriers. A 

squeeze, such as the one shown ih figure 22, can 

occur if rooms are developed too far ahead of 

extraction. 

7. Stresses produced by flexure of strata within the 

destressed zone of the compression arches are 

oriented perpendicular and parallel to the long 

direction of the mining panels, as are the compression arches above and below the mine 

workings. Both flexure and caving of strata into 

mine workings or mined-out areas tend to transfer an increasing amount of the overburden 

stresses from solid coal boundaries and barrier 

pillars back into the mine workings or mined -out 

areas as the arches migrate higher into the overburden. This may explain why miners commonly 

encounter reduced stress levels on mining faces 

and along pillar lines after caving occurs in 

mined-out areas behind the pillar lines. 

8. In multiple-bed coal mining, individual coal beds 

normally can be extracted more safely and completely if the beds are mined from top to bottom, 

provided the coal is uniformly extracted so that 

stresses cannot concentrate on isolated pillars 

and be thereby transmitted to underlying coal 

beds. However, it may be best to mine two beds 

concurrently using uniform methods if the beds 

are separated by only a thin (generally 25 - 50 ft 

(7.6- 15.2 m)) stratigraphic interval. It might 

also be best to reverse the sequence and mine 

coal beds uniformly from bottom to top in areas 

where the methane or water is abundant in 

nearby strata. If overburden rocks, for example, 

contained methane and water, large quantities 

of methane or water could accumulate in minedout areas; if these accumulations were above 

current mine workings, they might be tapped 

by subsidence fractures and perhaps pose a 

greater threat to mine safety and coal reserves 

than would reversal of the recommended mining 

sequence. 

9. The direction of dip of a coal bed relative to its 

location of outcrop can have a significant effect 

on mining safety and efficiency. If a coal bed is 

mined updip from where it crops out, even if the 

bedrock dips only a few degrees, a downdip 

decollement-type movement of the mine overburden is possible because the downdip component of gravitational force is unrestrained at 

the outcrop and the frictional resistance at the 

top of the coal bed is reduced due to mining. This 

could produce a severe hazard to life, coal 

reserves, and surface environment. For example, if the bedrock dipped westward instead 

of eastward at the same angle in the Geneva 

mine area (fig. 8), the entire block of overburden 

strata west of the fractures above the property 

barrier would be laterally unrestrained, and it 

could slide above mine workings, crushing 

pillars, men, and equipment beneath it. Needless to say, the effects on hydrologic regimen 

and on the surface environment also would be 

drastic. 

10. The subsidence ratio for the ground surface above 

the 6th Left mining panel (table 1, figs. 14, 16) is 

Smax/t=3.1ft/10ft =0.31, 

where 

S max = maximum surface subsidence, and 

t = mining height. 

Comparing subsidence ratio to the ratio of panel 

width (W) and average overburden depth (D) 

above the mined-out area (fig. 16) produces the 

following result: 

w /D =450ft/ 950 ft~410 ft' 

=450ft/680ft= 0.66. 

The subsidence ratio is considerably lower than 

that determined by Wardell (1971, p. 206; fig. 5) 

for the same WID ratio in many longwall mines 
36 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

of the United Kingdom. This could be because 

longwall mining induces greater surface subsidence, because a compression arch is bridging 

the 6th Left mining panel, or because the overburden strata are stronger at Somerset than in 

the United Kingdom. Complete room-and-pillar 

extraction should induce at least as much subsidence as complete longwall extraction under 

similar conditions. Weak overburden strata and 

interbedded coal in the Somerset district probably is as weak as those of the United Kingdom. 

Therefore, the low subsidence ratio further 

indicates that compression arches bridge the 

overburden strata across mining panels. Should 

the compression arches fail with time, the subsidence ratio may approach Wardell's (1971) 

curve. 

SUGGESTIONS TO MINE PLANNERS 

Previous and current subsidence studies indicate 

that a distinction must be made between subsidence 

and subsidence damage because most subsidence 

damage to the overburden and surface results from 

horizontal strain produced by differential vertical 

settlement of the mine overburden. Under geologic 

and mining conditions, such as those in the Geneva 

and Somerset mining areas, subsidence fractures can 

propagate through many hundreds of feet of strata. In 

order to estimate the probable effects of subsidence in 

areas of underground coal mining, mine planners 

should take into account such factors as (1) 

overburden thickness, lithology, and structure in the 

strata above and below the mine workings; (2) the 

geometry of mine workings; (3) the coal bed thickness; 

(4) the number of minable coal beds present; (5) the 

rate of mining; and (6) the natural and manmade 

environment at the surface. The first four factors vary 

with locality, and the last two factors often vary with 

time. The present and future conditions of an area, 

together with present and anticipated energy needs, 

determine the proper blend of extraction efficiency 

and preservation of the surface environment. 

Uniform coal extraction tends to cause the 

overburden to settle uniformly and, therefore 

minimizes subsidence damage. If, in addition, the coal 

bed could be completely extracted, this method would 

produce maximum yields. However, if complete 

extraction is planned by room-and-pillar methods, it 

may be impossible to mine uniformly during the 

development and extraction phases if the overburden 

is deep and stresses are high. Under these conditions, 

retreat lines must be close to solid coal or else a 

squeeze may develop (fig. 22), which would cause 

" 8" coal 

bed 

B 

0 

0 

50 FEET 

15METRES 

FIGURE 22.- Coal mine squeeze in "B" coal bed, Somerset district, 

Colorado. A, A mine opening that was closed by coal flowage in 

response to abutment stresses ahead of a pillar extraction area. 

The squeeze was caused by driving too many workings ahead of 

the area of pillar extraction. B, Cross section of mine workings 

and adjacent pillars before squeeze shown in photograph. C, 

Cross section of same area after squeeze. 

extensive loss of coal reserves. Therefore, in many 

instances uniform geometry can be attained only after 

mining is completed, provided that necessary coal 

barriers are thin enough to crush out so that stresses 

are relieved. The other alternative is to design for a 

uniform partial extraction procedure near solid coal, 

so that mining stresses are diffused and the possibility 

of a squeeze is minimized. The mine geometry, 
SUGGESTIONS TO MINE PLANNERS 37 

therefore, is a very important factor to consider when 

planning a new mining operation. 

In shallow overburden, where stresses are low, 

certain compromises might be made in designing mine 

geometry that would preserve the integrity of the 

overburden and surface without increasing the stress 

problems underground, whereas such compromises 

might prove hazardous and nonproductive where 

overburden ranges in thickness from 500 to more than 

1,000 feet (152 -305m). 

For example, room-and-pillar mine workings might 

be developed in a single coal bed beneath shallow 

overburden; then the pillar ribs could be uniformly 

sheared off in a multistage extraction sequence until a 

large percentage of the coal is extracted without 

damage to the overburden or surface. This is done 

locally in Japan, in areas where living space and 

energy are in an equally short supply. More than 90 

percent of a coal bed was extracted by this method. 

The harmonic method of extraction also is used locally 

in multiple-bed mining (Kaneshige, 1971; fig. 6). 

Beneath deep overburden, however, the bump and 

roof-fall problems could be very severe if more than 

two or three rooms were developed ahead of 

extraction or if harmonic mining procedures deviated 

from a rigid time and tonnage schedule; consequently, 

uneven mining procedures locally may be inevitable 

during the extraction phase unless partial extraction 

is planned. 

A uniform partial extraction procedure with a 

secondary recovery procedure was recently successfully implemented in the Bear mine, Somerset district, 

according to W. A. Bear, Bear Coal Co. (oral 

commun., 1974). Mining officials of the U.S. Geological 

Survey initially proposed that rooms and crosscuts be 

driven 16 feet (4.9 m) wide on 50-foot (15.2 m) and 

60-foot (18.3 m) centers, respectively, in a block of coal 

in the "C" coal bed in order to minimize stress effects 

to the underlying "B" coal bed, which was controlled 

by another company. Scientific counsel for the 

company controlling the "B" bed proposed an 

alternative procedure wherein rooms 16 feet (4.9 m) 

wide would be developed on 84-foot (25.6 m) centers to 

prohibit the possibility of a squeeze. Then a 10-foot 

(3.05 m) slab of coal would be cut off two adjacent 

sides of each mine pillar on the retreat, thereby 

widening all mine openings to 26 feet (7 .93 m) and, at 

the same time, reducing all pillars to 58 by 58 feet 

(17.7 x 17.7 m) (C. T. Holland, oral commun., 1971). 

This procedure, which was subsequently approved by 

mining officials of the U.S. Geological Survey, leaves 

uniform support pillars that diffuse overburden 

stresses and minimize stress effects to another coal 

bed, while at the same time yielding a reasonably high 

percentage (50 percent) for multiple-bed coal mining. 

A new modified longwall mining procedure is under 

development by Eastern Associated Coal Corp. (R. W. 

Thomas, vice president in charge of mine planning, 

oral commun., 1973) in one of their mines where 

changing geologic conditions make conventional 

longwall mining unsuitable. Basically, the procedure 

involves using a continuous miner to cut a longwall 

face 150-300 feet (45.7- 91.5 m) long beneath 

self -advancing, cantilevered, hydraulic roof -support 

machines. This method takes advantage of both the 

safety, economy, and productivity of the longwall 

method and the ability of the continuous mining 

machine to adapt to varying coal thicknesses and other 

changing geologic conditions. Using this procedure, 

the chain pillars probably can be mined in sequence 

with the longwall face advance, making it possible to 

produce a uniform final mine geometry. Of course, a 

decision must be made as to whether complete 

extraction is feasible and is in accordance with 

applicable mining laws in a particular mining area 

before this procedure is implemented. 

Subsidence rupture can be damaging to mining 

operations as well as to the overburden and surface 

environment. In some cases, particularly in sparsely 

settled areas, coal mines may be threatened more than 

the surface environment by subsidence rupture 

because of the threat of the sudden intrusion of 

methane gas or large volumes of water. This is 

particularly true in geologic environments where 

water and methane are abundant. Mining companies 

should, therefore, balance production with ongoing 

research on subsidence and other types of mine 

deformation for their own benefit as well as in the 

interests of conserving coal and protecting the 

environment. Accurate structural and lithologic maps 

with overlays showing mine workings and overburden 

configuration relative to the coal bed or beds will 

prove very useful in planning new mines or expanding 

existing ones. Periodic subsidence measurements 

designed to determine subsidence parameters, such as 

limit angle, break angle, and transient and stable 

compression-arch configurations, can yield subsidence 

parameters, which in turn can help identify areas of 

potential stress concentrations and attendant 

deformations that might occur near or within the mine 

workings. This information, in turn, could produce 

tangible returns in extraction efficiency and mine 

safety. 

It is evident that mine safety and efficiency and the 

effects of mining on the physical environment are 

interlocked with subsidence and other facets of mine 

deformation, such as roof falls, bumps, and squeezes. 

It also follows that mine design should be based not 
38 ENGINEERING FACTORS, COAL MINE SUBSIDENCE, UTAH AND COLORADO 

only on expected subsidence damage above a 

retreating room-and-pillar line or longwall face, as 

Ropski and Lama (1973, p. 109- 118) recognized, but 

also on subsidence effects above barriers, solid-coal 

mine boundaries, and any other uneven geometry 

called for in the mining plans. In the case of multiplebed coal mining, the overall success, in terms of mine 

safety, coal extraction ratios, and environmental protection, must be measured by the overall success of 

the multiple-bed mining operation, rather than by the 

overall successes of mining one bed. It also is evident 

that knowledge of overburden thickness, geology, and 

the environment are particularly critical to planning a 

multiple-bed mining operation that will produce a 

maximum amount of coal with a minimum risk to life 

and property. 

The sequence of mining coal beds also is very 

important because of the possibility of causing stress 

concentrations. Stresses produced in the strata by 

mining a coal bed will affect any coal beds present in 

overburden and, to a lesser but important extent, in 

the subjacent strata. A sequence of mining beds from 

top to bottom normally is safer and more efficient, 

paricularly if the final geometry is uniform so that 

stresses are not concentrated in isolated pillars and 

barriers and manifested downward to underlying coal 

beds; however, if the stratigraphic interval between 

two coal beds is generally 25- 50 feet (7 .62- 15.24 m) 

thick, concurrent, partial, uniform extraction of both 

beds may be the safest and most efficient method. 

Also, a sequence of mining from bottom to top might 

be less hazardous and more productive in certain areas 

where water or methane is abundant because mine 

voids in upper beds may store large amounts of water 

or methane that could later be a hazard to life and 

property if tapped by subsidence fractures induced by 

mining in lower beds. In short, the safest and most 

efficient method and sequence of mining depends on 

all the geologic and environmental factors present in 

the area. 

CONCLUSIONS 

Analysis of subsidence processes and their effects 

upon underground coal mining lends a new 

perspective to the effects of mining on safety, 

conservation of coal resources, and protection of the 

ground surface above mine workings. The studies 

show that the mine workings, the strata above and 

beneath the mine workings, and the ground surface 

form a delicately balanced system of often irreversible 

causes and effects that can be seriously affected by 

subsidence. The excavation of underground mine 

workings can set in motion an interrelated chain of 

environmental and mining problems that can not only 

affect the manmade and natural elements of the 

environment but also threaten the mining operation. 

The studies clearly reveal that the geology of the coal 

beds and overburden strata in a proposed mining area 

should be accurately known in advance of mine 

planning so that the mine can be designed in harmony 

with the physical surroundings. Such an approach is 

highly desirable in planning mines in one coal bed, but 

it is vital in multiple-bed mine planning if hazards to 

life, property, and the environment are to be reduced 

to a minimum. 

One of the key aspects of mine planning is to weigh 

the various mining, geologic, and environmental 

factors in order to insure that the balance between 

coal recovery and subsidence effects is responsive to 

current and future needs for both coal resources and 

protection of the environment in the area. It should be 

recognized that a timelag of months or even years 

probably will occur between the following events: (1) 

recognizing the need for subsidence research, (2) 

implementing and performing subsidence research, 

and (3) applying the results of research studies to 

subsidence control and formulating subsidence-control 

legislation. Subsidence-control regulations should be 

flexible enough to change as national needs change. 

Historically, mining legislation has been inflexible 

to changing demands for coal production and 

environmental protection. For example, if the land 

surfaces were more valuable than underlying coal 

deposits at the time subsidence control legislation was 

enacted, the requirements for preservation of the land 

surface normally would be more stringent than 

requirements for protecting underlying coal resources. It is clear, from recent energy shortages, that 

provisions for flexibility should be written into future 

mining regulations so that we can maintain a judicious 

blend of coal-extraction efficiency, mine safety, and 

environmental protection, as the changing times 

require. 

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