EFFECTS OF

COAL MINE SUBSIDENCE IN THE

SHERIDAN, WYOMING, AREA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1164
COVER.—Effects of past and present coal mining in the western Powder River Basin, as discussed in this report.
EFFECTS OF COAL MINE SUBSIDENCE 

IN THE SHERIDAN, WYOMING, AREA
:>j^*»

FRONTISPIECE.—Aerial obh'que views showing surface subsidence effects above abandoned coal mines 10-15 km 

north of .Sheridan, Wyo. A, Aerial obh'que view of subsidence depressions and pits above the Old Monarch mine 

in operation from 1904 to 1921 (May 1978). Rectangular 

depressions, some of which are bounded by pits or locally 

include pits, are evident at right. Some of the pits are 

sealed at the bottom and provide sufficient moisture to 

support trees (foreground). Overburden thickness is 

estimated to be approximately 10-15 m. The depressions 

occur where much of the coal is removed and the remaining 

coal cannot support the weight of the overburden. Pits at 

the margins of the depressions commonly are caused by 

piping failure and the local flow of surface water to

underground mines via subsidence cracks. B, Subsidence 

pits and troughs, above the Dietz coal mines (October 

1976). The Dietz mines were operated from the 1890's to 

the 1920's. Coal was mined from three different beds. The 

mine workings, which were abandoned in the early 1920's, 

are locally superimposed. The overburden comprises weak 

claystones, shales, and local thin, soft sandstones. Its 

thickness is estimated to range from about 5 m along the 

margins of the subsidence area (right side of photograph) 

to as much as 45 m (Barton, 1906, p. 111). Pits and troughs 

located in draws draining into Goose Creek (near 

background) disrupt or divert surface water to old mine 

workings. Bighorn Mountains are in far background.
Effects of Coal Mine Subsidence 

In the Sheridan, Wyoming, Area

By C. RICHARD DUNRUD and FRANK W. OSTERWALD

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1164

A summary of geology, subsidence, and 

other effects of past and present mining 

as related to the environment, coal 

resource management, and land use

Supplemental information is included 

from Beulah, N. Dak., Decker, Mont., 

Somerset, Colo., and Raton, N. Mex.

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1 980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication data

Dunrud, C. Richard.

Effects of coal mine subsidence in the Sheridan, Wyoming, area.

(Geological Survey Professional Paper 1164)

Bibliography: p. 48

Supt. of Docs, no.: 119.16:1164

1. Coal mines and mining—Environmental aspects—Wyoming—Sheridan region. 2. Mine subsidences—WyomingSheridan region. I. Osterwald, Frank W., joint author. II. Title. III. Series: United States Geological Survey 

Professional Paper 1164.

TN805.W8D86 557.3s [333.73] 79-607120

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

Washington, D.C. 20402 

Stock Number 024-001-032881
CONTENTS

———————————— Page

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------—•—-------»-"——----—-—-—----"--——--——--""•-"-•"••"-""--""•"""--""•"•••""•"" O

Geology and physiography ——-—--———....——...........————.——....—............—. 8

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OU.ri3.C6 SUDSldefiCe eiieCtS ""---——----——-•———-•—-—-——«—————-...-——•............... j_ f

Sheridan, Wyoming, area --——-——-—-———————.——.—.......—......—...—.... 17

l) i 1^1 ftT/'Yf"!' vl Ull Vi T~^ J_ydfi.<JLdf a Ir/vf- a ctl a 1*00 cd ......._............_..._......_................................................... -——• ».--...-... ...... - .... f? fr^j f\

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061 sniic dCtivity TMTM-TM-TM"———•—.——.—•..•...--.—«.-...—.-«.--•.—.—............—.....*.——...——— ^ j^

Environmental consequences of coal mining --—-————-—-———.——......——.— 41

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oiiixiixisry 3.110. conclusions --——-"-———"—•———————————————————.—«.-..—.... ^ ^

ILLUSTRATIONS

FRONTISPIECE. Aerial oblique views showing surface subsidence effects above abandoned coal mines.

FIGURE 1. Map of the Powder River Basin and adjacent areas ——————————————————————————————— 2

2. Photographs showing present and past mining in the Big Horn mine area ——————————————————— 3

3. Westward aerial oblique photograph of the west Decker strip mine near Decker, Mont. —-——-—— —— — ... — 4

4. Map showing the locations of abandoned underground mines and surface mines in the Sheridan, Wyo.,

5. Composite geologic section and geotechnical properties of the bedrock and coal in the Tongue River

Member of the Fort Union Formation from two core drill holes near the Big Horn coal mine ————— 10 

6-13. Photographs:

6. Westward aerial oblique view of the highwall at the abandoned Plachek surface mine near the Big

T-I/YI*n j. A Will TY^l'MO J.11111C .... .. __ 

-... .. ... ... .... 

.... ....... ........ ._ 1 .HJQ

7. Southward aerial oblique view of subsidence depressions, pits, and cracks above the abandoned roomdllU. Q Tl /I .T\1 ^Jllldi 1 1 0 1" ^\.L/lllc A/*TY^O TY^lTlO JJ.JJ.llc ————————————————— ....... . ....... ... -—--————— ... .... — — —.——-•—— ......... .......... -——...——— _ ................................ ........... ......... 1 ^f^A

8. Southeastward aerial view showing the effects of past and present coal mining along the Tongue

9. Ground views showing tensile and compressive features caused by subsidence ———————————— 18

10. Three holes in soil above subsidence crack in bedrock ————————————————————— — - — — — — — 20

11. Recent subsidence pit above the Old Monarch mine showing connecting underground cavity -- — — - — 21

12. Oblique aerial view of subsidence features located north of Beulah, N. Dak. ———————————————— 22

13. Eastward aerial view showing the effects of lignite mining near Beulah, N. Dak. — — —— — — — — — — - —— 23

14. Subsidence processes caused by mining different amounts of coal beneath overburden of different

dllU. OTifi f~Vii/^lrTioccoc Lrllll/lxllCOoCO ................................... •-————-—— ..................................... ......................_.............«......«...................................•......... ............. ....... ..... .. O^ ^4^
VI CONTENTS

Page

FIGURE 15. Maximum subsidence for selected coal mining areas in the United Kingdom and in the Western United

States —_•—__TM_••__•__~—__.----.._—————————..—.————.......—..—.—.......____............... 26

16-20. Photographs:

16. Eastward aerial oblique view of the surface effects of an underground coal mine fire in the northern

D£U*t OI t/llG A.CH16 H11I16 ------"----------------------------------•--»••---«----------«••———««««—•-«•«-««««—-«»•••—------—-«.- Q1

17. Northeastward aerial oblique view of the firepit located above the northern part of the Acme mine ——- 32

18. Southward views of firepit above the northern part of the Acme mine --————————-—-——-——-——- 34

19. Closeup views of the advancing fire front in the firepit ——-———-—————...——.............——.————.. 37

20. Firepit above the east-central part of the Acme mine ——-—-——————~—~~~~~~~~——.————— 33

21. Subsidence and fire above the northwest part of New Monarch mine —-—-——-——————-——-————— 39 

22-24. Photographs:

22. Effects of subsidence and fire above the New Monarch mine —-—-—————--———————-———.—— 40

23. Some pitfalls of fillinsr subsidence Dits -—--———————.————..—.———............................................................. 40

24. Fire in the southern part of the Acme mine ———-——~~~~~~~~——.................—————.——.——......... 44

25. Graph showing the daily number of small earth tremors at a seismometer station located near the firepit

METRIC-INCH-POUND EQUIVALENTS

Metric unit Multiply by To give inch-pound equivalent

Centimeter (cm) 0.39 Inch (in.)

Meter (m) 3.28 Feet (ft)

Kilometer (km) .62 Mile (mi)

Calories per gram (cal/g) 1.8 British thermal unit per pound (Btu/lb)

Meganewton per square 145 Pounds per square inch (psi)

meter (MN/m2) 

Grams per cubic 62.4 Pounds per cubic foot (lb/ft3)

centimeter (g/cm3)
EFFECTS OF COAL MINE SUBSIDENCE IN 

THE SHERIDAN, WYOMING, AREA

By C. RICHARD DUNRUD and FRANK W. OSTERWALD

ABSTRACT

Analyses of the surface effects of past underground coal 

mining in the Sheridan, Wyoming, area suggest that 

underground mining of strippable coal deposits may damage 

the environment more over long periods of time than would 

modern surface mining, provided proper restoration procedures are followed after surface mining. Subsidence 

depressions and pits are a continuing hazard to the environment and to man's activities in the Sheridan, Wyo., area 

above abandoned underground mines in weak overburden 

less than about 60 m thick and where the overburden is less 

than about 10-15 times the thickness of coal mined. In addition, fires commonly start by spontaneous ignition when 

water and air enter the abandoned mine workings via subsidence cracks and pits. The fires can then spread to unmined 

coal as they create more cavities, more subsidence, and more 

cracks and pits through which air can circulate.

In modern surface mining operations the total land surface 

underlain by minable coal is removed to expose the coal. The 

coal is removed, the overburden and topsoil are replaced, and 

the land is regraded and revegetated. The land, although 

disturbed, can be more easily restored and put back into use 

than can land underlain by abandoned underground mine 

workings in areas where the overburden is less than about 60 

m thick or less than about 10-15 times the thickness of coal 

mined. The resource recovery of modern surface mining commonly is much greater than that of underground mining procedures. Although present-day underground mining 

technology is advanced as compared to that of 25-80 years 

ago, subsidence resulting from underground mining of thick 

coal beds beneath overburden less than about 60 m thick can 

still cause greater damage to surface drainage, ground water, 

and vegetation than can properly designed surface mining 

operations.

This report discusses (1) the geology and surface and 

underground effects of former large-scale underground coal 

mining in a 50-km2 area 5-20 km north of Sheridan, Wyo., (2) 

a ground and aerial reconnaissance study of a 5-km2 coal 

mining area 8-10 km west of Sheridan, and (3) some environmental consequences and problems caused by coal mining.

INTRODUCTION

Thick coal deposits, representing hundreds of 

billions of tons of low-sulfur subbituminous 

coal, are present in the Powder River Basin of 

Wyoming and Montana (fig. 1). The coal occurs 

beneath overburden composed of weak bedrock 

and surficial deposits that range in thickness 

from a few meters to as much as 900 m. A controversy currently exists over the relative merits 

of mining strippable coal deposits by underground and surface methods. Many mining and 

land-use planners contend that the many billions 

of tons of strippable coal in the Powder River 

Basin can be mined more safely and much more 

completely by surface methods than by underground methods, particularly in areas where 

thick coal beds occur beneath soft bedrock and 

surficial material less than 60 m thick. On 

the other hand, environmental groups are concerned that surface mining might be more damaging to the environment than would underground mining.

Surface mining operations disturb the total 

land surface of the mining area during the mining 

cycle (figs. 2, 3). The topsoil and remaining overburden commonly are removed by earth-moving 

scrapers or by large draglines; the coal is then 

removed by draglines or power shovels, and 

trucks. In modern surface mining operations, in 

contrast to those of the past, the overburden is 

replaced and graded, the topsoil is replaced, and 

the land is revegetated (figs. 2, 3). In a few years 

or a few decades, depending on the climate and on
EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

108° 107° 106° 105° 104°

43

) 100 

I

200

I

300 k

I

I

100

EXPLANATION

Precambrian crystalline rocks

Precambrian metamorphic rocks--Dashes show 

dominant structure trend; Precambrian rocks 

in part of the Laramie, Owl Creek, and 

Hartville uplifts (no pattern) are undifferentiated

— Contact

High-angle fault--Rake teeth, where present, 

show direction of dip

200 MILES

Thrust fault- Saw teeth show direction 

of dip

Anticline 

Syncline

Structure contour, in feet, on top of 

the Dakota Sandstone -Datum is mean 

sea level. 1 ft=0.3048 m

Study area-Area shown in figure 4

FIGURE 1.—Map of the Powder River Basin and adjacent areas. Modified from the tectonic map of the United States (U.S. Geol. Survey

and Am. Assoc. Petroleum Geologists, 1961).
INTRODUCTION

FIGURE 2.—Present and past mining in the Big Horn mine 

area. A, Eastward oblique aerial view of the Big Horn surface mine (April 1976). Coal from two, and locally three, 

separate beds totaling 15-18 m in thickness is removed by 

power shovels and trucks after the overburden has been 

stripped by earth-moving scrapers. The highwall (near 

background), which is about 50 m high after coal is removed, ultimately will be as much as 75 m high in the active mining cycle. Note the variable thickness in rocks separating the coal beds. Geotechnical tests on core were 

made from drill holes located in left and middle background of photograph. Haul roads and spoil, in early 

stages of reclamation and revegetation, can be seen in the 

foreground. B, View of an underground mine opening of 

the abandoned Dietz No. 8 mine (fig. 4) in the Monarch coal 

bed (one of the beds mined in the Big Horn surface mine 

(right middle ground of A)), December 1971. Only about 2 

m of a 17-m-thick coal bed was removed in past underground mining. Photograph (July 1972) courtesy of Big 

Horn Coal Co.
EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

the physico-chemical and geotechnical properties 

of the overburden, the land surface normally can 

be restored to its original use or it can be used for 

other purposes (Hardaway, 1976, p. 1-76).

Underground mining, on the other hand, appears at first to disturb only a small part of the 

mining area. Mine portals, coal processing and 

loading facilities, and mine support buildings occupy only a small part of the area underlain by 

mine workings. However, beneath the surface the 

mine workings comprise a labyrinth of underground entries and rooms that are connected by 

coal haulageways and airways to mine portals or 

shafts at the surface (cover). The unmined coal 

pillars may be left to support the overburden, or

they may be extracted near the end of the mining 

cycle, which causes a local lowering of the ground 

surface. In modern mining operations, the pillars 

are extracted where possible, in order to make 

more efficient use of energy resources.

In the western Powder River Basin and in other 

areas, such as in underground mining areas in the 

Williston Basin of western North Dakota, studies 

by the authors revealed that the long-range effects of past underground mining, which include 

a local lowering of the land surface and attendant 

deformation, collectively termed subsidence, can 

cause environmental problems. In addition, coal 

fires in abandoned mine workings have locally 

damaged the land surface more severely than

FIGURE 3.—Southward aerial view of the west Decker strip 

mine near Decker, Mont. (May 1978). The overburden is 

removed by large draglines. The coal, which is about 17m 

thick, is removed by power shovels and trucks. The topsoil 

is removed by earthmoving scrapers and is stockpiled 

prior to stripping. The spoil is graded, covered with topsoil, and revegetated (green strips). The overburden comprises soil, colluvium, and soft bedrock of the Tongue 

River Member of the Fort Union Formation and ranges in

thickness from 8 to 37 m. Coal is hauled by trucks along 

radially oriented haul roads to the unit-train loading facility at left. Production of low-sulfur subbituminous coal 

from this mine totaled 9.3 million t (metric tons) in 1977, or 

about 1.5 percent of the total coal production of the United 

States in 1976 (based on a total of 609 million t (World 

Almanac, 1978, p. 97)). Bighorn Mountains and Cloud 

Peak are in right background.
ACKNOWLEDGMENTS

have surface mining activities on nearby land. 

Subsidence continues to be a problem many years 

or many decades after the mine workings were 

driven.

The purpose of this report is: (1) to discuss and 

evaluate the subsidence effects caused by past 

coal mining in the Sheridan, Wyo., area in underground mines that were worked beneath weak 

bedrock and surficial deposits 5-60 m thick; (2) to 

compare some of the long-range environmental effects of the underground and surface mining 

methods in these areas; and (3) to make this information a matter of public record in order that 

future mining activities may benefit from this 

knowledge.

These investigations are part of a series of 

engineering geologic studies of the western 

Powder River Basin, in connection with the U.S. 

Geological Survey's environmental studies of 

Energy Lands. Detailed subsidence investigations of a 50-km2 area 5-20 km north of Sheridan, 

Wyo. (fig. 4), were supplemented by a ground and 

aerial reconnaissance study of a 5-km2 mining 

area 8-10 km west of Sheridan along Big Goose 

Creek and an aerial reconnaissance of 65 km2 of 

land impacted by underground and surface mining near Beulah, N. Dak. Other investigations 

were made of strip mines in the northern Great 

Plains, particularly of the Decker strip mine in 

southern Montana. Some results of subsidence 

studies are included from currently operating 

coal mines near Somerset, Colo., and Raton, N. 

Mex., where the overburden is thicker than 60 m.

Large areas near Sheridan are underlain by 

abandoned mine workings in the Carney, Monarch, and Dietz coal beds located between about 5 

and 60 m beneath the ground surface (fig. 4). 

Underground mining began in about 1892 

(Kuzara, 1977, p. 55) and ended in 1953. The old 

mining towns or camps of Acme, Monarch, Kooi, 

Riverside, Kleenburn (Carneyville), and Dietz 

(Higby) are abandoned, demolished, or in the process of being demolished. In 1907 the population 

of the mining towns of Dietz, Kleenburn (Carneyville), and Monarch was about 2,000, 1,400, and 

700 respectively; and the towns of Kooi and 

Riverside were established. About 1,600 men 

were employed at the Dietz, Carney, and Monarch mines (Taff, 1909, p. 125). Many of the personnel employed at the current surface mines at

Big Horn and Decker now reside in Sheridan and 

other nearby communities.

The terms subsidence, overburden, and coal 

mine openings, as used in this report, require 

clarification. Subsidence is defined as the local 

lowering of the Earth's surface caused by subsurface removal or compaction of material. Coal 

mine subsidence includes the local lowering of the 

ground surface and all deformation processes 

within the overburden and at the surface that are 

produced by the movement of rock and surficial 

material into underground coal mine openings. 

Subsidence processes therefore include downward vertical movement, local depressions, pits, 

horizontal compressive and tensile strains produced by flexure of strata, compressive strain 

associated with compression arching, shearing 

across lithologic boundaries due to flexure of 

strata, and even upward movements that locally 

occur near depressions. The term overburden 

means all earth material that overlies the coal bed 

being mined. Coal mine openings include all 

underground cavities, entries, or workings 

created as coal is removed.

ACKNOWLEDGMENTS

The authors' work was aided by many individuals, organizations, and companies. Access 

to the field area and information was granted by 

Dan Scott and B. J. Spear of the Padlock Ranch 

Co., Dayton, Wyo. Personnel of Peter Kiewit 

Sons' Co., and the Big Horn Coal Co., Sheridan, 

Wyo., particularly T. P. Wollenzien, J. F. 

Ratchye, E. W. Temple, and W. M. Rosewarne 

provided drill core for geotechnical testing, maps, 

charts, and other useful information. Valuable information, mine mapping, and assistance in locating and identifying old coal mines were provided by S. A. Kuzara, Ray Bottomly, and W. F. 

Welch. Information on subsidence and coal mine 

fires was given by G. L. Mooney, Department of 

Environmental Quality, State of Wyoming; H. F. 

Alley, United Mine Workers of America, district 

15; and D. L. Donner, U.S. Bureau of Mines, 

Denver, Colo.

Geologic information on the western Powder 

River Basin was provided by W. J. Mapel, S. P. 

Kanizay, B. E. Law, and B. E. Barnum, U.S. Geological Survey. Subsidence information from New
EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA
ACKNOWLEDGMENTS
8 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

Mexico was made available to the authors by D. 

W. Gentry, and J. F. Abel, Jr., Colorado School of 

Mines, Golden, Colo.

Valuable suggestions were given by D. D. 

Dickey, S. P. Kanizay, and J. R. Smith, U.S. 

Geological Survey, and J. E. Hardaway, U.S. Office of Surface Mining.

GEOLOGY AND PHYSIOGRAPHY

The Powder River Basin, as defined by Keefer 

and Schmidt (1973), is a northwest-trending 

structural and topographic basin located in 

northeastern Wyoming and southeastern Montana (fig. 1). Structurally, the basin is strongly 

asymmetric, the deepest part being to the southwest only a few kilometers northeast of the 

Bighorn Mountains (Osterwald and Dean, 1961, 

p. 348). It is about 320 km long and 130 km wide 

and is bounded by elongate, blocklike uplifts of 

Laramide age, with minor younger movements, 

that commonly trend northwestward and are 

bounded by high-angle normal and reverse faults 

or folds (Osterwald and Dean, 1961, p. 338).

The Powder River Basin is bounded on the west 

and southwest by the Pryor uplift, the Bighorn 

uplift, and the Casper arch; on the east by the 

Black Hills uplift; and on the south by the 

Laramie and Hartville uplifts. Structural relief of 

the uplifts relative to the basin exceeds 5,500 m 

(U.S. Geol. Survey and Am. Assoc. Petroleum Geologists, 1961). Erosion has exposed crystalline 

and metamorphic rocks of Precambrian age in 

most of the cores of the uplifts. Paleozoic and 

Mesozoic rocks are exposed along the flanks of 

the uplifts; then they warp downward into the 

basins, where they are covered by thick sequences of rocks of early Tertiary age.

The topography of the Powder River Basin is 

characterized by low-rolling grass-covered hills 

with craggy, clinker-covered crests that are 

separated by intermittent and locally perennial 

streams. Five major streams drain the Powder 

River Basin: the Powder River and Tongue River 

to the north, the Belle Fourche River to the northeast, the Cheyenne River to the east, and the 

North Platte River to the southeast (fig. 1). Many 

tributaries of these major streams trend northwest and meander along extensive alluvial valley 

floors. This pattern tends to produce a pronounced ridge-and-valley topography oriented in 

a northwesterly direction. In some areas within 

the basin, however, the tributaries trend predominantly northeastward, northward, or westward. In these areas the ridges and valleys are 

oriented in these directions.

Elevations in the Powder River Basin range 

from about 910 m near the northern and northwestern margins of the basin to about 1,620 m in 

the Pumpkin Buttes area in the south-central 

part of the basin and about 1,560 m at Casper in 

the southwestern part of the basin. In the Sheridan area, elevations range from about 1,100 m at 

the former town of Acme to about 1,220 m near 

the Sheridan airport.

Precipitation—an important factor controlling 

restoration of surface-mined lands—is variable 

both locally and regionally in the Powder River 

Basin. On a regional basis, according to the U.S. 

Weather Bureau, the 30-year (1931-60) mean 

annual precipitation in the Powder River Basin 

ranges from 42 cm in the northwestern part, to 

36 cm in the northeastern and central parts, to 

about 30 cm in the southwestern part. Most of 

the Powder River Basin therefore has a semiarid 

climate. In the Sheridan area, according to a local 

rancher (Dan Scott, oral commun., 1975), the 

mean annual precipitation ranges from about 

25 cm at Acme to about 40 cm at Ranchester, 

20 km northwest of Sheridan.

BEDROCK AND COAL DEPOSITS

The bedrock in the western Powder River Basin 

comprises predominantly weak rocks of the Fort 

Union Formation of Paleocene age. Rocks of the 

overlying Wasatch Formation, which crop out 

farther east of the subsidence study area, commonly are lithologically similar to the rocks of the 

Fort Union except that they are somewhat softer 

and coarser and characteristically underlie more 

rounded hills and ridges and broader valleys (W. 

J. Mapel, U.S. Geol. Survey, oral commun., 1975).

The Fort Union Formation consists of soft 

siltstones, mudstones, shales, silty claystones, 

lenticular sandstones, and locally thick subbituminous coal beds. It is subdivided into three 

members by most workers in the area. They are: 

(1) the Tullock Member at the base, (2) the Lebo 

Shale Member, and (3) the Tongue River Member 

in ascending order (Barnum, 1974). The Tullock
GEOLOGY AND PHYSIOGRAPHY

Member, which is exposed about 6.5 km west of 

Ranchester, Wyo., is composed of interbedded 

mudstone and sandstone with some coal (Barnum, 1974). The Lebo Shale Member consists of 

soft mudstones, shales, and claystones, with interbedded discontinuous coal beds and thin soft 

sandstone lenses. Rocks of the Tongue River 

Member in the Acme area, which contain the major coal deposits in the area, consist of soft, 

argillaceous siltstones, mudstones, coal, and 

friable, lenticular sandstones totaling about 

365 m (B. E. Law, U. S. Geol. Survey, oral commun., 1976; fig. 5).

The coal beds, which are of subbituminous rank 

and contain low percentages of sulfur, thicken 

and thin within short distances and are separated 

by rocks that range in thickness from about 1 m 

to as much as 50 m. The coal beds vary in thickness from less than 1 m to about 17 m, and in 

some areas the rock interval between two coal 

beds varies in thickness from about 1 m to about 

10 m within short lateral distances.

Coal beds in the western Powder River Basin, 

which were extensively mined by underground 

methods near the old towns of Acme, Dietz, 

Kleenburn (Carneyville), and Monarch from the 

early 1890's to the early 1950's (fig. 4), include the 

Carney, Monarch, Dietz 3r Dietz 2, and Dietz 1 in 

ascending order (Taff, 1909, p. 129-130). Most of 

the coal has been produced by surface methods 

since the 1940's. Currently active or planned 

large-scale surface mines are located where a coal 

bed is locally thick or where two or more coal beds 

are locally thick and the intervening rocks are 

thin (figs. 2, 3).

SURFICIAL DEPOSITS

The surficial deposits in the Sheridan area consist of Holocene and Pleistocene colluvium, 

alluvium, and local pediment deposits with thin 

to moderately thick soil profiles developed on 

them (Barnum, 1974). The colluvium, which includes weathered clays, silts, and sands from 

bedrock of the Fort Union Formation and rounded to irregularly shaped granules, pebbles, and 

cobbles from adjacent uplifts and outcrops of preTertiary rocks, is concentrated locally on breaks 

in slope, in draws, and on valley margins.

The alluvium comprises clays, silts, sands, 

granules, and cobbles derived from Fort Union 

rocks, from Precambrian rocks in adjacent 

uplifts, from resistant pre-Tertiary rocks adjacent to the uplifts, and from colluvium. Alluvium, 

ranging from about a meter to about 15 m thick, 

blankets the valley bottoms and local terraces 

above the valley bottoms along major perennial 

streams and also the bottoms of draws containing small or intermittent streams. The upper portion of the alluvium along the Tongue River near 

the subsidence study area is predominantly clay, 

silt, and sand that is overlain by an organic soil 

that supports hay meadows and local grain fields.

BEDROCK STRUCTURE

Rocks of the Fort Union Formation generally 

dip l°-3° eastward in the Acme area. However, 

local undulations and local changes in thickness 

of lithologic sequences in short distances cause 

the rocks to dip more steeply, to dip in the opposite direction, or to dip in directions other than 

the general attitude. The rocks locally are disFIGURE 5 (overleaf).—Composite geologic section and 

geotechnical properties of the bedrock and coal in the 

Tongue River Member of the Fort Union Formation from 

two core drill holes near the Big Horn coal mine. The Big 

Horn mine is located about 13 km north of Sheridan, Wyo., 

in the major coal-bearing portion of the Tongue River 

Member of the Fort Union Formation of Paleocene age. 

Grain-size distribution—sand-size (wide bar), silt-size (narrow bar), and clay-size (line) particles—is shown graphically in percent of total disaggregated weight. Slake durability (Franklin and Chandra, 1972, p. 325-338) is in percent 

of material retained in drum after the first cycle of rotation 

at 20 rpm for 10 minutes (narrow bar) and percent of 

material retained in drum after second cycle of rotation at 

20 rpm for 20 minutes (wide bar). Atterberg limits of disaggregated core samples, which include plastic limit 

(smallest value of horizontal line) and liquid limit (largest 

value), are in percent of water relative to the dry weight of 

sample. Soil classification terms are described in Lambe 

and Whitman (1969, p. 35) in terms of plasticity (the 

numerical difference between liquid limit and plastic limit) 

as follows: (1) CL—inorganic, silty clays of low plasticity; 

(2) ML—silts with slight plasticity; (3) CH—inorganic 

clays of high plasticity; (4) SC—clayey sands, poorly 

sorted sand-clay mixtures; and (5) SM—silty sands, poorly 

sorted sand-silt mixtures. In MN/m2 (meganewtons per 

square meter), ls II and IS_L are point-load strength indices 

parallel and perpendicular to bedding, respectively.
10 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

LITHOLOGIC DESCRIPTION

DENSITY (G/CM 3) 

1.01.4 1.8 2.2 2.6 3.0

GRAIN SIZE

(WT PCT.)

10 30 50 70 90

SLAKE 

DURABILITY 

(PERCENT)

30 50 70 90

I i I i I i I i

ATTERBERG 

LIMITS

0 30 50

i I i I i I

SOIL

;LASS

10-

20-

Siltstone, shaly, argillaceous, becoming 

mudstone; calcareous

Dolomite interbed, light brown to 

gray, medium soft to hard, 

carbonaceous inclusions

Coal (Dietz 2), black to brown black, 

brittle

Shale, silty claystone, small coal 

interbed; medium hard, gray to 

gray-brown, carbonaceous inclusions; 

sandy at base

40-

50-

60-

70-

80-

90-

100-

1 10

120-

Sandstone, argillaceous

Mudstone, shaly, silty claystone; light 

to dark gray, carbonaceous inclusions

Sandstone, argillaceous; very fine to 

fine grained, light gray, carbonaceous 

stringers, laminae

Coal (Dietz 3); black, brittle, 

conchoidal fractures

Sandstone, very fine grained.Siltstone, 

shaly, argillaceous sandstone lenses; 

light to brown gray, calcareous to 

limy, carbonaceous laminae

(Monarch), black, brittle, 

conchoidal fractures

Interbedded argillaceous siltstones, 

sandstones, mudstoiies at bottom; 

very fine to fine-grained sand, light to 

dark gray, calcareous to 

noncalcareous, carbonaceous 

laminae, contorted laminae

(Carney); black, brittle, conchoidal 

fractures, shale partings

Siltstone, argillaceous, becoming 

sandstone,very fine to fine-grained

CL

CL

CL

CL-ML

CL 

CL 

ML 

CL

CH

CL 

SC 

SM

CL 

SC

SC-SM 

CL

CL 

CL

CL-ML

CL

CL-ML 

SC 

CL

CL 

CL 

CH

ML

CL

CL

CL

CL 

CL 

SC

- 10

- 20

- 30

- 40

- 50

60

- 70

- 80

- 90

-100

-110

-120
GEOLOGY AND PHYSIOGRAPHY 11

METERS

10-

20-

ou

40-

50-

60-

70

80-

90

100

110 

120

POINT LOAD 

STRENGTH 

I s ll , MN/m 2

0.5 1.0 1.5

I I

POINT LOAD 

STRENGTH 

Isl, MN/m 2

0.5 1.0 1.5

I

——

—

_

-

ANISOTROPY 

IS1/I S II

12345

i

6.2

6.2

6.0

14.5

——————— 7.5 

........ 11.0-^6.3

——— 17. C

—— 49.0

r~~ 103

—— 20 0

—— 10.0

= ——— 17.2

57.0

4«.C 

13.0

ESTIMATED COMP. 

STRENGTH 

RANGE II 

MN/m 2 

20 40

h-*H ' 

W

&

'-^ | ' I

t=r>

H 

H

tf

^_ ~^=^

tL, r^

^

H

' 1

1 —— i

t!=rf

H

! ,

h^=H '

1 J

^

w

H

ESTIMATED COMP. 

STRENGTH 

RANGE 1 

MN/m 2

20 40 i

, 1 ,

^ ———

^ —— I '

i ' i H ' ' ——— ! —— '

(d|

M-i i ' i '

tl

^^ i

t=M

i— i |

Sr^

ib==j |

i— i

i— i

H

1 ' 1

felq-1

I"1 . I-: —————————

\ —————— I 1

S^

UNCONFINED COMP. 

STRENGTH 1

MN/m 2

1 ,° , 3,° , 5,° I

218.5

156.5

100.6

METERS

10

20

— Tn

40

- 50

60

- 70

- 80

- 90

-100

-1 10 

-120
12 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

placed by discontinuous faults that commonly 

dip steeply to vertically and trend east-northeastward (B. E. Law, oral commun., 1976).

Joints and lineaments in the bedrock are expressed as individual fractures on outcrops or by 

the trends in stream drainage, and are revealed as 

lineaments on aerial photographs. Dominant 

trends are northwest, east-northeast, west, and 

north. Based on studies of the trends of drainages 

on aerial photomosaics and faults and joints 

mapped by high-resolution photogrammetric 

plotters, a close correlation exists among (1) the 

trends of joints and faults in the Fort Union 

rocks within the Powder River Basin, (2) the 

dominant trends of the uplifts, anticlines, and 

flexures adjacent to the Powder River Basin, and 

(3) the trends of foliation and joints and faults 

within the Precambrian rocks of the Bighorn 

uplift (fig. 1).

GEOTECHNICAL STUDIES

The geotechnical and geological properties of 

rock above and below the coal beds are important 

factors to consider in planning mining activities, 

because they control the behavior of the bedrock 

in response to surface or underground mining and 

also the behavior of the stripped overburden or 

spoil after restoration of surface-mined lands. The 

properties of the rocks at the outcrop commonly 

are so altered from their natural state that 

geotechnical test results on rock samples from 

outcrops can be misleading. Most weak, soft 

siltstones, shales, mudstones, and coal in outcrops are not preserved well enough to be tested 

and appear much weaker than core samples, 

whereas most outcropping sandstones are cemented more completely by near-surface ground 

water and therefore are stronger at the outcrop 

than in unaltered bedrock. Therefore, it is often 

necessary to test drill cores in order to obtain 

realistic geotechnical results (fig. 5). Some geotechnical properties of the coal and rock mass 

also can be determined from calibrated, downhole 

geophysical logging that includes gamma density, caliper, and sonic velocity measurements.

Rock masses commonly are much weaker than 

strength tests on core samples indicate, because 

bedding planes, joints, and faults weaken the 

rocks greatly. Cohesion across most of these

discontinuities is zero, or nearly zero. Therefore, 

bedded, jointed, and faulted rock masses derive 

strength only by confining stress and the resultant friction along the various fractures. This confining stress commonly is greatly reduced or 

altered by surface and underground mining activities.

Other factors that tend to weaken rock masses, 

particularly over periods of years or decades, are 

the effects of weathering, wetting, and drying by 

fluctuation and movement of ground water and 

exposure to air. An example of how rock masses 

are weakened by the movement of surface and 

ground waters can be seen in surface mining 

operations in the western Powder River Basin. 

Highwalls ranging from 7.5 m to more than 30 m 

high commonly stand nearly vertical for periods 

of weeks or months in operating surface mines 

(figs. 2, 3), but they weaken and fail by rockfalls, 

landslides, or other mass-gravity movements over 

periods of years or decades under the effects of 

alternate wetting, drying, freezing, and thawing 

of water along fractures and bedding surfaces

(fig. 6).

The stable slope angle of fractured and jointed 

bedrock on strip mine highwalls ultimately may 

be less than the stable slope angle of a brokenup pile of the same rock, because open joints and 

tension fractures behind the rims of highwalls 

provide avenues for water to flow, as well as to 

freeze and thaw, whereas the broken counterpart 

of the bedrock in spoil piles at the angle of repose 

appears to be less permeable and therefore less 

susceptible to the effects of water. Graded spoil 

material, however, might absorb surface water 

readily and fail, unless the graded slopes are designed in accordance with soil engineering properties of the broken-up and mixed overburden 

material (Terzaghi and Peck, 1967, p. 31-35; 

Lambe and Whitman, 1969, p. 33-38). Results of 

these tests should be considered, in addition to inplace soil properties, potential land use, and existing climatic conditions, before benching and final 

grading requirements of highwalls and requirements for grading restored spoil in surface mines 

are specified.

As an aid in evaluating the behavior of the Fort 

Union bedrock in response to past, present, and 

future coal mining, field and laboratory tests 

were conducted on cores from two drill holes in
GEOTECHNICAL STUDIES 13

FIGURE 6.—Westward aerial oblique view of the highwall at the abandoned Plachek surface mine near the Big Horn mine 

(April 1978). Rockfalls, landslides, and mass wasting caused by wetting, drying, freezing, and thawing may ultimately 

reduce the highwall slope below the angle of repose of the rocks of similar lithology and structure when broken up and 

mixed by surface mining. A coal fire, located in the black area near the west end of the highwall (middle ground at the 

right), started in about 1975 by spontaneous combustion. Since then, the fire has spread southward and eastward into coal 

near the base of the highwall.

the Big Horn mine area in March 1976 (figs. 2, 4, 

5). The core, which was made available for testing 

by Peter Kiewit Sons' Co., was stored in unsealed 

core boxes a few weeks prior to testing. The core 

had dried out some and locally was broken up by 

freezing and thawing.

Results of the field and laboratory tests on the 

Big Horn cores showed that the bedrock was 

weak to very weak, except for local thin lenses of 

dolomite. Characteristically the bedrock might be 

classified as overconsolidated muds, clays, and 

sands rather than rocks. Results of tests on core 

from a drill hole at Sheridan, Wyo., in the spring 

of 1975 (R. A. Farrow, written commun., 1977) revealed the same thing.

Few tests were conducted on the coal from the 

Big Horn cores, because the coal was broken up 

and put in bags before geotechnical testing 

began. Taff (1909, p. 133) described the fresh 

coals in the study area as laminated to massive 

with a black, shiny luster, which break into small 

angular blocks and fragments with hackly to conchoidal surfaces when mined. The coal, however, 

turns a dull-black to gray color and breaks readily 

into small fragments and dust when exposed to 

moisture and air. According to E. W. Temple (Big 

Horn Coal Co., oral commun., 1976), piles of coal 

commonly heat and ignite spontaneously when 

exposed to air and water, unless the fine dust is 

removed. Analyses of coal samples, on an as-
14 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

received basis, collected by Taff (1909, p. 135) 

from the Carney, Monarch, and Dietz coal beds, 

reveal that the coal is subbituminous in rank with 

heat values of about 5,000-5,400 cal/g 

(9,000-9,700 Btu/lb). Sulfur and ash contents 

ranged from about 0.2 to 0.8 percent and 3 to 6.5 

percent by weight, respectively, for the Carney 

and Monarch coal beds, and from about 0.4 to 1.2 

percent and about 4.5 to 8 percent by weight, 

respectively, for the Dietz coal beds.

The field tests, which were conducted in a 

mobile laboratory, included lithologic identification, point-load strength index, and slake durability tests (fig. 5). The point-load strength index 

test (Broch and Franklin, 1972, p. 669-693) is a 

simple, inexpensive method of estimating the tensile and compressive strengths of rocks. The 

point-load strength index approximates the tensile strength of the rocks, whereas an unconfined compressive strength can be estimated by 

multiplying the point-load strength by 15 and 35, 

based on tests by D'Andrea, Fischer, and Fogelson (1965, p. 7), Broch and Franklin (1972, p. 690), 

and on various tests run by the authors. Pointload index strength tests were run parallel and 

perpendicular to bedding in order to determine

- * -ViL-.' r^EW*^

FIGURE 7 (above and facing page).—Southward aerial oblique 

view of subsidence depressions, pits, and cracks above the 

south part of the abandoned room-and-pillar Acme mine 

(May 1976). The mine was operated from the early 1900's 

to 1943. The location and geometry of subsidence depressions and pits correspond to mining areas and to individual mine openings. A, General view of subsidence 

features (foreground), abandoned strip mine (left background), and currently operating Big Horn surface mine

along the Tongue River (center and left background). 

Dragline-dumped spoil piles in old abandoned strip mine 

are stable at the angle of repose; the vertical highwall in 

alluvium also is stable. Note that depressions enclosing 

pits (left middle ground) are rectangular in form near the 

abandoned strip mine, where overburden is about 12 m 

thick, to elliptical in form in left foreground, where the 

overburden is about 25 m. Some pits (left center and right 

center of photograph) are deeper than the reported
GEOTECHNICAL STUDIES 15

anisotropy or the ratio of strength perpendicular 

to bedding to strength parallel to bedding. The 

slake durability index test is designed to simulate, in an accelerated way, the wetting, drying, 

and abrasion of rocks subjected to weathering, 

erosion, and transport by streams. Two cycles 

were run on most samples to more accurately 

simulate wetting and drying.

Characteristically, the point-load strength indices of the siltstones ranged from 0.01 to 1 MN/m2 

perpendicular to bedding, with an anisotropy of 2 

to 10; the sandstones, 0.5 to 1 MN/m2 parallel and 

perpendicular to bedding, with an anisotropy of

approximately 1 (fig. 5). The slake durability indices of the siltstones, mudstones, and shales, 

when subjected to repeated test cycles, ranged 

from 30 to 90 percent in the first cycle and 15 to 

80 percent in the second cycle; the sandstones 

ranged from 25 to 85 percent in the first cycle and 

15 to 75 percent in the second cycle (fig. 5). Slake 

durability indices, according to Franklin and 

Chandra (1972, p. 337) of bedrock range from 

very low to high.

Laboratory tests conducted at U.S. Geological 

Survey laboratories in Golden, Colo., included 

grain-size distribution, density, unconfined comthickness of coal mined. On cold mornings steam is visible 

above many cracks and pits, indicating underground fires. 

The subsidence and fire that occurred above ground on 

January 22,1979, were located a few meters to the right of 

the road along the spoil where the road curves to the right. 

The grass is greener in depressions and pits where 

moisture accumulates. B, Closer view of subsidence 

depressions containing pits and bounded by tension

cracks. Pits of markedly different ages are present, ranging from bowl-like depressions blanketed by green grass, 

many years or decades old, to small pits with vertical or 

overhanging walls, only a few months or years old. Subsidence depressions, pits, and cracks occur sporadically on 

road in left foreground and present problems to stock and 

to vehicle travel.
16 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

FIGURE 8.—Southeastward aerial view showing the surface 

effects of past and present coal mining along the Tongue 

River (July 1977). Subsidence pits, troughs, depressions, 

and cracks have formed above the south part of the Acme 

mine. Dam in right foreground across Hidden Water Creek 

ruptured due to subsidence. The water now is diverted into 

subsidence depressions, pits, and cracks upstream from 

the dam. Garbage from the town of Acme was dumped into 

the large pit at the left of the photograph. Subsidence pits

pressive strength, and Atterberg limits (fig. 5). 

Grain-size and density test results showed that 

the rocks are primarily argillaceous siltstones, 

claystones, shales, and local sandstones with densities ranging from about 1.7 to 2.3 g/cm3 . The 

density of the coal ranged from 1.1 to 1.3 g/cm3. 

Results of the unconfined compressive strength 

tests reveal that, with the exception of the local, 

strong dolomite beds and a few other beds, compressive strength of the remaining bedrock ranged from 10 to 50 MN/m2 . Many of the weaker 

rocks, which were tested for point-load strength,

and troughs in middle of photograph are in alluvium. Note 

that the pits near the road and left of the draw do not occur 

in any noticeable depression; these pits are located above 

collapsed areas in the main haulageway or in spur 

haulageways of the Acme mine, where adjacent coal pillars 

are strong enough to support the overburden. Alluvium is 

being stripped at the Big Horn surface coal mine to extract 

the coal (background). Grading is beginning near the river 

(right background).

were too weak even to be prepared for the unconfined compressive strength test; such rocks were 

treated as strong soils.

Liquid limits and plastic limits were measured 

on ground up samples of the core. Plasticity indices, which are the numerical difference between 

liquid limit and plastic limit, provide a basis for 

the unified soil classification (fig. 5; Lambe and 

Whitman, 1969, p. 33-38). Based on this classification, the siltstones and mudstones contained 

clays of from slight plasticity (ML) to low to 

medium plasticity (CL), the local shales and
SURFACE SUBSIDENCE EFFECTS 17

claystones beneath the coal beds contained clays 

of high plasticity (CH), and the sandstones comprised poorly graded sand-clay mixtures (SC) and 

sand-silt mixtures (SM).

SURFACE SUBSIDENCE EFFECTS 

SHERIDAN, WYOMING, AREA

Surface subsidence effects caused by underground mining in the western Powder River 

Basin comprise local depressions, pits, troughs, 

tension cracks, and compression bulges (cover; 

frontispiece A, B\ figs. 7, 8). These effects also 

were observed and recorded in the Illinois coal 

fields in the early 1900's by Young (1916) and in 

the United Kingdom, for example, by Piggott and 

Eynon (1978, p. 749-780). The depressions overlie 

a number of rooms in room-and-pillar mining 

areas, where the remaining coal was not strong 

enough to support the weight of the overburden 

either because the coal pillars were partly removed or because the initial coal pilla L s were too 

small to support the load. In areas where the 

overburden is less than about 15 m thick, the 

depressions range in depth from about 30 cm to 

2.5 m; in plan view many are square or rectangular (frontispiece A) with rounded corners 

that outline the extent of the former mining 

areas. In overburden thicker than about 25 m, the 

corners of the depressions become more rounded, 

and the general outline of the depressions is more 

circular or elliptical in plan view. The areas encompassed by the surface depressions appear to 

be slightly larger than the mining areas beneath 

the depressions.

Tension cracks occur at the margins of subsidence depressions as a result of convex bending 

and associated stretching of the ground surface 

(figs. 7, 8, 9A; Dunrud, 1976a, p. 4-5). Compression ridges or bulges occur in the depressions 

where the ground surface is subjected to concave 

bending and attendant shortening of the ground 

surface (fig. 9B; Young, 1916, p. 36), although 

this damage is not as evident as the tension 

cracks because the soft bedrock and alluvium 

compress with less visible effect and compression 

features are not enhanced by erosion. Holes 

measuring 5 cm to as much as 3 m wide are present locally in soil and colluvium above tension 

cracks a few centimeters to a few meters wide

that occur in the shale and claystone overburden 

rocks below the soil and colluvium (fig. 10). In 

these areas the soil and colluvium stretched 

without fissuring, as the bedrock cracked in 

response to tensile stresses along the margins of 

local subsidence depressions. Consequently, soil 

and colluvium cover the underlying fissures except where holes were initiated by local piping or 

by the activities of man or animals. At the surface these holes look very much like the initial 

pits above individual mine cavities; however, 

below ground the initial pits commonly widen out 

into large cavities and are 3-4 m deep. The holes 

continually enlarge with time under the forces of 

erosion and small mass gravity movements, until 

the slope becomes stable or until the cracks are 

filled with material.

Subsidence pits and troughs occur above individual mine openings or above the intersections 

of mine openings. Field evidence indicates that 

the pits result from intermittent, sequential collapse of the overburden or an upward stoping process that is initiated by the collapse of mine roofs. 

In some areas the upward stoping process can be 

observed in subsidence pits that are adjacent to 

underground cavities. The roofs of these cavities 

commonly comprise arches and the floors consist 

of caved, broken rock or unconsolidated material 

(fig. 11). The cavities migrate upwards as roofs 

collapse and the caved material accumulates on 

the floor of the cavities. Troughs, which occur 

above elongate mine openings or rooms, probably 

form by the coalescence of individual pits. Collapse of mine roofs is governed by the width of 

mine openings, the strength of mine roofs, and 

the adequacy of roof support system used.

The occurrence of pits at the surface, therefore, 

depends on the time necessary for mine roofs to 

fail, on the thickness and strength of overburden 

materials, and on the width of mine openings. In 

the Acme area, the overburden consists of weak 

shales, mudstones, and local sandstones ranging 

in thickness from about 5 to 60 m. Pits commonly 

are more abundant in areas where mine openings 

trend northwest or northeast, parallel to the predominant trends of joints in the bedrock. New 

pits may form among old pits many years or 

many decades after initial pit formation (fig. 7). 

The occurrence of pits at the surface, even where 

overburden is of constant thickness and strength,
18 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

^^^^^^

FIGURE 9 (above and facing page).—Ground views showing tensile and compressive features caused by subsidence. A, Northwestward view of a large tension crack at the margin of a subsidence depression. Local pits to the right of the crack occur 

above individual underground mine openings. Snow and water accumulate in the larger cracks, which in turn enlarges the 

cracks by erosion. B, View of buckled and bulged siltstone and mudstone in the eastern part of the Acme mine above an 

underground fire. The bulge is about 1.5 m high. The effects of shortening are most pronounced in subsiding ground above 

coal mine fires because subsidence amounts are greater where more of the coal is removed than in past mining. The bedrock 

was stained a maroon color by oxidation of iron caused by heat from the fire below.

tends to be sporadic because collapse of mine 

roofs is sporadic.

The initial pits caused by sequential collapse of 

mine roofs commonly are nearly circular holes 

1-3 m wide and 2-5 m deep which have vertical to 

overhanging sidewalls (example of appearance in 

figs. 10, 14A). The pits widen and the slope angle 

becomes smaller through the processes of erosion, mass wasting, and deposition. In this area, 

these processes may continue for 10-25 years or 

more, depending on geologic conditions and geotechnical properties of the overburden. The bases 

and walls of the pits support range grass, vines, 

and woody plants a few years or decades after formation of the initial pits, depending on the nature 

of the near-surface material.

The geotechnical properties of the material in 

which the pits, troughs, and cracks form control 

the behavior of the rims and walls and their rate 

of failure. Pits, troughs, and cracks in sandstone 

or in alluvium, consisting of well-drained sands 

and gravels commonly exhibit vertical rims and 

walls for many years or even decades. However, 

the rims and walls of pits and cracks in poorly 

drained siltstones, mudstones, claystones, and 

shales—that contain clays of medium to high 

plasticity—may slake, slab, slough, or flow readily. These failures occur by mass gravity move-
SURFACE SUBSIDENCE EFFECTS 19

T ,*•

•* —-^ -^:

-^••f."1 '

v ,-*»«-, *v ,V:XT t» x ,\, ^-. * -w -% ^ ^- ' -.' r A v,- -x-

• -*?• «• ,>• '-»^*%'» -* • -»

•B

ments and mass wasting until pits and cracks 

attain a saucer shape or flattened trench shape in 

a few years or decades and begin to support 

vegetation.

Subsidence pits are present in the local depressions as well as in areas above room-and-pillar 

mine workings where no noticeable depression exists—or at least where depressions cannot be detected without making precise, periodic surface 

measurements (figs. 7, 8). Most pits within the 

depressions are not as deep as pits outside the 

depressions. The total depth of the depressions 

and the pits within them is roughly equal to the 

depths of pits that are present where no noticeable depression exists. On the basis of mine 

reports, the total depth of both types of these surface collapse features commonly is nearly equal 

to the thickness of coal mined. Locally, however, 

the total subsidence is greater than the reported 

thickness of the coal mined. In these areas some

of the collapsed material probably spreads, or is 

transported, laterally into adjacent mine openings or compacts more than the original state of 

the material, aided by the presence of water. 

Most pits caused by roof collapse occur where the 

overburden is less than about 10 times the original height of the mine workings, an upper limit 

suggested by studies in the United Kingdom by 

Piggott and Eynon (1978, p. 764-765). However, 

pits might occur in overburden as much as, or 

more than, 15 times the height of the mine workings in areas where the caved material moves 

laterally.

Pits within the local depressions usually form 

many years or many decades after the depressions, depending on geologic factors and mining 

conditions previously discussed; therefore, local 

depressions can serve as warnings to possible 

future pit collapse within them (figs. 7, 8). 

However, the elevation of the ground surface
20 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

may not noticeably change prior to pit collapse 

above mine openings that are bounded by pillars 

strong enough to support the overburden (figs. 8, 

11). In these areas the ground surface may collapse suddenly with little or no advance warning 

unless it is periodically monitored for minute 

movement. Pits and cracks are only beginning in 

some parts of the Dietz mine area, where the overburden is about 45 m thick (Darton, 1906, p. Ill) 

above mines that were worked from the early 

1900's to the early 1920's (frontispiece B; fig. 4).

BEULAH, NORTH DAKOTA, AREA

Spectacular surface subsidence features occur 

above abandoned underground coal mines in 

western North Dakota, particularly in the Beulah 

area (fig. 12). The subsidence features look like 

those in the western Powder River Basin except 

that subsidence troughs are more common. The 

overburden is similar in lithology and age to the 

coal-bearing rocks near Sheridan, Wyo., except 

that the bedrock appears to be somewhat softer 

and that glacial deposits—comprising clays, 

sands, silts, gravels, and local erratic 

boulders—locally are present in the upper few 

meters (D.E.Trimble, written commun., 1977).

Room-and-pillar mines operated in lignitic coal 

beds 2.5-9 m thick from 1884 to 1966. The area 

shown in figure 12 was mined by underground 

methods from 1918 to 1952 (Parker, 1973, p. 40). 

The coal bed is about 5 m thick and the overburden thickness averages about 25 m (Pollard 

and others, 1972, p. 21). The geometry of the 

room-and-pillar mine workings is vividly portrayed by subsidence pits and troughs. As in the 

Sheridan, Wyo., area, the troughs are believed to 

overlie elongate rooms and apparently formed by 

coalescence of individual pits.

Much of the lignite in the Beulah area was 

produced by surface mining in the past; currently lignite is produced only by surface mining 

(fig. 13). Restoration of surface-mined lands, although not done in the past, is now underway 

(Hardaway, 1976, p. 107-112) in compliance with 

the Mined Land Reclamation Law, 1969, which 

essentially requires that land mined by surface 

methods be restored in a manner that will 

minimize adverse economic and aesthetic effects

FIGURE 10.—Three holes in soil above a subsidence crack in 

bedrock. The holes are 30-45 cm in diameter and are 

located in soil and weathered bedrock approximately 1.2 m 

thick. Soil may initially bridge the tension crack without 

rupturing as fissure forms in bedrock below. Later, the 

holes form, either by natural piping associated with wetting and drying or by surface activities of animals. Note 

grass is greener and more prevalent on either side of crack 

than along it because crack dewaters soil and retards plant 

growth.

and will maximize the future use of the land 

(Pollard and others, 1972, p. 7-8). Based on an 

aerial reconnaissance study by the authors, operations appear to be hindered more by near-surface ground water than in similar surface mining 

operations in the Powder River Basin. Depressions may occur on restored land in the Beulah 

area, however, that could affect drainage because 

much of the land is nearly flat. There are fewer 

hills than in the Powder River Basin from which 

material can be used to compensate for the 

volume of coal removed (figs. 12, 13).
SUBSIDENCE PROCESSES 21

SUBSIDENCE PROCESSES

Studies of deformations within underground 

coal mine workings and at the surface in the 

United States (Peng, 1978, p. 1-280); in Utah and 

Colorado (Dunrud, 1976a, b; Osterwald, 1961, 

1962); in Wyoming (Dunrud and Osterwald, 

1978a, p. 175-190); in the Raton, N. Mex., area 

(Gentry and Abel, 1978, p. 191-220); in the 

Netherlands (Pottgens, 1979, p. 267-282); and in 

the United Kingdom (National Coal Board, 1966, 

1975; Shadbolt, 1978, p. 705-748) provide a basis 

for interpreting subsidence processes that are initiated when underground mining begins and that 

end with surface subsidence effects described in

the previous section. In this section, stresses and 

deformations are briefly described (1) within and 

near the mine workings, (2) in the mine overburden, and (3) at the land surface, in order to 

relate surface subsidence to underground mining 

operations. Additional information is available, 

for example, in Peng (1978).

Analyses of the behavior of coal and rock in the 

mine roofs, ribs (walls), and floors of underground 

coal mines by the authors and others (Schoemaker, 1948, p. 4-7; Dunrud, 1976a, p. 17-30) 

show that stresses caused by overburden load 

and by any existing tectonic stresses must readjust to the presence of underground openings. 

Stress concentrations after mining are greater in

FIGURE 11.—Recent subsidence pit above the Old Monarch mine (fig. 4) showing connecting underground cavity. Fencepost, 

about 2 m long, was left dangling in mid-air by collapse. The road from which photograph was taken collapsed in 1974 and 

had to be filled and graded in order to be used again. The cavity is migrating upward by intermittent collapse (stoping) of 

the mine roof. Original mine workings are estimated to be about 10 m below the surface. Note that the ground above and 

behind the pit appears to be unaffected by a large cavity only 2 m below, except that plant growth within about 25 cm of 

the rim is retarded due to the effects of soil dewatering.
22 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

the coal and rocks near the mine openings than 

they were before any opening existed in order to 

bear the stress previously borne by the material 

removed from the openings.

The shapes of the mine openings—in addition 

to such factors as overburden thickness, lithologic and structural character of the coal and 

associated rocks, and hydrologic conditions—control the state of stress around them. For example, 

tensile stresses in the roofs and floors of common 

rectangular or trapezoidal mine openings, can be 

nearly as great as the overburden stress, whereas 

compressive stresses two to three times greater 

than the overburden stress can occur on the ribs 

(Schoemaker, 1948, p. 4). However, stresses 

around elliptical openings, where the long axis of

the ellipse is oriented in the direction of maximum stress, are primarily compressive according to Fenner's analysis (presented in Schoemaker, 1948, p. 6-8). Elliptical openings therefore are 

much more stable than the common rectangular 

openings because, as shown in the geotechnical 

properties section (fig. 5), rocks are much 

stronger in compression than in tension.

Elliptical zones of compressive stress commonly occur around individual mine openings 

(fig. 14.A). They also occur around larger cavities 

created by extraction of pillars in room-and-pillar 

mining areas (Dunrud, 1976a, p. 23-30) or by 

longwall mining unless the elastic limit of the coal 

or rock is exceeded and viscoelastic or plastic 

deformation occurs, or the cavities are sufficient-

' %

*•*;• <

^

FIGURE 12.—Oblique aerial view of subsidence features located 1.5-6.5 km north of Beulah, N. Dak., looking south (October 

1976). Pits and troughs occur above elongate rooms in room-and-pillar mine workings in lignitic coal that was mined from 

1918 to 1952. The overburden averages about 15m thick and is composed of poorly consolidated claystones, siltstones, and 

lenticular sandstones overlain by local glacial deposits. The land surface above mine workings is of little use because of 

subsidence hazards. Wheat farming, the major industry in the area, is hazardous in the old mining areas because the vibrations and extra weight of equipment might trigger further collapse. Cultivated areas at right probably overlie areas of 

unmined coal.
SUBSIDENCE PROCESSES 23

FIGURE 13.—Eastward aerial view showing the effects of lignite mining near Beulah, N. Dak. (October 1976). Farmlands used 

for raising wheat are disrupted by subsidence pits and troughs (middle ground), spoil piles, and highwalls remaining from 

past underground and strip mining operations (foreground and background). Restoration, including grading and revegetation of spoil, has begun on later surface mining operations (background). Wheat is being grown between two abandoned 

strip mines in the foreground but not above i nderground mines (middle and near background).

ly wide so that the compression ellipses migrate 

to the ground surface (fig. 14A, D). Whittaker 

and Pye (1977, p. 306-307) discussed the transfer 

of overburden stresses above mine openings to 

adjacent coal by the presence of pressure arches 

and indicated that the height of the relaxed or 

destressed zone is approximately equal to the 

width of the mine opening.

Deformation of coal and rocks within the elliptical zones of compressive stress is controlled by 

such factors as (1) the height-to-width ratio of the 

openings, (2) the elastic and time-dependent geotechnical properties of the coal and rocks, (3) lithology and geologic structure of the roof and floor 

rocks, and (4) hydrologic conditions. Breaking 

and caving above mine openings are extensive

where heigh t-to-width ratios are near 1 (fig. 14A, 

enlarged view; 14£?, C), whereas the rock strata 

may only break one to three mining thicknesses 

above mine openings with very small height-towidth ratios (fig. 14D). The strata commonly flex 

downward into mine openings as more or less continuous units only a few mining heights above 

mine openings, with very small height-to-width 

ratios, and recompress the fragmented debris 

below. The rate of downward movement of overburden rocks into mine openings, or the voids 

above the openings, commonly is not uniform, 

because elliptical zones of compressive stress 

around the mine openings migrate upward and 

downward from openings that are widened by 

mining. This, in turn, may cause separation of
24 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

MAXIMUM 

COMPRESSION

FIGURE 14 (above and facing page).—Subsidence processes 

caused by mining different amounts of coal beneath overburden of different strengths and thicknesses. A, Block 

diagram showing subsidence pits above caved mine openings where adjacent coal is sufficiently strong to support 

the weight of the overburden (left), and subsidence depressions and subsidence pits where the remaining coal yielded 

partially to the weight of the overburden (right). An 

enlarged view of the elliptical zone of compressive stress 

around a rectangular mine opening shows buckled and 

heaved floor and caved roof rocks that are common in 

zones of reduced stress within the zone of compression. 

Subsidence pits, which are the end result of successive collapse of the mine roofs, are deeper where no subsidence 

depression occurs than are pits within the depression. A

plot of tilt, horizontal displacement, curvature, and strain 

caused by differential settlement is shown below the subsidence depression on right. ^ , limit or draw angle; ft, 

break angle; +, tensile strain; —, compressive strain. B, 

Caved opening above an entry in the Riverside mine (fig. 4) 

exposed on a surface mine cut (May 1978).Bedrock is soft, 

silty claystone with local thin carbonaceous zones. Entry 

is estimated to be about 3 m wide. The original height of 

the entries reportedly was about 2 m; the current limit 

of caving is about 9.5 m above the mine floor. C, 

Caved opening of an entry in the southern part of the
SUBSIDENCE PROCESSES 25

MAXIMUM TENSILE 

. STRAIN

* — '

MAXIMUM 

COMPRESSIVE

STRAIN 

INFLECTION POINTS

CRITICAL 

EXTRACTION

WIDTH

SUPERCRITICAL EXTRACTION 

WIDTH

D

Acme mine (fig. 4) (January 1979). Mine entry is about 3 m 

wide and 3 m high. Bedrock is moderately well cemented 

sandy siltstone. Note that the caved fragments are much 

larger and occupy a larger volume at the Acme mine than 

do the caved fragments at the Riverside mine. D, Block 

diagram showing subsidence depression above mine opening where overburden is thicker than about 60 m or greater 

than about 10-15 times the thickness of coal mined. Subsidence profile shows tilt, tensile and compressive strain, 

inflection points, and critical and supercritical mining widths above an opening where the coal was completely 

removed.
26 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

the rock units that are within or above the compression zones from those that are the compression zone, in (fig. 14A).

Subsidence commonly occurs above these elliptical zones of compressive stress because the 

weaker material, such as the coal, shale, and 

mudstone within the zone, yields to the increased 

compressive stresses (Dunrud, 1976a, p. 22-25). 

Surface subsidence therefore commonly begins 

above mining areas before the elliptical zones of 

compressive stress reach the surface. The zones 

may migrate upward after all mining activities 

have ceased, because of either time-dependent 

failure of the rock or coal within the zone of compressive stress or failure due to wetting and 

desiccation of the coal and rock with seasonal and 

other cyclical fluctuations of the position of the 

water table. When the zones of compressive 

stress reach the surface, the ground surface 

either settles downward to form subsidence 

depressions (fig. 14A, D) or it collapses to form 

pits, depending on the height-to-width ratios of 

the underground cavities (fig. 144, left). Pits 

eventually form at the surface in areas where 

these ratios are close to 1, unless the overburden 

is thick enough to contain cavities after they 

have become filled with caved debris (fig. 14B, C). 

Depressions commonly occur where the heightto-width ratios of the mine cavities are much 

smaller than 1 (fig. 14A, right; 14D).

The area mined necessary to cause maximum 

surface subsidence is called the critical area 

(fig. 14D; National Coal Board, 1975, p. 2-3). 

Critical area is dependent on overburden 

thickness. The thicker the overburden, the 

greater the mined area must be for the compression arch to reach the surface and allow maximum subsidence. Maximum subsidence commonly occurs when the horizontal dimensions of 

the mine cavities are greater than 0.8-1.2 times 

the overburden thickness (fig. 15). An area 

smaller than the critical area, where compression 

arches may still be present above the mine cavity, 

is called subcritical and a mining area larger than 

critical is called a supercritical area. The areas 

mined commonly are rectangular but most subsidence depressions are rounded (fig. 14A, D), apparently because of the formation of an elongate, 

horizontally oriented elliptical zone of compressive stress. Rounding of the corners of a rectangular mining panel was observed by Dunrud 

in western Colorado in 1976 as the roof rocks 

broke and collapsed behind the roof support jacks 

in a longwall coal mining operation.

100

0.2 1.6 1.8

FIGURE 15.—Maximum subsidence for selected coal mining areas in the United Kingdom and in the Western 

United States. Graph shows maximum subsidence (S), 

in percent of thickness of coal mined (t), versus the ratio 

of mining panel width (W) to overburden depth (d) (see 

inset for details) for longwall mines in the United Kingdom (upper solid line for backfilled (stowed) mines; 

lower solid line for caved or strip-packed longwall 

mines; data from Bell, 1975, p. 116; Shadbolt, 1978, 

p. 729) and for room-and-pillar mining at Somerset, 

Colo. (dashed and solid lines through circled data 

points), in rugged terrain underlain by moderately 

strong mudstones and sandstones of the Mesaverde 

Formation of Late Cretaceous age. The dashed curve 

shows surface subsidence upon completion of mining; 

the solid curve through circled data points shows subsidence amounts when the surface appears to have 

stabilized; difference between the two curves equals 

residual subsidence. The points enclosed by the square 

and diamond are for measurements made by Gentry 

and Abel (1978, p. 204) on a ridge and adjacent draw 

above a supercritical longwall panel in the York Canyon 

coal mine, in rugged topography near Raton, N. Mex., 

where the overburden consists of moderately strong 

mudstones and sandstones; the subsidence was 25-30 

percent less beneath a draw than it was beneath an adjacent ridge (Gentry and Abel, 1978, p. 203-204).
SUBSIDENCE PROCESSES 27

Although complicated by mining and geologic 

factors, such as adjacent mining areas, mining of 

vertically superposed coal beds, topography, 

lithology, and structure, the differential lowering 

of the overburden strata and the ground surface 

above subcritical, critical, and supercritical mining areas produces within the overburden and at 

the surface: (1) vertical displacement, (2) horizontal displacement, (3) tilt, (4) convex (positive) and 

concave (negative) curvature, and (5) tensile and 

compressive strain and local rupture (fig. 14A, D).

As vertical displacements occur within subsidence depressions, tensile stress and attendant 

strain occur at the margin of the depression due 

to positive curvature and reach a maximum at 

the point of maximum curvature (fig. 14A, D). 

The tensile strain reaches zero at the point of inflection, where the ground tilt reaches a maximum. The stresses and resulting strains become 

compressive inward from the point of inflection 

due to negative curvature and reach a maximum 

at the point of maximum negative curvature. 

Above critical or supercritical mining areas, 

negative curvature and tilt decrease inward and 

are zero where the curvature is zero. Compressive 

stresses and strains may be greater above subcritical mining areas than they are above critical 

or supercritical areas because two zones of compressive stress commonly are superimposed.

Tensile stresses and strains often are greater 

above coal barriers between two adjacent mine 

areas where tensile stresses, caused by positive 

curvature at the margins of the two adjacent 

depressions, are superimposed (Dunrud, 1976a, 

p. 4-5; Shadbolt, 1978, p. 727-728). As mining 

proceeds, the strata and surface are subjected to 

all these types of deformation until the mining no 

longer influences the surface. Shear stresses and 

strains also are generated along major lithologic 

boundaries by flexure of strata caused by downwarping of the rock mass into mine cavities. Further information on deformations produced by 

subsidence can be found, for example, in Shadbolt (1978, p. 705-748), in the Subsidence 

Engineers Handbook of the United Kingdom (National Coal Board, 1966, 1975), Dunrud (1976a), 

Peng (1978, p. 281-342), Singh (1979, p. 92-112), 

Salamon (1978, p. 187-208), and Pottgens (1978, 

p. 267-282).

Subsidence depressions may not occur, or at 

least may not be evident without the use of precision surveying instruments, above room-andpillar mine areas where only a small percentage of 

the coal is removed (figs. 8, 11, 14A, B, C). In 

these areas pits may eventually form if the overburden is not thick enough for the cavities to 

close by bulking of the caved debris. Subsidence 

pits can occur suddenly and without warning 

where depressions do not occur. These pits commonly are deeper than the pits within depressions. Pits within subsidence depressions indicate that coal pillars or parts of pillars are present 

in the mine openings below, but that these pillars 

are not strong enough to support the weight of 

the overburden completely (figs. 7, 8, 14A). Consequently, the overburden and surface subside, 

but not as much as if all the coal were removed. 

Pits form later, sometimes decades later, when 

the zones of compressive stress above individual 

mine openings or above the intersections of two 

mine openings migrate to the surface. The total 

depth of the depressions and pits may be nearly 

as deep as or sometimes even deeper than the 

height of the mine workings, because the collapsed material may spread laterally into adjacent mine openings, particularly where water is 

present in the mine workings to promote it.

The surface area affected by subsidence commonly is greater than the area mined where all or 

much of the coal is removed (fig. 14Z); A, right 

side), and particularly where the overburden is 

thicker than about 60 m. The draw angle or limit 

angle j is defined as the angle, referenced to the 

vertical, made by drawing a line from the margins 

of the depressions or affected surfaces at the surface down to the margins of the mine area causing 

the subsidence. Studies in abandoned coal mine 

areas north of Sheridan, Wyo., indicate that the 

draw or limit angle is steep to nearly vertical 

where the overburden is less than about 60 m 

thick. This angle is nearly 0 above barrier pillars 

between two adjacent mining panels and ranges 

from 10° to 25° above solid coal barriers in moderately strong Upper Cretaceous Mesaverde 

rocks in the Somerset district, Colorado (Dunrud, 

1976a, p. 34). However, the limit angle might 

range from 0° to 35° in many mines of the West, 

depending on overburden thickness, mining
28 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

methods, geology, and topography. In the Limburg coal mining area of the Netherlands, the 

limit angle is approximately 45° (Pottgens, 1978, 

p. 267), whereas it averages 35° in British coal 

fields, but may be less in deeper mines, according 

to Shadbolt (1978, p. 726). The limit or draw angle 

probably changes attitude with lithology, structure, strength, and depth of overburden.

The angle of break, or break angle /? —which is 

the angle of inclination of a line connecting points 

of maximum tensile stress and strain, from the 

edge of the mine workings to the surface—is less 

than the angle of draw. The break line—the line 

connecting points of maximum tensile stress and 

strain—normally is not straight, but steepens in 

thick, strong rocks and flattens in thin, weak 

strata. In the Sheridan, Wyo., area and in the 

Beulah, N. Dak., area, the break line appears to 

be nearly vertical to perhaps slightly negative; 

that is, dipping steeply away from the mine opening (fig. 14A). In the Limburg area of the 

Netherlands, the break angle is about 20° (Pottgens, 1978, p. 269). Knowledge of limit angle and 

break angle is important in land-use planning 

because these angles determine the limits of 

deformation.

Information regarding the rate and amount of 

subsidence and attendant surface strain that can 

be expected for mined-out areas of various widths 

and heights and for various overburden thicknesses of more than about 60 m can be found, for 

example, in Zwartendyk (1971); in the Subsidence 

Engineers' Handbook (National Coal Board, 

1966, 1975); in Salamon (1978, p. 187-208); 

Dunrud (1976a, p. 3-36); Brauner (1973a, b); and 

Pottgens (1978, p. 267-282). However, little or no 

specific information is available on the rupture 

limits in tension and compression of various 

types of bedrock or surficial material in any 

sources known by the authors. Studies to 1978 in 

the western Powder River Basin show that the 

rupture limit of bedrock (limit of strain at which 

rupture occurs)—consisting of weak, soft siltstones, claystones, shales, lenticular sandstones, 

and coal beds—is significantly lower than that of 

the soil and colluvium overlying the bedrock. 

Cracks as wide as 15-20 cm were observed in 

bedrock but were not observed in overlying colluvium or soil cover except where holes caused by 

piping failure or the activities of man or animals 

identified the underlying cracks (fig. 10).

Deformation of the soil and colluvium under 

tension was observed at the margins of subsidence depressions (figs. 7, 8, 9). Cracks were 

common in soil and colluvium where differential 

vertical settlement was more than about 0.8 m 

within a lateral distance of 3-6 m in overburden 

9-25 m thick. Cracks were rare in soil and colluvium at the margins of subsidence depressions 

where differential vertical settlement was less 

than about 0.6 m within 3-6 m in horizontal 

distance. It is not known whether or not the 

underlying bedrock is cracked beneath the soil 

and colluvium.

MODERN COAL MINING

Subsidence caused by modern underground 

coal mining beneath overburden less than about 

60 m thick, or less than about 10-15 times the 

thickness of coal mined, cannot be evaluated in 

the Powder River Basin and in western North 

Dakota, because no underground mines are currently operating in these areas. As far as is 

known by the authors, nearly all current mining 

in thin overburden is done by surface methods 

in the United States as well as in the rest of 

the world, because it is more feasible from an 

operational and economic standpoint. The recent 

Subsidence Engineers' Handbook from the NCB 

(National Coal Board of the United Kingdom, 

1975, p. 9), for example, does not predict subsidence amounts where overburden above longwall coal mines is less than about 50 m thick. 

Subsidence information from NCB also does not 

apply to pillar and stall (room-and-pillar) mining 

(1975, p. 40) because coal pillars remaining after 

mining as well as mine roofs and floors may fail 

for many years due to stress concentrations and 

(or) wetting of claystones and shales susceptible 

to deterioration by water. Information on the effects of modern coal mining by underground 

methods in overburden less than about 60 m 

thick appears to be rare or nonexistent.

In compliance with recently enacted Federal 

Coal Mining Operating Regulations (Federal 

Register, 1976, 30 C.F.R. 211, May 17, 1976, 

p. 20261-20273), underground mining of coal on 

Federal lands may be required in areas where the 

overburden is of variable thickness because of 

topography or structure, is locally less than
MODERN COAL MINING 29

about 60 m thick, but commonly is too thick to 

mine by surface methods. One primary purpose 

of the Coal Mining Operating Regulations 

(Federal Register, 1976, 30 C.F.R. 211.l(b), 

May 17, 1976, p. 20261) is to:

". . . assure the orderly and efficient prospecting, exploration, testing, development, mining, 

preparation and handling operations, and production practices without avoidable waste or 

loss of coal or other mineral resources or 

damage to coal-bearing or other mineralbearing formations. ..."

The first part of the section on "Underground 

mining, maximum recovery" (Federal Register, 

1976, 30 C.F.R. 211.30, May 17, 1976, p. 20268) 

stated that:

"Underground mining operations shall be conducted so as to yield a maximum recovery of 

the coal deposits consistent with the protection 

and use of other natural resources, sound 

economic practice, and the protection of the environment—land, water, and air."

Accordingly, if appropriate from an environmental and resource protection standpoint, mining 

plans for particular areas may include provisions 

for mining coal by underground methods beneath 

overburden less than about 60 m as adjacent coal 

beneath overburden thicker than about 60 m is 

mined. In these areas, unless a company planning 

underground mining operations could negotiate 

with a surface mining company to mine the coal 

beneath the thinner overburden or unless the 

underground company had surface mining equipment, the coal beneath overburden less than 

about 60 m thick might be mined by underground 

procedures.

In order to evaluate the overall effects on the 

environment and on resources caused by underground coal mining beneath thin overburden, 

future mining sites should be studied to determine such factors as (1) limit or draw angle and 

break angle, (2) subsidence amount and rate, and 

(3) local deformation. Studies by the authors 

(Dunrud and Osterwald, 1978a, b) and others (for 

example, Gentry and Abel, 1978, p. 202-203; 

Shadbolt, 1978, p. 729) in areas where the overburden is thicker than about 60 m show that subsidence above modern underground mines commonly equals 60-90 percent of the thickness of 

coal mined. In the Limburg coal mining area of

the Netherlands, maximum subsidence is as much 

as 96 percent of the extracted coal thickness 

(Pottgens, 1978, p. 269). Deformations, in the 

form of tension cracks, commonly occur at the 

margins of subsidence depressions, and compression bulges locally occur within the depressions 

provided that the areas mined are at least as wide 

and as long as the overburden is thick (critical 

mining area) and that the maximum amount of 

coal that can be safely and economically extracted is removed (fig. 15; Dunrud, 1976a, b).

Topography, lithology, structure, and amount 

of water present in overburden rocks control the 

rate and amount of subsidence as well as the 

nature of surface deformation. However, as indicated previously, the mining thickness, areal 

extent of mining, mine geometry, and overburden 

thickness commonly are the dominant controlling 

factors.

Tension cracks commonly are wider, more 

abundant, and more extensive near cliffs and 

steep terrain than they are in flat or gently rolling 

topography (Dunrud, 1976a, p. 12). Results of 

subsidence measurements above a supercritical 

longwall mining area in the York Canyon Mine 

near Raton, N. Mex., where rugged canyon 

topography is underlain by moderately strong 

mudstones and sandstones less than about 225 m 

thick, show that cracks are more extensive and 

abundant and the horizontal movement was 

much greater on steep slopes where mining progressed from beneath thicker towards thinner 

overburden than it was on slopes of similar grade 

which progressed from beneath thinner towards 

thicker overburden. In addition, the total amount 

of subsidence was from 25 to 30 percent greater 

on ridges than it was in valleys (fig. 15; Gentry 

and Abel, 1978, p. 203-204). Horizontal surface 

strain was as much as twice as great as predicted 

from studies in the United Kingdom.

A similar effect of topography on surface cracking was observed in the fall of 1977 in the 

Somerset, Colo., area, where moderately strong 

sandstones, mudstones, and shales 100-200 m 

thick underlie very rugged topography. Numerous tension cracks, some as much as 50 cm wide, 

occurred on a steep slope facing in the direction of 

pillar extraction above a room-and-pillar mining 

area where coal was mined from beneath a ridge 

towards a canyon bottom, whereas cracks were
30 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

rare and only a few centimeters wide on a similar 

slope facing in a direction opposite to the direction of pillar extraction, where mining progressed 

from the canyon bottom toward the next ridge.

In overburden consisting of strong rocks, such 

as well-cemented sandstones, the draw or limit 

angle steepens and the total amount of subsidence sometimes is less, compared to weaker 

shales and mudstones (Dunrud, 1976a, p. 5-7). 

Strong rocks, such as well-cemented sandstones, 

siltstones, and limestones, may also break into 

larger fragments than shales or mudstones. Subsidence amounts may be less in strong, massive 

rocks than in weak rocks because of higher bulking factors in caved zones near the mine workings.

Structural features, such as joints and faults, 

commonly localize subsidence movements. Open 

cracks can occur along joints in well-cemented 

sandstones and other strong rocks, particularly 

near cliffs (Dunrud, 1976a, p. 8-12). Subsidence 

can be localized and concentrated along faults, 

even in thick overburden. Extensive offset subsidence cracks were observed by the authors in 

Carbon County, Utah, in 1977; they occurred in 

strong overburden, along fault projections as 

much as 700 m above the elevation of room-andpillar mines that were driven in the late 1950's 

and early 1960's.

Backfilling mine openings with mine tailings or 

other available material during mining might 

reduce the amount of subsidence by about 50 percent (fig. 15) and perhaps reduce the rate of 

subsidence as well, which also commonly would 

reduce surface damage. This practice, however, is 

costly, particularly after the mines are abandoned. Abandoned coal mines that have not yet 

been affected by subsidence cover some 170,000 

hectares (418,000 acres) in the 25 coal-producing 

States. Costs to backfill these mines have been 

estimated at $12.5 billion by the U.S. Bureau of 

Mines (Johnson and Miller, 1979, p. 9).

The amount of water present in, or available to, 

the overburden from the surface also may control 

the amount and rate of subsidence. In areas 

where large amounts of ground water are reported to be common in the overburden, such as 

in the Netherlands (Pottgens, 1978, p. 267) and 

the United Kingdom (Piggott and Eynon, 1978, 

p. 752), subsidence amounting to 90-96 percent 

of the thickness of coal mined is common in

supercritical mining areas, whereas in the Somerset, Colo., and Raton, N. Mex., areas—where 

small amounts of ground water are present in the 

overburden—subsidence amounts to about 70 

percent of the mining thickness. Rock fragments 

in caved zones above the mine workings, particularly fragments of weak rocks such as shales 

and mudstones, may be significantly smaller 

where submersed in water than in dry or slightly 

wet zones.

COAL MINE FIRES

Coal mine fires are common in abandoned mine 

workings in the Sheridan, Wyo., area. The fires 

threaten both the environment and adjacent unmined coal deposits (fig. 16). According to the 

U.S. Bureau of Mines (Johnson and Miller, 1979, 

p. 19), about 250 uncontrolled fires are burning in 

abandoned underground coal mines in 17 States 

located primarily in the east and the west, of 

which about 195 are located in the Western 

States and approximately 150 in the States of 

Montana, Colorado, Wyoming, and North 

Dakota. Studies by the authors show that fires 

are burning in about a 3-km2 area in parts of at 

least five abandoned coal mines 10-20 km north 

of Sheridan, Wyo. Other areas of various underground mines may be on fire that have not yet 

been detected.

Most of the fires apparently started in these 

abandoned mines by spontaneous ignition when 

oxygen and water were introduced to the mine 

workings through subsidence cracks and pits and 

unsealed portals or shafts. Ignition probably occurred initially in piles of coal consisting of 

fragments of various sizes and fine dust that remained in the abandoned mines where oxygen 

and water were available in proportions conducive to combustion. Reports published during 

the past 50 years of laboratory studies-on spontaneous heating and ignition of coal (Kim, 1977, 

p. 2-6) reveal that (1) availability and flow of oxygen, (2) particle size, (3) rank of coal, (4) changes in 

moisture content, and (5) other factors, such as 

temperature, pyrite content, geologic structure, 

and mining practice, are the prime factors causing spontaneous heating and ignition. Coal rank 

and changes in moisture content appear to be two 

of the more important factors.
COAL MINE FIRES 31

Field studies indicate that, once the coal ignites, 

the fire can support combustion and spread by 

drawing fresh air in through open subsidence 

cracks and pits and exhausting gases via other 

cracks and pits. Voids created as the coal is 

burned produce further ground settlement, additional tension cracks, and more pits, which in 

turn provide more oxygen to the fire. Noxious 

gases, smoke, and steam produced by the fires 

are a major source of air pollution in the area.

A reconnaissance sampling of the gases that 

are exhausted along with smoke and steam

through tension cracks and pits was conducted 

by the authors in the Acme mine area in April 

1976 (fig. 4). Analyses by M. E. Hinkle, U.S. 

Geological Survey, and R. L. Kaplan, MSHA, 

revealed the presence of carbon disulfide, carbon 

oxysulfide, and an unknown sulfur compound. 

Methane in excess of 1 percent by volume was 

detected in a crack near one of the more intense 

fire areas. The exhausted gases also commonly 

contained considerably less oxygen and more 

helium and carbon dioxide than did the normal atmosphere. Carbon monoxide, ranging from a

• -<'"

FIGURE 16.—Eastward aerial oblique view of the surface effects of an underground coal mine fire above northern part of the 

Acme mine (November 1975). The firepit, shown in greater detail in figures 17,18, 19, and 20, is the brown spot in the left 

center of picture that is exhausting blue smoke and is bounded on the front (west) by plumes of steam. Steam fumaroles 

can be seen above subsidence pits and cracks throughout the valley. Most of the grass and trees in the foreground were 

burned when the fire first reached the surface. A coal bed that crops out in the draw about halfway between the road and 

the firepit (left foreground), will eventually be ignited by the advancing fire unless it is controlled.
32 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

FIGURE 17 (above and facing page).—Northeastward aerial oblique views of the firepit located above the northern part of the 

Acme mine. A, Position of firepit in November 1975. B, Position of firepit in April 1978; fire has advanced about 40 m during this period; total length of firepit in 1978 was about 110 m. Note anomalous green grass adjacent to scorched area 

around firepit. Fire, blue smoke, and steam issue from branching and offset cracks caused by subsidence above the burning 

coal. Crenulate cracks within and marginal to the pit divert surface runoff and ground water. The firepit area is within a 

larger subsidence depression that contains numerous old pits and is bounded by tension cracks as much as 3 m wide.

trace to as much as 0.35 percent by volume, also 

was locally detected.

Fires in the abandoned underground mines 

locally have breached to the surface or are close 

enough to the surface to produce ground temperatures that melt snow cover readily during the 

winter. Bedrock and surficial material weakened 

by steam and heat generated by fires in coal 

mines may collapse into underlying cavities 

without warning, particularly in areas where (1) 

coal fires have started recently, (2) the overburden is less than about 10-15 times the height 

of the mine workings, and (or) (3) thermal gradients have not been established and there are no 

elevated ground surface temperatures.

An underground fire above the northern part of 

the Acme mine breached to the surface in 1972 

and started a large grass fire that also burned a 

grove of juniper and pinon trees (figs. 4, 17, 18, 

19). In the breached area, which is locally called 

"the firepit," a vertical column of fire and molten 

rock 1.5-3 m in diameter and an estimated 15 m 

high supported combustion in the spring of 1976 

by burning other coal beds in the section or by 

burning the combustible products of the coal bed 

gasified by the fire, or by both processes (fig. 20). 

Temperatures in flame-filled cracks and chimneys 

averaged 925 °C, as measured with an optical 

pyrometer. Sulfur deposits are common along 

smoking cracks with a strong sulfur odor where
COAL MINE FIRES 33

the temperature ranged from 90° to 225 °C. Near 

the firepit, temperatures were as much as 150°C 

in cracks at the margins of scorched and dying 

grass. Temperatures in steaming cracks in green 

grass a few meters from the firepit were 

measured at about 60 °C.

Another firepit, which formed above the eastcentral part of the Acme mine from 1974 to 1977, 

afforded a good chance to study the processes of 

firepit formation (figs. 4, 20A, B). Elevated 

ground temperatures and fumes had begun to kill 

the vegetation in November of 1974 when the 

area was first visited by the authors. During 1975 

and early 1976 the ground began to sink into an 

elongate trench as much as 2.5 m deep, 15 m 

wide, and 25 m long. Temperatures as high as 

150°C were measured in cracks 30 cm down. At 

the collapse site in April 1976, a strong sulfur 

odor was detected from tension cracks along the 

margins of the depression.

The initial collapse occurred in May 1977 near a 

large tension crack on the east end of the depression. The resulting pit was a jagged hole measuring about 2.5 m long and more than 3 m deep. The 

pit was encompassed by crenulate, roughly concentric cracks. Collapse is the culmination of successive stoping of overburden rocks above 

cavities; it operates much like the stoping process 

above mine cavities that are not burning, except 

that the process probably is accelerated by the 

continual enlargement of the underground opening and by effects of high temperatures on the 

overburden material

The collapse occurred near a tension crack that 

had the highest temperatures and produced the 

most sulfur fumes in the area during 1976. Fire 

and sulfur fumes were issuing from the jagged 

hole. The hole may continue to enlarge, now that 

the initial pit has formed (fig. 2GB). A coal bed is 

believed to occur between the abandoned mine
34 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

workings, which are on fire, and the surface. This 

bed, if present, also is on fire.

A 1979 subsidence episode and fire in the northwest corner of the New Monarch mine (figs. 4, 

21 A, B) yielded considerable detailed eyewitness 

information about subsidence processes related 

to mine fires. Subsidence began with a single pit 

(pit 1, fig. 21 A) about 2 m wide and 6 m deep that 

occurred in 1977 (H. F. Alley, oral commun., 

1979). About a year after the hole was discovered, 

a fence was built around the hole as a safety 

precaution. Steam was first observed in the pit by 

H. F. Alley in mid-November 1978.

On December 1, 1978, DEQ (Department of Environmental Quality), State of Wyoming, received an air pollution complaint from a local resident concerning large quantities of acrid smoke

and steam in the Monarch area (G. L. Mooney, 

written commun., 1978). Subsequently the fire 

area was inspected by G. L. Mooney, DEQ, D. L. 

Donner, USBM (U.S. Bureau of Mines), and L. 

Hendrickson, OSM (U.S. Office of Surface Mining) on December 19, 1978 (fig. 21A, B). Steam 

and smoke were emanating from the initial hole 

(fig. 21 A, pit 1) on the site, which had enlarged to 

about 14 m in maximum diameter and 9 m deep. 

About half of the fence had collapsed due to 

enlargement of the pit. In addition, two new subsidence pits were found (fig. 2LA, pits 2, 3). Pit 3 

was located about 7 m north of a high voltage 

powerline serving the towns of Ranchester and 

Dayton and adjacent rural areas (T. P. Hammond, Electrical Superintendent, MontanaDakota Utilities, oral commun., 1979).

FIGURE 18 (above and facing page).—Southward views of the 

firepit above the northern part of the Acme mine. A, Bare 

ground surrounded by snow 0.25-1 m deep shows that the 

underground fire is much more extensive than the firepit

area (March 1978). B, The scorched, cracked, and collapsed 

ground of the firepit area is in stark contrast with the surrounding lush green grass (May 1978). Other bare spots 

can be seen in background where either high ground
COAL MINE FIRES 35

On January 2, 1979, an explosion shook the P. 

L. Vine residence located about 600 m south of 

the subsidence area (P. L. Vine, oral commun., 

1979). Black smoke and orange flames were visible above a 30-m-high ridge located between the 

subsidence area and the Vine residence. The 

flames appeared to be coming from pit 2. A 

second explosion was heard and felt by Mr. Vine 

about 5 hours later. Apparently, either methane 

present in the mine had suddenly ignited or 

steam in the mine workings was suddenly released under pressure.

On January 3, another steaming and smoking 

pit (fig. 21 A, pit 4) appeared about 30 m west 

of pit 3 and 3 m north of the powerline (T. P. 

Hammond, oral commun., 1979). This pit was 

oblong in plan, measuring about 4 X 8 m and as

much as 6 m deep (T. P. Hammond, oral 

commun., 1979). At this time, fire could be seen at 

ground level in pit 2. Flames 10-12 m high were 

observed by T. P. Wollenzien, geologist, Peter 

Kiewit Sons' Co., the evening of January 3 (fig. 

22^1; T. P. Wollenzien, oral commun., 1979).

G. L. Mooney, DEQ, visited the fire area on the 

afternoon of January 4 in response to another air 

pollution complaint from a resident living about 

6 km southeast of the fire area. Large quantities 

of smoke and steam were erupting from pit 2, 

which now was about 7 m in diameter (G. L. 

Mooney, written commun., 1979; fig. 22B). Only 

small amounts of smoke and steam were coming 

from pit 1. Steam and smoke also were emanating 

from pit 3, where no emissions were noted on 

December 19. At this time, pit 4 was emitting

temperatures or noxious fumes inhibit growth of vegetation. Blue smoke and fire issue from the firepit and adjacent branching and offset subsidence cracks. Steam is 

emitted from cracks about 2 m ahead of the fire front in

right foreground and also along ridge in background. 

Craters within the pit are either explosion craters caused 

by sudden releases of steam and other gases or are local 

secondary collapse features.
36 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

large amounts of steam and smoke, second in 

volume only to pit 2. Another pit (pit 5) was observed about 15 m north of pit 2. It was estimated that the fire was burning in about three 

rooms of the abandoned mine (fig. 2,1 A).

Fire control attempts began on January 6, 

when a local contractor began filling pit 4 to 

protect the powerline from fire damage (T. P. 

Hammond, oral commun., 1979). During the day, 

personnel walked across the ground surface between pits 3 and 4. The following morning a 

steaming pit (pit 6) had formed where persons 

had walked the previous day (fig. 21 A, B; T. P. 

Hammond, oral commun., 1979). The snow, which 

was about 0.5 m deep, had not melted nor was 

there any other indication of impending collapse 

(fig. 22D). At this time the contractor withdrew 

his equipment because of the hazard. It was 

decided to use an emergency fund supplied by 

OSM to contract for (1) drilling and gamma logging to define the mine workings and solid coal 

and (2) filling the holes after the mine workings 

and coal pillars were defined.

The fire area was first visited by Dunrud on 

January 9, 1979. Large volumes of steam were 

emanating from pit 5 with lesser amounts from 

pits 3, 4, and 6 (fig. 2L4, B). A small amount of 

smoke was present in pit 1; pit 2 was devoid of 

either smoke or steam. Pit 2 and the north side 

of pit 4 showed obvious signs of intense heat 

(fig. 22C). Bedrock and surficial materials in the 

pit were baked and stained red 1-3 cm inward 

from the surface of pit 2. The surface was blackened with soot a few millimeters thick. Sagebrush 

around pit 2 was burned or scorched. Interconnecting underground cavities 1-3 m high with 

arched roofs, located 1-4 m below the ground surface, were observed in pits 2, 4, and 6. Pit 4 had 

enlarged southward within the past few hours 

(fig. 22D).

A map prepared by D. L. Donner, G. L. 

Mooney, and C. R. Dunrud, using information 

from 10 drill holes and a map of the underground 

mine workings, revealed that the pits and interconnecting cavities occurred above northtrending rooms about 7.5 m wide that are separated by pillars 10-20 m long and 7.5 m wide; 

crosscuts between pillars are 3-5 m wide (fig. 

21 A). The mine workings, which are 5-6 m high, 

are in the Monarch coal bed (fig. 21B). Results of

drilling, gamma-ray logging, and inspection of 

the pit walls revealed that another 5.5 m of coal, 

containing five rock partings 50-75 cm thick, occurs above the mine workings; this coal in turn is 

overlain by 10-22 m of silty claystone with thin 

local siltstone lenses and a coal bed about 1 m 

thick located approximately 5 m above the base. 

Surficial deposits, 1-5 m thick, overlie the 

bedrock.

Filling of the pits began again on January 10, 

using a large bulldozer. By midafternoon, pits 4 

and 6 were filled. However, a pit suddenly developed beneath the bulldozer during the filling of 

pit 3, causing it to tip sideways (fig. 23^1). Inspection of the hole beneath the bulldozer revealed 

that it was connected northward toward pit 2 by 

an arched underground passageway 1-2 m high, 

2-3 m below the surface. The hole was filled using 

another bulldozer and backhoe (fig. 23B).

New pits continued to develop near the locations of the six original pits after repeated filling. 

On January 11, steaming pits were observed 

near, or above, all three pits filled the previous 

day. On January 12 pits were again observed near 

the south edge of pit 4 and the north edge of pit 6. 

Rumbling sounds were heard near pit 4. On January 24 seven new pits, ranging from 1 to 3 m in 

maximum width and depth, were observed by 

Mooney and Dunrud near the original pits (fig. 

21 A). Two more steaming and smoking pits were 

observed north of pits 3 and 5 on March 1, 1979. 

In addition, cracks 1-20 cm wide were observed 

around the original pits 1, 3, 4, and 6. By midMay three additional pits had developed—one circular pit immediately south of pit 2 and two 

elongate pits east of the original subsidence area 

(fig- 23Q.

A subsidence pit and fire also occurred above 

the south part of the abandoned Acme underground coal mine about 35 m west of a highwall of 

the Big Horn surface mine on the night of 

January 22, 1979 (figs. 4, 7A, 24). Flames were 

visible about 10 m above ground level for a few 

hours. Large volumes of steam and yellowishgray smoke emanated from a rapidly enlarging 

pit on January 24 and 25 when Dunrud visited 

the site (fig. 24A). The diameter of the pit enlarged from about 2.5 to 5 m and the volume of 

steam and smoke tripled or quadrupled in about 

20 hours. Acrid steam and smoke filled the
FIGURE 19.—Closeup views of the advancing fire front in the firepit (April 1976). A, Northward view across the edge of the 

firepit. The grass is dead near the pit from the effects of heat and noxious gases but is greener than normal in the 

background where the temperature promotes plant growth. Intense heat from the fire front scorches adjacent grass and 

shrubs and occasionally causes grass fires. B, Southward, view across the advancing edge of the firepit. A severe fire hazard 

exists in late summer and fall. The fire column is hot enough to melt the bedrock.

Tongue River Valley for a few kilometers 

upstream and downstream from the fire until the 

pit was filled on January 25.

In contrast to the fire above the New Monarch 

mine, the fire above the Acme mine produced 

flames above ground within minutes after the pit 

formed, perhaps because large quantities of air 

were available to the mine workings from mine 

openings exposed on the highwall of the nearby 

surface mine (fig. 24B). Like the fire above the 

New Monarch mine, large volumes of foulsmelling steam and smoke were emitted from the 

pit after the fire subsided below ground level. 

Steam was present in greater proportions at the

Acme site than at the Monarch site, perhaps 

because more water was available. The large 

quantities of steam probably contributed to the 

rapid enlargement of the pit.

Fires caused by spontaneous combustion also 

locally occur on high walls of abandoned surface 

mines or on exposed coal in active surface mines 

(Hertzberg, 1978, p. 47-49) in the Powder River 

Basin. The coal outcropping at water level in the 

abandoned Plachek surface mine near the currently active Big Horn mine caught fire, apparently by spontaneous combustion, in the early 

1970's (fig. 6). During 1975 and 1976 the fire had 

spread along the outcrop and was beginning to
*

• ^ it •* * \ "t i </*r.*•!•>• -.?»
COAL MINE FIRES 39

10 20 30 

METERS

EXPLANATION 

Caved bedrock, coal, and surficia! material; fire where shaded

Fire

Surficial material; weathered bedrock and soil

—^ Claystone, soft, silty, with local lenses of soft siltstone

Coal, Monarch coal bed overlain by coal and rock interbeds

FIGURE 21.—Subsidence and fire above northwest part of 

New Monarch mine. A, Idealized block diagram showing 

subsidence pits, lithology of overburden, and underground 

mine workings in the fire area on January 9,1979. B, Map 

showing relation of room-and-pillar mine workings to subsidence pits, filled pits, new pits, and estimated fire area 

(outlined by heavy dashed line), January 1979. Compiled 

by D. L. Donner, USBM, G. L. Mooney, DEQ. and C. R. 

Dunrud, based on taped traverses, drilling, geophysical 

logging, and inspection of pits. Voids 0.6 m high were encountered in drill holes 6 and 7; a void 6 m high and fire 

were encountered in drill hole 4.

O

EXPLANATION

Powei Iine--Montana-Dakota Utilities, 46,000 volts

New subsidence pit mapped 3-1-79

Subsidence pit-Dashed hachured lines where filled with 

earth material by 1-17-79; small ellipses, new pit 

mapped 1-24-79; cracks around pits, mapped 3-1-79

Coal pillar—Dashed line where probably burned, solid 

line where pillar probably in place as of January 1979

Coal pillar, mined by 1953

Drill hole; drilled 1-(8-9)-79, void encountered in coal 

Drill hole; drilled 1-(8-9)-79, no void encountered 

Subsidence pit as old as or older than pit 1

FIGURE 20.—Firepit above the east-central part of the Acme 

mine (May 1977). Broken, cracked, and locally blackened 

ground and scorched grass provide a stark contrast to the 

lush green grass surrounding this new firepit. A, Eastward 

view across firepit area. Meadowlands adjacent to the 

Tongue River are in the background. B, Southeastward 

view showing the jagged pit, which is bounded by 

crenulate, roughly concentric cracks. The depression (middle) is about 2.5 m deep. The pit, which is more than 3 m 

deep, contains fire and hot rocks, although they are not 

visible in picture because of the sunny day. Temperatures 

in the pit were measured at about 850 °C with an optical 

pyrometer.
40 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

n

FIGURE 22.—Effects of subsidence and fire above the New 

Monarch mine (January 1979). A, Steam and smoke (left) 

and fire (right) erupting from pits 1 and 2, respectively, at 

7:00 p.m. on January 3 (fig. 21.4). Flames are about 10 m 

high. Powerline is visible in foreground. Photograph courtesy of T. P. Wollenzien, geologist, Peter Kiewit Sons' Co. 

B, Eastward view showing column of smoke and steam 

erupting from pit 2 (fig. 21.4) on January 4, 1979. Base of 

column is about 7 m in diameter. Photograph courtesy of 

G. L. Mooney, DEQ, State of Wyoming. C, Northward 

view showing pits 1 (left middle ground), 2 (middle 

ground), and 4 (foreground). Note areas free of snow near 

pits 2 and 4, where heat from fire melted snow; fire also 

blackened the rim of pit 2 and the north edge of pit 4. D, 

Southward view of pit 4 showing melted area (foreground), 

overhanging rim, and freshly caved snow and soil near 

steaming area. Note that the snow shows no sign of 

melting at cave line in background on either side of steam.
ENVIRONMENTAL CONSEQUENCES OF COAL MINING 41

advance underground. Fires such as this can support combustion underground for considerable 

distances by intaking air and exhausting gases 

via tension cracks caused by collapse of the overburden into voids left when the coal burned. 

Steps should be taken to insure that adequate 

amounts of noncombustible material protects the 

coal from possible ignition during abandonment 

and restoration procedures in any future surface 

mining activities.

Fires also are a serious problem in currently 

active underground mines. Mine deformation 

studies in coal mines in Utah and Colorado by the 

authors (Dunrud and Osterwald, 1978b, p. 58) 

show that fires are common. The fires apparently 

start by spontaneous ignition, particularly in 

areas where stresses are high and heat is produced by resulting deformation of the coal and 

rock. The areas continue to burn even years after 

they have been sealed. Steps should therefore be 

taken to control fires during operation and abandonment of modern underground mines.

SEISMIC ACTIVITY

Small earth tremors commonly are generated 

by caving and stress readjustments in overburden rocks above underground mine workings 

and coal fires in the Sheridan, Wyo., area. The 

authors and other personnel of the U.S. Geological Survey monitored seismic activity above the 

Acme mine with a temporary seismic network 

between September 30 and November 4, 1975. 

The network consisted of 10 vertically oriented 

seismometers with a natural frequency of 1 Hz, 

which were connected by wires to a mobile recording laboratory. Ground motion velocity was 

recorded both on magnetic tape and on visual 

seismograms. Magnification of the system, with 

respect to ground-motion velocity, was about 

200,000 at 10 Hz. One seismometer was installed 

within about 185 m of the firepit that breached to 

the surface in 1972 (figs. 16,17,18,19) during the 

first half of the recording period and within 30 m 

of the firepit during the last half of the period.

In addition to earth tremors generated by 

blasting in the Big Horn and Decker surface 

mines (figs. 2, 3), 20-90 small earth tremors were 

recorded per day by the seismometer when it was

located about 185 m from the firepit, whereas 

550-800 small earth tremors per day were recorded when the seismometer was moved to 

within about 30 m of the firepit (fig. 25). Later 

seismic studies, during a 6-week period in the fall 

of 1976 above the Acme mine, showed a large increase in the seismic activity, which indicated 

that the fire was accelerating at a rapid rate.

At the site of the firepit, as well as in other 

areas of the Acme mine, most of the tremors appeared to be caused by breaking and caving of 

overburden rocks above and adjacent to areas 

where fires are burning intensely and perhaps 

have ignited overlying coal deposits. Another 

source of local seismic activity might be small 

underground explosions caused by steam suddenly released under high pressure.

A seismic network with audio recording capability and six vertical seismic stations linked to 

the mobile recording laboratory was installed 

above the fire area in the New Monarch mine on 

March 16, 1979. Preliminary results of the audio 

monitoring reveal thumping, rumbling, and hissing noises that sound like breaking and caving of 

the overburden and also movement and sudden 

releases of steam and air within the mine cavities.

ENVIRONMENTAL CONSEQUENCES OF 

COAL MINING

Adverse effects on the environment caused by 

underground mining or surface mining, in addition to potential hazards to life and property and 

effects on population growth, commonly comprise (1) disruption of the Earth's surface, 

bedrock, or other mineral deposits; (2) diversion 

or pollution of surface or underground water; and 

(3) spontaneous ignition of underground fires 

with the attendant land disturbance, water and 

air pollution, and surface fires. Past mining by 

both procedures has produced all of these effects 

in the Powder River Basin. However, in all cases 

known to the authors, the surface mining activities have produced less severe and shorter term 

problems than past underground mining. This is 

primarily because fire hazards are reduced by the 

extraction of most of the coal in the surface mining areas (compared to only a small percentage of 

the coal in the underground mining areas), and 

because problems such as damage to the surface,
42 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA
ENVIRONMENTAL CONSEQUENCES OF COAL MINING 43

FIGURE 23 (facing page and above).—Some pitfalls of filling 

subsidence pits. A, Track of large bulldozer (D-9 Caterpillar) fell into pit, which suddenly developed above an 

underground cavity (January 1979). B, Cavity beneath 

bulldozer initially was as much as 4 m deep.

water diversion, and pollution are open to view 

and can be examined, assessed, and corrected. In 

the underground mines, however, these problems 

may be hidden for many years or even many decades after mining is completed (figs. 2A, 7-11, 

16-24).

Modern surface mining activities in the Powder 

River Basin include well-planned sequential 

removal of topsoil, bedrock, and coal followed by 

replacement of spoil, topsoil, and revegetation 

(figs. 2, 3). Thus the land can be restored and put 

back into use in a few years or decades. Unstable 

conditions, locally common at the sites of older 

surface mines (fig. 6), and possible unstable 

conditions in restored spoil can be minimized

Backfilling operations beneath track took about an hour 

(January 1979). C, Southward aerial view of the subsidence 

and fire area (May 1979). Cracks and pits occur near the 

pits that were filled in January. Also new elongate pits 

occur to the east.

by grading and compaction in accordance with 

design specifications based on detailed, onsite geologic and geotechnical investigations 

of the bedrock and of broken and mixed rock in 

mine spoil. The fire hazard can be minimized by 

assuring that all coal outcrops and other remaining piles of coal are adequately blanketed by noncombustible material. Surface mine operators, 

therefore, can have good control of restoration 

procedures and can minimize long-range environmental damage if mining activities are properly 

planned and implemented from the initial cut to 

the final restoration of the highwall.

Many of these provisions and others are part of 

the Coal Mining Operating Regulations (Federal
44 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

Register, 1976, 30 C.F.R. 211.1 to 211.4, May 17, 

1976, p. 20261-20264), in the National Environmental Policy Act of 1969 (Public Law 91-190, 42 

U.S.C. 4321-4347, Jan. 1, 1970), as amended 

(Public Law 94-83, Aug. 9,1975), and the Surface 

Mining Control and Reclamation Act of 1977 

(Public Law 95-87) as implemented by the Surface Mining Reclamation and Enforcement Provisions (Federal Register, 1977, Part II, 30 C.F.R. 

700-725, December 13, 1977, p. 62639-62712).

Changes, diversion, and pollution of water are 

under study by other governmental agencies, but 

it appears to the authors that these effects can be 

minimized by proper planning based on on-site 

studies of geologic and hydrologic conditions. In 

any case, these effects are, in the opinion of the 

authors, less damaging than those often caused 

by modern underground mining of thick beds 

beneath thin overburden, wherein surface and 

ground-water flows may be changed and (or) 

diverted by surface depressions, tension cracks, 

and subsidence pits or troughs, and they may be 

locally contaminated by garbage and other refuse 

being placed in subsidence pits or troughs. 

Depressions resulting from extraction of thick 

coal beds by surface methods in the Powder River

FIGURE 24.- Fire in the southern part of the Acme mine 

(January 1979). A, Southward view of the fire area showing emission of a large column of steam and yellowish-gray 

smoke from a newly formed pit. Maximum height of spoil 

pile (background) is about 15 m. B, Westward view of 

highwall in Big Horn mine, located about 35 m east of 

steaming pit. Height of highwall is about 15 m. Two entries of the Acme mine (fig. 4) are exposed; the roof of the 

smaller one on the left (fig. 14C) has begun to collapse, 

whereas the wider one on the right still is intact.

Basin can perhaps be eliminated or minimized, in 

areas where topographic conditions and ownership or lease areas are amenable, by borrowing 

material from adjacent hills to compensate for 

the volume of coal removed, as is currently 

planned by Big Horn Coal Co. (figs. 2, 7, 8).

Environmental hazards caused by past underground mining in the Sheridan, Wyo., area include subsidence of the ground surface, diversion 

of surface and ground water, coal mine fires and 

attendant surface subsidence, and pollution of 

the air and water (frontispiece A, B; figs. 7, 8, 

16-20). Grasses and plants commonly die or are 

stunted near subsidence cracks, bulges, and 

along the margins of subsidence pits where the 

root systems are dewatered. Subsided lands are 

of limited use, and commonly are not useful for 

livestock grazing because stock can fall into pits 

and cracks (fig. 4). Also the noxious fumes 

emanating from cracks above burning coal mines 

have killed trapped cattle, in some cases, after exposure of less than an hour (Dan Scott, Padlock 

Ranch, Dayton, Wyo., oral commun., 1976).

Grass fires and forest fires also have been 

started when underground coal fires reached the 

surface. In 1972, a grass fire was ignited when a
ENVIRONMENTAL CONSEQUENCES OF COAL MINING 45

firepit in the northern part of the Acme mine first 

breached to the surface (figs. 16-20). The fire 

burned about 1.5 km2 of winter range grass and 

trees before it was brought under control.

All these environmental hazards limit the value 

of the land above these abandoned mine workings 

for agricultural, residential, or industrial development. Damage is compounded when the value of 

the lost coal is added to the damage done to the 

surface and to any overlying coal beds that often 

catch fire.

Periodic geologic and seismic studies of the 

Acme and New Monarch mine areas show that 

the fires are increasing in size and are spreading 

into unmined coal. It is estimated that at least 

five different fires are burning over a 3-km2 area 

(fig. 4). Other fires, such as reported in the Black 

Diamond mine in 1911 (Kuzara, 1977, p. 215-216), 

may still be burning although there presently is

800

700

[2 600

Z 

LU 

>

^ 500

o

CO

LU 400

CO

LL

0 300 

DC 

LLI

m

^ 200

Z

100 

0

-

-

-

—

_

-

-

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OCTOBER

20 2£> 30 1

1975

TIME (DAYS)

FIGURE 25.—Graph showing the daily number of small earth 

tremors at a seismometer station located near the firepit in 

the northern part of the Acme mine. The station was 

located about 185 m from the firepit from September 29 to 

October 19, 1975, then was moved to within about 30 m of 

the firepit. Note the approximate tenfold increase in 

number of events when the station was moved closer to the 

firepit. The earth tremors apparently are caused by the 

breaking and collapse of overburden material and perhaps 

by small underground explosions.

no surface evidence of a fire. Unmined coal beds 

of economic value locally overlie mine areas that 

are on fire, and they are threatened by the 

spreading fires below (figs. 4, 16-24). Seismic 

studies in the Acme and New Monarch mine 

areas also indicate that collapse and possibly 

underground explosions are increasing rapidly in 

these areas. Fires of this magnitude, in the opinion of the writers, may best be controlled by constructing a strip mine firebreak, isolation barrier, 

or isolation trench (Johnson and Miller, 1979, 

p. 18-21) around the burning area in order to cut 

off the supply of coal available to the fire. Carefully planned strip mining of the coal around fire 

areas, using mining procedures that protect personnel from subsidence and fire hazards, may be 

the best method to control fires in underground 

mines less than about 60 m, or about 10-15 times 

the coal thickness, below the surface and also 

might be economic in areas where large amounts 

of coal remain in place. Many local ranchers, coal 

operators, and others also favor timely strip mining around and within fire areas as the most effective control measure (figs. 16-24).

Water introduced from the surface seems only 

to increase the intensity of fires in this subbituminous coal. Other methods of control, such 

as hydraulically backfilling a slurry of retardant 

material through drill holes to the fire areas or 

operations to seal the fire, as done by the U.S. 

Bureau of Mines in several areas (for example, 

Whaite and Alien, 1975, appendix by Carlson), 

probably would not be cost effective on a fire the 

size of the Acme mine fire. Moreover, it would not 

recover any remaining coal.

Studies of coal mine fires suggest that the 

potential fire hazard also should be considered in 

planning in-place coal gasification or oil shale 

retorting activities. It would seem that advancing fire fronts can only be controlled as long as 

the supply of oxygen and withdrawal of gases are 

under controlled conditions. The presence of 

underground voids could create subsidence and 

cracks in the overburden that might allow the fire 

to intake air and exhaust gases independently 

from the designed system which, in turn, could 

result in out-of-control fires. The key factor is to 

control subsidence by either backfilling or designing the gasification or retorting operation
46 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

such that the cavities are small enough and the 

overburden is deep enough to provide a stable 

overburden above stable rubble-filled cavities 

(fig. 144). (See Shoemaker and others, 1979, 

p. 140-153, for examples of coal gasification 

model studies.)

HAZARDS IN RELATION TO LAND USE

The hazards to people, animals, and structures 

caused by coal mining activities can significantly 

affect future uses of the land because the severity 

of subsidence hazards may govern how mined 

lands are developed. Two of the basic hazards 

caused by coal mining—subsidence and landslides—also are identified as geologic hazards in 

"Warning and preparedness for geologic-related 

hazards, April 12, 1977" (Federal Register, 

1977a, Proposed procedures, p. 19292-19296). A 

geologic hazard is defined in this document as:

"... a geologic condition, process, or potential 

event that poses a threat to the health, safety, 

or welfare of a group of citizens or to the functions or economy of a community or larger 

governmental entity..." (p. 19292). 

Coal mine subsidence hazards to people include 

the sudden collapse of the ground surface above 

mine openings (figs. 7, 8, 11, 12, 13), above tension cracks bridged by soil (figs. 9,10), or collapse 

above cavities created by underground coal fires 

(figs. 16-20). Subsidence also can occur in spoil of 

surface mines, particularly in areas where increased surface loading and (or) rising water 

tables cause a reduction of porosity (Charles and 

others, 1978, p. 229-251). The greatest hazard to 

life and property in the Sheridan, Wyo., area is 

the sudden collapse of the ground surface into 

underground fires. In addition to ground collapse, the effects of tension, compression, and tilt 

due to the formation of depressions can damage 

structures beyond repair. Larger structures with 

small tolerances for deformation—such as multistory apartment buildings (Thorburn and Reid, 

1978, p. 87-99), schools (Stephenson and Aughenbaugh, 1978, p. 100-118), factories, and 

powerplants—commonly will sustain the greatest 

damage unless they are designed to withstand

the stresses and deformations caused by subsidence.

Landslides, which include rockfalls, slides, 

slumps, and earthflows on surface mine highwalls 

or reclaimed spoil material, can be a hazard to 

people and equipment during mining operations 

and also to people and structures after restoration, abandonment, and subsequent development 

of the mined land, unless proper grading, compaction, and vegetation procedures are followed. 

Small landslides on subsidence pits and cracks 

could damage adjacent structures or perhaps be a 

hazard to people and animals in the area. Landslides also may be triggered by subsidence on 

unstable slopes or, conversely, the increased surface loading from landslides above unstable mine 

openings may cause further subsidence.

Subsidence pits and cracks caused by underground mining and underground fires currently 

are a hazard to only a few persons traveling in the 

area and to animals. However, should lands 

underlain by underground mines be developed for 

either residential or industrial use, a hazard to 

many people and structures could exist in much 

of the area above abandoned underground mines 

(fig. 4). Abandoned coal mines with surface subsidence features are located in other areas in the 

western Powder River Basin that are not included in figure 4. Maps showing such aspects as 

present subsidence areas, fire areas, areas 

underlain by coal mines, thickness of overburden, 

and thickness of coal mined are needed to more 

precisely delineate current and potential hazard 

areas. (See also Ivey, 1978, p. 163-174.) Structures for residential or industrial use should not 

be sited above abandoned mines unless existing 

underground voids are located by systematically 

compiling appropriate subsidence and land-use 

maps and by conducting on-site engineering geologic studies, by drilling and geophysical studies 

where needed (Ivey, 1978, p. 163-174), or by other 

methods, and stabilized by such procedures as 

backfilling or grouting. Surface structures also 

might be designed to withstand the effects of 

subsidence in lieu of stabilization procedures. 

(For examples see National Coal Board, 1975, 

p. 64-95; Bell, 1978, p. 562-578; Johnson and
SUMMARY AND CONCLUSIONS 47

Miller, 1979; Geddes, 1978a, b; Shadbolt, 1978, 

p. 739-744; and Wood and others. 1978.

SUMMARY AND CONCLUSIONS

Subsidence studies in the western Powder 

River Basin and in western North Dakota indicate that, in overburden less than about 60 m 

thick or less than about 10-15 times the thickness of coal mined, the land surface can be reclaimed and returned to its original use, or to 

some other use, more quickly, easily, and cheaply 

if thick coal beds are mined by surface methods 

than if they are mined by underground methods, 

provided that proper restoration procedures are 

followed. Subsidence above room-and-pillar workings driven 25-80 years ago is still hazardous to 

man, animals, and the environment, and may continue to be a hazard for many years or many 

decades to come. Studies of subsidence above current underground coal mines indicate that subsidence effects—such as depressions, cracks, and 

bulges—would be caused by modern underground mining procedures, where the amounts of 

coal reserves extracted are acceptable in terms of 

economics, conservation, and hazard reduction. 

Depressions and cracks also disrupt the normal 

flow of surface and ground water.

Some subsidence pits, particularly in areas 

where the most dramatic and potentially hazardous collapses occur, may be deeper than the original height of the mine openings, because (1) the 

material spreads or is transported laterally into 

adjacent mine openings as it collapses, particularly in water-filled mine openings, (2) it compacts 

more than the state of the original material due to 

wetting and drying, or (3) it is subjected to both 

of these processes.

Coal recovery from thick coal beds in the weak 

rocks of the Tongue River Member of the Fort 

Union Formation is much greater, and long-range 

environmental problems are smaller, when 

modern surface mining procedures are employed 

rather than underground room-and-pillar mining, 

where the overburden is less than about 60 m 

thick, or where the overburden thickness is less 

than about 10-15 times the mining thickness. In 

view of current coal mining technology and the

subsidence effects from modern underground 

coal mining, surface mining may be the best way 

to produce coal in any area where coal thickness 

and overburden conditions make the operation 

economical, provided that proper restoration procedures are followed.

Coal mine fires, which often start by spontaneous ignition in the subbituminous and 

lignitic coal fields of Montana, Wyoming, and 

North Dakota, increase the damage to the environment manyfold compared to normal subsidence damage. The fires also consume large 

amounts of valuable energy resources as they 

create unsightly and hazardous steaming and 

smoking cracks and firepits, and pollute the air 

and water. Strip mine firebreaks appear to be the 

most effective way to control large underground 

fires beneath thin overburden, such as the large 

fires in the Acme and New Monarch mines. Most 

of the fires start spontaneously with the entry of 

oxygen and water through subsidence cracks. 

Once started, the fire spreads by creating larger 

cavities, increased subsidence, and additional 

cracks to intake oxygen. Careful consideration 

should therefore be given to subsidence and 

possible cracking in planned in-place gasification 

or retorting of combustible hydrocarbons such as 

oil shale.

Results of this report reveal a paradox. The 

land devoid of subsidence features, and therefore 

the most desirable for development, may be the 

most hazardous to develop if the area (1) is 

underlain by room-and-pillar mine workings, (2) 

the overburden depth is less than about 60 m 

thick or about 10-15 times the original height of 

the mine workings, and (3) sufficient coal remains 

underground adjacent to unstable mine openings 

to support, or partially support, the overburden. 

In these areas the land surface may be unaffected 

by subsidence for many years, or even hundreds 

of years, depending on the strength of the rock 

and the hydrologic conditions in the overburden. 

However, when and if surface collapse occurs as a 

result of successive collapse above the mine openings, the resulting pits can be much more of a 

threat to people, animals, and structures than the 

normal subsidence depressions and boundary 

cracks that commonly occur above mine work-
48 EFFECTS OF COAL MINE SUBSIDENCE IN THE SHERIDAN, WYOMING, AREA

ings where all, or nearly all, the coal was removed. 

In the opinion of the writers and others (for example, Ivey, 1978, p. 174), guidelines are needed 

to assure that all available mining and mine subsidence information is systematically assembled, 

evaluated, and made available to land-use planners before mined lands are developed for residential or industrial use or other uses involving the 

public welfare.

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OUS GOVERNMENT PRINTING OFFICE 1980—677.382