Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Geologic Hazards and Roof Stability

in Coal Mines

INFORMATION CIRCULAR 2003

IC 9466
`Information Circular 9466

Geologic Hazards and Roof Stability in Coal Mines

By Gregory M. Molinda

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Public Health Service 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, PA 

October 2003
ORDERING INFORMATION

Copies of National Institute for Occupational Safety and Health (NIOSH) 

documents and information 

about occupational safety and health are available from 

NIOSH–Publications Dissemination 

4676 Columbia Parkway 

Cincinnati, OH 45226-1998 

 FAX: 513-533-8573 

 Telephone:1-800-35-NIOSH 

 (1-800-356-4674) 

 E-mail: pubstaft@cdc.gov 

 Web site: www.cdc.gov/niosh 

DHHS (NIOSH) Publication No. 2003-152 

This document is the public domain and may be freely copied or reprinted. 

 _____________________________________________________ 

Disclaimer: Mention of any company or product does not constitute endorsement by NIOSH. 
CONTENT 

 Page 

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

Background ................................................................................................................................................................... 2 

The origin and history of the coal seam and overlying roof rocks .................................................................................. 3 

Geologic hazards............................................................................................................................................................. 5 

 Weak rock ................................................................................................................................................................ 5 

 Drawrock .......................................................................................................................................................... 6 

 Roof bolt anchorage ......................................................................................................................................... 6 

 Stackrock .......................................................................................................................................................... 8 

 Valley stress relief ............................................................................................................................................ 10 

 Head coal .......................................................................................................................................................... 10 

 Rider coal.......................................................................................................................................................... 10 

 Discontinuities ......................................................................................................................................................... 11 

 Clay veins ......................................................................................................................................................... 12 

 Pennsylvania example ............................................................................................................................... 12 

 Central Illinois example............................................................................................................................. 14 

 West Virginia example .............................................................................................................................. 16 

 Slickensides ...................................................................................................................................................... 16 

 Sandstone channels........................................................................................................................................... 18 

 Joints................................................................................................................................................................. 18 

Roof fall analysis ............................................................................................................................................................ 22 

 Timing of roof falls.................................................................................................................................................. 24 

 Case study................................................................................................................................................................ 28 

Summary ........................................................................................................................................................................ 30 

References ..................................................................................................................................................................... 31 

Appendix.–Glossary of terms.......................................................................................................................................... 33 

 

ILLUSTRATIONS 

 1. Injuries caused by the fall of roof and rib in U.S. coal mines, 1983-99................................................................... 2 

 2. Fatalities caused by the fall of roof and rib in U.S. coal mines,1983-99 ................................................................. 2 

 3. Deltaic model for the environment of deposition of a coalbed ................................................................................ 3 

 4. Coal-forming environment of deposition................................................................................................................. 4 

 5. Stacked paleochannels left from migrating stream channels in a deltaic distributary system.................................. 5 

 6. Potential discontinuities in coal measure rocks caused by changing depositional environments may pose roof 

 fall hazards............................................................................................................................................................ 6 

 7. Wedge-shaped lag deposit forms a weak drawrock in coal mine roof in southeastern Ohio ................................... 7 

 8. Weak drawrock begins to fall out between roof bolts; progressive deterioration occurs with time......................... 7 

 9. Head coal in an Ohio mine thickens in the roof, causing roof bolts to miss a strong limestone anchorage, which 

 reduces the CMRR from 72 to 30 ........................................................................................................................ 8 

10. As a weak seat earth thickens in a southern West Virginia mine roof, it causes roof bolts to miss anchorage in 

 a strong sandstone................................................................................................................................................ 8 

11. Stackrock showing weak partings between sandstone and shale beds..................................................................... 9 

12. Cutter forming in shale bed that overlies a thin sandstone in a stackrock sequence ................................................ 9 

13. Bedding plane faulting caused by lateral movement of valley floor members due to valley stress relief................ 11 

14. Rider coalbeds above the main bench of the Pittsburgh Seam................................................................................. 11 

15. Clay vein injected into the coalbed .......................................................................................................................... 12 

16. Roof damage caused by clay veins .......................................................................................................................... 12 

17. Roof guttering caused by fall of clay vein ............................................................................................................... 12 

18. Western Pennsylvania mine section without clay veins showing roof falls caused by horizontal stress ................. 13 

19. Western Pennsylvania mine with numerous mapped clay veins.............................................................................. 14 
ILLUSTRATIONS–Continued 

 Page 

20. Stacked clay veins infiltrated along bedding swell when exposed to moisture and cause roof falls by bulking 

 the strata................................................................................................................................................................ 15 

21. Rock fall occurred when swelling pressures in a roof clay vein broke a roof bolt................................................... 15 

22. Intersection spans are controlled by turning crosscuts only one way and dropping a crosscut in an already 

 overspanned track chute....................................................................................................................................... 15 

23. Clay vein damaging the roof and rib in an Illinois mine.......................................................................................... 15 

24. Roof falls in an Illinois mine are related to areas of limestone in the roof <1 ft thick and the occurrence of 

 clay veins .............................................................................................................................................................. 16 

25. Roof fall rate increases dramatically with reduced CMRR and clay vein occurrence ............................................. 16 

26. Slickensides on a rock fallen from a coal mine roof ................................................................................................ 17 

27. Slickensides are formed when a massive roof rock (sandstone and stackrock) compresses a softer claystone, 

 causing small internal faults.................................................................................................................................. 17 

28. Progressive failure ("horseback") caused by sagging head coal and exposure of slickensided shale above............ 17 

29. Metal screen supporting roof scale formed in highly slickensided shale ................................................................. 18 

30. Roof rock supported by metal screen....................................................................................................................... 18 

31. Illinois coalfields showing system of paleochannels with disruption or erosion of the coalbed.............................. 19 

32. Erosional paleochannel in a western Pennsylvania mine causing severe coalbed and roof disruption.................... 20 

33. Complete washout of the coal by a paleochannel in a West Virginia mine terminated numerous headings ........... 20 

34. When a paleochannel approached the roof in a southwestern Pennsylvania longwall mine, roof falls and roof 

 pots were common ................................................................................................................................................ 20 

35. Systematic jointing in the shale roof of a southwestern Pennsylvania mine continued only about 6 in vertically 

 and terminated against a siltstone ......................................................................................................................... 20 

36. Roof fall terminating against a large joint face ........................................................................................................ 21 

37. Roof fall caused by large joints spaced 6-10 ft and overspanned intersection......................................................... 21 

38. Hillseam near mine portal presents a roof hazard .................................................................................................... 21 

39. A hillseam is a zone of numerous extensional fractures .......................................................................................... 21 

40. High-angle thrust faults cause poor roof conditions in a southwestern Virginia mine............................................. 22 

41. High horizontal stress oriented east-west causes extensive north-south roof falls in an Illinois mine..................... 23 

42. Regular roof shears in a southwestern Virginia mine cause roof falls when crosscuts are aligned parallel to them 24 

43. Roof fall report by a geologist at an Ohio mine....................................................................................................... 25 

44. Roof fall data sheet developed by NIOSH............................................................................................................... 26 

45. Roof fall caused by broken rock falling between bolts............................................................................................ 27 

46. Two views of "glove-fingered” bolts that provide little mechanical interlock to support strata.............................. 27 

47. Method for measuring intersection diagonals .......................................................................................................... 27 

48. Standup time for roof falls at a western Kentucky mine.......................................................................................... 28 

49. Standup time for roof falls at an Illinois mine ......................................................................................................... 28 

50. A portion of a southwestern Pennsylvania longwall mine showing roof falls, sandstone channels, and CMRR..... 29 

51. Height of roof falls at a southwestern Pennsylvania mine ....................................................................................... 29 

52. Height of roof falls over bolts at a southwestern Pennsylvania mine ...................................................................... 29 

53. Location of roof falls at a southwestern Pennsylvania mine.................................................................................... 30 

54. Roof falls versus roof bolt type at a southwestern Pennsylvania mine .................................................................... 30 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

 ft foot km kilometers 

 ft3 cubic foot mi miles 

 gal/min gallon per minute min minutes 

 in inch psi pound per square inch 
GEOLOGIC HAZARDS AND ROOF STABILITY IN COAL MINES 

By Gregory M. Molinda1 

ABSTRACT 

The U.S. underground coal miner faces a continuing hazard from the fall of roof. At the root of many 

injuries and fatalities are weak or defective roof strata. Throughout mining history, millions of miles of entry 

have provided exposure of every conceivable geologic roof hazard. This report describes the geologic 

origin, association, and potential danger from the most common hazards. Discussions of weak rock include 

drawrock, rider coals, head coal, stackrock, and stream valley effects. Discontinuities, or roof defects, are 

described including, clay veins, slickensides, joints, and paleochannels. A number of examples from U.S. 

coalfields are used to document geologic structure and associated hazards. Roof fall analysis is a 

methodology used by NIOSH for hazard recognition and prevention; its application and benefit to the 

industry are discussed. 

1

Research geologist, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 
2 

BACKGROUND 

 Although roof and rib fall fatalities in U.S. coal mines 

have greatly decreased because of technological advances 

(e.g., roof bolting, canopies, automatic temporary roof supports (ATRS), longwall mining, rib support, and screening), 

fatalities (figure 1) and injuries (figure 2) continue to plague 

U.S. undeground coal miners. In 1999, 745 injuries resulted 

from 2,087 reported roof falls [MSHA 1999]. Many of these 

fatalities and injuries are due to weak roof strata and unexpected discontinuities in the roof. Within the accident narratives filed by operators to the Mine Safety and Health Administration (MSHA) are numerous references to "slips," 

"horsebacks," "faults," "coal streaks," "clay veins," "joints," 

and "rolls".2

 These are all geologic defects or weaknesses in 

the roof that were not anticipated or adequately supported. 

The ground control program at the Pittsburgh Research Laboratory for more than 30 years has focused on numerous safety 

issues. These include roof bolt effectiveness, bump control, 

strata deformation, subsidence, retreat mining, and horizontal 

stress. Integral to each one of these topics and many more is 

geology. Indeed, the behavior of sedimentary sequences to 

the applied stresses of mining is central to both short- and 

long-term opening stability. Almost every aspect of the creation of underground excavations will require a description 

and analysis of the local strata to provide a framework for an 

engineering solution. This strata description is necessary to 

explain ground reactions and to attempt to capture the effect 

of a material variable that can change drastically on a micro 

and macro scale. 

 The National Institute for Occupational Safety and Health 

(NIOSH) has had success in using a rock mass classification 

called the Coal Mine Roof Rating (CMRR) to evaluate the 

strength of coal measure roof sequences. A number of 

successful engineering solutions have been applied to rock 

mechanics problems using the CMRR. These topics include 

multiple-seam design guidelines, gateroad design, and 

selection of tailgate support [Mark et al. 1994]. The CMRR 

considers discontinuities in the bolted roof sequence and the 

weakness they impart to the rock mass. With simple field and 

lab tests, the relative strength of any coal mine roof rock can 

be determined. The important rock properties include the 

uniaxial compressive strength (UCS), the effects of moisture 

deterioration of the rock, and the bedding strength of the core. 

Researchers have developed other rock mass classifications 

as well [Bieniawski 1988; Kester and Chugh 1980; Buddery 

and Oldroyd 1992]. Some systems concentrate on geologic 

structures and their effect on roof quality [Moebs and 

Stateham 1985; Milici et al. 1982]. 

 Although this classification approach has proved valuable 

and has resulted in numerous engineering solutions to ground 

control problems, much geologic information falls outside the 

partition of classification. Mining geologists have long 

known that specific geologic structures can occur either in a 

systematic or isolated manner and can significantly impact 

 

2 Refer to the glossary at the end of this report for definitions. 

 Figure 1.–Injuries caused by the fall of roof and rib in U.S. 

coal mines, 1983-99. 

Figure 2.–Fatalities caused by the fall of roof and rib in U.S. 

coal mines, 1983-99. 

safety and productivity. These structures weaken the roof 

beam and can cause damage that is not prevented by 

conventional roof support. In addition, the occurrence of the 

geologic structures may not be obvious to the untrained 

observer. For these reasons, NIOSH has undertaken to study 

and document the nature and occurrence of common geologic 

structures that may form hazardous roof when exposed. The 

purpose of this study is to present to the industry real field 

experiences that have been documented through underground 

visits and examinations of other exposures, including 

highwalls. In this way, the NIOSH ground control program 

addresses regular geology through the CMRR and anomalous 

geology through individual description. 

Numerous researchers have described the various geologic 

environments and rock types that form the coal mine roof 

[Damburger et al. 1980; Ferm 1974; Iannacchione et al. 1981; 

Ingram and Chase 1987; Milici et al. 1982; Hylbert 1978; 

Molinda and Ingram 1989; Sames and Moebs 1991; Greb 

1991; Nelson 1991]. Geologic models and classifications 

have been used for the purpose of constructing hazard maps 

and mine plans. The tracking of hazardous roof layers is 
3

important, but can be difficult when exploratory drillholes are 

limited and widely spaced. It is almost impossible to 

recognize and project geologic defects (clay veins, shears, 

horsebacks, etc.) in the roof from only exploratory core. 

Many times recognition and projection of features occurs by 

close underground observation. 

 There are three parts to safely mining beneath weak roof: 

(1) recognizing and understanding the origin, nature, extent, 

and potential hazard of the geology, (2) predicting/ anaticipating and tracking the feature in the roof, and (3) adequately 

supporting the roof or avoiding the hazard. This report discusses aspects of all three areas. It also discusses the geologic environment of formation. Some of the more common 

geologic roof hazards that occur in coal mine roof are presented, but the report is not intended to be an exhaustive 

reference on all roof hazards. 

THE ORIGIN AND HISTORY OF THE COAL SEAM AND OVERLYING ROOF ROCKS 

 The strength or weakness of coal seam roof rocks can be 

explained by the depositional environment in which the coal 

seam was formed and its subsequent burial and tectonic 

history. James Hutton's Principle of Uniformitarianism states 

that the processes governing the deposition of sediments 

today can be used to explain the deposition of the rock record 

of the past. Operators can use the modern-day coastal delta 

system as a model for coal measure rocks of the past. 

 Minable coal seams, especially the widespread and 

continuous coal seams of the Appalachian and Illinois basins, 

represent unusual geologic conditions not typically found in 

the geologic record. These coal seams developed from peat 

swamps that existed, relatively undisturbed, at sea level for 

several thousands of years. By necessity, the vegetation of 

the peat swamp required a stable subaerial platform (delta) 

for this length of time or sea flooding would have terminated 

the swamp and prevented significant peat formation. 

Conversely, an emergence of the swamp (by sea level reduction or tectonic uplifting) would have drained the swamp and 

precipitated its removal by erosion and weathering. 

 The Mississippi delta system can be used as a model for 

ancient coal seam deposition in order to envision the 

formation and preservation of a coal seam. The model also 

explains some of the disturbances and weakness in coal mine 

roof rocks. The familiar birdsfoot Mississippi delta complex 

is a stable structure that is building out into the Gulf of 

Mexico. It is at this interface where new land is created and 

swamps are formed. The vegetation on the delta is sustained 

by influxes of freshwater from the distributary system mixed 

with seawater. If undisturbed, thick widespread sequences of 

peat can form (figure 3).

 The distributary river system feeding the delta is free, at 

sea level, to meander about the coastal plane because it is 

unconfined by valley walls. In addition, lower stream 

velocities, due to low gradients, also free the stream from 

channelization. Periodically, the river floods and breaches its 

banks (crevasse splay). A new river channel and delta 

complex is established and the previous delta is abandoned 

(figure 4). This pattern of delta formation and abandonment 

is repeated countless times in a large delta system (hundreds 

of square miles in area). In this way, channels are commonly 

eroded into the peat swamp. Abandoned channels can also 

fill with sand, silt, and clay, which can disrupt coalbed 

continuity and become future mining hazards. Channel 

systems are often "stacked" by overlapping deposits from 

flooding river systems, causing more roof hazards (figure 5). 

 Most of the familiar roof rocks of present day coal mines 

can be explained by this fluvial-deltaic model. As large 

open-water bays between fingers of the distributary river 

system become filled with silt from seasonal flooding, they 

emerge and peat swamps are established. As a crevasse splay 

floods the peat swamp with silt and clay, the future coal mine 

roof is formed (siltstones, shales, claystone). If the channel is 

abandoned, the swamp may drain enough to again establish 

vegetation using the underlying clay as a seat earth. The result is a rider coalbed with a weak drawrock beneath, again 

presenting future roof problems. 

 Thick blanket sandstones, which make excellent roof, may 

be the result of several different environments. The alluvial 

plains upstream of the active delta contain more sand, which 

comes in direct contact with a peat swamp. As rivers meander back and forth over the plane, peat swamps are covered 

by thick sand blankets, eventually becoming strong, massive 

sandstone. Another environment is at the distributary mouth 

where sands are dumped at the end of the river. These sands 

are then reworked by wave action and spread out as thick 

beach deposits much like the modern barrier islands of New 

Jersey. They can overlap adjacent peat swamps and form the 

future sandstone roof of a coal mine. Limestone roof is more 

frequent in southeastern Ohio and Illinois mines. These marine deposits are formed offshore of the delta mouth or in 

marine embayments between delta lobes. Limestone roof is 

more uniform and laterally consistent because it is formed in 

quieter offshore waters. 

 Figure 3.–Deltaic model for the environment of deposition of a 

coalbed. 
4 

 Figure 4.–Coal-forming environment of deposition. A, Advancing active delta wedge with an adjacent abandoned wedge (modified 

from Ferm [1974]). B, Detailed cross-section of a coal-forming wedge showing a stream channel dissecting a coal-forming sequence. 
5

 Figure 5.–Stacked paleochannels left from migrating stream channels in a deltaic distributary system. Mining under or near these 

sand bodies presents a roof fall hazard [Ferm and Cavaroc 1968]. 

GEOLOGIC HAZARDS 

 Roof deterioration can range from a few inches of scale 

between bolts to complete failure in the form of a roof fall 

that could run for hundreds or thousands of feet. MSHA defines a reportable roof fall as any roof failure that (1) causes 

injury that has reasonable potential to cause death, 

(2) disrupts regular mining activity for more than an hour, 

(3) occurs at or above bolt anchorage, (4) impairs ventilation, 

or (5) impedes passage [30 CFR3

 75.223]. The goal of 

primary roof support is to build a stable roof beam or to 

suspend immediate roof from a competent overlying roof 

beam between coal pillars. Roof falls can occur when the 

roof beam is damaged or severed. The geologic hazards that 

can damage or destroy the roof beam are here grouped into 

two categories: weak rock and structural discontinuities. 

WEAK ROCK 

 Most of the injuries related to roof falls in U.S. coal mines 

are due to the fall of small pieces of rock between bolts or 

from pieces dislodged by the ATRS or the bolting operation 

itself [Bauer and Dolinar 2000]. These failures are known as 

"roof skin falls" and are the subject of current NIOSH 

research. As expected, they occur much more frequently in 

weak rocks because of weak bedding cohesion and the 

tendency to weather more easily. Generally, coal measure 

rocks are weaker than other sedimentary rocks because they 

have been deposited in shallow near-surface waters and have 

been subjected to a number of disturbances from rapidly 

changing environments (peat swamp flooding causing seam 

splits, channel erosion causing slumping, burrowing, and 

 

3 Code of Federal Regulations. See CFR in references. 

differential compaction from burial loading) (figure 6). 

Sediments deposited in quiet waters (lagoons and brackish 

water interdistributary bays) are clay and mud-rich and have 

weakly bonded laminations. When overbank floodwaters 

introduce coarser sediment, the bedding contacts are 

particularly weak. Within coal measures, the UCS varies 

greatly from crystallized sandstone (UCS > 20,000 psi) to 

crumbling clay shales (<300 psi), but most roof rocks are 

shales, claystones, sandy shales, siltstones, stackrock, or other 

variations of mudrocks. 

 The CMRR rates the roof rock from 0 (weak) to 100 

(strong) by considering— 

 • Bedding strength 

 • An index test for the UCS 

 • Moisture sensitivity 

 • The frequency and strength of discontinuities 

 • A strong bed adjustment 

The CMRR is used in this report to indicate the strength of 

roof sequences. 

 There are a number of geologic explanations for weak rock 

in the immediate roof. These include weak, plastic clay 

compositions; closely spaced laminations; water-sensitive 

shales; disturbed bedding; and bedding defects like coal 

stringers, slickensides, and lag deposits. These features are 

usually depositional and inherent to the rock fabric. They are 

distinguished from structural discontinuities, which are 

generally due to erosion, differential compaction, structural 

faulting/fracturing, and horizontal stress. Weak types of roof 

strata are explained below. 
6 

Drawrock 

 Drawrock is generally defined as soft shale or mudrock 

that falls soon after mining or is taken down by the miner 

because it is difficult to support. Geologically, this rock is 

often a seat earth, underclay, or paleosoil and represents the 

beginning of another peat swamp established on top of the 

main coal seam. The rock may be the base of an overlying 

rider coalbed that was not preserved. The unit rating (UR) of 

drawrock falls into the 20-35 range (a UR is the strength 

given to each roof unit in the CMRR calculation) [Molinda 

and Mark 1996]. In a recent roof bolting survey, 36% of all 

reporting mines (17 of 47) stated that they mined drawrock 

from the roof or that drawrock on occasion formed the 

immediate roof.4

 Figure 7 is an example of a potentially dangerous 

drawrock. It is a lag deposit that significantly disrupts the 

roof in an eastern Ohio mine. Large basal cobbles are 

intermingled with sand, silt, and clay matrix, forming an 

immediate roof that is barely self-supporting (UR = 28 

(weak)). The rock can be crumbled by hand. Differential 

compaction has caused slickensides throughout the sequence 

(up to 4 ft long). This lag is the unsorted product of bed load 

carried by a fast-moving stream. Here, the lag sequence has 

damaged the roof over an area of 14 acres and resulted in a 

roof fall rate of more than 17 falls per 10,000 ft of drivage 

(high roof fall rates are 2.0 falls per 10,000 ft). 

 Several mines report using the color of the drawrock as a 

rule of thumb to indicate its strength. In general, they note 

that the lighter the shale, the weaker the rock. Although dark 

color locally can indicate stronger rocks, black shales are 

often weak on bedding. Usually the drawrock will fall to a 

more competent unit, which will form a more stable bolted 

roof beam or anchorage horizon. The difficulty comes when 

the thickness of the drawrock increases rapidly, resulting in 

an unacceptable dilution of the coal. When the drawrock 

reaches 18 in or more, it may become necessary to bolt 

through the unit and try to support it. This often results in 

uneven roof where some drawrock has been taken down right 

next to thick drawrock that has been bolted. The uneven roof 

decreases the strength of the roof beam by reducing its 

thickness. The exposed brow also acts as a conduit for 

moisture. Drawrock is often highly moisture-sensitive and, 

with time, may result in 6-12 in or more of "chandelier bolts" 

as the rock "rats out" between the bolts (figure 8A). In severe 

cases of "chandelier bolts" (8-12 in), the roof beam is 

effectively severed (or at least drastically reduced) and must 

be rebolted (figure 8B). Roof scale can be controlled by steel 

screen, plastic mesh, or geogrids applied to the roof. One 

mine in southeastern Ohio reported problems with a weak 

black shale in the immediate roof spalling out between the 

shields and in front of the shield tips. This rock is already 

weak and is additionally fractured when the shields are 

pressured against it. Large pieces roll off the face, causing an 

uneven surface for shield setting as well as presenting a 

 

4 Data available on request from G. Molinda. 

hazard to the operators. Another mine reports that tensioned 

bolts can apply forces large enough to fracture the brittle roof. 

 Weak drawrock can affect the length of cut taken. Grau 

and Bauer [1997] report that at one mine where the 

immediate roof was more stable, more extended cuts were 

taken on that side of the section than on the side of less stable 

roof. Remote-control mining machines are damaged from 

small pieces of drawrock. Drawrock also makes roof bolting 

more difficult and dangerous due to ragged roof. Weak 

drawrock is a cause for limiting the length of cut, especially 

when it is known that bolter delays will prevent quick roof 

bolting. 

 Drawrock is more common in fluvial depositional 

environments. This is the environment more affected by the 

main stream channel and seasonal flooding. These rapidly 

changing upper deltaic environments produce high contrasts 

in rock types: micaceous sandstone on shale, wild coal on 

silty shale, etc. These contrasts make for weak contacts and 

weak roof. In marine-dominated roof sequences (Illinois 

basin), thick shales or limestones make stronger and more 

persistent roof. Another 59% of reporting mines (22 of 37) in 

the roof bolting survey also had slaking problems in the 

immediate roof, which opens the potential for deterioration 

between bolts. 

Roof Bolt Anchorage 

 In cases where thick drawrock must be bolted up, it is 

important to try to suspend it from a strong rock unit. The 

importance of a relatively strong rock member within the 

bolted interval is known [Molinda and Mark 1994]. It is 

preferable to support weak roof in suspension rather than try 

to build a roof beam entirely in weak rock. This may mean 

longer bolts, and it is essential to locate anchorage either by 

observation of drill cuttings or by borescope. Although 

immediate roof that is supported in suspension may only pot 

out up to the stronger anchorage, roof support that is 

attempting to build a beam entirely in weak rock has the 

potential for roof failure above anchorage due to anchor slip 

and progressively upward delamination and moisture 

infiltration. 

 Figure 6.–Potential discontinuities in coal measure rocks 

caused by changing depositional environments may pose roof 

fall hazards. 
7

 Figure 7.–Wedge-shaped lag deposit forms a weak drawrock in coal mine roof in southeastern Ohio. A, Close-up view; B, longrange view. 

Figure 8.–A, Weak drawrock begins to fall out between roof bolts. B, Progressive deterioration occurs with time. 

 Thick drawrock can cause roof bolts to miss their target 

anchorage. At an Ohio mine, a drawslate with a UR of 17 

usually falls out with mining and the roof is formed in the 

overlying rider coal. Figure 9 shows the roof sequence. The 

main limestone in the roof was the target anchorage for a 6-ft 

point-anchored resin-assisted bolt. If this anchorage is 

achieved, the CMRR = 72 and the roof is effectively supported. If the roof coal or the overlying claystone gets thick, 

the bolts fall short of the limestone anchorage and the CMRR 

drops to 30. The loss of the limestone as an anchorage is 

dramatic to the strength of the bolted roof. For this reason, an 

8-ft point-anchored resin-assisted bolt is now used. In 

addition, if the roof coal thins and falls or is inadvertently cut 

through, the highly moisture-reactive and slickensided claystone is exposed and pots out. When this occurs, the mine 

uses metal screen to support the weak, exposed claystone. 

There are many examples of stable roof when strong rock 

members are close enough to provide good bolt anchorage. 

Limestone units, more common in Ohio and the Illinois basin, 

are generally more uniform and of consistent thickness and 

location than sandstone bodies. It is always preferable to take 

down weak rock instead of bolting it, but taking down weak 

drawrock can present another hazard. With the immediate 

top down, the drawrock forms the upper rib and sits 
8 

 Figure 9.–Head coal in an Ohio mine thickens in the roof, causing roof bolts to miss a strong limestone anchorage, which reduces 

the CMRR from 72 to 30. 

unsupported above the main seam. It is generally highly 

slickensided, and fatalities have occurred from rib rolls close 

to the roof. 

 In rolling sandstone top, it is critical to monitor anchorage 

height by drilling blind holes every 20 ft. This requirement is 

written into many roof control plans. In MSHA District 5, 

test holes are required regardless of strata type [McLoughlin 

2001]. One mine in Illinois records the height to a limestone 

bed on the rib at each intersection. The data are then 

recorded and plotted on base maps to determine trends. 

 At a mine in Logan County, WV, the immediate roof contains a 

brown seat earth with almost no strength (figure 10). This roof 

member can range from 2 in to 4 ft thick. In this case, bedding has 

been destroyed by burrowing and differential compaction and the 

rock fractures in clods. Mostly this drawrock is taken down, but as it 

thickens locally it must be bolted up. When it is bolted, it tends to 

fall out between bolts, but the greatest problem is that, commonly, 

the 6-ft bolt misses a strong anchorage in the overlying sandstone. 

Unless the bolter can adjust and install longer bolts, the roof beam 

will be formed in an extremely weak bed. 

Stackrock 

 Stackrock is often associated with weak or poor-quality 

roof, especially when subjected to high horizontal stress. 

Stackrock is a coalfield name for a sequence of rock 

composed of interbedded sandstone and shale. The "stack" 

looks like a tall column of telephone books or newspapers 

 Figure 10.–As a weak seat earth thickens in a southern West 

Virginia mine roof, it causes roof bolts to miss anchorage in a 

strong sandstone. 

due to its peculiar bedding breakage. It is also referred to as 

"catalogue rock" or "transition rock." The rock is really a 

mixture of sandstone and shale in widely varying proportions. 

It grades into a shale with sandstone streaks as one end 

member and a sandstone with shale streaks as the other end 

member (figure 11). Geologically, the stackrock sequence 

represents alternating periods of high-energy water movement 

(depositing sandstone) with quiet low-energy standing water 
9

(depositing shale). Springtime surges of floodwaters in 

deltaic rivers may break through natural river levees in a 

crevasse splay, depositing blankets of sand. Silts and muds 

are deposited on top of the sand as the surge wanes. 

Subsequent surges of water through the levee in a subsequent 

flood deposit more sand and repeat the cycle. As the river 

meanders across its valley by abandoning its channels, 

stackrock sequences fan out and overlap vertically. These 

sequences can be 100 ft or more thick and cover hundreds of 

square miles (figure 4B) [Pettijohn et al. 1973]. Stackrock 

sequences are flatlying, indicating their discharge into open 

areas (peat swamps and lakes). At their feather edge, dips are 

<1°. They often grade laterally into thick sandstone wedges 

in the direction of the paleochannel. Chances are good that a 

mine that reports channel sandstones in the roof or eroding 

the seam will also encounter stackrock at some location on 

the mine property near the channel. The size of the channel 

source will determine how far the stackrock wedge will reach 

(100 ft to several miles). 

 At first glance it may seem that stackrock would form a 

stronger beam when reinforced with roof bolts. In structural 

support, an engineered composite beam is stronger than a 

beam formed from a single member. The problem with roof 

bolts is that it is difficult to exert enough compression on the 

stackrock beam to prevent its laminas from shearing 

horizontally. Once this delamination occurs, the strength of 

the beam becomes the strength of the individual laminations. 

Stackrock can be strong axially (12,000-15,000 psi) 

depending on the proportion of sandstone in the mix, but is 

typically weak along bedding. The CMRR of stackrock 

averaged 40-48 from 90 samples tested. Although average 

axial strengths ranged from 4,653 to 11,335 psi, average 

tensile bedding strength ranged only from 1,482 to 2,508 psi 

in samples with a Ferm number of 322, 323, 332 [Molinda 

and Mark 1996; Ferm and Smith 1981]. Even small amounts 

of roof sag can break bedding bonds and begin an unraveling 

of the sequence. In this situation, quick bolting and an active 

tensioned system may help reduce sag. 

 When subjected to horizontal stresses, stackrock makes 

notoriously bad roof. The sandstone beds can act as stress 

concentrators and cause tensional delamination with adjacent 

shale beds as they buckle individually. Weaker shale 

interbeds may crush and exhibit characteristic "overshoots" or 

rock "flour." (figure 12). The lower sandstone bed (2-3 in 

thick) did not shorten and cut because of its higher stiffness. 

It failed because of stress concentration, falling away to 

reveal the above shale. The shale above shows feathering and 

rock floor damage, which is more typical of softer shale. 

Stress-induced roof falls are often higher and more localized 

to intersections in stackrock than shale roof. Shale roof cuts 

and gutters more easily along the rib line because it is less 

stiff. Falls tend to "run" for long distances depending on the 

orientation of the entry to the principal stress. Stackrock falls 

because sandstone interbeds concentrate stress and "bulk," 

then delaminate vertically until a stable stratum is reached, 

resulting in a flat-topped fall. In thinly laminated stackrock 

with a high proportion of shale, it is important to maintain 

close control on entry width and bolt the roof as soon as 

possible to prevent initial sag and delamination. In stackrock 

with a higher percentage of sandstone and thicker shale beds, 

a staggered-length bolt pattern may prevent accidental 

anchorage in the weak shale. If roof sag occurs at the weak 

shale band, the longer staggered bolts will act to suspend the 

separated roof beam, provided a stiff anchorage is found. In 

areas of high horizontal stress where bulking and 

delamination cannot be prevented, cable bolts in intersections 

will provide a softer support and allow some movement. 

Figure 11.–Stackrock showing weak partings between sandstone and shale beds. 

 Figure 12.–Cutter forming in shale bed that overlies a thin 

sandstone (2-3 in) in a stackrock sequence. The lower sandstone acts as a stress concentration and falls away to reveal the 

shale.
10

Valley Stress Relief 

 The effects of mining under valleys are manifested in two 

ways. First, a valley can act to reorient and concentrate an 

active regional stress field as the horizontal stress attempts to 

deflect underneath the valley. This can concentrate stresses at 

the bottom of a valley and cause roof damage if the mine is 

close enough to the valley bottom. Van der Merwe [2000] 

reports the K ratio (ratio of horizontal stress to vertical stress) 

is much higher at depths <650 ft (up to 5.7), which can occur 

beneath stream valleys. Secondly, stress relief prior to 

mining can damage roof rocks as the valley is downcut and 

confinement removed. The second type is discussed here. 

 Roof damage due to stress relief under valleys is well 

documented [Molinda et al. 1992; Fergusen and Hamel 1981; 

Fergusen 1967; Moebs 1977; Moebs and Stateham 1985]. 

There is evidence that wide, flat-bottomed valleys have 

experienced more premining valley floor buckling due to less 

confinement than V-shaped valleys. (The latter are more 

likely to have experienced active mining-induced stress and 

to have cutter damage.) The effects of valley stress relief 

have been observed at 300 ft of cover and deeper [Fergusen 

and Hamel 1981] in the form of small overthrust faults, fault 

gouge, slickensides, and fractured rock. Molinda et al. [1992] 

found that 52% of all unstable roof areas in five mines 

occurred beneath valley bottoms. Twenty-four of thirty-one 

mines (77%) that responded to a survey had roof problems 

while mining under stream valleys. Problems include cutter 

roof, swags, fractured roof, large shears, swelling clays, and 

water inflow. Some operators had roof problems sporadically 

under valleys; others had problems under every valley. 

 At a mine in Boone County, WV, a manshaft was being 

sunk in a deep valley about 800 ft wide. At 43 ft down, a 

4-ft-wide natural bedding plane separation was encountered 

in stackrock. Horizontal drilling showed the opening to 

extend 8 ft away from the shaft wall. The opening made 80 

gal/min of water for 6 days. Blasting to 10 ft below the 

separation stopped the water inflow. Horizontal movement 

caused by the transfer of load from the adjacent valley walls 

(900 ft of relief) seems to have caused the separation even 

under the confining overburden load. It is impossible to 

determine the cover at the time of valley stress relief. 

 Roof damage was documented beneath a side valley at the 

same mine. Open bedding planes in the stackrock roof were 

exposed under 160 ft of cover. Roof falls over 100 ft long 

and 20 ft high were common. Fault gouge 1/8 in thick was 

present in roof bedding planes, indicating premining failure 

of the shale members of the stackrock, a result of valley stress 

relief (figure 13). Roof sag occurred as a result of the 

removal of the coal support during mining, causing bolts to 

load and fail. No active stress was observed due to complete 

valley stress relief. 

Head Coal 

Head coal is coal that is left in the top part of the minable 

bench and forms the immediate roof. There are several 

reasons for leaving head coal. (1) If the seam is too high to 

mine easily, some coal is left to reduce mining height. 

(2) Often the upper part of a coal seam is of poor quality due 

to sulfur or ash content and is left for economic reasons. 

(3) The most common reason for leaving head coal is to 

protect a moisture-sensitive mudrock above. Fourteen of 

thirty-seven mines surveyed (37%) in the aforementioned 

roof bolt study reported leaving head coal in the immediate 

roof. The thickness of head coal ranged from 4 to 60 in and 

averaged about 8 in. Head coal can form an effective barrier 

to moisture and, because it is generally resistant to moisture 

effects, will not slake with seasonal variations in intake air. 

However, because it is also relatively weak (<1,000 psi), it 

may sag and peel away with time, especially if it has well 

developed cleat. Typically, "bright coal" (coal with a higher 

vitrain content) is weaker than dull coal because cleat is better 

developed in bright coal. Despite low compressive strength, 

head coal can contribute to the effectiveness of the roof beam. 

At one Wyoming mine, the gateroads are oriented along 

strike and the panels pulled downdip. This leaves a wedge of 

2-5 ft of head coal across the entry. The head coal is the 

upper part of the Hanna No. 80 Seam and, although not 

strong, is significantly stronger (UR = 40) than the overlying 

carbonaceous mudstone (UR = 27). The carbonaceous 

mudstone above is thick, and no adequate anchorage is to be 

found. Where the head coal thickness is <2 ft, roof 

conditions were noticeably worse [Mark et al. 1994]. This 

coal forms the strong bed in this roof sequence and 

significantly increases the CMRR when thick. 

 At a large mine in western Pennsylvania where head coal 

has been inadvertently cut down, the roof is significantly 

slabbier and more uneven. At this mine, some roof falls may 

be related to areas with no head coal left, although this is 

difficult to document. At an older mine in Utah, the 24 in of 

roof coal left is considered the primary support. It is strong 

enough to be left unsupported and for many years was the 

only U.S. mine with no artificial roof support requirement. 

 Difficulties arise in a rolling (or dipping) coal seam when 

trying to leave a consistent thickness of head coal. Without a 

consistent marker bed in the seam, it is easy for the 

continuous mine operator to inadvertently cut out the top coal 

or leave too much. Either situation is undesirable. If too 

much top coal is left, bolts may not hit their target anchorage 

and be ineffective. In one Colorado mine, all of the falls in 

the mine were in an area of thick head coal (>3 ft) 

[Mark et al. 1994]. Cutter head-mounted gamma sensors 

have been used successfully for indicating top coal thickness, 

but are not common. When leaving head coal, it is important 

to drill roof test holes every intersection or, more often, to 

document head coal thickness and keep records. 

Rider Coal 

 In the United States, coal measure rocks are deposited in 

stable, slowly subsiding, continental basins. This low 

gradient allows prograding streams in deltaic systems to 

slowly meander back and forth across the basin surface. As a 
11

result, the depositional environments often occur in repeating 

cycles (cyclothems) as the basin subsides. This is why 

coalbeds typically occur in stacked sequences (West Virginia 

has more than 60 minable coal seams [Lotz 1970]). Rider 

coalbeds are minor coalbeds (6 in to 4 ft thick) that are the 

result of peat swamps that have been reestablished on top of 

the main seam. The term "rider coal" is loosely defined and 

may refer to a coal split at the top of the main seam (figure 

14). In the southern Appalachian basin, rider coals are less 

likely to be preserved than in the northern part of the basin. 

This is because the southern basin is deeper and subsided 

faster, resulting in thicker sequences of sandstone being 

deposited [Ferm 1974]. As a result, minor peat swamps were 

not preserved and there are fewer rider coals. Shales with a 

high carbonaceous content or coal streaks are often part of a 

rider coal roof package. Called "rash", this sequence is 

generally weak and has a CMRR range of 28-40, depending 

on the amount of coal or strength of the shale. This roof 

sequence is characteristic of the Pittsburgh Coalbed, widely 

known for weak roof. 

 Rider coals may cause a number of roof control problems. 

The most significant is a result of the difficulty in tracking 

their location and thickness. The thickness, location, and 

spacing of rider coals can be highly variable. In its worst 

case, the Pittsburgh Coalbed has up to five rider coalbeds in 

varying thickness and spacing. Rider coals in the bolted 

interval are a common occurrence depending on geographic 

location. In 17 of 21 mines surveyed, rider coals were 

reported within the bolted interval. If roof bolts are 

systematically anchored in a rider coal, separation and roof 

fall can occur. Ground penetrating radar has shown promise 

in mapping rider coals, but regular (every 20 ft) test holes 

with records are still the most reliable and inexpensive 

method [Molinda et al. 1996]. An operator's roof support 

program may revolve entirely around the location and 

thickness of the rider coal(s). Often a roof bolt operator will 

carry longer bolts on the machine and will have authorization 

to install these bolts based on test holes. There are a number 

of other support options, such as narrow entries, staggered 

crosscuts, timbers, or beams. 

 Roof falls often top out in the overlying rider seam (10-30 

ft up). This does not necessarily mean that the roof beam first 

separated at the weak rider. It may only mean that the 

arching fall found a convenient place to stop at the last weak 

roof member. 

DISCONTINUITIES 

Coal measure rocks, both weak and strong, can be affected 

by defects that disrupt the lateral continuity of bedded rocks. 

These can range from features on a microscopic scale to major faults offsetting beds for miles [Molinda and Ingram 

1989]. The size of the features that most often affect coalbeds can be measured from inches to tens of feet. Artificial 

support of flatlying coal measure rocks relies on the strength 

properties of a clamped beam or the suspension of a clamped 

beam. When a structural defect (crosscutting or parallel to 

 Figure 13.–Bedding plane faulting caused by lateral movement of valley floor members due to valley stress relief. 

Figure 14.–Rider coalbeds above the main bench of the 

Pittsburgh Seam. 

bedding) partially or completely severs or splits the beam, 

roof failure can occur. A roof fall rate for a typical mine is 

generally 1.0/10,000 ft of drivage or less [Molinda et al. 

2000]. This means that 99.5% of the roof in even a mine with 

poor top has been successfully supported. It is suspected that 

roof falls in a mine with otherwise solid roof and no abnormal 

loading are often due to random defects or discontinuities in 

the roof. Other causes include improperly installed roof support or use of roof support systems inconsistent with the 

strata. 

 Most of the structures that occur in coal mine roof were 

created during deposition and compaction or shortly thereafter. These include paleochannels, clay veins, kettlebottoms, 

slips, shears, pinchouts, lag deposits, concretions, rolls, or 

small faults [Moebs and Stateham 1985; Ingram and Chase 

1987; Kertis 1985; Chase and Ulery; 1987 Sames and Moebs 

1989; Iannacchione et al. 1981]. Other structures are due to 

tectonic events (folds and faults) [McLoughlin 1986; Molinda 

and Ingram 1989]. Some, such as joints, are believed to occur both during and long after deposition. In Virginia, many 

"horsebacks" are formed as a result of drag along bedding 

plane faults [McLoughlin 2001]. Some of the more common 
12

coalbed discontinuities and their effect on roof stability are 

discussed below. 

Clay Veins 

 Clay veins or clay dikes occur when soft, ductile roof or 

floor mudrocks are squeezed into tension fractures in the 

coalbed. They can also be injected parallel to bedding, or 

they may occur as a combination feature as in a stairstepping 

pattern (figure 15). Clay veins and their associated mining 

problems (water and gas barriers, roof damage) have been 

well documented [Chase and Ulery 1987; Hill 1986]. On a 

smaller scale, clay veins have squeezed into existing joint 

systems. The clay-filled tension features can then form the 

boundaries of roof falls. Clay veins can range in thickness 

from millimeters to tens of feet, and the dikes can occur from 

a single one to one every 10 ft or more often. Roof damage 

can be severe (figures 16-17). Roof rock dissected by clay 

veins is already failed upon exposure by mining and can fall 

out before bolting, covering the continuous miner and limiting cuts to only a few feet before cleanup and bolting. When 

a clay vein cuts through a pillar corner, large rib rolls can occur. When excess horizontal stress is added to a clay veinbroken roof, mining can be a nightmare. Conversely, there is 

some indication that clay veins can act as shock absorbers for 

horizontal stress. They can soften roof enough to prevent 

some massive roof falls that may have occurred in stiffer 

roof. 

Pennsylvania Example 

At one Pennsylvania mine with numerous clay veins, horizontal stress produced a number of falls in an area devoid of 

clay veins (figure 18). This is the only area with significant 

horizontal stress damage in the mine. It may be that horizontal stress, present throughout the mine, was relieved by 

Figure 15.–Clay vein injected into the coalbed. Note inclusion

of “floating” coal pieces. 

roof softening due to clay veins. Figure 19 shows a section of 

the same mine infested with clay veins. The mapped clay 

veins represent only a portion of the sinuous and bifurcating 

structures. Numerous reportable (above anchorage) roof falls 

and very ratty top result. A number of headings were stopped 

due to roof damage from clay veins. It is impossible to detect 

Figure 16.–Roof damage caused by clay veins. 

Figure 17.–Roof guttering caused by fall of clay vein. 
13

 Figure 18.–Western Pennsylvania mine section without clay veins 

showing roof falls caused by horizontal stress. 
14

 Figure 19.–Western Pennsylvania mine with numerous mapped clay veins. A number of headings were 

stopped by clay veins and associated roof falls.

clay vein occurrence with exploratory drilling and difficult to 

anticipate mining into a clay vein on a working section. 

Although many clay veins seem random, their trend can 

sometimes be projected for hundreds and even thousands of 

feet, giving early warning before mining interception. In one 

mine in western Pennsylvania with hundreds of roof falls, 

90% of them have been caused by clay veins. The source of 

clay vein material is the black clay shale roof. This material 

is weak and moisture-sensitive (CMRR = 36-39). When 

exposed to water, the soft mud filling swells and bulks the 

roof shales below, causing bolt failure (figure 20). At the 

shaft bottom, a number of point-anchored resin-assisted bolts 

were broken at the coupler. The fall at this area showed at 

least five stacked clay veins parallel to bedding in 15 ft of 

exposed roof. The combined load from these bulking clay 

veins was enough to overload the bolts. The broken bolts 

could not be explained by excessive dead loads, only by the 

high pressures exerted by swelling clays (figure 21). If water 

cannot be excluded from clay vein areas, it may be necessary 

to accept bulking and roof sag and control the roof in 

intersections with a softer system using cable bolts or slings. 

 Clay veins at the western Pennsylvania mine are more 

prevalent beneath stream valleys. Poor rock quality occurs in 

the form of horsebacks and large slips. Clay veins may have 

infiltrated this damaged rock and are aggravated by water 

inflow. This suggests that the clay veins may have formed 

after the development of the surface drainage. Mining is very 

difficult beneath any stream valley in this mine. 

 The mine has attempted to control the clay veins by 

controlling the intersection spans (figure 22), by only turning 

crosscuts toward the belt and track, and by dropping crosscuts 

when they would result in wide spans. The mine has also 

used fully grouted bolts to stop moisture entry, roof bolting is 

completed as quickly as possible, and mining is avoided 

under stream valleys if possible. 

Central Illinois Example 

 Clay veins dominate roof stability at another mine in 

central Illinois. Operators here attribute over 90% of the falls 

to clay vein disturbances. The roof typically consists of 3-6 ft 

of weak, black, carbonaceous shale (UR = 32) overlain by 12-

18 in of strong limestone (UR = 97) (figure 23). The roof 

support goal is to anchor the fully grouted bolt at least 1 ft 

into the limestone in order to form a strong beam and 

anchorage for shale suspension. The limestone varies in 

thickness and locally it splits. Figure 24 shows a large 

section of the mine where limestone thickness has been 

mapped. Roof falls are also shown. When the limestone is 

<12 in thick, the roof fall rate increases over 5.5 times 

(figure 25). Clay veins ranging from 6 in to 6 ft thick occur 

sporadically about the mine. When these structures penetrate 

the limestone, a cantilevered beam can occur, resulting in a 

roof fall. 

 The mine has a well-developed plan for controlling the 

roof. Through systematic roof hole monitoring, the thickness 

of the shale and limestone is painted on the intersection rib. 

Figure 24 shows the variability of roof geology at this central 

Illinois mine. This volume of data over such a large area is 

rare, but illustrates the need for systematic roof geology 

exploration if roof support selection is tightly linked to 

geology. A bolt 1 ft longer than the shale thickness is used to 

obtain 1 ft of anchorage in the limestone (e.g., when there is 

5 ft of shale, a 6-ft bolt is used). When limestone is <1 ft 
15

 Figure 20.–Stacked clay veins infiltrated along bedding swell 

when exposed to moisture and cause roof falls by bulking the 

strata. 

 

 Figure 21.–Rock fall occurred when swelling pressures in a 

roof clay vein broke a roof bolt.

 Figure 22.–Intersection spans are controlled by turning 

crosscuts only one way and dropping a crosscut in an already 

overspanned track chute.

 Figure 23.–Clay vein damaging the roof and rib in an Illinois 

mine. 

thick, an 8-ft bolt is used regardless of the shale thickness. 

When significant clay veins are encountered, a minimum of 

6-ft bolts are used and trusses in the intersections. Bolt 

spacing is reduced and straps are used to support exposed 

clay veins. The roof bolter has the immediate responsibility 

to select bolts. This strategy has helped to dramatically 

reduce roof falls. The mine routinely installs roof sag monitoring devices in intersections and has developed support 

procedures triggered by preset sag action levels. The operators also have a "roof failure rating system" for alerting the 

section boss to expected problems. The system incorporates a 

point-based rating that uses the size and orientation of clay 

veins, the limestone thickness, and shale damage to evaluate 

the roof fall hazard. 
16

 Figure 24.–Roof falls in an Illinois mine are related to areas of limestone in the roof <1 ft thick and the occurrence of 

clay veins. 

 Figure 25.–Roof fall rate increases dramatically with reduced 

CMRR and clay vein occurrence.

West Virginia Example 

 Another factor in the stability of clay vein-plagued roof is 

timing. At a mine in northern West Virginia, 56% (10 out of 

18) of the falls in 1 year were caused by clay veins. The 

average standup time of all falls was 3.9 months (n = 39) 

(standard deviation = 3.1 months). This may represent the 

average time it takes the clay filling in the clay veins to swell 

and bulk the roof enough to cause roof falls. Monitoring of 

roof sag is an effective way to determine the critical amount 

of sag necessary for the roof to fail. In this way, the mine can 

use fall timing to indicate when to install additional support. 

Slickensides 

 Slickensided rock is common to coal mine roof. It is one 

of the structures that seriously weakens the strength of coal 

mine roof. In addition, slickensided roof is responsible for 

large numbers of injuries on the working section due to the 

fall of small rocks between roof bolts. A slickenside, slip, or 

shear is actually a failure surface on which there has been 

lateral movement of shale or other clay-rich rocks (figure 26). 

Striations often indicate the plane and direction of movement. 

These structures occur less often as rocks coarsen into 

sandstones because of the ability of coarser grains to resist 

shearing. Slickensides occur early in the rock's diagenesis as 

a result of differential compaction, or later as a result of high 

confinement and slippage due to local or tectonic faulting. In 

any case, slickensides are already present in the roof at the 

time of mining and are not caused by mining-induced 

stresses. 

 Because mudrocks are soft and can be compressed and 

dewatered, differential-compaction slickensides are common 

to shales. These commonly occur beneath the margin of 

paleochannels and wherever stiffer rocks abut softer rocks. 

Slickensides are common in the flatlying immediate shale 

roof of the Pittsburgh Coalbed. Figure 27 shows how 

slickensides are formed in a Pittsburgh Seam mine in 

southwestern Pennsylvania. The Pittsburgh Sandstone commonly occurs as a flatlying, tabular, crevasse splay deposit 0-

20 ft up in the roof. It acts as a platen on the weak rash roof 

package below. As the overburden load was applied, the 

muds sheared horizontally along bedding and subvertically 

into random slickensides, millimeters to inches long. This 

zone, locally called "slickrock," occurs immediately above
17

Figure 26.–Slickensides on a rock fallen from a coal mine roof. 

 Figure 27.–Slickensides are formed when a massive roof rock

(sandstone and stackrock) (A) compresses a softer claystone,

causing small internal faults (B).

the coalbed and is about 4 ft thick. This rock often falls out 

on mining and, when bolted, sloughs out between bolts with 

time. These small falls are sometimes named "horsebacks" 

because of their shape (figure 28). It is essential to bolt this 

roof with straps or mesh immediately after exposure to reduce 

roof sag and moisture exposure. With the introduction of the 

integrated continuous miner and roof bolter, mining has been 

revolutionized in the Pittsburgh Seam. The notoriously weak 

Pittsburgh roof can be controlled by narrower entries and 

immediate roof support. 

 Figure 28.–Progressive failure ("horseback") caused by 

sagging head coal and exposure of slickensided shale above.

 Slickensided black shale or "slickrock" occurs in an Ohio 

mine with thick limestone in the main roof. The weak black 

shale below the limestone is highly slickensided due to 

vertical loading by the limestone "platen." When possible, 

this rock is taken down, but if it becomes too thick, it must be 

bolted. Even when bolted, this unit tends to fall out in blocks. 

It is also a problem because it tends to fall out in front of 

shield tips on the longwall face. 

In a longwall mine in western Maryland, the roof rock 

appears wet because of its high reflection. On closer 
18

inspection, however, the roof is a highly slickensided shale. 

Its numerous intersecting slickensided planes give the 

appearance of broken panes of glass. This roof falls out up to 

40+ inches immediately after mining. It can only be mined in 

5- to 8-ft cuts. Where it does not fall out, it is taken down by 

the miner. Mining is only possible because metal screen is 

installed on cycle (figures 29-30). Another problem caused 

by the removal of 3-4 ft of slickensided roof is unstable high 

ribs. This brow and rib is controlled by 4-ft angled planks 

bolted on 6-ft centers. The rib bolts can be retightened with 

Crosby clamps after sloughage. The mine is testing a new 

four-head walk-through bolting machine with a rib bolter 

mounted on the back for quick rib support. These 

innovations have allowed mining to continue under extremely 

weak, slickensided rock. 

Sandstone Channels 

Much has been written about sandstone channels and their 

adverse effects on the stability of adjacent and underlying 

coal mine roof [Molinda and Ingram 1988; Ingram and Chase 

1987; Kertis 1985; Krausse et al. 1979; Roen and Farrel 

1973]. In the fluvial-deltaic distributary environments of 

deposition typical of coal measure rocks, disruption of the 

continuity of roof rocks by channel scour and fill is common. 

Major paleochannel systems, including the Walshville, Anvil 

Rock, and the Galatia channel, cause roof hazards and mining 

disruptions in southern Illinois [Krausse et al. 1979]. 

Figure 31 shows the extent of paleochannels in the Herrin 

No. 6 Seam in southern Illinois. A major paleochannel 

system also disrupts the Pittsburgh Seam. These channel 

disruptions are mostly located by mining. They can extend 

for hundreds of miles depending on the size of the alluvial 

valley or delta distributary system. Figure 4 shows an 

erosional channel in relation to the surrounding depositional 

environments. A number of adverse features are associated 

with paleochannel cutouts or depressions of coal seams. 

Severe disruptions including faulting, crushed rock, slickensides, shears, rolls, water, clay veins, rotated cleat, and 

slumped structures can occur. Figure 32 shows a crosssection of a paleochannel disruption in a mine in Washington 

County, PA [Ingram and Chase 1987]. The crushed and 

faulted zone fell out immediately upon exposure and required 

significant supplemental support. 

 Some disruption occurs during the deposition of the peat

swamp. As the channel scours the peat, slumping occurs on

the cut bank and crossbedding occurs on the point bar side of

the channel. Both features can cause hazards in the future

roof strata. Slump surfaces become slickensided by loading.

The resulting disrupted, rotated blocks fall between bolts.

After burial, even more disruption of the paleochannel and

surrounding rocks occurs because of differential compaction.

The channel fill acts to point load the soft peat, which flows

away from the base of the paleochannel. The result is thick

coal sequences and highly sheared and friable rocks on the

margin of the paleochannel. Figure 33 shows the effect of a

paleochannel system on mining in a mine in the Upper

 Figure 29.–Metal screen supporting roof scale formed in 

highly slickensided shale.

Figure 30.–Roof rock supported by metal screen.

Freeport Seam in Preston County, WV. At least seven 

separate developments were prematurely terminated due to 

complete seam washout or poor roof quality and difficult 

mining conditions (rolls and splits). 

 Mapping at one large longwall mine in southwestern 

Pennsylvania shows numerous roof falls and pots when one 

paleochannel, from a large ancient river system, came near 

the roof (figure 34). Weak rock beneath and along the margin 

of the structure potted out when mining approached too close 

to the sandstone. Associated hazards included horsebacks, 

low-angle faults, ragged sandstone, and water dripping. 

When the paleochannel approaches the mined opening from 

the roof, less head coal could be left, allowing roof sag and 

falls up to the bottom of the sandstone. 

Joints 

 Joints are vertical or near vertical fractures in coal mine 

roof caused by tension. These discontinuities typically form 

planar surfaces and can extend from 6 inches to tens of feet.
19

 Figure 31.–Illinois coalfields showing system of paleochannels with disruption or erosion of 

the coalbed [modified from Krausse 1979].

Joints can occur early in a rock's formation because of local 

warping, or later when large regional forces shape 

anticlines and synclines. Coal cleat is a type of joint that 

can be formed relatively shortly after peat burial. Cleat 

distortion near paleoslumping is evidence of early cleat 

formation [Ingram and Chase 1987]. Other cleat systems 

parallel the axes of major folds, indicating their formation 

by later regional forces [McCulloch et al. 1975]. 

 Rocks have different strengths and respond to tension in 

different ways. Joints are more closely spaced in mudrocks 

than in clastic rocks. In a roof sequence, joints may continue 

only for the thickness of the host strata and abut vertically in the 

upper or lower strata. This situation in a southwestern 

Pennsylvania mine resulted in small pieces (2-3 in thick) being 

isolated in the immediate roof shale, but only extending upward 

the thickness of the black shale (6 in) (figure 35). Conversely,
20

 Figure 32.–Erosional paleochannel in a western Pennsylvania mine causing severe coalbed and roof disruption. 

Slump features along the margin of the channel created poor roof conditions.

 Figure 33.–Complete washout of the coal by a paleochannel 

in a West Virginia mine terminated numerous headings.

 Figure 34.–When a paleochannel approached the roof in a 

southwestern Pennsylvania longwall mine, roof falls and roof 

pots were common. 

 

 Figure 35.–Systematic jointing in the shale roof of a 

southwestern Pennsylvania mine continued only about 6 in

vertically and terminated against a siltstone.

joints may be persistent, extending through numerous roof 

strata, effectively severing the roof beam. At one western 

Pennsylvania mine, joints formed the severed termination of a 

number of roof falls (figure 36). The fall pictured in the 

intersection had two turnouts, contributing to its oversized 

intersection span of 74 ft and likely triggering the fall 

(figure 37). 

 Systematic joint sets near outcrop are called hillseams. 

These are the result of valley stress relief through 

downcutting and removal of confinement [Sames and Moebs 

1989]. As confinement is removed, the valley walls move 

outward and fracture vertically in tension. The trace of 

hillseams are often parallel to outcrop, and they can curve 

around points of land. They can form zones of failed rock of 

more than 4 ft thick (figures 38-39), allowing surface water to 

infiltrate and expand weathered zones within the hillseam. 

Mining near outcrop through hillseams can be extremely 

hazardous. Most hilltop mines must leave at least l50 ft of 

barrier between the mine and outcrop. At one mine in 

western Pennsylvania, a fatality occurred when mining 

proceeded within 100 ft of the outcrop because of a surveying 

error. The factors contributing to the death included wet, 

weathered, clay-filled hillseams and sheared coal ribs.
21

 Figure 36.–Roof fall terminating against a large joint face. 

A, Diagram; B, photograph. 

 Figure 37.–Roof fall caused by large joints spaced 6-10 ft and 

overspanned intersection. 

Figure 38.–Hillseam near mine portal presents a roof hazard. 

 Figure 39.–A hillseam is a zone of numerous extensional 

fractures. 
22

ROOF FALL ANALYSIS

 One of the most important diagnostic tools for evaluating 

roof failure is the systematic analysis of the roof fall itself 

[Ferm et al. 1978; Moebs and Stateham 1985]. Often only a 

cursory investigation of the roof fall is made. This reveals 

little to prevent or avoid similar falls. A review of roof fall 

accident reports filed by operators to MSHA often lists "roof 

slips" or "cracks" in the roof as the cause. Most likely the 

reason for this lack of information is the observer is untrained 

in what to look for and how to document it. 

 The roof fall analysis should focus on two areas. First, the 

accurate mapping of roof falls is not only an MSHA 

requirement, but can reveal important trends that can only be 

seen on a large map scale. Figure 40 shows that the failure of 

roof in different parts of one mine and between several mines 

in different seams were related to two thrust faults [Molinda 

and Ingram 1988]. Without mapped data showing the roof 

failures were spatially related, their common source might be 

obscured. Figure 41 shows linear, oriented roof falls that are 

the result of a high east-west horizontal stress in an Illinois 

mine. At a map scale of 1 in = 400 ft, the direction of the 

most entry damage (north-south) becomes obvious. 

 Secondly, the investigation of the individual falls should 

be systematic and capture information that can be used to 

determine the performance of the support, the strength of the 

roof, and the immediate cause of the fall and to identify any 

extenuating circumstances at the site. 

 Obviously, a cleaned-up fall provides the best access for 

investigation, but information can also be gained (from a safe 

distance) at falls that will not be loaded out. Sometimes the 

cause of a fall can be immediately determined. Crosscutting 

defects in the roof are often the cause of destroyed roof 

beams. If a large slip or fault is cutting through an 

intersection, the smooth failure plane may be obvious. The 

plane may be obscured, but fault gouge (thin layer of clay) 

present on the plane may provide a clue. The fact that the 

roof stayed up long enough to be bolted shows that it was at 

least temporarily self-supporting or that bolts were installed 

to bridge the slip. In this case there may be a timedependency factor to the fall. The standup time of each fall 

should also be documented. 

 Large slips often occur in swarms and may be mapped in 

adjacent entries. If a slip is present in the fall, it should be 

mapped out of the fall to determine its orientation and the 

stability of adjacent entries. At one mine in Virginia, large 

slips in the roof related to regional structure were regularly 

spaced at about 300 ft. The crosscut was turned at the same 

orientation as the slip plane. When the crosscut coincided 

with the slip plane, roof falls occurred (figure 42). 

 It is important to make a sketch of the exposed lithology, 

along with a description and measured or estimated 

thicknesses. A handheld spotlight will greatly aid in the 

observation. Weak rock units (rider coals, underclays, coal 

streaks, ironstone lenses, etc.) may have inadvertently 

become anchorage zones, and their location could explain the 

 

 Figure 40.–High-angle thrust faults cause poor roof 

conditions in a southwestern Virginia mine.

fall. Note should be made of the presence and location of 

water entering the roof fall cavity. The relationship of the 

water to moisture-sensitive members should be noted. The 

CMRR should also be calculated from the roof fall exposure. 

This is usually the best exposure of the roof strata. 

 Figure 43 is an example of a roof fall report from a mine in 

southeastern Ohio. Other data sheets developed by NIOSH 

are available that include additional relevant information 

(figure 44). The sheet provides a space for the plan view, as 

well as a lithologic cross section. The plan view shape of the 

fall can indicate an orientation. Oriented falls can indicate 

horizontal stress influence. Cutter roof and roof damage 

adjacent to the fall can be sketched here. The cross-sectional 

shape of the fall (stairstepped, flat-topped, arched, or coneshaped) may highlight the various units of the bolted interval. 

 An assessment of the performance of the primary roof 

support must be made at the scene of a roof fall. Broken roof 

bolts are a sign of exceptional loading. If bolts have loaded 

and failed, they may be under capacity. Bent plates and 

popped bolt heads may also be signs of excessive stress. If 

bolts have not taken load, then the problem may be failure of 

key blocks, which allows rocks that are bounded by slips to 

fall between bolts, thus severing the beam (figure 45). Bolts 

that have not loaded may also indicate that the beam is 

isolated by sag above the bolt horizon, causing tensional 

breaks on the beam. Finally, another cause of roof failure 

without bolt loading may be slip due to faulty anchorage. 

 Horizontal stress damage to a roof sequence has received a 

lot of attention in the last 10 years. Previously thought to be 

roughly one-third of the vertical stress, horizontal stress is 

now believed to be regularly much higher and to be 

responsible for many more roof falls. An assessment of the 

contribution of horizontal stress to the fall should be made. 
23

Figure 41.–High horizontal stress oriented east-west causes extensive north-south roof falls in an Illinois mine.
24 

Figure 42.–Regular roof shears in a southwestern Virginia mine cause roof falls when crosscuts are aligned parallel to them.

Indicators of horizontal stress are many. These include cutters or guttering, roof "stitching" or rock flour, long running 

falls, "snap top," oriented roof falls, and offset bolt holes 

[Mucho and Mark 1994; Ingram and Molinda 1988]. Shortening of the roof due to horizontal stress may also cause roof 

blocks to fall between bolts. Stiff roof members, like the 

sandstone interbeds in a stackrock, can concentrate stresses 

and, by bending, they can delaminate and fail underlying layers in tension. 

 Defective bolts may be the cause of more roof falls than is 

widely believed. Although sometimes hard to document, it is 

worth the effort to examine roof bolts in roof fall debris. 

Poor roof bolt installation can result from a number of practices [Mazzoni 1996]. “Glove-fingered” bolts occur when the 

bolt spins within the resin cartridge, failing to break the bag 

and preventing resin contact with the wall of the roof bolt 

hole (figure 46). At one mine in southern Illinois, nearly all 

of the bolts observed (n = 10) in several roof falls were 

"glove-fingered." "Glove-fingered" bolts can be eliminated 

by reducing the resin annulus and spinning the bolt as it is 

inserted. This can be done by drilling a smaller diameter hole 

or installing a larger bolt in the hole. One support 

manufacturer has developed a rebar that has a serrated 

surface, which ensures complete resin tube penetration during 

initial inserts of bolt in hole. If mechanically anchored bolts 

are undertensioned, roof failure can result. If the same bolts 

are overtensioned and high-strength bolts are used, crushing 

of the rock could occur, resulting in lost tension. Bolt 

corrosion should also be documented. This can be identified 

by rust-through, but also by a characteristic coning of the 

corroded end. This is a result of progressive corrosion. 

 The intersection span (sum of the diagonals) should 

always be recorded. A previous study showed that over 70% 

of roof falls occurred in intersections [Molinda et al. 1998]. 

In addition, the spans of intersections around the fall 

(n = 8-10 intersections) should be recorded to determine if a 

systematic overspan contributed to the roof fall. At a mine in 

southwestern Pennsylvania, the roof fall rate for the 20-ftwide belt entry was six times the fall rate for the other 16-ftwide entries. Figure 47 shows the method for measuring an 

intersection span. Diagonals were taken from the midpoint 

on the pillar corner between the spalled rib and the original 

cutpoint of the corner. 

TIMING OF ROOF FALLS 

 Unplanned roof falls are always a potential hazard. Falls 

that happen on the working section expose more miners to 

injury and death than those in outby areas. Weak roof that 

falls immediately upon mining is troublesome, but can be 

cleaned up and bolted. Falls that occur several crosscuts back 

from the face are more dangerous. 
25 

Figure 43.–Roof fall report by a geologist at an Ohio mine.
26 

Figure 44.–Roof fall data sheet developed by NIOSH.
 27 

Figure 45.–Roof fall caused by broken rock falling between bolts. 

 Figure 46.–Two views of "glove-fingered" bolts that provide 

little mechanical interlock to support strata. 

Figure 47.–Method for measuring intersection diagonals.

 At one western Kentucky mine over a 3-year period, only 

12% of 161 total falls occurred inby the feeder breaker, but 

46% of the falls occurred within 1 month of development 

when mine workers are still frequently exposed (figure 48). 

This mine had more than 100 reported roof falls in 1996, the 

highest in the country. The roof lithology of this mine 

consists of a siltstone (UR = 32) 0-8 ft up in the roof with a 

3-in-thick, highly reactive mudband 15-18 in up in the roof. 

In numerous falls, it seems that swelling of this member as 

well as the siltstone itself may be responsible. The siltstone 

"rats" out around the bolts because of pressure from the 

mudband, which causes "chandelier" bolts and much debris in 

the intakes. In addition, the use of wooden half headers as 

plates on some bolts has caused the loss of applied tension to 

the torque tension bolts used in the mine. Sixty-four percent 

of bolts tested, using wood headers, showed torque bleedoff 

(some of these bolts were as close as one to two breaks from 

the face). Only 28% of bolts with steel plates showed torque 

bleedoff. One possible solution to such conditions is the use 

of a fully grouted torque tension bolt with a steel plate to help 

prevent swelling and roof ratting. Another might be to leave 

6-8 in of head coal. By doing this, the mine may be able to 

help seal the reactive zone in the roof. 

 The source of roof failure may sometimes be revealed in a 

distribution of roof fall standup time. At a mine near Hanna, 

WY, 21 out of 28 documented roof falls occurred more than 

18 months after entry development. The average standup 
28 

time was 4.1 years. At this mine, 2-5 ft of head coal was 

typically left to protect an extremely weak carbonaceous 

mudstone roof. If the head coal that was left was thinner (in 

the 6-12 in range), then it tended to sag and separate with 

time, exposing the moisture-sensitive mudstone and causing a 

roof fall. 

 At one Illinois mine, most falls occur shortly after mining 

(figure 49), but a significant number occur many months or 

even years after development (average standup time is 34.8 

months). In this mine the roof shale (Dykersburg Shale) is 

moisture-reactive. Although many falls occur within 1-2 

months of development because of high horizontal stress, 

which is characteristic of the Illinois basin, moisture slaking 

continues, causing roof falls long after development. 

 Figure 48.–Standup time for roof falls at a western Kentucky 

mine. 

Figure 49.–Standup time for roof falls at an Illinois mine. 

CASE STUDY 

A roof fall analysis was done at a mine in southwestern 

Pennsylvania in the Pittsburgh Coalbed. Figure 50 shows the 

southern part of the mine with roof falls and CMRR values. 

The mine had a total of 207 roof falls, 108 of which were 

formally reported to MSHA. Following is a summary of roof 

fall characteristics from roof fall reports. 

 (1) Average height of falls: 8.1 ft (figure 51). 

(2) Average height of falls over bolt anchorage: 2.5 ft

(figure 52). 

 The average height of the falls gives insight into the extent 

and nature of the damage. Inordinately high roof falls (20-30 

ft) indicate that high horizontal stresses are acting on the roof 

and bulking the roof strata, resulting in a high roof fall. 

(3) Location of falls: Eight-six percent of all falls were in 

intersections or in both intersections and entries (figure 53). 

 The location of the falls is important to the resulting 

control. If falls are predominantly in intersections, there are 

several possible solutions: 

 (a) Intersections are overspanned. Reduce the span by 

minimizing turnouts in critical belts or travelways. 

 (b) Increase the support, including longer bolts, cable 

bolts, or standing support for pillar corners. 

 (c) Reduce the number of intersections by staggering 

crosscuts. 

 (4) Average volume of falls: 6,258 ft3

.

 This value is important for estimating cleanup costs. 

 The performances of various roof bolts used were analyzed for a relationship with roof falls. The following is a 

distribution of roof falls by roof bolt type. 

Roof bolt type Falls

5-ft resin........................................................ 152 

6-ft point-anchored resin-assisted ................ 25

8-ft point-anchored resin-assisted ................ 1

8-ft conventional ........................................... 16

Mixed pattern................................................ 13

The number of roof falls normalized to feet of drivage per 

bolt type gives a more representative look at the relationship 

between roof falls and roof bolts (figure 54). The two bolt 

categories with the highest roof fall rate are the mixed pattern 

and the 8-ft conventional bolt. The mixed pattern (5-ft resin 

bolt with 8-ft point-anchored resin-assisted bolt) roof fall rate 

can be explained by the fact that this combination of bolts 
29 

was used only in areas of poor roof. The mine had a 

significantly higher roof fall rate using the 8-ft conventional 

bolt early in its life and quickly discontinued its use. Also 

statistically significant was the markedly lower roof fall rate 

realized by using the 8-ft point-anchored resin-assisted bolt. 

This apparent improvement in roof control with the 8-ft 

point-anchored resin-assisted bolt may be due to its greater 

length, higher load-bearing capacity, tensioned roof support 

mechanism, or some combination of effects. The analysis 

also shows that there is no significant improvement in the fall 

rate with the 6-ft point-anchored resin-assisted bolt. 

Figure 50.–A portion of a southwestern Pennsylvania longwall mine showing roof falls, sandstone channels, and CMRR. 

 Figure 51.–Height of roof falls at a southwestern Pennsylvania 

mine. Figure 52.–Height of roof falls over bolts at a southwestern 

Pennsylvania mine. 
30 

 Figure 53.–Location of roof falls at a southwestern Pennsylvania mine. Figure 54.–Roof falls versus roof bolt type at a southwestern 

Pennsylvania mine. 

SUMMARY

 The safety of underground coal miners is of great concern to NIOSH. One of the most hazardous areas of mining 

remains the instability of exposed roof. During 1983-2000, 

245 coal miners were killed by roof falls. In addition, an 

average of 980 injuries per year were attributed to roof falls 

during the same period. Unlike construction materials for 

which there are known strengths and other physical properties, roof geology, and thus rock stability, is highly variable. Current roof exploration methods reveal little about 

the strength of the roof in advance of mining. Therefore, 

engineers and operations personnel must be able to recognize and support weak rock as they advance. Toward the 

goal of recognition and projection of weak and defective 

roof, it is important that strata control personnel understand 

the origin, nature, extent, and potential hazard of "bad 

geology." From numerous field investigations, NIOSH has 

compiled field examples of weak and hazardous roof. 

Geologic hazards are grouped into two categories: weak 

rock and discontinuities. The depositional and structural 

origin, associations, and mining implications for drawrock, 

stackrock, rider coal, and head coal have been discussed, 

along with field examples of associated hazards. The roof 

lithologies are often available in advance in the form of 

roof bolt hole logs. The systematic collection and analysis 

of the information may provide advance warning of weak 

roof rock. 

 Discontinuities, or defects in the roof, are the most dangerous geologic structures found in coal mine roof. Unseen 

breaks in otherwise solid roof may provide little warning of 

impending failure. Clay veins, slickensides, sandstone channels, and joints are the most common of such discontinuities. It 

is a rare coal mine that has never experienced some type of 

geologic roof disturbances. An understanding of the origin and 

occurrence of these features will greatly aid in their tracking 

and prediction. Support measures can be applied more appropriately if the roof damage and resulting loads are better 

understood. 

 Much benefit can be realized from careful and systematic 

roof fall analysis. Roof falls are the best exposures of roof, 

especially weak and defective roof. Uncovering the cause of 

roof falls can reveal trends, including regular shear patterns, 

weak bolt anchorages, damage from horizontal stress, bolt 

failures or poor bolt installations, overspanned intersections, 

and water swelling in clay fault gouge. A CMRR should be 

determined for each roof exposure and can be plotted against 

roof fall rate to indicate the threshold of weak roof for that 

mine. The mapping of falls can reveal their correlation with 

paleochannels, multiple-seam mining, or stream valleys. The 

tracking of standup times of roof falls can help identify the 

personnel most at risk, as well as provide early warning for 

remedial support application. A roof bolt performance using 

the roof fall rate as the outcome variable, roof bolting can be 

optimized based on real mine experiences. 

 Often a roof fall seems to defy explanation. However, with 

careful examination and a historical record of the geology of the 

mine roof, the cause of the fall may be determined. An 

understanding of coal mine roof geology can only lead to an 

increased awareness of the hazards facing the coal miner. 
31 

REFERENCES Bauer ER, Dolinar DR [2000]. Skin failure of roof and rib and support 

techniques in underground coal mines. In: Mark C, Dolinar DR, Tuchman 

RJ, Barczak TM, Signer SP, Wopat PF, eds. Proceedings: New 

Technology for Coal Mine Roof Support. Pittsburgh, PA: U.S. 

Department of Health and Human Services, Public Health Service, 

Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication No. 2000-

151, IC 9453, pp. 99-109. 

 Bieniawski ZT [1988]. Rock mass classification as a design aid in 

tunnelling. Tunnels and Tunnelling July:19-23. 

 Buddery PS, Oldroyd DC [1992]. Development of a roof and floor 

classification applicable to collieries. In: Proceedings of the Eurock '92 

Conference. London: Thomas Telford, pp. 197-202. 

 CFR. Code of Federal regulations. Washington, DC: U.S. 

Government Printing Office, Office of the Federal Register. 

 Chase FE, Ulery JP [1987]. Clay veins: their occurrence, 

characteristics, and support. Pittsburgh, PA: U.S. Department of the 

Interior, Bureau of Mines, RI 9060. NTIS No. PB 87-204517. 

 Damberger HH, Nelson WJ, Krausse HF [1980]. Effect of geology on 

roof stability in room-and-pillar mines in the Herrin (No. 6) Coal of 

Illinois. In: Proceedings of the First Conference on Ground Control 

Problems in the Illinois Coal Basin. Carbondale, IL: Southern Illinois 

University, pp. 14-32. 

 Fergusen HF [1967]. Valley stress release in the Allegheny plateau. 

Bull Assoc Eng Geol 4(1):63-71. 

 Fergusen HF, Hamel JV [1981]. Valley stress relief in flatlying 

sedimentary rocks. In: Proceedings of the International Symposium on 

Weak Rock (Tokyo, Japan, Sept. 21-24), pp. 1235-1241. 

 Ferm JC [1974]. Carboniferous environmental models in eastern 

United States and their significance. In: Briggs G, ed. Carboniferous of 

the southeastern United States. Geological Society of America Special 

Paper 148, pp. 79-95. 

 Ferm JC, Cavaroc VV Jr. [1968]. A nonmarine sedimentary model for 

the Allegheny rocks of West Virginia. In: Klein GD, ed. Late Paleozoic 

and Mesozoic continental sedimentation, northeastern North America. 

Geological Society of America Special Paper 106, pp. 1-19. 

 Ferm JC, Smith GC [1981]. A guide to cored rocks in the Pittsburgh 

basin. U.S. Bureau of Mines contract No. J0188115. Library of Congress 

No. 81-51290. 

 Ferm JC, Melton RA, Cummins GD, Mathew D, McKenna L, Muir C, 

et al. [1978]. A study of roof falls in underground mines on the 

Pocahontas #3 Seam, southern West Virginia and southwestern Virginia. 

University of South Carolina. U.S. Bureau of Mines contract No. 

H0230028. NTIS No. PB 80-158983. 

 Grau RH, Bauer ER [1997]. Ground control worker safety during 

extended-cut mining. In: Peng SS, ed. Proceedings of the 16th 

International Conference on Ground Control in Mining. Morgantown, 

WV: West Virginia University, pp. 283-288. 

 Greb SF [1991]. Roof falls and hazard prediction in eastern Kentucky 

coal mines. In: Peters DC, ed. Geology in coal resource utilization. 

Fairfax, VA: American Association of Petroleum Geologists, pp. 245-262. 

 Hill JL III [1986]. Cutter roof failure: an overview of the causes and 

methods for control. Spokane, WA: U.S. Department of the Interior, 

Bureau of Mines, IC 9094. NTIS No. PB 87-116976. 

 Hylbert DK [1978]. The classification, evaluation, and projection of 

coal mine roof rocks. Min Eng 30(12):1667-1676. 

 Iannacchione AT, Ulery JP, Hyman DM, Chase FE [1981]. Geologic 

factors in predicting coal mine roof rock stability in the upper Kittanning 

coalbed, Somerset County, Pa. Pittsburgh, PA: U.S. Department of the 

Interior, Bureau of Mines, RI 8575. NTIS No. PB 82-145376. 

 Ingram DK, Chase FE [1987]. Effects of ancient stream channel 

deposits on mine roof stability: a case study. Pittsburgh, PA: U.S. 

Department of the Interior, Bureau of Mines, RI 9092. 

 Ingram DK, Molinda GM [1988]. Relationship between horizontal 

stresses and geologic anomalies in two coal mines in southern Illinois. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 

9189. NTIS No. PB 90-268814. 

 Kertis CA [1985]. Reducing hazards in underground coal mines through the 

recognition and delineation of coalbed discontinuities caused by ancient channel 

processes. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, 

RI 8987. NTIS No. PB 86-156205. 

 Kester WM, Chugh YP [1980]. Premining investigations and their use in 

planning ground control in the Illinois coal basin. In: Proceedings of the First 

Conference on Ground Control Problems in the Illinois Coal Basin. 

Carbondale, IL: Southern Illinois University, pp. 33-43. 

 Krausse H-F, Damberger HH, Nelson WJ, Hunt SR, Ledvina CT, Treworgy 

CG, et al. [1979]. Roof strata of the Herrin (No. 6) coal member in mines of 

Illinois—their geology and stability: summary report. Champaign, IL: Illinois 

State Geological Survey, Illinois Minerals Notes 72. 

 Lotz CW [1970]. Probable original mineable extent of the bituminous coal 

seams in West Virginia. Morgantown, WV: West Virginia Geological and 

Economic Survey. 

 Mark C, Molinda GM, Schissler AP [1994]. Evaluating roof control in 

underground coal mines with the coal mine roof rating. In: Peng SS, ed. 

Proceedings of the 13th International Conference on Ground Control in Mining. 

Morgantown, WV: West Virginia University, pp. 252-260. 

 Mazzoni RA [1996]. A trouble-shooting guide for roof support systems. 

Pittsburgh, PA: U.S. Department of Labor, Mine Safety and Health 

Administration, Informational Report 1237. 

 McCulloch CM, Diamond WP, Bench BM, Deul M [1975]. Selected 

geologic factors affecting mining of the Pittsburgh coalbed. Pittsburgh, PA: 

U.S. Department of the Interior, Bureau of Mines, RI 8093. NTIS No. PB 249 

851. 

 McLoughlin TF [1986]. Explanation of the regional tectonic map of the 

southwestern coalfield of Virginia. U.S. Department of Labor, Mine Safety and 

Health Administration, Informational Report 1177. 

 McLoughlin TF [2001]. Telephone conversation between G. M. Molinda, 

Pittsburgh Research Laboratory, and T. F. McLoughlin, U.S. Department of 

Labor, Mine Safety and Health Administration, District 5, Roof Control 

Department, Norton, VA. 

 Milici RC, Gathright, TM, Miller BW, Gwin MR [1982]. Geologic factors 

related to coal mine roof falls in Wise County, Virginia. Appalachian Regional 

Commission, contract No. CO-7232-80-I-302-0206. 

 Moebs NN [1977]. Roof rock structures and related roof support problems 

in the Pittsburgh Coalbed of southwestern Pennsylvania. Pittsburgh, PA: U.S. 

Department of the Interior, Bureau of Mines, RI 8230. NTIS No. PB 267 922. 

 Moebs NN, Stateham RM [1985]. The diagnosis and reduction of mine roof 

failure. Coal Min 22(2):52-55. 

 Molinda GM, Ingram DK [1988]. Case evaluation of a surface seismic 

reflection technique for delineating coalbed discontinuities. Pittsburgh, PA: 

U.S. Department of the Interior, Bureau of Mines, RI 9147. NTIS No. PB 88-

229885. 

 Molinda GM, Ingram DK [1989]. Effects of structural faults on ground 

control in selected coal mines in southwestern Virginia. Pittsburgh, PA: U.S. 

Department of the Interior, Bureau of Mines, RI 9289. NTIS No. 90-196809. 

 Molinda GM, Mark C [1994]. Coal mine roof rating (CMRR): a practical 

rock mass classification for coal mines. Pittsburgh, PA: U.S. Department of the 

Interior, Bureau of Mines, IC 9387. NTIS No. PB 94-160041. 

 Molinda GM, Mark C [1996]. Rating the strength of coal mine roof rocks. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, IC 9444. 

NTIS No. PB 96-155072. 

 Molinda GM, Heasley KA, Oyler DC, Jones JR [1992]. Effects of 

horizontal stress related to stream valleys on the stability of coal mine openings. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 9413. 

 Molinda GM, Monaghan WD, Mowrey GL, Persetic GF [1996]. Using 

ground penetrating radar for roof hazard detection in underground mines. 

Pittsburgh, PA: U.S. Department of Energy, RI 9625. NTIS No. PB 96-188123. 

 Molinda GM, Mark C, Bauer ER, Babich DR, Pappas DM [1998]. Factors 

influencing intersection stability in U.S. coal mines. In: Peng SS, ed. 

Proceedings of the 17th International Conference on Ground Control in Mining. 

Morgantown, WV: West Virginia University, pp. 267-275. 

 Molinda GM, Mark C, Dolinar DR [2000]. Assessing coal mine roof 

stability through roof fall analysis. In: Mark C, Dolinar DR, Tuchman RJ, 

Barczak TM, Signer SP, Wopat PF, eds. Proceedings: New Technology for 
32 

Coal Mine Roof Support. Pittsburgh, PA: U.S. Department of Health and 

Human Services, Public Health Service, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, DHHS 

(NIOSH) Publication No. 2000-151, IC 9453, pp. 53-71. 

 MSHA [1999]. Quarterly employment and coal production, 

accidents/injuries/illnesses reported to MSHA under 30 CFR 50. Denver, 

CO: U.S. Department of Labor, Mine Safety and Health Administration, 

Office of Injury and Employment Information. 

 Mucho TP, Mark C [1994]. Determining horizontal stress direction 

using the stress mapping technique. In: Peng SS, ed. Proceedings of the 

13th International Conference on Ground Control in Mining. 

Morgantown, WV: West Virginia University, pp. 277-289. 

 Nelson WJ [1991]. Faults and their effect on coal mining in Illinois. 

Champaign, IL: Illinois State Geological Survey, Circular 523, pp. 1-40. 

 Pettijohn FJ, Potter PE, Siever R [1973]. Sand and sandstone. New York: 

Springer-Verlag, pp. 453-466. 

 Roen JB, Farrel DE [1973]. Structure contour map of the Pittsburgh 

Coalbed, southwestern Pennsylvania and northern West Virginia. U.S. 

Department of the Interior, U.S. Geological Survey open-file map. 

 Sames GP, Moebs NN [1989]. Hillseam geology and roof instability near 

outcrop in eastern Kentucky drift mines. Pittsburgh, PA: U.S. Department of 

the Interior, Bureau of Mines, RI 9267. NTIS No. PB 90-157439. 

 Sames GP, Moebs NN [1991]. Geologic diagnosis for reducing coal mine 

roof failure. In: Peters DC, ed. Geology and coal resource utilization. Fairfax, 

VA: American Association of Petroleum Geologists, pp. 203-223. 

 Van der Merwe NJ [2000]. Horizontal stress: the root of all evil? In: Peng 

SS, ed. Proceedings of the 19th International Conference on Ground Control in 

Mining. Morgantown, WV: West Virginia University, pp. 129-136.
33 

APPENDIX.—GLOSSARY OF TERMS 

 Chandelier bolts.—When moisture-sensitive shales fall 

between roof bolts, leaving 2 in to 2 ft of exposed roof bolt. 

Clay vein.—A body of soft or somewhat indurated clay 

that can either cut through or follow bedding in the coal 

seam. The clay is usually injected into the cleat or coal 

fractures from the clay forming the roof or, less frequently, 

the floor. Clay veins range from inches to many feet in 

lateral extent. 

Cleat.—A vertical or rotated systematic joint set found 

in coal seams formed by tension during compaction or 

during large-scale structural movement. 

Coal streak.—Small piece of coal (2-10 in) found in a 

matrix of shale in the roof. The coal streak may be formed 

in place or washed in as a result of flooding. 

Cutter.—A linear fracture in the roof that usually occurs 

along the roof and rib contact caused by compression due 

to horizontal stress or roof sag. 

Decollement.—Small faults or folds occurring in roof 

rocks and caused by the sliding of overlying or underlying 

strata along bedding. 

Drawrock.—A soft shale or claystone, about 2 in to 4 ft 

thick, above the coal that falls with the coal or shortly after 

mining. 

Fault.—A failure surface along which there has been 

movement. Movement can be the result of sediment 

compaction during burial or larger scale strata movement 

due to major regional stresses. Often used to describe any 

interruption in the lateral continuity of the coal seam. 

Gutter.—A cavity formed by the fall of roof rock 

crushed out by a cutter. 

Hillseam.—A systematic joint or joint set formed by 

extensional movement of rock toward a stream valley. Sets 

can range from 2 in to 10 ft wide and can crosscut the coal 

seam or can be confined only to the roof strata. 

Horseback.—A mass of rock found in the roof with a 

smooth surface and resembling a horse's back. A ridgelike 

body that is the result of erosion and filling with sediment. 

Also, the slickensided cavity in the roof of a rock mass that 

has fallen out of the roof. 

Joints.—A system of fracture surfaces that cut 

perpendicular or subperpendicular to bedding and along 

which there has been no movement. Usually found in the 

roof, but can include cleat in the coal seam. 

Pot.—A fall of roof, not reaching to the roof bolt anchorage, 

that can range from 2 in to 5 ft up into the roof. 

Rash.—A sequence of rock consisting of carbonaceous 

shale, dirty coal, thin riders, and coal streaks found above a coal 

seam. 

Rider coalbed.—A thin (2 in to 4 ft), minor coalbed that 

occurs above the main coal seam and results from the 

reestablishment of coal swamps on top of the flooded main 

seam. Several rider coalbeds can be present above the main 

seam. 

Rolls.—A local thickening of the roof, floor, or interior 

parting strata causing thinning of the coal seam. This could be 

due to a washout or coal split. The term is also used to refer to 

a localized uplift or downdrop of a coal seam. 

Scale.—Small slabs or pieces of roof rock that are left as the 

coal is removed, or formed by separation and sagging of 

bedding in roof rock. 

Seat earth.—A bed representing an ancient soil, usually 

containing abundant rootlets, under the coal seam. 

Shear.—A fault surface in roof, coal, or floor. 

Shear bodies.—Large roof features (feet to tens of feet) that 

are isolated cylindrical bodies formed by slippage in the roof. 

Slaking.—The disintegration of clay-rich rocks due to 

flaking and swelling when exposed to moisture. 

Slickensides.—A polished, or sometimes striated surface 

found in a coal mine roof produced by rubbing during faulting 

or compaction. 

Slip.—Small fault characterized by slickensides and a glassy, 

glazed appearance. Usually found in the roof or interseam 

binder and due to soft sediment deformation, such as slumping 

and compaction with burial. Can range from inches to 10 ft. 

Snap top.—Roof rock that, when subjected to compressional 

loading, works and fails audibly by making a snapping sound. 

Stackrock.—A sequence of roof rock consisting of 

alternating shale and sandstone layers that resembles a stack of 

newspapers or books. 

Stitching.—A zigzag fracture in the roof due to compression 

and shortening of the roof beam that resembles cloth stitching. 

Swag.—A depression in the roof or floor caused by a roll in 

the coal seam. 

Wild coal.—Thin rider coal above the main seam. 
Delivering on the Nation’s Promise:

Safety and health at work for all people 

Through research and prevention

For information about occupational safety and health topics contact NIOSH at:

DHHS (NIOSH) Publication No. 2003-152

1-800-35-NIOSH (1-800-356-4674)

Fax: 513-533-8573

E-mail: pubstaft@cdc.gov

www.cdc.gov/niosh

SAFER HEALTHIER PEOPLE