ABSTRACT 

 Mines with exceptionally low-strength roof (UCS <3,500 psi 

and CMRR <40) are much more likely to struggle with roof falls 

than other mines. Weak roof is a particular problem for many room 

and pillar mines in the Midwestern and Northern Appalachian coal 

basins. Traditional roof support techniques are often not up to the 

challenge at these operations. 

 This paper focuses on two mines, one operating in the Upper 

Freeport seam and the other in the Herrin No. 6 seam. Together, 

the two mines had more than 300 roof falls during a recent 5 year 

period. Each has experimented with a variety of roof bolt types and 

lengths, and with different supplemental supports. Detailed 

statistical analysis was conducted to determine which support 

combinations have proven to be most effective. Geology, 

horizontal stress, and time are also important at both mines. 

Successful control techniques have included: 

• Identifying particularly troublesome lithologic roof units; 

• Installing longer and stronger roof bolts; 

• Installing supplemental support in intersections, and; 

• Limiting the duration that a panel remains open. 

 The lessons learned from these mines, and others like them, 

could help improve ground control safety for the entire U.S. mining 

industry. 

INTRODUCTION

 Roof falls continue to be one of the greatest hazards in 

underground coal mines. Although fatalities from roof falls 

reached an all-time low of two in 2003, more than 1,400 major roof 

collapses were reported to the Mine Safety and Health 

Administration (MSHA). These roof falls can threaten miners, 

damage equipment, disrupt ventilation, and block critical 

emergency escape routes. 

 Roof falls are not evenly distributed between coal mines. 

Analysis of MSHA statistics from the years 1995-2002 show that 

nearly 60% of roof falls occurred in mines accounting for just 20% 

of all hours worked underground. Miners at these operations were 

six times more likely than other miners to be exposed to a roof fall. 

Indeed, just 20 mines accounted for one-fourth of all roof falls 

during the period. 

 The probability of a roof fall varies from region to region. 

Alabama, Utah, and northeast Kentucky all have roof fall rates that 

are about half the national average. Certain mines in Virginia and 

Northern Appalachia have high roof fall rates, though on the whole 

those regions have a moderate risk of roof falls. Roof falls are 

most troublesome in the Illinois basin, where the rate is 

approximately twice that in the rest of the country (figure 1). 

 The variation in roof fall rates is clearly related to geologic 

conditions. One recent study compared the uniaxial compressive 

strength of roof rocks in the Illinois basin with those in southern 

West Virginia (Rusnak and Mark, 2000). The study involved more 

than 800 rock units and 10,000 laboratory strength tests. It 

concluded that for each of the three major roof rock types (shale, 

siltstone, and sandstone), the strength of the Illinois basin variety 

was less than half that of the same rock in West Virginia (see 

figure 2). 

 The Coal Mine Roof Rating (CMRR) can also be used to 

compare roof strength in different regions. The National Institute 

for Occupational Safety and Health (NIOSH) has been measuring 

the CMRR in its ground control studies for more than a decade, and 

the NIOSH data base now includes nearly 300 CMRR 

observations. Figure 3 shows that nearly all the weak roof 

observations (CMRR<41) have been made in either the Northern 

Appalachian or Midwestern coal basins. 

Preventing Falls of Ground in Coal Mines with Exceptionally 

Low-Strength Roof: Two Case Studies 

Christopher Mark, Section Chief, Rock Mechanics 

Gregory Molinda, Research Geologist 

Lisa M. Burke, Mining Engineer 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 

Figure 1. U.S. underground coal mining non-injury roof 

fall rates, by region.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

Northern

Appalachia

Central

Appalachia

Alabama Midwest West

Region

Roof Falls per 200,000 hrs worked
 High roof fall rates can also be caused by inadequate support, 

wide spans, and/or high horizontal stress (Mark and Barczak, 

2000). However, Mark et al., (2001) found that for the weakest 

roof, heavy roof support may not be able to prevent roof falls. The 

Analysis of Roof Bolt Systems (ARBS) data base contains 13 case 

histories in which the CMRR was less than 40. None of these 

mines were able to achieve a “satisfactory” roof fall rate (which 

was arbitrarily defined in the study to be less than 1 roof fall per 

25,000 ft of drivage), despite support densities 50% greater than the 

other mines in the study. 

 It seems that today’s roof control technology may not be fully 

protecting miners working in low-strength roof environments. 

NIOSH is currently in the initial phase of a research effort aimed at 

improving safety for these miners. The first step is to obtain a 

clearer understanding of the fundamental issues. 

 This paper describes preliminary studies that were conducted at 

two mines with low-strength roof. One of these mines is located in 

the Illinois basin, the other in northern Appalachia. Specific issues 

that are evaluated include: 

• The adequacy of existing techniques for geologic 

characterization; 

• The presence of seasonal and long-term effects; 

• The role of horizontal stress; 

• The performance of different roof bolt patterns, and; 

• The effectiveness of secondary support. 

MINE A 

 Mine A is located in the Illinois basin. Two continuous miner 

sections are extracting the Herrin No. 6 seam, using room-andpillar methods with no secondary extraction. The seam height is 

about 5.5 ft and the depth of cover is about 250 ft. According to 

MSHA data, there have been 12 rock fall injuries at Mine A during 

the past 2.5 years. 

Geology 

 Three immediate roof types have been identified at Mine A: 

• Typical shale which has a uniaxial compressive strength of 

about 3,000 psi, appears massive but splits easily along 

bedding, and is moisture sensitive; 

• Weak shale with a UCS of about 2,000 psi, is highly 

laminated and splits very easily along bedding, and is 

moisture sensitive, and; 

• Sandstone overlying one of the two shale facies, usually 

accompanied by groundwater. 

All of these roof types are quite weak, with CMRR values of 35 or 

less. However, the weak shale roof is much more troublesome than 

the typical shale, particularly when it is greater than 6 ft thick. 

Because the characteristics of the two shale facies are so similar, 

with the weak shale gradually coarsening upward into the other, it 

is very difficult to judge a definitive consistent contact between the 

two units whether underground or in cores. Instead, the weak shale 

thickness is determined in exploration holes primarily based on 

RQD (rock quality designation) and lithological observations, and 

then estimated between holes using a contouring package. The 

cores are observed fresh in the box and then again approximately 

48 hr later, with the final determination of the thickness of the weak 

shale being made when the rock mechanics data become available. 

Unfortunately, the accuracy of the estimate undoubtedly diminishes 

rapidly as the distance from a borehole increases. 

 The sandstone unit has been troublesome when it approaches to 

within 20 ft of the top of the seam. It can degrade the competence 

of the underlying roof shales by causing compaction slips and 

shears in the transition zones. Because it often carries water, 

sandstone may also reduce roof strength by increasing the shale’s 

moisture content. In addition, since sandstones are generally much 

stiffer than shale, they may concentrate horizontal stress. It is not 

clear which of these mechanisms is the most disruptive, and 

perhaps they all work together to reduce roof stability in sandstone 

roof areas. 

Horizontal Stress 

 The effects of horizontal stress are quite evident in Mine A. 

Cutters or kink zones often occur in the advancing face as it is 

being cut. Other times, cutters work their way up from outby into 

the roof, creating hazardous conditions for roof bolt operators. 

Often, cutters in a particular section seem to follow a consistent 

pattern, developing only on the right or the left side of the entries. 

Over time, the cutters can continue to worsen until they result in a 

roof fall. Once started, roof falls tend to start in non-intersection 

areas and run for a couple of breaks. 

Figure 2. Unconfined compressive strength of roof rock in the 

Midwestern and Southern Appalachian coal basins (after 

Rusnak and Mark, 2000). 

Figure 3. CMRR observations in the NIOSH data base, by 

region. 

0

20

40

60

80

<=40 41 - 50 51 - 60 61 -70 >70

Coal Mine Roof Rating (CMRR)

No. of Observations

Northern Appalachia Southern Appalachia Midwest

CO,WY,NM Alabama Utah

N = 274
 While no overcoring stress measurements have yet been made 

at Mine A, a compilation of measurements from elsewhere in the 

Illinois basin indicate that the regional maximum horizontal stress 

is typically between N70°E and E-W (Mark and Mucho, 1994). 

This direction also agrees with the data available from the World 

Stress Map Project (2004). 

 Underground stress mapping has been used to estimate the 

stress direction at Mine A. Roof cutters are the only features that 

were mapped because they are so common. Since cutters that 

follow a rib are not very helpful, the focus was on ones that 

traveled across an intersection or entry from rib to rib (see circles 

on figure 4). These cutters should be perpendicular to the major 

principal stress. 

 Figure 5 shows that the stress mapping measurements cluster 

around a mean orientation of N10°W. Taking all this information 

together, it appears that N80°E is probably the best estimate for the 

direction of the maximum principal horizontal stress at Mine A. 

This orientation explains the directional patterns of cutting that 

occur in the faces. The cutters have consistently occurred in the 

“leading edge” (the corner of the entry that is the first to contact the 

stress) of the entries being mined as illustrated in figure 6. 

N10W

W E

H

N

Figure 5. Formation of roof cutters at the “leading edge” 

of the faces at Mine A.

Figure 4. Stress mapping conducted at Mine A. 

8-22-03

SEALED

6+00

5+40

4+80

4+20

3+60

3+00

2+40

1+80

1+20

0+00

2+10

1+40

0+70

0+00

Rm.1

Rm.2

Rm.3

Rm.4

Rm.5

Rm.6

Rm.7

Ent.7

Ent.6

Ent.5

Ent.4

Ent.3

Ent.2

N

Cutters

Roof Falls

Key

8-22-03

SEALED

6+00

5+40

4+80

4+20

3+60

3+00

2+40

1+80

1+20

0+00

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Rm.1

Rm.2

Rm.3

Rm.4

Rm.5

Rm.6

Rm.7

Ent.7

Ent.6

Ent.5

Ent.4

Ent.3

Ent.2

N

Cutters

Roof Falls

Key
 The next issue is the magnitude of the horizontal stress. Recent 

NIOSH research has shown that the stress magnitude is largely 

determined by the stiffness (Young’s Modulus) of the rock 

(Dolinar, 2003). Therefore, it is probably “low” in the roof shales, 

at least compared to some measurements in sandstone or limestone. 

However, relative to the strength of the rock, particularly the shear 

strength of the laminations in the weak shale, the stress magnitude 

is clearly quite high. In fact, even the minimum horizontal stress 

may be large enough to cause rock failure in some instances. 

Roof Support 

 Three main roof support patterns have historically been used at 

Mine A: 

 1. Bolt Pattern 1: Three, 8-ft long, #7 resin-assisted pointanchor bolts; with two, 6-ft long, #5 fully-grouted rebar 

(ARBS=17); with 16 ft steel straps; 

 2. Bolt Pattern 2: Two, 12-ft long, #7 resin-assisted pointanchor bolts; with three, 5-ft long, resin-assisted pointanchor bolts (ARBS=14); with either 14 ft steel straps or 

wire mesh (5'x15' panels, of 8 Gauge wire with 4" square 

openings), and; 

 3. Bolt Pattern 3: 6 ft long, #5 fully-grouted rebar, 5 per row 

(ARBS=8.5). 

Bolt patterns (1) and (2) have been used in the main drivages, 

pattern (3) has been confined to short-life panels only. 

 In addition, steel straps, cable trusses, mesh, and wood posts 

have been employed in some critical entries such as primary 

escapeways, beltways, travelways, intakes, and returns. Figure 7 

shows the approximate areas in the mine where each of these 

support systems were used. 

 Underground observations indicated that a typical sequence of 

events leading to a roof fall might be as follows: 

• On development, a cutter forms on one side of the entry 

(due to the horizontal stress, as discussed above). 

• As the cutter works its way up into the roof, the rock around 

the roof bolt plates nearest the cutter tends to unravel. If the 

bolts are point-anchor, they may lose their effectiveness 

when the bearing plate no longer contacts the roof. 

• If the cutter works its way above the bolts, a roof fall may 

occur. In cases where the only the rib side long bolt had 

lost its plate load, the fall may be just 6 ft high, and extend 

half way across the entry. 

An important, non-traditional “roof support” employed at Mine 

A is an air conditioning system. Air conditioning demonstrated its 

effectiveness at a nearby mine where it dramatically reduced the 

number of long-term roof falls (Laswell, 1999). The air 

conditioning system at Mine A employs three, 400-ton air cooled 

chillers. The units are typically operated five months per year, 

from May 1st to October 1st. The temperature is controlled to 

achieve dew point on the air conditioning coils to remove moisture 

from the mine intake air and reduce slaking of the mine roof rock. 

Typical temperature drops are in the 20 to 25 °F range. 

Roof Fall Analysis 

 A total of 110 roof falls are included in the Mine A database1

. 

Each of these falls was identified as to: 

• Roof bolt pattern; 

 

1

 Only about half of these roof falls met the criteria in the Code of Federal 

Regulations (30 CFR 50.20-5) to be reportable to MSHA. The others were 

documented by the mine. 

Figure 7. Sketch map of Mine A showing the areas of 

different roof bolt patterns and thick weak shale. For 

details on the roof bolt patterns, please refer to the text. 

Figure 6. Sketch showing the "leading edges" of entries 

driven in different directions at Mine "A". 

H

H

N65 W°

°

S 25 W°

°

N 80 E

N 25 E°

S 65 E
• Geology; 

• Drivage orientation; 

• Stand-up time, and; 

• Number of intersections or entry/crosscut segments 

involved. 

 The statistics indicate that 70% the falls at Mine A have 

occurred within 6 months after mining, and 83% have occurred 

within 18 months. 

 Geology: The statistical analysis indicated that roof falls were 

approximately twice as likely beneath thick weak shale roof than 

beneath the typical shale (table 1). In addition, the relatively small 

area where sandstone affected the underlying shale roof, the roof 

fall rate was several times greater than the rest of the mine. 

Table 1. Roof fall data from Mine A. 

Geology Bolt Pattern Drivage 

(10,000 ft) Roof Falls1

 Falls/10,000 ft 

1 6.2 37 6.0 

2 9.0 50 5.6 Typical 

Shale 3 7.2 25 3.5 

1 10.9 19 1.7 

2 6.6 72 9.2 Weak 

Shale 3 3.2 81 25.3 1

Each fallen intersection, entry segment, or crosscut segment is 

counted as one roof fall. 

 Roof bolt pattern: Statistical analysis confirmed the importance 

of bolt pattern at Mine A. In the weak shale roof, where the 6 ft 

bolts (bolt patterns) were used, the roof fall rate was nearly 5 times 

higher than the rate for the rest of the mine. This was true even 

though the 6-ft bolts were only used in panels that are open for 

relatively short periods of time. Bolt pattern (2), with 2-12 ft bolts, 

was also significantly less successful than bolt pattern (1). It seems 

the roof failure may quickly extend to the top of the thick weak 

shale, above the 6 ft bolts. Bolt pattern 1 may be most successful 

because it has three bolts that extend beyond 6 ft. 

 In contrast, in the typical shale roof, all three patterns had 

similar roof fall rates (in fact, the 6 ft bolts had the lowest rate, 

perhaps attributable to their short exposure time). Here it seems 

that the 6 ft bolts may be able to fully contribute to the performance 

of the roof support system. 

 Drivage Orientation did not appear to be a significant factor, 

once other variables were controlled. Most of the mine’s drivage is 

oriented either N65°W or N25°E. Assuming that the maximum 

horizontal stress is oriented N80°E, then the N65°W drivage should 

be a slightly more favorable direction. The statistics show that in 

the typical shale roof, or where the bolts were at least 8 ft long, the 

N25°E drivage was at most 20% more likely to be involved in a 

roof fall. 

 However, one significant observation was that in either drivage 

direction, falls were more than twice as likely in entries than in 

crosscuts. One explanation is that the stress damage that occurs in 

the faces on development may be more severe in the entries, 

making them more prone to subsequent collapse. Similar trends 

have been observed at other mines (Mucho and Mark, 1994). 

 Intersection span also seemed to be a factor. Mine A typically 

turns crosscuts from entry No. 2 and entry No. 5 in their 7-entry 

development sections. The turnout intersections have spans (sumof-the-diagonals) averaging about 63 ft, versus 54 ft for the typical 

intersections. The data indicates that turnouts were nearly twice as 

likely to be involved in roof falls than typical intersections. 

 Surprisingly, three-way intersections were nearly 2.5 times as 

likely to collapse as four-ways. The most likely explanation is that 

three-ways are located at the edge of panels, where the horizontal 

stress seems to concentrate. 

MINE B 

 Mine B is a room and pillar mine located in the Northern 

Appalachian coalfields. After rooms are developed, the pillars are 

extracted by secondary mining. It is a drift mine with cover ranging 

from 0-480 ft. There have been 17 roof fall injuries in the last 5 

years. 

Roof Geology 

 The mining height is 52 in, including approximately 16 in of 

weak drawrock that would otherwise collapse before it could be 

bolted. The roof rock is 10-12 ft of weak, laminated clay shale, 

grading into a 10-12 ft thick sandy shale unit. Locally known as 

“soapstone”, the weak shale (CMRR 30-32) contains frequent clay 

veins and slickensides. The roof rock is very moisture-sensitive 

and dissolves quickly when immersed in water. Underground, the 

rock degrades with time and exposure to humidity, resulting in very 

slabby, ragged roof. Hanging roof bolts are common where the 

roof has unraveled between them (figure 8). 

Horizontal Stress 

 There are no horizontal stress measurements available at Mine 

B, though the presumed regional horizontal stress is approximately 

N70°E. (Mark and Mucho, 1994). Horizontal stress is not as 

evident at Mine B as at Mine A, though there are some cutters and 

rock “stitching” that occurs near the face. Roof that has been 

exposed 3-4 months shows heavy guttering and scaling between 

bolts. Stress does not appear strongly directional, but only low 

stress may be required to crush this weak rock. Once guttering 

occurs, the entire roof beam is susceptible to fracturing. 

Figure 8. Photograph of roof bolts exposed by weathering of 

weak shale at Mine B.
Roof Support 

 As the mine started in from the outcrop, fully-grouted, 6 ft long 

tensioned rebar bolts were used until out of the influence of outcrop 

and hillseams. At that point the primary support in the mains was 

switched to 6 ft long, 5/8 in diameter non-tensioned, fully-grouted 

resin bolts. After numerous roof falls occurred, a test panel was 

driven to compare 5 ft point-anchored, resin-assisted bolts with a 

6 ft non-tensioned fully-grouted resin bolt. Afterwards, 5 ft fully 

grouted tensioned rebar bolt became the primary support in the 

mains, while 6 ft, 3⁄4-in-diameter fully grouted resin bolts were used 

in the panels. In addition, supplemental support consisting of eight, 

14 ft long cable bolts in the intersections and roof screen was 

routinely installed throughout the belt, track, and primary 

escapeway entries. 

Roof Fall Analysis—Short-Term 

 In the 6 years the mine operated through May, 2003, a total of 

206 roof falls were carefully tabulated. Many of these falls 

occurred in inactive parts of the mine and so did not meet the 

criteria for a “reportable” roof fall as defined by MSHA. As in 

mine A, a number of ground fall-related variables were tabulated 

throughout the mine. The drivage in the mains was partitioned into 

individual cases by the following variables: 

• Depth of cover 

• Primary roof bolt 

• Supplemental support 

• Mine orientation 

 The stand-up time was also determined for each fall. At Mine 

B, a number of falls have occurred long after mining, and these 

long-term falls were evaluated separately. In the short-term 

analysis, only roof falls which took place within 18 months of 

development were considered. Panels that were open less than 

18 months, and their associated roof falls, were not included in the 

analysis. 

 Depth of cover: At Mine B, the workings under shallow cover 

have been much more troublesome. For this analysis, the mine was 

divided into two zones: 

• Shallow cover (less than 240 ft), and; 

• Deep cover (more than 240 ft). 

 Statistical analysis showed that the overall roof fall rate when 

mining under less than 240 ft of cover is more than 3 times higher 

than when mining with more than 240 ft. In one area where all the 

other variables were constant, the roof fall rate under shallow cover 

was more than 5 times greater than when mining under deeper 

cover. 

 Shallow cover can be associated with mining near outcrop or 

beneath stream valleys. Poor rock quality, concentrated horizontal 

stress, and water inflow have all been documented beneath stream 

valleys (Molinda et al., 1992). Additionally, joint sets and 

weathering are commonly found near the outcrop. 

 Primary Roof Bolts: Since the depth of cover has such a large 

affect on the roof fall rate, it is necessary to remove this variable 

before evaluating the other variables. Only the deep-cover areas 

were used in the analysis of the primary roof bolt systems. 

Two outcome measures were used to compare roof bolt 

effectiveness: 

• Drivage roof fall rate (roof falls/10,000 ft of drivage), and; 

• Four way intersection roof fall rate (number of four-way 

intersection roof falls/total number of four way intersections 

developed). 

 Three bolt types were considered in the analysis: 

1. 6 ft long, fully grouted, untensioned rebar (ARBS=6.1); 

2. 5 and 6 ft tensioned rebar, fully grouted (ARBS=7.4), and; 

3. 5 ft point-anchored, resin-assisted (PAR) bolts, installed 

with 3 ft of resin (ARBS=9.2). 

 Using the drivage measure, the non-tensioned rebar bolt 

performed slightly better than the other two systems (table 2). 

However, using the 4 way intersection roof fall rate, the PAR bolts 

and the tensioned rebar performed slightly better. Overall, it does 

not appear that any of the bolt systems was significantly more 

effective. 

Table 2. Roof fall data from Mine B. 

Depth of 

cover Bolt pattern Drivage 

(10,000 ft) Roof falls1

 Falls/10,000 ft 

1 7.1 14 2.0 

Deep 2 17.3 46 2.7 

3 4.1 16 3.9 

Shallow - 7.5 57 7.6 1

Each fallen intersection, entry segment, or crosscut segment is 

counted as one roof fall. 

Supplemental Support was systematically installed on cycle or 

very close to on cycle in the belt, track, and primary escapeway 

entries in about half of the mains drivage. When supplemental 

support was used, it normally consisted of eight, 14-ft cable bolts in 

the intersections, with welded steel screen added for skin control. 

 The analysis showed that the supplemental support reduced the 

4 way intersection roof fall rate to less than half of what it was 

where no cable bolts were installed on cycle. However, it appears 

that at least 5 intersections have collapsed even with supplemental 

support. 

 Mine orientation: Two main drivage directions have been 

used at Mine B, N28°W and N66°E. The N66°E drivage is very 

nearly parallel the presumed major horizontal stress, and therefore 

might be expected to be a more favorable mining direction than 

N28°W. However, the roof fall rates in both directions were almost 

identical. The shallow cover drivage directions were N28°W and 

N60°W, but again there was no significant difference between roof 

fall rates. It appears that the stress field at Mine B is not strongly 

biaxial (i.e., it is approximately the same magnitude in all 

directions). 

Roof Fall Analysis—Effect of Time

 The roof falls at Mine B exhibit two types of time-dependency. 

First, the roof fall rate during the humid summer months is more 

than twice what it is during the fall and winter (figure 9). Second, 

while most of the falls have occurred within 12 months of mining, 

many continue to occur several years after mining (figure 10). 

 The roof fall rate is the highest in the initial mains drivage near 

the portal. Mine openings there have been open nearly 7 years, and 

the effect of humidity on the moisture-sensitive “soapstone” is 

clearly evident. Incoming ventilation air has no chance to 

equilibrate in temperature and adversely affects the weak rock. 
Progressive roof falls have created very difficult conditions in the 

beltway and intake air courses. 

 Since most of the falls occur in the first 12 months of exposure, 

the mine moved to shorten the panels which allows the operator to 

develop and retreat panels quickly. In this weak roof, less panel 

exposure time means less chance for roof falls. In the mains, which 

must remain open for a longer time, the installation of supplemental 

support on cycle has resulted in more stable intersections. The 

success of these measures can be seen in figure 11, which shows 

that the number of short term roof falls at mine B have been 

dropping year to year. 

 Evidence for another potentially successful control technique 

can be found in the initial mains development. Nearly 1,300 ft of 

the track entry roof was sprayed with gunite when it was 

developed, and this section of entry has experienced only 1 roof 

fall. In contrast, the adjacent belt entry roof was not sealed, and 13 

roof falls have taken place in the same distance. 

DISCUSSION AND CONCLUSIONS 

 Ground control is a significant challenge in exceptionally lowstrength roof conditions. Reducing the number of roof falls 

requires concerted efforts in a number of areas: 

• Improved geologic characterization: At mines like mine A, 

more detailed knowledge of the thickness of the extremely 

weak roof strata could be a big help in pinpointing areas 

that might benefit from more support. Mechanical tests 

(like UCS) can be difficult to conduct in weak, moisture 

sensitive rocks. An alternative might instead be to measure 

in situ moisture content, which has been shown to be 

strongly correlated with UCS in the Illinois basin (Bauer, 

1982). However, surface borings may seldom be spaced 

closely enough to accurately define the thickness of weak 

units (Mark et al., 2004). Better techniques for identifying 

rock characteristics underground could prove very useful 

(Peng et al., 2003). 

• Better definition of horizontal stress: Although horizontal 

stress magnitudes seem relatively low in weak rock, due to 

the lower rock strengths they can affect ground control 

significantly. Orientation did not appear to be a major 

factor at either of these mines. In fact, the case of Mine B 

indicates that sometimes the horizontal stress may not have 

a preferred orientation. Nonetheless, site-specific 

knowledge of the horizontal stress would be very helpful in 

the mine planning stage, because reorientation of an 

operating mine is not easily accomplished. 

• Optimized roof support: Both of these operations expend 

considerable effort on roof support. Mine A’s experience is 

that using longer, stronger primary bolts in pattern can 

reduce roof fall rates, while Mine B has achieved success 

with cable bolts installed on cycle. In both cases, however, 

the roof support patterns depend upon the heavier supports 

working with shorter, lighter, fully grouted bolts. Basic 

scientific and systematic testing of such combination roof 

support systems in coal mines would be very useful, 

especially for low strength roof rock. 

• Both mines have also found that unraveling of the roof skin 

can reduce the effectiveness of even heavy roof bolt 

supports. Achieving skin control with mesh (or even straps 

or large plates) can therefore help reduce the number of roof 

falls while also protecting miners from falling rock. 

 Another important conclusion from this study is the importance 

of moisture sensitivity to ground control in weak roof. It seems 

that roof falls at these mines can be divided into two classes: 

1. Falls that occur soon after mining, which may be related to 

a combination of roof defects (clay veins, slickensides, 

weak bedding, and other defects) and horizontal stress. 

2. Falls occurring years after development, often related to 

progressive unraveling upward in cutters and between roof 

bolts. The high moisture sensitivity of the weak shale, 

“soapstone,” and clay veins causes them to deteriorate over 

time, and perhaps to swell and induce internal pressures. 

Figure 9. Roof falls at Mine B, by month. 

0

5

10

15

20

25

30

35

40

Jan.

Feb.

March

April

May

June

July

August

Sept.

Oct.

Nov.

Dec.

No. of Roof Falls

0

10

20

30

40

50

1997 1998 1999 2000 2001 2002 2003

No. of falls

Standup

time less

than 12

months

Standup

time more

than 12

months

Figure 11. Roof falls at Mine B, by year in which the entries 

were driven.

Figure 10. Roof falls at Mine B, entry by stand-up time. 

0

20

40

60

80

100

120

140

160

1yr 2yr 3yr 4yr 5yr 6yr

Roof Fall standup time (yrs)

No. of falls
 Preventing this second category of roof falls is essential for the 

long-term viability of mines in low-strength roof environments. 

Spray-on sealants seemed to show their potential at Mine B, while 

air conditioning appears to have been helpful at Mine A. 

 Now, more than ever, coal is essential to the energy security of 

the United States. The easy reserves have been depleted, and future 

mining will of necessity require dealing with more difficult 

conditions. The mines described in this paper are currently in the 

forefront of the struggle to develop better methods for controlling 

roof falls. Their strides and success will serve as an example for 

future mines with similar geologic conditions. 

REFERENCES 

Bauer, E.R. (1982). The Loss of Natural Moisture Content and Its 

Effect on the Mechanical Properties of Some Pennsylvanian Shales 

from the Illinois Basin. Proceedings, 1st Conference. on Ground 

Control Problems in the Illinois Coal Basin, SIU, Carbondale, IL, 

pp. 89-94. 

Dolinar, D.R. (2003). Variation of Horizontal Stresses and Strains 

in Mines in Bedded Deposits in the Eastern and Midwestern United 

States. Proceedings, 22nd International Conference on Ground 

Control in Mines, Morgantown, WV, Aug. 5-7, pp. 178-185. 

Laswell, R.E. (1999). The Effects and Economics of 

Dehumidifying mine air at the Riola Mine, Vermilion County, 

Illinois. Proceedings, Annual Meeting Illinois Mining Institute. 

Mark, C., McWilliams, L., Pappas, D., and Rusnak, J. (2004). 

Spatial Trends in Rock Strength – Can They Be Determined from 

Boreholes? Proceedings, 23rd International Conference on Ground 

Control in Mines, Morgantown, WV, Aug. 3-5, in press. 

Mark, C. and Mucho, T.P. (1994). Longwall Mine Design for 

Control of Horizontal Stress. Proceedings, New Technology for 

Longwall Ground Control, U.S. Bureau of Mines Technology 

Transfer Seminar, USBM SP 94-01, pp. 53-76. 

Mark, C. and Barczak, T.M. (2000). Fundamentals of Coal Mine 

Roof Support. Proceedings, New Technology for Coal Mine Roof 

Support, NIOSH Open Industry Briefing, NIOSH IC 9453, pp. 23-

42. 

Mark, C., Molinda, G.M. and Dolinar, D.R. (2001). Analysis of 

Roof Bolt Systems. Proceedings, 20th International Conference on 

Ground Control in Mines, Morgantown, WV, Aug. 5-8, pp. 218-

225. 

Molinda, G.M., Heasley, K.A., Oyler, D.C. and Jones, J.R. (1992). 

Effects of Horizontal Stress Related to Stream Valleys on the 

Stability of Coal Mine Openings. U.S. Bureau of Mines RI 9413, 

26 pp. 

Mucho, T.P. and Mark, C. (1994). Determining Horizontal Stress 

Direction Using the Stress Mapping Technique. Proceedings, 13th 

International Conference on Ground Control in Mines, 

Morgantown, WV, Aug. 2-4, pp. 277-289. 

 

Peng, S., Tang, D., Mirabile, B., Luo, Y. and Wilson, G. (2003). 

Mine Roof Geology Information System (MRGIS). Proceedings, 

22nd International Conference on Ground Control in Mines, 

Morgantown, WV, Aug. 5-7, pp. 127-135. 

Rusnak, J., and Mark, C. (2000). Using the Point Load Test to 

Determine the Uniaxial Compressive Strength of Coal Measure 

Rock. Proceedings, 19th International Conference on Ground 

Control in Mines, Morgantown, WV, Aug. 8-10, pp. 362-371. 

World Stress Map Project (2004). Website http://www.worldstress-map.org.