25th International Conference on Ground Control in Mining 

57 

ABSTRACT 

 Historically, miners have known that roof rocks originally 

composed largely of mud were prone to slaking and deterioration 

when exposed to water and humidity. Mine Safety and Health 

Administration (MSHA) records show that roof falls are more 

prevalent in summer and late fall. Field data shows higher bolt 

loads and increased room convergence in summer months. 

Laboratory data shows that very high internal pressures can be 

generated by significant strain in clay-rich rocks. In summer 

months humidities ranging from 70-100% can cause rock strains up 

to 7%, and rock pressures of 14,000 psi, more than enough to break 

apart weak shales and fireclays. 

 The National Institute for Occupational Safety and Health 

(NIOSH) conducted wet/dry immersion cycling of clay-rich rocks. 

Results show that the rocks most prone to moisture deterioration 

are fireclays, claystones, and sandy shales, all of which have 

disrupted bedding. Of the 7 mudrocks with a weatherability index 

above 40%, none showed consistent fissile bedding. Black shales 

were surprisingly resistant to wet/dry water immersion cycling. 

Generally, mixed rocks (stackrock), typically susceptible to 

horizontal stress, are more resistant to moisture-deterioration. 

Mine roof rocks tested with moisture sensitivity ratings above 50% 

(moisture sensitivity index ranges from 0-100 with 100 being the 

most sensitive) all came from mines which had roof skin problems 

and used supplemental support to prevent rock fall injuries. Some 

roof rocks showed lateral variability of moisture sensitivity values, 

suggesting the need for more samples in highly varying roof rocks. 

Swelling strains of mudrocks showed that almost all of the swelling 

occurred in the first 24 hours after immersion. The wet/dry 

immersion cycling test used provides an accurate, quick and 

inexpensive method for estimating the moisture sensitivity of 

mudrocks. With this information mine operators can begin to 

anticipate hazardous ground and prepare appropriate roof support. 

BACKGROUND 

 Mudrocks are generally composed of both clay and silt-sized 

particles and are the result of deposition in quiet standing water, 

interrupted periodically by flooding and clastic inflow (Krumbein 

and Sloss, 1963). Approximately 80% of all sedimentary rocks are 

shales or mudrocks (Krumbein and Sloss, 1963). Mudrocks are 

also very common in rock sequences forming the roof of coal 

mines. Since clay minerals are relatively weak, it generally follows 

that mudrocks, lacking contact quartz grains and siliceous cement, 

would also be weak (Molinda and Mark, 1996). Weak mudrocks 

contribute heavily to roof falls and roof fall injuries. In one study 

the roof fall rate was higher for weak rock than for stronger rock as 

defined by the Coal Mine Roof Rating (CMRR) (Molinda et al., 

2000). One of the factors which make mudrocks particularly weak 

is their susceptibility to degrade on contact with moisture. 

Numerous studies have shown that some roof shales degrade when 

in contact with humid mine air (Aughenbaugh and Bruzewski, 

1973; Conrad, 1936; Cummings et al., 1983; Cummings and Singh, 

1981; Fletcher and Cassidy, 1931; Haynes, 1975; Herbert, 1940; 

Holland, 1956; Huang et al., 1986; Lucas, 1975; Sames, 1985; 

Unrug and Padgett, 2003). Although some shales may be 

completely unaffected by moisture exposure, the MSHA typically 

issues alerts when summer is approaching to warn of roof of 

instability related to moisture deterioration. The number of rock 

fall injuries was 15% higher in June, July, August, September and 

October for the time period 1995-2004 than the annual average for 

that same time period (MSHA, 2004) (figure 1). 

 Shale deterioration is not only a skin control problem, but may 

lead to weakening in the rock mass and large scale roof falls. In 

one study mine, roof falls were much more frequent in August than 

any other month (Mark et al., 2004). Stateham and Radcliffe (1976) 

report that the frequency of roof falls is related to absolute humidity 

IDENTIFYING MOISTURE SENSITIVE ROOF ROCKS IN COAL MINES 

Gregory M. Molinda, Research Geologist 

David C. Oyler, Mechanical Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

Hakan Gurgenli, Graduate Research Assistant

University of West Virginia 

Morgantown, WV, USA 

0

100

200

300

400

500

600

700

January

February

March

April

May

June

July

August

September

October

November

December

Number of Injuries

Figure 1. Rock fall injuries increase in humid months 

(1995-2004). 
25th International Conference on Ground Control in Mining 

58 

and lags it by 14 days. This is an indication of the average amount 

of time needed for humidity to act to deteriorate rocks and cause a 

roof fall. Some mudrocks are largely stable through time and 

moisture exposure, and some are highly sensitive to moisture. 

Swelling clay minerals can absorb water rapidly and generate 

pressures which can break apart weakly bonded rocks (Huang et al., 

1986). This degradation can occur months or years after exposure 

or it can occur within days. A NIOSH study has documented the 

distribution of roof falls in a West Virginia mine. In this mine, 

numerous falls continue to occur up to 6 years after roof exposure 

(Mark et al., 2004). The CMRR recognizes the danger of slaking 

roof rocks by degrading the CMRR as much as 15 points. 

 Numerous studies show that it is the cycling of wetting and 

drying that occurs with seasonal weather changes that causes rocks 

to deteriorate. Some shales will not weaken significantly when 

immersed in water, but will fail if subjected to repeated wetting and 

drying (Aughenbaugh, 1981). In the dry air of the Utah coalfields 

such deterioration is not as severe, but in the eastern and 

Midwestern coalfields summer humidity causes weak shales to 

“rain rock” in beltways, travelways and other outby areas. 

 Other studies have focused on the changes of mechanical rock 

properties as rocks are exposed to moisture. Aughenbaugh and 

Bruzewski (1976) report confined swelling pressures of 4,000 psi 

regularly and up to 14,000 psi developed in humidity tests on 

shales. Kelly (1969) reports swelling pressures of approximately 

5,000 psi, well beyond the tensile and compressive strength of 

many shales. Shale samples were found to lose weight when 

subjected to low humidities and gain weight at high humidities 

(>80% relative humidity), indicating moisture absorption and 

increased pore pressures. Expansion of claystone samples reached 

7% at 100% humidity and samples actually shrunk (contraction) at 

0% humidity. At lower humidities (<70%) swelling strains were 

greatly diminished (Cummings and Singh, 1981). Fireclays and 

claystones represent “overcompacted” rocks which can be activated 

by moisture that releases trapped strain energy and causes swelling. 

 Negative pore pressures then “suck” in water and advance the 

reaction (Duncan et al., 1968). Other mudrocks, black shales and 

gray shales, are stronger and not overcompacted and are more 

stable. Chugh and Missavage (1980) compiled data from several 

investigators and report that even in sandstones, unconfined 

compressive strength (UCS) and modulus were reduced by as much 

as 75% from the original values when rocks were subjected to 

immersion or humidity changes (Fabjanczyk and Gale, 1999). 

Matsui et al., (1996), and Zhang et al., (2004) also reported a 

significant reduction in UCS with increasing moisture content. 

Bauer (1980) reports UCS values of oven-dried samples were 2-3 

times greater than rocks saturated 100% with water. Aughenbaugh 

and Bruzewski (1973) found that average sclerscope hardness and 

fracture toughness decreased with increasing relative humidity. 

 Time dependent deterioration of mudrocks can be seen in the 

corebox. Unrug and Padgett (2003) showed that RQD averages a 

42% decrease as time increases from the “barrel to the box to the 

lab”. Some rocks deteriorate to the point where testing is 

impossible. Conversely, it is clear that some rocks gain strength as 

they dry out (Bauer, 1980; Van Eeckhout and Peng, 1975). 

 There are a number of mechanisms which have been offered to 

explain the failure of mudrock with moisture exposure. These 

include secondary mineralization, chemical deterioration, pore 

pressure increase, frictional reduction, fracture energy reduction, 

and capillary tension increase (Van Eeckhout and Peng, 1976). As 

rock is unloaded, i.e., stream valley downcutting, tunneling, or 

mining; relaxation causes internal tension and opening of new 

fractures or movement adjacent to existing ones. At a large scale, 

the tension cracks are seen as hillseams, and at a smaller scale, 

relaxation of slickensides occurs, causing expansion of rock masses 

and opening conduits for moisture. Seismic wave velocity 

increases away from the walls of tunnels, indicating that weaker, 

fractured rock has moved in response to lower confinement 

(Duncan et al., 1968), with the effect diminishing away from the 

opening. With this opening of the rock mass, water conduits are 

created allowing exposure to mine air and rock deterioration. 

Water is drawn into the rock by the action of strong capillary forces 

which compress air in its path, resulting in disruption of the rock 

(Anwar et al., 1999). 

 Seedsman (1986) identified two mechanisms for the swelling of 

shales when immersed in water; 1) compression of entrapped air 

and 2) osmotic swelling. If muds are exposed to air and dry, 

shrinkage causes tension cracks to form. As the water evaporates it 

is replaced by air. When water is reabsorbed by shale during burial, 

air is trapped and compressed in capillary openings causing more 

tensile stresses and rock deterioration (Holland, 1956). In the lab, 

samples with high quantities of montmorillinite generally show the 

greatest volume change. In the field, roof rocks with high 

quantities of montmorillinite have been associated with bad roof 

(Holland, 1956). The presence of expansive clay minerals is only 

part of the answer since rapidly deteriorating mudrocks are also 

known which contain no expansive clay minerals. Holland found 

that only one of 38 roof shales from southwest Virginia, Illinois, 

and Indiana had detectable amounts of montmorillinite. Pyrites, 

found in roof shales and coals, can oxidize in the presence of 

moisture and form ferrous sulphates which absorb water and swell 

considerably (Chugh and Missavage, 1981). This may well be the 

more important mineralogic source of swelling. Although some of 

these mechanisms remain the subject of debate, the outcome is still 

the same; mudrocks deteriorate with exposure to moisture. 

 There is abundant evidence that undermined coal measure roof 

rock masses respond to changes in humidity in mine air (Unrug and 

Padgett, 2003). Cripps and Taylor (1981) reports that 

overconsolidated clays will relax in a time-dependant way upon the 

removal of confinement by mining. This relaxation causes micro 

tension failures in weak shales and allows for the increased 

infiltration of moisture. Increased moisture exposure results in 

swelling and more tension failures, leading to progressive 

deterioration. This mechanism may explain the time dependant 

roof failures of some rocks which may occur years after mining. In 

fact, this relaxation deterioration may occur even without moisture 

infiltration. Carboniferous rocks which begin to “weather,” show 

marked reductions in modulus and failure transitions from brittle to 

plastic. Depending on the composition, this “weathering” can take 

place in the real time between excavation and roof fall (0-6 yrs), 

and beyond. 

 Aughenbaugh and Bruzewski (1976) showed that roof failure in 

moisture-sensitive rock can occur by anchor slippage as bulk 

swelling in the roof loads up roof bolts. Roof failure also occurred 

by rock fracturing between bolts with no prior bolt loading, also 

due to bulk swelling. This indicates that weak rock fractured 

before any load could be transferred to the roof bolt. Slip of pointanchored bolts has been more prevalent in highly humid summer 

months than dryer winter months. Bolt loading, as determined by 

lengthening and shortening of the bolt due to rock mass swelling, 

also increases in summer months due to higher humidity 

(Aughenbaugh and Adam, 1980). Matsui et al. (1996) reports 

vertical closure in wet entries was 40-60 cm, whereas in dry entries 
25th International Conference on Ground Control in Mining 

59 

it was only 5-15 cm. Some of this closure was due to heave of 

floor members. Cummings found roof convergence to be 

seasonally related; increased convergence in summer and decreased 

convergence in winter (Cummings and Singh, 1981) 

 Aughenbaugh and Adam (1980) found that holes drilled into 

samples to simulate open roof bolt holes greatly facilitated access 

to moisture. This indicates that open holes, even when protected by 

roof plates, may provide excellent conduits for moisture. Moisture 

also moves much more readily along bedding planes than 

perpendicular to it. Young’s modulus and Poisson's ratio of shales 

increased rapidly perpendicular to bedding as compared to parallel 

to bedding. Van Eeckhout and Peng (1975) likened this to adding 

water between the cards in a deck and compressing it. The deck 

has swelled more perpendicular to bedding and thus deforms more 

in that direction, indicating that moisture has infiltrated parallel to 

bedding. In mine openings this can translate into roof bulking and 

sag. 

 Recent data shows an unexpectedly high percent of fully 

encapsulated resin roof bolts have lost resin to the rock mass 

(Compton and Oyler, 2005). When roof bolts were inserted under 

high pressures (5,000-6,000 psi), overcoring showed resin injected 

into both vertical joints and horizontal bedding planes. Uphole loss 

of resin can result in several inches to several feet of open hole at 

the bottom of the roof bolt. In moisture-sensitive rock, this 

exposure may result in unraveling from the roof line upwards. 

 A rock classification which characterizes the swelling potential 

of shales was proposed by Huang et al. (1986). The model uses the 

Moisture Activity Index because of its close association with other 

physical properties and because it can be defined by simple 

laboratory testing. Mudrocks were subjected to varying amounts of 

humidity and the subsequent weight gain was measured and used as 

an index to describe the swelling capability of various mudrocks 

(Huang et al., 1995). Using the Moisture Activity Index, the 

plasticity of shale, the configuration of particle arrangement, and 

the percent weight of clay size particles, shales are partitioned by 

their response to low, medium, and high humidities. The swelling 

pressure and strain are predicted for stable, mildly susceptible, and 

moisture sensitive shales. Olivier (1979) related Duncan free 

swelling values and uniaxial compressive strength to generate a 

“geodurability” classification which groups rocks into categories 

from very low strength to very high strength. With this relationship, 

the swelling potential of mudrocks may be related back to the 

unconfined compressive strength. Other work documents efforts to 

evaluate index tests which would aid in the prediction of moisturesensitive roof rocks. Shakoor and Sarman (1992) measured 13 

mudrock properties to determine if swelling properties could be 

predicted. He developed a “swelling property index” broken into 5 

categories and based on combinations of the 13 properties, 

including grain size distribution, clay content, clay mineralogy, 

texture, absorption, adsorption, Atterberg limits, specific gravity, 

density, slake durability, compressive strength, volumetric increase, 

and swelling pressure. 

 There is convincing evidence, both in the laboratory and in the 

field, to show that some mudrocks can be highly reactive to 

moisture exposure. This exposure can lead not only to hazardous 

roof skin failures but to large roof falls years later. NIOSH is 

undertaking a two part investigation into the problem of moisture 

sensitive shales. The first part is a program of rock testing to 

identify moisture sensitive mudrocks. The second part will 

correlate these rocks to roof falls and roof damage in order define 

the expected standup time for purposes of mine planning and roof 

support design. 

 The ability to accurately predict the long term stability of 

common mudrocks would be a valuable tool for mine design and 

planning. If moisture sensitive rock types can be identified, a 

number of roof controls may be considered. These include leaving 

head coal to seal off moisture, taking down sensitive drawrock, 

installing additional support necessary to prevent time dependent 

falls, and applying skin control to prevent injuries. 

IDENTIFICATION OF MOISTURE SENSITIVE ROOF 

ROCKS 

 As discussed above, there are a number of ways to attack the 

problem of identifying moisture sensitive rocks. These approaches 

have included, measuring the capacity of the rock matrix to absorb 

moisture, tracking the change in mechanical properties of rocks 

with humidity changes, and measuring the fractions of swelling 

clays which may contribute to rock deterioration (Shakoor and 

Sarmon, 1992; Zhang et al., 2004; Bauer, 1980). For this study 

NIOSH is focusing on utilizing a practical index test which will 

provide an estimate of the deterioration of common roof rocks with 

exposure to moisture. An index test can often provide the desired 

measure of a rock property in a relative sense. If rock deterioration 

is the property of interest, then a test which measures it directly 

may be a valuable tool for support design. 

 NIOSH has begun testing roof rock using the wet/dry cycling 

test developed by Unrug (1997). This test simulates the exposure 

of roof rocks to the seasonal humidity changes in mine ventilation 

air. Exposed core was subjected to a cycle of immersion in water 

for one hour and then air dried for six hours. This cycle was 

repeated 3 times. The weatherability (WAI) Index is then 

calculated as follows: 

where: WAI = Weatherability Index, %, 

Wini = Initial Wt of sample, grams. 

Wrem = Weight of the largest remaining fragment of a 

sample, grams. 

 Samples of roof rock were collected from 14 mines from 

various coal basins around the U.S. (figure 2). Lump samples from 

roof slabs were also tested. Figure 3 shows the distribution of 

water sensitivities for 26 common roof rocks. The rock types are 

distinguished using the Ferm rock classification system (Ferm and 

Smith, 1981). Rock numbers and descriptions are included in the 

Appendix. The Ferm rock classification system is based on a 

number of rock properties including bedding fracture, mineralogy, 

grain size, color, and inclusions. It is intended to provide a numeric 

value to aid in the field distinction between rock types. The 

continuous variation of the above properties means that the Ferm 

classification can provide only an approximation of the actual rock 

type. The values are plotted as an average for each rock type. 

Roof rock moisture sensitivities ranged from completely unreactive 

to complete disintegration of the sample. In some cases this 

occurred in only 60 seconds. Figures 4 and 5 show samples before 

immersion and after immersion. Significant deterioration has 

occurred after three cycles of wet and dry. 

WAI W W

W

 * 100 ini rem

ini

= −
25th International Conference on Ground Control in Mining 

60 

 Of the rocks tested, the most water sensitive rock types are the 

mudrocks. The term “mudrock” is here used to group rocks which 

consist predominantly of clay minerals (Krumbein and Sloss, 1963). 

 These include: sandy shale (324, 325), dark gray sandy fireclay 

(327), sandy claystone (347), sandy fireclay with limestone nodules 

(437), dark gray sandy fireclay with limestone modules (427), and 

gray fireclay (127). All of these rock types had a water sensitivity 

of 40% or above (figure 3). For clarity, the various rock types have 

been consolidated into groups based on rock fabric and mineralogy 

(figure 6). Of the 6 rock types, fireclays and sandy shales were the 

most water sensitive. 

 The feature which is common to all of these rocks is that they 

all have some degree of disturbed bedding. Ferm distinguishes 

fireclays and claystones as having an “irregular fracture or irregular 

streaks” and sandy shale as having a “massive or uniform” 

structure, essentially lacking fissile bedding. It appears that it may 

be this irregular bedding which is most common to the water 

sensitive rocks. Clay minerals have a phyllosilicate, or platy 

structure. This platiness gives the shale its flat or fissile bedding. 

Flat bedding may act as a barrier to vertical water infiltration, 

allowing transmission of water along bedding, with transmission 

across bedding more difficult (figure 7). When bedding is 

disrupted due to rooting, burrowing, or overcompaction, water can 

more easily moved by capillary action across bedding (figure 8). 

This progressive infiltration may allow the exposure of swelling 

Figure 5. Roof samples after water immersion test. Water 

sensitivity values (WAI) are attached showing extreme 

deterioration (WAI = 100%). 

Figure 4. Roof samples before water immersion test. 

0

10

20

30

40

50

60

70

80

90

100

32

113

114

117

122

124

127

322

323

324

325

327

347

437

427

444

543

547

564

742

748

802

804

894

904

906

Rock Type

Water Sensitivity (WAI) 

%

See Appendix 1 for 

rock descriptions

Figure 3. Average water sensitivity (WAI) of rock samples 

identified by Ferm numbers. 

Figure 2. Weatherability samples were collected from roof rock 

at the above mine locations. 

Mine locations

Figure 6. Moisture sensitivity of coal measure roof rocks. 

0

10

20

30

40

50

60

70

Black Shale

and fireclay

Shale Fireclay Sandy

Shale

Sandstone Limestone

Water Sensitivity (WAI) % .
25th International Conference on Ground Control in Mining 

61 

clays and begin the deterioration of the rock mass (figure 9). Rock 

falling between bolts provides no load transfer to the bolts. 

 In addition, many of the water sensitive rock types we tested 

also have a sandy component. Unlike “mixed rocks” which have 

alternating layers of sandstone and shale to act as barriers to water 

infiltration, sandy shales have sand “floating” in a matrix of clay. 

This matrix is disturbed compared to fissile bedding. 

 The bedding of each rock sample was classified prior to the 

wet/dry cycling test. The bedding ranged from the flat fissile 

bedding of a quiet water lagoon deposit to the highly slickensided, 

chaotic bedding of a rooted underclay. Figure 10 shows that 

disturbed and massive unbedded rocks were the most water 

sensitive. The water sensitivity test utilizes a wet/dry cycling tank 

to simulate the seasonal change that roof rock may be subjected to 

from ventilation air. Fireclays can be over-compacted, and perhaps 

the repeated swelling and shrinking can have a cumulative effect 

that weakens the rock fabric. These rocks are often weak to begin 

with and cannot take much stress on the disoriented and disturbed 

rock fabric. This can result in a progressive failure leading to rock 

falls between bolts. 

 Sandstones and limestones are usually the least moisture 

reactive rocks due mainly to their inert mineralogy. Black shales 

and black fireclays are relatively non-reactive (figure 6). Holland 

(1956) cites data that show that rocks containing organic 

compounds like fatty amine acetate were less plastic, shrink and 

swell less, and are more resistant to swelling. Coal and bone coal 

are commonly used to protect moisture sensitive mudrocks in the 

roof above. 

 In order to identify the most reactive rock types, the proportion 

of samples tested of each rock type with a water sensitivity (WAI) 

greater than 75% was calculated. Figure 11 shows that the sandy 

fireclays, claystones, and sandy shales tested have 30-50% of their 

samples in the highest quartile of water sensitivity (>75% WAI). 

While showing some variability within a rock type, these data may 

suggest which rock types should be identified as likely to present 

problems with long term exposure to moisture. While data on the 

long term standup time of various rock types is not yet available, 

these test results may indicate that fireclays and sandy shales may 

deteriorate with long exposure to ventilation humidity. 

0

10

20

30

40

50

60

Disturbed Massive Hackly Irregular Flat

Bedding Type

Water Sensitivity (WAI) % .

Figure 10. Disturbed and massive bedded rocks are the most 

water sensitive. 

Figure7. Water moves more readily along bedding than across

bedding. 

Figure 9. Water sensitive rock falls around bolts in an Illinois

coal mine. 

Dark Gray Fireclay—Ferm No. 127

Figure 8. Fireclay showing disturbed bedding and water moving 

easily across bedding. 
25th International Conference on Ground Control in Mining 

62 

SWELLING PROPERTIES OF MUD ROCKS

 The deterioration that occurs in the wet/dry water immersion 

cycling test as described above is primarily due to absorption of 

water by clay minerals making up the fabric of mudrocks. Water is 

absorbed into the platy clay minerals, causing swelling and 

disagregation of the minerals, and subsequent disintegration of the 

rock. 

 Swelling strain was measured on roof rock core that was 

immersed in water. Figure 12 shows sample swelling in strain 

versus time for four core samples fully immersed in water. The 

samples were all shales and laminated shales (Ferm 122, 124) from 

the roof of two coal mines, one in northern WV (A) and one in 

southern WV (B). These rock types are common to the roof of 

many coal mines. The data shown are typical for 10 of the 12 

samples tested. Most samples showed rapid swelling during the 

first 8 to 24 hours, followed by either a much reduced swelling rate, 

or more commonly, a slow reduction in swelling strain over time. 

The peak strains for the typical samples ranged from -0.1 to .95%, 

averaging about 0.7%. 

 The implication is that sensitive rocks react quickly to moisture 

exposure and may result in scale formation at the face in the humid 

summer months. The long term implications relate to progressive 

upward unraveling of the roof. As roof members sag and 

delaminate with time, tensile fractures open, as well as relaxation 

on slickensides, and allows pathways for moisture access. Swelling 

occurs quickly causing more fractures and rock mass damage. 

MOISTURE SENSITIVITY AND ROOF CONDITIONS 

 Samples of roof rocks were collected from 14 mines from 

various coal basins around the U.S. (figure 2). The wet/dry cycling 

test was conducted on samples from each mine. Figure 13 shows 

the distribution of the average moisture sensitivity (WAI) from 

each mine. Moisture sensitivities ranged from completely 

unreactive to roof rock which disintegrated to mud within 60 

seconds of water immersion. Samples from six of the mines 

averaged nearly 50% or greater water sensitivity (WAI). All 6 of 

the mines had significant roof problems related to scaling and 

slaking. In response, all six mines were using some type of skin 

control to prevent injuries from rock falls. These controls included 

full and partial coverage of the roof with steel screen, cyclone fence, 

and the use of air conditioning to remove humidity from the air 

before it entered the mine. The mines below the 50% limit 

generally had less scaling and deterioration problems. 

SUMMARY 

 MSHA accident statistics show an increase in roof falls as well 

as rock fall injuries in the summer and early fall (MSHA, 2004). 

NIOSH has also reported significant seasonal deterioration of roof 

requiring the installation of skin controls 

 NIOSH has begun a rock testing program aimed at identifying 

the most moisture sensitive rocks. Moisture sensitivity has been 

measured using the Unrug weatherability apparatus. This index 

test simulates the wet and dry cycling which occurs with seasonal 

changes in humidity. Testing of nearly 400 samples of common 

roof rocks found that those rocks with disturbed or non-fissile 

bedding are the most moisture sensitive. These include: sandy 

shale (324), dark gray sandy fireclay (327), green sandy claystone 

(347), sandy fireclay with limestone nodules (437), dark gray sandy 

fireclay with limestone nodules (427), and gray fireclay (127). 

These data suggest that the mechanism of rock deterioration is at 

least somewhat related to internal rock discontinuities rather than 

solely due to the presence of swelling clays. Disrupted bedding 

allows moisture to move across bedding, finding access to 

0

10

20

30

40

50

60

32

113

114

117

122

124

127

322

323

324

325

347

427

437

444

543

547

742

748

802

804

894

904

906

Rock Type

Pct. of rock samples tested >75% WAI

Figure 11. Proportion of rock samples tested for each rock type 

that had a water sensitivity (WAI) greater than 75%. 

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15

Time, Days

Strain, pct

Mine A Sample 1

Mine A Sample 2

Mine B Sample 3

Mine B Sample 5

Figure 12. Strain profile of mudrocks shows most swelling

occurs in the first 24-48 hours.

0

10

20

30

40

50

60

70

80

90

100

A B C D E F G H I J K L M N

Mine

Water Sensitivity (WAI) %

Figure 13. Average water sensitivity (WAI) for roof rocks 

sampled from selected U.S. mines. 
25th International Conference on Ground Control in Mining 

63 

slickensides. More surface area becomes available for wetting and 

subsequent swelling can break apart weak rocks. Additionally, 

disrupted bedding does not provide the benefit of lamination 

beaming and the rock is more easily disaggregated by swelling. 

 Moisture sensitivity testing of rocks from 14 mines showed that 

all 6 mines with WAI at or over 50% had poor roof conditions and 

rock slaking. All these mines were using some type of 

supplemental support or other remedy to control rock falls. 

Swelling strains were measured in 12 core samples from two mines 

in WV. The profiles showed that most of the strain occurred in the 

first day after water immersion, with rock swelling subsiding 

quickly afterward. This may suggest that highly moisture sensitive 

rocks may need to have surface control installed quickly after 

mining in order to catch falling rock. NIOSH data has shown that 

installing steel screen on cycle with primary support can 

significantly reduce rock fall injuries. Roof bolters and CM 

operators suffer the most rock fall injuries (Molinda et al., 2003). 

When highly moisture sensitive rocks (fireclay and sandy shale) are 

encountered, on-cycle surface control provides the most protection 

for these miners. 

 The Unrug weatherability apparatus provides a good index test 

for the deterioration of mudrocks. It correctly simulates the wet 

and dry cycling which can damage roof rocks. 

 Remedies to prevent the fall of moisture sensitive roof rock 

include: covering the strata by leaving head coal, removing the 

hazardous draw rock, sealing the strata with chemical sealants, 

conditioning the ventilation air to remove humidity, or using high 

coverage surface control like straps, pans, or screens. Planning for 

these controls can best be accomplished when the occurrence of 

moisture sensitive roof rocks can be projected in advance through 

routine rock testing. 

REFERENCES 

Anwar, H.Z., Shimada, H., Ichinose, M., and Matsui D. (1999). 

Slake Durability Behaviour of Coal Mine Shales. Proceedings, 

1999 International Symposium on Mining Science and Technology 

Xie and Golosinske (eds), Bejing, China, Aug. 29-31, pp. 343-346. 

Aughenbaugh, N.B, and Bruzewski, R.F. (1973). Investigation of 

the Failure of Roofs in Coal Mines. Final Report, U.S. Bureau of 

Mines Contract No. H0111462, Department Mining, Petroleum and 

Geological Engineering, University of Missouri-Rolla, Rolla, 

Missouri. 

Aughenbaugh, N.B. (1981). Effects of Humidity on Ground 

Control in Mining and Tunneling. In: Y.P. Chugh (editor), Rock 

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APPENDIX

Ferm No. Rock Description Ferm No. Rock Description 

32 Bone Coal 427 Dark gray sandy fireclay with limestone nodules 

113 Black shale with coal streaks 437 Light gray green sandy fireclay with limestone nodules 

114 Black shale 444 Green sandy shale with limestone nodules 

117 Black fireclay 543 Gray sandstone with shale steaks, rippled 

122 Dark gray layered shale 547 Gray rooted sandstone 

124 Dark gray shale 564 Hard massive sandstone 

127 Dark gray fireclay 742 Gray shale pebble conglomerate 

322 Dark gray shale and interbedded sandstone 748 Gray sandstone with coal spars 

323 Dark gray shale with sandstone streaks 802 Layered fine grained shaly limestone 

324 Dark gray massive sandy shale 804 Massive fine grained shaley limestone 

325 Dark gray massive churned sandy shale 894 Massive fine grained shaley limestone with fossil shells 

327 Dark gray sandy fireclay 904 Massive fine grained limestone 

347 Green sandy claystone 906 Nodular fine grained limestone