COMPARATIVE ANALYSIS OF MOISTURE SENSITIVITY INDEX TESTS FOR COAL MINE ROOF 

T. Klemetti, NIOSH, Pittsburgh, PA 

G. Molinda, NIOSH, Pittsburgh, PA 

ABSTRACT 

Moisture deterioration of clay-rich roof rocks causes high numbers 

of roof falls in coal mines in humid summer months. Rocks with high 

moisture content are generally weaker and can further deteriorate 

when subjected to wetting and drying cycles. As a result, it is 

important to evaluate the moisture-sensitivity of roof rock prior to 

mining. The National Institute for Occupational Safety and Health 

(NIOSH) has evaluated three moisture-sensitivity index tests on roof 

rocks from 23 U.S. coal mines. The three tests were the University of 

Kentucky Weatherability test (Weatherability test), the Consol Energy 

Water Sensitivity test (Water Sensitivity test), and the NIOSH

Immersion test (Immersion test). Of these three tests, the

Weatherability and Water Sensitivity tests are more reliable in

classifying the moisture-sensitivity of roof rocks. 

Disclaimer: The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational Safety 

and Health and should not be construed to represent any agency 

determination or policy. 

INTRODUCTION 

During the past 5 years, over 2,200 U.S. coal miners have been 

injured by rock falls. Despite reductions in the rate of occurrence, the 

number of injuries remains at over 400 annually. There have also 

been 82 ground fall fatalities since 1995, and of these, 15 have been 

attributed to skin failure (Mark and Pappas, 2009). Skin failures 

involve rocks that fall between roof supports. The reasons for roof skin 

failures include weak bedding planes, slickensided surfaces, and 

moisture-deterioration of mudstones (Molinda et al., 2008, Molinda and 

Klemetti, 2008). This report addresses the moisture-deterioration of 

clay-rich roof rocks. 

Coal-forming environments are usually near sea level and 

resemble modern day coastal and flat-lying locales. As such, quiet 

water deposition often prevails, resulting in predominantly clay-rich 

mudrocks which form the roof of coal mines. Some of these rocks are 

highly reactive to moisture and can deteriorate with exposure, resulting 

in poor roof conditions and roof falls. A large body of evidence exists 

detailing the hazardous effects of moisture on clay-rich mudrocks 

(Molinda, et al., 2006, Molinda and Klemetti, 2008). An extensive 

literature review can be found in Molinda, et al. (2006) giving detailed 

information on the following concepts. It is well known that during 

humid summer months roof fall rates increase. In addition, roof 

swelling and damage has been measured with instrumentation. As a 

result, mine operators have successfully responded with high coverage 

skin controls, including roof screen and spray-on sealants. 

For mine planning and support design, it is critical to conduct an 

accurate assessment of the moisture-sensitivity of roof rocks in both 

the exploration and mining phases. Laboratory testing of core samples 

can provide the first evaluation of the future roof integrity. Tests for 

rock moisture-sensitivity can range from simple immersion to slake 

durability to strain-gaging rock samples in highly controlled humidity 

chambers. Individual tests have benefits and disadvantages related to 

cost, difficulty, and accuracy in correctly simulating the mining 

environment. 

The purpose of this research is to evaluate three commonly used 

moisture-sensitivity tests for similarities and differences in determining 

the relative deterioration of roof rock samples. The three tests 

evaluated are the Immersion test as modified by Molinda and Mark 

(1994) from Sickler’s Submersion test (Sickler, 1986), the 

Weatherability test proposed by Unrug (1997), and the Water 

Sensitivity test, a modified slake durability test proposed by Hasenfus 

and Su (2005). 

MOISTURE SENSITIVITY TESTING 

Samples of roof rock were collected from coal mines throughout 

the U.S. The moisture reactivity of these samples was tested using the 

three tests mentioned previously. The results from each test will be 

compared along with their advantages and disadvantages. 

The three tests require approximately the same preparation and 

sample size, thus for this study the same procedures were used for 

collecting and preparing the samples for all test procedures. The 

majority of samples for this research were taken from roof slabs 

collected underground, but in two cases exploration cores were used. 

The slabs and exploration cores were collected from within the primary 

support horizon of the mine roof. A number of mines had several 

samples collected throughout the mine to represent variable roof 

geology. 

All the specimens from the roof slabs were prepared by breaking, 

cutting, or coring the collected samples. All test procedures allowed all 

three methods of specimen preparation. The exploration cores were 

cut into approximately 2-in specimens. The specimens were weighed 

to ensure that they fell within the specifications of each specific test 

procedure. The specimens were then tested using one of the three 

test methods. The specimen results were averaged to give a sample 

result for each test. 

The Immersion test was performed on specimens from 43 

samples from 17 mines. The Weatherability test was conducted on 

specimens from 50 samples from 24 mines. The Water Sensitivity test 

was performed on specimens from 31 samples from 18 mines. The 

results of the tests using these samples are presented. The samples 

with all three test results were then compared to determine which they 

indicates the likelihood of moisture deterioration. Additionally, the tests 

are compared with one another when both tests are performed on the 

splits from the same sample. The following account is a description of 

each of the test procedures. 

The Immersion Test 

The Immersion test (Molinda and Mark, 1994) is the simplest and 

most straight forward of the three tests. It is a more qualitative than 

quantitative test, leading to more variation of results due to human 

interpretation. As shown in figure 1, the Immersion test only requires 

fist-sized specimens, beakers, and water. The Immersion test 

procedure is as follows: 

1. Select sample(s) - ~ hand sized. 

2. Test for hand breakability. 

3. Rinse specimen (to remove surface dirt, dust, etc.). 

4. Immerse in water for 24 hours. 

5. Observe and rate water appearance, talus formation, and 

cracking of the sample. 

6. Sum rating for Immersion Index. 

7. Retest for hand breakability. 

8. Determine the Breakability Index. 
9. The final Immersion Test Index is the greater of the 

Breakability Index or the Immersion Index. 

The final Immersion Test Index is a value between 0 and 15. A 

detailed description of the Immersion test indices and procedures can 

be found in Mark et al. (2002). 

Figure 1. Immersion test showing specimen deterioration and original 

sample. 

The Weatherability Test 

The Weatherability test is a three part test, requiring specimens 

either from cores or roof slabs weighing between 500 and 2,000 grams 

(Unrug, 1997). This test is more quantitative than the Immersion test 

and requires more time and equipment to perform. This test requires a 

special testing apparatus, consisting of a tank with grates on the 

bottom to lift the samples off the floor of the tank and a fan, as shown 

in figure 2. A programmable data logger is used to control the wetting 

and drying cycles. A schematic of the testing apparatus can be found 

in earlier works of Unrug (1997). This apparatus allows for the 

specimens to be subjected to cyclic immersion and drying. This 

wetting and drying cycle simulates the seasonal wetting and drying that 

attacks the coal mine roof. The Weatherability test procedure is as 

follows: 

1. Oven-dry (70-80 degrees Celsius) specimens to constant 

weight (i.e. no longer losing moisture and record weight. 

2. Place samples in apparatus, label, and photograph. 

3. Soak samples for 1 hour. 

4. Air dry samples for 6 hours. 

5. Repeat steps 3 and 4 for a total of 3 cycles. 

6. Take photographs for comparison. 

7. Oven dry specimens to constant weight and record weight of 

the largest intact fragment. 

Figure 2. University of Kentucky Weatherability test setup. 

This Weatherability test procedure was modified slightly. The 

initial drying period was removed due to the limited impact on the 

overall result while reducing the time required to complete the test. 

Secondly, the specimen size requirements were relaxed to include 

specimens as small as 100 grams, while aiming for 500 gram 

specimens. This size reduction did not appear to impact the results of 

the test and allowed for a larger number of samples to be tested. 

The resulting Weatherability Index is a ratio of the degraded 

material, based on the largest remaining intact fragment’s weight, to 

the initial specimen’s weight. The Weatherability Index is calculated as 

follows: 

W − W WAI = ini rem ×100 (1)

Wini

Where: 

WAI = Weatherability Index, % 

W = Initial weight of specimen, grams, and ini

W = Weight of the largest remaining fragment of a rem

specimen, grams 

The Water Sensitivity Test 

The Water Sensitivity test involves more time, physical handling

and mechanical manipulation than the two other tests used in this 

study, but the equipment is simple. This test requires the use of two 

sieve sizes, a 19-mm and a 2-mm. This test is also quantitative in 

nature (Hasenfus, 2005). The selection of specimens is similar to the 

other tests in that specimens are fist-sized, weighing 300-500 grams. 

The Water Sensitivity test procedure is as follows: 

1. Weigh specimens. 

2. Oven dry specimens (100-105 degrees Celsius) for 24 to 48

hours. 

3. Weigh specimens (Wd). 

4. Immerse in water for 24 to 48 hours. 

5. Wet sieve (stacked sieves +19-mm and +2-mm) and retain 

+19-mm and +2-mm portions as shown in figure 3. 

6. Oven dry (100-105 degrees Celsius) separate +19-mm and 

+2-mm portions 24 to 48 hours. 

7. Weigh remnant sample portions separately (W19 and W2). 

Figure 3. Wet sieving of the sample during the Consol Energy Water 

Sensitivity test procedure. 

The resulting Water Sensitivity indices are ratios of the weight of 

the material that passes each screen to the initial weight as determined 

via the remnant weights. The water sensitivity indices are calculated 

as follows: 

WS19 = 100 × (Wd − W19 ) / W (2) d (19 - mm index)
WS (3) 2 = 100 × (Wd − W2 ) / Wd (2 - mm index)

Where: 

WS = water sensitivity index for 19 – mm screen, 19

WS = water sensitivity index for 2 – mm screen, 2

W = initial dry weight of specimen, d

W = weight of remaining fragments on 19 – mm screen, 19

and 

W = weight of remaining fragments on 2 – mm screen. 2

Moisture Sensitivity Test Results 

The moisture sensitivity results are presented in three ways: (1) 

individual test procedure results, (2) compilation of all test procedure 

results, and (3) comparative analysis of test procedure results. 

Average results for all the specimens from the same sample tested are 

reported in all cases. As an example, one roof slab from a coal mine 

would be drilled to produce 10 specimens. The moisture sensitivity 

values for each of the specimens were averaged and reported as the 

sample moisture sensitivity. The samples were collected and prepared 

as discussed previously. Only those samples that were tested in all 

procedures are included for comparison purposes. The first 

assessment is a distribution of the results from each test separately. 

The Weatherability test, then the Water Sensitivity test, and finally the 

Immersion test results are presented. 

The distribution of average sample test results for the 

Weatherability test can be seen in figure 4. A total of 50 samples were 

tested. According to Molinda et al. (2006), rocks are moderately to 

highly sensitive to moisture if the index for this test is above 40%. 

There were 17 samples (34 %) that tested on average above 40%, 

which designates moderate to high moisture sensitivity as designated 

by the redline in figure 4. 

0%

25%

50%

75%

100%

0 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100

Weatherability Index WAI (%)

% Samples

Frequency (%)

Cumulative

Frequency (%)

N=50

Low Sensitivity

Moderate to High 

Sensitivity

Figure 4. University of Kentucky Weatherability test results. 

The Water Sensitivity results can be seen in figure 5. A total of 31 

samples were tested. The WS19 index was used since it is the primary 

measure of moisture sensitivity. The WS2 index provides additional 

indication of the clay content in the sample. The test procedure 

indicates a moderate level of water sensitivity when the WS19 index 

falls above 25% and a high level of water sensitivity above 58%. The 

red lines in figure 5 indicate these partitions. A total of 11 samples 

(35% of the total samples tested) were in the moderate to severe 

range. 

The results from the Immersion test are shown in figure 6. A total 

of 43 samples were tested. There are two lines on the chart 

representing different moisture sensitivity levels. The first red line at a 

Total Immersion Index of 5 represents moderate sensitivity and the 

second red line at a Total Immersion Index of 9 represents high 

sensitivity to moisture (Mark et al., 2002). This test resulted in 47 % 

samples in the moderate sensitivity range and 14% in the high 

sensitivity range. 

0%

25%

50%

75%

100%

0 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100

 Water Sensitivity Index WS19 (%)

% Samples

Frequency (%)

Cumulative

Frequency (%)

N=31

Low 

Sensitivity

Moderate 

Sensitivity

High 

Sensitivity

Figure 5. Consol Energy Water Sensitivity test results. 

0%

25%

50%

75%

100%

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Total Immersion Index

% Samples

Frequency (%)

Cumulative

Frequency (%)

N=43

Low 

Sensitivity

Moderate 

Sensitivity

High 

Sensitivity

Figure 6. Immersion test results.

The results of each of the three tests show the wide variation of 

moisture sensitivity that can be found throughout coal mines across the 

country. A compilation of the results from all three test methods is 

presented next. It only includes the samples which were tested using 

all three test procedures. In this way, the results of the three tests on 

splits of the same sample can be compared side by side. 

The sample results (N=22) shown in figure 7 are the average of 

all specimens from the same sample tested using all three test

procedures. Some results represent multiple roof samples from the 

same mine. The results for the Immersion test in figure 7 have been 

multiplied by a factor of 6.67 to adjust the rating scale to 0-100 to 

match the other two tests. In general, each test showed similar 

sensitivity to moisture for a given sample. In only 2 of the 22 samples, 

the resulting moisture sensitivities were off by 2 or more categories. In 

6 of the 22 samples, the moisture sensitivity fell in the same category. 

Overall, the three tests showed similar results when the moisture 

sensitivity was either extremely high or low. In 14 of the 22 samples, 

the Immersion test recorded the highest sensitivity to moisture when 

compared to the results from the other two test procedures. When the 

results fell in the moderate range, the Immersion test typically showed 

greater deterioration of the roof rock samples. This indicates that the 

Immersion test can overestimate the moisture deterioration in a 

specimen. 

Figures 8-10 compare the test results from each test method 

against each of the other test methods. Only the samples which were 

tested using both methods are presented in these three figures. In 

each figure, there are two intersecting lines which partition the 

moderate to high moisture sensitivity values for each test, according to 
test procedures. The samples which resulted in low moisture 

sensitivity indexes for both tests can be found in the lower left corner. 

This indicates agreement between the 2 tests. The samples which 

resulted in moderate to high moisture sensitivity indexes for both tests 

can be found in the upper right corner. This also indicates agreement 

between the tests. The remaining samples had conflicting test results. 

0

25

50

75

100

Samples

Water Sensitivity (LOW)

Water Sensitivity (MODERATE TO HIGH)

Weatherability (LOW)

Weatherability (MODERATE TO HIGH)

Immersion (LOW)

Immersion (MODERATE TO HIGH)

Moisture Sensitivity Index

N=22

Figure 7. Compilation of the three moisture sensitivity test results:; the 

Weatherability test, the Water Sensitivity test, and the Immersion test 

conducted from splits of the same sample are presented. 

0

20

40

60

80

100

0 20 40 60 80 1

Average Weatherability Index WAI (%)

00

Average Water Sensitivity Index W 19 (%)

Poor 

Agreement

Good Agreement

Good 

Agreement

Poor 

Agreement

Figure 8. Comparison of moisture sensitivity results from the Water 

Sensitivity test and the Weatherability test conducted on splits from the 

same sample. 

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0 20 40 60 80 1

Average Weatherability Index WAI (%)

00

Average Immersion Index

Poor 

Agreement

Poor 

Agreement

Good 

Agreement

Good 

Agreement

Figure 9. Comparison of moisture sensitivity results from the 

Immersion test and the Weatherability test conducted on splits from the 

same sample. 

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0 20 40 60 80 1

Average Water Sensitivity WS19 (%)

00

Average Immersion Index

Good 

Agreement

Good 

Agreement

Poor 

Agreement

Figure 10. Comparison of moisture sensitivity results from the 

Immersion test and the Water Sensitivity test conducted on splits from 

the same sample. 

The first is a comparison between the Water Sensitivity test and 

the Weatherability test (figure 8). This plot shows a good correlation 

between the moisture sensitivity measures from the two tests. Three 

samples (10%) produced conflicting moisture sensitivities and 27 

samples (90%) showed similar moisture sensitivity with 17 (57%) 

having low moisture sensitivity and 10 (33%) moderate to high 

moisture sensitive. This agreement indicates that these two tests 

provide similar results. 

Figure 9 compares the Immersion test and the Weatherability test. 

The comparison shows fewer samples with the same results and 

substantially more conflicting results (13 or 33%). There were 15 

samples (39%) showing low moisture sensitivity in both tests and 11 

samples (28%) showing moderate to high moisture sensitivities. 

These two tests rarely produced consistent test results for a single 

sample. These results indicate that there is a higher risk of 

overestimating the moisture sensitivity when using the Immersion test, 

as compared to the Weatherability test. The variation is likely due to 

the subjective observations required by the Immersion test. 

The third figure compares the Immersion test and the Water 

Sensitivity test. Figure 10 shows a similar trend to figure 9 in that 

given the same sample, the two tests can produce conflicting levels of 

moisture sensitivity. There were 8 (36%) not sensitive or slightly 

sensitive to moisture and 7 (32%) moderate to highly sensitive to 

moisture according to both tests. The Immersion test and Water 

Sensitivity test had 7 (32%) conflicting results from the samples tested. 

Again, the Immersion test was more likely to overestimate the moisture 

sensitivity of roof rock, as compared to the Water Sensitivity test. 

Conflicting results seen could be due to the mechanical interaction with 

the samples during the Water Sensitivity test, or again, errors in 

observations made for the Immersion test. 

The Weatherability test and the Water Sensitivity test correlate 

with each other. Thus, either the Weatherability test or the Water 

Sensitivity test will produce similar conclusions when assessing the 

potential moisture deterioration of the roof. 

TEST RESULTS FROM SPECIFIC COAL MINES 

To evaluate the variability in moisture sensitivity that can be 

observed within a single rock type at a given mine, the Weatherability

test was performed on 89 specimens prepared from 11 samples 

collected from the immediate roof at various locations throughout a 

mine in central Ohio. The immediate roof rock was a gray silty shale. 

Figure 11 is a distribution of the results The average of the tests was 

49%. While a majority (65%) of the values fall into the moisturesensitive range (>40%), there are a number of specimens that are not 

highly reactive to moisture. This extreme variability is due to natural 

variations in rock composition and is not apparent upon visual 

inspection. From an exploration standpoint, it is unlikely that enough 

core samples could be obtained to demonstrate this degree of 
variability. For this reason, in roof rocks that are highly variable, it is 

important to sample and test more frequently. 

0.0

20.0

40.0

60.0

80.0

100.0

Weatherability Index (WAI) %

Moisture Sensitive

Figure 11. Variability in the moisture sensitivity of roof rock from a 

central Ohio mine (Samples ranked by Weatherability Index WAI). 

CONCLUSIONS 

Roof deterioration due to moisture sensitivity continues to cause 

roof falls and requires extensive entry rehabilitation. Diagnostic 

testing, both at the exploration phase and during underground mining, 

would provide data for support design. Three index tests for moisture 

sensitivity have been evaluated. Of the three tests, the Weatherability

test and the Water Sensitivity test have been shown to provide 

reasonable agreement in estimating the potential for moisture 

deterioration of the roof. While still providing a baseline estimate of 

moisture sensitivity, the Immersion test tends to overestimate the 

likelihood for future deterioration of the roof. One approach would be 

to perform the Immersion test first. If the results show moisture 

sensitivity, then use one of the other tests. 

Depending on the lithologic composition of the rock, significant 

variability can be present within roof samples from a coal mine, or even 

a single specimen. Some potential causes of this variability are clay 

mineralogy, grain size, lamination spacing, and cement. As a result, 

large numbers of tests are preferred when roof variability is high. For 

this reason a number of tests of splits from a single roof rock may be 

required to capture this variability. The Weatherability test has the 

advantage because batch testing of up to 30 samples at a time is 

possible. The Water Sensitivity test can utilize batching of similar 

sample quantities during the drying and wetting periods, but sieving 

must be done individually. 

If, during the testing regime moisture sensitivity is detected, there 

are several engineering controls which can aid in the safe recovery of 

the coal. These controls include screen, sealants, increased support 

density, leaving top coal, removing moisture sensitive roof rock, 

narrower entries, shorter panel life, rib meshing, and conditioning the 

air. 

REFERENCES 

ASTM Standard D 4644-08, 2004, “Standard Test Method for Slake 

Durability of Shales and Similar Weak Rocks,” ASTM 

International, West Conshohocken, PA, 

http://www.astm.org/Standards/D4644.htm. 

Hasenfus, G. and D. Su (2005), “Test Method for Assessing Water 

Degradation Potential of Coal Measure Rocks for Analyses of 

Floor, Roof, and Ribs,” Proceedings of the 24th International 

Conference on Ground Control in Mining, Morgantown, WV, pp. 

169-178. 

Mark, C., G.M. Molinda, and T.M. Barton, (2002). “New Developments 

with the Coal Mine Roof Rating,” Proceedings of the 21st 

International Conference on Ground Control in Mining, 

Morgantown, WV, pp. 294-301. 

Mark, C. and D.M. Pappas, (2009), “Current Trends in Reducing 

Groundfall Accidents in U.S. Coal Mines,” SME Annual Meeting, 

Denver, CO (in press). 

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A Practical Rock Mass Classification for Coal Mines,” U.S. 

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25th International Conference on Ground Control in Mining, 

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Molinda, G.M., and T. Klemetti, (2008), “Diagnosing and Controlling 

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Geotechnical Engineering (EJGE), V. 13, Bundle A. 

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Proceedings of the 5th International Conference on Ground 

Control in Mining, Morgantown, WV, pp. 221-223. 

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Mines," Proceedings of the 16th International Conference on 

Ground Control in Mining, Morgantown, WV, pp. 259-266.