Klemetti, T.

Oyler, D.

Molinda, G.

 

 

 

Time Dependent Roof Deterioration at a Central Ohio Coal Mine 

Ted Klemetti, Research Engineer
 

Dave Oyler, Research Engineer
 

Greg Molinda, Lead Research Scientist
 

NIOSH Pittsburgh Research Laboratory
 

Pittsburgh, PA
 

ABSTRACT 

Roof deterioration in weak moisture-sensitive rock leads to 

roof falls in coal mines. An observation site was set up in intake 

air at an underground coal mine in central Ohio to evaluate the 

time-dependent deterioration of moisture-sensitive roof rock. By 

periodically collecting and weighing the rock falling from the roof, 

the National Institute for Occupational Safety and Health (NIOSH) 

was able to quantify the deterioration of the moisture-sensitive 

shale roof. The measurements showed that more roof rock is 

falling in the dry winter months. Movements in an intersection 

were monitored in order to determine if deformation correlated 

with seasonal changes in temperature and humidity. The rate of 

intersection roof-to-floor convergence was found to be cyclical 

and related to seasonal changes, with more convergence during the 

months of April through September. At the study site intersection, 

the roof beam did not show deflection indicating movement most 

likely due to mine floor deformation. The bolt load also increased 

in the warmer months before shedding load, possibly due to rock 

failure. Temperature and relative humidity sensors, placed in holes 

in the roof show that mine air does not mix with the air in the holes. 

Consistent saturation levels of relative humidity were measured 

in the holes and it appears that high levels of moisture may have 

migrated into the bolt holes from roof shales. A roof sealant applied 

to a portion of the test roof appears to be preserving the roof and 

reducing the amount of rock fall into the entry, while the screened 

area virtually eliminated rock falling from the roof. 

InTRODuCTIOn 

During the past 5 years, 2,300 U.S. coal miners have been 

injured by rock falls. There have also been 82 ground fall fatalities 

since 1995, and of these, 15 have been attributed to skin failure 

(Mark et al., 2009). Skin failures involve rocks that fall between 

roof bolts. 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). 

Although there are numerous causes of roof skin failures, moisture 

sensitivity is the primary focus of this research. 

Several moisture sensitivity index tests have been proposed 

in the past and laboratory comparisons of the tests have been 

performed (Klemetti and Molinda, 2009), yet there has been limited 

field evaluation of these indices and their relationship to the mining 

conditions. A more thorough evaluation of the moisture sensitivity 

index test results in relation to the condition of the mine’s roof are 

still needed. 

There is no established standard for controlling or characterizing 

moisture sensitive roof. Numerous mines and researchers have 

developed and utilized engineering controls to improve the 

condition of mines with weak, moisture-sensitive roofs. The 

primary function of these engineering controls is to provide 

maximum coverage of the roof to minimize roof skin failures 

(Compton et al., 2007). Gadde et al., (2006) recommends using roof 

screen to control roof skin failures when the roof is weak. Similar 

to roof screen, spray-on sealants have been developed for and 

utilized in mining applications. A successful application of a sprayon sealant was described by Mark et al., (2004). Numerous other 

sealants have been developed for mine roof coverage, including 

one developed by Kot Unrug at the University of Kentucky (Unrug, 

1997). This sealant is a mining emulsion sealant designed to create 

a non-permeable membrane that will prevent moisture changes and 

therefore, protect the rock against weathering. 

In mines with weak, moisture-sensitive roof rock, bolts must be 

fully grouted (Unrug et al., 2004). Roof bolts with exposed annulus 

allow moisture penetration parallel with bedding plans leading to 

much faster degradation of the roof rock. Degradation may also 

be accelerated by unsealed test holes in the mine roof. It has been 

speculated that roof damage is increased due to mine air entering 

open bolt and test holes (Unrug, 1997; Chugh and Missavage, 

1980). 

There is abundant evidence that the time an entry remains open 

and the humidity variations of the mine air affect the condition 

of the roof (Unrug and Padgatt, 2003). This is especially true in 

weak moisture-sensitive roof rocks and regions of the world where 

temperature and humidity change drastically throughout the year 

(Mark et al., 2004). The effect of these conditions on the mine 

roof can range from small roof rocks falling to larger roof falls. 

Moisture-sensitive rock can have an immediate impact, but more 

often is seen as a time-dependent deterioration. The stability of the 

mine openings can impact the safety and performance of the entire 

mine. 
 

Due to these factors that can influence roof stability, NIOSH 

researchers designed a field study to evaluate these issues. The two 

primary goals were to evaluate (1) the long-term stability and (2) 

the seasonal affects on mine entries with highly moisture-sensitive 

roof rock. Secondary goals included: 

• evaluating some engineering controls for weak moisturesensitive roof rock; 

• developing a case history to correlate laboratory moisturesensitivity indices to mine roof conditions; 

• studying the mixing and infiltration of entry air into drill holes 

in the roof. 

The paper describes the study site, presents the instrumentation, 

evaluates the rock for moisture sensitivity through laboratory 

testing, presents the results from the field study, and presents the 

key issues determined from the study. 

STuDy SITe DeSCRIpTIOn 

The subject mine of this study is located in eastern Ohio in the 

#7 seam. The mine is a three-year-old room-and-pillar operation, 

with a life of ~20 years. The mains are driven from a box cut on 

the surface. The depth of cover ranges from 100 to 300 ft. The 

coal seam thickness ranges from about 4.5 to 5.5 ft. The immediate 

roof consists of a shale layer of variable thickness overlain by 

a relatively thick, strong sandstone layer. A generic depiction of 

the immediate roof is shown in Figure 1. Throughout the mine, 

there are locations where the sandstone rolls down to the top of the 

coal causing the shale layer to vary in thickness between 0 and 12 

feet. The shale layer is weak and moisture sensitive, whereas the 

sandstone is strong and not moisture-sensitive. 

Sandstone 12 ft 

Shale 

5 ft 

#7 Seam 

0 ft 

Figure 1. Generalized roof at the study. 

The mine layout can be seen in Figure 2. The area to the 

north on the map is the old workings, which are now sealed. The 

entries are typically 19 ft wide. The main pillars in the mine are 

generally 60 x 60 ft on centers. The primary roof support consists 

of 5/8-in, fully-grouted, 5-ft-long bolts installed 4 per row with a 

4-ft spacing. In addition, four 10-ft-long cable bolts were installed 

in the intersections. When the mine began operation, screen was 

installed in four entries; 1 intake, 1 return, travelway, and the belt. 

Screen was not installed in the cross-cuts. 

The study site is located on the East Mains as seen in Figure 2. 

The layout of the study site, including instrumentation locations 

and roof rock collection areas, can be seen in Figure 3. In order to 

insure that the seasonal changes in temperature and humidity would 

propagate to the study site, a location approximately 700 ft inby the 

mine portal was chosen. Previous research has demonstrated that 

air is tempered after the air travels through the mine (Molinda and 

Klemetti, 2008). The study site was mined less than 3 months prior 

to the beginning of the study. Additionally, this study site was used 

to evaluate engineering controls for skin control in highly moisture 

sensitive roof rock. Two specific controls were evaluated in this 

study, roof screen and a sealant. Because screen was installed on 

cycle in 4 of 8 entries, the opportunity to evaluate this control was 

available. The sealant1 

1 Mention of company name or product does not constitute endorsement by the National 

Institute for Occupational Safety and Health. 

used in this study was an emulsion fluid 

developed by Kot Unrug at the University of Kentucky to create 

a non-permeable membrane around the exposed rock. The sealant 

was applied to the roof with a spraying device similar to a garden 

sprayer at concentrations of 50% and 100%. It appears blue 

immediately upon application and shortly dries to form a surface 

barrier. 

 

 

 

 

 

 

 

 

 

 

 

 

 

E A S T M A I N S 

E A S T M A I N S 

B o x C u t 

R o c k S a m p l e 2 0 0 F t O l d W o r k i n g s R o o f F a l l 

C o l l e c t i o n L o c a t i o n 

S e a l s E A S T S U B M A I N S 

S t u d y 

S i t e 

EAST MAINS 

R e c e n t l y M i n e d 

Figure 2. Study mine layout. 

 

 

 

 

Humidity Sensor at Roof Line 

1 50% Bolt w / Load Cell 

2 100% 3 pt Extensometer 

3 NONE Convergence Meter 

Humidity Sensor Up Bore 

Hole 

1 Roof Rock Collection Area 

4 NONE Numbers 

Figure 3. Study site layout with instrumentation and area 

locations. 

This study used three methods to monitor the condition of the 

roof at this mine site: fallen roof rock, instrumentation, and visual 

observation. 

evAluATIOn TeChnIqueS 

Roof Rock Collection 

Fallen roof rock can be an indicator of the time dependent 

deterioration due to moisture. The roof rock collection component 

consisted of four distinct areas where the floor was covered with 

tarps to capture the rock falling from the roof throughout the study 
as shown in Figure 4. The tarps covering the floor were used to 

separate the fallen roof rock from that already on the floor. The 

roof rock on the tarps was collected in buckets and weighed with a 

portable scale. The resulting weights were recorded for each area 

during the visits to the study site. 

Figure 4. photo of roof rock collection areas 8-16-07, location 1. 

Roof rock collection area 1 was sprayed with a 50% 

concentration of roof sealant, and area 2 was sprayed with a 

100% concentration, as seen in Figure 3. The 50% concentration 

consisted of 50% sealant and 50% water. The remaining two 

areas were not sprayed with the sealant. All of the roof rock fall 

collection areas were originally quite similar in appearance. 

Instrumentation 

The instrumentation deployed at the study site consisted of a 

roof-to-floor convergence monitor, roof sag extensometer, bolt 

load cell, and humidity and temperature sensors as illustrated in 

Figure 3. The convergence monitoring was intended to show the 

seasonal effects on the roof and floor. The swelling and shrinking 

of the immediate rock surrounding the openings may be captured 

in the convergence measurements. The mechanical extensometer 

and the convergence rod may allow for determination of the roof 

and floor convergence individually. The bolt load may be another 

indicator of the swelling and shrinking occurring in the roof as 

well as the separation occurring in the roof. The relative humidity 

and temperature sensors were installed to document the seasonal 

changes in humidity and temperature in the entries as well as 

determine if there was communication of humidity from the entries 

into the roof. These sensors were used to investigate changes in 

grouted bolt holes, un-grouted bolt holes, and tests holes resulting 

from varying weather conditions. 

There were two instruments used to measure convergence of the 

mine opening, the roof-to-floor convergence monitor and the roof 

sag extensometer. The roof-to-floor convergence monitor was a 

convergence rod installed in the center of the intersection between 

the roof rock collection areas. The convergence rod was simply 

installed between the roof and floor with no anchors. The roof 

sag extensometer used in this study was a three-point mechanical 

extensometer. The instrument has three string potentiometers 

to measure displacement. The potentiometers are connected to 

anchors installed at 2 feet, 8 feet, and 15 feet above the roof line. 

The anchor furthest up the hole defines the horizon of measurement, 

with the box holding the potentiometers connected to the roof. 

The two intermediate anchors can show where the separation is 

occurring. Depending on the spacing of the anchors, the separation 

can be located to within half the spacing of the anchors. The 

bolt load cell, a Geokon Model 3000, was a strain gage type cell. 

The cell was installed between the bolt head and bolt plate, with 

an initial load of approximately 17,000 lbf. The bolt was a 5-ft 

mechanical-anchored bolt installed centered between the primary 

bolt rows which were already installed. 

The humidity and temperature sensors were positioned in two 

distinct locations, one within the entry and one up drill/bolt holes. 

There were initially three sensors installed in the entry hung from 

bolt plates along the roof of the study site. Additionally, 9 sensors 

were installed in the three holes drilled in the roof. The first hole 

had the sensors and setting rod up the hole, remained open, and is 

referred to as the open hole. The second hole was covered with 

a bolt plate attached to the roof with anchors. The third hole was 

sealed with clay to simulate a grouted hole. The sensors in the 

drill/bolt holes were located at 2-ft, 8-ft, and 12-ft horizons above 

the roof line. 

Visual Observation 

The visual observation component consisted of photographic 

documentation of the aging of the study site and the adjacent 

entry. The benefit of both the screen and sealant as well as the time 

dependent roof deterioration, can be seen through the photographs 

taken over the study period. The locations of the photos taken 

during each visit can be seen in Figure 5. 

6 7 8 

Figure 5. Study site layout with photograph locations. 

MOISTuRe SenSITIvITy TeSTInG 

The University of Kentucky Weatherability test (Weatherability 

test) is a three part test, requiring special testing apparatus and 

specimens either from cores or roof slabs weighing between 500 

and 2000 grams (Unrug, 1997). The first part is oven drying to 

a constant weight and weighing of the samples. The second part 

is the wetting (1 hour) and air drying (6 hours) of the samples. 

The final part is oven drying and weighing of the samples. The 

multiple wetting and drying cycles of the test simulate the seasonal 
wetting and drying that attacks coal mine roof. Details of the 

Weatherability test procedure are available in previous publications 

(Unrug, 1997). The resulting Weatherability Index is a ratio of the 

degraded material weight, based on the largest remaining intact 

fragment’s weight, to the initial specimen’s weight expressed as a 

percentage. 

Weatherability Test Results 

The Weatherability test was performed on 89 specimens obtained 

at several locations throughout the study mine, as shown in Figure 

2. Figure 6 is a distribution of the results of the Weatherability 

Index of the 89 specimens of gray shale (Ferm No. 124) from 

the mine’s immediate roof. The overall Weatherability Index 

average of the shales tested was 49%. While a majority (65%) 

of the values fall into the moisture sensitive range (>40%), there 

are a number of specimens that are not reactive to moisture. The 

variation in results may be attributed to the inherent variability 

of rocks. Rock characteristics vary widely throughout a mine as 

well as in a smaller sample. The variation may also be attributed 

to variations in bedding and spacing in the individual specimens. 

Another potential cause for the variation is the presence or absence 

of swelling minerals in the individual specimen samples. Weatherability Index (WAI) % 

100.0 

80.0 

60.0 

Moisture Sensitive 

40.0 

20.0 

0.0 

Figure 6. Moisture sensitivity variability of the immediate roof 

rock (gray shale, Ferm no. 124). 

Spray Sealant Testing 

Prior to the field study, samples from the study site area were 

evaluated using the Weatherability test, with and without being 

coated in the lab with the spray-on sealant used in this study. 

These tests were performed as a baseline for the results from the 

field study. Table 1 compares the moisture sensitivity indices 

from coated and uncoated samples collected at the study site. The 

location where the samples were collected can be seen in Figure 

2. The coated samples showed a drastic decrease in the moisture 

sensitivity. The average Weatherability Index for the uncoated 

samples was 29.8% and for the coated samples was 0.7%. This 

indicates that the coated samples are no longer highly moisture 

sensitive. Therefore, it was anticipated that the study site would 

show a similar protection against moisture exposure where the 

sealant was applied. 

Table 1. Study Site Weatherability Index Test Results, 

Coated vs. uncoated. 

Sample Condition Weatherability 

Index % 

Uncoated 

Average 

Maximum 

29.8 

69.8 

Minimum 2.2 

Coated 

Average 

Maximum 

0.6 

0.7 

Minimum 0.6 

 

 

STuDy SITe ReSulTS 

Roof Rock Collection 

The fallen roof rock collection provided the most intriguing 

results from this study as presented in figure 7. The normalized 

average weight of roof rock collected per day was approximately 

0.0069 lb/ft2 in both coated areas, compared to about 0.0230 lbs/ft2 

for the uncoated areas. The areas sprayed with sealant yielded 2.9 

and 3.8 lbs/ft2 of roof rock collected during the first year of 

monitoring. The areas with no sealant yielded 9.2 and 15.5 lbs/ft2 

during that same period. The two uncoated areas showed at least 

2.5 times and as much as 4.8 times the fallen rock of the two coated 

areas. The uncoated areas had varying roof rock falling rates 

throughout the study, whereas the coated areas had similar trends, 

as seen in figure 7. Some of the rocks collected from the uncoated 

areas were much larger than the rocks collected from the coated 

areas. There were two periods of increased roof rock fall amounts, 

December 2007 through March 2008 and December 2008 through 

February 2009.

 

 

 

 

0 

5 

10 

15 

20 

25 

Cumulative Rock Fall (lb/ft2 ) 

Sprayed - 50% Conc. 

Sprayed - 100% Conc. 

Unsprayed - Inby 

Unsprayed - Outby 

08/07 10/07 12/07 01/08 03/08 05/08 07/08 09/08 11/08 01/09 02/09 

Figure 7. Cumulative roof rock collection results. 

The results demonstrate the improvement expected from sealing 

roof rocks when compared to the areas that were not coated with 

the sealant. There does not appear to be any distinct seasonal trends 

associated with the falling roof rock. The results also indicate that 

this sealant can be diluted with 50% water and still maintain the 

same level of protection. Since the spray sealant has no strength, 

it should be applied as quickly after mining as possible for the 

greatest effect prior to roof sag. A photo of the roof rock collection 

area taken on March 3, 2008 prior to rock collection can be seen in 

Figure 8. 

Convergence 

The cumulative roof-to-floor convergence for the study site 

can be seen in Figure 9. The plot shows that the convergence is 

continually increasing with time. There are four distinct periods 

in the figure. Periods one and three have a much higher rate of 

convergence compared with periods two and four. The increase in 
the rate of convergence corresponds to the warmer humid months 

of April through September. This increase in humidity can cause 

swelling of the roof and floor leading to the increasing rate of 

convergence. Another possible factor in the changing rates is bed 

separation of the weakly bedded immediate roof and floor. 

Figure 8. photo of the rock collection area, looking inby from 

the intersection, taken on March 3, 2008. 

1.2
 

Period 1 Period 2 Period 3 Period 4

1.0 

Convergence (in) 0.8 

0.6 

0.4 

0.2 

0.0 

8/07 10/07 12/07 1/08 3/08 5/08 7/08 9/08 11/08 12/08 2/09 

Date 

Figure 9. Convergence measurements in intersection of study 

mine. 

Another instrument deployed at the study site was the 3-point 

mechanical extensometer intended to document the roof sag over 

the entire study period. The mechanical extensometer never 

measured any roof sag. With the extensometer showing no roof 

sag and the convergence rod measuring roof-to-floor convergence, 

swelling of the floor is probably the cause of the convergence. 

Bolt Load 

The bolt load cell shows an almost immediate reduction in load. 

The initial load loss is typical with a mechanical anchor bolt. This 

initial drop off was followed by a steady decline from September to 

January and a steady increase from January until mid July. In mid 

July there was a steep reduction in the bolt load. After this steep 

reduction, the bolt load has been gradually decreasing ever since. 

This trend, as seen in Figure 10, does not correspond to the trends 

of the convergence measurements. This steep reduction in July 

2008 could be due to anchor slippage or roof deterioration around 

the bolt plate. It appears that the bolt loading is somewhat related 

to seasonal trends. The mechanical bolt appears unable to sustain 

the full design load in this roof.

18,000
 

16,000
 

14,000
 

12,000
 

Bolt Load (lbf) 10,000 

8,000 

6,000 

4,000 

2,000 

0 

8/07 10/07 12/07 1/08 3/08 5/08 7/08 9/08 11/08 12/08 2/09 

Date 

Figure 10. Bolt load in intersection of study mine. 

Temperature and Humidity 

The temperature and humidity sensors showed a constant relative 

humidity of 100% throughout the entire year on the sensors located 

in all three holes at 2-ft, 8-ft, and 12-ft horizons above the roof 

line. A single sensor was placed approximately 1 ft above the roof 

line in the open hole on 28 April 2009 and has maintained 100% 

relative humidity ever since. The sensors in the entry varied in 

relative humidity from less than 40% to 100% throughout the year, 

reflecting the ambient conditions. The constant relative humidity 

in the holes, including the open hole, shows that no air transfer 

is occurring between the entry and the hole. It indicates that the 

humidity is coming from the formation rather than the humidity in 

the entries’ air. Thus, the roof is still subjected to moisture trapped 

in the formation which may still be damaging the roof. The relative 

humidity measurements up the hole are the overlapping lines at 

100% in Figure 11. 

12 ft above roof 8 ft above roof 

2 ft above roof Entry 120 

Relative Humidity, % 100 

80 

60 

40 

20 

0 

8/07 10/07 12/07 2/08 4/08 6/08 7/08 9/08 11/08 1/09 

Figure 11. Open hole and entry humidity. 

The temperature in the holes was also extremely constant on 

a daily basis compared with the entry measurements. The daily 

variation was typically less than one degree Fahrenheit in the hole 

sensors compared to an average of around seven degrees Fahrenheit 

for the entry sensors. There was also very little variation in the 

temperature and relative humidity measurements among the three 
open and sealed holes. A sample of the temperature results can 

be seen in Figure 12. The temperature measurements are thicker 

smoother lines for the sensor up the open hole. The thinner line 

represents the temperature in the entry. Temperature, °F 

12 ft above roof 8 ft above roof 

80 2 ft above roof Entry

70 

60 

50 

40 

30 

20 

8/07 10/07 12/07 2/08 4/08 6/08 7/08 9/08 11/08 1/09 

Figure 12. Open hole and entry temperature. 

Visual Observation 

The final method used in evaluating the deterioration of the 

roof was visual observation documented with photography. 

Several photographs were taken during each visit to document the 

time dependent deterioration of the roof. The locations of these 

photographs can be seen in Figure 5. The photograph in Figure 

13 was taken on August 16, 2007, at onset of study, from the 

intersection in the study site looking inby, point 5 in figure 5. This 

photo shows minor roof deterioration and no roof rock on the tarps 

as would be expected at the beginning of the study. Figure 14 is 

a photo of the same location taken on June 25, 2008. Figure 14 

depicts a much greater level of roof deterioration and roof rock on 

the tarp than that in Figure 13. 

Figure 13. photo taken 8-16-07 looking inby at study site, 

location 1. 

Figure 14. photo taken 6-25-08 looking inby at study site, 

location 1. 

In addition to the photographs demonstrating the changing 

conditions in the instrumented area, photographs of the adjacent 

entry were taken. This adjacent entry is also in intake air and the 

roof is screened. As seen in photo in Figure 15 taken during the 

10th month of the study, the screen contained the roof rock and 

subsequently maintained better overall conditions. Several previous 

studies have shown the benefit to roof screening in moisture 

sensitive roof control (Molinda et al., 2003; Compton et al., 2007).

Figure 15. photo taken 6-25-08 looking inby in adjacent entry 

with screen, location 15. 

COnCluSIOnS 

The roof rock volumes, along with several other measurements, 

demonstrated the importance of time to the deterioration of roof 

rock in coal mines. Seasonal changes appear to have some impact 

on the condition of the roof. Weak moisture-sensitive roof rock 

at the study mine began deteriorating within 1 year of the mine 

opening. 

From the convergence measurements, it appears that the floor 

swells substantially during the hot humid months of the year and 

may continue moving slightly during the cold dry months. The 

extensometer also showed that the roof was either not acting as 

a beam or that the anchors are unable to maintain solid contact. 
Since the extensometer showed no movement but the convergence 

rod did record displacement during the study period, the movement 

recorded is most likely due to floor heave. The floor heave appears 

to be related to seasonal temperature and humidity changes in the 

mine air. The bolt load has not regained its peak load since last 

summer, which may be related to anchor slippage or loss bolt 

tension due to creep yielding or deterioration of the roof rock most 

likely move the bolt plate. 

The other interesting outcome from the instrumentation at the 

study site came from the humidity and temperature monitoring. The 

results showed that the intake air moisture does not affect moisture 

content in drill or bolt holes, one foot above the roof line. All three 

open and sealed tests holes maintained 100% humidity from two 

feet above the mine roof to the top of the hole and a single sensor 

maintained 100% relative humidity less than one foot above the 

roof line in the open hole. This lack of communication between 

the moisture in the entry and open holes indicates that if open bolt 

holes cause a problem, it is probably not due to penetration of the 

mine air as some have speculated, but rather to the effect of in situ 

groundwater. The temperature and relative humidity measurements 

do not explain the seasonal loading and increased rate of roof falls 

typically seen in mines with weak, moisture-sensitive roof rock 

during the summer. The exposed rock at the skin of the opening 

may well be affected by the seasonal temperature and humidity 

changes in the mine air through absorption of moisture from the 

mine air as proposed by Kot Unrug (1988). 

As for the engineering controls, the sealant coated entry 

proved to limit the amount of roof rock falling substantially over 

the year long study. Through instrumentation, measurement and 

visual observation the emulsion coating appeared very beneficial, 

at least in the short term, for protecting moisture sensitive roof. 

Using the results from the roof rock collection, it is estimated that 

approximately 1 ton of rock will fall to the floor per 50 ft of drivage 

every year for the roof sprayed with sealant, assuming that the roof 

sealant maintains the same level of protection. For the roof without 

sealant, the amount of rock falling to the floor per 50 ft of drivage 

was approximately 4 tons per year. Additionally rocks collected 

from the floor of uncoated areas appeared to be larger than those 

from the coated areas. The difference in the roof condition of the 

instrumented unscreened entry and the adjacent entry, which was 

screened, can be seen through photographs and visual observations. 

The screened entry maintained its original condition much better 

than the entry with the sealant and the entry with neither screen nor 

sealant. 

The findings from this study indicate that weak moisturesensitive roof rock can deteriorate rapidly, as soon as six months. 

There is also evidence that these seasonal effects are related to the 

surface of the opening and in the short term do not affect the general 

stability of the roof based on the extensometer data. Screening may 

well be the single best engineering control available for controlling 

moisture deterioration in weak roof rock. For short lived panels, 

sealants appear capable of maintaining adequate protection from 

deterioration and may protect for longer periods if reapplied. 

CuRRenT SuppORT DeSIGn 

After observing the progressive roof deterioration, the mine 

operator changed its support plan. Instead of installing screen in 

four entries, now the entire entry system is being screened. When 

the screen does not reach both ribs, along the rib where screen is 

not in contact, a 4-ft bolt is installed between the end of the screen 

and the rib. Along with the increase in screen, straps and one 10-ft 

cable bolt is being installed in every other row. They are also now 

installing six 10-ft-long cable bolts per intersection instead of 

four. To date this new support plan seems to be adequate and no 

subsequent roof falls have been reported. 

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 agency 

determination or policy. 

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Compton, C., Gallagher, S., Molinda, G., Mark, C., and Wilson, 

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