Evaluation of deep-seated crib 

block fires and direct application 

fire suppression agents

M.A. Trevits and A.C. Smith

Senior research scientist and manager Fire Prevention Section, respectively, 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania

J.E. Urosek and M.P. Valoski

Chief Mine Emergency Operations and chief Toxic Agents Branch, respectively, 

Mine Safety and Health Administration (MSHA), Pittsburgh, Pennsylvania

Abstract

Unlike other types of mining accidents, where an incident generally involves only a few workers, the 

danger of a mine fire extends to every person working in the underground environment. Mine fires 

can trap or kill those working underground by blocking avenues of escape and rapidly creating heat, 

asphyxiating gases and toxic fire products that can quickly spread throughout the mine. The National 

Institute for Occupational Safety and Health (NIOSH), in partnership with the Mine Safety and Health 

Administration (MSHA), is conducting research to test, evaluate and improve or modify coal mine firefighting strategies and methodologies through large-scale tests. Because wood is the second most abundant 

fuel available during a coal mine fire, it was decided that a series of large-scale wood crib fire tests was 

needed to measure the products of combustion and to observe the capability of commonly available direct 

application fire suppression equipment, namely, fire extinguishers, water and gas-enhanced foam. This 

paper discusses the wood crib fire tests, provides insight into the products of combustion and describes 

observations made during the application of the fire suppression agents.1

 1 The findings and conclusions in this report are those of the authors and do not necessarily represent the views of NIOSH.

Background and introduction

When possible, during the early stages of a mine fire, common suppression agents such as fire extinguishers, water or 

foam are directly applied to contain or extinguish the fire. If 

the fire cannot be quickly brought under control, the mine is 

then evacuated to reduce the chance of injury or death due to 

exposure to the products of combustion. Once safe evacuation has been achieved, the entire mine or fire zone is sealed 

or contained to rob the fire of oxygen. Fire suppression and 

ultimate extinguishment then involves the remote injection 

of agents, including water, inert gas, gas-enhanced foam and 

possibly jet engine exhaust gases. Unfortunately some of these 

technologies are being used at mine fire sites without a fundamental knowledge of the technology and a comprehensive 

understanding of the proper application techniques. 

In 2001, NIOSH and MSHA agreed to partner in research 

studies to develop new understandings of the characteristics 

of mine fires and the capabilities and limitations of mine fire 

suppression technologies. Since that time, it is believed that 

this partnership has served the mining industry well, as NIOSH 

and MSHA have worked together in the field at actual mine fire 

sites and in the laboratory and each has gained new insights 

into the science of mine fires and fire control and suppression 

technology. As a result of this partnership, research projects 

have been conducted and are ongoing in the areas of remote 

installation of mine seals and evaluation of remote suppression 

technologies (NIOSH, 2001; Trevits and Urosek, 2002; Mucho 

et al., 2005; Trevits et al., 2006a; Trevits et al., 2006b). 

In late 2006, NIOSH initiated a new research project called 

“Remote Methods for Addressing Coal Mine Fires.” The 

objective of this research project is to test, evaluate, improve 

or modify remote fire-fighting methodologies for coal mine 

fire control and suppression and to directly transfer these 

improvements and modifications to the coal mining industry. 

The research is focused on the completion of testing of remote 

mine seal technology (including the evaluation of rigid foam 

technology) and on the evaluation of fire-suppression technology. The work plan calls for large-scale, controlled deep-seated 

(firmly established) coal fire tests to be conducted underground 
in the NIOSH Lake Lynn Experimental Mine. A series of largescale, deep-seated wood crib fire tests was added to the work 

plan because the use of wood is so prevalent in underground 

coal mines. It was thought that the wood crib fire tests would 

yield significant information about the combustion products 

of wood components that are typically consumed in a mine 

fire and could offer a platform for evaluating the capability of 

directly applied fire suppression technology. 

Previous research was conducted by the U.S. Bureau of 

Mines on wood crib fires in an intermediate-scale fire tunnel 

(Trevits, 2006b). The previous work involved various Douglas 

fir crib block configurations and generated critically important 

information on carbon monoxide (CO) and carbon dioxide 

(CO2) gas concentrations, smoke particle characteristics and 

heat release rates. The present study serves to expand and 

supplement the previous work using large-scale mixed wood 

species crib block fire tests. 

Objective and approach

The objective of this study was to conduct a series of largescale, deep-seated, wood crib fire tests to measure the various 

combustion products and to observe the capability of commonly 

available direct application fire suppression technology, namely, 

fire extinguishers and water and gas-enhanced foam.

Fire suppression facility

The crib block fire tests were conducted at the Fire Suppression Facility (FSF), and MSHA provided supplemental gas 

monitoring equipment for the tests along with technical experts 

to operate the equipment. The FSF is part of the NIOSH Lake 

Lynn Laboratory (LLL), which is located approximately 100 

km (60 miles) southeast of Pittsburgh, Pennsylvania. The LLL 

is a world-class, highly sophisticated surface and underground 

facility where large-scale explosion trials and mine fire research 

is conducted (Egan and Litton, 1986).

The FSF was configured to simulate a 46-m- (150-ft-) long 

mine entry. The interior height of the simulated mine is 2.2 m 

(7.2 ft) and the width is 2.4 m (8 ft). The roof of the simulated 

mine is made of corrugated steel bridge planks, the ribs are 

made of 203-mm- (8-in.-) thick mortared solid concrete blocks, 

and the floor is made of reinforced concrete. The interior roof 

is covered with a 50.8-mm- (2-in.-) thick layer of Fendolite 

M-II® (a specialized fire resistant mixture of vermiculite and 

Portland cement) and a 25.4-mm- (1-in.-) thick layer of Fendolite M-II® has been placed on the ribs.2

 2 Reference to product or trade names does not imply endorsement by NIOSH and MSHA.

For ventilation, a 

1.8-m- (6-ft-) diameter variable-speed axivane fan (equipped 

with a pneumatic controller to adjust fan blade pitch) has been 

installed at one end of the simulated mine. The fan can provide 

blowing sustained airflow up to about 5.84 m/s (1,150 fpm) 

over the cross-section of the entry. Two doors, which permit 

access to the inside of the FSF, are located about 14 m (47 ft) 

from the fan. Figure 1 shows the exterior of the FSF.

Figure 1 — Fire suppression facility.

The FSF is equipped with an array of chromel-alumel 

thermocouples (type-K) projecting 30 mm (1.2 in.) down from 

the mine roof. The thermocouples are spaced at 3 m (10 ft) 

intervals starting about 3 m (10 ft) from the fan leading along 

the centerline to the end of the simulated entry. The thermocouples are connected via a wire network to a computer-based 

data-acquisition system. During the crib fire tests, data were 

collected at 10-second intervals and radiation corrections were 

not made in the temperature data. Video images of the tests 

were recorded using a camera that was positioned on the one 

side of the mine entry about 6.4 m (21 ft) upwind from the 

crib block sets.

The components of the fire gases were measured using three 

gas-monitoring arrays. The first was a nine-point array located 

6.46 m (21.2 ft) from the trailing edge of the crib block sets. 

The array consisted of an interconnected network of 12.7-mm- 

(1/2-in.-) diameter black iron pipe set across the width of the 

mine entry. A total of nine 3.175-mm- (1/8-in.-) diameter holes 

were drilled into vertical sections of the pipe to sample the fire 

gases. The holes in the pipe were spaced equally apart from 

the roof-to-floor and across the width of the entry (Fig. 2). 

Figure 2 — Layout of a gas-monitoring array. (Circled areas 

show gas monitoring points and thermocouples.)

A 

thermocouple was also positioned at each gas sampling point 

to measure the temperature of the fire gases. The gases collected at each sample point were mixed together in a manifold 

that penetrated the FSF roof. The manifold was connected to 

a Tygon® tubing line that led to the MSHA gas monitoring 

truck. The second gas-sampling array was configured in the 

same manner as the first and was located 29.1 m (95.5 ft) from 

the trailing edge of the crib block sets. The third gas-sampling 

array was positioned near the second gas-sampling array and 

consisted of four 3.175 mm (1/8 in.) holes drilled into 25.4-
mm- (1-in.-) diameter black iron pipe (spaced equally from 

the roof-to-floor). The pipe was positioned vertically at the 

centerline of the entry and was connected to a line that led 

to a set of NIOSH infrared gas analyzers. The gas analyzers 

measured O2, CO and CO2 gas concentrations and the resultant 

data was collected at 10-second intervals and recorded by a 

computer-based data-acquisition system. 

In the MSHA gas monitoring truck, the gas samples were 

analyzed using a combination of infrared and electrochemical gas analyzers (the gases analyzed included O2, CO, CO2

and methane (CH4)). Throughout each test, the data from the 

MSHA gas analyzers were collected at 2-minute intervals and 

were recorded in a computer database. In addition, gas samples 

were collected periodically over 3- to 4-minute intervals from 

the first gas-sampling array and were analyzed using gas 

chromatography. The analysis of each of the gas samples by 

the chromatograph took about 10 minutes to complete. Therefore, during each test, a sample was collected and analyzed 

approximately every 13 to 15 minutes. The gases analyzed 

included H2, O2, CO, CO2, CH4, acetylene (C2H2), ethylene 

(C2H4) and ethane (C2H6). The gas chromatography results 

were verified by back-up gas bag samples that were periodically collected from the same sample point and analyzed using 

a different chromatograph at a MSHA laboratory. The NIOSH 

and MSHA gas analyzers and the MSHA gas chromatograph 

were calibrated before each test (the calibration standard was 

5 ppm H2, C2H2 and C2H4 and 10 ppm C2H6). After each 

test, the MSHA gas analyzers were checked with fresh air to 

determine instrument drift.

Composition of the crib blocks

The crib blocks were comprised of a variety of mixed hardwood species, including beech, cherry, maple, white oak, red 

oak and poplar. To insure that the crib block sets would burn 

evenly and produce a more intense fire, the crib block sets were 

composed of an even mix of older blocks (about 6 to 8 months 

old) that had been stored underground at the LLL and newer 

blocks that were delivered to the LLL about two months prior 

to the tests. All blocks were obtained from the same vendor, 

and the wood was milled from two days to one month before 

shipment to the NIOSH. Ten crib blocks (five newer and five 

older blocks) were randomly collected and 76-mm (3-in.) cubeshaped samples of wood were cut from each of the selected 

crib blocks. The samples were sent to an independent testing 

laboratory for moisture, ultimate analysis and heating value 

determinations. The averaged laboratory 

data from the 10 samples analyzed are 

shown in Table 1.

Table 1 — Laboratory data from crib block samples.

Average Standard deviation

 Sample condition: AR Dry DAF AR Dry DAF

 Moisture, % 25.52 – – 7.11 – – 

 Ultimate analysis, % 

 Hydrogen 6.68 5.11 5.13 0.30 0.21 0.21 

 Carbon 37.55 50.49 50.72 3.40 0.47 0.45 

 Nitrogen 0.09 0.11 0.11 0.01 0.02 0.02 

 Sulfur 0.06 0.09 0.09 0.03 0.03 0.03 

 Oxygen 55.29 43.76 43.95 3.12 0.29 0.31 

 Ash 0.33 0.45 – 0.12 0.16 –

 Heating value (Btu/lb) 6,424 8,652 8,691 470 259 265

AR = as received

DAF = dry ash-free

From Table 1 it can be observed that 

the wood contained an average of about 

25.5% water. This average moisture value 

and the associated high standard deviation 

was expected because, as discussed above, 

half of the crib blocks were “green,” and 

these blocks did not have much time to lose 

moisture prior to the tests. Fuel moisture is 

an important factor with regard to the crib 

fire tests. If high moisture content wood is 

burned, the resulting fire will take longer 

to reach ignition temperature, the fire will 

be less intense because much heat energy 

will be used to convert the moisture to 

steam, and the fire will produce more particulates and smoke. On the other hand, if 

low moisture-content wood is burned, the 

resulting fire reaches the ignition temperature faster, is more 

intense and produces less smoke and particulates. Finally, 

because the samples of the wood were a combination of newer 

and older samples, the variation in the moisture content of the 

wood caused a variation in the heating value from an average 

of about 14.9 MJ/kg (6,400 Btu/lb), with a 1.1 MJ/kg (470 

Btu/lb) standard deviation. 

Crib block set geometry

Table 2 provides information about the crib blocks used in this 

study. Each crib block set was constructed on a foundation 

consisting of eight solid concrete blocks, with each concrete 

block measuring 152 mm high, 203 mm wide and 406 mm 

long (6 in. high, 8 in. wide and 16 in. long). A total of 34 crib 

blocks were stacked vertically to build each crib set (17 of the 

older crib blocks and 17 of the newer crib blocks). Each row 

of blocks was oriented perpendicular to the adjacent row. The 

bottom row contained four crib blocks (two newer blocks on 

the outside and two older blocks on the inside) oriented with 

the 152 mm (6 in.) side facing down, the next row contained 

four crib blocks (two newer blocks on the inside and two older 

blocks on the outside) oriented with the 152 mm (6 in.) side 

facing down. The next row contained three crib blocks (two 

older blocks on the outside and one newer block on the inside) 

oriented with the 152 mm (6 in.) side facing down. The next 

row contained three crib blocks (two newer blocks on the 

outside and one older block on the inside) oriented with the 

152 mm (6-in.) facing down. The remaining rows contained 

two crib blocks each, alternating between the new and older 

blocks in each row with the 127 mm (5 in.) side facing down. 

The final row of blocks was adjusted with the 127 or 152 mm 

(5 or 6 in.) side facing down, depending on the relative closeness of the mine roof. 

The crib blocks were secured to the roof with wedges as 

needed. To insure the maximum stability of each crib set, for the 

longest period of time, each crib set was wrapped with poultry 

fencing that was affixed to the wood with staples. Figure 3

shows the typical crib block set-up prior to a test.

FSF test set-up

The layout of the FSF for the crib block tests is shown in Fig. 

4. The crib blocks were located 15.8 m (52 ft) from the fan 

and 10.4 m (34 ft) from a simulated conveyor structure in the 

FSF. The crib block sets were placed 0.45 m (1.5 ft) apart and 

were positioned near the center of the entry. As mentioned 
above, the crib block sets were constructed on solid concrete 

blocks and located in the middle of the blocks was an impinged 

natural gas burner that was equipped with 60 stainless steel 

jets (with a rated heat output of 44 to 114 kW). The burners 

were set on the simulated mine floor with the jets positioned 

about 50 mm (2 in.) below first row of crib blocks. Natural gas 

was used instead of an accelerant (e.g., diesel fuel) to assist 

in starting the fires, because it was more readily consumed by 

the fire and left no residue that could have altered the wood 

fire combustion products. A series of preliminary experiments 

showed that the natural gas burners would need to operate 

for about 50 minutes to ensure that the crib fires would burn 

without the need to relight the burners.

Table 2 — Crib block information.

 Parameter Data

 Block height, in. 5

 Block width, in. 6

 Block length, in. 30

 Average block weight, lbs (S.D., lbs) 27.2 (3.05)

 Number of blocks used per crib block set 34

 Number of crib block sets used per test 4

 Number of crib blocks used per test 136

S.D. = Standard deviation

Prior to igniting the fires, the ventilation flow rate was measured in front of the crib block sets, at the first gas sampling 

array and at the location of the second and third gas-sampling 

array. At each location, the ventilation airflow rates were 

measured at nine points in the cross-sectional area of the mine 

entry and an average flow rate was determined. Table 3 shows 

the ventilation airflow rate data for the tests. 

Crib block fire tests

The crib fire tests were designed so that four sets of crib blocks 

would be burning simultaneously as the combustion products 

were measured. As the tests were being conducted, the estimated 

heat release rate of the fire was calculated from the gas data and 

the intensity of the fire (and smoke rollback) was monitored 

remotely using the video camera. The estimated heat release 

rate or estimated energy output of the fire was determined using the amount of CO and CO2 in the combustion product air 

stream, the rate of consumed oxygen and the relative increase 

in temperature. The methodology used for making the estimated 

heat-release rate calculations is defined in detail in Smith et 

al. (1995). The heat-release rate estimations were made using 

the data collected by the MSHA infrared and electrochemical 

equipment at gas sample Arrays 1 and 2 (CO, CO2 and O2) and 

the NIOSH thermocouple data collected at the same locations. 

The final estimate of the heat release rate was calculated by 

averaging the estimated heat-release rates from the amount of 

CO and CO2 in the combustion product air stream and the rate 

of consumed oxygen. The estimated heat-release rate that was 

calculated using the temperature data was 

inordinately low due to significant heat 

loss to the FSF. As a result, these values 

were not used in overall heat-release rate 

determinations. 

In this study, a total of five tests were 

conducted, including two crib fire control 

tests. The first control test involved crib 

block sets that were stacked in a normal 

manner, and the second control test involved crib block sets that were normally 

stacked and enclosed in a steel framework. 

The framework was designed to make the 

crib block sets stand upright for a maximum period of time. The control tests were 

designed to serve as the base case for the 

extinguishment tests and the fires were 

permitted to burn until the estimated heat 

release rate declined significantly. Three 

crib set fire tests with extinguishing agents 

(fire extinguishers, water and gas-enhanced 

foam) were also conducted. These fires 

were permitted to burn until they were 

emitting a stable and steady amount of 

Figure 3 — Typical crib block set-up prior to a test.

Figure 4 — FSF typical set-up for a crib fire test. 

heat (as determined by the heat release-rate calculations) at a 

consistent level of intensity with minimal smoke rollback (as 

determined by the video camera) before the fire-extinguishing 

agents were applied. If a crib block set fell prior to application of the fire suppression material, an interval of time was 

needed to allow the fire to stabilize before the fire suppression 

material was applied. In this manner, the effect of applying the 

extinguishing agents to the fire could be readily observed in 
the heat-release rate data would not be perturbed by dramatic 

variations caused by changes in the geometry of the blocks or 

intensity of the fire. Below are summaries of each test.

Table 3 — Average ventilation flow rate information for the crib fire tests.

Test mumber

 Description 1 2 3 4 5

 In front of the crib block sets, ft/min 290 300 395 310 300

 (Changed rate, ft/min)1 (340)1

 MSHA Gas sample array No. 1 - air velocity, ft/min 330 300 280 295 270

 (Changed rate, ft/min)1 (320)1

 MSHA Gas sample array No. 2 - air velocity, m/sec 315 320 285 310 280

 (Changed rate, ft/min)1 (330)1

 NIOSH Gas sample array - air velocity, ft/min 315 315 285 310 280

 (Changed rate, ft/min)1 (330)1

1Severe smoke rollback required an increase in the flow rate.

Figure 5 — Control test – Burning crib block sets.

Crib block Tests 1 and 2 (control tests). In Test 1, after the 

gas supply was turned off it was observed that the back set 

of blocks were burning more intensely than the front set of 

blocks, most likely because they were sheltered from the ventilation air flow by the front set of crib blocks. Although the 

fire grew vertically through each crib block set, the ventilation 

air currents caused the fire to burn more intensely towards the 

backside (downwind side) of each crib block set (Fig. 5). It 

was determined that this burning condition would most likely 

cause the crib sets to topple over rather than collapse vertically. 

The fire burned unremarkably until 94 minutes into the test, 

when the rear left crib block set fell down. About 18 minutes 

later (112 minutes into the test) the rear right crib block set fell 

down. About 30 minutes later (142 minutes into the test) the 

front left crib block set fell down into the debris of the left rear 

crib block set and about 20 minutes later (162 minutes into the 

test) the front right crib block set fell down into the debris of 

the rear right crib block set. Each time a crib block set fell, the 

intensity of the fire increased for a short period of time. 

As discussed above, Test 2 was designed to serve as the 

alternate control base case for the extinguishment tests and 

involved four crib block sets that were enclosed in a steel 

framework. The fire burned unremarkably until 123 minutes 

into the test, when the fire in the left rear crib set intensified 

dramatically with flames reaching the simulated mine roof and 

subsequently engulfing the right rear crib set. Some smoke 

rollback was observed, extending towards the video camera 

location. This event lasted for about five minutes and then 

the fire reduced to about the same level observed before the 

event occurred. At about 147 minutes into the test, the front 

left crib set fell into and toppled the rear left crib set. The fire 

then burned more intensely, and about two minutes later the 

rear right crib set fell (about 149 minutes of elapsed time). 

The fire continued to burn and eventually reduced in size over 

the next 20 minutes. At about 203 minutes into the test, the 

right front crib set fell, and the fire again intensified. No other 

significant events occurred. 

A plot of the estimated heat release rate and the products 

of combustion for both tests are shown in Figs. 6a, 6b and 

6c. From the figures, it appears that Test 1 emitted a larger 

amount of heat and produced a larger amount of CO2 and less 

CO, suggesting more complete combustion of the crib blocks 

than in Test 2. Also, the gas chromatograph data (Fig. 6c) 

shows that H2, C2H2, C2H4 and C2H6 gases were observed in 

the combustion product air stream during both tests, and these 

gases are most likely the products of incomplete combustion 

and pyrolysis of the wood. During Test 1, the concentration 

of hydrogen and the hydrocarbon gases appeared to follow 

the general trend of the CO gas concentration. This same relationship was observed in the H2, C2H2, C2H4 and C2H6 gas 

concentrations measured during Test 2.

Each time a crib block set fell, the geometry of the wood 

changed, and the fire burned more intensely for a period of 

time. The change in the intensity of the fire is reflected as an 

increase in the estimated heat-release rate immediately following one of these events along with the associated increase 

in the products of combustion. The increase in fire intensity 

is most likely due to the close proximity of the fallen burning 

crib blocks and the associated heat build-up.

As planned during Test 2, the steel framework did indeed 

hold the crib sets upright for about 50 minutes longer than 

the unconfined crib block sets in Test 1. It was decided that 

framework system, though useful for keeping the crib sets 

erect, did not substantially extend the life of the fire tests and 

represented a considerable deviation from typical crib block 

configuration used in a coal mine setting. The steel framework 
system was, therefore, not used in any of 

the subsequent extinguishment tests. 

Figure 6a — Plot of estimated heat release rate and products of combustion 

for crib block Tests 1 and 2.

Crib block test (water suppression agent 

applied).This test was designed to observe 

the effects of water application on a deepseated crib set fire because water is the most 

commonly available fire suppression technology. Water is an effective suppression 

and extinguishing agent because it cools 

the wood surfaces and therefore reduces 

rate of pyrolysis. For this test, water was 

supplied from a fire truck to a fire hose that 

was equipped with an industrial plastic fog 

nozzle (the type that is typically available 

in a coal mine) that was adjusted to form 

stream flow. A total of 9.07 m3 (2,400 gal) 

of water was available from the truck for 

the test. Water was supplied through the 

fire hose to the nozzle at a rate of 4.7 L/s 

(75 gpm) at 690 kPa (100 psi).

The test was initiated in the same manner 

as the control tests. At about 41 minutes into 

the test, the fire grew to the mid-height in the 

right and left rear crib block sets, and over 

the next four minutes there were periods of 

increased intensity with smoke rollback. At 

46 minutes into the test, the right rear crib 

block set was burning intensely at the top 

with flame spread across the roof towards 

the rib areas. This condition continued for 

about two minutes and eventually included 

all crib block sets, with periods of heavy 

smoke rollback. At 49 minutes into the test, 

the fire had reduced in size and the smoke 

rollback problem had diminished. The fire 

then burned without any remarkable events 

until about 70 minutes into the test, when 

a small increase in intensity was observed 

in the lower portions of the front left and 

rear right crib block sets. This event lasted 

for about two minutes and then the fire 

reduced to about the same level observed 

before the event occurred. It was decided to 

apply the water extinguishing agent at about 

76 minutes into the test, because the fire had 

reached a steady-state burning rate. 

The individuals selected to apply the 

water to the fire were NIOSH technicians 

who had previously served as coal miners 

and had training and experience in the use of a fire hose. The 

technicians were instructed to apply the water until they observed that the fire had been extinguished. For personal safety 

reasons, the technicians were also instructed not to progress 

past a line that had been drawn on the entry floor (perpendicular 

to the trend of the entry) about 457 mm (18 in.) upwind from 

the leading edge of the crib block sets. This location was selected because it would keep the technicians out of the smoke 

and away from the heat of the fire. Water was applied to the 

fire for about four minutes when the technicians decided the 

fire had been extinguished, and a total of 1,135 L (300 gal) 

of water was used (Fig. 7). Observations of the wood showed 

that the horizontal surfaces appeared to be wet and water was 

observed to be actively dripping from the vertical surfaces. 

About four minutes after the water application was completed, 

hot spots (glowing embers) were observed in the rear left and 

rear right crib block sets. At about 52 minutes after the water 

application was stopped, small flames were observed in the 

rear left crib block set. The embers and flames eventually selfextinguished and no other events were observed. 

Figures 8a and 8b are plots of the estimated heat release 

rate and the products of combustion for this test. As observed 

in the control tests, the fire showed an increase in heat output 

and CO2 production during the intense burning event or when 

the crib block sets shifted or fell. CO production increased at 

a steady rate with only slight changes when the fire intensified or when the crib block set shifted or fell. As expected, the 

heat output of the fire and the production of combustion gases 

declined dramatically as the water was applied (refer to Figs. 

8a and 8b). As observed in the controls tests, the fire also pro-
duced H2, C2H2, C2H4 and C2H6 gases, which are considered 

to be the products of incomplete combustion and pyrolysis 

of the wood. Unfortunately, because this test was of a short 

duration and only a very limited set of gas chromatograph data 

was collected during this test, it is therefore difficult to make 

comparisons with the other gas species. 

Figure 6b — Gas chromatograph data for Test 1. Figure 6c — Gas chromatograph data for Test 2.

Figure 7 — Use of fire hose during crib block fire Test 3.

Crib block Test 4 (gas-enhanced foam suppression agent 

applied). This test was designed to observe the effects of 

a portable gas-enhanced foam fire suppression technology. 

Compressed air or nitrogen gas, when added to a mixture of 

foam concentrate and water, creates foam that can expand up 

to twenty times the original liquid volume. Foam addresses 

a fire condition through evaporation of contained water and 

through cooling by energy removal. It serves to blanket the 

combusting material and isolate it from oxygen. As the foam 

collapses, water is released and the temperature of the water 

increases by absorbing heat and eventually turns the water 

into steam. Water is released from foam either during bubble 

rupture and because this process takes time, foam can act as 

a water reservoir, releasing water at a rate that allows absorption into the fuel, rather than running off the surface (The 

Snuffer Corporation, 2006). If nitrogen gas is used to make 

the bubbles, then the resulting foam can serve to remove two 

components of the fire tetrahedron by robbing the fire of heat 

and by removing or displacing oxygen.

The system used was a Sentinel 120RS unit supplied by 

USF Equipment and Services (a research partner in this test). 

The Sentinel 120RS is a self-contained fire-fighting system 

that uses the patented Hellfighter® mixing chamber to create foam (USF Equipment and Services, 2006). The system 

used was capable of producing 9.07 m3 (2,400 gal) of gas 

enhanced foam using a mixture of water, a small amount of 

Mine Fire Fighting Foam® (MFFF) concentrate and nitrogen 

gas (Fig. 9). The system can discharge up to 18.9 L/s (300 

gpm) of gas-enhanced foam. Prior to the entry into the FSF, 

the NIOSH technicians were instructed on the proper use of 

the Sentinel unit and application of the foam product on the 

fire. As in the previous test, the technicians were instructed to 

apply the gas-enhanced foam until they observed that the fire 

had been extinguished. 

The test was initiated in the same manner as the previous 

tests. At 44 minutes into the test, the fire began to grow and 
within three minutes, the fire had substantially increased and 

was burning at the top of both the right and left rear crib sets 

with periods of severe smoke rollback. At 50 minutes into the 

test, the burners were turned off and the fire intensity subsided; 

however, the smoke rollback problem continued. At about 68 

minutes into the test, the fire intensified followed by active burning at the top of the right and left rear rib block sets. This event 

continued for about four minutes with periods of heavy smoke 

rollback. At about 71 minutes into the test, the smoke rollback 

problem became so intense that it was decided to increase the 

ventilation flow rate (refer to Table 3). Within a few minutes 

the increase in the ventilation rate cleared the smoke rollback 

problem, and it was decided to let the fire stabilize to the new 

ventilation conditions before attempting to extinguish the fire. 

At about 80 minutes into the test, the rear left crib block set 

fell followed by a brief increase in fire intensity. 

Figure 8a — Plot of estimated heat release rate and products 

of combustion for crib block Test 3.

Figure 8b — Gas chromatograph data for Test 3.

Figure 9 — USF Sentinel® unit used during crib block fire 

Test 4.

Figure 10 — Use of gas-enhanced foam during crib block 

Test 4.

Gas-enhanced foam application began 103 minutes into the 

test, and about 1 minute later, the right rear crib block set fell 

down (Fig. 10). Foam application continued for about 8 minutes 

when the technicians decided that the fire had been extinguished. 

A total of 9.07 m3 (2,400 gals) of gas-enhanced foam (the 

entire capacity of the system) was used. At about 40 minutes 
after the foam application had been completed, the foam had 

degraded to the point were it was no longer observed on the 

vertical surfaces and 120 minutes after the foam application was 

completed the foam was no longer observed on the horizontal 

surfaces (on the crib blocks or the simulated mine floor). From 

a fire extinguishing perspective, at about 63 minutes after the 

foam application was completed, a small flame was observed 

in the lower portion of the front right crib block set and this 

flame continued to burn until the completion of monitoring for 

this test. A close inspection of this crib block set area indicated 

that it was sheltered from foam application and the flame was 

most likely a result of a glowing ember. 

Figures 11a and 11b are plots of the estimated heat release 

rate and the products of combustion for this test. 

Figure 11a — Plot of estimated heat release rate and products of combustion for crib block Test 4.Figure 11b — Gas chromatograph data for Test 4.

As observed 

in the previous tests, the fire showed an increase in heat output 

and CO2 production during the intense burning event or when 

a crib block set fell. CO production increased at a steady rate 

with only slight changes when the fire intensified or when 

the crib block set fell. The heat output of the fire and the 

production of combustion gases declined dramatically when 

the foam extinguishing agent was applied (refer to Figs. 11a 

and 11b). As in the previous tests, the fire produced H2,C2H2, 

C2H4 and C2H6 gases. The change in concentration of these 

gases appeared to follow the same general trend of the CO2

gas concentration.

Crib block Test 5 (fire extinguisher agent applied). This 

test was designed to observe the effects of using fire extinguishers on a deep-seated crib block fire. Fire extinguishers 

are rated according to the extinguishing agent’s effectiveness 

in controlling one or more classes of fire. An ABC-rated fire 

extinguisher is capable of addressing A, B and C class fires 

(combustible solid materials, flammable liquids and energized 

electrical equipment) and is commonly found in underground 

coal mines. 

ABC-rated fire extinguishers contain monoammonium 

phosphate and/or ammonium sulfate powder that leaves a 

nonflammable layer of material (which melts at a low temperature) and blocks gas and heat transfer at the fuel surface. 

For this test, four 13.6 kg (30 lb) fire extinguishers were placed 

in the FSF upwind of the crib block sets. To ensure that the 

extinguishers were in proper working order, they were filled 

with ABC powder and were outfitted with new CO2 cartridges 

the day before the test.

The fire test was initiated in the same manner as the previous 

tests. Once the gas burners were turned off, the fire for some 

unknown reason did not achieve a stable condition for a long 

period of time. At about 121 minutes into the test the rear left 

crib set fell, whereupon the fire intensified with some smoke 

rollback. At about 123 minutes into the test, the rear right crib 

set shifted into the front right crib set followed by an increase 

in fire intensity as flames were observed at the roof with smoke 

rollback. Approximately 3 minutes later, the fire returned to 

about the same level observed before the event occurred. At 

128 minutes into the test, the rear right crib set fell followed 

by an increase in fire intensity shortly thereafter. Use of the 

fire extinguishers on the fire began at 161 minutes into the test 

when the estimated heat release rate of the fire had generally 

stabilized. The extinguishing work continued for about four 

minutes until the all of the available material was applied (Fig. 

12). A total of about 45 kg (100 lbs) of ABC powder was used 

on the fire. The powder was observed on the horizontal surface 
and some of the vertical surfaces. At about 15 minutes after the 

application of the extinguishing agent was completed, small 

flames were observed in the debris from the left rear crib set 

and glowing embers were observed in the lower area of the 

front left crib set. The flames and embers continued to burn 

until the end of the test. Figures 13a and 13b are plots of the 

estimated heat release rate and the products of combustion 

for this test.

As shown in Fig. 13a, the increase in heat output of the fire 

and CO2 production was related to the changing geometry of 

the wood pile as a crib block set fell. CO production increased 

at a steady rate with only slight changes when the fire intensified or when the crib block set fell. The heat output of the fire 

and the production of combustion gases declined dramatically 

as the extinguishing agent was applied. As in the previous 

tests, the fire produced H2,C2H2, C2H4 and C2H6 gases (refer 

to Fig. 13b). The change in concentration of these gases appeared to follow the same general trend as the CO2 and CO 

gas concentrations. Figure 12 — Use of fire extinguishers during crib block 

fire Test 5.

Discussion

Wood cribs are used throughout underground coal mines as 

a secondary means of roof support. A mine fire can quickly 

propagate through the timbered passageways, producing 

life-threatening hazards such as toxic fumes, smoke and heat 

(Egan and Litton, 1986). During the crib fire tests, significant 

increases in heat output and CO2 production were observed 

whenever a crib block set fell and the geometry of the burning 

wood pile changed or if an intense burning event occurred. CO 

gas production followed a gradual build-up in all of the tests 

and then declined as expected in the control tests as the fire 

reduced in size. In all of the tests, significant levels of H2 were 

detected and small concentrations of C2H2, C2H4 and C2H6

gases were observed in the combustion product air stream. 

However, because of the variety of wood species used in these 

tests and the fact that the fire within each crib set was burning 

differently at the bottom of the set as compared to the top, it is 

impossible to determine exactly where the gases originated. As 

the crib block set fires evolved, the change in gas concentration of these gases appeared to follow the same general trend 

as either CO2, CO or both gases together. 

It was observed that the intensity of a crib block set fire 

could grow quickly creating localized conditions similar to 

that seen during a flashover event. A flashover is a term that 

describes an event that can occur in when a fire is burning in a 

closed room. Combustible gases produced by the fire and that 

are not wholly consumed forms a superheated gas layer at the 

ceiling of the room that grows laterally and downward heating nearby material to their auto-ignition temperature. Given 

enough oxygen, a flashover event occurs and everything in the 

room breaks out into open flames, instantly producing a large 

amount of heat, smoke and pressure. Though the conditions 

in the FSF during the crib block set fires test were different 

that those generally observed during a structural fire flashover 

event, the sudden uncontrolled intense burning of the crib block 

sets at the roof level, followed by fire growth towards the rib 

areas with intense smoke rollback appeared to mimic that of 

a very localized flashover burning event even in the presence 

of air flow. The crib block set fire events last for only a few 

minutes and the fire did not readily consume the crib blocks. 

This is most likely because the combustion products (due to 

incomplete combustion and pyrolysis of the wood material) 

migrate upwards through the blocks and accumulate at the roof 

level (in the relatively calm air spaces) along with heat from 

the fire. The fire then grows quickly upwards through the crib 

block sets and consumes the combustion products near the 

mine roof. Once the combustion products are consumed, the 

intensity of the fire is reduced to about the same level observed 

before the event occurred.

All of the fire-extinguishing agents tested appeared to be 

capable of extinguishing a deep-seated crib block fire. Each 

extinguishing agent performed as expected when applied to a 

burning surface. The fact that none of the fire-extinguishing 

agents completely extinguished any of the fires is attributed 

to the crib block set geometry and the procedure used to fight 

the fires, rather than limitations of the fire-extinguishing 

agents. The complicated geometry of the lower portion of the 

crib block sets was needed to create the fires; however, this 

geometry also created small inconspicuous areas. These areas 

were difficult or impossible to access with fire suppression 

agents given the fact that the fires could only be fought from 

the front and to a limited degree from the sides of the crib 

block sets. Furthermore, the poultry fencing used to support 

the crib block sets also prevented the placement of the spray 

nozzles inside the crib block sets thus limiting the application 

of the extinguishing agents. 

When a crib block set fell, the geometry of the crib blocks 

became much more complicated and the NIOSH technicians 

could not directly access and separate the fallen crib blocks 

to apply the extinguishing agents. The need to separate the 

burning blocks is considered to be critically important to the 

process of completely extinguishing a crib block set fire. This 

also emphasizes the need to watch over an assumed extinguished crib block fire, as undetected and unseen hot spots 

could reignite the fire.

Results and conclusions

In this study, a series of large-scale, deep-seated wood crib fire 

tests were conducted in partnership with MSHA at the NIOSH 

Fire Suppression Facility to measure the various combustion 

products and to observe the extinguishment capability of commonly available fire suppression equipment, including dry 

powder fire extinguishers, water and gas-enhanced foam. These 

tests are part of a large-scale, deep-seated fire test program that 

is ongoing at the NIOSH Lake Lynn Laboratory. 

During the crib block set fire tests, combustion gas information was collected in an attempt to characterize the fires. 
Figure 13a — Plot of estimated heat release rate and products of combustion for crib block Test 5.

Figure 13b — Gas chromatograph data for Test 5.

In all of the tests, H2, C2H2, C2H4 and C2H6 were observed 

in various concentrations in addition to the gases that were 

expected (CO2 and CO) in the combustion gas air stream. The 

gases were most likely the product of incomplete combustion 

and pyrolysis of the wood. The presence of H2, C2H2, C2H4

and C2H6 in the crib block fire combustion product air stream 

could offer a viable explanation as to why these gases have 

also been detected at some of the mine fire sites. Analysis of 

gas samples collected during the planned deep-seated coal fire 

tests will undoubtedly provide additional insight.

All of the extinguishing agents applied to the crib block 

fires (water, gas-enhanced foam and ABC powder) appear 

to be capable of extinguishing the fires if they are placed on 

all burning surfaces. During our tests, after the extinguishing 

agents were applied, the severity of the fire was significantly 

reduced to the point where safe evacuation from the fire area 

was possible and additional fire-fighting resources could have 

been accumulated and used to watch over the debris from the 

fire, separate the blocks and extinguish any and all remaining 

“hot spots.” 

Acknowledgments

The authors recognize Brian Blashich, Melanie Calhoun, Mark 

Schroeder and Donald Sulkowski, MSHA; Alden Ozment, 

USF Equipment and Services; Richard A. Thomas, John 

Soles and Eric S. Weiss, NIOSH; and the NIOSH staff at the 

LLL for their professionalism, dedication and assistance in 

the conduct of the field work needed to complete this study. 

We would also like to recognize Thomas A. Gray, GAI 

Consultants Inc.; Mark Schroeder, MSHA; and Dr. Liming 

Yuan, NIOSH, for their expertise in providing the technical 

review of this paper.

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