PROGRESS TOWARD IMPROVED ENGINEERING OF SEALS AND SEALED AREAS OF COAL MINES

R. K. Zipf, NIOSH, Pittsburgh, PA

J. F. Brune, NIOSH, Spokane, WA

E. D. Thimons, NIOSH, Pittsburgh, PA

ABSTRACT

Recent scientific studies by researchers at the National Institute

for Occupational Safety and Health (NIOSH) and the U.S. Army Corps

of Engineers (USACE) presented the worst-case explosion pressures

that can develop if a methane-air mixture of a certain composition and

dimension accumulates and ignites within a sealed area of a mine.

The Mine Safety and Health Administration (MSHA) issued its final rule

on sealing of abandoned areas which considers potential explosion

pressures and the risk of achieving the necessary conditions. The new

MSHA seals regulation has increased the pressure design criteria an

order of magnitude beyond the old standard of 140 kPa (20 psi) to 800

kPa (120 psi). However, there still are major knowledge gaps

pertaining to seals and sealed areas including: (1) the composition of

the sealed area atmosphere and how it changes in time and space,

(2) explosion pressures that might actually develop within sealed

areas, (3) engineering procedures to follow when designing seals and

seal installations, and (4) guidelines to manage the atmosphere within

sealed areas with monitoring or inertization. NIOSH has initiated a

seals research program with collaborators from Safety in Mines

Testing and Research Station (SIMTARS) in Australia, West Virginia

University (WVU), the Naval Research Laboratory (NRL), and USACE.

Highlights of progress so far include: (1) construction of a gas

explosion tube at Lake Lynn Laboratory for studying large methane-air

explosions, (2) computer simulation of the fluid dynamics of high

pressure methane-air explosions, (3) development of seal analysis

methods considering construction materials, reinforcement and the

seal foundation, and (4) development and testing of innovative seal

designs that utilize inexpensive, readily-available rock rubble to resist

high explosion pressures. NIOSH researchers aim to provide sound

engineering guidelines to better address the new MSHA seal

regulations.

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

The Sago Mine disaster January 2, 2006 caused by an explosion

within a recently sealed area precipitated many changes to mining

regulations pertaining to seals. Mandates from the Mine Improvement

and New Emergency Response Act (the MINER Act) of June 2006

required MSHA to increase seal design pressures by the end of 2007.

Scientific studies of gas explosions within sealed areas provided a

basis for the new MSHA regulations on sealing of abandoned areas

(Federal Register, 2008). However, the behavior of the sealed area

atmosphere and possible explosions within sealed areas is still fraught

with uncertainties. The new regulations have moved the mining

community to use engineering methods to design seals; however,

professional engineers lack accepted guidelines to follow for seal

design.

This paper will summarize the following:

1. New scientific knowledge and new regulations for sealed

areas.

2. Knowledge gaps with seals and sealed areas of coal mines.

3. Research in progress to address these issues.

NEW SCIENTIFIC KNOWLEDGE AND NEW REGULATIONS OF

SEALED AREAS

Upon enactment of the MINER Act, NIOSH researchers

conducted scientific studies (Zipf et al., 2007) of methane-air

explosions within sealed areas of coal mines by first considering the

formation of potentially explosive gas mixtures that can develop in

sealed areas upon sealing. Starting with an atmosphere that is pure

air, the methane concentration may increase; the oxygen concentration

may decrease; or some combination of these changes may occur.

Figure 1, which plots oxygen concentration versus methane

concentration, shows the various paths that an atmosphere may take

toward some later sealed area atmosphere composition, beginning

with normal air containing 21% oxygen and 0% methane and ending

with lower oxygen and higher methane concentrations. Figure 1 also

shows “Coward’s triangle” where mixtures of oxygen and methane can

ignite. As methane concentration increases and oxygen concentration

decreases, the sealed area atmosphere may become explosive for

some time while the average methane concentration is between about

5 to 16% and the oxygen concentration is greater than 12% as shown

by paths A to B or A to C. In some cases, the sealed area atmosphere

could become inert and never cross through the explosive range as

shown by path A to D. If a mine pumps inert gas into a sealed area,

the path toward a fuel-rich inert atmosphere may follow a path similar

to A to E to F, and also never cross through the explosive range. The

composition path followed by any particular sealed area atmosphere is

most likely unique for each particular coal mine. Unfortunately, a

methane-rich, oxygen-lean inert atmosphere may not necessarily

remain inert. Such an inert sealed area atmosphere can become

explosive again if air leaks into the sealed area through seals.

Changes in atmospheric pressure caused by normal diurnal variation

or passing high pressure weather systems can cause air to leak into a

sealed area. Pumping methane from within sealed areas can also

induce air leakage into the sealed area, which could then create a

potentially explosive atmosphere behind seals.

Based on thermodynamics, chemistry, and physics, NIOSH

researchers conducted a worst-case analysis of methane-air

explosions within the sealed areas of coal mines and presented

several important facts about possible explosions within sealed areas:

1. Combustion of stoichiometric (about 10%) methane-air mix

in a closed volume increases the pressure about 807 kPa

(117 psig). This pressure is called the constant volume (CV)

explosion overpressure. The CV explosion overpressure is

greatest for a stoichiometric mix and decreases for fuel-rich,

fuel-lean, oxygen-deficient, or carbon dioxide-rich mixtures.

2. Combustion of coal dust in air in a closed volume produces a

somewhat lower CV explosion overpressure of about 690 to

790 kPa (100 to 114 psig).

3. Due to dynamic effects, explosions in tunnels produce

transient pressure waves that are greater than the CV

overpressure.

4. When a blast-created shock wave with a quasi-static

overpressure impacts a structure, it reflects with a transient
reflected wave overpressure that is 2 to 8 times greater than

the incident quasi-static overpressure.

5. If detonation of a methane-air mix develops, the maximum

detonation wave overpressure for is 1.66 MPa (241 psig).

When a detonation wave impacts a structure, it reflects at a

pressure of about 4.40 MPa (640 psig) which is about 2.54

times greater than the incident detonation wave pressure.

Figure 1. Coward triangle (Coward and Jones, 1952) showing

explosive zone for methane in air mixtures and different paths toward

an inert atmosphere.

The NIOSH report (Zipf et al., 2007) also presented simple

numerical model calculations of explosion pressures within mine

tunnels. Using the gas explosion models AutoReaGas and FLACS,

which are commonly applied throughout the oil, gas, and chemical

industries, researchers calculated explosion pressure at a seal. Figure

2 shows the simple mine layout and the calculated explosion pressure.

The 160-m-long (525-ft-long) gas cloud that filled 3 entries and the

cross-cuts developed explosion pressure ranging from about 2.4 to 3.3

MPa (350 to 480 psi). Independent calculations by explosion experts

at USACE also produced explosion pressures of similar magnitude

(McMahon et al., 2007). USACE engineers used a sophisticated

computational fluid dynamics program called SAGE (SAIC Adaptive

Gridding Eulerian) hydrocode (Gittings et al., 2005) to compute

possible explosion pressures that developed during the explosion at

Sago Mine. In the first simulation with infinitely strong seals and a

homogeneous, near stoichiometric methane-air mixture, the USACE

calculations produced reflected explosion pressures at the seals of 8.8

MPa (1,300 psig). In the second simulation with seal failure at 140 kPa

(20 psig) and the same mixture, the calculated reflected pressure at

the seals was 5 MPa (700 psig). In the third simulation where the

sealed area contained a layered methane-air mixture that was inert

near the seals, the calculated reflected pressure at the seals was 1.65

MPa (240 psig).

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Figure 2. Calculated gas explosion pressure from numerical models at

a seal from a 160-m-long gas cloud. The model gas cloud geometry is

shown above. (Zipf et al., 2007).

To demonstrate the possibility of the high explosion pressures,

recent experiments by NIOSH researchers at the Lake Lynn

Experimental Mine produced such pressures from very small explosive

gas clouds (Sapko et al., 2009). In one experiment as shown in Figure

3, a methane and coal dust cloud with an effective length of 38-m-long

(125-ft-long) developed an incident quasi-static blast wave pressure of

324 kPa (47 psig) which then produced a reflected explosion

overpressure of about 1.124 MPa (163 psig) on an experimental

structure.

NIOSH researchers (Zipf et al., 2007) then presented a threetiered recommendation for explosion pressure design criteria for seals

as shown in Figure 4. Application of these criteria depends on the

monitoring regimen applied. If the sealed area is monitored

continuously during and after sealing and if the potential size of

explosive mixture is limited to a less-than-5-m-long space (about 15 ft)

right behind a seal, then a 345 kPa (50 psi) explosion pressure-time

curve applies. However, if the sealed area atmosphere is not

monitored, then much larger explosive gas volumes and much higher

explosion pressures can develop. If the open entry behind the seal is

small with a length less than 50 m (about 150 feet), then the 800 kPa

(120 psi) pressure-time curve applies. If the open entry behind the

seal is large with a length more than 50 m (150 ft), then the 4.4 MPa

(640 psi) explosion pressure-time curve applies. Note that it is not

necessary for an explosive methane-air mixture to detonate to achieve

high explosion pressure. As shown in Figure 3, non-reactive blast

waves from ordinary deflagrations can easily develop reflected

pressures greater than 1 MPa (145 psi) as the experiments at LLEM

have demonstrated for gas clouds as small as 26-m-long (85-ft-long).

Based in part on the analyses presented by NIOSH researchers

and an assessment of the risk of developing high explosion pressure

within sealed areas, MSHA issued its final rule on sealing of

abandoned areas. According to the new regulation, the design

explosion pressure for seals also has three tiers:

 At least 50-psi overpressure when the atmosphere in the

sealed area is monitored and maintained inert.
 Overpressures of at least 120 psi if the atmosphere in the

sealed area is not monitored and is not maintained inert.

 Overpressures greater than 120 psi if the atmosphere in the

sealed area is not monitored and is not maintained inert and

other conditions exist which could lead to higher explosion

pressures such as likelihood of detonation or pressure piling.

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Reflected explosion pressure

Figure 3. Measured quasi-static blast waves and reflected explosion

pressure on a structure from a gas cloud equivalent to a 38-m-long

(125 ft) methane-air mixture (Sapko et al., 2009).

Figure 4. Design criteria for various sealing conditions (Zipf et al..

2007).

Seal design applications must address the design pressure-time

curve, engineering design and analysis, material properties,

construction specifications, quality control, and other considerations.

New seal designs and their construction require certification by

professional engineers and the local mine management for quality

control.

The new MSHA regulation on seals has increased the pressure

design criteria for seals an order of magnitude beyond the old 140 kPa

(20 psi) standard. Prior to the Sago Mine disaster, the mining

community relied upon a “build and test” approach (Gadde et al.,

2007A) to develop seal designs to meet the old standard; however, to

meet the new standard, the mining community has used engineering

methods under the auspices of a professional engineer to analyze and

design seals. In addition, the new standard requires quality control

and accountability throughout the life of the seal from design through

construction and operation, again under the supervision of professional

engineers and the mine management.

KNOWLEDGE GAPS WITH SEALS AND SEALED AREAS OF

COAL MINES

The recent NIOSH study (Zipf et al., 2007) coupled with the

USACE report (McMahon et al., 2007) provided complementary,

independent analyses of worst-case explosion pressures that could

develop in sealed areas of coal mines. The discussions in the mining

community ensuing from these reports and comments on the new

seals regulation identified numerous unknowns and knowledge gaps

with seals and the sealed areas of coal mines. NIOSH researchers

categorize these knowledge gaps into three main themes:

1. Science of sealed areas including the sealed area

atmosphere composition, explosion processes, and the

explosion pressures that could develop.

2. Seal engineering including failure mechanisms of seals,

analysis and design of seals, and engineering methods to

account for the seal foundation and convergence in seal

design.

3. Management of sealed areas and their atmospheres

including ways to plan mines for future sealing along with

monitoring and inertization techniques for sealed areas.

The composition of the sealed area atmosphere is not well

understood. Prior to the Sago Mine disaster, few researchers had

measured the composition of the atmosphere within sealed areas. The

U.S. mining community erroneously assumed that the sealed area

atmosphere would become inert rapidly upon sealing and then

remained inert. In comments regarding the MSHA ETS on “Sealing of

Abandoned Areas,” Gadde et al. (2007B, 2009) presented the first

composition data from thousands of sealed area atmosphere samples

from a few mines. More than 99% of the samples were inert and less

than 1% were potentially explosive with oxygen concentration above

10% and methane concentration between 8 and 12%. Thus, it

appears that the probability of encountering a potentially explosive

atmosphere within a sealed area is low. However, the 12 documented

explosions within sealed areas that occurred between 1986 and 2006

demonstrate the inadequacy of assuming an inert atmosphere.

To comply with the new regulations on sealed areas, mining

companies and MSHA now sample the atmosphere behind seals on a

regular basis. Questions remain concerning 1) whether potentially

explosive gas mixtures really do exist within sealed areas, 2) how

extensive such mixtures might be, 3) how the composition changes

over time, 4) whether methane layering exists within sealed areas, and

5) how homogeneous the atmosphere is within extensive sealed areas.

Answers to these questions may in turn answer questions concerning

the adequacy of sampling techniques and sampling frequencies for

sealed area atmospheres. Understanding the sealed area

atmosphere’s composition both spatially and temporally is a key

component in assessing the risk of explosion behind seals.

Explosion processes within sealed areas are not well understood.

Some question whether the high explosion pressures presented in the

recent NIOSH and USACE studies could ever occur in a mine. Others

question whether detonation of methane-air mixtures is a physical

possibility. The process by which a weak spark ignition grows from a

laminar flame to a deflagration and then possibly a detonation is not

well understood for methane-air. Whether deflagration-to-detonation
transition (DDT) can occur in methane-air mixtures and whether the

process can occur in a mine requires further study. Only two

laboratory-scale studies exist in which researchers achieved

detonation of methane-air (Bartknecht, 1993; Kuznetzov et al., 2002).

Experimental apparatus of insufficient size may have hampered these

experiments from developing and sustaining a true detonation. No

experiments in full-scale mine tunnels have ever produced detonation

beginning from a weak spark. Prior experiments to produce detonation

initiated the detonation directly with a small quantity of high explosive

and therefore do not represent realistic in-mine conditions. The largest

experimental explosions at NIOSH’s Lake Lynn Experimental Mine

have only used limited quantities of methane-air mixture with an

equivalent length of less than 26 m (85 ft). Such small lengths and

quantities of methane-air may not be sufficient to develop DDT.

Numerical gas explosion model calculations by researchers at USACE

(McMahon et al., 2007) indicate the possibility of methane-air

detonation. Finally, MSHA investigators of the Blacksville No. 1

explosion in 1992 (Rutherford et al., 1993) back-calculated explosion

pressure of about 6.9 MPa (1,000 psi) which also suggests the

possibility of detonation. However, Gadde et al., (2007B) presented a

dynamic structural analysis using sophisticated numerical analysis

methods and concluded that the back-calculated explosion pressure is

much less than that reported by MSHA. Additional research should reexamine this failure and implied explosion pressure using the best

available material properties and construction details for the shaft

collar.

Guidelines for seal engineering require extensive development.

As mentioned earlier, prior to 2006, the mining industry used a “build

and test” approach for seal design, but now the community must follow

engineering methods to design seals in a manner similar to building or

dam design. NIOSH researchers recommended a four-phase

approach to seal design: (1) information gathering to locate appropriate

seal sites and assess their foundation, convergence, and air leakage

characteristics, (2) seal engineering to choose the appropriate design

pressure-time curve, analyze the seal design, and specify all

dimensions and construction materials, (3) seal construction with

quality control, and (4) post sealing inspections to ensure continued

seal integrity and performance within the assumed design conditions

(Zipf et al., 2007). The MSHA final rule on seals provides two separate

design methods to apply for approval of a seal design: (1) an

engineering design application and (2) an application based on fullscale explosion tests. A testing-based application could utilize fullscale, hydrostatic tests of seals to produce pressure loads on the seal

equivalent to the required design pressure-time curve (Sapko et al.,

2008). An engineering design application shall “address gas sampling

pipes, water drainage systems, methods to reduce air leakage,

pressure-time curve, fire resistance characteristics, flame spread

index, entry size, engineering design and analysis, elasticity of design,

material properties, construction specifications, quality control, design

references, and other information related to seal construction” (Federal

Register, 2008). The mining community needs engineering guidelines

for professional engineers to follow during the design, evaluation, and

approval of mine seals.

Additional guidelines to manage sealed areas also require

development. Often, mine sealing plans are developed long after the

initial mine plans. Lack of adequate pre-planning may result in

complex seal lines, convergence, and leakage-prone sealed areas,

and more seals used than necessary. Better pre-planning of sealed

areas could eliminate many safety hazards and explosion dangers.

The NIOSH seals report and the MSHA final rule on sealing of

abandoned areas provided an option for 345 kPa (50 psi) seals as long

as monitoring of the sealed area showed that it remains inert at all

times after sealing. Use of this seal design option may necessitate

artificial inertization of the sealed area immediately after sealing and

periodically thereafter. Continuous monitoring of sealed areas requires

the use of sample-draw systems, for example tube-bundle systems to

sample and analyze the atmosphere within sealed areas. Such

monitoring systems and inertization systems are routinely applied at

virtually all underground coal mines in Australia. NIOSH researchers

plan to demonstrate these critical technologies to facilitate their

possible adoption by the U.S. coal mining industry.

NIOSH RESEARCH PROGRESS TO ADDRESS SEALING ISSUES

Composition of Sealed Area Atmosphere

Understanding the composition of the sealed area atmosphere

and how it changes across the sealed area and over time is important

for understanding the blast pressure that could develop from an

explosion and assessing the risk associated with this danger. NIOSH

researchers have acquired from SIMTARS in Australia a monitoring

system that is capable of measuring the composition of the sealed

area atmosphere and tracking its evolution continuously over time.

NIOSH researchers seek an appropriate coal mine with sealed areas

in which to deploy this system for research and technology

demonstration purposes. NIOSH researchers would place sampling

tubes throughout an abandoned area prior to seal construction and

final sealing.

Figure 5 shows several photographs of a tube-bundle system for

continuous monitoring of a mine atmosphere. Systems similar to the

one shown are deployed in virtually all underground coal mines in

Australia. The top left of Figure 5 shows a typical monitoring shed

located on the surface above a mine. The monitoring tubes enter the

mine via a borehole to the left of the shed. Typical tube-bundle

installations will monitor from 20 to 40 points or more, with about half

located in the active mining areas and the other half in the sealed

areas. The top right of Figure 5 shows a close-up of a seven-tube

bundle. The pumps shown in the bottom right of Figure 5 draw air

samples continuously from each monitoring point. Left of the pumps is

where the sample tubes enter the shed for analysis. Inside the shed is

a solenoid-valve-manifold system activated by a programmable logic

controller. The bottom left of Figure 5 shows the on-line gas analyzers,

programmable logic controller, and some of the sample tubes inside

the monitoring shed. Samples are sequentially directed to an on-line

gas analyzer and analyzed for carbon monoxide, carbon dioxide,

methane, and oxygen. It is assumed that nitrogen and argon comprise

the balance. A typical tube-bundle system provides a gas analysis at

each monitoring point every 1-3 hr or at intervals that can be

individually programmed for each point. Real-time data are displayed

at the mine’s control center, where trained operators can respond as

necessary.

Figure 5. Continuous atmospheric gas monitoring system used in

Australia. Top left: Monitoring shed over mine showing borehole and

sample tubes. Top right: Close-up of sample tube bundle. Bottom right:

Sample tube pumps. Bottom left: Inside monitoring shed showing gas

analyzers and controller (Zipf et al., 2007).

Figure 6 shows the layout of a longwall mine with several active

mining faces, several mined-out and sealed panels, and the sample

point locations for the tube-bundle system. Sample points are typically

located in both intake and return airways of each working section.
Each set of seals usually has one monitoring point about 5 m (16 ft)

behind the seal within the sealed area.

Figure 6. Typical longwall mine layout showing sample points for tube

bundle monitoring systems throughout the active mine and within

sealed areas.

Green–sample points in intake airways

Red–sample points in return airways

Black–sample points behind seals within the sealed area.

Continuous monitoring of the mine atmosphere both in the active

mine and within sealed areas enables mine operators to identify

potentially dangerous trends early enough to take proactive safety

measures if necessary. The SIMTARS continuous monitoring software

can display compositional trends on a Coward diagram as shown in

Figure 1 or on an Ellicott diagram as shown in Figure 7. The Ellicott

diagram is a modification of the Coward triangle where the X axis

represents relative fuel concentration and the Y axis represents

relative oxygen concentration. The Ellicott diagram has four quadrants

to represent mixtures of methane, oxygen, and inert gases – (1) nonexplosive, (2) fuel-lean potentially explosive, (3) fuel-rich potentially

explosive, and (4) explosive. Upon sealing, when the methane

concentration begins to increase, the trend may look similar to curve

“A” on Figure 7. If a seal leaks air into a methane-rich atmosphere, the

trend can look similar to curve “B”. Plotting these trends also enables

mine safety personnel to estimate when a dangerous composition may

form in a sealed area.

Understanding the process by which ignition of a methane-air

mixture with a weak spark grows into a deflagration or detonation is

crucial for understanding the explosion pressures that could develop

and, in turn, for estimating the potential blast loads that could impact

seals. NIOSH researchers are collaborating with researchers at the

NRL to calculate explosion pressures from methane-air mixtures under

various conditions and to design full-scale verification experiments

(Oran and Boris, 2001; Oran and Gamezo, 2007).

To simulate high pressure methane-air explosions in coal mine

entries, NIOSH researchers have constructed a gas explosion tube

with a diameter of 1048 mm (42 in) and a length of 73 m (240 ft) as

shown in Figure 8 (Zipf et al., 2008A). This tube is almost twice the

diameter of a tube used for similar experiments by Kuznetsov, et al.,

(2002), and it should have sufficient diameter to support a true

detonation of methane-air. The objectives for experiments with this

tube are (1) to measure maximum explosion pressures for various

mixtures of methane, air and inert gases, (2) to measure the distance

required to accelerate a flame, achieve high pressure and develop

possible detonations, and (3) to validate numerical gas explosion

models of these processes. In addition to varying the explosive gas

mixture composition, the experimental program will vary the number of

turbulence-generating obstacles in the tube along with the blockage

ratio.

Potentially Explosive

Zone (fuel-lean)

Explosive Zone

Non-Explosive

Zone

Potentially Explosive

Zone (fuel-rich)

+Y

-X +X

The Ellicott Diagram

-Y

Path A

Path B

Figure 7. The Ellicott diagram divides mixtures of methane, oxygen

and inert gases into four zones – non-explosive, potentially explosive

(fuel-lean), potentially explosive (fuel-rich) and explosive. Measured

compositions when plotted on the Ellicott diagram can show potentially

dangerous trends.

Figure 8. 1,048-mm-diameter (42 in) gas explosion tube at NIOSH

Lake Lynn Laboratory.

NRL researchers have recently simulated deflagration-todetonation transition (DDT) of methane-air mixtures using state-of-theart reactive flow programs (Kessler, et al., 2008). The NRL model

solves numerically the Navier-Stokes equations for fluid dynamics

using an explicit, second-order finite difference scheme and

considering the viscosity, diffusion, and thermal conductivity of

methane-air and the reaction products. To calculate turbulence and

shock waves in the fluid, the model uses an adaptive mesh refinement

scheme. A single-step Arrhenius burn model is used to describe the

kinetics of energy release. Based on fundamental physics and

chemistry, the NRL reactive flow model calculates laminar flame
properties such as the flame speed and detonation properties such as

the detonation wave speed that agree well with experimental

measurements of the same. Figure 9 compares calculated flame

velocity to measured flame velocity from experiments using two

different tube sizes. Flame velocity increases with distance from the

ignition point. These calculations based on first principles agree well

with experimental determinations.

Figure 9. Calculations of pressure, temperature and turbulent flame

speed for reacting mixtures of methane-air in channels agree with

experimental measurements. Top – 174-mm-diameter tube, 12-mlong. Bottom – 520-mm-diameter tube, 34-m-long (Kessler et al,

2008). Note – The sound speed in hot combustion products is about

1,000 m/s. Flame velocity greater than 1,000 m/s indicates possible

detonation.

Figure 10 shows calculations of a turbulent flame, shock wave

development, and the initiation of a detonation in methane-air. The

first two frames show the turbulent flame front and a shock wave

traveling at the local sound speed ahead of the flame. When this

leading shock wave impacts an obstacle (the second baffle), it ignites a

detonation as shown beginning in the third frame. In frames 4, 5, and

6, the detonation front travels supersonically and rapidly overtakes the

leading shock wave. Beginning in frame 7, the reaction continues as a

detonation. Thus, the subsonic turbulent flame or subsonic

deflagration has become a detonation in the process known as DDT.

The fundamental calculations shown in Figure 10 also predict correctly

the approximate distance required to develop detonation from a weak

ignition source. Calculations of this caliber when coupled to sound

experiments create a deep understanding of the process leading to

DDT in methane-air and the factors controlling the behavior.

Figure 10. Calculations based on fundamental physics correctly

predict distances to deflagration-to-detonation transition for

stoichiometric methane-air mixtures in channels with obstacles

[Kessler et al. 2008] Blue represents unburned mixture; the line

between green and blue is a shock wave, and the line between yellow

and blue is a detonation wave.

Seal Engineering

Developing engineering procedures for the mining community to

use for new seal design is another key component of on-going

research. NIOSH researchers recommended a four-phase approach

to seal design and MSHA’s final rule on seals provides the framework

for engineered seal design applications. However, the details require

additional development in order to produce engineering guidelines.

NIOSH researchers are collaborating with researchers at USACE and

WVU to produce these guidelines. Major efforts to date include: (1)

cataloging existing 20 psi seal test data, (2) developing basic seal

analysis methods, (3) developing seal foundation analysis methods,

and (4) developing simple, cost-effective seal designs to resist high

explosion pressures.

Compendium of Seal Test Data

Prior to 2006, NIOSH researchers conducted full-scale explosion

tests at Lake Lynn Experimental Mine (LLEM) on a wide variety of old

140 kPa (20 psi) seal designs. The structural response and failure

data obtained from these tests enable calibration and verification of

numerical models of seal behavior at the 140 kPa (20 psi) design level,

which then enables more reliable structural analysis of seal designs

that meet the new explosion pressure design criteria of 345 kPa (50

psi) and 800 kPa (120 psi).

NIOSH researchers have organized the seal testing data into 6

broad categories of seal structures:

1. Concrete-like materials with steel reinforcement and

reinforcement bar anchorage to rock.

2. Pumpable cementitious materials of varying compressive

strength with no steel reinforcement and no hitching to the

surrounding rock.
3. Articulated structures such as solid concrete block seals and

ventilation stoppings made of solid and hollow-core concrete

blocks.

4. Composite polymer and aggregate materials without hitching

to the surrounding rock.

5. Wood crib block seals with and without hitching.

6. Articulated structures such as lightweight cementitious

blocks with and without hitching.

The NIOSH compendium of 20 psi seals test data presents the

applied pressure-time curve and the measured displacement-time

response from over 100 explosion tests on 52 distinct seal structures

(Zipf, et al., 2008B).

Figures 11 and 12 provide examples of how this seal test data is

used to verify structural analysis models of seal response. Figure 11

from researchers at WVU shows the measured and calculated

displacement response of a Category 1 seal constructed of 0.3-m-thick

(12 in) reinforced concrete and subjected to a pressure-time curve with

a peak pressure of about 400 kPa (58 psi). Calculated peak

displacement using the finite element method program ABAQUS is

2.54 mm (0.10 in) compared to a measured peak displacement of 2.03

mm (0.08 in). Note that the measurements only capture displacement

motion up to the peak and are not able to record subsequent vibration

of the structure due to the nature of the instrument deployed. Figure

12 from researchers at USACE shows measured and calculated

displacement response for a Category 2 seal made from 0.75-m-thick

(2.5 ft) pumpable cementitious material. Peak pressure of the applied

pressure-time curve is 198 kPa (28.7 psi). The displacement response

is calculated with the Wall Analysis Code (WAC) which is a singledegree-of-freedom, equation-of-motion program with pre-defined

resistance functions for different structures. Measured peak

displacement response is 54.4 mm (2.14 in), and calculated

displacement response is 55.9 mm (2.20 in).

0

0.5

1

1.5

2

2.5

3

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Time - seconds

Displacement - mm

0

0.02

0.04

0.06

0.08

0.1

Displacement - inch

Measured displacement

Calculated displacement

Figure 11. Calibration of seal analysis methods to seal tests at Lake

Lynn Experimental Mine. Measured displacement of a reinforced

concrete seal compared to calculated displacement using the finite

element program ABAQUS. [Peng et al. 2008].

Further analysis of the structural data presented in the

compendium of 20 psi seal test data will produce additional validations

of different structural analysis methods for various types of seals.

Additional analyses of this data may also produce generalized failure

criteria for different seal types.

Seal Analysis Methods

USACE researchers recognize three distinct analysis methods for

seals: (1) bending beams and plates shown in Figure 13, (2) shear

plugs shown in Figure 14, and (3) arching shown in Figure 15. A

bending-beam analysis applies when the thickness-to-span ratio for

the structure is less than 4. The failure mode for a bending structure

varies, but may involve tensile failure of the outer fibers opposite the

applied load, compressive failure on the same side as the applied load

or shear failure near the supports. All three failure possibilities require

analysis and design consideration. As shown in Figure 13, the quality

of the surrounding foundation rock influences the behavior of seal

structures in bending. Seal structures located in weak foundations

showed significant damage around the perimeter necessitating the use

of engineered foundations. Seal structures located in competent

foundation rocks survived without engineered foundations.

0

10

20

30

40

50

60

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time - seconds

Displacement - mm

0

0.5

1

1.5

2

Displacement - inches

Calculated displacement

Measured displacement

Figure 12. Calibration of seal analysis methods to seal tests at Lake

Lynn Experimental Mine. Measured displacement of a pumpable

cementitious material seal compared to calculated displacement using

WAC from the USACE (Walker et al. 2009).

Weathered Shale

Roof and Floor

Loading

Direction

Symmetry

Plane

Foundation

Failure

Figure 13. Seal analysis as a bending beam or plate. Red color

indicates foundation failure; blue color indicates no foundation failure.

Weaker foundation materials show significant damage and may require

the use of extended seal foundations (O’Daniel et al., 2009).

Shear plug analysis applies when the thickness-to-height ratio for

the seal is greater than 1. The failure mode is either via shear failure

through the seal material or through the surrounding foundation rock.

Figure 14 shows two shear plug analyses. In stronger foundations, the

blast side of the shear plug displaces about 125 mm (5 in), but

otherwise, the plug survives with little shear damage. However, in

weaker foundations, failure occurs around the plug and the entire plug

moves. The plug analysis shown in Figure 14 assumes weak

foundation material on all four edges of the seal, which is not realistic

in practice. Future analyses will consider weak material of finite depth

in the floor only, which may be representative in general. However,

seal structures located on weak foundations may require engineered

foundations to survive the required design explosion pressures.

Note that for thickness-to-span ratio between 4 and 1, the failure

mode may become a complex combination of bending and shear

failure. Again, all failure modes require analysis and consideration in

proper seal design.

Arching analysis applies to articulated structures where the

thickness-to-span ratio ranges from about 2 to 5. As shown in Figure

15, the arching failure mechanism is via compressive failure of the seal

material at the supports in the outer fibers opposite the applied load

and also at the mid-span on the same side as the applied load.

Development of the ideal arching mechanism requires an infinitely stiff

or rigid foundation. USACE researchers investigated the effect of
weaker foundations on the arching failure mechanism. Weak

foundations prevent the development of arching, and average

foundation conditions in a coal mine may only develop limited arching.

Seal designers should not use an arching analysis to determine the

load bearing capacity of a seal since; in general, the method tends to

overestimate the strength of a seal and is therefore not conservative.

However, if the soft foundation rock is excavated to competent

material, then an arching analysis could apply.

Loading

Direction

Limited shear damage

With strong foundation

Global failure around plug

with weak foundation

Loading

Direction

Figure 14. Seal analysis as a shear plug. In weaker foundation

materials, failure occurs around the gob plug and the entire plug

moves. In stronger foundation materials as shown, blast side

displacement of about 5 inches occurs. The plug survives with limited

shear damage (O’Daniel et al., 2009).

Pi ttsburgh

Coal

Articulated

seal structure

Roof Rock

Floor Rock

Loading

Direction

Roof Rock

Articulated

seal structure

Loading

Direction

Floor Ro ck

No arching in

weak foundations

Limi ted arching in

Strong foundations

Figure 15. Arching analysis of articulated seal structures. Weaker

foundations prevent development of arching. Arching cannot be

assumed to determine load capacity of a seal (O’Daniel et al., 2009).

The prior examples illustrate the different seal failure mechanisms

involving bending, shearing, or arching. When using analysis tools,

seal designers need failure criteria in order to assess the adequacy of

a structure. Failure criteria options include maximum stress, allowable

displacement, tensile cracking, and others. Researchers at WVU

presented a method to assess overall failure damage in a structure

based on a “weighted average damage factor” which is a single value

describing the overall damage in a seal. Based on finite element

analysis, the damage factor for an element varies from 0 to 1,

depending on the level of plastic strain within the element. A damage

factor of 0 indicates no failure, and a damage factor of 1 indicates

complete failure. The weighted average damage factor (WADF) is

then computed as:

(element volume  element damage factor)

WADF 

(element volume)

As shown in Figure 16, the WADF provides a means to compare

quantitatively different seal designs. A 30-cm-thick (12 in) seal has

about 44% damage and has failed, whereas a 50-cm-thick (21 in) seal

has minimal damage of about 5%. Figure 16 also indicates that little

reduction in damage occurs in going from a 50 to 75-cm-thick (21 to 30

in) seal for this particular design.

The WADF expresses structural damage as a single value, and

that value may or may not indicate overall structural failure, since the

WADF does not consider the location of the damage. Conceivably, a

structure with low WADF value could fail if all the damage were

concentrated around the seal perimeter for example. Evaluation of the

integrity of a structure requires consideration of stress and

displacement failure criteria to fully assess integrity.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 10 20 30 40 50 60 70 80

Seal thickness - centimeter

Weighted average damage factor

WADF

Figure 16. Weighted average damage factor for seals of different

thickness (Peng et al., 2008).

Seal Foundation Analysis

Just as a seal structure requires analysis and design, a seal

foundation requires similar engineering consideration. While the seal

structure must resist the internal stresses induced by the design

explosion pressure-time curve, the seal foundation must adequately

anchor the seal to the surrounding rock. In effect, the seal structure

must remain connected to the seal foundation during the explosion

loading. Proper seal foundation engineering should consider the

quality of the surrounding rock mass and its strength. The seal

foundation design must specify the depth of any hitching or excavation

into the surrounding rock and the length and size of any rock bolts or

reinforcement bars.

Using well-accepted foundation analysis methods, USACE

researchers have developed preliminary bearing capacity

recommendations for seal foundation design. As shown in Table 1, the

bearing capacities of the seal foundation are related to the

compressive strength of the foundation rock whose quality is described

by the Coal Mine Roof Rating (CMRR) which is an accepted method to

quantitatively describe the quality of coal mine rocks. From these

recommended bearing capacities, seal designers can engineer the

required seal foundation including depth of hitching and reinforcement.

Innovative Seal Designs

NIOSH researchers in collaboration with USACE and the West

Virginia Office of Miner’s Health Safety and Training (WVOMHST)

have developed several concepts that should provide simple, costeffective seal designs to resist the explosion pressures specified in the

new MSHA final rule on seals or even higher worst-case explosion

pressures. Figure 17 shows a Gob Seal with Load Collectors concept
developed by military experts at the USACE. This seal uses drystacked, large concrete block “load collectors” to compress a gob pile.

Upon compression from an explosion loading, the gob pile expands

laterally and locks into the surrounding rock. Preliminary analyses by

USACE researchers showed that a 2.4-m-high (8 ft), 5.6-m-thick (18 ft)

gob pile with the load collectors could resist the 800 kPa (120 psi)

design pressure. Constructing this seal would require about 100 to

150 tons of gob, which could be placed in several work hours using an

underground mine scoop.

Table1. Foundation analysis and design for seals. Bearing capacity

design applies to seal foundations. The required depth of hitching

depends on the bearing capacity of the rock which depends on the

rock quality (Walker et al., 2009).

Material

Name Description CMRR Lab UCS

(MPa)

Bearing

Capacity

qu (MPa)

Soil 1 Paste N.A. 0.04 0.020

Soil 2 Very soft soil N.A. 0.07 0.041

Soil 3 Soft soil N.A. 0.14 0.081

Soil 4 Firm soil N.A. 0.29 0.160

Soil 5 Stiff soil N.A. 0.63 0.349

Soil 6 Very stiff soil < 30 – very weak

rock 3.6 2.008

Rock 1 Claystone, fireclay 31 – weak rock 6.4 3.558

Rock 2 Black shale 33 – weak rock 11 6.043

Rock 3 Black shale, gray

shale 38 – weak rock 18 10.163

Rock 4 Gray shale 47 – average rock 25 14.127

Rock 5 Siltstone, gray

shale 56 – average rock 34 19.204

Rock 6 Siltstone 65 – average rock 48 26.628

Rock 7 Siltstone,

sandstone 72 – strong rock 63 34.641

Rock 8 Sandstone,

limestone 78 – strong rock 77 43.297

Rock 9 Sandstone 82 – very strong

rock 95 52.660

Rock 10 Limestone 86 – very strong

rock 139 78.504

Coal 1 Banded, bright coal N.A. 3.6 2.037

Coal 2 Banded coal N.A. 6.3 3.464

Coal 3 Banded, dull coal N.A. 12 6.716

Coal 4 Dull coal N.A. 17 9.742

Roof Rock

Floor Rock

Loading

Direction

Gob Pile

Block Load Collectors

Figure 17. Innovative seal designs for the mining industry showing

schematic side view of gob seal with block load collectors. This design

from the USACE uses concrete-block “load collectors” to compress a

gob pile when loaded by an explosion. Expansion of the gob transfers

the blast load to the surrounding rock. (NO SCALE) (Walker et al.,

2009).

Figure 18 shows the Sapko-Hieb Mine Blast Attenuator concept

advanced by the WVOMHST. NIOSH researchers (Sapko et al., 2009)

constructed a prototype attenuator at LLEM by filling an entry with

broken rock as close as possible to the mine roof. The attenuator

measured about 6-m-long (20 ft) near the roof line and about 12-mlong (40 ft) at the floor. A test explosion subjected the Mine Blast

Attenuator to an upstream or inby, quasi-static pressure of 324 kPa (47

psig) as shown by the test data in Figure 18. Downstream or outby of

the attenuator, the test data showed that the pressure was less than

4.8 kPa (0.7 psig). This initial test of the Mine Blast Attenuator concept

shows that simple constructs using inexpensive, readily-available

broken rock can reduce downstream blast pressures by a factor of 10

or more. NIOSH researchers will continue work developing similar

innovative seal designs.

30-ft. 25 20 15 10 5 0

B Limestone "floor gob"

ROOF

FLOOR

41"

16"

45 30"

45

30

7 6 45 3 12

A

0 - 5" GAP

RAMP

Outby Inby

Pressure

Transducer

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7

Time, seconds

Explosion Overpressure - MPa

0

20

40

60

80

100

Explosion Overpressure - psi

Upstream quasi-static pressure

Downstream quasi-static pressure

Figure 18. Innovative seal designs for the mining industry showing the

blast wave attenuator concept which may reduce the downstream blast

pressure by a factor of 10 or more (Sapko et al., 2009).

Management of Sealed Areas

NIOSH researchers have advanced several concepts and

technologies relevant to the management of sealed areas: (1) mine

planning practices, (2) monitoring practices and (3) inertization

methods. The increases in explosion pressure design criteria for new

seals have increased the cost of seals which now require more

material and labor to construct. The mining community has expressed

concern that the cost of seals may lead to abandonment of sealing and

continued ventilation of mined-out areas. Alternatively, NIOSH

researchers suggest that better planning for future sealing can

decrease the number of seals required for lesser net increase in the

cost of sealing an area. Better pre-planning for sealed areas can

decrease the number of seals required and decrease the ventilation

pressure differential across sealed areas. NIOSH researchers plan to

document improved mine layouts that facilitate mine sealing.

Continuous monitoring behind seals to ensure that the sealed

area atmosphere remains inert enables the use of 345 kPa (50 psi)

seals. As mentioned earlier, NIOSH researchers have acquired a

tube-bundle gas monitoring system from SIMTARS in Australia to

study the atmosphere within sealed areas and to demonstrate this

technology in U.S. coal mines. This type of monitoring system is

utilized at every underground coal mine in Australia, and NIOSH

researchers hope to hasten its adoption by U.S. coal mining

companies.

Deployment of tube-bundle systems as an integral part of the

normal daily monitoring regimen at mines could have a positive impact

on mine rescue and response following an explosion, fire or other

catastrophic event. If a sampling and analysis system similar to that

shown in Figure 5 were part of the routine mine monitoring system at

an operation, such a system should survive a catastrophic event

largely intact and would continue to provide valuable data on the status
of the underground mine atmosphere. The analysis system located on

the surface should survive unaffected, and significant portions of the

underground sampling tubes should also survive. Thus the system

should continue to provide near-real-time data on the composition of

the mine atmosphere at many points throughout the mine. Such

atmospheric composition information should enable safer and more

rapid mine rescue and response decisions in an emergency.

To facilitate the development and adoption of inertization

technology for sealed areas, the NIOSH Office of Mine Safety and

Health Research contracted with On Site Gas Systems (OSGS) to

build an in-mine mobile gas generation plant to extract nitrogen gas

from the mine atmosphere. The demonstration unit shown in Figure 19

uses pressure-swing-adsorption separation technology, produces

about 8.5 m3(300 ft3) per minute, is less than 3.3-m-high (4 ft), and fits

on a standard shield carrier used in underground coal mines. In recent

tests conducted at the NIOSH Safety Research Coal Mine, OSGS in

collaboration with NIOSH researchers successfully inertized a sealed

area of the mine using the demonstration unit (Trevits et al., 2009).

Figure 19. Photograph of On Site Gas Systems in-mine nitrogen gas

generation plant (Trevits et al., 2009).

CONCLUSIONS

NIOSH is in its first year of a major new research effort on seals

and sealed areas of coal mines. Recent scientific studies of

explosions within sealed areas and the new regulations have made the

mining community aware of the potential danger posed by seals and

sealed areas of coal mines. Knowledge gaps include:

1. Composition of the sealed area atmosphere and how the

atmosphere varies within sealed areas and changes

composition over time.

2. Explosion processes within sealed areas and how the

ignition of a flammable mixture develops into a deflagration,

produces shock waves and reflected shock waves, and

possibly transitions into a detonation.

3. Seal engineering procedures and how engineers should

design seals that consider the explosion pressure, the

surrounding foundation rock, and convergence effects on

seal structures.

4. Sealed area atmosphere management procedures and how

the mining community should plan mines for future sealing,

monitor the atmosphere within sealed areas, and ensure that

sealed area atmospheres are inert.

NIOSH researchers are collaborating with numerous research

groups to address issues and knowledge gaps with seals and sealed

areas of coal mines. Highlights include:

1. Acquisition of a tube-bundle system from SIMTARS in

Australia to study the sealed area atmosphere and its

evolution. NIOSH researchers seek a collaborating mine in

which to demonstrate this technology and encourage its

adoption by U.S. coal mines.

2. Construction of a gas explosion tube to study large-scale

methane-air explosions and detonations. NIOSH

researchers aim to measure the explosion pressures that

can develop under various conditions similar to a sealed

area.

3. Simulation of gas explosion processes and calculation of

explosion pressures under varying conditions. NIOSH

researchers are collaborating with gas explosion experts at

the Naval Research Laboratory to study high pressure

methane-air explosions based on first principles from

physics and chemistry.

4. Compilation of all the 20 psi seals test data from Lake Lynn

Experimental Mine for subsequent structural analysis. This

compendium presents the applied pressure-time curve and

the measured displacement-time response on 52 distinct

seal structures organized into six different broad categories.

5. Development of methods for engineering design of seals.

Researchers at WVU and USACE are analyzing seals as

bending, shear plug or arching structures. Seal designers

should not rely on arching to develop except in areas where

the foundation conditions are rigid and unyielding.

6. Development of seal foundations analysis methods.

Researchers at USACE presented a method to assess the

bearing capacity of seal foundations that is related to the

CMRR used to assess the quality of coal mine rocks.

7. Development of innovative seal designs to resist explosion

pressures. Sapko et al., presented the Sapko-Hieb Mine

Blast Attenuator concept that was tested at NIOSH’s Lake

Lynn Experimental Mine, and USACE researchers presented

the concept of a gob seal with load collectors. In both

concepts, a mine entry is filled for a length of about 6 m (20

ft) from floor to roof.

8. Development of new inertization technology. On Site Gas

Systems, under contract to NIOSH, has successfully tested

a new nitrogen gas generator for inerting sealed areas of

coal mines.

NIOSH researchers aim to provide engineering guidelines for

meeting all aspects of the new MSHA regulations. The new guidelines

should provide multiple solutions for sealing that go beyond a “onesize-fits-all” approach. In developing proper engineering design codes

for seals and sealed areas, the authors advocate seal designs to resist

pressure-time curves that are based on an understanding of the risk

involved and not just a simple worst-case analysis.

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