Passive mine blast attenuators 

constructed of rock rubble for 

protecting ventilation seals
�

M.J. Sapko 

Sapko Consulting Services, LLC, Pittsburgh, Pennsylvania 

M.R. Hieb 

West Virginia Office of Miners’ Health Safety and Training, Charleston, West Virginia 

E.S. Weiss, R.K. Zipf Jr., and S.P. Harteis 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania 

J.R. Britt 

Applied Research Associates, Inc., St. Joseph, Louisiana 

Abstract 

The design requirements for mine ventilation seals have undergone a radical transformation in recent 

years, and these revisions have greatly increased the cost of the seal designs and their construction. For 

example, in the past two years, new federal regulations have increased the minimum design requirement 

to withstand explosion pressures from 138 kPa (20 psig) to 345 kPa (50 psig) or 827 kPa (120 psig), 

depending on whether the sealed mine volume is monitored continuously or not. Moreover, there is still 

a possibility that under certain conditions (such as detonations), even higher pressure requirements may 

be necessary. The ability of a monolithic, stand-alone mine seal to reliably withstand the full range of 

possible explosion pressures is becoming increasingly difficult and expensive. In an effort to develop a 

practical alternative, the West Virginia Office of Miners’ Health Safety and Training (WVOMHST) and 

the National Institute for Occupational Safety and Health (NIOSH) have collaborated in a research effort 

to develop a practical, economic and safe mine sealing technique that can enable mine seals to meet the 

full range of new explosion pressure design requirements. The basic idea is to use a barrier of common 

mine gob and rubble, in combination with a conventional mine seal, so that the pressure resulting from 

a gas explosion is reflected, absorbed and attenuated, and the pressure on the ventilation seal is reduced. 

This paper discusses some concepts and preliminary test results for a “passive mine blast attenuator” 

(PMBA) that can provide a useful alternative to increasingly larger and stronger stand-alone mine 

seals. Numerical models and full-scale experiments conducted at the NIOSH Lake Lynn Laboratory 

Experimental Mine (LLEM) show that the use of a PMBA can signficantly reduce the blast pressure 

and impulse on conventional ventilation seals. 

Introduction 

Recent explosions at the Sago Mine in West Virginia and 

the Darby Mine in Kentucky in 2006 have greatly heightened 

awareness of the need for high quality ventilation seals that 

can protect miners from the effects of violent explosions and 

also from the toxic gases resulting from ignitions inside the 

sealed volumes of underground mines.

These disasters have led state and federalregulators to adopt 

morestringentperformancestandardsformineventilationseals, 

including, most notably, the Mine Safety and Health Administration (MSHA) Final Rule 30 CFR Part 75.335 (April 18, 

2008) on Sealing of Abandoned Areas. This new rule requires 

that a mine seal shall be designed, constructed and maintained 

to withstand: (1) at least 345 kPa (50 psi) overpressure for a 

specified pressure-time curve, when the atmosphere inside the 

sealed volume is monitored and kept inert; (2) overpressures 

of at least 827 kPa (120 psi) for a specified pressure-time 

curve if the atmosphere is not monitored or is not kept inert; 
loading on a ventilation seal and reduce the magnitude of the 

explosion pressure on the final containment seal.

Although granular filters are often used to protect against 

the large but short-impulse shocks produced by explosive 

detonations (~207 MPa [30,000 psi]), limited information is 

available about how they might influence mine explosions that 

produce relatively low shock pressures (~4.4 MPa [640 psi]) 

of longer duration. The most relevant results are those related 

to attenuation of shock waves by partitions, grids, and granular 

materials (Britan et al., 2001; Britan et al., 2004; Gelfand et 

al., 1987; Lind et al., 1999) and by using mine gob plugs (Hieb 

and Sapko, 2008; Hieb and Wooten, 2008; Unrug et al., 2008).

Conceptually, the overpressures on a seal may be reduced 

by placing a passive barrier in the tunnel between the putative explosion and the seal. When a shock wave impacts the 

barrier head-on, one portion of the wave is reflected, some is 

absorbed and the rest is transmitted through the open or porous 

parts of the barrier. These processes are referred to as shock 

wave reflection, absorption and transmission (or diffraction), 

respectively. By the proper choice of barrier geometry and 

construction, the peak pressure and the rate of pressure loading 

on the outby (or downstream) side of the containment seal can 

be significantly reduced. Further, PMBAs used in conjunction 

with 827 kPa (120 psi) mine seals have the potential not only 

to protect miners against 827 kPa (120 psi) overpressures, but 

also against the “worst-case” methane-air detonation scenario 

described by Zipf et al. (2007), wherein direct pressure loadings can reach and exceed 4.4 MPa (640 psi). 

Conceptual designs for a passive mine 

blast attenuator 

Twobasic approacheshavebeenconsideredfor constructing 

a PMBA in an underground mine; both utilize rock rubble or 

gob materials that are commonly found underground.

The first is a “shot-rock” design (Fig. 1), wherein a small 

section of the mine roof is shot down to close off a mine entry.

Conceptually, this should be done between two mine crosscuts, rather than in a mine intersection, to reduce the amount 

of shot material that is needed to provide full closure of the 

entry. Optionally, and for the same reasons, mine gob can be 

placed on the mine floor and then ‘walked-in’ with a mine 

scoop or raised by other means to within approximately 1.1 

m (3.5 ft) of the mine roof. The shot holes can be pre-drilled 

to a prescribed depth and shot such that a complete or nearlycomplete closure of the mine entry is achieved.

The second approach is a “fully-stowed” design (Fig. 2), 

wherein the gob material is placed and compacted fully up 

to the mine roof without shooting down any of the mine roof 

itself. Although this approach requires significantly more material handling and more attention to the details of placement 

and compaction in order to achieve uniform closure along the 

roof and ribs, it was developed (and used) primarily for the 

convenience of conducting full-scale explosion experiments 

without permanently altering the LLEM test facility.

It is the authors’ opinion that the “shot-rock” PMBA design (Fig. 1) is likely to be the most economical and effective 

design in practice. Shooting down the roof generates larger 

interlocking rubble, the weight of which increases normal 

forces to and confinement of the underlying particles, and 

this, coupled with roof hitching at the cavity walls, increases 

(3) overpressures greater than 827 kPa (120 psi) if the ventilation seals are constructed around mine volumes that are not 

monitored and are not inert, and if any one of the following 

three conditions occurs: (i) a homogeneous explosive atmosphere exists, (ii) pressure piling could produce overpressures 

exceeding 827 kPa (120 psi) or (iii) detonation is likely. If the 

latter situation exists, then seal strengths must be designed to 

withstand more than 827 kPa (120 psi), as determined by a 

professional engineer. These regulations reflect a worst-case 

explosion pressure scenario that was discussed in a NIOSH 

study entitled, Explosion Pressure Design Criteria for New 

Seals in U.S. Coal Mines (Zipf et al., 2007).

Given these new, more rigorous standards, it is necessary 

and desirable to explore new ideas and concepts for sealing 

mine gob volumes in ways that will protect miners against explosions. One such concept is to use a supplemental structure, 

such as a passive mine blast attenuator, or PMBA, to reflect, 

absorb and attenuate the explosion pressure before it arrives 

at the ventilation seal rather than relying on a single seal to 

contain the entire impulse. The West Virginia Office of Miners’ Health Safety and Training (WVOMHST), the National 

Institute for Occupational Safety and Health (NIOSH) and 

Sapko Consulting Services, LLC, have collaborated on a research effort to evaluate practical, economic and safe methods 

for sealing abandoned volumes in underground mines. In the 

PMBA method, common mine gob and mine rubble are used 

to construct barriers that will reflect, absorb or otherwise attenuate the high explosive pressures that are produced by gas 

or dust explosions, before those pressures come into contact 

with the ventilation seals. The feasibility of using such an 

approach began with three-dimensional simulations based on 

the Second-Order Hydrodynamic Automatic Mesh Refinement Code (SHAMRC) and culminated with three large-scale, 

exploratory experiments on blast wave attenuators that were 

conducted at the Lake Lynn Experimental Mine (LLEM). 

SHAMRC is an Eulerian finite difference code widely used 

by US government agencies, particularly the Department of 

Defense (DoD), for calculation of air blast (and other transient 

gas dynamics) effects. Preliminary results show that the use 

of PMBAs in underground mines is a viable and potentially 

useful approach when conditions warrant protecting against 

pressures up to and exceeding 827 kPa (120 psi). 

Passive blast wave attenuators in defense 

applications

Passive blast wave attenuation systems have been used for 

many years to protect against explosions in underground munitions complexes. The primary purpose of the attenuators in 

such applications is to reduce and absorb the explosive shock 

waves that emanate from a blast on the surface and prevent 

propagationofthose forcesdowntheventilationshafts andinto 

underground bunkers. Most such attenuators are composed of 

sand or rocks that permit the passage of some air for ventilation 

and cooling into complexes yet will significantly attenuate the 

shock waves produced by surface bombs. Blast attenuators are 

passive systems that reflect and absorb energy in such a way 

that they reduce the dynamic loading on the final containment 

structures, any pressure sensitive systems underground and, 

most importantly, personnel. When applied in a mine environment, passive attenuators can potentially reduce the dynamic 
 

 

 

 

resistance to particle displacement and erosion during the gas 

blow-by phase that accompanies the reduction of pressure inby 

the PMBA. With the ‘shot-rock’ method, any gob on the bottom will also facilitate blocking from the roof with minimum 

drilling depth. When the seam height exceeds 1.8 m (6 ft), it 

may be desirable to reduce the mine opening to about 1.1 m 

(3.5 ft) prior to shooting down the roof. As a practical matter, 

the drill holes could be pre-drilled as a first step, followed by 

gob placement and loading/shooting as the second and third 

steps, respectively. Conceptually, the drilling depth should be 

about 2.5 times the distance of the remaining entry height. It 

was not possible to test the “shot rock” design experimentally 

at the LLEM, but the “shot rock” design was evaluated using 

sophisticated computer simulations. 

Figure 1 — The “shot-rock” optimum attenuator design, where a 

section of the mine roof is shot down with explosives using holes 

drilled prior to placement of an optional foundation layer of borrowed gob material. 

Figure 2 — The “fully-stowed” alternate attenuator design with sections of corrosion resistant wire roof mesh to provide confinement 

and passive reinforcement to minimize material erosion during the 

pressure blow down. This design was tested and evaluated in fullscale explosion tests at the NIOSH Lake Lynn Experimental Mine, but 

without the wire mesh screen. 

Preliminary numerical simulations for 

a passive mine blast attenuator

The geometry of the LLEM three-entry system (see Figs. 3 and 4) was used in the computer 

simulations, so that the detailed pressure histories 

obtained in previous LLEM full-scale explosion 

experimentscouldbeusedtovalidateandcalibrate 

the computer model. The SHAMRC code was 

used to predict the pressure profile of a “worst 

case” explosion where the A-, B- and C-drifts of 

the LLEM were all filled with a mixture of 9.5% 

methane/air and ignited at the face of the C-drift. 

Attenuators consisting of simulated ridge rock 

piles with 20% void spaces, distributed in 6.56 cm 

(2.58-in.) wide computational cells (Fig. 3), were 

located 164 m (509 ft) outby the face in A-, B- and 

C-drifts and inby the three seals, so as to intercept 

themethane-airdetonationshockpressurebeforeit 

arrives at the seals. The simulated rigid seals were 

located 195 m (640 ft) outby the ignition or about 

30 m (100 ft) outby the attenuator. Figure 4 shows 

the relative orientation of the simulated attenuators and rigid 

seals that were placed in the A-, B- and C-drifts. It was also 

assumed thattheregion between theattenuator and sealwas air. 

The simulation produced a deflagration-detonation-transition 

(DDT) event with about 107 m (350 ft) of flame propagation 

beyond the point of ignition. The detonation pressure (~1.7 

MPa or ~240 psig) continued to propagate along the tunnel 

before it impacted the simulated PMBA. The impact of the 

detonation shock and its reflection produced a total pressure of 

~4.4MPa(~640psig). Figure5shows thecalculatedexplosion 

pressure versus time just inby the attenuator (C-510) and just 

inby the seal (C-640) in the C-drift for the geometry shown 

in Fig. 4. The pressure computations for both the A- and Bdrifts are very similar to what is shown in Fig. 5. The reflected 

detonation pressure just inby the attenuator decays 

rapidly, and then stabilizes to a value that is close to 

the constant volume (CV) pressure of ~827 kPa (120 

psi) in about two seconds. Combustion gases vented 

through the porous PMBAproduce an initial reflected 

pressure of ~140 kPa (~20 psig) on the outby seal. As 

the gases continue to pass through the attenuator, the 

pressure on the outby seal rises more gradually to 353 

kPa (57 psig) at ~two seconds, where the simulation 

was terminated. If the simulation was continued for 

another two seconds, the pressure between the attenuator and the seal would likely continue to rise to an 

equilibrium CV pressure of ~827 kPa (~120 psig). 

These preliminary numerical simulations, although 

not complete, clearly show that the use of a PMBA

has the potential to reduce both the incident shock 

pressure and the subsequent average rate of pressure 

loading on the downstream containment seals. The 

simulations also show that a significant reduction in 

the explosion impulse can be achieved with relatively 

simple barriers. 

Test facility and boundary conditions

In order to supplement and validate the numerical 

simulations of PMBAperformance, several full-scale 
explosion experiments were performed at the 

LLEM in the summer of 2008. Since it would 

not be prudent to create permanent cavities in 

the roof of the LLEM test facility, a PMBA was 

constructedusingthe “fully-stowed”gobbarrier 

method as shown in Fig. 2, but without the wire 

roof mesh. The PMBA was constructed using 

limestone gob materials borrowed from various 

parts of the test mine and supplemented (in the 

first test only) by 15-cm (6-in) by 0-cm material 

that was brought in from outside the facility.

The LLEM facility (Triebsch and Sapko, 

1990) is part of the NIOSH Lake Lynn Laboratory, which is located about 80 km (50 miles) 

southeast of Pittsburgh near Fairchance, in 

Fayette County, PA. The explosion tests were 

conducted in the A-drift entry area (Fig. 6). The 

entry and crosscuts are approximately 6.1m (20 

ft) wide by about 2 m (6.5 ft) high, with crosssectional areasof12-13m2 (130-140ft2). Figure 

6 gives a close-up, plan view of the seal test area 

in the multiple-entry area of the LLEM. In this 

example, there are pre-existing structures in the 

first four crosscuts from the face or closed end 

of A-drift. Note that the first crosscut is the one 

nearest the face (closed end). The flammable 

natural gas-air volume (ignition zone) is limited 

to a 26-m (85-ft)-long section of the ~30-m 

(100-ft)-long butt entry located inby crosscut 

1 in the A-drift. The ignition zone is contained 

by a plastic diaphragm attached to a wooden 

perimeter framework.

Figure 3 — The attenuator configuration used in the SHAMRC calculations. The lowest meter is assumed to be well-packed gob material and the upper composed of large rocks with 20% void spaces. 

Figure 4 — Lake Lynn Experimental Mine layout of a passive mine 

blast attenuator used in SHAMRC numerical detonation simulations. 

Figure 5 — SHAMRC-calculated pressures just inby the 

simulated attenuator (C-510) and just inby the simulated 

seal (C-640) located in C-drift of the Lake Lynn Experimental 

Mine (see Figure 4). 

Solid concrete block structures 400 mm (16 

in.) thick, with a 800-mm (32-in.) x 400-mm 

(16-in.) center pilaster are located in crosscuts 

1-4 between the A- and B-drifts. Each of these 

structures has been constructed as close as possible to the A-drift rib-line. The structure in 

crosscut 1 has a blast-resistant door located between the center 

pilaster and the inby crosscut rib. The structures in crosscuts 2 

and 3 were reinforced along their contact perimeter with heavy 

steel angle securely bolted to the roof, ribs and floor on the 

B-drift side and along the floor on the A-drift side. Vertical 

steel angles reinforcements were placed on the B-drift side 

mid-width between the pilaster and rib.

For the explosion tests, 31 m3 (1,100 ft3) of natural gas (98% 

methane) was injected into the 26-m (85-ft)-long gas zone and 

mixed with air using an electric fan with an explosion-proof 

motor housing. The combustible gas mixture filled 22% of 

the (simulated) sealed gob volume. The gas was ignited at the 

face by three sets of two 100 J (~0.1 BTU) electric matches 

that were equally spaced at mid-height across the closed end 

or simulated face. Five water-filled barrels were placed ~8 

and ~14 m (25 and 45 ft) from the closed end of the A-drift. 

The barrels were equally spaced across the entry at each 

location and served as turbulence generators. In addition to 

the 26-m (85-ft) gas zone, eight dust shelves were suspended 

from the mine roof; two shelves each at 27, 30, 33 and 36 m 

(89, 99, 109 and 119 ft). The total dust load was 14.5 kg (32 

lbs) of Pittsburgh pulverized coal dust which, if suspended at 

a nominal dust concentration of 100 g/m3 (0.1 oz/ft3), is the 

approximate equivalent of an additional 12 m (40 ft) of gas 

zone. Therefore, the total equivalent length of the gas zone 
 

 

 

 

 

 

was 38 m (125 ft), which represents about 32% 

of the total volume between the mine face and 

the attenuator. 

 

 

Figure6 —LakeLynnExperimentalMinelayoutofthemineblastattenuatortested against rathersmall methane-airdeflagrationsinthe A-drift. 

Each drift in the LLEM drift has ten datagathering (DG) stations recessed into the rib 

wall and each DG station houses a strain gauge 

transducer to measure the explosion pressure and an optical sensor to detect the flame 

arrival. A wall pressure sensor is mounted 

perpendicular to the gas flow and measures 

the quasi-static or side-on pressure. Another 

pressure sensor was mounted in the middle of 

the entry just inby the attenuator to measure 

the total explosion pressure on the attenuator. 

A video camera recording at 35 frames/s was 

mounted on the rib 15 m (50 ft) downstream 

from the attenuator to record initial movement 

of the attenuator. A high-speed, PC-based 

computerdata acquisitionsystemcollectedthe 

data from the various pressure transducers and 

flame sensors distributed along the A-drift, at 

a sampling rate of 5,000 samples per second.

A fully-stowed PMBA was installed in the 

A-drift starting 114 m (375 ft) from the closed 

end. It was constructed of limestone rubble 

stowed rib-to-rib and floor-to-roof (Fig. 7a) 

using a battery-powered mine scoop. The 

base (~0.9-1.2 m [~3-4 ft] high), shown as lifts 

“A” and “B,” consisted of limestone rubble 

generally less than 46 cm (18 in.) in diameter. 

The top ~1 m (3 ft) consisted of processed 

limestone with particle sizes ranging from 15 

cm (6 in.) in diameter to 0 material. The lifts 

were constructed with 15-cm by 0 limestone 

and are shaded in light gray in Fig. 7a. Lift 6 

was constructed with 50% “limestone floor 

gob” (46 cm by 0) and 50% of 15-cm by 0 

processed limestone and is represented by the 

patternedareainFig.7a. ForLLEMTest#525, 

the overall thickness of the PMBA at mine 

floor level was approximately 15 m (50 ft) 

and tapered to about 4.6 m (15 ft) at the roof.

Thebulkdensityofthestowedgobmaterial 

 

Figure 7 — Configuration of mine blast attenuator before (a) and after 

(b) the first test (LLEM Test #525). White: 46-cm (18-in.) x 0 limestone 

rubble; light gray: 15-cm (6-in.) x 0 crusher-run limestone; patterned: 

mixed 15-cm (6-in) x 0 and limestone; dark gray: Re-deposited material 

(undifferentiated). 

for LLEM Test #525, as well as the subsequent two tests, was 

measured periodically with a Troxler gamma density tool. The 

average maximum dry density of the 46 cm by 0 rock rubble 

was determined to be 2,146 kg/m3 (134 lb/ft3) and the actual 

averagedensityofthematerialasplacedduringconstructionwas 

approximately 1,800 kg/m3 (112 lb/ft3). The moisture content 

ranged between 3.5% and 4.2%. The average maximum dry 

densityofthe15-cmby0processedlimestoneusedtoconstruct 

the upper half of the PMBA for Test #525 was 2,130 kg/m3 

(133 lb/ft3) and the average density of the as-placed material 

was about 1,900 kg/m3 (119 lb/ft3). The moisture content 

ranged between 3.1% and 4.6%. The method of determining 

the maximum density values was the modified roller-pass 

method. During construction, a reasonable effort was made 

to achieve compaction, although this was hindered by the floor 

being solid concrete, combined with a general deficit of fines 

in the limestone rubble material used. Back-blading using the 

scoop bucket was not useful for compaction; however, using 

the weight of a fully loaded scoop bucket, combined with the 

downward pressure while producing side-to-side pivoting action, was useful. For this test, 15 cm by 0 rock was used for 

the top lift in order to facilitate full roof coverage. To assist in 

compaction, a small ram extension mounted on the stab-jack 

housing of the scoop bucket was employed. 

Full-scale explosion testing of the passive mine 

blast attenuator 

Three full-scale explosion tests (LLEM Tests #525 through 

#527) were conducted to evaluate the actual performance of 

a PMBA. 

First Test (LLEM Test #525). The results of the first explosion test are shown in Fig. 8. The peak quasi-static (side-on) 

explosion pressure was 379 kPa (55 psig) at 86 m (283 ft) and 

324kPa(47psig)at108m(355ft), asmeasuredfromtheclosed 

end ignition zone. The incident explosion pressure produced 

a total reflected (head-on) explosion pressure of about 1,124 

kPa (163 psig) on the PMBA at A-375.

When an air shock strikes a rigid structure head-on, such 

as the face of a seal or a PMBA, the theoretical reflected overpressure can be approximated by: 
(1) 

where 

pS = incident quasi-static side-on overpressure, 

po = initial atmospheric pressure and 

pR =reflectedoverpressure (ZucrowandHoffmann, 

Figure 8 — Quasi-static side-on pressures inby the attenuator (A-283,

A-355) and reflected explosion pressure at the attenuator (A-375)

during the first test (LLEM Test #525) (see Fig. 6). 

Figure 9 — Reflected explosion pressure inby the attenuator (A-375

and the quasi-static pressure 15 m (50 ft) outby the attenuator (A-453

during the first test (LLEM Test #525). Displacement of the attenuato

as measured with LVDTs is also shown. 

Table 1 — Maximum quasi-static side-on pressures and impulses recorded 6 m (20 ft) inby and 15 m (50 ft) outby 

the attenuator. 

Test LLEM Test #525 LLEM Test #526 LLEM Test #527 

Location (ft) 6 m inby (20) 15 m outby 

(50) 

6 m inby 

(20) 

15 m outby 

(50) 

6 m inby 

(20) 

15 m outby 

(50) 

Max pressure 

kPa (psig) 

628 

(91) 

4.8 

(0.7) 

524 

(76) 

145 

(21) 

621 

(90) 

~6 

(0.85) 

Impulse 

kPa-s (psi-s) 

448 

(65) 

6 

(0.9) 

131 

(19) 

14 

(2) 

372 

(54) 

5.5 

(0.8) 

1985).

Equation (1) predicts an overpressure of 1,259 kPa 

(182 psig), due to the head-on impact of a 324-kPa 

(47-psig) incident shock wave, and the measured 

overpressure was lower than this by about 11%. This 

discrepancy is due in part to the fact that the incident 

shock wave had an impact angle of about 45 degrees 

instead of 90 degrees (head-on). Figure 8 shows that 

the reflected wave propagated back towards the face 

and decayed with distance. Behind the PMBA were 

two linear variable displacement transducers (LVDTs) 

mounted to the posts via strings of 5.5 m (18 ft) and 

attached to 1.3-cm (0.5-in.)-diameter rods driven into 

the gob material. One LVDT was located vertically at 

0.3 m (1 ft) and the other at 0.9 m (3 ft) from the roof. 

The LVDTs were used to measure the displacement 

of the outby side of the gob pile relative to the inby 

causedbypressure loading. The reflectedshockwave 

loaded the attenuator to ~1,124 kPa (163 psig) and 

then decayed to about 276 kPa (40 psig) in about 0.1 

seconds, while part of the energy was reflected inby 

and part was absorbed by the attenuator. During this 

time, no movement was detected at the opposite end 

of the PMBA as indicated by the LVDTs (Fig. 9). At 

about 0.77 seconds, the outby end of the attenuator 

started to move and it stopped at about 0.94 seconds; 

the LVDT located 0.3 m from the roof showed a 

displacement of ~9 cm (3.6 in.), while the LVDT

located 0.9 m from the roof showed a displacement 

of ~5 cm (2 in.). At 1.43 seconds, the roof LVDT

registered a rapid displacement to 15 cm (6 in.), the 

maximum range of the LVDT. The LVDT located 0.9 

m (3 ft) from the roof showed a more gradual rise to 

its maximum range of deflection and that occurred at 

twoseconds. Thepressure inbythe attenuatorshowed 

multiplepulsesthatwereproducedbyreflectedwaves 

of pressure traveling between the PMBAand the face 

) at ~114 m (375 ft). At about three seconds, a ~0.7 kPa 

) (0.1 psig) side-on rise in pressure was detected ~15 

r m (50 ft) outby the PMBA (Fig. 9), indicating that 

gas was starting to vent through the attenuator. The 

maximumquasi-staticside-onpressurerecordedoutby 
 

 

 

 

was 4.8 kPa (0.7 psig) at ~4.9 seconds, while the 

corresponding inby pressure had decayed to ~35 

kPa (~5 psig). If a rigid seal had been placed 15 

m(50ft)outbythePMBA,thecalculatedreflected 

pressure on this seal, as determined from Eq. (1), 

would be ~10 kPa (~1.4 psig) from the impact 

of 4.8 kPa (0.7 psig).

Videofootageforthistestconfirmedthemovement of the PMBA pile indicated by the LVDTs. 

Initially, the top third of the attenuator started to 

displace slightly outby and then it stopped moving for ~0.4 seconds as shown in Fig. 9 (for the 

LVDT located ~0.3 m [1 ft] from roof), while the 

compressed gases vented along the roof on the 

outbyside. Gasventingcontinuedwhiletheinby 

pressure fluctuated from ~55 kPa (8 psig) to 207 

kPa (30 psig) due to multiple pressure reflections 

between the attenuator and the face. During this 

initial venting process, dust or small particles of 

rubble were displaced from the roof on the outby 

side and then this process progressed inby. The 

erosion rate increased with gas venting until the 

peak pressure outby the seal rose to a maximum 

of 4.8 kPa (0.7 psig) as shown in Fig. 9. This 

erosion is similar in appearance to the scouring 

that occurs during bedload transport in stream 

hydraulics. Particle movement appears to begin 

near where the roof contacts the attenuator on the 

outby side, where particles were least confined 

and readily set in motion by escaping gases. It 

takes approximately 2.5 seconds for the erosion 

to start and once started, it takes another 1.5-2 

seconds to reach conclusion. Erosion appears 

to have proceeded generally from the outby side 

of the PMBA toward the inby side and from the 

top down. Similar to bedload transport, it is the 

material that is smallest and least confined that 

moves most readily. The majority of the transported material stayed within ~8-15 m (25-50 ft) 

of the attenuator, despite the floor being concrete 

and sloping about 5% to the outby side.

This rudimentary PMBA, without erosion 

protection, effectively reduced the 6-m (20-ft) 

inby peak side-on pressure of 628 kPa (91 psig) 

to a maximum side-on outby pressure of 4.8 kPa 

(0.7 psig). The corresponding pressure impulse 

was reduced from 448 kPa-s (65 psi-s) inby to 

about 6 kPa-s (0.9 psi-s) at 15 m (50 ft) outby as 

shown in Table 1. 

Figure 7b shows the PMBA configuration after 

LLEM Test #525. About 55,800 kg (123,000 lb), 

or 30%, of the pile (primarily the 15 cm by 0 fine 

dust materials) had been eroded from the top and 

folded over the outby end of the attenuator. The 

amount of 15 cm by 0 material that was removed 

or redistributed, as shown in Fig. 7b, is a numerical average of measurements of the resultant roof 

gap made along each profile line, at 0.6-m (2-ft) 

intervals (dashed lines). 

Figure 10 — Configuration of the mine blast attenuator before (a) and 

after (b) the second test (LLEM Test #526). 

Figure 11 — Incident quasi-static pressures inby the attenuator (A-283, 

A-355) and reflected explosion pressure at the attenuator (A-375) during 

the second test (LLEM Test #526). 

the quasi-static pressure 78, 175, 274 and 432 ft outby the attenuator 

during the second test (LLEM Test #526). 

Figure 12 — Reflected explosion pressure inby the attenuator and 

Second Test (LLEM Test #526). A second test 

was conducted without rebuilding the PMBA, using the same explosive force to initiate the pressure 

loading. Figure 10a (identical to Fig. 7b) shows the 
 

 

 

 

 

 

Figure 13 — Configuration of the mine blast attenuator before (a) 

and after (b) the third test (LLEM Test #527). 

Figure 14 – Incident quasi-static pressures inby the attenuator 

(A-283, A-355) and reflected explosion pressure at the attenuator 

(A-375) during the third test (LLEM Test #527). 

Figure 15 — Reflected explosion pressure inby the attenuator and 

the quasi-static pressure 15 m (50 ft) outby the attenuator during 

the third test (LLEM Test #527). 

configuration of the PMBA before the second test. 

This test was done to determine the effectiveness of a 

PMBA when it only partially blocked the mine entry. 

As shown in Fig. 11, this test generated an incident 

quasi-staticside-onexplosion pressureof 352 kPa (51 

psig) at 283 ft (86 m) and 345 kPa (50 psig) at 355 ft 

(108m)beforeitimpactedtheattenuator. Theincident 

explosion pressure produced a reflected pressure that 

peaked at about 1,048 kPa (~152 psig). Figure 12 

shows plots of the inby reflected explosion pressure 

and the side-on explosion pressures as a function of 

time outby the attenuator. The inby pressure transducer failed shortly after impact, causing the signal to 

end abruptly at about 0.7 seconds. The peak side-on 

pressure 15 m (50 ft) outby the attenuator was 145 

kPa (21 psig) and the corresponding impulse was 14 

kPa-s (2 psi-s). Figure 10b shows the configuration 

of the PMBA after the second explosion test and it 

should be noted that it indicates additional erosion 

of the 15-cm-by-0 material. It should also be noted 

that during the second test, there was a reduction in 

the peak side-on pressure by a factor of 3.8 and a 

reduction in the side-on impulse by a factor of 9.5 

(Table 1), even after ~0.6 m (2 ft) of the top of the 

attenuator had been eroded by the first explosion test. 

If a stand-alone ventilation seal had been constructed 

15 m (50 ft) outby the attenuator and impacted with a 

145 kPa (21 psig) quasi-static side-on peak pressure, 

thecalculatedreflectedpressure(Eq.1)wouldbeabout 

435 kPa (63 psig). Some additional erosion of the 

15-cm (6-in.)-by-0 limestone aggregate (light gray) 

occurred along the entire top portion of the PMBA, 

some of which was re-deposited on the leeward side 

(dark gray), but the PMBA was still effective in 

lowering the explosion pressure. 

Third Test (LLEM Test #527). For the third 

test, the attenuator was modified as shown in Fig. 

13a. Most of the 15-cm-by-0 material from prior 

tests was removed and some 46-cm-by-0 limestone 

floor gob (“C”) was added on top of the original 

base layers (“A” and “B”). A ramp, 15-cm-by-0, 

was constructed with waste to accommodate the 

scoop and some of this surplus material was stacked 

against “C.” This was done to delay the removal of 

the material until completion of the test, but it served 

little, if any, structural purpose. Note in Fig. 13a that 

the best roof closure only existed for about 1.5 m (5 ft) 

(between the 4.6-m [15-ft] and 6.1-m [20-ft] marks), 

while along the ribs there were larger openings. As 

shown in Fig. 14, the third test generated a side-on 

incident quasi-static explosion pressure of 338 kPa 

(49 psig) at 86 m (283 ft) and 393 kPa (57 psig) at 

108 m (355ft) before impact with the attenuator. This 

incidentquasi-staticpressureproducedareflectedpeak 

explosion pressure of ~1,048 kPa (152 psig). Figure 

15 shows the inby peak explosion pressure and the 

outby quasi-static side-on explosion pressure 15 m 

(50 ft) from the attenuator. Again, the inby pressure 

transducer failed shortly after impact; therefore, this 

signal ends approximately at 0.8 seconds. The peak 

quasi-static side-on pressure 15 m (50 ft) outby the attenuator was ~6 kPa (~0.85 psig) with a corresponding 
 

 

 

 

 

 

impulse of 5.5 kPa (0.8 psi-s) (Table 1). Figure 13b shows 

a schematic of the attenuator after the third test (LLEM Test 

#527). About 54,400 kg (120,000 lbs), or 30%, of the top material had been eroded and was mostly folded over the outby 

end of the attenuator and smaller material traveled ~45 to 60 

m (150 to 200 ft) outby. If a seal had been constructed 15 m 

(50 ft) outby the attenuator and impacted with a peak ~6 kPa 

(0.85 psig) quasi-static side-on pressure, the approximate peak 

reflected pressure, as calculated by Eq. (1), would be ~12 kPa 

(~1.7 psig). The amount of material eroded off the top of the 

PMBA was comparable to the first explosion test (LLEM Test 

#525), except for some isolated “large rocks” that did not move 

from their location. 

As noted above, a comparable volume of material was 

eroded from the top of the PMBA during the third test (LLEM 

Test #527) and was eroded during the first test (LLEM Test 

#525). Better compaction and roof coverage were achieved 

for the top half of the PMBA in LLEM Test #527. Notably, 

several relatively large rocks, + 45 cm (18 in.), on the explosion side of the attenuator were not displaced, which indicates 

that the larger rock sizes had a higher level of resistance to 

displacement during the erosion that began at the roof line 

during gas venting.

These test results clearly show that even a rudimentary 

PMBA constructed inby a mine seal has the ability to significantly reduce the peak explosion pressure, the rate of pressure 

loading and the impulse load on outby ventilation seals. We 

are not proposing that a PMBA or any other type of blast 

wave attenuator replace mine seals, but clearly these attenuators could be used in conjunction with classic, time-tested 

seals, i.e., designs such as the hitched Mitchell/Barrett seal, 

to contain larger explosion pressures than would be possible 

withoutthem. The400-mm(16-in.)-thick,solidconcreteblock 

Mitchell/Barrett seal has demonstrated the ability to withstand 

static pressures of at least 655 kPa (95 psi). While this seal has 

recently fallen into disfavor with federal regulators because of 

an uncertainty about its ability to resist shearing effects during 

the initial shock loading at higher design pressures, the PMBA

appears to be generally capable of mitigating this hazard. Over 

time, as well as in the case of multiple explosions, PMBAs in 

conjunction with ordinary mine seals may be more durable and 

dependable than current mine seal designs, which place the 

entire burden of containing both the explosion and the gases 

on a single, stand-alone seal. Further, should design engineers 

determine that i) a homogeneous explosive atmosphere exists, 

(ii) pressure piling could produce explosion pressures exceeding 827 kPa (120 psi) or (iii) detonation is likely, the use of 

PMBAs would provide a practical alternative solution where 

none currently exists. 

Conceptual attenuator designs for future 

research studies 

One enhancement that in our opinion warrants further testing is to incorporate corrosion-resistant wire mesh between 

successive lifts in the top half of the attenuator (see Fig. 2). 

This would add passive resistance to particle movement and 

reduce transport by erosion during the gas blow-by phase that 

accompanies the reduction of pressure inby the PMBA. This 

concept is similar to the use of rock basket gabion walls to 

inhibit bank erosion along streams.

Onemethodmightemploystandardwelded10-cmby10-cm 

(4-in by 4-in) wire roof mesh installed and secured between 

selected lifts to keep rock particles confined during pressure 

venting (Fig. 2). This wire mesh could be attached to existing 

roof bolts and bent down over the previous lift. Asecond sheet 

of roof mesh could be applied over the previous sheet and the 

mine bottom could be covered with gob. The next lift could 

be applied and the mesh layering repeated. The wire mesh 

serves to prevent the erosion which starts from the outby side 

of the attenuator and moves inby. Other designs could utilize 

a well-anchored gabion basket wall that is backed by stowed 

mine rubble. The gabions fill material and rubble pile may 

also be bound together with injected grout or cementitioustype material. 

Proposed theory of operation: analogy with 

blast hole stemming

We have proposed a passive mine blast attenuator that responds in a manner similar to the stemming in a blast bore hole, 

but at much lower shock pressures and impulses. Resistance 

to flow in the center of the stemming is limited because of the 

interlocking or wedging of the particles (Konya et al., 1981). 

A certain wall friction is essential to start the wedging action, 

but once this action is started, the resistance quickly increases. 

The interlocking of particles produces lateral forces that tend 

to pack the outer material against the wall of the blast bore 

and in doing so create material arches that are similar to those 

that cause “hang-ups” in ore passes. Total movement of the 

column of stemming occurs only if the particles against the 

borehole wall fail in shear or slip.

Asimilarprocess occurredinthefirsttest(LLEMTest#525) 

with the “fully stowed” PMBA. The shock impact (~1,138 kPa 

or ~165psig)initiallycompressedtheinbyendof theattenuator 

while the outby end did not move. The interior of the attenuator moved faster than the face near the edges of contact (at the 

ribs, floor and roof), because wall friction opposed movement 

of the edges during the inter-particle momentum transfer. This 

initial compression produced material arches and large lateral 

forces as it dissipated the shock energy. Further, lateral forces 

increased the overall frictional and shear resistance to plug 

movement. After the pressure decayed to about 138 kPa (20 

psig), compressed gases began venting along the roof and this 

eroded particles from the attenuator starting at the outby end 

and moving inby. 

Conclusions 

The results of these initial experiments in rather small combustible methane-air volumes clearly show that a rudimentary 

passive mine blast attenuator constructed of common mine gob 

and mine rubble can significantly reduce the peak explosion 

pressure transmitted to outby mine structures such as ventilation seals. The use of a PMBA barrier in conjunction with a 

conventional mine seal shows great promise as a means of 

protecting against the effects of 827 kPa (120 psig) and higher 

gob gas explosions. Because these tests were conducted in a 

limestone mine where the surrounding boundary conditions 

were likely to be more stable than those found in coal mines, 

future research should focus on the potential differences between these mine types and their implications for attenuator 

performance. AlthoughthesecomputationalandLLEMexplosion test results are preliminary, on-going experimental and 

modeling research efforts within the NIOSH and WVOMHST

will aim to refine the concept of passive mine blast attenuators and develop more design data and recommendations for 

optimum deployment. 
 

 

 

 

Disclaimer 

This information is distributed solely for the purpose of 

pre-dissemination peer review under applicable information 

quality guidelines. It has not been formally disseminated by 

the National Institute for Occupational Safety and Health 

(NIOSH). It does not represent and should not be construed 

to represent any agency determination or policy. 

Acknowledgments

The authors thank the following NIOSH contractors: 

Timothy W. Glad, James R. Rabon, Bernard T. Lambie and 

Brandin Lambie, Mechanical Technicians for the Ki Corporation, for construction of the attenuator and assistance in the 

underground materials handling and extensive cleanup operations. The authors acknowledge the following PRL personnel 

for their contributions: Kenneth W. Jackson and Walter P. 

Marchewka, electronic technicians, for sensor calibrations 

and for their participation in the data collection and analyses, 

and James D. Addis, Frank A. Karnack and Donald D. Sellers, 

physical science technicians, for their extensive participation 

in the installation of the sensors and the setup and conduct of 

the explosion tests. Special thanks to John Cruse, Technical 

Analyst III, WVOMHST, for his contributions to the oversight 

and documentation of construction. 

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