Practical techniques to improve the air quality in underground

stone mines

R. H. Grau III, T. P. Mucho, S. B. Robertson, A. C. Smith & F. Garcia

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

PA, USA

ABSTRACT: Researchers working for the National Institute for Occupational Safety and Health (NIOSH) at the

Pittsburgh Research Laboratory are developing ways to protect the health of miners. Part of that effort is devoted

to improving the air quality in underground stone mines by developing ventilation techniques that can be used

in these types of operations. The air quality in these large opening nonmetal mines can be significantly improved

by using diesel particulate matter (DPM) controls along with sufficient ventilation quantities to remove

contaminants. Practical methods of ventilating these underground stone mines can be accomplished by using mine

layouts that course and separate ventilation air through the use of stoppings. The design, construction, and

maintenance of effective stoppings in large openings have been a real challenge to mine operators. Several

different types of stoppings have and can be used for this application. The choice of stopping design, material

used, and construction techniques should be dependent upon a number of factors such as the intended life and

effectiveness desired.

1 INTRODUCTION

The National Institute for Occupational Safety and

Health (NIOSH) conducts research into various

mining health and safety issues to provide the basis for

improvements to U.S. miners’ health and safety. As

part of this role, researchers at the NIOSH Pittsburgh

Research Laboratory (PRL) are developing methods

and technologies to improve the air quality for large

opening underground metal/nonmetal mines. This

paper discusses NIOSH/PRL research dealing with

ventilation techniques that will be applicable to large

opening mining operations. Furthermore, the paper

describes concepts that can be incorporated into the

overall ventilation design of these mines. The most

common underground large opening mines are

underground stone mines followed by underground

rock salt mines. Surveillance data from the Mine

Safety and Health Administration (MSHA) for the

year 2000 shows that there were 162 active nonmetal

underground mines in the United States, of which, 117

were stone mines and 13 were rock salt mines. 

The continuing and emerging air quality issues in

metal/nonmetal mines include silica dust, diesel

particulate, fog and fumes. The concentration of these

contaminants can be effectively reduced by utilizing

various control technologies along with adequate air

quantities and proper ventilation methods. A growing

concern by various health agencies is the health risks

associated with exposure to diesel particulate matter

(DPM). It is generally accepted by various regulatory

agencies, ACGIH (2001), NIOSH (1988), EPA (2000),

and confirmed by the United States Congress, as to the

health hazards of exposure to diesel particulate matter.

As this concern grows, the mining community is

confronted with new DPM regulatory exposure limits.

MSHA recently addressed these health concerns by

promulgating underground diesel regulations for coal

and metal/nonmetal mines, MSHA (2001). The

standard was developed to reduce the health risks

associated with exposure to DPM. Our view is that the

metal/nonmetal DPM exposure limits proposed by the

regulations of 400 tc :g/m3

 on July 19, 2002 and a

more stringent limit on January 12, 2006 to 160 tc

:g/m3

 will impel the use of diesel emissions control

technology, and in many cases, some form of

ventilation improvement to meet these new air quality

standards. The most common ventilation knowledge

and techniques that are utilized in coal and some metal

mines are not readily adaptable to large opening

mines. The large openings in many mines offer little

ventilation resistance to air flow. However, this low

resistance permits large air quantities to move through

the large opening mines at extremely small mine (fan)

pressures. From an engineering design prospective,

this large air quantity, small pressure scenario should

play an integral part in the overall mine ventilation

design scheme. 
2 FUNDAMENTALS OF IMPROVING

 VENTILATION IN LARGE OPENING MINES

Previous literature (Head 2001; Grau 2002) has

documented the necessity for the large air volumes

that are required to effectively dilute DPM

concentrations to meet the proposed regulatory

standards established by MSHA. In addition to the

large air requirements, effective planning for the

placement of ventilation equipment and control

devices, such as fans and stoppings are necessary to

effectively ventilate the large opening mines.

Determining the required air quantity throughout the

mine is the first and most important elements for

planning effective underground mine ventilation.

Although many mining activities produce

contaminants that enter the mine air, the greatest

concern is with the DPM created from the diesel

engines used to power the equipment operating in

these U.S. mines. Most likely, if the DPM

concentrations are reduced or diluted to concentrations

that comply with the proposed regulatory standards,

the other contaminant concentrations will also be in

compliance. The research at NIOSH indicates that

there is no single fix or approach to reduce DPM

concentrations within these large opening mines,

however, providing at least the minimum ventilation

quantities to areas with operating diesel equipment

plays a crucial role in diluting DPM concentrations.

Therefore, we believe, that for the foreseeable future,

the eventual DPM regulatory exposure limits will be

the dominant parameter driving ventilation

requirements for these mines.

3 DESIGNING EFFICIENT VENTILATION

 SYSTEMS

The fundamental principle of mine ventilation is that

air movement is caused by differences in air pressure.

The pressure difference results from either natural

ventilation pressures or a mechanical fan(s) or a

combination of both. There are currently large

variations in the methods used by U.S. underground

large opening mine operators to develop air

movement. The methods vary from reliance on natural

ventilation forces to the use of main mine fan(s) or

combinations of both. In addition, auxiliary jet fans

(free standing) are often used in most of these systems

for local areas or to assist and direct the main mine

currents. Since natural ventilation is a product of the

differences in densities of air columns in and around

mine openings, natural ventilation is largely variable

and uncontrolled. The direction and magnitude of

natural ventilation will change frequently, often

several times in a day and certainly seasonally in

temperate climates. Therefore, mines that rely solely

on natural ventilation as the primary source of

ventilation have a highly uncontrolled ventilation

system. It should be noted that natural ventilation is

better than no ventilation and natural ventilation may

provide satisfactory air exchanges in some

circumstances or in some parts of the mine. Natural

ventilation has been helpful in some large opening

drift stone mines with multiple entries and in parts of

mines that have been extensively benched. Even with

small differences in elevation, natural ventilation

alone can promote large volume air movement and

mine air exchanges, although in an uncontrolled

manner. In areas that have become extensively

benched, the large void created may actually create an

“air reserve.” Although this air reserve can become be

gradually contaminated with DPM, the natural

ventilation does provide some ventilation relief during

working hours and clean out the system during off

shift times. Jet fans positioned in proper locations may

enhance this exchange process. However, jet fans in

other portions of the mine are often positioned

working against the natural ventilation flow direction.

This results in inadequate air flow and uncontrolled

recirculation. In most cases, using natural ventilation

as a primary ventilation source is a haphazard affair

usually with unknown results.

To effectively improve the air quality in these

underground mines, sound ventilation planning needs

to be incorporated into the overall mine planning

process. For instance, mechanical main mine fans,

auxiliary fans, stoppings, and a general ventilation

concept should to be integrated into mine layouts and

mining sequences. Also, special ventilation considerations, such as production faces, shops, benching

areas, and haulage routes should be considered in this

mine planning process. Criteria for proper fan

selection, installation and operation for both main

mine fans and auxiliary fans should be considered. Fan

characteristics of pressure and quantity should be

matched for the operation. Fan effectiveness is

increased dramatically when used in conjunction with

stoppings. Utilizing stoppings to build air walls helps

control the mine ventilation flow, i.e., efficiently

directing the air to where it’s needed the most. The air

walls also separate the intake and return airways.

Stoppings can be made from man-made materials,

leaving areas of intact rock to act as stoppings, or by

filling an opening with waste material.

Fan and stopping locations need to be an integral

part of the mine layout. Stopping and air wall

locations will often need to be built, taken down or

moved with changes in mining areas and/or in concert

with a predetermined sequence of a mining and

accompanying ventilation scheme. This would include

methods to ventilate the active faces, while providing

adequate ventilation to any special needs area noted

above. The overall ventilation concepts for these types

ventilation concepts are discussed more fully in Grau

(2002). Other important factors that reduce DPM at
Figure 1. Jet fan.

the face area are selecting cleaner burning diesel

engines and planning the truck haulage routes.

Effective planning of haulage routes will reduce DPM

from truck haulage which is the single largest source

of DPM in many underground stone mines.

4 DETERMINING SUFFICIENT AIR

 REQUIREMENTS

The first step to designing an effective ventilation

system in underground stone mines is to determine the

total air quantity that is needed for effective dilution of

DPM and other contaminants. As previously noted,

although many different mining activities emit noxious

contaminants and require dilution, the result of the

new DPM regulations will be that the overriding

ventilation design parameter is for the dilution of

DPM. In addition, even though the total theoretical air

quantity needed to dilute these contaminates can be

estimated for adequate dilution, sufficient quantities of

air must be distributed to areas where contaminates are

being generated. Therefore, certain mining operations

may require auxiliary fans to adequately dilute the

DPM at the source. Methods to determine the mine air

requirements for DPM dilution are described by both

Haney (1998); Grau (2002). Grau (2002) reported that

the estimated air quantity required for the equipment

currently operating in an underground stone mine

producing 113 million metric tons (1.25 million tons)

is 401 m3

/s (850,000 cfm) to dilute to a 400 tc :g/m3

concentration and 990 m3/s (2,100,000 cfm) to dilute

to a 160 tc :g/m3

 concentration. These conclusions

were based on the current equipment, controls, etc

being used. The air quantities may be too high for

practical mine ventilation, however the required air

quantity is highly dependent upon the engines in use

and as previously described, the extremely large

volume of the bench area may reduce the air flow

required. It should be noted that engines of an older

vintage are less efficient. As an engine ages, the

combustion process degrades, which lowers the fuel

economy and promotes higher emissions. Mine

operators can dramatically decrease air requirements

by selectively replacing the engines with a lower DPM

emissions or by adding control measures to engines

that emit the most DPM. This significant difference

defines why additional research is needed to define

more accurate estimates of air requirements.

The goal for many mine operators in the near

future will be to have their mine be in compliance with

the DPM regulations. We expect that, over time, this

will be a process of implementing both DPM control

measures and ventilation techniques. Operators are

looking at different scenarios in both areas to

determine where the most DPM reduction can be

achieved in the best practical way. As they move

through this iterative process, they will likely make

ventilation changes to their mine and also gradually

replace the older high DPM emitting engines with new

cleaner burning engines. The operators should factor

these scenarios into their mine planning process. 

5 FAN SELECTION

Many underground limestone mines are drift mines

developed from previous quarry operations. Typically,

these room and pillar mines have entries that are 6.1 m

(20 feet) or higher and at least 12.2 m (40 feet) wide.

These large dimensions lead to a very small pressure

loss, even when significant air quantities move

through the mine. This is especially true of the drift

mine operations where our observations found that

pressure differences of less than a 24.9 Pa ((0.1 in of

water gauge, (w.g.)) are not uncommon, no matter

whether these mine are ventilated by natural

ventilation, a mechanical fan(s) or combinations of

both. Our observations also indicate that the

underground stone mines with slope/decline and shaft

operations that are less than 70 m (200 ft) in depth,

have small mine pressure differences, usually less than

746 Pa (3 in w.g). These differences are or could be

much lower if the proper consideration was given to

the contribution that the slope/decline and shaft

provide to the overall mine resistance. 

The low pressure loss present in these large

opening mines is actually an advantage compared to

other type mines and should be treated as such. The

ventilation principles, concepts and techniques used to

ventilate these mines are different from the techniques

used in mines with larger pressure losses. For

example, axial vane fans have predominately been

used where higher pressures are required. However, in

large opening mines with low pressure requirements,

propeller fans offer an alternative. The propeller fans

can develop large air volumes under low pressure

conditions. Propeller fans can be used as either main

mine fans or as free standing auxiliary (jet) fans. Freestanding fans are commonly used to promote air

movement as shown in Figure 1. 
Figure 2. Stopping locations in a typical room and pillar stone

mine.

Ventilation studies by Matt et al. (1978), Agaipito

(1985), Goodman (1992) and Foster-Miller (1980)

have measured the performance of jet fans (usually

axial vane free standing) either in single headings or

ventilating portions of the main airways. The research

found that the most important aspect for jet fan

performance is that the jet fan should be positioned in

the intake incoming main air stream so that there is

sufficient intake air for the fan. Other important results

from these tests showed that the performance of these

fans was enhanced by adding a nozzle to the fan.

Results were also significantly improved by angling

the fan upward and located against a rib when

ventilating a dead-ended opening. 

6 VENTILATION CONTROLS (STOPPINGS)

In order to adequately deliver proper air flows to the

face areas, good air controls in the form of stoppings

are necessary. Stoppings are physical barriers that

separate the intake air from the return air. Since air

flows through a mine due to differential pressure

between travel points, a pressure difference always

exits between the intake and return airways. The

stoppings act as a barrier allowing for this pressure

differential to exist and circumvent short circuiting of

intake air to return air. Currently, in most U.S. large

opening mines, stoppings and fans are the only control

measures used. Most of these operations are currently

using or strive to produce a primary, single mine air

current to the active mining faces. However, there are

a number of variations, especially for drift operations

where natural ventilation and sometimes a number of

openings, yields secondary air currents. This single

split concept currently eliminates the need for other

control measures such as overcasts, regulators and air

doors. In many underground mines with large

openings, the auxiliary fans are the only control

devices used to distribute the air to the face working

area.

Stoppings have not been widely used in large

opening stone mines. Unfortunately, capital expense,

construction, and maintenance problems have impeded

this segment of the mining industry from building

stoppings. This is particularly problematic in the

larger, more established mines. In those mines,

stoppings were never incorporated into the mining

plan. Retrofitting the mines with stoppings to course

the air requires building many stoppings with a

corresponding investment in time and construction

cost.

Design criteria for stoppings include minimizing

the leakage between the intake and return air,

withstanding the fan pressure differentials and

withstanding or relieving the pressure from face

production blasting. Table 1 shows the criteria that are

the most important in different parts of the mine.

There are three main areas of the mine to consider in

determining the type or quality of stopping, the main,

intermediate, and the face areas. These areas are

shown in Figure 2 for a typical underground stone

mine. The stoppings in the main airways will typically

have less blast pressure, but since they are usually

located near the main mine fan, they are subject to the

highest constant pressure differential and thus have the

potential for the highest leakage. The stoppings in the

main entry will also need to survive the life of the

mine, hopefully requiring little maintenance.

Minimizing leakage in the main airways prevents a

direct short circuit of air to or from the fan. For these

reasons, the stoppings located in the main areas of the

mine should be substantially constructed. For these

stoppings, some form of pressure relief may be needed

from production face shots, especially early in their

life. This need will often diminish as the active mining

advances further away, causing the blast pressures to

dissipate with ventilation relief (other openings) and

distance.

Table 1 - Stopping criteria for locations in an

underground stone mine.

Location in

the mine

Fan pressure

difference

Blast

pressure

Acceptable

leakage

Main Greatest Little Low

Intermediate Significant Some Intermediate

Face Area Lowest Greatest Moderate

For underground large opening stone drift mines with

multiple entries, the pressure across intake and return

air is generally less than 62 Pa (0.25 inch w.g.) as

found by Grau (2002). From theoretical ventilation

calculations, this pressure differential is greatest near

the fan. 

Pressures from face production blasts far exceed

the ventilation pressure. Tests performed by NIOSH,

(Mucho, 2001) found pressures from two different
Figure 3. Schematics of tests for measure pressure from face

production shots.

Figure 4. Used mine belt used pressure relief.

production face shot, ranged from 8.2738 kPA

(1.20 psi) to 9.3769 kPA (1.36 psi) at distances of 200-

500 ft from the face shot as shown in Figure 3. The

face shots were generated with 400 lbs of ANFO,

169 lb of dynamite and 50 lb of Datagel. Research is

continuing at NIOSH to further bracket expected

blasting pressures that stoppings could be expected to

experience in these types of mines and to define the

controlling parameters such as distance and the impact

of venting to adjacent openings.

Some mines have had success in developing stoppings

designed to provide relief from blast pressure.

Techniques such as leaving the brattice loose at the

floor (and sometimes ribs), using tear away VELCRO

strips (Timko 1987), creating openings in the

stoppings prior to blasting, and using a combination of

used mine belt and brattice have been used. The

brattice left loose at the floor simply allows the

brattice to fly up when the face shot pressure passes by

and returns to the floor when the pressure is through.

This technique has been used in some mines near face

areas where leakage is not as critical and pressure

differentials are lower. Brattice stoppings sealed with

VELCRO strips have been developed and used on

brattice stoppings in oil shale mines (Timko 1987) and

in the NIOSH Lake Lynn Laboratory (Mayercheck

2002). The VELCRO strips separate during the impact

of the face shot but they immediately reseal. If sealing

is not immediately accomplished, the VELCRO strip

seals are manually reconnected after the mine blast.

Although they exhibited good success in the Lake

Lynn conditions, at least one mine has discontinued

their use because of mud and dirt filling the VELCRO

and reducing the sealing effectiveness.

7 TYPES OF LARGE OPENING STOPPINGS 

Stoppings are built from a variety of construction

materials. The construction materials are chosen based

upon the desired performance, construction time and

ease, and material cost. Construction materials that

have typically been used in these mines for stoppings

include steel sheeting, cementious-covered fiber

matting, mine brattice cloth, used mine belting and

piled waste stone. 

Used conveyor belting that is no longer useful for

material transport can be used to make stoppings. The

combination of used belting and brattice have been

used effectively in stoppings for both sealing,

production face shot relief, and flyrock or other

physical damage protection. It has been successfully

used as blast relief in a main mine fan bulkhead. Prior

to utilizing the mine belt as shown in Figure 4, the

mine had several stoppings blown over during

production face shots. The mine belt weight and

strength allow it to be strong enough to withstand the

pressure wave from the face shot but flexible enough

to give and act as a pressure relief. Belting hung in this

manner should be hung in an overlapping concave

pattern to promote interlocking of belting. This

technique will minimize air leakage. Figure 5 shows

used mine conveyor belt supplementing conventional

mine brattice in a stopping. This combination

minimizes leakage while providing protection, blast

relief, and a more substantial stopping. Conveyor belts

could also be used to shield conventional brattice

stoppings from the fly rock damage shown in Figure 6.

Certainly one of the most durable, but also the most

costly, for both construction and materials are the

corrugated steel panels reinforced with a steel frame as

shown in Figure 7. This is the most durable stopping

and can be effectively sealed on roof and rib by

making a template of the rib and cutting the corrugated

sheet to match. The remaining spaces can be filled

with expanding foam. One advantage of this stopping

is that a swing door can be incorporated into it. This

allows for personnel and equipment passage, as well

as for blast relief. Besides the cost and time required

to install, a disadvantage of this door is that leakage

can occur at the door bottom. This might be corrected

by adding some type of door sweep. 
Figure 6. Fly rock damage in brattice cloth.

Figure 7. Stopping made for corrugated steel panels reinforced

with a steel frame.

Figure 5. Used mine conveyor belt

supplementing conventional mine brattice in a

stopping.

Figure 8. Fabric-grid material sprayed with

cementious material.

A less elaborate, but still rigid, stopping is a

fiber/mesh covered with cementious grout as shown in

Figure 8. This type of stopping is currently being

evaluated in an operating underground limestone

mine. This stopping is installed by hanging fabric

backed by grid and then sealed by spraying with a

water-based cementious grout on both sides using high

pressure grout pumps. Stoppings of this type are still

being evaluated for effectiveness by NIOSH

researchers. 

A prototype stopping being researched by NIOSH

is a tension brattice stopping. The stopping is similar

to the tension membrane construction methods used to

create various fabric covered, large dome stadiums

throughout the country. In this stopping, currently

being installed and tested at NIOSH’s Lake Lynn

Laboratory, a brattice material is tensioned and

attached to the various steel framework supports,

thereby increasing the strength of the structure. 

8 NATURAL ROCK STOPPINGS

Leaving rock in place to form natural rock stoppings

has several advantages. By using the natural rock

stopping, leakage, construction, and maintenance costs

are eliminated. The rock stoppings are created by

leaving at least the last face shot that would normally

break through two adjoining openings. This keeps a

natural rock integrity between the two adjoining

pillars. Similar to constructed stoppings, natural rock

stoppings between future independent pillars can be

strategically oriented to direct the ventilation air. In
order to direct the air, the rock stoppings are oriented

parallel to the ventilation flow. Stone production may

be temporarily compromised because the stone in the

rock stopping is not immediately mined. However, the

rock stoppings can be pre-drilled and mined through at

a later time for stone recovery, or for other reasons

when the particular stopping line is no longer required

to course the air. 

When using lines of rock stoppings to separate and

course the air, openings need to be created every few

crosscuts to meet practical mining needs. However,

often the natural rock can be left in place along the

ribs and back of the final cut that creates these long

pillars to serve as a natural framework for the

stoppings and to minimize the size of the stoppings.

These too can be pre-drilled for future enlargement to

normal opening size when the stopping line is no

longer needed and/or the area is to be benched. A

caution when using this method is the mining horizon

for the top or back rock must be carefully chosen so

that a ground control problem is not created.

9 CONCLUSIONS

NIOSH is researching various ways to improve

ventilation in large opening mines in an effort to assist

with methods and techniques to improve the air

quality in these mines and therefore the health of

miners. NIOSH is currently focusing on fan

applications, air coursing, intake and return airway

separation using stoppings, and implementing mine

ventilation techniques and concepts into the mine

planning to accomplish this goal. 

Many U.S. underground stone mines are large

opening mines that generally feature small ventilation

head losses compared to other types of underground

mining. Propeller fans are generally well suited to

efficiently produce large air quantities under low

pressure requirements. Stoppings are necessary to

direct and control the mine air. A variety of stopping

choices exist for these types of applications and

depend upon the quality of the stopping needed.

Different portions of the mine may be better suited to

different types of stoppings. The use of stone

stoppings is being investigated, especially as it relates

to their deployment in various stages of the mine

 layout. Operators of all underground stone mines

should find that this information will improve their

ventilation in the underground workings.

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for chemical substances and physical agents and

biological exposure indices. 

Grau, III, R.H., Robertson, S.B., Mucho, T.P., Garcia,

F., & Smith, A.C. 2002. NIOSH research

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Mining, Metallurgy and Exploration Annual

Meeting, Feb. 26-28, Phoenix AZ.

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Manager, April 2001: 6(3):17-19.

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Department of Labor 2001. Final Rule 30 CFR

Part 57. January.

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Practical mine ventilation. Presentation at the

Safety Seminar for Underground Stone Mines,

Louisville, KY, Dec. 5.

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(NIOSH), Department of Health and Human

Services 1988. Carcinogenic effects of exposure to

diesel exhaust. Publication 88-116.

Timko, R.J. & Thimons, E.D. 1987. Damage resistant

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Engineering Mining Journal: 188(5):34-36.

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Report EPA/600/8-90/057E, July.