Ventilating large opening mines
 

Roy H. Grau III and Robert Krog, U.S. Department of Health 

and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and 

Health, Pittsburgh Research Laboratory, Pittsburgh, PA USA 

Abstract 

The National Institute for Occupational Safety and Health 

(NIOSH) has conducted research to improve the ventilation 

of large-opening mines whose entries are at least 93m2 

(1,000 ft2). The ventilation of large-opening mines is unique 

compared to other types of mines because (1) it is 

challenging to keep airflow velocities high enough to effectively remove or dilute airborne contaminants because the 

entries are so large, (2) large air volumes can be moved 

through the mines with little static pressure drop, and (3) 

stoppings that are used to direct ventilation airflows are 

costly to construct and to maintain. The research results 

suggest that the ventilation of large-opening mines is 

improved significantly by incorporating ventilation planning 

into the mine planning process, using propeller fans to 

direct airflow, developing new stopping materials and 

construction methods, and using long pillars to eliminate 

crosscuts where possible. 

Introduction 

Some metal/nonmetal mines in the United States have 

large openings or entries that are at least 93 m2 (1000 ft2) in 

cross-sectional area. Limestone aggregate is the most 

common type, with 101 of these mines operating in the 

United States in 2006 (MSHA, 2006). Underground 

limestone aggregate mines use room-and-pillar mining 

techniques with entries that are about 12 m (40 ft) wide 

and 8 m (25 ft) high, often followed by two bench cuts that 

increase the height to as much as 24 m (80 ft). Other types 

of large-opening mines include marble, dimension stone, 

salt, and lead/zinc. 

This paper discusses air quality concerns and methods 

to improve the ventilation of large-opening mines. Due to 

recent Mine Safety and Health Administration (MSHA) 

regulations, the primary air quality concern in large-opening 

mines is diesel particulate matter (DPM). Other 

contaminants of concern are diesel equipment exhaust 

gases, welding fumes, silica dust, and nuisance conditions 

of fog and inert dust. After lengthy litigation, the MSHA rule 

establishing DPM exposure limits in underground 

metal/non-metal mines was upheld by the courts. On May 

18, 2006, MSHA promulgated a final rule that changed the 

interim DPM limit to 350 μg/m3EC (elemental carbon) 

effective on January 20, 2007. On May 20, 2008, the limit 

was reduced to 160 μg/m3TC (total carbon). 

The strategy used by many operators to reduce workers' 

exposures to these contaminants is to employ engineering 

controls such as increasing the air quantity entering the 

mine, using stoppings and auxiliary fans, and developing 

mine plans that specifically consider ventilation needs 

(Grau et al., 2004a, 2006a, 2006b; Krog et al., 2004) and 

methods that direct airflow to the face areas. Previous 

literature (Head, 2001; Grau, 2004b) has documented that 

large airflow volumes are required to dilute DPM in order to 

meet the MSHA regulatory standards. 

It is important to consider ventilation requirements while 

developing mining plans. Until recently, ventilation was not 

a major part of the underground limestone mine planning 

processes (Head, 2001; Grau et al., 2002; Krog et al., 

2004). The length and orientation of long stone pillars and 

the principle of dividing the mine into smaller units are vital 

means to improve ventilation. Mine planning should 

consider ventilation plans incorporating main mine fans, 

auxiliary fans, and stoppings and these plans should focus, 

in particular, on face areas, maintenance shops, crusher 

areas, and truck haulage routes. Since truck haulage is the 

largest contributor of DPM in the underground mine 

environment, effective planning of haulage routes will 

reduce workers' exposure to DPM. It is generally best for 

truck haulage to take place in return air, allowing the truck 

exhaust to exit the mine and thus preventing contamination 

of the air used to ventilate the face units. Truck operators 

are usually protected in pressurised and filtered cab 

systems; however, scaler operators, blasters, drillers, 

mechanics, and laborers who work outside these cab 

systems in face areas are exposed to airborne 

contaminants. Finally, significant decreases in DPM 

emissions can be realised by using cleaner burning 

engines, catalytic converters, filters (Schnakenberg et.al., 

2002), and alternative fuels. 
Understanding the ventilation of large-opening 

mines 

Since many limestone mines operate at shallow depths, 

drift portals are common at these operations. Operations at 

depths up to 61 m (200 ft) are ventilated using airshafts or 

slopes, the latter also providing access. Still deeper largeopening mines such as lead/zinc mines and salt mines use 

shafts for access and ventilation. The type of opening to the 

surface largely determines the main mine fan type in largeopening mines. Mines with drift portals have extremely low 

mine ventilation head loss; therefore, propeller fans are a 

viable option for whole-mine ventilation in those mines. 

Drift stone mines have some common characteristics 

that influence the ventilation. Features such as multiple 

portal openings with small elevation differences between 

portals are common, creating weak but noticeable natural 

ventilation. From observation, truck movements and 

outside wind conditions are factors that appear to create air 

currents independent of mechanical ventilation that impact 

air movement in large-opening mines. These air currents 

may enhance ventilation when the trucks are moving in the 

direction of the mine ventilation and reduce the mine 

ventilation when the trucks are moving against the 

ventilation airflow direction. The large void volume created 

from benching operations influences the air quality 

because the void is available for diluting air contaminants, 

essentially reducing the required ventilation air quantity. In 

addition, the high roofline in the benching area allows the 

diesel equipment's hot exhaust gases to rise away from 

workers, reducing their exposure to DPM. 

Ventilating large-opening mines poses some specialised 

challenges that include providing sufficient airflow to dilute 

airborne contaminants to statutory levels, while 

simultaneously using auxiliary fans or stoppings to control 

and direct the airflow to face areas where diesel equipment 

is operating. Offsetting these challenges is the extremely 

low resistance to ventilation airflow because of the large 

cross-sectional area of the openings. From an engineering 

design prospective, the large air quantity/low pressure 

scenario plays an integral part in the overall mine ventilation 

design of large-opening mines, because large air quantities 

can be moved through large distances with very small 

pressure drops. 

Estimating air requirements 

The first step in designing an effective ventilation system 

in large-opening mines is determining the total air quantity 

necessary for effective dilution of DPM and other 

contaminants. Although there are a number of noxious 

airborne contaminants, for most mines, the overriding 

ventilation design parameter is the dilution of DPM. 

However, the actual air quantity necessary is dependent 

upon how efficient the air is distributed to the faces using 

auxiliary fans and stopping lines. 

To help operators estimate how much ventilation air 

quantity is necessary in their mine based upon DPM 

exposure limits established by MSHA (MSHA, 2001), 

NIOSH developed an air quantity estimator software 

programme (Robertson et al., 2004). For the equipment 

operating in an underground stone mine that annually 

produced 1.13 million metric tons (1.25 million tons), Grau 

(2004b) reported that an air quantity of 400 m3/s (850,000 

cfm) was required to dilute DPM to a 400TC μg/m3 

concentration, and 990 m3/s (2,100,000 cfm) was required 

to dilute DPM to a 160 TC μg/m3 concentration. However, 

these conclusions were based on the existing controls and 

the equipment operating in the mine. In fact, the required 

air quantity can be reduced significantly by replacing older 

engines with cleaner-burning engines. Mine operators can 

dramatically decrease air requirements by selectively 

replacing older less efficient engines with newer engines 

that produce less DPM emissions or through the 

introduction of other controls which reduce DPM 

emissions. 

Ventilation considerations of underground 

limestone mines 

As the mining industry strives to improve the air quality 

in underground large-opening mines, there is an 

observable transition from reliance on natural ventilation to 

an increased use of mechanical ventilation. Mechanical 

ventilation with stoppings offers a more-controlled 

ventilation system compared to natural ventilation. 

Operators of limestone drift mines have often relied on 

natural ventilation because even with small differences in 

elevation, natural ventilation can produce large (though 

uncontrolled) air volume movements and mine air 

exchanges. Furthermore, natural ventilation is helpful in 

some large-opening drift limestone mines where larger 

openings are created during benching. From observation, 

it appears that even with inadequate ventilation, DPM 

concentrations develop more gradually in these areas 

because of the larger available space. During the night 

shift, when no mining is occurring, the natural ventilation 

also refreshes the air reserve. 

Temperature differences between air pockets in the mine 

can create barriers preventing effective ventilation. If 

conditions allow the air to short circuit these areas, 

inadequate ventilation will result. This is clearly observable 

as mines often have more difficulty maintaining air quality 

when the outside temperatures are warmer. Particularly 

problematic is mining up-grade as shown in Figure 1. In 

this situation, adequate face ventilation is more difficult 

because the hot exhaust fumes rise and accumulate in the 

upper face corner. One way to improve the ventilation in 

these areas is to ensure auxiliary fans are properly 

positioned to direct the air to the specific mine areas. As 

shown, mining down-grade enhances air quality as hot 

exhaust fumes rise and follow the roofline to the return 

entry. 

The appropriate selection of auxiliary fans and main 

mine fans should be based upon their operating 

parameters and the mine characteristics. Low-pressure 

propeller fans move large airflow volumes efficiently in 
large-opening drift mines and in some shallow slope or 

shaft mines (Grau et al., 2004b; Krog et al., 2004). Further, 

mine ventilation effectiveness is substantially increased 

when used in conjunction with directional control devices 

such as stoppings or rectangular pillars. The rectangular 

stone pillars are named "long pillars" because their length is 

over ten times their width. The long pillars are oriented to 

create continuous walls, often called "air walls," that 

separate intake airways from return airways and, with 

stoppings, direct the air to desired locations. Other 

methods to separate airways permanently are stoppings 

constructed of fly ash block or piled waste stone. 

Alternatively, fabric stoppings can be moved in concert with 

mining advances to create needed ventilation changes. 

Figure 1: Air quality variations when mining up and down 

dip. 

From NIOSH observations, it appears that most U.S. 

large-opening limestone mining operations use a primary 

single mine air split to ventilate the active mining faces. This 

single split concept eliminates the need for other control 

measures such as overcasts, regulators, and air doors. 

Ventilation techniques in underground limestone mines 

have evolved from using auxiliary fans alone to auxiliary 

fans with stoppings to direct air to the faces. It appears that 

the combination of auxiliary fans and stoppings is 

becoming more frequently used to direct the ventilation 

airflow. Many operators of older mines that do not have 

long pillars directing the ventilation air are now faced with 

installing stoppings, requiring a considerable investment in 

time and construction costs. 

Using long stone pillars to increase ventilation 

efficiency 

Reducing DPM concentrations requires that a high 

percentage of air generated by the main mine fan be used 

to ventilate the faces where much of the diesel powered 

equipment operates. This percentage of air is represented 

by the mine ventilation efficiency, which equals the airflow 

at the face divided by the total mine ventilation airflow. 

Ventilation airflow must be directed to the faces using 

stopping lines that separate intakes from returns. Using 

long pillars, rather than leakage-prone constructed 

stoppings, dramatically increases ventilation efficiency. It is 

clear that long stone pillars reduce the building and 

maintenance of constructed stoppings, reduce leakage, 

and increase the percentage of total mine ventilation air 

reaching the working faces. Unfortunately, due to mine 

layout and vehicle movement, directing ventilation air using 

only long stone pillars is not always practical. Constructed 

stoppings are often necessary, even though they are 

difficult and costly to construct and maintain in large 

openings. Poor installation techniques and inferior 

construction materials lead to stopping failure, resulting in 

large air quantity leakage. 

Grau (2006a) demonstrated the dramatic differences in 

mine ventilation efficiencies found between an older mine 

retrofitted with fabric stoppings and a newer mine that used 

long stone pillars from mine start. In the former case, 

controlling leakage was difficult due to tears in the brattice 

material from repetitive motion and blast damage. Due to 

this leakage, only about 33% of the ventilation air from the 

fan was available for the faces at 800 m (2,625 ft) from the 

main mine fans. In comparison to the older mine with the 

brattice stopping wall, the newer mine using the longer 

stone pillars to direct the air delivered 77 pct of air at the 

main mine fan to the last open crosscut located 680 m 

(2,230 ft) from the fan. Mines using fabric stoppings to run 

long air walls will find that as the mine expands, requiring 

additional stoppings, the ventilation system may be 

incapable of delivering adequate fresh air quantities to the 

working areas. This will necessitate the addition of a 

ventilation shaft or portal located closer to the workings to 

deliver the needed ventilation air. These results suggest 

that newer mines can benefit from using long stone pillars 

instead of brattice curtain stoppings to maintain adequate 

airflow to the face. 

Increasing face ventilation 

Although the preceding example shows that it is 

possible to deliver large airflows to the last open crosscut, 

the airflow must also be effectively distributed within the 

face. Total mine ventilation efficiency includes delivery of 

the air toward the face and then properly distributing this 

air to the face using an auxiliary fan. Generally, in large 

opening mines, air is distributed to the face using auxiliary 

fans as shown in Figures 2. The fan used in these analyses 

was a freestanding vane-axial fan with a diameter of 0.91m 

(36 in), powered by a 19kW (25 hp) motor, and mounted 

with a reducer at the outlet with a discharge diameter of 

0.58 m (23 in). 

The location of the auxiliary fan makes a big difference in 

the ventilation efficiency. Figure 3 shows the improper 

location of an auxiliary fan inby the last open crosscut. A 

fan located inby the last open crosscut increases air 
recirculation at the face thereby reducing total mine 

ventilation efficiency. Fans must be positioned in the intake 

airflow to ensure that a substantial quantity of fresh intake 

air is moved and directed to the face. 

Figure 2: uxiliary Fan type used near face. 

Figure 3: Incorrect Auxiliary Fan Placement 

Figure 4 shows the 

proper placement of the auxiliary fan, positioned in the 

intake air stream, outby the last open crosscut. 

Figure 4: A Properly Positioned Auxiliary Fan 

The proper 

placement of the auxiliary fan inby the last open crosscut 

makes a significant ventilation improvement by increasing 

the percentage of fresh air delivered to the face thus 

significantly increasing the total mine ventilation efficiency. 

Figure 5 shows the ventilation efficiency of a largeopening stone mine along the stone pillar air wall to the la st 

open crosscut and through the face where the highest 

efficiency drop occurs. 

Figure 5: Comparison of ventilation airflow efficiencies for 

new and 15-yyr old mine. 

The positioning of the auxiliary fan 

makes little difference in mine ventilation efficiency up to 

the last open crosscut. The differences to total mine 

ventilation efficiency and face ventilation efficiency is foun d 

by the placement of an auxiliary fan or fans near the last 

open crosscut. Without the auxiliary fan, the air naturally 

follows the path of least resistance and bypasses the face 

through the last open crosscut. This resulted in a mine 

ventilation efficiency of 5 percent at only 73 m (240 ft) from 

the last open crosscut. Similar poor efficiency but at a 

slightly further distance is obtained with the fan being 

located inby the last open crosscut. The authors have 

frequently observed auxiliary fans improperly positioned 

inby the last open crosscut in large-opening mines. Tests 

showed that an auxiliary fan positioned in by the last open 

crosscut provided a mine ventilation efficiency of 5 percent 

at 122 m (400 ft) inby the last open crosscut. Contrasting 

with this, an auxiliary fan properly positioned in the intake 

air, significantly increases the fresh air quantity that moves 

to the face. Correct positioning of the fan led a mine 

ventilation efficiency of 45 pct 122 m (400 ft) inby the last 

open crosscut. As DPM regulations are becoming more 

stringent, the use of all available air will be necessary to 

meet the regulatory mandates. 
Ventilating unit mining operations 

Unit mining is a recognised form of mining in largeopening mines (Grau et al, Krog et al) and is generally 

used in combination with other ventilation plans such as 

split ventilation systems. Unit mining consists of sectioning 

off specific zones for development using rectangular pillars 

arranged in sections that permit a controlled ventilation 

system. Ventilation air enters and exits the units in a few 

entries allowing auxiliary fans to be strategically positioned 

at air entrance locations. This type of face ventilation 

configuration permits better face ventilation and reduces 

ventilation recirculation than mining situations where there 

are wide mine expanses where many faces are present. 

Also, at some time in the mine's life, these units can be 

sealed and separated from the main ventilation flow of the 

mine, thus reducing demands on the main ventilation 

circuit. 

Alternative stopping designs 

Even when using long stone pillars, constructed 

stoppings are a necessity in some locations. However, they 

should be designed and constructed to minimise air 

quantity leakage. NIOSH designed, constructed and blast 

tested (Grau, 2006b) two novel stoppings at its Lake Lynn 

Laboratory (LLL): the Super Stopping and the EZ-Up 

Curtain Stopping (Grau et al., 2006b; NIOSH, 2006a, 

2006b). These stoppings are a means to control and direct 

ventilation airflows throughout large-opening mines. The 

Super Stopping was designed as a permanent stopping 

for use in the main entries of the mine, while the EZ-Up 

Curtain Stopping was portable, more economical, and 

easier to install. 

The Super Stopping (Figure 6) was constructed from 

oversized Omega Blocks1, which are low-density, 

composite cement and fly ash blocks manufactured by 

Burrell Mining Products, Inc. 

Figure 6: Placing Blocks in Place for Super Stopping. 

The individual blocks are 

1.22 m (48 in) long by 1.22 m (48 in high) by 0.81 m (32 in) 

wide and weigh approximately 544 kg (1,200 lbs) each. 

The Super Stopping constructed at LLL was 17.4 m (57 ft) 

wide by 9.1 m (30 ft) high. 

After a footing was built, the Super Stopping was 

constructed using an extended reach forklift to lift and 

position the blocks in the standard masonry pattern of 

staggered blocks on successive layers. Two factory cast 

holes are in place on one side of each block to receive 

forklift tines. In order to maintain stability and vertical 

trueness, a forklift with an articulating head is necessary to 

align the blocks in the structure. Since the stopping will be 

subjected to blast pressures, securing each block with a 

high-grade polyurethane construction adhesive strengthens 

the stopping. Gaps present between the stopping and roof 

were sealed using an expansion foam product that was 

applied using a handheld apparatus. The expansion foam 

acted to conform to the shape of both the cut blocks and 

surfaces of the mine opening, thus increasing structure 

stability. 

Ventilation curtain stoppings in large-opening mines are 

cumbersome to construct and they often deteriorate from 

blast pressures or from repetitive flap damage. NIOSH 

researchers have investigated new techniques to construct 

stoppings and alternative fabric materials for use in underground large-opening mines. A variety of fabrics can be 

used for the EZ-Up Curtain Stopping (Figure 7). 

Figure 7: Raising the EZ-Up Stopping. 

The most 

common type of fabric observed in underground stone 

mines is brattice cloth. However, other materials that are 

lighter in weight are available and offer viable alternatives 

to brattice cloth. Evaluations were performed on EZ-Up 

Curtain Stoppings that were composed of two different 

fabrics: NOVA-Shield RU88XFR-6, a high-density 

polyethylene woven fabric, manufactured by Intertape 

Polymer Group, Inc., and Dura-Skrim D15CFB, which is 

composed of two layers of film laminated with polyethylene 

that sandwiches a scrim reinforcement, manufactured by 

Raven Industries, Inc. The sewn-in loop allowed the tubing 

to be easily laced into the curtain while it was laid out on 

the ground. The tubing and curtain were then hoisted to 

the mine roof with a strap and ratchet mechanism system. 

The stopping was attached to the mine roof with four 

ratchet assemblies, each anchored by a roof bolt. 
The stopping was secured to the walls using two sets of 

vertical boards with dimensions of 3.8 cm (1.5 in) by 8.9 

cm (3.5 in) by 3 m (10 ft). The first set of boards was bolted 

to the ribs from the floor to the roof in line with the edge of 

the curtain. The excess fabric on the curtain sides was then 

wrapped onto the second set of boards. This wrapping 

prepares the curtain for attachment to the existing wallmounted boards to create a sandwich effect. The wrapped 

curtain boards on the LLL prototype stopping were 

attached to the wallboards using 0.6-cm (0.25-in) diameter 

by 10-cm (4-in) long lag screws. 

The curtain stopping was secured to the floor by laying 

sand bags on the excess curtain material that was draped 

on the floor. The sand bags seal the stopping from 

excessive leakage and provide pressure relief to reduce 

stopping damage in the case of a face blast that produces 

excessive pressures against the stopping. In an over 

pressure scenario, the curtain will be blown free from the 

sand bags, thus preventing damage; however, the 

sandbags need to be replaced to re-establish the seal. 

Another option for holding the curtain down is jugs filled 

with water or sand that are attached to the curtain and 

would move with the curtain during excessive over 

pressures from blasting but still hold the curtain down 

during non-blasting conditions. 

A brief overview of costs in 2006 U.S. dollars of the two 

stoppings is shown in Table 1. 

Table 1- Estimated costs for EZ-Up Stopping and Super 

Stopping. 

Estimated Capital Investment and Labor Costs for 

EZ-Up Stopping $ 

Ratchet Winch, weld-on storage, bottom mount, 4 @ $33.51 ea 134 

Yellow polyester webbing, 4 @ $23.81 ea 95 

Flag Bolts, 1 box 27 

Washers, 1 box 13 

10 ft x 1.5 in rigid conduit, 6 @ $36.00 ea 216 

0.25 in x 1.5 in lag screws, 50 @ $0.33 ea 17 

0.25 in x 1 in fender washers, 50 @ $0.12 ea 6 

Fabric, 57 ft x 30 ft @ $1.35/yd2 257 

Labor, 20 hrs @ $28.00/hr 560 

Total estimated cost for EZ-Up Stopping 1,325 

Estimated Capital Investment and Labor Costs for 

Super Stopping $ 

Footer, 44 ft wide x 32 in depth x 6 in thick, @ $74.5/yd3 162 

Omega Blocks 443, 48x48x32 in, 110 @ $78.00 each 8,580 

Omega Block 384, 24x8x16 in, 418 @ $4.50 ea 1,881 

Block Delivery 515 

Wood framing (reusable) 112 

Glue, 8 @ $14.20 each 114 

Mine Seal foam, 4 @ $199 each 796 

Miscellaneous items 110 

Labor, 410 hrs @ $28.00/hr 11,480 

Total estimated cost for Super Stopping 23,750 

The stopping size for the 

EZ-Up Stopping was 17 m (55 ft) wide by 9 m (28 ft) high. 

The Super Stopping was 13 m (44 ft) wide and 9 m (30 ft) 

high. The total cost in 2006 was US$1,325 for the EZ-Up 

Stopping and US$23,750 for the Super Stopping. Super 

Stopping costs did not include the rental price for an 

extended forklift, which many mines already have. The 

rental cost for an extended forklift was $200/day for 2 days 

plus $150 for combined pickup and delivery charge. The 

overwhelming cost for the Super Stopping was for the 

blocks and labour. A reduction in labor time will be realised 

as the workers gain experience in building the stopping, 

thus significantly reducing the stopping costs. 

Summary

Due to recent MSHA regulations, the primary air quality 

concern in large-opening mines is DPM. Some strategies 

available to operators to reduce workers' exposure to DPM 

are to increase the air quantity entering the mine, use long 

stone pillars, stoppings, and auxiliary fans to direct the air, 

consider ventilation in the mine planning process, and 

replace high-emitting DPM engines with cleaner-burning 

engines. An important step in reducing DPM 

concentrations is to direct a high percentage of air 

provided by the main mine fan to the faces. This paper 

demonstrates that the percentage of air reaching the face 

is increased by using long pillars that reduce crosscuts 

between intake and return airways and by building 

stoppings that are constructed to allow for low leakage 

amounts. Long stone pillars reduce the building and 

maintenance of constructed stoppings, reduce leakage, 

and dramatically increase the percentage of total mine 

ventilation air reaching the working faces. Building and 

maintaining stoppings are costly and persistent issues in 

large-opening mines. In response, NIOSH has developed 

several stoppings that provide mine operators with 

alternatives for efficiently installing stoppings and reducing 

stopping maintenance. 

Disclaimers

The findings and conclusions in this report are those of 

the authors and do not necessarily represent the views of 

NIOSH. Mention of any product or company name does 

not constitute endorsement by NIOSH. 

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