Horizontal air-blocking shelf reduces 

dust leakage from surface drill shroud

J.D. Potts and W.R. Reed

Mining engineers, Respiratory Hazard Control Branch, National Institute for 

Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania

Abstract

Installation of a horizontal air-blocking shelf around the inside perimeter of the surface mining drill 

deck shroud is proposed to improve dust capture. Without the use of this shelf, a strong Coandӑ effect 

airflow pattern results in dust leakage to the environment, especially at the shroud-to-ground interface. 

Bailing air exits the drill hole and travels along the drill steel to the underside of the table, where it fans 

out across the bottom of the drill table and continues down the sides of the shroud. This dust-laden 

bailing air then strikes the ground and fans out under gaps between the shroud and the ground. The 

proposed shelf prevents dust-laden air from striking the ground by redirecting it toward the center of 

the enclosure, where it can be captured. Laboratory tests demonstrated that the shelf reduced dust levels 

outside of the shroud by 81% under operating conditions consisting of a 1.9:1 collector-to-bailing-airflow 

ratio and a 51-mm (2-in.) gap between the shroud and ground. The shelf was 152-mm (6-in.) wide and 

was located at a height equal to 2/3 the distance measured from the bottom of the shroud to the bottom 

of the drill table. The air-blocking shelf was also capable of overcoming the adverse impact of vertical 

leakage along the shroud seams as long as this leakage was confined to an area below the horizontal 

plane defined by the shelf location. 

Introduction

Overexposure to respirable crystalline silica dust can result in 

the development of silicosis, a serious and often fatal respiratory disease. The U.S. Mine Safety and Health Administration 

(MSHA) regulates the exposure of coal miners to hazardous 

substances and has set the permissible shift exposure limit for 

respirable dust at 2.0 mg/m3 when the crystalline silica level 

does not exceed 5%. If silica in the sample exceeds 5%, the 

respirable dust standard is reduced to a level equal to the quotient of ten divided by the percentage of quartz in the sample, 

resulting in an effective silica exposure limit of 0.1 mg/m3. 

Surface-coal drill operators have had some of the highest 

frequency for noncompliance, with the respirable dust standard 

exceeding the limit 15% to 19% of the time during the period 

from 2001 through 2006 (Joy, 2007). Compliance problems are 

exacerbated by both the level of respirable dust generated by 

the drilling machines and the fact that they are often being used 

to cut silica-bearing materials such as sandstone and shale. One 

study found that time-weighted average respirable dust levels 

in areas surrounding the drill shroud could exceed 95 mg/m3 

(Organiscak and Page, 1995). Depending on wind direction 

and worker location, rotary drills can potentially expose all 

surface workers to elevated silica levels. A recent voluntary 

study of Pennsylvania surface miners showed that nearly 7% 

had radiological evidence of silicosis (Tyson et al., 1997). 

As demonstrated by the data above, additional technology is 

needed to reduce the exposure of surface miners to silica. A 

recent research project initiated by the National Institute for 

Occupational Safety and Health (NIOSH) identified improvement of the drill shroud design as one of its main objectives 

for reducing the exposure of surface miners to respirable 

dust. Priority was assigned to designs that would minimize 

operational resources and maintenance. A method utilizing a 

horizontal shelf attached to the inside perimeter of the shroud 

to disrupt the Coandӑ airflow pattern under the shroud shows 

promise for reducing dust leakage to the environment.

Background

Many surface mining operations use dry drilling techniques 

to eliminate problems associated with wet drilling, including 

maintaining a constant water supply, short bit lives and freezingrelated issues in colder climates. Dry drilling requires a dust 

collection circuit to remove dust from the bailing air. Components 

common to all dry dust collection systems are shown in Fig. 1, 

including a shroud to enclose the area under the drill deck, an 

inlet and flexible duct to route the dusty air to a filtration device 

and an exhaust fan to power the system. Dust emissions from this 

circuit can result from several sources, including the collector 
exhaust, drill stem bushing leakage, shroud leakage and the dust 

collector dump discharge (Organiscak and Page, 2005). From a 

control standpoint, poor collector exhaust quality and drill stem 

leakage are largely a function of improper maintenance, which 

can be easily remedied. In addition, an effective dust-control 

technique, which makes use of a simply constructed shroud, 

has already been identified for minimizing collector discharge 

dust generation (Reed et al., 2004). Therefore, NIOSH research 

efforts have focused on minimizing leakage from the shroud 

with field evaluations demonstrating that openings or gaps in 

the shroud can significantly diminish dust capture (Zimmer and 

Lueck, 1986; Zimmer et al., 1987). Oftentimes these gaps are 

unavoidable due to uneven and sloping ground surfaces and 

outward forces created by the accumulation of drill cuttings at 

the base of the shroud.

Figure 1 — Dry dust collection circuit.

Drill shrouds commonly found in the field can be categorized 

into two main types: those constructed of a single piece of material wrapped around the drill table and those constructed of 

sectional pieces. A typical sectional arrangement is constructed 

of four separate pieces of conveyor material hung from each of 

the four sides of the drill table. Another common arrangement is 

a single piece of material around the table with a hinged access 

door on the front side of the shroud. The access door allows 

the operator to move the drill to the next hole without forcing 

the cuttings into the hole. Shrouds constructed of a single piece 

of material are subject mainly to horizontal leakage under 

the shroud, whereas sectional configurations are additionally 

subject to vertical leakage along the seams.

Airflow patterns within the shroud enclosure were investigated in a full-scale drill table simulator (described in the 

next section) to determine if dust-capture could be improved 

through simple modifications. A qualitative airflow model was 

developed using smoke tubes and a tuft grid technique (SAE, 

1986), which uses a mesh and streamers to visualize the flow 

directions. One side of the simulator’s shroud was constructed 

of transparent acrylic plastic to allow observation of the area 

enclosed by the shroud. The tuft grid was created by attaching lightweight vinyl streamers to fencing material, which 

was cut to the same dimensions as the horizontal parametric 

cross-section of the shroud enclosure. Airflow patterns were 

observed at various heights under the shroud during operation of the drill simulator to develop the qualitative model 

shown in Fig. 2. As a result of a strong 

Coandӑ airflow effect, the bailing air exits 

the drill hole and travels up the middle of 

the shrouded area, maintaining its course 

along the drill steel to the underneath side 

of the drill table, where it fans out across 

the bottom of the drill table and continues 

down the sides of the shroud. A dead air 

zone was found between the drill steel and 

sides of the shroud. Dust emissions from 

horizontal leakage occurred when the air 

traveling downward along the sides of the 

shroud struck the ground and fanned out 

underneath the enclosure. This leakage was 

observed when smoke tubes were aspirated 

at the base of the shroud. 

A concept to reduce dust emissions 

caused by the above process was to disrupt this airflow and to redirect it toward 

the center of the enclosure to prevent air 

from striking the ground. This was accomplished, as shown in Fig. 3, by installing 

a 152-mm- (6-in.-) wide horizontal shelf 

along the inside perimeter of the shroud. This paper describes 

the laboratory development of the concept and provides important design criteria for its successful application.

Testing facility

All preliminary and designed experimentation was conducted 

in NIOSH’s full-scale drill table simulator located at the Pittsburgh Research Laboratory. The simulator is shown in Fig. 4

and described in detail in previous publications (Organiscak 

and Page, 2005). Operating parameters similar to a mediumsized rock drill can be produced in the lab. The dimensions of 

the shroud, which is located in the center of the dust chamber, 

are 1.52 m wide by 1.22 m deep by 1.22 m high (5 x 4 x 4 

ft). The dust chamber dimensions are 3.66 m wide by 3.05 m 

deep by 2.44 m high (12 x 10 x 8 ft). The base of the shroud 

is fitted with hinged sections of plywood, allowing simulation 

of horizontal leakage at the ground surface with shroud gap 

heights of 51, 203 and 356-mm (2, 8 and 14-in.). To generate 

bailing airflow velocities typical of those found in the field, 

a compressor capable of generating 0.326 m3/s (690 cfm) at 

758 kPa (110 psi) is used to force air down the inside of a 

152-mm (6-in.) schedule 40 pipe, which simulates the drill 

steel. The 152-mm (6-in.) pipe is concentrically located and 

suspended within a capped 203-mm (8-in.) schedule 80 pipe, 

which simulates the drill hole. In this way, air is forced through 

the end of the 152-mm (6-in.) pipe and then upward between 

the outside diameter of the 152-mm (6-in.) pipe and inside 

diameter of the 203-mm (8-in.) pipe at velocities exceeding 

30.5 m/s (6,000 fpm). 

To simulate dust generation, a Vibra-Screw feeder is used 

to meter a constant flow of limestone dust (100% < 200 mesh) 

into the bailing airflow using a compressed air eductor. The 

dust collector utilized in the simulator is capable of providing 

a maximum airflow of 0.944 m3/s (2,000 cfm). The collector 

inlet is rectangular, measuring 457 by 191 mm (18 x 7.5 in.) 

and is located in the corner of the drill table, which is typical 

of field conditions. 

During operation of the simulator, the dust concentration 

within the test chamber reaches a state of equilibrium because 

make-up airflow equals collector airflow and dust feed is 

constant. Adjustable louvers on top of the chamber ensure that 

make-up air velocities are significantly high to prevent dust 
leakage from the chamber. This chamber configuration allows a 

direct comparison of the efficacies of various simulated drilling 

parameters and control technologies on shroud leakage.

Figure 2 — Coanda ̆ air effect under drill shroud. Figure 3 — Air-blocking shelf concept.

Figure 4 — NIOSH’s full-scale drill table simulator.

Preliminary testing

Preliminary testing was conducted to determine if the horizontal 

shelf warranted additional experimentation. It was also used 

to determine whether basic design criteria, such as shelf width 

and location, as well as material loading on top of the shelf, 

played any role in the effectiveness of the shelf for reducing 

dust concentrations.

Preliminary testing was conducted with two GCA real-time 

aerosol (RAM) instruments located outside 

the shroud at the front side (on inlet) and 

rear side (off inlet) of the drill table. Tests 

were conducted using an A-B format over 

a 30-minute sampling period, where the 

horizontal shelf was utilized during the first 

15 minutes of the test and then removed for 

the last 15 minutes. All preliminary testing 

was conducted using a collector to bailing 

airflow ratio (airflow ratio) of 2:1, and 

shroud gap height at 51-mm (2-in.). 

The first series of tests were conducted 

to determine if blocking width affected 

dust capture. Horizontal shelves were 

constructed of plywood at widths of 152, 

89 and 38 mm (6, 3.5, and 1.5 in.). Each 

shelf was tested at a height of 813 mm (32 

in.) above the bottom of the shroud, which 

equated to 2/3 of the total shroud height. 

All of the boards significantly improved 

capture, decreasing average dust levels 

outside of the box by 92%, 86% and 81%, 

respectively. Because decreasing the board 

width appeared to slightly diminish the effectiveness of the horizontal shelves, a 152-mm (6-in.) board 

width was used for a subsequent two-level factorial designed 

experiment.

A second series of tests were conducted to determine if 

board location would influence capture efficiency. For the 

first test, all of the 89-mm (3.5-in.) boards were placed at a 

lower location about 64-mm (2.5-in.) above the ground. During this test, dust levels were reduced about 72%, indicating 

that this is a less desirable location because the 89 mm (3.5 

in.) boards lowered dust 86% at the higher location. Also, the 

higher location is less likely to interfere with the cuttings pile 

generated during drilling; therefore, no further testing was 
conducted at the lower position. For the second test, the positions of the 152-mm (6-in.) boards were offset to determine 

if gaps in placement would diminish performance. The front 

sideboard was lowered 102 mm (4 in.), while the rear sideboard was raised 102 mm (4 in.). During this test, dust levels 

outside of the shroud were reduced by 86%, indicating that 

locating the boards on the same horizontal plane was not an 

important design criterion. This may be a beneficial attribute 

for installation because operators will not have to be concerned 

with placing all of the blocking shelves on the same horizontal 

plane as long as they cover the parametric cross-section of the 

shroud. However, for final testing, the 152-mm (6-in.) boards 

were kept on the same horizontal plane. 

The last preliminary test was conducted to determine if the 

buildup of material on the shelf would negate the dust-reduction 

effects of the horizontal shelf by altering the path of airflow 

within the shroud. For this test, sections of 208-mm- (8.2-in.-) 

wide cardboard were secured between the sides of the shroud 

and the outer edges of the 152-mm (6-in.) boards to simulate 

material collected on top of the shelf at a 43° angle of repose. 

During this test, dust levels were reduced 76%, indicating a 

somewhat diminished but still dramatic reduction in dust under 

the loaded condition. The unloaded condition was used for the 

designed experiment because any material buildup would fall 

off when the drill mast is lowered.

Table 1 — Two-level factorial experiment matrix and 

average gravimetric results.

 Gravimetric 

 Airflow Horizontal Blocking average, 

 ratio leakage shelf Test No. mg/m3

 1 1 0 14 8.48

 1 1 0 15 8.87

 1 1 0 19 7.7

 1 0 0 2 21.04

 1 0 0 8 21.29

 1 0 0 17 21.43

 0 1 0 1 18.83

 0 1 0 5 15.39

 0 1 0 10 16.61

 0 0 0 7 27.04

 0 0 0 21 34.67

 0 0 0 24 36.28

 1 1 1 9 1.67

 1 1 1 12 1.25

 1 1 1 20 1.85

 1 0 1 3 5.57

 1 0 1 18 5.65

 1 0 1 23 7.77

 0 1 1 13 4.7

 0 1 1 16 3.15

 0 1 1 22 3.9

 0 0 1 4 18.72

 0 0 1 6 19.54

 0 0 1 11 22.57

1 = 1.9:1 airflow ratio, 51 mm (2 in.) shroud gap, with blocking shelf

0 = 1.2:1 airflow ratio, 203 mm (8 in.) shroud gap, without blocking shelf

Two-level factorial experiment

As a comparison to the successful testing detailed above, a twolevel factorial experiment was designed to determine the dust 

reduction potential of the horizontal shelf under more severe 

operating conditions. Collector-to-bailing airflow ratios of 2:1 

and 1.25:1 were tested, as well as shroud gap heights of 51 and 

203-mm (2 and 8-in.). Using these parameters and conducting 

experiments both with and without the use of the horizontal 

blocking shelf resulted in a matrix of eight (2 x 2 x 2) different 

tests. Each test was run three times for a total of 24 tests. All 

24 tests were conducted in random order (Table 1).

Airflow ratios of 2:1 and 1.25:1 were attempted by targeting 

collector volumes of 0.472 and 0.295 m3/s (1,000 and 625 cfm), 

while keeping the bailing airflow volumes constant at 0.236 

m3/s (500 cfm). After adjusting for temperature and pressure 

changes, actual bailing airflow volumes averaged 0.242 m3/s 

(513 cfm) and ranged from 0.236 to 0.245 m3/s (500 to 519 

cfm). For tests targeting a collector volume of 0.472 m3/s 

(1,000 cfm), the actual levels varied between 0.437 and 0.464 

m3/s (925 and 984 cfm) and averaged 0.450 m3/s (953 cfm), 

resulting in an average airflow ratio of 1.9:1. Tests targeting 

a collector volume of 0.295 m3/s (625 cfm) varied between 

0.278 and 0.300 m3/s (589 and 636 cfm) and averaged 0.288 

m3/s (610 cfm), resulting in an average airflow ratio of 1.2:1. 

Experimental airflow ratios were slightly lower than targeted 

ratios due to a slight drop in collector airflow observed during 

the course of each test, which occurred due to material loading 

on the baghouse filters.

Respirable dust levels outside of the shroud were measured 

using samplers consisting of 37-mm filter cassettes and 10-mm 

Dorr-Oliver cyclones operated at a flow rate of 2 L/min. Two 

samplers were placed on each side of the box, approximately 

406 mm (16 in.) from the box and 610 mm (24 in.) above the 

ground. The RAM samplers described in the preliminary testing section were also used to determine when dust levels in the 

chamber had stabilized. At this point, the gravimetric samplers 

were energized for a period of 30 minutes for each test. The 

limestone dust feed rate averaged 23.8 g/min during the factorial 

experiments, ranging from 21.4 to 25.0 g/min. This level allowed 

sufficient gravimetric filter loading during low dust concentration tests. The final chamber dust concentration was determined 

from the average of the eight gravimetric samples.

Analysis of two-level factorial experiment

Use of a two-level factorial design experiment facilitated an 

analysis of variance (ANOVA) to determine the relative importance of the independent variables, including airflow ratio, 

shroud gap height, and the use of the blocking shelf on the 

dependent variable of dust concentration outside the shroud. 

Table 1 shows the experiment matrix and average gravimetric 

results. Table 2 shows the ANOVA for the experiment. Each 

independent variable was found to be highly significant (95% 

confidence interval did not include zero) and had a similar 

impact, in terms of magnitude, on dust levels outside of the 

shroud. Results from the experiment suggest that the use of a 

horizontal blocking shelf can overcome the detrimental effects 

on dust concentrations resulting from increasing the horizontal 

gap between the bottom of the shroud and the ground from 51 

to 203 mm (2 to 8 in.) or resulting from decreasing the airflow 

ratio from 1.9:1 to 1.2:1. Figure 5 shows dust levels under 

various operating conditions both with and without the use of 

the air-blocking shelf. Figure 6 shows relative percentage dust 

reductions measured when using the shelf. At an airflow ratio 

of 1.9:1 and a 51 mm (2 in.) shroud gap height, the use of the 

shelf decreased dust levels outside of the shroud by 81%. The 

reduction achieved with the shelf was 70% at a ratio of 1.9:1 

and a 203-mm (8-in.) gap and was 77% at a ratio of 1.2:1 and a 

51-mm (2-in.) gap. A reduction of 38% was achieved through 
the use of the shelf under poor operating conditions consisting 

of an airflow ratio of 1.2:1 and a 203-mm (8-in.) shroud gap 

height. The independent variables examined for this experiment accounted for approximately 90% (adjusted R2) of the 

difference in measured dust levels outside of the shroud.

Figure 5 — Dust concentrations outside the shroud with 

and without the air-blocking shelf, standard deviation bars 

shown.

Figure 6 — Measured dust reductions using the air-blocking 

shelf.

Table 2 — ANOVA for two-level factorial experiment.

 Regression statistics

Multiple R 

R square 

Adjusted R square 

Standard error 

Observations 

0.958

0.918

0.905

3.17798

24

ANOVA

 Degrees of Sum of Mean 

 freedom squares square F-ratio 

Significance 

F

 Regression 3 2252.438 750.813 

 Residual 20 201.991 10.100 

 Total 23 2454.430

74.341 0.000 

 Standard 

 Coefficients Error t-value 

Significance 

t

 Intercept 30.553 1.297 

 Airflow ratio -9.069 1.297 

 Shroud gap -12.431 1.297 

 Air blocking shelf -11.774 1.297 

23.549 

-6.99 

-9.581 

 -9.075 

0.000

0.000

0.000

0.000

Analysis of the blocking shelf when vertical 

leakage occurs

Two separate experiments were conducted to test the effectiveness of the air-blocking shelf for reducing dust when vertical 

leakage was present in the shroud design. As discussed above, 

vertical leakage scenarios can arise when sectional conveyor 

belt pieces are incorporated into the shroud design. The first 

experiment examined full-seam vertical leakage, where the 

vertical leakage extended from the base of the shroud to the 

bottom of the drill table. This condition 

was simulated by attaching hinges to the 

topside of the plywood on the off-inlet side 

of the shroud and propping the shroud open 

a distance of 152 mm (6 in.) at the base. The 

second experiment examined partial vertical seam leakage, where the vertical leakage extended from the base of the shroud 

to a point just below where the blocking 

shelves were installed. Figure 7 shows the 

shroud configurations for both the full and 

partial vertical leakage scenarios. During 

these experiments, both the airflow ratio 

and shroud gap height were kept constant 

at 2:1 and 51 mm (2 in.), respectively. A 

total of six tests were conducted for each 

experiment, alternating between using and 

not using the air-blocking shelf.

For the full vertical leakage experiment, 

dust concentrations outside of the shroud 

averaged 10.69 mg/m3 (0.50 standard deviation) when using the shelf, and 10.61 

mg/m3 (0.44 standard deviation) when 

not using the shelf. Calculation of the 

95% confidence interval (0.08 ± 1.04) for 

comparing the difference between two means with equal variance could not exclude zero, indicating that no true difference 

between the means existed. Therefore, the air-blocking shelf 

was not effective for controlling dust under the condition of full 

vertical leakage. Researchers hypothesized that air traveling 

down the sides of the shroud struck the shelf and was forced 

out through the gap created to simulate vertical leakage during 

the experiments.

For the partial vertical leakage experiment, dust concentrations outside of the shroud averaged 2.06 mg/m3 (0.26 standard 

deviation) when using the shelf, and 9.60 mg/m3 (0.65 standard 

deviation) when not using the shelf. Calculation of the 95% 

confidence interval (-7.54 ± 1.11) for the difference between 

these two means excluded zero, indicating that a true difference 

did exist. The air-blocking shelf reduced dust levels outside 
the shroud by 79% when the vertical leakage was contained 

below the shelf plane. 

The results from the vertical leakage experiments underscore 

the importance of maintaining a tight shroud seam seals at 

least to the horizontal plane created by the lowest air-blocking 

shelf. Angle iron installed at the seam corners may be suitable 

for creating these seals.

Figure 7 — Shroud configurations for both (a) partial and (b) full vertical leakage configurations.

Conclusions

One objective of a recently initiated NIOSH project was to 

develop an improved shroud design for lowering dust liberation 

from surface mine drilling machines. Emphasis was placed on 

a design that would minimize resources and maintenance. During the course of the project, researchers discovered a Coandӑ 

effect airflow pattern under the shroud enclosure created by the 

bailing air, which traveled upward along the drill steel, fanning 

out across the bottom of the drill table and flowing down the 

sides of the shroud. This dust-laden bailing air struck the ground 

and fanned out under gaps between the shroud and the ground, 

resulting in respirable dust leakage to the environment. 

Laboratory tests showed that installation of a horizontal 

air-blocking shelf around the inside perimeter of the shroud 

disrupted this Coandӑ pattern and improved dust capture under the shroud. The blocking shelf was 152-mm (6-in.) wide 

and located at a height equal to 2/3 the distance measured 

from the bottom of the shroud to the bottom of the drill table. 

Under operating conditions consisting of a 1.9:1 collector to 

bailing airflow ratio, and a 51-mm (2-in.) shroud gap height, 

the air-blocking shelf reduced dust levels outside the shroud 

by approximately 81%. The air-blocking shelf was also shown 

to be capable of overcoming the adverse impact of vertical 

leakage along the shroud seams, as long as this leakage was 

confined to an area below the horizontal plane defined by the 

shelf location. Performance of the shelf was diminished to a 

38% reduction in dust levels when the operating conditions 

were poor, including a 1.2:1 airflow ratio and a 203-mm (8-in.) 

shroud gap height. 

Laboratory testing also identified several important design 

criteria for a horizontal air-blocking shelf. First, a 102- to 152-

mm (4- to 6-in.) shelf width appeared to be sufficient. Second, 

lowering the shelf did not improve performance. Third, the 

shelves, which were located on each side of the shroud, did not 

have to be placed on the same horizontal plane in order to be 

effective. Rather, coverage of the parametric cross-section of 

the shroud seemed to be the important design criteria. Finally, 

testing also demonstrated that material loading on top of the 

shelf only slightly diminished performance. Future research 

efforts will focus on evaluating the air-blocking concept for 

dust control on actual surface drilling machines. 

References

Joy, J., 2007, personal contact about surveillance analysis of MSHA’s dust 

sampling database, (National Institute for Occupational Safety and Health, 

Respiratory Health Control Branch: Pittsburgh, PA).

Organiscak, J.A., and Page, S.J., 1995, “Assessment of Airborne Dust Generated from Small Truck-Mounted Rock Drills,” U.S. Bureau of Mines, Report 

of Investigations 9616. 

Organiscak J.A., and Page, S.J., 2005, “Development of a dust collector inlet 

hood for enhanced surface mine drill dust capture,” International Journal 

of Surface Mining, Reclamation, and Environment, Vol. 19, No. 1, March, 

pp. 12-28.

Page, S.J., and Organiscak, J.A., 2004, “Semi-empirical model for predicting 

surface coal mine drill respirable dust emissions,” International Journal of 

Surface Mining, Reclamation, and Environment, Vol. 18, No. 1, pp. 42-59.

Reed, W.R., Organiscak, J.A., and Page, S.J., 2004, “New approach controls dust 

at the collector dump point,” Coal Age, Vol. 109, No. 6, June, pp. 20-22.

Society of Automotive Engineers, 1986, “Aerodynamic Flow Visualization 

Techniques and Procedures,” SAE Information Report HS J1566 Jan86, 

Warrendale, Pennsylvania. 

Zimmer, R.A., and Lueck, S.R., 1986, “Investigation of Quartz Dust Sources 

and Control Mechanisms on Surface Mine Operations, Vol. I – Results, 

Analysis, and Conclusions,” U.S. Bureau of Mines Final Contract Report, 

NTIS: PB 86-215852.

Zimmer, R.A., Lueck, S.R., and Page, S.J., 1987, “Optimization of Overburden 

Drill Dust Control Systems on Surface Coal Mines,” Int. J. Surf. Mining, Vol. 

1, No. 2, pp. 155-157.