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TESTS OF FIBER-REINFORCED SHOTCRETE AT THE CHIEF JOSEPH 

MINE, BUTTE, MONTANA 

Lewis A. Martin and John P. Dunford 

Spokane Research Laboratory National Institute for Occupational Safety and Health 

Spokane, WA USA 

Mary M. MacLaughlin and Robert R. Cronoble 

Montana Tech of the University of Montana. Butte, MT USA 

Abstract 

 Researchers from the Spokane Research Laboratory, 

National Institute for Occupational Safety and Health, in 

cooperation with engineers from the Department of Mining 

and Geological Engineering, Montana Tech, Butte, MT, 

conducted tests to evaluate the tensile strength, fiber count, 

and adhesion of shotcrete applied to panels at the Chief 

Joseph Mine, a research and training facility operated by 

Montana Tech. The tests were conducted in the mine ramp 

45 m (150 ft) inward from the access portal at a 15.25-m- 

(50-ft) long test area divided into five 3-m- (10-ft) long 

sections. The shotcrete was applied with an Aliva 240.5 

machine fitted with a predampener. Different amounts of 

fiber were entrained for each panel. Background data for 

the tests were obtained from earlier tests in Nevada, where 

the typical amount of fiber in the shotcrete is 6.54 kg/m3

(11 lb/yd3

). All shotcrete was applied to a minimum 

thickness of 76 mm (3 in). Cylindrical cores were drilled at 

the wall-rock interface, and adhesion was tested after 28 

days of curing. The goal of this research is to reduce the 

need to install multiple supports at the advancing face by 

creating a safer mining system that limits the time miners 

are exposed to unprotected roof. 

Introduction 

 Researchers from the Spokane Research Laboratory, 

National Institute for Occupational Safety and Health 

(NIOSH), in cooperation with engineers from the 

Department of Mining and Geological Engineering, 

Montana Tech of the University of Montana, Butte, MT, 

conducted tests to evaluate the tensile strength, fiber count, 

and adhesion of shotcrete applied to panels at the Chief 

Joseph Mine, Butte, MT, a research and training facility 

operated by Montana Tech. The goal of the research study 

was to determine the effectiveness of the primary support, 

which consists of synthetic-fiber-reinforced shotcrete. 

Previous studies conducted at an underground mine in 

Nevada showed that fiber-reinforced shotcrete could be 

used instead of unreinforced shotcrete and wire mesh mats 

as ground support in rocks with a low rock mass rating 

(RMR) (1). By conducting round-panel tests to determine 

compressive strength, adhesion, and modulus, engineering 

data were collected that will allow rock mechanics 

engineers to improve mine design. 

 Prior to the tests, the mine site was thoroughly mapped 

to establish the orientation and number of joints, fractures, 

and faults; RMR; and rock quality (Q) values to establish 

the best conditions for applying the shotcrete. Samples were 

collected and tested to determine the unconfined compressive strength of the rock, which was found to range from 12 

to 234 MPa (1740 to 34,000 psi) (2). Representative cell 

mapping rather than scan line mapping was used to collect 

the data because this method generates a better data set, 

particularly at sites of limited size such as in this mine. 

 RMR values resulted in grouping the rock into five 

categories (very good, good, fair, poor, and very poor) on 

the basis of scores of 68, 62, 57, 80, and 82, respectively 

(2). Cell 3, which was rated as fair, would correlate with 

RMR's found in Nevada underground mines. This section 

also contained three dominate joint sets. 

 The tests were conducted in the mine ramp 45 m (150 

ft) inward from the access portal at a 15.25-m- (50-ft) long 

test area divided into five 3-m- (10-ft) long sections. The 

shotcrete was applied wall to wall by a certified nozzle man 

at a thickness of 76 mm (3 in). In addition, fiber count and 
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adhesion tests were conducted at each section and then 

correlated to the rock mass. 

 The shotcrete was mixed at a plant in Big Timber MT, 

using a mixture (SUPERSTICK1

) developed by Thiessen 

Team USA for Nevada and Montana mines and applied 

with a dry system. The shotcrete mix consisted of portland 

cement, microsilica, coarse aggregate, fine aggregate, sand, 

water reducer, and bar-chip fiber (4). 

Australian Round Panel Flexure Test 

 Shotcrete round panels were constructed according to 

American Society for Testing and Materials (ASTM) 

standard c1550 and were mounted on pallets for ease of 

handling. The shotcrete was applied to the panels following 

a sequence in which fiber densities were increased in 0.3-

kg/m3

 (0.5-lb/ft3) increments, beginning with 5.9 kg/m3 (10 

lb/ft3) through 7.13 kg/m3 (12 lb/ft3). The panels were 

washed down and degreased before the shotcrete was 

applied; following application, the panels were covered 

with plastic and wet burlap for 3 days and then taken to 

cure in the mine haulageway for 28 days. 

 Fifteen panels (three for each quantity of fiber added) 

were tested with a portable round-panel test frame and a 10-

ton ram fitted with an electronic pressure transducer and 

pressure gauge to duplicate readings (figure 1). Displacement data were gathered by a string potentiometer attached 

to the frame and ram pedestal. All data were stored on a 

Campbell Scientific 21 datalogger. Scan rates were taken at 

1-sec intervals. Typically, each panel was loaded to failure 

(or just 75 mm [3 in] of deformation) in about 15 min. The 

graphs in figure 2 (next page) show each test sequence. 

 Analysis of the graphs shows a repeatable loading 

trend in the data that supports the use of this field test 

frame. This finding encourages researchers to think that 

similar tests could easily be conducted at other mine sites. 

An analysis of the data shows that the initial breaking 

strength of the samples tends to deteriorate as fiber count 

increases. This result was not unexpected because the fiber 

is designed to absorb energy after initial failure of the 

shotcrete. Also, as the amount of fiber dosage increases, the 

ultimate carrying capacity of the panel also increases. 

Graph C in figure 2 shows that there may be a point of 

diminishing return on adding more fiber. Published articles 

mention that dosage rates between 9 to 18 kg/m3

 (15 to 30 

lb/yd3

) are used in mining applications (5). In these and 

other tests conducted in Nevada mines, a dosage of 6.5 

kg/m3

 (11 lb/yd3) of the bar-chip fiber appears to be optimal 

in maintaining residual strength of the round panels. 

 

1 Mention of specific products and manufacturers does not 

constitute endorsement by the National Institute for Occupational 

Safety and Health. 

 In figure 3, the initial dose of 6.5 kg/m3 (11 lb/yd3) 

provides a residual strength of 12 kN (2700 lb). The shotcrete panel shows higher load-carrying capacities, which 

indicates that cost savings could be achieved by using less 

fiber and still provide ultimate safety. The peak strength of 

the panels is lower with higher fiber counts than the 

strength of the panel with the lower fiber counts; however, 

a residual load over 76 mm (3 in) of deformation has 

greater energy absorption with increased fiber dosages up to 

the optimal amount of 6.5 kg/m3

 (11 lb/yd3). 

Figure 1.—Portable shotcrete round panel tester 

Figure 3.—Averaged peak load versus displacement for 

initial and residual strengths of round panel tests 
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Figure 2.—Load versus displacement in round panel tests. A, Test 1 at fiber dosage of 5.9 kg/m3 (10 lb/yd3); B, test 2 at fiber dosage of 6.2 

kg/m3

 (10.5 lb/yd3); C, test 3 at fiber dosage of 6.5 kg/m3 (11 lb/yd3); D, test 4 at fiber dosage of 6.8 kg/m3 (11.5 lb/yd3); E, test 5 at fiber 

dosage of 7.1 kg/m3

 (12 lb/yd3).
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Table 1.—Fiber count information in five test cells 

 Cell A Cell B Cell C Cell D Cell E 

 West East West East West East West East West East 

 12 5 3 5 10 5 7 8 7 6 

 7 7 3 9 6 9 7 8 7 6 

 5 8 3 2 5 12 7 7 8 5 

 5 9 4 7 5 7 6 6 9 13 

 6 3 6 6 3 6 6 10 4 8 

Average fibers 

 7 6.4 3.8 5.8 5.8 7.8 6.6 7.8 7 7.6 

Accumulative average 

 6.7 4.8 6.8 7.2 7.3 

Table 2.—Fiber count information with outliers removed 

 Cell A Cell B Cell C Cell D Cell E 

 West East West East West East West East West East 

 7 8 4 5 6 9 7 8 7 6 

 6 7 3 6 5 7 7 8 7 6 

 5 5 3 7 5 6 6 7 8 8 

Average number of fibers 

 6 6.7 3.3 6 5.3 7.3 7.6 7.6 7.3 6.7 

Accumulative average 

 6.3 4.7 6.3 7.2 7 

Fiber Count Analysis for In-Place Shotcrete at Chief 

Joseph Mine 

 The analysis of fiber density was trying. An engineer 

working in the Nevada mines brought up the need to count 

fiber in place to determine if the mixing rate was what the 

design engineer specified. It would be desirable to have a 

nondestructive method to determine fiber count and thus 

establish a relationship to the overall physical properties. 

The method tried in this study was to mark an array of five 

10.2-cm (4-in) squares on both sides of the entry in each 

section where the amount of fiber in the shotcrete changed. 

The squares were inspected visually, and the fibers within 

each marked square counted. They were then were logged 

for statistical analysis. 

 As can be seen in table 1, a visual relationship between 

the amount of fiber added to the mix and the number of 

fibers counted was not apparent. However, outlier numbers 

were seen in some sets. Another comparison was made by 

throwing out the high and low counts in each set (table 2). 

Again these data sets do not define a pattern that can be 

used to determine the amount of fiber visually. 

 Another count was done using a 25-cm (10-in) square 

in a five-set array (table 3). These data sets showed an 

increasing trend in fiber count with simple averaging. 

Table 3—Fibers counted in five-set array in 25-cm (10-in) square 

 Cell A Cell B Cell C Cell D Cell E 

 West East West East West East West East West East 

 22 13 7 10 34 19 18 28 36 17 

 13 15 11 23 20 12 22 19 28 17 

 1 5 11 13 22 29 18 10 23 13 

 8 10 13 17 15 23 27 25 32 120 

 2 13 27 9 12 11 31 29 26 15 

Average fibers 

 13 11.2 13.8 14.4 20.6 18.8 22.2 22.2 29 16.4 

Accumulative average 

 12.1 14.1 19.7 22.7 
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 Further analysis was conducted in which the high and low fiber counts were thrown out (table 4). 

Table 4—Fibers counted in five-set array in 25-cm (10-in) square with high and low values eliminated 

 Cell A Cell B Cell C Cell D Cell E 

 West East West East West East West East West East 

 13 13 11 10 20 12 18 19 26 17 

 8 10 11 13 22 19 22 25 32 17 

 21 13 13 17 15 123 27 28 28 15 

Average fibers 

 14 12 11.7 13.3 19 18 22.3 17.6 28.6 16.3 

Accumulative average 

 13 12. 18.5 19.9 22.3 

 Based on just the core number of fibers, the upper 

dosage was more defined, but the lower dosages started to 

map each other. A larger population should be analyzed so 

that the outliers would become less significant and fiber 

readings could give a rough example of the dosage used. 

Continued research in this area is needed. 

Adhesion Pull Tests 

 A portable drill with a thin-walled coring bit was used 

to develop a simple, portable method to obtain information 

on in-mine shotcrete adhesion. Adhesion of the shotcrete to 

the mine roof and walls was determined by conducting pull 

tests with a disk-type puller glued to the shotcrete surface of 

the wall with epoxy. A diamond core drill was then used to 

overcore the shotcrete until a full kerf of mine wall had 

been drilled. The apparatus was applied, and tests 

commenced. The RMR for each section, along with 

location and sample condition, are shown in table 5. 

Table 5—Panel information 

 Panel location in mine 

 0-2.5 m 2.5-5 m 5-7.5 m 7.5-10 m 10-12.5 m 

RMR 69 34 61 74 69 

Status Broke 

sample 

NA Broke 

sample 

NA Broke 

sample 

NA = Not applicable. 

 Upon running the tests at the mine, it was determined 

that another drill support platform needed to be developed, 

as the current drill does not generate enough axial force to 

penetrate the mine rock without jeopardizing the integrity 

of the shotcrete sample. 

 With an RMR less than a critical value, which appears 

to be approximately 50 at the Chief Joseph Mine, the rock 

itself is deteriorating, not the bond of the shotcrete to the 

wall and roof. Further research is necessary to understand 

fully this apparent critical RMR value with respect to rock 

versus bond deterioration and its influence on support 

systems in underground mines. 

Dynamic Modulus of Shotcrete 

 Tests were conducted on cores taken from the round 

panels using an apparatus called the Hopkinson bar. The 

Hopkinson bar is a device that shoots projectiles at a test 

specimen. Velocity and strain readings are taken from the 

reactive bar. From these data and the calculations, a 

dynamic modulus can be determined. For shotcrete in 

which the amount of fiber equaled 6.5 kg/m3

 (11 lb/yd3), 

the dynamic modulus E was 41 GPa (6 × 106 psi).

Conclusions 

 The panel tests conducted at the Chief Joseph Mine 

showed consistent results for initial round panel failure and 

energy absorption. With this test program, the characteristics of the shotcete being used at a specific site could be 

determined for various curing times without transporting 

the specimens to an independent lab. The data suggest that 

the 6.5-kg/m3

 (11-lb/yd3) dosage is optimal with respect to 

ultimate load capacity. Fewer fibers result in less reinforcement of the shotcrete, and a greater number of fibers 

interferes with the strength of the shotcrete. 

 Counting the fibers shows promise if a 25-cm (10-in) 

square is used and enough samples are taken. The 10 

sample areas used in these tests showed delineation, but 15 

would be more statistically significant. The man-hour 

requirement is small (less than a minute a square), so 

quality control could be obtained through an inexpensive 

test. 

 Adhesion pull testing is a slow and arduous process 

that is not to be taken lightly. Development or use of proper 
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drill equipment is imperative to make sure that the drilling 

is done without creating vibrations as the drill bit passes 

through the interface. 

References 

1. Brady, T., L. Martin, and D. Thibadeau. Overview of 

Weak Rock Mass Assessment in Nevada. Presentation 

at Elko Mining Days, Elko, NV, June 2002. 

2. MacLaughlin, M., and R. Conoble. Site Characterization of a 50-Foot Section of the Chief Joseph Decline, 

Butte Montana. Final Report, April 5, 2004, 15 pp. 

. Pakalnis, R. Site Characterization Design of Underground Mine Openings. Notes taken during short 

course at the Spokane Research Laboratory, May 28-

29, 2002. 

4. Schumacher, J. Shotcrete Specifications for Superstick 

Shotcrete. Shotcrete Handbook, USA. Theissen Team 

USA. 

5. Spearing, S. Shotcrete as an Underground Support 

Material. Foundations for Design. In Handbook of 

Underground Mining Methods, W. Husterlid and R. 

Bullock, eds. Soc. of Mining Engineers, 2001, pp. 

563-568.