FIELD TEST WITH STRAIN-GAUGED FRICTION BOLTS AT THE GOLD HUNTER MINE, 

MULLAN, IDAHO, USA 

Jeffrey Johnson, Ted Williams, Carl Sunderman, and Stephen Signer 

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, Washington, USA 

Douglas Bayer, Hecla Mining Company, Mullan, Idaho, USA 

ABSTRACT 

 To measure the loading behavior of friction bolts, researchers at 

the Spokane Research Laboratory of the National Institute for 

Occupational Safety and Health (NIOSH) installed strain gauges on 

the interior of friction bolts and developed a battery-powered 

miniature data acquisition system (MIDAS) that fits inside the 

hollow portion of the friction bolt. The advantages of this system 

are that it is protected from face blasts and eliminates the need for 

external wiring. Laboratory pull-out tests showed that friction bolts 

installed in a concrete block with resin were loaded to about 1.8 

tons/ft of bolt length; bolts installed without resin were loaded to 

1.3 tons/ft. Three strain-gauged friction bolts were installed at 

Hecla Mining Company’s Gold Hunter Mine, Mullan, ID, in the 

wall or rib of a mechanized cut-and-fill stope. The object of these 

tests was to establish an installation procedure for the bolt-andMIDAS combination and to evaluate performance. The stope was 

advanced in three 15-ft increments and strains induced in the rock 

bolts by stress changes in the rock were recorded every 30 minutes. 

The MIDAS proved to be rugged enough to withstand the shock 

and vibration from nearby (5 ft) face blasting. Data from MIDAS 

showed that the friction bolt installed with resin was the most 

sensitive to each face advance. It showed a steady increase in 

loading to a maximum value of 1000 microstrain. The friction bolt 

installed without resin showed stick-slip behavior with loads to a 

maximum of 400 microstrain before the stope was completed. 

INTRODUCTION 

 Ground control is the most important aspect of safety in 

underground mining. Today, rock bolting is the most effective and 

economical means of ground support. A commonly used rock bolt 

is the friction bolt, invented by Dr. James Scott in 1972 (Scott, 

1981). In 1973, Scott sold the rights to the Ingersol Rand 

Company, who registered the name “Split-Set” as a trademark in 

1977. An elastic-plastic analysis of a Split-Set bolt by Davis 

(1979) showed that loading on a friction bolt was not a simple 

uniform frictional stress distribution around its circumference, but a 

complicated mix of friction, point, and free-surface loading 

conditions. Willaescusa and Wright (1977) directed their research 

at increasing the support load of friction bolts by filling the cavity 

of the friction bolt with cement. The conclusions of their work 

state that “grouted bolts are likely to provide up to four times the 

initial bond strength per meter of embedment length provided by 

the ungrouted bolts.” 

 Although the use of cement grout is a very effective method of 

increasing bond strength, it is not as convenient as placing epoxy 

resin cartridges into the hole. However, resin cartridges need to be 

mixed properly to develop full strength. The jackleg drill 

commonly used to install friction bolts is not able to mix the resin 

completely, but rather punctures and thrusts its way into the resin. 

Although the resin is not properly mixed, the amount of mixing the 

resin does receive is sufficient to cause the resin to harden and 

increase pull-out load on the bolts. 

 Friction bolts have been used for 30 years, but little is known 

about their actual mechanical behavior when installed in mine rock. 

To learn more about such behavior, researchers at NIOSH’s 

Spokane Research Laboratory (SRL) developed a miniature data 

acquisition system (MIDAS) (figure 1) that would fit inside the 

center of an installed 46-mm friction bolt (Sunderman, personal 

communication, 2001 ). To fit inside a 39-mm bolt, the first 12 in 

of the bolt hole must be overdrilled or reamed with a 1-5/8-in bit so 

that the wall of the friction bolt will not collapse. 

LABORATORY TESTS 

 Laboratory tests consisted of uniaxial tension tests in a loading 

machine and pull-out tests in a large concrete block performed on a 

39-mm in diameter, 5-ft-long friction bolt equipped with 16 strain 

gauges. The bolt had a measured thickness of 0.086 in, an average 

diameter of 1.453 in, an average gap width of 0.666 in, and a crosssectional area of 0.335 in2

. Nine axial gauges were attached to the 

bolt beginning 6 in from the ring and spaced every 6 in to the end 

of the bolt. The circumferential gauges were started 12 in from the 

ring and spaced every 6 to 12 in from the end of the bolt. 

 The bolt was then attached to a Tinius-Olsen stiff testing 

machine. An initial load of 1000 lb was applied to set the bolt, 

followed by increases in load in 1000-lb increments up to 12,000 

lb. A yield load of 16,000 lb and an ultimate load of 20,000 lb was 

assumed for the bolt. Figure 2A shows axial strains as a function of 

applied load. All axial strain gauges 12 in from the end of the bolt 
Figure 1.─Minature data acquisition system (MIDAS) 

show a linear response with an r-squared value of 0.96. Measured

elastic modulus was about 10% higher than steel’s nominal value

of 30 million psi. 2B shows circumferential strains as a function of 

applied load. The circumferential strains were also linear, with

Poisson ratios ranging from 0.20 to 0.25 as computed from pairs of 

axial and circumferential gauges. Circumferential strains are all

negative, indicating that radially the bolt is in compression and 

getting smaller in diameter. This aspect will be important for 

explaining the effects of resin in pull-out tests. 

The pull-out tests were performed using a large concrete block

and installing the bolts with and without resin. The first test 

(without resin) used the same bolt as used for the uniaxial tension 

test. A 1-3/8-in hole was drilled into the concrete block, and the 

bolt was inserted. All the radial gauges showed extremely large

strains, and most did not survive insertion. Because of the low 

survival rate of the circumferential gauges and to keep the costs of 

the experiment at a minimum, all friction bolts were then 

constructed with only axial strain gauges. Figure 3A shows axial 

strain versus pull-out load for a 5-ft-long friction bolt installed 

without resin. A zone in the center shows most of the axial gauges 

behaving similarly. The gauges at both ends, the ring, and the 

tapered end are outside the center zone. 

The center zone shows that along most of the length of the 

friction bolt, an approximately equal amount of strain is developed. 

This finding is different than that shown during pull tests on 

grouted bolts, where a decrease in load is seen with distance from 

applied pull load. Pull-out load is about 6.5 tons, or 1.3 tons/ft, 

which is a normal value for a pull test on a friction bolt. Calculated 

stiffness of the bolt using the pulling force and the measured strains 

is 33.3 million psi, which is about 11% higher than the nominal 30 

million psi value.

Figure 2.─Uniaxial tension (positive) test on strain-gauged friction 

bolt. A, Axial strains; B, circumferential or radial strains. 

Resin cartridges were used in two pull-out tests. Two sticks of

1-1/8- by 24-in tubes of medium-setting resin were placed in the 

drill hole and a strain-gauged friction bolt was inserted. Only three 

strain gauges were installed on the bolts, one in the middle of the 

bolt and the two others in the middle portions of the top and bottom 

halves. 
Figure 3.─Pull-out test on strain-gauged friction bolts. A, Without 

resin showing uniform axial strains along most bolt lengths; B, with 

resin showing axial strain related to distance from applied load. 

 Figure 3B shows axial microstrain versus pull-out load for a 

friction bolt installed with resin. Loading behavior is similar to that

with grouted bolts, such as reinforcement bars (Serbousek et al. 

1987; Signer 1990) and for cable bolts (Martin et al. 2002). 

The increased strain in the friction bolt may come from two 

conditions: bonding of the rock–bolt interface and filling the 

hollow portion of the bolt at the back end of the drillhole. When 

the friction bolt is inserted into the resin cartridge, it first pushes the

cartridge back into the hole. It then punches holes in the cartridges, 

spreading the resin between the bolt and the rock, but also packing 

a large portion of the resin into the hole and filling the center 

cavity. When a friction bolt takes axial load in tension, the

diameter of the bolt is decreased according to Poisson's ratio,

allowing the bolt to come out of the hole more easily. The resin in

the back of the hole fills the cavity and the slot that prevents the

bolt from getting smaller in diameter, as demonstrated in the 

laboratory uniaxial tension test. The resin has an effect similar to

filling the cavity with cement grout to increase pull-out load. 

FIELD TESTS

On October 10, 2002, four strain-gauged friction bolts were installed in the Gold Hunter Mine. The Gold Hunter is accessed from

the Lucky Friday Mine, b

(figure 4). The mining

method is underhand

mechanized-cut-and fill.

The purpose of the field

test was to test the

MIDAS and measure the

strains developed in friction bolts during actual 

mining. All bolts were 5

ft long; two were 39-mm 

in diameter, and two

were 46 mm in diameter.

For these tests, each bolt

was equipped with four

strain gauges placed 1 ft

apart, as shown in figure 

5. The strain gauges

were numbered sequentially (1, 2, 3, and 4) from 

the ring end and read using the MIDAS. 

Figure 4.─Location o

oth owned by the Hecla Mining Co. 

f Gold Hunter Mine. 

Three bolts were installed in a vertical row approximately 5 ft

Figure 6 shows the advance of the stope (dashed lines) and the 

back from the face on the 4520 level of the mine and 3, 5 and 6 ft 

above the floor (section AA, figure 6). A 39-mm bolt was installed 

without resin at the 6-ft position and with resin at the 5-ft position.

A 46-mm bolt was installed without resin at the 3-ft position. All 

bolts were installed during the regular mining cycle, and no special 

training or equipment was needed. However, the size of the 

MIDAS required that the first foot of the 39-mm bolts had to be 

reamed with a 1-5/8-in bit to allow the MIDAS to be inserted into 

the friction bolt. A newer model of MIDAS will fit inside the 39-

mm friction bolt without the need to ream the bolthole. All bolts

were installed with the friction bolt slot down to allow water to 

drain from the hole and to keep the gauges as high as possible on 

the back side of the drill hole. A fourth bolt was installed near an

ore pass about 100 ft back from the face. The ore pass was chosen 

by mine personnel because the area was being loaded and causing 

the end rings of the friction bolts to yield. No resin was used in the

ore pass hole. 

date corresponding to the position of the face so they can be 

compared to the bolt strain data. Each advance was about 15 ft. 

Data from the strain gauges were collected from October 10 to 

October 30, 2002, at which time the MIDAS was removed from 

each bolt, and the stope was backfilled.
22nd International Conference on Ground Control in Mining 

Figure 5.─A, Strain-gauged friction bolt with MIDAS attached to laptop computer; B, Diagram of friction bolt showing locatin of strain gauges; C, 

diagram showing installation of 39-mm strain-gauged friction bolt and reamed section of hole. 

FIELD DATA AND RESULTS

Figure 7 shows the data plots of axial strains versus time. Since 

the MIDAS was zeroed out, the first movement in strain resulted

from the first advance of the drift, resulting in a new face (October 

11 on the figure). A day later, the second advance was made 

(October 12 on the figure). The third advance was 5 days later, 

resulting in the final face of the stope on October 17. 

 Figure 7A shows the strain on the 39-mm friction bolt installed

with resin. Strain gauge 3, at 3 ft back from the ring end, shows the 

greatest amount of initial and final strain. Tensile strain is positive. 

The initial instantaneous strain shown by gauge 3 is 400 

microstrain and resulted from the first face advance. The sharply 

rising increase to 700 microstrain resulted from an additional 300

microstrain of tensile strain from the second face advance. A 5-day 

pause in mining resumed with the third and final face advance on

October 17, which completed mining of the stope and resulted in

another increase of slowly creeping tensile strain of 300

microstrain. Final strain reached 1000 microstrain. The final stress 

in the bolt at gauge 3 is found by multiplying the strain by the 

Young’s modulus of the steel (30 million psi), resulting in a bolt

stress of 30,000 psi that is about one-half the yield strength of the 

bolt. 

Strain gauge 2, 2 ft back from the ring end, showed behavior

similar to that of gauge 3, but lower in magnitude, with

microstrains of 150 (initial), 300 (secondary), and 400 (final). 

Gauge 1, in the reamed out portion of the hole, responded to the 

initial advance with a slight negative or compressive strain and was 

flat until the third advance, where it showed a slowly creeping

tensile load increase of 100 microstrain. Gauge 4 showed an initial 

instantaneous tensile response of 200 microstrain that then declined

gradually but steadily to 50 microstrain at the end of mining. 

 Figure7B shows the 39-mm friction bolt installed without resin. 

The most prominent feature is the “stick-slip” pattern of the second

gauge, which occurred near the time of the third face advance. A 

slowly creeping stick-slip resulted in a final of value of about 400 

microstrain. Gauge 3 showed a pattern similar to that of gauge 2 

without the stick-slip for the second and third face advances. This 

gauge went into compressive strain with the first face advance.

Gauge 4 showed an initial tensile strain of 200 microstrain that 

decreased to about 100 microstrain at the end of mining. Gauge 1 

showed an initial strain of 325 microstrain that deceased quickly to

a value of 200 microstrain for most of the mining. 

 Figure 8A shows the 46-mm bolt installed without resin. One or 

two of the lead wires of strain gauge 2 were apparently torn off 

during installation. Gauges 1, 3, and 4 went into compression (-100

microstrain) during the first face advance. The second face 

advance caused gauges 3 and 4 to go slightly into tension while 

gauge 1 continued to increase in compressive strain to -400

microstrain. The reason for this compressive state is thought to be 
Figure 6.─Plan view of 4520 level. Section A-A shows location of three strain-gauged friction bolts installed 5 ft from stope face on October 10,

2002. Three advances of 20, 13, and 16 ft were made and the date of the new face labeled. A fourth bolt was installed near the ore pass. 

that the bolt was bending. The third face advance shows all

operating gauges undergoing slow tensile creep for an additional

100 microstrain.

A summary of the readings from the friction bolts at the face is

that these bolts showed a slowly creeping tensile strain increment

of 100 microstrain from the third face advance. The 39-mm

friction bolt installed with resin was the most sensitive to each face

advance and underwent the largest amount of strain. The 39-mm 

bolt installed without resin showed stick-slip loading, but generally 

showed the same pattern of loading as the bolt installed with resin.

In general, the gauges at the center of the bolts (2 and 3) carried

more load than the end gauges (1 and 4). The smaller strains on

gauge 1 on the 39-mm bolts may have resulted from the reamed 

borehole. The reason for the lower strains at gauge 4 is not known. 

The fourth friction bolt was installed in the rib near an ore pass 

about 100 ft from the initial stope face. Because some of the rings 

on the friction bolts were coming off in the mine, a bolt was placed 

there to determine the amount of loading taking place near the ore 

pass. Figure 8B shows that the initial face advance did not load the 

bolt significantly. A slight amount of loading (from 50 and 75 

microstrain) occurred near the third face advance for gauges 2 and 

1 respectively. The ring damage on the friction bolts may have 

resulted because bolts installed early in the mining cycle of the 

stope are initially loaded from the face advance and then are loaded 

by slow creep that ultimately exceeds the strength of the ring weld

on the friction bolt. 
Figure 7.─Axial strain versus time for 39-mm friction bolts installed 5 

ft from face. A, With resin; B, without resin. 

CONCLUSIONS

Strain gauges installed on friction bolts can be useful for

studying loading conditions. In a uniaxial test, friction bolts were

loaded as simple rods. In a pull-out test in which no resin was 

used, the bolt was loaded uniformly along its length. In a similar 

test with resin, bolt load was a function of the distance from the

applied load. In the field, strain gauges showed stick-slip behavior

in friction rock bolts installed without resin and no slippage when 

installed with resin. The latter bolt developed a higher load than 

the former. However, it should be noted that pull-out loads 

increase on friction bolts with time due to the effects of rock movement interlocking with the bolt and other environmental factors. 

Future work needs to be done to determine the effects of bending

on friction bolts and strain gauges. 

ACKNOWLEDGMENTS 

Special thanks go to Dr. Alan Campoli of Fosroc, Inc., for 

information and resin cartridges; staff of Hecla Mining Company 

who assisted in the installation of the strain-gauged friction bolts; 

Figure 8.─Axial strain versus time for 46-mm friction bolts installed

without resin. A, Bolt installed 5 ft from face and 3 ft above floor; B, 

bolt installed 100 ft from face and 5 ft above floor. 

Tom Brady, Lewis Martin, and Mike Jones of SRL for laboratory

assistance with drilling the concrete test holes and with the pull-out 

tests; Dr. Rimas Pakalnis and his students for discussions of friction

bolts; and John Goris of Thiessen TEAM and Terry Karlsen of

Inter-national Rollforms, Inc., for supplying the bolts. 

REFERENCES 

Davis, R. L. Split-Set Rock Bolt Analysis. International Journal of 

Rock Mechanics and Mineral Science and Geomechanical 

Abstracts, vol. 16, 1979, pp. 1-10. 

Martin, L., R. Pakalnis, and D. Milne. Determination of Physical

Properties of Cable Bolts in Cement Grout Pull Tests Using Instrumented King Wires. Presentation at CIM conference, April 30-

May 3, 2002, Vancouver, British Columbia, Canada. 

Scott, J. J. Ground Control Technology Transfer Problems in 

Metal and Coal Mines of the U.S.A. 22nd U.S. Symposium on 

Rock Mechanics, June 29-July 2, 1981, pp. 518-523. 
Serbousek, M. O., and S. P. Signer. Linear Load-Transfer Mechanics of Fully Grouted Roof Bolts. U.S. Bureau of Mines Report of 

Investigations 9135, 1987, 17 pp. 

Signer, S. P. Field Verification of Load Transfer Mechanics of 

Fully Grouted Roof Bolts. U.S. Bureau of Mines Report of 

Investigations 9301, 1980, 13 pp. 

Villaescusa, E., and J. Wright. Permanent Excavation Reinforcement Using Cement Grouted Split Set Bolts. In Proceedings of the 

Australasian Institute of Mining and Metallurgy, vol. 1, 1997. 
FIELD TEST WITH STRAIN-GAUGED FRICTION BOLTS AT THE GOLD HUNTER MINE, 

MULLAN, IDAHO, USA 

Jeffrey Johnson, Ted Williams, Carl Sunderman, and Stephen Signer 

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, Washington, USA 

Douglas Bayer, Hecla Mining Company, Mullan, Idaho, USA