IC 9469 

INFORMATION CIRCULAR/2004 

Design and Testing of a 

Nondestructive Friction 

Bolt Tester 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
Information Circular 9469 


Design and Testing of a Nondestructive Friction Bolt Tester 


By Lewis Martin, John Goris, and Lauren Roberts 

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES 


Centers for Disease Control and Prevention 


National Institute for Occupational Safety and Health
 

Spokane Research Laboratory 


Spokane, WA 


March 2004 
_____________________________________________________ 

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DHHS (NIOSH) Publication No. 2004-131 

CONTENTS 

Page 

Abstract ................................................................................................................................................................... 1 


Introduction ................................................................................................................................................................... 2 


Development ................................................................................................................................................................... 2 


Installation and test procedure......................................................................................................................................... 2 


Field tests ................................................................................................................................................................... 5 


Correlating rock mass rating to pull-out load.................................................................................................................. 5 


Conclusions ................................................................................................................................................................... 6 


References ................................................................................................................................................................... 6 


ILLUSTRATIONS 

1. Diagram of friction bolt puller assembly............................................................................................................... 3 


2. Installation of pull claw on friction bolt ................................................................................................................ 3 


3. Complete test assembly installed on friction bolt.................................................................................................. 3 


4. RMR versus pull-out strength................................................................................................................................ 5 


TABLES 

1. Maximum loads during pull tests with pull claw and pull collars ......................................................................... 4 


2. Results of pull tests of friction stabilizers.............................................................................................................. 4 


3. Predicted values for neural network ...................................................................................................................... 5 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


kN kilonewton ft foot

m meter lb pound

mm millimeter 
DESIGN AND TESTING OF A NONDESTRUCTIVE FRICTION BOLT TESTER 

By Lewis Martin,1

1

Mechanical engineer, Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA. 


John Goris,2 

2

Technical representative, Thiessen Team USA, Spokane, WA.
 

and Lauren Roberts3 

3

Chief engineer, Barrick Goldstrike, Meikle Mine, Carlin, NV.
 

ABSTRACT 

The Spokane Research Laboratory (SRL) of the National Institute for Occupational Safety and Health (NIOSH) 

and Thiessen Team USA, Spokane, WA, have developed a pull claw that can be attached to any accessible friction 

bolt ring to test the performance (i.e., load-carrying capacity) of bolts without the use of a pull collar. This would 

allow true random testing of friction bolts, which need to be evaluated to determine if they are still providing an 

adequate level of support months or years after installation. Such support is critical to maintaining safe working 

conditions for underground miners. 
2 

INTRODUCTION 


The Spokane Research Laboratory (SRL) of the National 

Institute for Occupational Safety and Health (NIOSH) and 

Thiessen Team USA, Spokane, WA, have developed a bolt 

puller that can be attached to any accessible friction bolt ring 

to test the performance (i.e., load-carrying capacity) of bolts. 

Lauren Roberts of Barrick Goldstrike’s Meikle Mine 

developed the first prototype of this device. The goal was to 

allow any friction bolt installed in the mine to be tested 

without the use of a pull collar. This would allow true random 

testing of friction bolts. The present practice of asking a 

miner to install a certain number of bolts with test collars 

does not result in an unbiased sample population because the 

miner could cluster the bolts in one easy-to-reach area. True 

random testing would lead to a better understanding of the 

field performance of friction bolts, and this information could 

be used to improve installation practices. In addition, bolts 

are often installed without test collars. Such bolts need to be 

evaluated to determine if they are still providing an adequate 

level of support months or years after installation. Ground 

water, acidic gases, and other conditions can corrode these 

bolts and reduce their effectiveness. This new tool will allow 

any friction bolt to be tested as dictated by field conditions. 

Hard-rock mines in Nevada and Montana that use friction 

bolts as support systems are currently testing the capabilities 

of the pull claw. The test systems were obtained from 

Thiessen Team USA, which also provided technical 

assistance for use of the pull claw and interpretation of the 

pull tests. 

DEVELOPMENT 


In the first-generation pull claw, a device was inserted into 

the friction bolt past the ring and plate, and then a cone 

assembly was expanded. In stiff rock, the rock’s confining 

force kept the device from pushing the wall of the friction 

bolt apart. However, the measurement reading of the initial 

pull-out load was false. In softer rock, the expander would 

tear the bolt apart in situ. Tests were conducted using the 

plate and ring as confinement, but not enough surface area 

was present for radial forces to overcome the pull-out 

strength of the bolts. 

During tests of the second-generation pull claw, it was 

discovered that a ring welded onto a friction bolt would need 

a minimum contact area of 67% to begin a pull test without 

causing premature failure of the ring. Failure occurred when 

the ring was bent, ripped, or torn off the bolt. Slippage 

failures occurred when the pull claw did not have enough of a 

“bite” around the ring or the jaw half was not tightened 

adequately and the pull claw slipped off the ring. 

Another problem was that the small amount of clearance 

left between the ring on the bolt and the backing plate after 

the friction bolt had been installed left only a small area of 

contact between the bolt and the rock surface, resulting in 

high shear loads on the lip of the jaws. Thus, to prevent shear 

failure, heat-treated, high-strength steel had to be used in the 

manufacture of the jaws. 

The next piece of equipment was a two-piece pull claw 

that encompassed the welded ring on a friction bolt. This 

proved to be a sound concept, and the bolts could be pulled to 

ultimate failure in the same manner as pull collars. When this 

system was tested in the mine, however, investigators 

discovered that four bolts would be required instead of two if 

enough force were to be exerted on the shell to drive the claw 

around the welded ring. 

The final production design is illustrated in figure 1. The 

device has three basic components: the jaws (No. 4), the 

carrier crow’s foot, and the jack stand (No. 7). The jaws are 

available in standard 33-, 39-, and 42-mm sizes that allow 

pull tests to be conducted on different sizes of friction bolts 

using the same crow’s foot. The screws on the crow’s foot 

are designed to bring the two halves of the puller together 

and encompass the ring on the friction bolt, just as a standard 

pull collar works. The jack stand is made of aluminum to 

reduce the weight of the system. The pull screw is fitted with 

a fast-threading system for ease of installation. 

INSTALLATION AND TEST PROCEDURE 


If the back can be reached with a short ladder, the test 

equipment is light enough for a single person to carry. 

Special care should be taken when testing an overhead 

friction bolt, because the bolt might fail or the puller, jack, 

and stand could fall and injure test personnel. 

Proper installation of the pull claw prior to testing is 

crucial. The bolt ring must first be inspected to determine 

whether it can be pulled or not. If the ring is accessible to 

about two-thirds of the circumference of the inner jaws, a 

test can be performed. Next, it must be determined whether 

the wall angle dips too steeply away from the ring. An adaptor 

set has a 15° capability to account for discrepancies in the wall 

and back rock. 

The test assembly with the pull screw is first installed on the 

friction bolt ring. Prior to beginning the test, load must be 

parallel to the bolt. The four screws are just started in the jaw 

halves, then the pull claw is slipped over the ring with the 

carrier side at a slight angle to the axis of the bolt. The test 

assembly is then straightened so that the adjustable jaw side 

encompasses the ring (figure 2). An initial setting pressure of 
3 

a few hundred kilonewtons of torque is applied to the bolts, 

and then the system is checked for alignment and contact of 

the hardened jaws on the ring of the bolt. With either a power 

drill or ratchet wrench, the bolts are tightened in a criss-cross 

pattern until the proper torque is reached. 

Figure 1.–Diagram of friction bolt puller assembly. 

Figure 2.–Installation of pull claw on friction bolt. 

With the test assembly securely fastened to the bolt, the 

angle rings and jack stand are put into place against the 

mine wall and kept from slipping with one hand. This is 

possible because the stand is light. Next, the hydraulic jack 

and speed nut are taken out and installed on the system with 

only a light hand torque on the nut. Now, the rings can be 

adjusted to keep the pull bolt in line with the friction bolt. 

With a slight twist of the nut, the test is ready to be 

commenced (figure 3). 

Figure 3.–Complete test assembly installed on friction bolt. 

During the test, displacement is measured to 1 mm in 

4.4-kN increments. Loading continues until a displacement 

occurs without an increase in load or the yield point is 

reached, whichever occurs first. Load is released and the 

equipment is removed. 

After the test is completed, a wedge should be placed 

behind the plate of the tested bolt to assure that the bolt 

remains useful as an active ground control member. 
4 

Table 1. –Maximum loads during pull tests with pull claw and pull collars 

Brand of bolt Pull claw Pull collars 

Kilonewtons Pounds Kilonewtons Pounds 

B .................... 

B .................... 

A .................... 

A .................... 

A .................... 

B .................... 

A .................... 

B .................... 

64.5 

64.5 

66.7 

71.2 

71.2 

80.1 

86.8 

102.35 

14,500 

14,500 

15,000 

16,000 

16,000 

18,000 

19,500 

23,000 

62.3 

68.4 

69.0 

71.2 

75.6 

82.3 

84.5 

102.35 

14,000 


15,500 


15,500 


16,000 


17,000 


18,500 


19,000 


23,000 


Table 2.—Results of pull tests of friction stabilizers 

Tonnes/meter Pounds/foot RMR Tonnes/meter Pounds/foot RMR 

Mine 2, 7/21/1999, 5-ft-long-bolt 


3.1 1,958 


3.5 2,162 


3.2 2,026 


3.2 2,026 


3.2 2,026 


3.2 2,026 


Mine 2, 7/21/1999, 5-ft-long-bolt 


1.0 625 


1.4 875 


1.3 813 


1.1 688 


2.0 1,250 

1.1 688 


Mine 3, 8/5/99, 5-ft-long-bolt 


2.9 1,800 

4.5 2,800 

7.7 4,800 

6.7 4,100 

5.3 3,300 

Mine 4, 1/19/2000, 5-ft-long-bolt 

2.8 1,720 

3.1 1,920 

3.4 2,120 

3.6 2,240 

3.5 2,180 

Mine 4, 9/06/2000, 5-ft-long bolt 

1.6 960 


2.4 1,440 

3.8 2,400 

3.8 2,400 

2.6 1,536 

4.0 2,400 

1.6 960 


4.0 2,400 

0.9 520 


3.5 2,112 

4.0 2,400 

3.2 1,920 

3.3 1,968 

2.7 1,632 

2.4 1,440 

3.0 1,824 

3.2 1,920 

4.2 2,496 

3.7 2,208 

3.8 2,304 

4.2 2,496 

3.9 2,016 

70 


70 


70 


70 


70 


70 


50 


60 


50 


50 


60 


50 


85 


85 


85 


85 


85 


80 


80 


80 


80 


80 


60 


60 


60 


60 


60 


60 


60 


60 


60 


85 


85 


85 


85 


85 


85 


85 


80 


80 


80 


80 


80 


80 


3.4 2,400 

4.0 1,536 

2.6 1,440 

2.4 960 

2.2 1,344 

3.3 1,968 

1.8 1,056 

2.4 1,440 

1.8 1,104 

1.8 1,056 

2.0 1,200 

1.2 720 

2.6 1,584 

2.9 1,728 

4.0 2,400 

3.6 2,160 

4.0 2,400 

2.0 1,200 

0.8 480 

2.3 1,392 

2.7 1,632 

Mine 5, 2/9/2000, 6-ft-long-bolt 


2.3 1,467 

2.7 1,700 

2.6 1,600 

1.9 1,200 

Mine 6, 6/21/2000, 6-ft-long bolt 


2.5 1,583 

2.0 1,250 

2.9 1,783 

2.9 1,783 

2.9 1,783 

2.6 1,595 

2.9 1,783 

2.9 1,783 

2.9 1,783 

2.9 1,783 

Mine 7, 6/04/2002, 6-ft-long bolt 


2.7 1,700 

1.9 1,166 

2.7 1,666 

2.4 1,500 

2.4 1,500 

0.8 500 

2.9 1,800 

1.9 1,166 

2.4 1,500 

2.4 1,500 

80 


80 


80 


80 


80 


80 


80 


80 


80 


80 


80 


80 


80 


80 


75 


75 


75 


75 


75 


75 


75 


65 


65 


65 


65 


60 


60 


60 


60 


60 


60 


60 


60 


60 


60 


55 


40 


55 


55 


55 


30 


55 


55 


55 


55 

___________________ 

5 

FIELD TESTS 


Extensive tests of the 39-mm friction bolts were carried out 

at six different underground mines in three different states. All 

the tests were conducted with the cooperation of mine personnel. 

The primary objectives of these tests were to (1) determine the 

reliability of the pull claw compared to conventional methods of 

testing using pull collars and (2) establish a reliable procedure for 

installing the pull claw assembly properly on bolts. 

Table 1 shows results from pull tests conducted with both the 

pull claw assembly and pull collars on two different brands of 6-ftlong friction stabilizer bolts. Each bolt was installed with a pull 

collar and then tested with both the claw and the collars. 

For each test, load was applied to the bolt until slippage 

occurred and the load dropped off, after which the bolt was re-tested 

using either the pull claw or the pull collars. Based on prior tests 

conducted on friction bolts, the industry standard of an average load 

of 1 tonne per meter of length in a 35-mm in diameter hole was 

recommended as a starting point for a mine pull-test program 

(Tomory et al. 1998; Min. Cong. J. 1959; ASTM 1999). As shown 

in table 2, each mine will have its own average pull-out strength, 

which will be higher or lower than the standard. The bolts are 

working properly in a given mine if the bolts are providing frictional 

resistance. The pull collar was also the industry standard. 

The results show that the bolts exhibited approximately 

the same pull-out resistance regardless of the pulling 

mechanism used, that is, the collar-tested bolts and the pullclaw-tested bolts produced similar results. To produce similar 

results randomly on any bolt is of great benefit to mine 

personnel and increases safety, installation quality, and the 

accuracy of ground support analyses. 

Additional tests using only the pull claw assembly were 

then conducted at the six mines. Table 2 shows the results of 

the tests. These data are representative of pull-out loads from 

multiple headings in underground mines. These pull-out loads 

were also correlated to the rock mass rating (RMR) 

(Bieniawski 1976) at the test location. 

The field tests also led to a number of improvements, 

such as redesigning and heat-treating the inner jaws. In 

addition, the tests helped establish a reliable procedure for 

properly installing the puller on bolts and showed the 

limitations of the collarless test system. On average, mine 

personnel were able to conduct tests on about 40% of the 

bolts in any given area. The deciding factor was the condition 

of the ring on the friction bolt. If the bolt was installed at an 

acute angle to the bearing plate, then the ring is usually forced 

hard against the plate and at least one of the inner jaws of the 

pull claw will not be able to slip behind the ring. This 

condition is very common, especially for bolts placed in the 

ribs of an entry. Friction bolts placed in the back are less of a 

problem because the bolts are usually installed perpendicular 

to the bearing plate. 

CORRELATING ROCK MASS RATING TO PULL-OUT LOAD 


While testing friction bolts for pull-out loads, a rating of 

RMR 76 was calculated in the areas where the tests took 

place. Correlating RMR to pull-out load was important in 

determining if there were a trend in loads associated with 

different RMR values. Mines in Nevada use over 400,000 

Split-Set4 

4

Mention of specific products or manufacturers does not imply 

endorsement by the National Institute for Occupational Safety and Health. 

friction bolts as primary support. The data were analyzed for 

statistical relevance with an R-square test, but the data were too 

scattered. The data were then analyzed using a neural network (Ward 

System 2003) to normalize an expected curve that the data should fit. 

The neural net was superimposed over the data to determine trend and 

load support predictions (figure 4). 

Figure 4.–RMR versus pull-out strength. A neural trend line 

is superimposed. 

The use of this neural curve data 

set will enhance a mine engineer’s ability to determine the load 

expected from frictional ground support in a rated rock mass (table 3). 

Table 3.–Predicted values for neural network 

RMR Tonnes/meter Pounds/foot 

0 1.06 666 

10 1.07 672 

20 1.10 693 

30 2.07 1,295 

40 2.08 1,303 

50 2.53 1,583 

60 2.66 1,663 

70 2.78 1,740 

80 3.25 2,034 

90 3.30 2,080 

100 3.40 2,124 
6 

CONCLUSIONS 


The friction bolt pull claw met the original design criteria 

of a mine engineer, that is, that the pull claw can be used to 

perform random tests on friction bolts to evaluate their 

performance. If the proper installation techniques are followed, a miner can evaluate the condition of bolts in areas 

suspected of being unstable. Unlike visual inspections that do 

not give a load profile of the ground support, the pull claw 

can indicate places where support is weak. A ground control 

engineer can also use the friction bolt pull claw to analyze the 

effectiveness of different bolt types, lengths, and hole sizes in 

a specific rock type in a given mine. 

Weak ground conditions at several locations in six mines 

were documented with RMR values to determine a 

correlation between pull-out strength and RMR. The 

collection of these data will aid support engineers in 

determining support performance in specific rock types in 

their mines. 

The friction-bolt pull claw will be able to produce results 

similar to those obtained using a pull collar test on any 

accessible bolt, which will be of great benefit to mine 

personnel and will enhance safety, installation quality, and 

ground support analyses. 

REFERENCES 

American Society of Testing Materials (ASTM). Standard Specifications of 

Roof and Rock Bolt Accessories. Designation F432-99, vol. 15.8, 1999, p. 15. 

Bieniawski, Z.T. Rock Mass Classifications in Rock Engineering. In

Proceedings: Symposium of Exploration for Rock Engineering. 

Johannesburg, S. Africa, 1976, pp. 97-106. 

Mining Congress Journal. Standard Roof Bolt Anchorage Testing 

Procedure. A Report of the AMC Coal Division Committee on Roof 

Action. Dec. 1959, 2 pp. 

Tomory, P.B., M.W. Grabinsky, J.H. Curran, and J. Carvalho. 

Factors Influencing the Effectiveness of Split-Set Friction Stabilizer Bolts. 

CIM Bulletin, vol. 91, No. 1018, 1998, pp. 205-214. 

Ward Systems Group (Frederick, MD). NeuroShell Version 2.01. 

2003. Available at http:wardsystem.com.