Field Tests of Cable Bolts Using Instrumented King Wires

Lewis A. Martin 

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

R. Pakalnis

University of British Columbia, Vancouver, BC

Richard P. Curtin

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

ABSTRACT

Instrumented cable bolts developed at the Spokane Research Laboratory of the National Institute for Occupational

Safety and Health were used in conjunction with existing ground control to monitor rock mass loads at various field sites

(T.G. Soda Ash, Getchell, Meikle, SSX, K-2, and Stillwater). Axial and shear loads were determined by strain gages

to levels of instrument accuracy of ±5 N or ±5 microstrain as these loads were transferred to the instrumented cable bolts.

These gages were 12.5 mm long and embedded into a remanufactured king wire that replaced the conven-tional king

wire. Cable bolt performance, quality of grout, and installation techniques were also assessed. By using instrumented

cables, a mine operator can determine axial load along the cable at predefined gage locations. By monitoring load on

and displacement of the rock mass, more effective ground support can be selected and installed, which will lead to safer

working conditions for miners.

BACKGROUND

Until recently, external measurement devices such as the tensmeg or resistance wire cable strain cell gages (Hyatt et al.

1997) were used to determine load on cable bolts. In 1996, the “stretch measurement to assess reinforcement tension”

(SM.A.R.T) cable was developed and patented by researchers at Queens University (Hyatt et al. 1997). This system

replaces the king wire of a cable bolt with multipoint extensometers (MPBX’s) enclosed in a tube to measure cable

elongation, thereby enabling deformation and load to be determined (Bawden et al. 1998). 

Researchers have previously expressed the view that an internal gage would be too expensive to use with cables as

an instrument within mines. Another issue is that external gages on cables may fail because of water seepage through

the grout before the cable begins to take on load (Goris et al. 1993). The usefulness of a ground control instrument

having gages mounted in the center of the cable or on the king wire was shown by Signer (1990) in research on the use

of resin-grouted rebar bolts in coal mines. 

Rock mass, grout quality, effective anchorage length, and cable load can now be determined with cement- and

epoxy-grouted cables. In 1998, researchers at the Spokane Research Laboratory (SRL) of the National Institute for

Occupational Safety and Health (NIOSH) developed an “Instrumented King Wire Cable Bolt” (patent applied for). This

instrument uses a different approach during the manufacturing process in which the king wire is molded from epoxy

around a flat piece of steel containing strain gages. 

CONCEPT

The instrumented king wire cable bolt has 12.5-mm-long strain gages embedded in the king wire at positions defined

by the mine operator. Presently, 10 gages can be installed along a resin-grouted bolt up to 5 m long or a cement-grouted

bolt up to 7 m long. These constraints are largely due to current manufacturing processes and not to limits on the system.
Figure 1.—Calibrated curve for instrumented king wire cable

bolt.

Figure 2.—Instrumented cable bolt assembly with and without

pretension head.

The gages can either be set in pairs to detect shear

load in the same plane or the flat stock can be set at

90° angles to detect loads in three directions. The

cables are capable of sustaining an ultimate load of

251 kN and can produce reliable readings to 179

kN, as shown by the calibration curve in figure 1. 

To create an instrumented king wire cable bolt,

strain gages are first installed on both sides of an 8-

by 2.4-mm metal strip at various user-specified

positions on the bolt (figure 2). The individual

strain gages are connected to an instrument plug by

30-awg wire. This apparatus is then placed in a

steel mold and injected the total specified length of

the bolt with a two-part epoxy. This new king wire

measures 8.3 mm in diameter. 

The original cable is uncoiled strand by strand,

the original center wire is replaced by the new

epoxy-filled king wire, and the outer six strands are rewound around it, resulting in an instrumented cable bolt. The gage

wires protruding from the center are then inserted into a 12-pin connector. This connector is recessed into a 4.4-cm hex

head attached to the cable by means of a barrel-and-wedge assembly (figure 2).

The cables are fitted with special heads so they

can be inserted by either a jackleg drill or a roof

bolter. An instrumentation plug is used to monitor

the gages. A data acquisition system is required to

read the loads. 

The system has been tested on a Vishay strain

indicator box, Campbell Scientific system, and an

Omni Data system. 

MATERIAL DESCRIPTION

Standard cable specifications were the American

Society of Testing Materials (ASTM) standard

A416, 0.6-in, grade 270 K, low-relaxation, sevenstrand cable having a minimum breaking load of 258

kN with 3.5% elongation. Upon removing the

original king wire and replacing it with the manufactured one, minimum breaking load was reduced

to 214 kN with reliable strain readings to 179 kN.

Calibration tests at SRL provided correlation of microstrain to load to enter into the data collection system (figure 1).

This technique was used successfully throughout the elastic load-strain range on instrumented rebar bolts by Signer et

al. (1997). 

CABLE DESIGN

Laboratory tests were conducted on instrumented king wire cable bolts embedded in concrete blocks to obtain calibration

curves for converting microstrain to load in kilonewtons. Pull tests were also conducted to determine the load-carrying

characteristics of the bolts. The strain gages were mounted 0.15, 0.35, 0.61, 0.91 and 1.22 m from the head of the bolt
Figure 3.—Calculated load and strain on grouted cable bolt

with instrumented king wire.

Figure 4.—Instrument locations in trona mine.

(figure 1). The cables were loaded to 179 kN using

hydraulic pull test equipment. A strain-to-load curve

is shown in figure 3 and is typical of what Goris et al.

(1994) observed in pull tests with conventional cable

bolts. 

These tests aid in explaining how load on cable

bolts is transferred from the cable to the grout and

then into the rock mass. A “shelling” effect is shown

whereby the grout-cable interface breaks down and

the gage becomes unbonded as load is increased and

is taken fully by the cable. Figure 3 shows results of

the pull tests as a load profile along the cable length.

The curve shows that a 128-kN (~12.8-tonne) pull

load on the collar of the cable resulted in measured

strain loads of 123 kN along gages 1 and 6 at a distance of 0.15 m from the collar. Researchers interpreted this measurement (5 kN) as the amount of load the cable was not sensing 0.15 m from the collar. At 1.22 m from

the collar, the cable did not sense any load, which indicates critical bond length. Other researchers (e.g., Goris et al.

1994). have reported critical bond length to be approximately 1 m for standard cable. This load capability enables an

operator to measure failure in real time as debonding of the grout-cable interface is initiated. In addition, the difference

between cable load and applied load indicates grout quality, which can be a better field test of the quality of cable bolt

installations than field pull tests.

CASE STUDIES

Cable bolts with instrumented king wires were tested at six mines. The results of these tests are described below. For

simplification, the terms “cable bolts” will mean bolts in which the conventional king wire has been replaced with the

new remanufactured king wire.

Case Study 1: Trona (T.G. Soda Ash)

Cable bolts were installed in the roof of T.G. Soda Ash’s trona mine, a room-and-pillar operation in Wyoming, to

measure roof loads in the intersection of 350S and 12371E (figures 4 and 5). This location was chosen because of drill

access ahead of the mining cycle and because loads would be induced in the cable bolts as ore was extracted toward the

intersection. Mine personnel were interested in determining the potential for instability in the back and the extent of any

unwedged blocks. 

Ground support was installed after development and during rehabilitation. Strain

gages were positioned along rebar and cable bolts at depths that would ensure that

loads in the resin grout would be measured. In addition, a set of gages was mounted

at an ungrouted section to measure dead weight loads in the rock mass. Figure 6 shows

the position of the strain gages with respect to the collar. The longest cable bolts were

installed at the center of the intersection because these bolts would receive the greatest

loads, while shorter cable bolts were installed north of the intersection closer to the

pillar front. Resin-grouted rebar bolts were placed in the south part of the intersection

to measure loads where the density of rebar bolts was highest. Sagmeter stations were

set at three locations in the intersection (figure 5), and holes were drilled to depths of

2.4, 3.2 and 3.8 m. These sagmeter depths were chosen to correspond with the bolt

gage profiles (figure 6), allowing correlations to be made between deformation

recorded by the strain gages to deformation recorded by the 
Figure 6.—Positions of strain gages on bolts

(note two at each position).

Figure 5.—Bolt and sagmeter locations.

sagmeter stations. This is important because slip expected to occur

between the cable-grout-rock interface can be compared to actual rock

deformation.

Results of Measurements on Instrumented King Wire Cable Bolts

Loads on the cable bolts were measured from April 29 through July 11, 1999. Mining activities influencing the intersection were progressing from northeast of the site to the south (figure 4). On June 9, mining reached a point one pillar

width from the intersection (8 m). Figure 7 shows all load profiles measured by the 4.27-m-long bolts (bolts 1, 2, 4, and

5) and the 3.66-m-long bolts (bolts 6, 8, 9, and 11). Approximately the last 1.5 m of the bolt was bonded with resin

grout.

What follows is the authors’ interpretation of loads on eight bolts. By recording actual point loads in the cable, it

will be possible to understand more readily the likelihood that instabilities may be forming or that there may be problems

with the quality of the installation.

• Gages 2 and 7 on bolt 1 were indicating loads up to the yield limit of the steel. These loads were higher than the

axial dead weight loads on the ungrouted section of the bolt and loads on the grouted gages, indicating that some

delamination was occurring close to the gage. The higher readings were most likely a result of the fact that concentrated

load is measured over the gage length by a strain gage, whereas an ungrouted gage measures the sum of all delamination

cracks over the length of the ungrouted section, which was 2.8 m. The result is an average load much lower than that

of a bonded cable over a given distance.

• Bolt 2 was inspected by mine personnel, who determined the bolt had broken. 
Figure 7.—Load profile versus time on 4.27-m-long bolts (1, 2, 4, and 5) and 3.66-m-long bolts (6, 8, 9, and 11).
Figure 8.—Displacement of back over 

time.

• Bolt 4 shows a load shelling profile similar to that shown in the laboratory (see figure 3). The greatest load was

recorded 3.1 m above the back by gages 3 and 8. The ungrouted length (2.8 m) shows a low load primarily because load

was distributed over a large distance, as discussed previously. It must be noted that loads were lower on the gages on

either side of the bolt at 3.1 m. This implies that a crack was close by.

• Bolt 5 shows the classic load shelling profile. The highest load was recorded in the ungrouted section within 2.8

m of the back. The next highest load was at the bonded gage 2.9 m from the back. Readings again indicate that

delamination was largely controlled by cracks within 3 m of the back.

• Loads on bolt 6 were uniform, with an average of 40 kN, excepting for gages 2 and 10, which were located 2.3 m

above the back. This implies that delamination may have been occurring at this location. Note that this bolt was close

to the pillar boundary and therefore the depth of cracking would be less.

• Gage sets 2 and 7 (2.3 m deep), 3 and 8 (2.5 m deep), and 4 and 9 (2.7 m deep) on bolts 8 and 9 showed a load

profile approaching, and in most cases reaching, the yield strength of the bolt. This reading indicates that delamination

was occurring throughout the horizon or that rock mass loading was higher. These two bolts were placed in the northcenter section of the site, which experienced high loads such as those on bolt 4, which had also been placed in the center

of the intersection. The deepest gages (5 and 10) at 3.1 m in the back recorded low values, indicating that the bond at

depths between 2.7 and 3.1 m was sufficient to absorb most of the load.

• Bolt 11 was located at a pillar boundary and therefore was receiving more support from the mine rock, resulting in

less delamination. Therefore, loads were lower. Representative load curves for the 3.1-m-deep gage resulted in the

greatest amount of load. The second set of gages, at 2.3 m deep, showed the next highest load, and the first set of gages,

at 2.59 m deep, had the lowest loads. Delamination was, however, occurring, albeit at low loads, approximately 3 m

above the back.

Results of Measurements at Sagmeter Stations

Three sagmeter stations were installed at three different depths (2.4, 3.2,

and 3.8 m) at the intersection of 350S and 12,371E (figure 5). Figure 8

shows that sagmeters at the three stations generated similar curves with

displacements ranging from 0.5 to 2.0 cm. In general, the most centrally

located station had the greatest displacements at all three depths.

Generally, the north-northwest and south-southeast sagmeter stations,

which were near the boundary pillars, showed the least amount of

deformation. Delamination was observed throughout all horizons with the

largest amount of movement between 2.4 and 3.8 m of the back. This roof

movement corresponded with data collected from the strain gages,

showing that delamination was generally 3 m above the back. Sagmeter

readings from the 3.2-m-deep hole at the center of the inter-section would

be comparable to readings from a 4.3-m-long cable bolt with 1.5 m of

resin embedment. This would mean 2.8 m of unbonded cable length. 

Hook’s law as applied to a cable with a steel modulus (E) equaling

2.3 × 1011 N/m2

 and a cable area of 0.00011 m2 would yield an elongation

of 2 cm over 2.8 m. The calculated force would be approxi-mately about

180 kN. This force correlates with data from the cable bolt for the

unbonded section of bolt 5, gages 1 and 6 (figure 7), and bolt 4, gages 2

and 7. Based on this information, it is likely that the resin bond was

destroyed as a result of loading.

The above case history is being further evaluated with respect to

deformation, mining sequence, and behavior of the back. The depth of

failure most likely is 3 m. Employing a support pattern of bolts placed on 1.2- by 1.2-m spacings and a specific gravity
Figure 10.—Load on bolt 4 at Getchell Mine. Figure 9.—Plan view of 4950 stope

showing locations and depths of

strain gages.

of 2.6 for the sandstone back would yield a dead load of approximately 110 kN, which is within the range of loads as

measured by the bolts. Readings from these cable bolts enable researchers to understand overall support-rock interaction.

Case Study 2: Getchell Mine, 4950 Stope Areas (Placer Dome)

Ten 3.6-m-long instrumented cable bolts were installed at the Getchell Mine, which is a transverse, open-stoping

operation in Nevada. Gages were spaced 0.6 m apart at five positions on the cable bolt. A tensionable head was placed

on the bolt (figure 9), and the bolts were grouted with a cement-based grout into the back and ribs at the locations shown

in figure 9. The bolts were then tensioned 10 kN on the cables and bearing plates. The instruments were placed after

the primary cuts had been backfilled and the secondary drifts to the ore were about to be mined. This enabled analysis

of the effects of secondary and tertiary mining effects.

Preliminary data indicate that bolt 4 in the stope is showing a load higher than 10 kN (figure 10) while the rest of

the bolts are not showing any significant loading (under 1 kN). Loading on bolt 4 suggests that a crack is most likely

developing between gages 1 and 6 (0.6 m deep) and gages 2 and 7 (1.2 m deep). A borescope survey was conducted

as part of this study and located a crack 0.9 m above the back. Based on a 1.2- by 1.2-m bolting pattern and a depth of

0.9 m (specific gravity = 2.6), a dead load of 38 kN is likely. Measurements and interpretations of the strain data were

reinforced by subsequent visual observations intended to provide a higher level of safety for the operator. This area of

the mine was reinforced.

Case Study 3: Meikle Mine, Nevada (Barrick Goldstrike)

An instrumented king wire cable bolt was placed into the shop area at the 1250 level of the Miekle Mine. This area was

selected because a dike in the roof was delaminating, and mine personnel wanted to know how much load was being

transmitted to the ground support. The 6-m-long cable bolt was placed perpendicular to the crack created by the dike.

Strain gages were placed on the bolt so that when it was installed, the gages were 1.2 and 0.61 m deep on either side of

the crack and one at the crack location. Half the instrumented bolts lost the common ground wire to the strain gages,

and no readings were obtained from that side of the bolt. Loading on the 1.2- and 0.61-m-deep gages reached 178 kN

while a gage at the dike showed minimal load. Cable strain was reaching failure, so the area was reinforced.

Case Study 4: K-2 Mine (Echo Bay Minerals)
Figure 11.—Load on bolt 14 at K-2 Mine.

The K-2 Mine is a longhole open stoping operation

in Washington. The purpose of the project was to

determine loads in the backs of stopes and brows.

One of six instrumented cable bolts to be placed at

the mine have been installed and is producing data.

This bolt is 3.7 m long with five sets of gages spaced

at 0.61-m intervals. 

The cable bolt was placed into the mine back

directly above the sublevel open stope. Preliminary

data show low loads on all gages along the bolt (figure 11). The cable is approaching yield at gage 5 (3

m deep), and delamination appears to be occurring

close to 3 m above the back. The immediate back

exhibits minor loading (gage 1 at 0.6 m deep). The

area where the cable bolt is installed is inaccessible

since the open stope has yet to be backfilled. There

is no concern for personnel because the stope below is nonentry.

Other Installations

A series of three cable bolts was installed at AngloGold’s SSX Mine, NV, in a transverse stope. Two of the bolts were

placed into the back above the upper stope and a third into the brow. When the stope was shot, the data cables were

damaged by blast rock, and so the instruments cannot be read until backfilling operations are completed.

The Stillwater Mine, which is a sublevel open stoping operation in Montana, plans to install five 3.7-m-long cables

bolts with gages spaced at 0.61 m.

CONCLUSIONS

In all installations where the gages have been read, cable bolts were responsible for delivering information about loads

in the immediate rock mass. Grout quality could be determined by relating bonding capacity of these cable bolts to that

of the load profile of the ungrouted bolts. The cable bolt was found to be reliable in warning of instabilities and has been

calibrated to deformation of the adjacent rock mass. 

The dangers of assuming that deformation is linear across a bonded cable were also suggested as the load profile

showed the relationship of the degree of grout bonding to load along the cable. The simple relationship of strain to load

calculated with cable modulus is only valid for the ungrouted sections of cable. However, the grouted sections of the

cable should be calculated using calibrated data to determine the strain relationship. The above interpretation is only

possible if point strains and loads on a cable are known.

By monitoring load on and displacement of the rock mass, more effective ground support can be selected and

installed, which will lead to safer working conditions for miners.

ACKNOWLEDGMENTS

If not for the cooperation of the mining companies and their mine engineers, this research would not have been

conducted. We would like to thank Catherine Dreesbach, T.G. Soda Ash; Pat McCann, Getchell Mine; Lauro Lacerda,

Miekle Mine; Ken Boie, K-2 Mine; and Colin Connors, SSX Mine, for their valued assistance. We would also like to

thank Steve Signer, engineer at SRL, for his expertise in the application of Campbell Scientific systems, and Dennis Cox,

Richard Rains and Joanne Johnson, engineering technicians at SRL, for their dedication in constructing the instrumented
bolt systems.

REFERENCES

BAWDEN, W.F., HYATT, A.J., LAUSH, P.M, MOOSAVI, M., DEGRAAF, P., and REUST, M., 1998

The S.M.A.R.T. cable bolt: Results from two initial field trials. Presentation at 100th ann. general meeting of Can. Instit.

Min. and Metall., Montreal, PQ, May 3-7, 1998, 8 pp. Available from author and on CD-ROM from CIM, Montreal,

PQ.

GORIS, J.M., BRADY, T.M., and MARTIN, L.A., 1993

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GORIS, J.M., NICKSON, S.D., and PAKALNIS, R., 1994

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HYATT, A.J., BAWDEN, W.F., REUST, M., and PAHKALA, M., 1997

The S.M.A.R.T. cable bolt: An assessment for the determination of tension in seven-wire strand cable bolts.

International Symposium on Rock Support: Applied Solutions for Underground Structures, ed. by E. Broch, A.

Myrvang, and G. Stjern (Lillehammer, Norway, June 22-25, 1997).

SIGNER, S.P., 1990

 

Field verification of load transfer mechanics of fully grouted roof bolts. U.S. Bur. Mines Rep. of Invest. 9301, 13 pp.

SIGNER, S. P., COX, D., and JOHNSTON, J., 1997

A method for the selection of rock support based on bolt loading measurements. International Symposium on Rock

Support: Applied Solutions for Underground Structures, ed. by E. Broch, A. Myrvang, and G. Stjern (Lillehammer,

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BIBLIOGRAPHY: LEWIS A. MARTIN 

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

WA

Since he graduated from Washington State University, Pullman, WA, in 1988, Mr. Lewis Martin has been involved

in work on cable bolts. His first project was with the Spokane Research Center of the U.S. Bureau of Mines (now the

Spokane Research Laboratory of the National Institute for Occupational Safety and Health) assisting with the installation

of cable bolts at the Homestake Mine in South Dakota. Another project at coal mines in Colorado led to the acceptance

of cable bolts as secondary support in these mines. During this time, Mr. Martin has worked to establish minimum

embedment lengths and safe installation practices for grouted cable bolts. Cooperative projects with Thiessen Team has

resulted in the development of jackleg-installed resin-grouted cable bolts and the instrument king wire cable bolt.