Case Studies 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 (FMC Granger, Getchell, Meikle, SSX, K-2, and Stillwater). Axial and shear loads 

were determined by strain gauges to levels of instrument accuracy of ±5 N or ±5 microstrain as these loads were 

transferred to the instrumented cable bolts. These gauges were 12.5 mm long and embedded into a 

remanufactured king wire that replaced the conventional 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 gauge 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 gauges 

(Hyett 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 (Hyett 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 gauge would be too expensive to use with 

cables as an instrument within mines. Another issue 

is that external gauges 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 gauges 

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 cementand 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 60/076,138). 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 gauges. 

CONCEPT 

The instrumented king wire cable bolt has 12.5-mmlong strain gauges embedded in the king wire at positions defined by the mine operator. Presently, 10 

gauges 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. The 

gauges 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. 
Figure 1.—Calibrated curve for instrumented king wire 

cable bolt. 

To create an instrumented king wire cable bolt, strain 

gauges 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). 

Figure 2.—Instrumented cable bolt assembly with and 

without pretension head. 

 The individual strain gauges 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 epoxyfilled king wire, and the outer six strands are rewound 

around it, resulting in an instrumented cable bolt. The 

gauge 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 gauges. A 

data acquisition system is required to read the loads. 

The system has been tested on a Vishay strain 

indicator box, a 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, seven-strand 

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 

Laboratorytests 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 gauges were mounted 0.15, 0.35, 

0.61, 0.91 and 1.22 m from the head of the bolt (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. 

Figure 3.—Calculated load and strain on grouted cable bolt 

with instrumented king wire. 
 

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 gauge 

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 gauges 1 and 6 at a distance of 

0.15 m from the collar. Researchers interpreted this 

measure-ment (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 (FMC Granger) 

Cable bolts were installed in the roof of FMC 

Grangers’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). 

Figure 4.—Instrument locations in trona mine. 

Figure 5.—Bolt and sagmeter locations. 

 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 gauges 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 gauges was mounted at an ungrouted 

section to measure dead weight loads 

in the rock mass. Figure 6 shows the position of the 

strain gauges 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. Resingrouted 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 gauge profiles (figure 6), allowing correlations to 

be made between deformation recorded by the strain 

gauges to deformation recorded by the sagmeter stations. 

Figure 6.—Positions of strain gauges on 

bolts (note two at each position). 

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. 

• Gauges 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 gauges, 

indicating that some delamination was occurring close 

to the gauge. The higher readings were most likely a 

result of the fact that concentrated load is measured 

over the gauge length by a strain gauge, whereas an 

ungrouted gauge 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. 

• 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 gauges 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 gauges on either side of 

the bolt at 3.1 m, implying 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 gauge 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 gauges 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. 

• Gauge 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 north-center 

section of the site, which experienced high loads such 
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). 
as those on bolt 4, which had also been placed in the 

center of the intersection. The deepest gauges (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-mdeep gauge resulted in the greatest amount of load. 

The second set of gauges, at 2.3 m deep, showed the 

next highest load, and the first set of gauges, 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. 

Figure 8.—Displacement of back over time. 

In general, the most centrally located station had 

the greatest displacements at all three depths. 

Generally, the north-northwest and south-southwest 

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

readings from the 3.2-m-deep hole at the center of the 

intersection 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 × 10 11 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 approximately 

about 180 kN. This force correlates with data from 

the cable bolt for the unbonded section of bolt 5, 

gauges 1 and 6 (figure 7), and bolt 4, gauges 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 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 supportrock 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, openstoping operation in Nevada. Gauges 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 second-ary 

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 gauges 1 

and 6 (0.6 m deep) and gauges 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. 

Figure 9.—Plan view of 4950 stope showing 

locations and depths of strain gauges. 

Figure 10.—Load on bolt 4 at Getchell Mine. 

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 gauges 

were placed on the bolt so that when it was installed, 

the gauges 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 gauges, and no readings were obtained from that 

side of the bolt. Loading on the 1.2- and 0.61-m-deep 

gauges reached 178 kN while a gauge 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) 

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 gauges 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 gauges along the bolt (fig-ure 11). 

The cable is approaching yield at gauge 5 (3 m deep), 

and delamination appears to be occurring close to 3 m 

above the back. The immediate back exhibits minor 

loading (gauge 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. 

Figure 11.—Load on bolt 14 at K-2 Mine. 
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 gauges spaced at 0.61 m. 

CONCLUSIONS 

In all installations where the gauges 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 un-grouted 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 John 

McDonnell, FMC Granger; 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. 

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