EVALUATION OF INSTRUMENTED CABLE AND REBAR BOLTS AS GROUND SUPPORT AT A
 


TRONA MINE 

L. A. Martin, Spokane Research Laboratory, National Institute for Occupational Safety and Health,
 


Spokane, WA, USA 
 

C. A. Dreesbach, FMC Granger, Granger, WY, USA
 


R. Pakalnis, University of British Columbia, Vancouver, BC, Canada
 


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 systems 

to monitor rock mass loads at Tg Soda Ash’s trona mine, Granger, 

WY. Axial and shear loads were determined to levels of strain gauge 

accuracy of ±5 N or ±5 microstrain. These gauges were embedded in 

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. These data provide necessary input for an operator to 

design a safer working environment for miners. 

BACKGROUND 

Until recently, external measurement devices such as the 

“tensmeg” or resistance-wire cable strain gauges (Hyett et al. 1997; 

Windsor 1987) were used to determine load on cable bolts. Previously, researchers thought that an internal gauge would be too expensive to use with cables as an instrument within mines. Another issue 

has been 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). In 1996, the “stretch measurement to assess reinforcement tension” (S.M.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 

enclosed in a tube to measure cable elongation, thereby enabling deformation and load to be determined (Bawden et al. 1998). 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. 

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 pending). 

This instrument is manufactured by molding the king wire from epoxy 

around a flat piece of steel containing strain gauges. Instrumented 

cable bolts will aid in evaluating geologic conditions, grout quality, 

effective anchorage length, and cable load. 

CONCEPT 

The instrumented king wire cable bolt has 12.5-mm-long 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 250 kN and can produce reliable readings to 180 

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 a 0.24- by 5-mm metal strip at various 

user-specified positions on the bolt (figure 2). 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 

5.3 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 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). 

Figure 2.—Instrumented cable bolt assembly with and 

without pretension head. 

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 werethe American Society of Testing 

Materials (ASTM) standard A416, 0.6-in, grade 270 K, lowrelaxation, 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, ultimate load was reduced 

to 215 kN. Strain readings were reliable to 180 kN with 3.5% strain 

on the cable at failure. 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 loadstrain range on instrumented rebar bolts by Signer et al. (1997). 

CABLE DESIGN 

Laboratory tests were conducted on instrumented king wire cable 

bolts embedded with resin grout 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 3). The 

cables were loaded to 180 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 

gauge becomes debonded 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. 

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

instrumented king wire. 

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 measurement, a difference of 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, indicating that 

1.22 m was the 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. 

REBAR DESIGN 

The resin-grouted rebar roof bolts used in these tests were standard, 

grade 60, No. 6 rebar that had been milled with a 6.35-mm-wide by 

3.18-mm-deep slot along each side (figure 4). Axial load tests showed 

that the average yield load of the bolts was 96 kN and the average 

ultimate load was 158 kN. Prior to milling the slot, yield load had 

been 107 kN and ultimate load had been 176 kN. Thus, slotting 

caused a 10% reduction in strength (Signer 1990). 

Strain gauges were installed into the slots as shown in figure 4. The 

strain gauges were then calibrated in a uniaxial test machine to 

correlate voltage change to load change. A typical load-strain curve 

from a tested bolt is shown in figure 5. When the data from the instrumented rebar bolts were reduced, the correlation coefficients from the 

axial calibrations were used to convert voltage changes to load 

changes. This process was accurate to ±22 N. When the load levels 

exceeded the yield point of the steel, voltage readings were converted 

to strain readings to estimate the degree of plasticity (figure 5). 
Figure 4.—Cross section of resin-grouted rebar bolt machined 

for installation of gauges.

Figure 5.—Typical load strain profile for instrumented rebar 

bolt. 

CASE STUDY: TG SODA ASH, FMC GRAINGER 

Mine Geology 

The mine is located in the Green River Basin of southwest 

Wyoming. Trona deposits in the basin were formed during the 

Eocene period within the Wilkins Peak Member of the Green River 

Formation. The repeated evaporation and recharge of Lake Gosiute, 

a playa lake occupying the basin during Eocene time, created 

evaporite layers of trona intermixed with marlstone and alternating 

layers of oil shale (Leigh 1998). 

The areal extent of the basin is marked by the Uinta Mountains in 

the south, the Rock Springs uplift in the east, the Wind River and 

Gros Ventre ranges in the north, and the Wyoming Range in the west. 

There are 42 known trona beds, of which 25 range from 240 to 670 

m thick. These beds are numbered 1 through 25, according to the 

order in which they were deposited (bed 25 being closest to the 

surface). Beds 1 through 18 consists mainly of fine-grained trona, 

while beds 19 through 25 are composed of a coarse crystalline trona. 

Mining at Tg Soda Ash takes place in bed 20 approximately 425 m 

below the surface. The area in which the field study was conducted 

is overlain by an additional 60 m of overburden. 

Mining Method 

Trona is mined with a drum-and-borer-type continuous mining 

machine. These machines, traditionally used for coal mining, are 

custom built to cut the harder trona ore. Tg Soda Ash uses only drumtype continuous miners. Cutting heads are typically 5.3 m wide, but 

the panel in which the field study was conducted was mined with a 

5.6-m-wide cutting head. 

The Tg Soda Ash mine uses a modified room-and-pillar mining 

method. In the field study area, the panel was developed as a fourentry system. An additional two entries were later driven parallel to 

the existing workings, with an eight-room retreat mined in a chevron 

configuration 78 m wide by 103 m deep with cross cuts on 12.2-m 

centers. The retreat consisted of pulling chevrons, a method in which 

pillars are cut on 12.2-m centers 60° from the direction of advance. 

This method maximizes the rate of extraction by minimizing ground 

control problems. 

The field study site was chosen for its rapidly changing ground 

conditions and its proximity to mining activity in this panel. It is 

believed that the wider cutting head, extra overburden, and weaker 

geologic material of the roof and floor in the panel contributed to 

rapidly deteriorating ground conditions in the outside entries of the 

system, making the area ideal for a field study. During the course of 

the study, the face was approximately 215 m from the test area and 

retreated to within 45 m of the test area. 

Instrumented Bolt Installation 

For simplification in the rest of this paper, the term “cable bolt” 

means a bolt in which the conventional king wire has been replaced 

with the new remanufactured king wire. 

Cable bolts and rebar bolts were installed in the roof of the mine to 

measure roof loads in the intersection of 350S and 12371E (figures 6 

and 7). This location was chosen because of drill access ahead of the 

mining cycle and because loads would be induced in the 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 unstable 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 on the resin-grouted 

support 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 8 shows the location 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. 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 7), 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 8), allowing correlations to be made between deformation recorded by the strain gauges to deformation recorded by the 

sagmeter stations. This is important because slip expected to occur 

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

deformation. 
Figure 6.—Instrument locations in mine. 

Figure 7.—Bolt and sagmeter locations. 

Figure 8.—Positions of strain gauges on bolts (note 

two at each position). 

Results of Measurements on Instrumented Bolts 

Loads on the cable bolts were measured from April 29 through July 

11, 1999. Mining activities influencing the intersection progressed 

from northeast of the site to the south (figure 6). On June 9, development mining began within one pillar width (8 m) of the intersection. 

Figure 9 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. 

Figure 10 shows load profiles for the 2.44-m-long, fully grouted rebar 

bolts (bolts 201-204). 

What follows is the authors’ interpretation of loads on 12 bolts. By 

recording actual point loads in the bolts, 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 cable bolt 1 indicated 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. 
Figure 9.—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 10.—Load profile versus time on a 2.4-m-long, fully grouted rebar bolt. 

The higher readings were most likely a result of the fact that concentrated load is measured over the gauge length by a strain gauge, 

where-as an ungrouted gauge measures all delamination cracks over 

the length of the ungrouted section, which was 2.8 m. The result is 

an average load much lower than the load on a bonded cable over a 

given distance. 

• Bolt 2 was inspected by mine personnel, who determined that it 

had broken. This could have happened because the bolt was defective 

or because of unexplained shearing of the bolt. 

• Bolt 4 shows a load shelling profile similar to that generated in the 

laboratory (see figure 3). The greatest load was recorded 3.1 m above 

the roof by gauges 2, 3,7, 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 2.9 to 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 into the back. 

The next highest load was at the bonded gauge 2.9 m into the back. 

Readings again indicate that delamination was largely controlled by 

cracks within 3 m into 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 the fractured zone 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 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 into 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 and 

lower loads. Representative load curves for the 3.1-m-deep gauge 

show the greatest amount of load. The second set of gauges, at 2.3 m 

deep, took the next highest load, and the first set of gauges, at 2.59 m 

deep, took the lowest loads. Delamination was occurring, albeit at low 

loads, approximately 3 m above the back. 
• Typical axial load curves are shown for bolt 201. These types of 

curves usually result when conducting laboratory pull tests on 

instrumented bolts. The curves for the northwest corner bolts also 

show a quick loading rate, which should be expected because the bolt 

was closest to active mining on the east side of the intersection. 

• Strain gauges on bolt 202 were reading both positive and negative 

strain, which indicate bending of the bolt. Gauges 4 and 9 and 5 and 

10 indicated that bolt bending on one side was in the yield zone of 

that bolt. Loads on gauge 5 showed the same loading horizon as the 

cable bolts at 2 to 5 m. The remainder of the gauges showed that the 

nonbending sections of the bolt were under low loads compared to the 

sections of the bolt showing bending. 

• The tip on bolt 203 was undergoing high axial loads in the yield 

zone. This could be interpreted as indicating that the bolt was just 

reaching the load horizon at 2 to 3 m into the back. The rest of the 

gauges were being loaded at relatively low load rates. 

• The load curve on bolt 204 was typical except for readings from 

two gauges. The bolt was supporting ground at all instrumented 

horizons, with somewhat higher loads at the center of the bolt. These 

load profiles also indicate that the resin grout was developing a good 

bond along the entire length of the hole. It is possible that a crack in 

the back caused grout delamination that affected gauge set 3 and 7. 

Total lower load readings along the bolt could have resulted from a 

greater bolt density in the surrounding area. 

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 

7). Figure 11 shows that sagmeters at the three stations generated 

similar curves with displacements ranging from 0.5 to 2.0 cm.

Figure 11.—Displacement of back over time. 

 In 

general, the most centrally located station had the greatest 

displacement 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 gauges, showing that 

delamination was generally 2.4 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. 

The above case history is being further evaluated with respect to 

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

of failure was most likely 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 

marlstone-trona-shale back would yield a dead load of approximately 

110 kN, which is within the range of loads as measured by the bolts. 

Although several of the measured loads exceeded the calculated dead 

load significantly, which could have been the result of mining-induced 

loading, readings from these cable bolts enable researchers to 

understand overall support-rock interaction. 

CONCLUSIONS 

In all installations where the gauges were read, cable bolts provided 

information about loads in the immediate rock mass. Grout quality 

could be determined by relating bonding capacity of these cable bolts 

to the load profile of the ungrouted bolts. The instrumented cable bolt 

was found to be reliable in indicating load changes and has been 

correlated 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. 

This instrumented bolting system is used to aid in determining 

proper bolt spacings, bolt lengths, and load-carrying characteristics of 

the bolts, thus allowing a mine manager to design a safer roof support 

system for miners. 
ACKNOWLEDGMENTS 

If not for the cooperation of Tg Soda Ash and mine engineers John 

McDonnell, Christopher Pritchard, and Scott Lehman, this research 

would not have been conducted. We would also like to thank Steve 

Signer, engineer at SRL for his expertise in the application of 

Campbell Scientific systems; and Richard Curtin, Dennis Cox, 

Richard Rains and Joanne Johnson, engineering technicians at SRL, 

for their dedication in constructing the instrumented bolt systems. 

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