EFFECTS OF BOLT SPACING, BOLT LENGTH, AND ROOF SPAN ON BOLT LOADING IN A

TRONA MINE

S. P. Signer and R. Raines

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

Abstract

Researchers from the Spokane Research Laboratory of the

National Institute for Occupational Safety and Health installed 39

instrumented, fully grouted bolts at six test sites in a trona mine

retreat panel to study mine roof stability for the improvement of

workplace safety. Variables at each test site included bolt spacing,

bolt length, roof span, and location in the panel layout. At most test

sites, two rows of instrumented bolts were installed, one in or near the

intersection created during development and the other in or near the

intersection created during second-pass mining. 

The most significant factor affecting bolt load was roof span. The

highest loads were on the bolts installed in the intersections of the 6-

m-wide entry. Mining-induced stress resulting from panel layout was

the next most significant factor affecting bolt loading. Minor variations in bolt loading could be attributed to changes in bolt spacing

and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressionalloading during gas bleed-off.

INTRODUCTION

Strain gauges were first used by researchers in the mid-1970's to

measure loading on fully grouted roof bolts (Dunham 1974; Farmer

1975; Sawyer and Karabin 1975) and have been proven to be quite

successful under both laboratory (Serbousek and Signer 1985, 1987;

McHugh and Signer 1999) and underground (Gale 1987) conditions.

The Bureau of Mines conducted a study on grouted bolt loading in

1988 (Signer and Jones 1990) and found that loads on grouted bolts

were much higher than loads predicted by suspension theory. Followup tests confirmed the initial findings (Maleki et al. 1994; Larson et

al.1995; Signer et al. 1993; Signer and Lewis 1998). The Bureau of

Mines published a procedures guide for the fabrication of this type of

instrument (Johnston and Cox 1993), and instrumented bolts were

commercialized in the mid-1990's. 

In 1995, researchers at the Spokane Research Center began a series

of studies at five coal mines, two trona mines, and four metal/

nonmetal mines to measure loading along the length of fully grouted

supports. This paper reports on the roof bolt research done at one of

the trona mines. The approach was to use fully grouted bolts

instrumented with strain gauges to measure axial and bending loads

on each bolt in the pattern, the points at which these loads developed,

and how much anchorage length was available to provide support.

This information can be used to select the most efficient bolt

diameter, grade, length, and spacing for a given mining condition.

INSTRUMENTS

Thirty-nine bolts were instrumented with strain gauges and

installed during the normal mining cycle. Twelve were 2.1 m (7 ft)

long, 15 were 1.5 m (5 ft) long, 2 were 1.2 m (4 ft) long, and 10 were

0.9 m (3 ft) long. These bolts were modified by milling a slot 6 mm

(1/4 in) wide and 3 mm (1/8 in) deep along the bolt and attaching five

strain gauges on both sides, as shown in table 1 and figure 1. Tension

tests showed that the average yield load of the bolts was 96 kN

(21,500 lb) and the average ultimate load was 158 kN (35,500 lb)

(figure 2). Prior to slotting, the yield load had been 106 kN (24,000

lb), and the ultimate load had been 176 kN (39,500 lb). Thus, slotting

caused a 10% reduction in strength. The typical area of an unslotted

bolt was approximately 2.6 cm2 (0.4 in2), while that of a slotted bolt

was 2.3 cm2 (0.36 in2). 

Table 1: Position of gauges on bolts

Bolt length, m Distance from bolt head, cm

D1 D2 D3 D4 D5 L

0.9 ......... ......... . 15.3 30.5 45.6 61.0 76.3 91.4

1.2 ......... ......... . 20.3 40.6 61.0 81.3 102 122

1.5 ......... ......... . 25.4 50.8 76.2 102 127 152

2.1 ......... ......... . 35.6 71.1 107 142 178 213
Figure 1.—Instrumented roof bolt.

Figure 2.—Typical test of No. 6, grade 60 roof bolt.

Two types of data acquisition systems were used to measure bolt

loads and roof movements. A Campbell-Scientific1

 data acquisition

system, which has the capability of measuring all instruments on a

continuous basis, collected readings daily on 12 of the instrumented

bolts. A Omnidata Polyrecorder data acquisition system was used to

read the remaining bolts immediately after installation and periodically throughout the test.

GEOLOGY

Trona is a sodium bicarbonate that forms when sodium and

calcium are deposited in the presence of carbon dioxide. Sources of

sodium and calcium are volcanic ash. The mineral shortite is present

in the shales above and below bed 17, which is the horizon of mining

interest. Shortite weakens the shale by introducing planes of

fracturing.

The trona beds near Green River, Wyoming, are in the Wilkins

Peak Member of the Green River Formation. These beds were deposited during the Eocene about 50 million years ago as Lake Gosiute

evaporated. The Green River Formation is overlain by the Bridger

Formation and underlain by the Wasatch Formation. Within the

Wilkins Peak Member, 25 trona beds have been identified, beginning

with bed 1 (lowest) to bed 25 (highest). Bed 17 is 450 m (1,500 ft)

below the surface and ranges from 2.7 to 3.0 m (9 to 10 ft) thick. 

From 46 to 56 cm (18 to 22 in) of trona overlain by a green shale

were found in the immediate roof. The shale contained fine-grained

shortite that became coarser higher in the section. In places, the

shortite was concentrated along bedding planes, which caused the

shale to part easily. About 1.5 m (5 ft) above the top of the trona lay

the spar trona bed, locally called the Jewell Seam, which ranged from

a few centimeters to 0.3 m (1 ft) thick. This seam of trona is very

pure and of a semitransparent, clear to light-brown color. This was

the easiest bed to correlate in the core. Above the Jewell Seam was

0.6 to 0.9 m (2 to 3 ft) of shale very similar to the shale below the

Jewell Seam, although the upper shale contained less shortite. It

graded to a dark brown, finely laminated oil shale with parallel to subparallel bedding. Carbonaceous partings were observed along many

of these bedding planes. In the floor immediately below the trona was

a 0.3- to 0.6-m- (1- to 2-ft-) thick bed of oil shale like that in the roof;

this oil shale graded into green shale that contained very little shortite.

The amount of shortite increased with depth in each of the core holes.

In the area of test sites 1, 2, 3, and 4, 3-m- (10-ft-) long NX core

holes were drilled in the roof and 1.5-m- (5-ft-) long holes were

drilled in the floor. Averaged results from unconfined compressive

strength tests on the core are shown in table 2.

TEST SITES

The support plan called for the 0.9-m- (3-ft-) long bolts to be installed with a Beaver drill on bore advance at 1.2-m (4-ft) spacings

approximately 9 to 12 m (30 to 40 ft) behind the borer miner. After

the miner was retracted, a roof bolter installed 1.5-m- (5-ft-) long

bolts on 1.2- by 1.2-m (4- by 4-ft) spacings. At intersections, the 1.5-

m- (5-ft-) long bolts were replaced with 2.1-m- (7-ft-) long bolts. The

bolts were typically grade 60, No. 6 rebar and installed in a 25-mm

(1-in) in diameter hole with a full column of fast-setting polyester

resin grout. The instrumented bolts were installed in the same size

hole, but used a slow-setting resin to facilitate installation.

1Mention of specific products or manufacturers does not imply

endorsement by the National Institute for Occupational Safety and

Health.
Figure 3.—Plan view of test sites.

Table 2: Results from tests on NX core

Material Ultimate strength,

MPa

Modulus, GPa Poisson’s

ratio

Green shale 28 7.0 0.20

Oil shale . . 32 5.2 0.25

Trona .... 41 25 0.50

‘

Thirteen rows of instrumented bolts were installed as part of the

primary support in the panel. Four test areas were in the No. 5 entry,

and two were in the No. 3 entry. A plan view of a part of the panel

with test sites is shown in figure 3. At most test sites, one row of

bolts was installed at mid-pillar, and one row was installed in or near

the intersection. In addition, one row of bolts was installed in a passand-a-half entry between the No. 3 entry and the 4 room at test site 5.

The trona is mined with a borer miner that creates an entry width of

approximately 4.3 m (14 ft). Additional space was created in the No.

3 entry for the conveyor belt by mining another half pass, which

created an entry width of 6 m (20 ft). Both types have a height of

approximately 2.4 m (8 ft), and ribs are cut in a semicircle. After

development mining, the panel was retreat mined by two additional

cuts in each small pillar and four additional cuts in each large pillar.

This additional mining was not enough to cause caving.

At test site 1, typical bolt lengths and spacings were used, while at

test site 2, a bolt row with spacings of 1.5 m (5 ft) was installed along

the entry. At the third test site, 1.2-m- (4-ft-) long bolts were installed

in place of the 1.5-m- (5-ft-) long bolts using typical spacings.

Typical bolt lengths and spacings were used again at the fourth test

site. Test sites 5 and 6 were located in the No. 3 entry using typical

bolt spacings and lengths.

Holes were drilled to relieve built up gas pressure in the immediate

roof. These holes were 4.6 m (15 ft) deep, drilled in the center of the

entry, and spaced 15.2 m (50 ft) along the length of the drivage. The

gas pressure was measured at several locations with a packer placed

2.4 m (8 ft) from the roof line with a pipe connected to an air pressure

gauge at the roof line. A valve was attached adjacent to the pressure

gauge to allow pressure to bleed off between readings. 

TYPES OF BOLT LOADING

Bolt loads can be axial, bending, and/or shear. Axial loading is

generally the primary force on a steel bolt, although under some

situa-tions, high bending moments and/or shear stresses can cause

bolt fail-ure. Shear loads are impossible to estimate with this type of

instru-ment because of the nature of the loading mechanisms and the

uncer-tainties of load locations. However, when joint movements are

pres-ent, shear loading can be critical in the design of bolt systems,

and further research will be required for a better understanding of this

loading mechanism.

Axial loads cause fiber stress in the bolts according to the equation

Fa = P ÷ A (1)

where Fa = axial stress, Pa,

P = load, N,

and A = area of steel, cm2.

A design engineer should consider several factors when

calculating bending moments measured by instrumented bolts. The

location of maximum bending moments may be localized and thus

may not be accurately represented. Bending is measured in only one

plane, but can take place in other directions, especially if high

horizontal stress fields are present. (At our test sites, bolts were

oriented during instal-lation to measure the highest estimated plane

of bending.) Bending moments can also be caused by joint

movement, large-block rotations, and/or differential loading in mats

and meshes. Bending loads cause axial fiber stress in the bolts

according to the equation

Fb = M ÷ Sx(2)

where Fb = bending stress, Pa,

M = moment, N-m,

and Sx = section modulus, cm3

.

The total fiber stress then becomes

Ft = Fa ±Fb # Fmax (3)
where Ft

 = total fiber stress

and Fmax = allowable fiber stress.

Total stress must be less than or equal to a design stress that has

been selected on the basis of the amount of variation in geology,

geometry, and in situ stress. 

Another, equally important, aspect of selecting bolts for roof

support is the evaluation of strain levels in the bolt (figure 2). Typical

engineering design limits strain to a percentage of the yield point.

Previous evaluations of strain measured on roof bolts show that, in

many cases, the yield point of the steel bolt is exceeded where the

roof remains stable. Fully grouted roof bolts are a stiff support system

in which loads increase quickly as the bolted strata move. Rebar bolts

are made from a ductile steel that can reach strain levels of 100,000

to 160,000 microstrain at ultimate load. Yielding occurs at approximately 2,000 microstrain on grade 60 rebar. Immediately after yielding, the bolt will continue to stretch with very little increase in load

until the steel begins to work-harden. Thus, design strain limits

should take bolt loading mechanisms—axial, bending, and/or

shear—into consideration. Bolts loaded axially with little bending or

shear load can reach higher strain levels than bolts subjected to high

shear and bending forces. 

MEASURED BOLT LOADS

Data from each instrumented bolt were evaluated in several

different ways. Each bolt was calibrated in a uniaxial test machine to

correlate voltage change to load change. This calibration factor was

used below the steel yield point to convert voltage data to load at each

of the 10 strain gauges. If a bolt section exceeded the steel’s yield

point, strain was calculated and load estimated from the stress-strain

relationship shown in figure 2. 

Axial loading was calculated by averaging the load on each side of

the bolt at each gauge location. Bending moment was calculated

according to equation 2. The section modulus used to calculate

bending was determined both experimentally and mathematically. If

a strain gauge failed, then neither axial load nor bending moment

could be calculated at that bolt location. Strain was calculated

directly from the voltage readings using the equation

, = 4)V ÷ (GF)(ExV) (4)

where , = strain, mm/mm,

)V = change, V,

GF = gauge factor,

and ExV = excitation voltage.

Axial load, bending moments, and axial load per roof area for each

instrumented bolt are shown in table 3. The bolt loads selected for

this table were the maximum levels before the area was mined during

the second pass. Several electrical connectors were sheared by the

borer during development, and several were sheared during retreat.

Those sheared during retreat showed the highest load prior to failure.

Bolt loading can also be shown as a cross-sectional view of the mine

entry. Figures 4 through 6 show typical axial loads on each bolt at

one moment in time.

RESULTS

Maximum loading was observed on the instrumented bolts installed in the No. 3 entry intersections. The average of the maximum load

on each of the 2.1-m- (7-ft-) long bolts was 90.6 kN (20,400 lb), and

the highest load was approximately 111 kN (24,900 lb). Five out of

six bolts installed in this area reached or exceeded the yield point of

the steel. The average of the maximum load on each of the bolts in

or near the intersections in the No. 5 entry was 45.4 kN (10,200 lb).

All but two of these bolts were near the edges of the intersections.

However, the two bolts in the center of the intersection showed

approximately the same amount of load as those near the edge. The

average of the maximum load on each of the mid-pillar bolts was

20.0 kN (4,500 lb) in the No. 3 and 34.7 kN (7,800 lb) in the No. 5

entry.

Figure 7A shows the average of maximum load of each bolt as a

function of entry width, and figure 7B shows the maximum load as

a function of test site. Intersection sites 5 and 6 in the No. 3 entry

had the highest bolt loads. Variations in loading at intersection sites

1 through 4 in the No. 5 entry were statistically insignificant. It

appears that maximum load increased from site 1 to site 4 at the midpillar locations. Variations from site 1 to site 3 would be expected

based on changes in bolt spacing and length and grout length.

However, site 1 and 4 are identical, yet show the largest difference in

bolt load. This could be due to the fact that site 4 is closer to the

center of the panel and would have had higher rock stresses. All

maximum bolt loads were within the capacity of the bolts.

Maximum strain values were measured on the 2.1-m- (7-ft-) long

bolts in the intersections in the No. 3 entry. Maximum measured

strain valuewas36,000 microstrain; however, several gauge locations

on two bolts lost continuity, which could be due to exceeding the

capacity of the strain gauge (50,000 microstrain). The loss of continuity might also indicate shearing conditions that could have led to

severing of the gauge lead wires. The high microstrain readings were

after second-pass mining had already passed the test site. 

Stress increases caused by second-pass mining did not increase

load on the bolts until the test sections were inby the borer. In some

cases, load increased significantly, but in only a couple of strain

gauge locations did the load approach the yield point of the steel.

Bending did not appear to have a significant effect on overall bolt

load. The 0.9-m- (3-ft-) long bolts appeared to have the highest

maximum bending moments.

EFFECTS OF GAS PRESSURE

After installation, approximately 50% of the instrumented bolts

had one or more strain gauges loaded in compression instead of

tension. A typical compression loading pattern is shown in Figure 8.

The magnitude of compression load change varied from 0.5 to 33 kN

(100 to 7,400 lb). The average change for all of the bolts was 9 kN

(2,000 lb). After 41 days, the increase in compression loading

stopped and changed to tensile loading. The average time period for

this transition zone was 20 days. Only 14% of the 0.9-m- (3-ft-) long

bolts showed effects from gas pressure, whereas 70% of the 1.5-m-

(5-ft-) long bolts and 63% of the 2.1-m-(7-ft-) long bolts were

afffected by gas pressure.
Table 3: Bolt load data

Length, m Test site Load, kN Moment, N-m

2.1 1 intersection 49.5 112

2.1 1 intersection 65.7 5

0.9 1 mid-pillar 21.3 65

1.5 1 mid-pillar 9.8 30

1.5 1 mid-pillar 8.0 65

2.1 2 intersection 39.5 27

2.1 2 intersection 46.0 28

1.5 2 mid-pillar 29.4 43

1.5 2 mid-pillar 24.9 96

2.1 3 intersection 29.4 39

2.1 3 intersection 60.4 89

0.9 3 mid-pillar 36.0 288

1.2 3 mid-pillar 29.0 41

1.2 3 mid-pillar 40.0 85

2.1 4 intersection 19.3 22

2.1 4 intersection 51.8 40

1.5 4 mid-pillar 54.6 165

1.5 4 mid-pillar 56.2 29

0.9 4 mid-pillar 72.1 114

0.9 5 intersection 95.6 78

2.1 5 intersection 95.6 D*-35

2.1 5 intersection 95.6 118

0.9 5 mid-pillar D D

1.5 5 mid-pillar D D

1.5 5 mid-pillar D D

0.9 6 intersection D D

0.9 6 intersection 50.0 D*-353

2.1 6 intersection 95.8 97

2.1 6 intersection 110.9 14

0.9 6 mid-pillar D D

0.9 6 mid-pillar 16.0 253

1.5 6 mid-pillar 24.2 D*-144

1.5 6 mid-pillar D D

1.5 6 mid-pillar D D

0.9 7 49.0 145

1.5 7 31.6 35

1.5 7 53.0 D*-22

1.5 7 33.5 44

1.5 7 11.6 20

D = Electrical connector destroyed on development. D* = Electrical connector destroyed on retreat.

When instrumented bolts show compressional loading, it means

that the rock surrounding the bolt is compressing as well. This is an

unusual rock behavior in the immediate roof, and can be explained by

gas pressure in the immediate roof. The measured gas pressure at two

locations showed readings up to 0.34 MPa (50 psi). When the roof

bolter was drilling gas relief holes, a noticeable amount of gas was 

released, varying by location. The gas would bleed off over a

considerable amount of time, causing rock pressure to subside, which

in turn would cause the roof support to compress. The 0.9-m- (3-ft-)

long bolts showed the least effect of this behavior, which could be

because (1) they were installed closest to the face and carried more

initial load and (2) they were farther from the zone of compression.
Figure 4.—Axial bolt loads at site 1 intersection.

Figure 7.—Axial bolt load by (A) entry width and (B) test site.

Figure 5.—Axial bolt loads at site 1 mid-pillar.

Figure 6.—Axial bolt loads at site 5 intersection.

Figure 8.—Initial loading of 1.2-m-long instrumented bolt that

showed compressional loads.

CONCLUSIONS

The highest loads were on the 2.1-m- (7-ft-) long bolts installed in

the intersections of the No. 3 entry. Average axial load on these bolts

was 90.7 kN (20,400 lb), and maximum load was approximately 111

kN (24,900 lb). The ultimate strength of the grade 60, No. 6 rebar

bolt used as a typical roof support was approximately173 kN (39,000

lb). However, most of these bolts reached or exceeded the yield point

of the steel. Bolt loads in the No. 5 entry intersections were significantly less than in the No. 3 entry intersections. However, many of

these bolts were not positioned at the center of the intersection. Bolt

loading at the mid-pillar locations was approximately one-third the

yield point of the steel and one-fifth the ultimate strength. Minor

variations in bolt loading could be attributed to changes in bolt spacing and bolt length. Geological variations and stress changes could

also have been a factor in these slight differences. Overall, readings

from the instrumented bolts indicated that the roof was stable. 

REFERENCES

Dunham, R. K. Anchorage Tests on Strain-Gauged Resin-Bonded

Bolts. Tunnels and Tunneling, vol. 8, no. 6, Sept. 1974, pp 73-76.

Farmer I. W. Stress Distribution Along a Resin-Grouted Anchor.

Intern. J. of Rock Mechanics & Min. Sci., vol. 12, 1975, pp. 347-351.
Gale, W. J. Application of Field Measurement Techniques to the

Design of Roof Reinforcement Systems in Underground Coal Mines.

Commonwealth Scientific and Industrial Research Organization

(CSRIO), Australia, 1987. 

Johnston, J. L., and D. J. Cox. Instrumentaion Procedures for Fully

Grouted Rock Bolts. U.S. Bureau of Mines Inform. Circular 9341,

1993, 10 pp. 

Larson, M., C. Stewart, M. Stevenson, M. King, and S. Signer. A

Case Study of a Deformation Mechanism Around a Two-Entry

Gateroad System Involving Probable Time-Dependent Deformation.

Paper in Proceedings of 14th International Conference on Ground

Control in Mining, ed. by S. S. Peng (Morgantown, WV, Aug. 1-3,

1995). Dept. of Mining Engineering, WV Univ., 1995, pp. 295-304.

Maleki, H., S. P. Signer, M. E. King, and P. A. Edminster.

Evaluation of Support Performance in a Highly Stressed Mine. Paper

in Proceedings of 13th International Conference on Ground Control

in Mining, ed. by S. S. Peng (Morgantown, WV, Aug. 2-4, 1994).

Dept. of Mining Engineering, WV Univ., 1994, pp. 9-17.

McHugh, Ed, and Steve Signer. Roof Bolt Response to Shear Stress:

Laboratory Analysis. Paper in Proceedings—18th International

Conference on Ground Control in Mining, ed. by S. S. Peng and C.

Mark (Morgantown, WV, Aug. 3-5, 1999). Dept. of Mining

Engineering, WV Univ., 1999, pp. 232-238.

Sawyer S. G., and G. J. Karabin. The Development and Use of

Instrumentation for In Situ Measurement of Axial Loads in a Fully

Resin-Grouted Roof Bolt. Paper in Proceedings of 1st Symposium on

Underground Mining. NCA/BCR Coal Conference & Exposition II

(Louisville, KY, Oct. 21-23, 1975). Vol. 2, 1975, pp. 90-103.

Serbousek, M. O., and S. P. Signer. Load Transfer Mechanics in

Fully-Grouted Roof Bolts. Paper in 4th Conference on Ground

Control in Mining, ed. by S. S. Peng (Morgantown, WV, July 22-24,

1985). Dept. of Mining Engineering, WV Univ., 1985, pp. 32-40.

Serbousek, M. O., and Signer, S. P. Linear Load Transfer Mechanics

of Fully-Grouted Bolts. Bureau of Mines Rep. of Invest. 9135, 1987,

17 pp.

Signer, S. P., and S. D. Jones. A Case Study of Grouted Roof Bolt

Loading in a Two-Entry Gate Road. Paper in 9th International

Conference on Ground Control in Mining: Proceedings, ed. by S. S.

Peng (Morgantown, WV, June 4-10, 1990). Dept. of Mining

Engineering, WV Univ., 1990, pp. 35-41

Signer, Stephen P., and John L. Lewis. A Case Study of Bolt

Performance in a Two-Entry Gateroad. Paper in Proceedings, 17th

International Conference on Ground Control in Mining, ed. by S. S.

Peng (Morgantown, WV, Aug. 4-6, 1998). Dept. of Mining

Engineering, WV Univ., 1998, pp. 249-256.

Signer, S. P., C. Mark, G. Franklin, and G. Hendon. Comparisons of

Active Versus Passive Bolts in a Bedded Mine Roof. Paper in

Proceedings of 12th Conference on Ground Control in Mining, ed. by

S. S. Peng (Morgantown, WV., Aug. 3-5, 1993). Dept. of Mining

Engineering, WV Univ., 1993, pp. 16-23.