48th CONFERENCE ON GROUND CONTROL IN MINING

KEY

1 Displacement at joint yield

[@_ Displacement at ultimate joint strength

RAI Pretensioned bolt load

14 TrTrtT Tt
| A
§ 12 100
E10}
Z 9 Z
08 | ~
a Cone)
O06 F Oo
bal 40 4
a 04 |
Oo
B02 20
o WW 0
120
100
80 Z
4
60 O
<
fe}
40 4
20
t)

1423.45 67 B 9 1011121314 15 1617
TEST NUMBER

Figure 5.—Relation of initial tension to shear displacement, A,
Displacement at joint yield strength; B, displacement at ultimate
joint strength.

yield strength. Displacement at ultimate strength ranged from 2.83
to 4.28 cm (1.113 to 1.684 in) and showed no correlation to initial
axial tension; average displacement at ultimate shear strength was
3.30 cm (1.300 in). These relationships are shown in figure 5

Strain-Gauge Data
Axial and bending strains were calculated for the seven gauge

pair positions along the bolt for the duration of each test according
to the equations—

EgyiatTM (Ende 1 + Esse 2/2 (2)
and Ei (Esue 1 ~ Ennat)0/Y G)
where — d= bolt radius
and y =d- depth of the slot

Because of the bolt-and-block configuration, the maximum
effects of applied shear loads consistently occurred between
positions 3 and 4 on the instrumented bolts (figure 1). The gauges
at position 4 were at the center of the bolt and therefore in the plane
of shearing; electrical connection to these gauges was commonly
lost before ultimate strain levels were reached. Signals were

236

T

normally not lost for gauges at position 3, and these gauges showed
the most consistent maximum strains.

Axial strains at position 3 for all bolts were used to relate
pretensioned load to applied shear load. Pretensioned axial strains
on the bolts at position 3 ranged from 146 to 1617 microstrain. At
shear yield strength of the joints, axial strains ranged from 777 to
2438 microstrain; yield strength of the steel (about 2000
microstrain) was exceeded in only two bolts (tests 7 and 13). Axial
strains at ultimate shear strength of the joints ranged from 11,888
to 40,846 microstrain, all well above yield strength of the steel,
Variability among tests obscures a generalized correlation between
pretensioned bolt loads and axial strain at both yield and ultimate
shear strengths of the joints, Mean values were calculated for bolts
grouped into those with low (six bolts), moderate (six bolts), and
high (five bolts) pretensioned loads (figure 6A).

Bending strains at shear yield strength of the joints were quite
uniform and apparently not influenced by initial pretensioned bolt
load; yield strength of the steel was exceeded at position 3 for 12
of the bolts. For the grouped data, figure 6B shows a correlation in
bending strain between pretensioned bolt loads and ultimate joint
shear strength. Bending strains reached as high as 64,029
microstrain at ultimate joint shear strengths.

Pretensioned load:

Low < 4.48 kN
Moderate 44,48 - 80.07 KN
Hig
100,000 gh 2 80.07 KN
A- Axial strain
z —oo
< L
Pa 10,000 4
E
Q
fe)
c
OQ 1,000 4
=
100 - -
50,000 - -
B- Bending strait
40,000 g strain 4
z
< 30,000 + KEY
£ oe Initial
Q L
6 70,000 —O- Yield
c
Q 10,000 F- Ultimate
=
ok __—Xmr———————_Cos “dj
-10,000 - -

Low Moderate High
PRETENSIONED BOLT LOADS

Figure 6.—Axial and bending strains for pretensioned load groups



i
i
|

18th CONFERENCE ON GROUND CONTROL IN MINING

Table 1._Instrumented roof bolt pretensioned loads, shear strengths, and displacement.

Test No. Pretensioned axial bolt

Joint yield strength

Ultimate joint strength

load, KN (bf) Applied shear load, Block displacement, Applied shear load, Block displacement,
. KN (Ibf) cm (in) KN (Ibf) cm(in)
9.19 (2,065) 125.90 (28,305) 0.528 (0.208) 242.73 (54,570) 3.091 (1.217)
22.17 (4,985) 99.21 (22,305) 0.635 (0.250) 222.78 (50,085) 3.475 (1.368)
24.25 (5,451) 103.02 (23,160) 0.610 (0.240) 216.91 (48,765) 3.226 (1.270)
bic 28.68 (6,448) 128.04 (28,785) 0.411 (0.162) 259.14 (58,260) 3.162 (1.245)
5. 35.37 (7,951) 93.43 (21,006) 0.767 (0.302) 199.63 (44,880) 3.744 (1.474)
6. 39.08 (8,786) 116.36 (26,160) 0.406 (0.160) 220.44 (49,560) 2.827 (1.113)
Teen 53.28 (11,979) 111.49 (25,085) 0.300 (0.118) 217.51 (48,900) 3.185 (1.254)
Bo 56.93 (12,799) 120.39 (27,066) 0.721 (0.284) 223.91 (50,340) 3.307 (1.302)
Qo. 62.68 (14,091) 92.95 (20,898) 0.470 (0.185) 213.77 (48,060) 3.261 (1.284)
10.. 66.35 (14,917) 114.62 (25,770) 0.472 (0.186) 227.98 (51,255) 3.012 (1.186)
" 70.88 (15,935) 92.61 (20,820) 0.330 (0.130) 235.12 (52,860) 3.294 (1.297)
12.. 75.37 (16,944) 112.14 (25,212) 0.335 (0.132) 220.44 (49,560) 3.495 (1.376)
1. 83.87 (18,855) 103.26 (23,214) 1.293 (0.509) 240.99 (54,180) 4.277 (1.684)
Wo. 92.41 (20,776) 102.56 (23,058) 0.441 (0.162) 200.16 (45,000) 2.865 (1.128)
Bo... 98.49 (22,142) 111.76 (25,125) 0.330 (0.130) 230.25 (61,765) 3.228 (1.271)
16... 100.87 (22,678) 129.10 (29,025) 0.262 (0.103) 233.79 (52,560) 3.175 (1.250)
Wa. 107.44 (24,155) ___ 102.95 (23,145) 0.549 (0.216) 227.58 (51,165) 3.518 (1.385)
* Bolt did not break at maximum shear displacement.
KEY
Applied shear load KEY
Strain: Pretensioned bolt load
Position 3A
Postion 38 [1 Appiied toad at joint yieta
Position 4A, HW eoplied toad at uttimate joint strength
_—— Position 48 300
Z 210 - Ho 120
5 ‘80 Ultimate strength "7 % 250
< Yield point vy (8 z F 200
G 150 + oa c ray
a a e a,
yao b 72 2 %°
¢ fa) im
w iva w
& so to S Zz 100
= a
9 80} < < 50
4 0 3
z % 7 0
& 0 yy -30 123 46 6 7 8 9 101112131415 1617
} 3 4 TEST NUMBER

1 2
DISPLACEMENT, cm

Figure 3.—Load and displacement profile, test 14

Wis 109.4 KN (24,595 Ibf), The point at which the curve beyond
the yield shear strength again departs from an approximate straight
line because of either bolt failure or rapid loading of the system, is
Gesignated as the ultimate shear strength of the joint. Mean
ultimate shear strength was 225.5 kN (50,692 Ibf). No discernable
Sorelation was found between pretensioned axial loads on the bolt
andeither yield shear strength or ultimate shear strength of the joint
‘gure 4). As shear loading progressed, each bolt was deformed
m0 an S bend, with the center part of the bend at an angle to the
PIN surface between the blocks. In seven cases, applied shear load
®NCeeded the ultimate strength of the bolt. This was attributed to

235

Figure 4.—Pretensioned bolt loads and shear yield/ultimate strength
of joint

the bolt being forced through the confined block, crushing the
concrete surrounding the hole, which was enclosed on five sides by
the steel shear box. Applied shear loads in these cases reached as
much as 658 kN (148,000 Ibf). In all but three cases (tests 3, II,
and 13), the bolt ultimately broke before the limit of shear
displacement was reached.

Displacements at the yield strength of the joint ranged from
0.33 to 1.29 cm (0.13 to 0.509 in) and averaged 0.52 cm (0.204 in),
The data are not definitive, but a slight correlation is shown
between higher initial axial tension and lower displacement at shear


@

Swain and load data for gauges at positions 3 and 4 from a
representative test (table 1, No. 14) demonstrate how the bolts
responded to shear loading (figure 3). Axial strain data for these
and other gauges are shown in figure 7. Bolt 14 was pretensioned
10.92.41 KN (20,776 Ibf) before it was tested. Initial strains on
individual gauges ranged from 917 to 1665 microstrain. The
effects of applied shear were greatest at gauge sites nearest the
center of the bolt. However, signals were lost at positions 4 and 5
before ultimate loads were reached. At the shear yield strength of
the joint, axial strain exceeded yield strength of the steel only at

sition 4. At the ultimate strength of the joint, the yield strength
of the stec] was exceeded even at position 1, which was 20.3 cm (8

in) from the joint surface.

POSITION NUMBER

— Initial
~~~ Yield
——— Ultimate

10 100
103 MICROSTRAIN

Figure 7.—Axial strain, test 14

Examination of the broken bolt showed that it had been offset
2.3 cm (0.92 in) across the $ bend during shearing. About 0.2cm
(0.1 in) of extension was measured in the top 14.1 em (9.5 in) of
the bolt from the bolt end to gauge position 3 (about 10,500
microstrain), The longitudinal centerline of the bolt through the S
bend and the rupture extended at least 0.58 cm (0.23 in) over the
10.2 cm (4 in) between positions 3 and 5 (>580,000 microstrain)
before the bolt broke. No measurable extension was noted from
position 5 to the other end of the bolt.

The gauges al position 3 (figure 8) showed pronounced
convex bending on the side of the bolt toward the shear ram (the
direction from which shear stress was applied), and strain at gauge
3A increased quickly beyond the yield strength of the steel, On the
concave side of the bend (away from the shear ram), strain was
compressional, or negative. As shearing proceeded, negative strain
decreased to a minimum (-18,000 microstrain for gauge 3B), then
reversed and began to increase. Bending at gauge position 4 was
reversed from that at position 3, or concave (compressional) on the
shear ram side of the bolt, Negative strain decreased to a minimum
of -24,000 microstrain at gauge 4A, then began to rise. Strain on
the convex side of the bolt (gauge 4B) had already increased to
29,000 microstrain when electrical connection to the gauge was
lost. Gauges at position 5 showed bending much reduced, but in
the same direction as at position 4.

237

18th CONFERENCE ON GROUND CONTROL IN MINING

As was common in all tests, gauges near the ends of the bolt
at positions 1 and 7 (20.3 cm {8 in] from the joint surface)
responded to shear loading with a slight initial decrease in strain,
then an increase at all gauges. No bending was apparent. Gauges
at positions 2 and 6, which lay 10.2 cm (4 in) from the center in
each direction, also showed eventual increases in axial strain, but
also small amounts of bending in the same direction as at positions
3 and 5. Negative strains mirror positive values for gauges on the
opposite side of the bolt, but at lesser magnitudes.

CONCLUSIONS

Shear loading of the bolts and the bending that results created
tensional (positive) and compressional (negative) strain at the
bending site. The effect of applied shear stress was observed
throughout the length of the bolt, but was translated to tensional
stress at gauges farthest from the joint sur face (positions | and 7).
Bending was observed at gauges 10.2 cm (4 in) from the joint
surface (positions 2 and 6), but continuing shear displacement
resulted in tensional strain on both sides of the bolt. In two tests,
bending strain at positions 2 and 6 exceeded the yield strength of
the steel. For all bolts, the magnitude of negative strain was
consistently less than the opposing positive strain on the other side
of the bolt and was reversed as shearing continued. With increased
displacement, strain on both sides of the bolt increased. Eventually
applied shear force exceeded ultimate joint strength, and the bolt
essentially failed in tension.

The tests suggest that axial loading has little effect on a joint’s
resistance to shear loading. Yield joint shear strengths may have
a minor correlation to axial loading on the bolt, but ultimate joint
shear strength is apparently unaffected. Joint shear strength at yield
averaged about 76% of expected axial strength of the bolt alone.
Field tests in underground coal mines have shown that roof bolts
commonly have axial loads beyond their rated yield strength, a
condition not duplicated by pretensioned loads in these tests.
However, the tests suggest that bolts subjected to these high loads
can still provide resistance to shear loading.

Although concrete blocks are similar in compressive strength
to the type of host rocks common in coal mines, the hardness of
host rocks plays a role in how a bolt responds to shear loading.
Concrete on one side of the bolt crushes as it hends, and a void is
created on the other side. Very hard host rock would likely tend to
slice a bolt off at a joint face rather than allow the bolt to bend. In
such cases, bolts may provide less resistance to shear loading.
Laboratory tests have been designed to test the effects of shear
stresses in harder materials.

Pretensioning of the bolts effectively minimized separation of
the joint faces during shear displacement. Unlike fully grouted
bolts in mine installations, the nut and plate anchors at the ends of
the test bolts ensured that grout failure would not be a factor in load
and displacement profiles. Both separation of the shearing faces
and plastic failure of grout would tend to propagate bending forces
for greater distances along the bolt.


48th CONFERENCE ON GROUND CONTROL IN MINING

Roof Bolt Response

to Shear Stress: Laboratory Analysis

Ed McHugh, Geologist
Steve Signer, Mining Engineer
National Institute for Occupational Health and Safety
Spokane Research Laboratory
Spokane, WA

ABSTRACT

Recent studies by researchers from the National Institute for
and Health indicate that shear loading
contributes significantly to failure of bolts used for rock
reinforcement in coal mine roofs. Laboratory tests on 17 bolts
were conducted to study the behavior of roof bolts subjected to
shear loading over a range of axial bolt loads, Fourteen strain
gauges were attached to each bolt to measure axial and bending
loads at seven locations along its length. The instrumented bolts
were grouted through two high-strength concrete blocks, and axial
tension was applied to as much as 75% of yield strength. With the
block interface acting as a failure plane, shear loads were applied
to the blocks at a constant rate of displacement to the ultimate
strength of the bolts. The tests characterized the relationship of
axial bolt loads to shear forces across a rock bedding plane and
measured the distribution of axial and bending strain along the
length of the bolts. These results will improve the selection of roof
reinforcement in mine arcas where high shear stresses are present,
thus improving the safety of miners working in these areas.

Occupational Safety

INTRODUCTION

Resin-grouted roof bolts are standard elements in ground
control systems at most underground coal mines in North America.
In general, sagging of the mine roof and yielding of pillars result in
vertical and horizontal stresses in the mine roof, imparting both
axial and shear forces on roof bolts. Combined tensile and shear
forces are at times sufficient to cause failure of the bolts. Mine
Safety and Health Administration (MSHA) statistics show that
more than 400 miners are injured in roof and rib falls annually; in
the 5 years from 1994 through 1998, 53 miners died in such
accidents. Twenty-two fatalities occurred in underground coal
mines during 1998; of that number, 14 deaths (64%) resulted from
falls of ground.

The Spokane Research Laboratory (SRL), a division of the
National Institute for Occupational Safety and Health, is

232

conducting research on roof bolt loading to reduce support failure
and subsequent ground falls. The intent of this study was to
examine the effects of shear forces on rock bolts.

Farly laboratory shear tests were conducted by Haas et al,
(1976) using various bolt types and anchors in blocks of limestone
and shale; they found that resistance to shear stress was increased
about 3.7 times when fully grouted bolts were used to secure a
natural fracture. Radcliffe and Stateham (1980) installed
instrumented bolts in a Colorado coal mine. Each bolt had three
pairs of strain gauges, one pair at each end of the bolt and one in
the center. Most of the bolts exhibited bending at the center gauges,
and about a third of the bolts were subjected to strain beyond the yield
point of the steel. Haas (1981) applied laboratory shear test data
to estimate safety factors for reinforcing a natural shear plane with
grouted bolts. Turner (1987) found that appreciable opening of
fractures prior to shear movements resulted in double bends and
failure of rock bolts by tension. Bolts across unopened fractures
had been actually cut (guillotined) during rock bursts. Pellet et
al,. (1996) related theoretical and experimental analyses of rock
bolt shear strength and found that bolts installed perpendicular to
a joint plane allowed the greatest displacement along the joint before
failure, but that displacement at failure decreased rapidly as the angle
between the bolt and joint plane decreased. They also found that
harder rock led to bolt failure at smaller displacements. Goris et al.,
(1996) used essentially the same procedures as followed in this study
to conduct direct-shear tests on conventional steel cable bolts. Signer
and Lewis (1998) used axial loads on instrumented bolts in an
underground coal mine as evidence of shear stresses that contributed
to roof bolt failure

Methods

The bolts tested were 58.4-cm-long (23-in), standard grade 60,
Dywidag No. 7 Threadbars.' Continuous, rolled-in threadlike de-

‘The mention of specific products or manufacturers does not imply
endorsement by the National Institute for Occupational Safety and Health.



18th CONFERENCE ON GROUND CONTROL IN MINING

103 INITIAL AND YIELD
MICROSTRAIN

4 2 0 2 4 6 8

rT TT 4

KEY =

Initial
Yield
Ultimate

POSITION NUMBER

| | | | |

-40 +20 0 20 40 60 80

103 ULTIMATE/MAXIMUM
MICROSTRAIN

BIA

A

Gauge position

Surface
zone

6
Applied shear

Figure 8.—Bending strain and bolt deformation, test 14.

REFERENCES

Goris, J. M., L.A. Martin, and R. P. Curtin. Shear Behavior of Cable
Bolt Supports in Horizontal, Bedded Deposits. Paper in Proceedings,
15th International Conference on Ground Control in Mining, ed.
by L. Ozdemir, K. Hanna, KY. Haramy, and S. Peng (Golden,
CO, Aug 13-15, 1996). Colorado School of Mines, 1996, pp. SIl-
521,

Haas, C. J. Analysis of Rock Bolting to Prevent Shear Movement
in Fractured Ground. Min. Engineering, June 1981, pp. 698-704.

Haas, C. J., R. L. Davis, C. J. Keith, J. Dave, W. C. Patrick, and J.
R. Strosnider, Jr. An Investigation of the Interaction of Rock and
Types of Rock Bolts for Selected Loading Conditions, Third
Annual Report. Rock Mechanics and Explosives Research Center,
Univ. of Missouri, Rolla, U.S. Bur. Mines Contract Report
HO122110, 1976, 283 pp.

Johnston, J. L., and D. J. Cox. Instrumentation Procedures for
Fully Grouted Rock Bolts. U.S. Bur. Mines Inform. Cir. 9341,
1993, 10 pp.

Pellet, F., P. Egger, and A. M. Ferrero. Contribution of Fully
Bonded Bolts to the Shear Strength of Joints: Analytical and
Experimental Evaluation. Paper in Mechanics of Jointed and
Faulted Rock. Proceedings of the International Conference on
Mechanics of Jointed and Faulted Rock, ed. by H. P. Rossmanith
(Tech. Univ. of Vienna, Vienna, Austria, April 18-20, 1990).
Balkema, 1990, pp. 873-878.

Radcliffe, D. E., and R. E. Stateham. Stress Distribution Around
Resin-Grouted Bolts. U.S. Bur. Mines Rep. Invest. 8440, 1980, 39

pp

Signer, $. P., D. Cox and J. Johnston. A Method for the Selection of
Rock Support Based on Bolt Loading Measurements. In Proceedings
of 16th International Conference on Ground Control in Mining, ed.
by S. S. Peng (Morgantown, WV, Aug, 5-7, 1997). Dept. of Min.
Engin., WV Univ., 1997, pp. 183-190

Signer, S. P., and J. 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 Min. Engineer, WV
Uriv., 1998, pp. 249-256

Turner, P. A. Guillotining of Rock Tendons During Rockbursts
Due to Shear Displacements on Fracture Planes. Paper in
Proceedings of International Society for Rock Mechanics,
Symposium on Design of Rock Reinforcing: Components and
Systems(Johannesburg, S. Africa, Nov. 1987). Int. Soc. for Rock
Mech., 1987, pp. 59-65.

238



18th CONFERENCE ON GROUND CONTROL IN MINING

formation along the length of a bolt allows it to be tensioned to its
full yield strength through the use of nuts and couplers, A 6.4-mm-
wide by 3.2-mm-deep (0.25 by 0.125 in) slot was milled along the
smooth sides of each bolt. After milling, the bolt’s cross-sectional
area was 3.47 cm? (0.5375 in’), and section modulus was 0.816 cm*
(0.196 in’), Tests have shown that the slotting process reduces axial
bolt strength by approximately 10% (Signer et al., 1997, p. 184).
Seven strain gauges were installed along each side of the bolt as
described by Johnston and Cox (1993). The strain gauges were
arrayed so that one pair of gauges was at the center and additional
pairs of gauges were at 5, 10, 20 cm (2, 4, and 8 in) from the center
in each direction (figure 1). The instrumented bolts were calibrated
in a uniaxial test machine to correlate voltage change to load change
at bolt load levels below the yield point of the steel. Voltage readings
were also converted to strain according to the equation—

€=4AV/GFEV ay

where AV = change in voltage,
GF = gauge factor (2.075),
and EV = excitation voltage (5 V).

The instrumented bolts were installed in pairs of 29-cm (11.5-
in) square concrete blocks made from a fine sand-cement mix
having an average 28-day compressive strength of 85.5 MPa
(12,400 psi). The first block of the pair was cast by pouring the
mix in a steel mold with a flat aluminum bottom to create a smooth
shearing surface of 780 cm? (121 in?). A 2.86-cm (1.125-in) in
diameter rod was centered in the mold before casting. The rod was
removed after about 24 hours to provide a hole through the block
for placement of the bolt to be tested. A recess about 8.9 cm (3.5
in) square and 8.9 cm deep was also cast into the top of each block
to allow room for a plate and a nut, and to permit access to the end
of the bolt when it was to be tensioned. The second block of the
pair was cast using the bottom surface of the previously cast block
to create a matched shearing surface. This block was also cast with
acentered rod and a bolt-end recess.

The instrumented bolt to be tested was positioned through the
aligned holes so that the strain gauges faced one side of the block
assembly. A polyester resin anchoring grout (Fosroc Anchortite)
was used to assure direct contact between the bolt and the concrete
block. The grout has an unconfined compressive strength of at
least 69 MPa (10,000 psi) and a setting time of about 60 min. The
bolt was coupled through an A-frame extension to an 18-mt (20-
ton) hydraulic jack and tensioned against a plate and a nut on one
end of the bolt. A through-socket and wrench were then used to
tighten a nut at the other end to a specified torque to retain the
desired load.

After the grout had set, the block assembly was placed in steel
Shear boxes and oriented so that the gauges faced in the direction
of shear displacement. The shear boxes allow a 0.63-cm (0.25-in)
Bap between the walls of each box and the surfaces of the blocks.
This annular space around the blocks was filled with quick-curing
8ypsum cement, which has a compressive strength of about 28 MPa
(4,000 psi), to ensure complete contact with the shear boxes.

233

2.2-cm diam 0.64 em

Faces parallel to block edge

0.92 em
Section A-A

t 58.4 cm
mal ft 8.9m
| 20.3m 20.3 cm 1
I
Fi

Wo

(center) Nut

Gauge Position

Figure 1.—Instrumented roof bolt
Testing

Tests were conducted in a Gilmore direct-shear, materials-
testing machine, This high-pressure hydraulic test frame has a
capacity of 1,334-KN (300,000 Ibf) along both the shear and normal
axes. The shear box assembly with the concrete blocks and a
tensioned, instrumented bolt was placed on rollers in the test frame
with the bolt aligned to the normal (vertical) axis. Figure 2 shows
the shear box assembly ready for testing. An MTS TestStar II
control system allowed for computer management of test
parameters through templates that determined loads, displacement,
and rates in both the shear and normal directions. For this series of
tests, a constant load of 8.90 KN (2,000 Ibf) was applied along the
normal axis of the block assembly, principally to help maintain the
position of the box assembly during shearing.

During the test, the upper box was held immobile against a
backstop in the frame while shear loads were applied at a constant
displacement rate of 0.55 mm/min (0.0216 in/min) on the lower
box. With the stecl shear boxes in the test frame, maximum shear
displacement for the machine was 4.6 cm (1.8 in). Applied load
and displacement data were collected at 3-sec intervals within the
TestStar system and at 15-sec intervals with an external datalogger
(Campbell Scientific Model 21X-QM). The datalogger also
collected information from the array of strain gauges on the
instrumented bolt.

SHEAR TEST DATA

The program was designed to test 15 instrumented bolts
pretensioned at five different axial bolt loads. An initial test on an
uninstrumented bolt was conducted to establish reasonable shear
displacement rates, load ranges, and applied-load sampling
intervals. Two tests on instrumented bolts were added to establish
strain gauge sampling methods and intervals. Strain gauge data


48th CONFERENCE ON GROUND CONTROL IN MINING

Normal foad piston
4

NS

3.3-cm nut

,

iI een

1\ 2B Bs
a B te
of yoy
2

3
aay
Bearing

plate

Yl

i<@-— Stee! shear box

, Hydrostone

Roof bolt

Back plate

Grout

Joint

Shear load piston

Roller assembly

Figure 2.—Shear blocks and test assembly

from all 17 tests on instrumented bolts were considered valid and
so are included here.

Calculated loads from all functioning gauges (signals from 3
of the 238 gauges failed prior to tensioning) were used to determine
average pretensioned load on the bolt. Special care was required
to minimize bending at the ends of the bolts where the plates and
nuts secured them to the blocks. The bolts were pretensioned to
axial loads ranging from 9.2 to 107.4 kN (2,065 to 24,155 Ibf), or
as much as 75% (approximately 143.4 kN (32,250 Ibf) of the
estimated yield strength of the bolts. Tensile tests of No. 7 slotted
rebar bolts showed a mean yield point of 163 KN (36,600 Ibf) and
an ultimate strength of 233 KN (52,300 Ibf). Since a vertical load
of 8.90 kN (2,000 Ibf) had been applied to the block assembly prior

234

to shearing, effective load on the tensioned bolt was reduced by
that amount. Pretensioned bolt loads, applied shear loads, and
shear block displacement for each of the 17 bolts are shown in table
1.

Applied Shear Loads and Displacements

The concrete blocks, the joint surfacc between the blocks, and
the pretensioned bolt together comprise a system that responds to
shear loading applied during the test. The general relationship
between applied shear load and displacement is shown in a profile
from test 14 (figure 3). The point at which the curve departs from
the initial steep slope to a lesser slope is here designated the yield
shear strength of the joint. Mean yield shear strength for these tests