Table 3. Incremental strain change after blasts, bolt 1, pm/m_

Gauge Blast 1 Blast 2 Blast 3
pair Side! Side2 Average “Side |_Side2 Average Side] Side2 Average
1 113 142 127 “0 46 32 3 8 “I
2 120 145 133 156129 142 iF 10 Wl
3 163 107 135 o 102 96 9 4 2
4 NA 43 NA NA 1 NA NA “17 NA
5 47 58 53 5 19 12 2 -l -l
6 ST 55 56 16 16 16 4 -l 2
NA - Not available,
Table 4. Incremental strain change after blasts, bolt 2,_prm/m_
Gauge Blast 1 Blast 2 Blast 3
pair Side _Side2 Average Side |_‘Side2 Average Side! Side2 Average
1 187 45 116 51 15 -18 -28 0 -14
2 258 4a 150 263 6 135 2 2 9
3 2 68 139 180 34 107 15 21 18
4 al 58 50 3 -l 2 4 -10 7
5 256 55 155 208 i 110 13 3 5
6 270 205 237 -23 161 69 -38 13 -12
Key
@ Blast 1, bolt 4 Blast 1, bolt 2
@ Blast 2, bolt 1 C_ Blast 2, bolt 2
4 Blast 3, bolt 1 4 Blast 3, bolt 2
BOLT DISTANCE INTO RIB, in
t) 10 20 30 40 50
T T T T T
4 4A
300 a Aa 4
A a
£ . )
€
 200- 4
z ce}
t ie)
iv Q
5 ; °
100 + 4
rN
s
fa] e
0 1 1 1 n 1
0 25 50 75 100 125 150

BOLT DISTANCE INTO RIB, cm

Figure 5, Average accumulated axial strain versus rib distance after blasts 1, 2, and 3


Table 2. Accumulated strain change after blasts, bolt 2, ym/m

Gauge Blast 1 Blast 2 Blast 3
pair Side Side2 Average Side! Side2 Average Side 1 Side2 Average
1 187 45 116 150 64 107 128 69 99
2 257 43 150 556 52 304 598 55 327
3 211 68 140 415 96 256 470 108 289
4 41 59 50 40 2 56 41 B 37
5 255 54 155 492 70 281 523 14 299
6 270 ____ 206 238 258388 323 230_ 435 333
Key

@- Gauge 1 —@ Gauge 4

-O- Gauge 7 —O- Gauge 10

—¥- Gauge2 —d&— GaugeS —y Gauges —d— Gauge 11
i Gauge3 —O- Gauge6 -{+ Gauge9 —®- Gauge 12

200

3
8

STRAIN, jzm/m

-100

300

200

STRAIN, jzm/m
8

T

30 40

100 +

Blast 1

Blast 2

Blast 3

10

20

30 40

TIME, hr

50 60

Figure 4. Incremental strain changes after blasts. A, Bolt 1; B, bolt 2

500


so that the gauges were in the horizontal plane, as
shown in Figure 1. The grout was allowed to cure for
about 2 weeks before the drift was advanced. Because
the bolts were placed so close to the face, the
instrument cables were placed inside a 5-crn (2-in) in
diameter steel pipe, covered with old conveyor belts,
and bolted to the rib for protection against blasting.
The drift was extended in three blasts, each by a
distance of the drift radius of 2 m (6 ft).

4 DATA AND RESULTS

Figure 3 shows the accumulated strains measured on
bolts | and 2 by each gauge over time. Measurements
were collected by the data acquisition system begin-
ning at midnight on August 5, 1998. The first blast
took place between hours 3 and 4, the second between
hours 18 and 19, and the third blast between hours 38
and 39. Tables 1 and 2 show accumulated individual
and average strains on bolts | and 2 after each blast.

Incremental strain was calculated by subtracting
the strain reading for the current hour from the
reading for the previous hour. Figure 4 shows
incremental strain changes over time during advance
of the drift since midnight of August 5, 1998. Large
changes in strain were measured after the first and
second blast. Very little change in strain occurred
between blasts and after the third blast. Tables 3 and
4 provide incremental changes in strain on bolts 1 and
2 after each blast. Compressive or negative strain on
side | of pair | on both bolts indicates bending from
the second blast at a distance of 19 cm (7.5 in) into
the rib. Fach bolt in pairs 2 and 3 shows almost the
same response as shown following the first blast. Pair
4 gave an anomalous reading and showed only small
changes in strain for all blasts. Pairs 5 and 6 of bolt |
showed small changes in strain while the same pairs
‘on bolt 2 showed a much larger change in strain, Pair
6 even showed some bending from the second blast.
Figure 5 is a plot of the average accumulated strain
data versus position of the gauges on the bolt.

5 CONCLUSIONS

‘Two strain-gauged rock bolts were grouted in the
middle of a rib near the face of a 3.7- by 3.7-m (12- by
12-ft) drift. The drift was advanced in three 2-m (6-ft)
blasts of one-half drift wide. The average response in
terms of axial strain to the first and second blasts was
about the same for the gauge pairs 42 and 65 cm from
the head of the bolt or the rib. Bending strains were
observed on the gauge pair located 19 cm from the

rib, mostly as a result of the second blast. There was
very little change in strain from the third blast.
Dynamic strains resulting from blasting were not
measured. The grouted bolts cannot be recovered and
checked to see if any damage occurred during
blasting.

REFERENCE
Serbousek, M. O. & S. P. Signer 1987, Linear Load-

Transfer Mechanics of Fully Grouted Roof Bolts. ,
U.S. Bur. Mines Rep. Invest. 9135, 17 pp.

502


Flock Mechanics for Industry, Amadei, Kranz, Scott & Smeallie (eds) © 1999 Balkema, Rotterdam, ISBN 90 5809 052 3

Use of strain-gauged rock bolts to measure rock mass strain during drift

development

J.C. Johnson, T. Brady & M. Larson

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

R. Langston
Stillwater Mining Company, Nye, Mont., USA

H. Kirsten

Steffen Robertson and Kirsten, Johannesburg, South Africa

ABSTRACT: An experiment is described in which instrumented rock bolts were used to measure strain in the
rock mass during drift development at the Stillwater Mine, Nye, Montana, USA. Two strain-gauged rock bolts
were grouted into the middle of the rib next to the face. The drift was excavated in three advances, each one-half
drift wide. Strains were recorded hourly by a datalogger. Results show that the average change in axial bolt
strain for the first and second advance was about the same and that there was almost no change in strain after
the third advance. Bending strain was observed 19 cm from the head of the bolt.

1 INTRODUCTION

Unstable ground around openings continues to be a
safety concern in underground mines, and ground falls
are a major contributor to underground injuries and
fatalities. Ideally, stability and design analyses should
be conducted early in a mine’s life so that problems
can avoided or at least minimized to the greatest
extent possible. The National Institute for Occupa-
ional Safety and Health (NIOSH) is advancing
methods to validate computer models of a rock mass
in order to assist in the design of a safer work
environment for miners.

‘One method of validating computer models in rock
mechanics studics is to compare field strains to
calculated strains. Strain-gauged rock bolts can
measure strains close to the skin of underground
openings. By placing the gauges close together, strain
gradients can be determined.

An experiment was conducted in which strains
were measured during the advance of an underground
drift. The purpose of this paper is to describe the
instrumented bolts, the installation procedure, and the
measured strains.

2 INSTRUMENTS

Serbousek & Signer (1987) describe the construction
of the rock bolt used in this experiment. Although the
strain-gauged rock bolts used in this experiment were
constructed at SRL, they can be purchased commer-

497

cially. Figure 1 shows the positioning of the strain
gauges on the bolt. They are numbered | through 6
from the head of the bolt on side 1, and 7 through 12
on side 2. Strains are analyzed from pairs of gauges.
Pair 1 is composed of gauges | and 7, pair 2 is com-
posed of gauges 2 and 8, etc. A positive value indi-
cates tensile strain. A negative valuc indicates com-
pressive strain. Uniform tension would be indicated
if both gauges of a pair have the same positive magni-
tude of strain. Differences in the magnitude of strain
indicate some degree of bending. Axial strain is ob-
tained from calculating average strain from each pair.

Each gauge on the bolt was calibrated. The
calibrations showed that the response of all strain
gauges was linear (r° = 0.96) except on gauge 4 of bolt
1, The gauges were read with a Campbell Scientific
21X datalogger.

3 INSTALLATION PROCEDURE

‘The test site was on the 3200 level of the mine at the
eastern end of the footwall drift (Fig. 2). Two |.5-m-
(S-ft) long strain-gauged bolts were spaced 30 cm (1
ft) apart vertically and 15 cm (6 in) back from the face
in the southern rib. The bolts were installed in exactly
the same manner to ensure reproducibility of field
data. A jackleg drill was used to drill holes 3.8-cm
(1.5-in) in diameter, 1,5 m (5 ft) deep, and about 5°
below horizontal.

The holes were pumped full of a nonshrinking
grout, after which the bolts were inserted and oriented


3.8-0m diam 0.64cm

(grout filled)

Leads A

Cross-section
2 c 2
3 s 3 2 2 2
3
BF &€ & & &
= o © * rs °
2 2 rH 3
<8 of of 28 of of
Se sg 82 & S3 53
&8 #5 ¢% ¢€& && as
Bolt 2
rh Bolt 1

=F

Leo}

119.05 cm!
1. -| 41.9cm
¢—— 64.8 cm ——
+ 87.6 cm
110.5. cm |
133.4. om
152.4cm 1

Figure 1. Location and orientation of strain gauges on bolts.

Section A-A

\7

Scale, m
0 30 60
—
Scale, m
oO 5 10
a on |
Bolt 2
Bolt 1

1.8.m (6+tt)
advance

Figure 2. Locations of bolts and advance of drifts.

498


Key
—@ Gauge! —@- Gauge4 —O- Gauge7 —O- Gauge 10
—¥- Gauge 2 —@- Gauge5 —V- Gauge 8 —4— Gauge 11
—TM@ Gauge3 —O- Gauge6 —Ck Gauge9 —@- Gauge 12

400 p+ T T T T T
300 F A
E
ES 200F
z 400 F
& oF
too F — [Blast 1 [Blast 2 Blast 3
1! 1 1 1 l 1
t) 10 20 30 40 60
600 F
500 F
E
E 400F
Ba
Zz 300F
é
- 200F
oO
100 F
oF
TIME, hr
Figure 3. Measured strains versus time. A, Bolt 1; B, bolt 2.
Table 1. Accumulated strain change after blasts, bolt 1, jm/m
Gauge Blast | Blast 2 Blast 3
pair__Sidel Side2 Average Side 1_Side2__ Average Side Side 2 Average
1 113 142 128 “14 188 87 330 181 16
2 121 147 134 301 295 296 328 320 324
3 163 108 136 261 229 236 263 269 266
4 NA 44 NA NA 53 NA NA 37 NA
5 48 58 53 55 82 15 537 87 2
6 57 55. 56. 79 74 75 80. 76. 78

NA ~ Not available because of nonlinear response during calibration

499