1.905 cm (0.75 in) diam 0.635 cm (0.25 in)

Faces parallel to ribs ——_———+]

Z

0.3175 em (0.125 in) ——
Section A-A

Leads

Electrical strain gauges.

A
n 1 2 3 ‘ ce | 5 6
—— ——— a ee

Cy 15.24.cm
(6 in)
le— 45.72 cm (18 | A
76.2 cm (30 in) {
|~——___—__—. 106.68 cm (42 in)

{a} 187.16 cm (54 in}
167.64 cm (66 in)

182.88 cm (72 in)

C

Figure 1.—Instrumented roof bolt

180 T 1 40
. Sitel

z 135 430 2
=~ 2 The first test site was located in a two-entry gate
) 90 J20 TM, road where the coal seam is approximately 2.5 m
Oo 2 thick, and overburden is between 210 and 240 m
+ 45 10 9 deep. The mine roof is a competent mudstone inter-
spersed with very fine grained sandstone and silt-
ul A 6 stone. Grouted bolts 1.8 m long, spaced in a square

0 .

0 0.08 010 0.15 pattern approximately 1.2 by 1.2 m were used as the

main roof support. Axial and bending loads shown in
figure 3 form the load profile after the pass of the
Figure 2.—Stress-strain curve of slotted Second longwall panel. Bending moments measured
roof bolt were primarily caused by rock loading in the mesh.
Maximum fiber stresses are shown in table 1.

STRAIN, mm/mm


AXIAL LOAD, kN

P+0.7 mer

BENDING LOAD, N-m

Figure 5.—Load profiles for test site 3


Table 1—Maximum fiber stress for test site 1, megapascals

Bolt Strain gauge station

1 2 3 4 5 6
5. 300 365 493 415 316 70.0
6. 250 418 539 410 161 45.9
7... 172 451 420 352 129 110
8... 298 451 422 352 132 65.5

The selected design stress for this test site was 414 MPa. Every roof bolt exceeded this value.
Reducing the spacing between bolt rows, increasing the number of bolts per row, and increasing the
diameter of the bolt are three ways to reduce fiber stress. Using these methods and equation 6
produced the figures shown in table 2.

Table 2.—Roof bolt design alternatives

Number of bolts S,,,m Sy, m_ Bolt diameter, mm

qlee 1.22 0.90 18, No. 6 rebar
See 1.02 1.08 18, No. 6 rebar
4 oo. 1.22 1.32 22, No. 7 rebar

These selections assume an equal spacing of roof bolts. If there were sufficient data to suggest a
consistent pattern of bolt load profiles, then irregular bolt spacings could be used. In this case, the
safety factors should be increased to provide for variations that might overload the roof bolts. Each
of these options would limit bolt stress to less than 414 MPa. The stress distribution along the bolt
length indicates that there would be adequate anchorage length if a 1.5-m-long bolt were used in
place of the 1.8-m-long bolt.

Site 2

The second test site was located in a four-entry gate road with yield abutment pillars. Overburden
at the test site is approximately 670 m. A 30-cm-thick layer of incompetent drawrock overlies the
coal seam. The immediate mine roof is formed by a fossiliferous shale that grades into thinly inter-
bedded shales and coals. This seam is overlain by a 30-cm-thick coal seam. Where the drawrock is
thin, it falls during extraction of the coal together with the coal rider seam. The main roof above the
rider seam consists of 30 to 60 cm of competent siltstone overlain by massive sandstone. Instrument-
ed bolts were installed in areas where the rider seams remained intact. Row spacings were approxi-
mately 1.22 m.

Axial and bending loads (figure 4) show the load profile after entry development. Bending moments
were significantly higher at this test site. High horizontal stresses produced lateral movements at the


AXIAL LOAD, kN
5

8 7 6
15 26 1 7
31 31 38 75
tam Mudstone 83 83 98 99
101 100 129 116
106 107 100 85
68 31 56 67
0.9 m- | 1.4m 1,3 m- 1.5 m: | 1.0
BENDING LOAD, N-m
8 7 6 5
-12 4 1
8 5 4 5
17 -13 oO oO
2 al 1 37
21 0 -32
-57 78
0.9 m- 1.4m 1.3m | 1.5m | 1.0 m-

Figure 3.—Load profiles for test site 1


rider seam location, which contributed to these bending stresses. Maximum fiber stress was 513 MPa
on bolt 3 at 46 cm from the mine roof.

The selected design stress for this test site was 331 MPa. A lower stress limit was selected because
maximum bending loads may be reached between strain gauges. Also, the amount of data at this test
location was not extensive. Using six No. 6 bolts per row, the spacing between the rows was reduced
to 0.83 m, and the spacing between the bolts in the row was 0.86 m. If five No. 7 bolts were used
per row, then the bolt spacing would be 1 by | m.

Site 3

The third test site was in a two-entry gate road. The coal seam is approximately 4.3 m thick and has
a 16° pitch. Overburden at the test site is approximately 335 m deep. The entry was cut
approximately 3 m high, and top coal was left on the roof and the floor. The top coal was
approximately 0.5 m thick on the downdip side of the entry and 1.5 m on the updip side. Above the
top coal, the immediate roof was a very low strength carbonaceous mudstone.

Row spacing of the roof bolts was approximately 1.2 m. Axial and bending loads (figure 5) show the
load profile after the pass of the first longwall panel. Maximum fiber stresses are shown in table 3.

Table 3.—Maximum fiber stress for test site 3, megapascals

Bolt Strain gauge station

1 2 3 4 5 6
1. 156 423 402 411 343 315
2. 108 554 319 406 382 682
3. 117 225 434 420 413 339
4. 104 252 432 412 410 196
5. 130 426 411 416 435 400

The selected design stress for this test site was 414 MPa. The ultimate strength of the instrumented
bolts was approximately 684 MPa. Station 6 on bolt 2 showed a combined stress that was very close
to failure. It should be noted that the accuracy of the strain gauges decreased considerably at greater
elongations, so the high stress readings should be taken as estimates.

The main concern at this test site was the lack of adequate anchorage length at the ends of the roof
bolts. Cutters would form on the downdip side of the entry, and the bolts would appear to loose
anchorage in the weak mudstone. The bolt load profile shows that high stresses were too close to
the end of the bolts. Increasing bolt diameter, reducing bolt spacing, and/or increasing the number
of bolts probably would not be enough to prevent this type of roof failure. A longer bolt that is
angled over the ribs could solve the anchorage problem.


A METHOD FOR THE SELECTION OF ROCK SUPPORT BASED ON BOLT
LOADING MEASUREMENTS

Stephen P. Signer
Dennis Cox
Joanne Johnston
Spokane Research Center, National Institute of Occupational Safety and Health,
Spokane, Washington, USA

ABSTRACT

A method to assist in the evaluation and selection of roof bolts using in situ measurements of roof
bolt loading has been developed by researchers of the Spokane Research Center, National Institute
of Occupational Safety and Health. Both axial and bending forces are measured by strain gauges at
many locations along the length of fully grouted bolts during various stages of mining. This
information is then used to (1) predict bolt loading for variations in bolt spacing, grade, and diameter,
(2) calculate the design changes necessary in zones where bending loads are high, and (3) determine
if bolt length is adequate. Results from several case studies of full-column bolt loading in coal mine
gate roads are used to illustrate the design method. This knowledge will give design engineers a tool
for the selection of roof support systems that will improve underground safety by reducing roof bolt
failures.

INTRODUCTION

In this paper, a method is described for selecting roof bolt diameter, spacing, and length based on in
situ measurements of bolt loading. It is not meant to be used to the exclusion of other design
methods and means of analysis. Rather, it should be used as another tool in a well-rounded approach
to rock support selection and design. An excellent discussion of rock support design methods is
available in Choquet and Hadjigeorgiou (7).

Selection of rock reinforcement depends on many factors that require engineering judgment, such as
geology, stress field, geometry, factor of safety, standup time, and estimated risk of roof failure. The
intended use of the underground opening will dictate the selection of design loads. In mines, some
areas are expected to remain open only for a matter of months, while other areas must remain open
for many years. Therefore, more support must be installed where safety or economic interests require
greater roof stability.

Civil engineering design approaches include a factor of safety to ensure stability for the life of the
project. This is done by limiting the allowable stresses to a percentage of the yield strength or ulti-
mate strength of the material. Mining often allows for the yielding of structures so that the material
can be extracted at a minimum cost. This often results in a minimum amount of support installed, and
previous research has shown that roof bolts are often loaded well past yield loads (2-4).

The use of strain gauges to measure loading on fully grouted roof bolts has been used successfully
to study support-rock interaction mechanics (5-//). Effective use of strain gauges for long-term


Serbousek, M. O., and S. P. Signer. “Load Transfer Mechanics in Fully Grouted Roof Bolts.”
Paper in Fourth Conference on Ground Control in Mining (Morgantown, WV, July 22-24,
1985). Dept. Min. Eng., WV Univ., 1985, pp. 32-40.

Serbousek, M. O., and S. P. Signer. Linear Load Transfer Mechanics of Fully Grouted Bolts.
U.S. Bur. Mines Rep. Invest. 9135, 1987, 17 pp.

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. Min. Eng.,
WV Univ., 1994, pp. 9-17.

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. Min-Eng.,
WV Univ., 1995, pp. 295-304.

Farmer, I. W. “Stress Distribution Along a Resin Grouted Anchor.” Jnt. J. Rock Mech. Min.
Sci. & Geomech, v. 12, 1975, pp. 347-351

Dunham, R. K. “Anchorage Tests on Strain-Gaged Resin Bonded Bolts.” Tunnels and
Tunneling, v. 8, No. 6, Sept. 1974, pp. 73-76.

. Gale, W. J., and M. W. Fabjanczyk. “Application of Field Measurement Techniques to the

Design of Roof Reinforcement Systems in Underground Coal Mines.” Proc., 13th Cong. of
Council of Instit. Min. and Metal., Singapore, pp. 135-141.

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

. Littlejohn, $. “Overview of Rock Anchorages.” Paper in Comprehensive Rock Engineering:

Principles, Practice, and Projects. V. 4, Pergamon, 1993, pp. 413-450.


Every roof bolt exceeded the design stress value so other corrective action is required. Reducing the
spacing between bolt rows, increasing the number of bolts per row, and increasing the diameter of
the bolt are three ways to reduce fiber stress. Using these methods and equation 6 produced the
results shown in table 4.

Table 4.—Roof bolt design alternatives for test site 3
Number of bolts Sa,m Sy,m Bolt diameter, mm

Slee 1.02 1.07 18, No. 6 rebar
4. 1.22 0.89 18, No. 6 rebar
4. 1.22 1.28 22, No. 7 rebar

SUMMARY

All of the design selections incorporated an evenly spaced bolting pattern. The design loads
combined maximum axial and bending loads. Additional refinements could be made by allowing
uneven spacings across the entry and using the exact load profile for the design loads. If this
approach is used, the factor of safety should be increased to allow for variations and anomalies. After
redesigning the bolting pattern, the pattern should be measured to see how close estimated loads
compare with actual loads. In situ tests should also be conducted to establish anchorage capacities
in the roof rock. This method could be used effectively for the development of an engineered solution
having a selected factor of safety for selecting bolt diameter, spacing, and length. The end result will
be fewer roof falls and safer conditions underground.

REFERENCES

1.

Choquet, P., and J. Hadjigeorgiou: “The Design of Support for Underground Excavations.”
Paper in Comprehensive Rock Engineering: Principles, Practice, and Projects. V. 4,
Pergamon, 1993, pp. 313-345.

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: Proceed-
ings, ed. by Syd S. Peng (Morgantown, WV, June 4-10, 1990), Dept. Min. Eng., WV Univ.,
1990, pp. 35-41.

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. Min. Eng., WV Univ.,
1993, pp. 16-23.

Signer, S. P, “Field Evaluations of Grouted Roof Bolts.” Paper in New Technology for Long-
wall Ground Control. Proceedings: U.S. Bureau of Mines Technology Transfer Seminar,
comp. by C. Mark, R. J. Tuchman, R. C, Repshar, and C. L. Simon. U.S. Bur. Mines Spec. Pub.
01-94, 1994, pp. 91-101.


AXIAL LOAD, kN

Not instrumented

2 3 4 5 6
4 7 10 12 Sandstone
a 6 78 Eg
Siltstone
44 54 46 na
66 62 98 76 Coal
68 ye 119 27
Shale
14-5 28 16
4.0m | T 0.7 mr | 1.1m | 4.4m 1.0
| 0.6m | 07m
BENDING LOAD, N-m Not instrumented
1 2 3 4 5 6
9 -af 4 13
“aT zc) 8 wes
1379 10 94 nla
26 118 -1 “47
"76 -29) 54 181
8 -30ff -97 59
1.0 m | 0.7 m 1A im | 4.4 1.0m
0.6m 07m

Coal

Figure 4.—Load profiles for test site 2


S4 = new bolt spacing across the opening,
S:4 = new bolt spacing down the entry,
and PM, = new load and moment values.

By solving for P,and M, and substituting equations 4 and 5 for | and 2, and solving for equation 3,
then—

124 124
a +4 4 <6
“3s, 4 SS, 5,

x

SiS, P_ Sus

Both P and M are measured axial forces and bending moments derived from the instrumented bolts.
The values of A and S, are derived from the member selected for design considerations. 6, must be
less than or equal to the selected maximum stress. Total stresses should be calculated for each bolt
location to determine the highest value and must be less than or equal to a design stress based on the
factor of safety selected. Safety, engineering judgment, and cost considerations will result in a design
selection based on variations in bolt diameter, strength, and spacing.

Bolt diameter and bolt length are two of the major variables changed in the design process. These
factors can have a significant effect on roof stability. Adequate anchorage length is also critical to
ensuring the stability of the bolted horizon (73). Anchorage length can be evaluated by examining
load distribution with respect to the distance from the embedded end of the roof bolt and comparing
to estimates of rock anchorage in that type of stratum.

CASE HISTORIES

To illustrate the use of this method, three sets of data from different coal mines were evaluated. Bolts
used in these tests were standard grade 60, No. 6 rebar bolts milled with a 6.4-mm-wide by 3.2-mm-
deep slot along each side. Strain gauges were installed as shown in figure 1. After milling, the cross-
sectional area was 2.39 cm? and the section modulus was 0.338 cm’. The instrumented bolts were
calibrated in a uniaxial test machine to correlate voltage change to load change. During installation,
the bolts were oriented with the strain gauges parallel to the ribs to measure localized bending effects
inthe mine roof. The data collection system was an Omnidata Polyrecorder 516-C. A completion
box was made that provided 5-V excitation and completion resistors for the Wheatstone bridge
circuits used to measure strain gauge voltage changes.

Five grade 60 bolts were tested to failure to determine the strength and yield point of the bolt after
the slots had been milled. The slotting process reduced bolt strength by approximately 10%. A
typical load strain curve from one of these tests is shown in figure 2. When the data from the
instrumented bolts were reduced, the correlation coefficients from the axial calibrations were used
to convert voltage changes to load changes. This process was accurate to +0.4 KN. When bolt load
levels exceeded the yield point of the steel, voltage readings were converted to strain readings, and
these values were used in figure 2 to estimate bolt loading. However, converting voltage to strain
is not as accurate as converting voltage to load.


P
and A

load. N.
steel area,

Bending moments measured by the instrumented bolts involve several factors that a design engineer
should consider. Maximum bending moments can be localized and may not be accurately measured
with an instrumented bolt. Bending is measured in only one orientation because of the limitations
of the instrument. If possible, the bolts should be rotated so that they measure the highest estimated
bending moment. Bending moments can be caused by joint movement, rotation of large blocks,
and/or differential loading in mats and meshes. Considering the source and prevalence of bending
loads when using the design formulas is important. Bending loads cause fiber stress in the bolts
according to the formula—

o, =
aa (2)
where oO, = bending stress, Pa,
M = moment, N-m,
and S, = section modulus, m’.
Total fiber stress then becomes—
0,=0, 40,59... (3)

total fiber stress
allowable fiber stress.

where G,
and G,

max

Bolt spacing is the primary variable manipulated in the design process. Both axial and bending loads
measured with instrumented bolts can be estimated when using a new bolt spacing based on the
following equations—

Pi. Pa @)
5\5 SS,
and
Moma
5S, 5,5 6)
2a
where S; = bolt spacing across the opening of instrumented bolts,

a
"

bolt spacing down the entry of instrumented bolts,


underground monitoring requires proper installation to eliminate moisture and long-term creep of the
glue line (/2). Instrumented bolts should be calibrated and cycled many times to check that the gauge
has been properly installed. During field investigations, rock deflections should be measured to detect
separations in the horizon above the roof bolts.

DESIGN METHOD

The following assumptions and generalizations are made as part of this design method. The amount
of roof bolt loading required to stabilize the immediate roof can vary with respect to distance from
ribs and into the roof. This relationship will be referred to as “support load profile.” The bolt load
depends on amount of area of roof exposed per bolt and the support load profile required for stability.
This means that a change in bolt spacing will change bolt load as a function of support load profile.
The support load profile consists of both axial and bending loads.

A support load profile can show various aspects of roof behavior. The amount of rock loading that
roof bolts are required to support is a function of geology, geometry, and in situ stress fields. In
some applications, the skin needs to be controlled while in other areas, roof bolts alone may not be
adequate to maintain entry stability. Timing of support installation and distance from the face can
also change support loading and roof behavior.

Factors that contribute to bolt loading can be complex and variable. This makes analytical and
numerical approaches difficult. A method based on bolt load measurements can be useful to supple-
ment these methods to obtain a better understanding of particular support and rock interactions.
Because loading pattems can vary significantly with respect to location, the selection of design loads
requires that enough sites be tested to develop confidence in the relationship between bolt load pro-
files and mining location, geology, geometry, and stress fields. These load profiles can then be ex-
trapolated to an entire area or separate design load profiles can be used when mining conditions vary.

Engineering design typically limits stress to either a percentage of the yield strength or a percentage
of the ultimate strength. This safety factor is then applied to prevent material failure. Bolt loads can
be axial, bending, and/or shear. Total fiber stress will be a combination of axial loading and bending
moments. Axial loading is generally the primary force on a steel bolt. The nature of loading
mechanisms and the uncertainties of determining just where loading is taking place makes it impos-
sible to estimate shear loads. When movernent takes place along joints, shear loading could be critical
in the design of bolt systems, and additional research is required to develop a better understanding
of this loading mechanism.

The development of bolt loading profiles requires the installation of instrumented roof bolts during
mine development to allow normal rock movement and stress redistribution to load the bolts. The
measured bolt locations and axial and bending loads are used to create bolt load profiles. Axial loads
Cause fiber stress in the bolts according to the formula—

o,=2. 1
a5 ()

where o, = axial stress, Pa,

a