ANCHORAGE PULL TESTING FOR FULLY GROUTED ROOF BOLTS

Christopher Mark, Rock Mechanics Team Leader

Craig S. Compton, Engineering Technician

David C. Oyler, Mechanical Engineer

Dennis R. Dolinar, Mining Engineer

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

Fully-grouted roof bolts comprise more than 80% of the primary

roof supports used in U.S. coal mines. However, nearly 1,500 MSHA

reportable, non-injury roof falls occur each year, and most of these

are attributable to failure of the roof bolt system. Anchorage failure

is one failure mechanism for fully-grouted bolts. As roof deformation

works its way upward, the bolts can become heavily loaded near their

upper ends. If the applied load exceeds the anchorage, the bolts will

simply pull out. Research dating back 30 yrs indicates that this type

of anchorage failure is most likely when the roof rock is weak, just

where roof support is most critical. In soft shale or coal roof, the

small amount of data available in the literature indicates that 20-30 in

of resin anchorage may be required to achieve the full capacity of the

bolt. In other words, the “full resistance zone” of a 60 in bolt may

actually be just 30-40 in.

Despite its potential importance, there is no widely accepted

anchorage test for fully grouted bolts. Standard pull tests have

sometimes been employed, but they provide no information on the

anchorage near the top of the bolt. An alternative, first described

more than 25 yrs ago in the U.S., is the short-encapsulation pull test

(SEPT). With this test, the bolt is installed with only a short (1 ft or

less) tube of resin. In recent years variations of this test have become

international standards. 

This paper describes recent studies using short encapsulation pull

tests in the U.S. Tests were conducted in the National Institute for

Occupational Safety and Health (NIOSH) Safety Research Coal Mine

at Bruceton and at underground mines in Pennsylvania and West

Virginia. The study found that the SEPT can be used to make a

simple evaluation of resin bolt anchorage. Suggested procedures for

conducting SEPT are included. The study also confirmed that poor

anchorage can be an issue, particularly where the roof rock is very

weak. Some simple techniques for improving anchorage, and thereby

the effectiveness of fully grouted bolts, are discussed.

INTRODUCTION

Each year, about 10,000 miles of underground entries are

developed in U.S. coal mines. Approximately 80 million roof bolts

are installed to support these roadways, at an installed cost of more

than $500 million (1). The vast majority of the bolts, more than 80%

of the total, are fully grouted (2). 

Despite this effort and investment, MSHA statistics show that

nearly 1,500 non-injury roof falls are reported each year. The big

majority extend higher than the anchorage horizon of the bolts. Each

of these large roof collapses represents a failure of the roof bolting

system.

NIOSH has been working to develop better techniques to help

mine operators select and evaluate their roof bolts. These include:

• Analysis of Roof Bolt Systems (ARBS) Computer Program

evaluates the length, capacity, and density of a roof bolt pattern.

It is based on statistical back-analysis of roof fall experience at

a broad spectrum of coal mines (3), and allows mine operators

to compare their current design to an industry-wide benchmark.

• Trouble-Shooting Guide for Roof Support Systems (TGRSS)

Computer Program is based on the MSHA handbook written by

Mazzoni et al. (4). It provides a comprehensive, logical, stepby-step method for improving roof bolt performance.

• Field evaluations of roof bolt performance using instrumented

fully-grouted roof bolts and a standard suite of roof monitoring

have been presented (5, 6).

A fourth technique is described in this paper. It focuses on a

seldom-recognized, but potentially serious, issue of the anchorage of

fully grouted bolts. When a mine is experiencing roof fall problems,

the first response is often to change the length of the bolts. If the

anchorage is poor, however, improving the anchorage might be a

more appropriate response. A simple field test to evaluate the

anchorage, the short-encapsulation pull test (SEPT), is described

here.

FAILURES OF FULLY GROUTED BOLTS

Fully grouted bolts are loaded by movement of the rock. The

movement may be vertical sag, shear along a bedding plane, or

dilation of a roof layer buckled by horizontal stress (figure 1). The

movements cause tensile forces in the bolt, usually combined with

bending stresses. 
Figure 2. Failure mechanisms of a fully grouted roof bolt. (A)

Roof movement near head; (B) Roof movement in central portion;

(C) Roof movement in anchorage zone.

Figure 1. Loads in fully grouted roof bolts caused

by roof movements. (Top) Tension resulting from

bed dilation or bed separation; (Bottom) Tension

and bending caused by slip on a bedding plane.

Figure 3. Photograph showing fully grouted bolts pulled from

their holes in a roof fall.

Depending on where the roof movements are concentrated, the

bolts can fail in one of three ways, as shown in figure 2 (7, 8):1

• The head or the plate can fail;

• The rod may break, either in tension, or a combination of tension

and bending; or

• The anchorage may fail.

The anchorage can fail when roof movement occurs near the top of

the hole, as shown in figure 2c. If the load applied to the bolt exceeds

the strength of the grout anchor, the top of the bolt will be pulled out

of the hole. If the bolt had been suspending weak or failed lower roof

from intact upper rock, a roof fall can follow.

Roof falls can sometimes provide clues as to the type of bolt failure

that took place. If broken bolts can be seen, the anchorage was

probably adequate, and the problem may have been that the capacity

of the bolts was inadequate to resist the loads applied by the roof (as

in figure 2b). But if the tops of resin bolts can be seen protruding

from the top of the muck pile after a fall, then inadequate anchorage

should be suspected (figure 2c and figure 3).

ANCHORAGE OF FULLY GROUTED BOLTS

A fully grouted bolt anchors itself by frictional interlock between

the resin and the rock. The effectiveness of the interlock is measured

by the “Grip Factor,”2

 which is defined as the bolt’s resistance to

pullout per inch of bolt length. 

1

In addition, the roof bolts may be intact, but the support system can fail if the

bolts are too short, allowing the roof to fail above them; or the bolts fail to

provide adequate skin control, allowing loose rock to create a hazard.

2

In the literature, what this paper calls the “Grip Factor” has also been referred

to as the “Bond Factor” or the “Anchorage Factor.”
Figure 4. The Short Encapsulation Pull Test. (A)

Normal hole; (B) Reamed hole. Figure 5. Effect of the Grip Factor on the resistance

available from 10-ton roof bolts to react against roof loads.

(A) Grip Factor = 0.5 tons/in; (B) Grip Factor = 1.0 ton/in.

Table 1 Grip factors for fully grouted resin bolts.

Rock type Country Grip Factor,

(tons/in)

Length for 10 tons of

anchorage, (in) Reference

Coal, shale . . . . . . . . . . . . . . . . . . . . Australia 0.7-2.1 4-12 Yearby (14)

Hard sandstone, limestone . . . . . . . . Australia 2.3-5.8 1.4-3.6 Yearby (14)

Minimum allowable1 . . . . . . . . . . . . U.K. 1.1 8.9 H&SE (10)

Soft rock . . . . . . . . . . . . . . . . . . . . . . U.S.A. 0.5 20 Peng (15)

Strong rock . . . . . . . . . . . . . . . . . . . . U.S.A. 2 5 Peng (15)

1

Over at least 50% of the bolt length.

The Grip Factor must be determined by loading the upper portion

of the grouted bolt. This is accomplished with short encapsulation

pull tests (SEPT), in which only the top 12 in of the bolt is grouted

(figure 4). The Grip Factor (tons per inch) is calculated as:

Grip Factor = Maximum SEPT Load (tons)/12 in

SEPT have been used since the earliest days of resin bolts (9).

They are widely employed internationally today, and are even

required in the U.K. (10). In the U.S., Karabin and Debevec (11)

reported some valuable results obtained from SEPT, and

recommended that “pull tests of approximately one foot of grouted

length should be made from time to time, to ensure that the resin

used is of good quality.”3

Figure 5 illustrates the effect of the Grip Factor on bolt

performance. Within the anchorage zone (the upper portion of the

bolt), the bolt’s available resistance to loading from rock movement

may be considerably less than its nominal yield strength. The

length of the anchor (LAnch, in in) is the bolt’s yield load (Y, in tons)

divided by the Grip Factor (GF):

LAnch = Y / GF

Obviously, a bolt with a larger Grip Factor will have more

available resistance over a longer length, as shown in figure 5b. In

fact, the “Full Resistance Length” (LFR) of a fully grouted bolt,

which is the zone in which the force available to resist rock

movement is at least equal to the yield strength of the bolt, is the

total bolt length L minus the length of the anchor:

LFR = L - LAnch

CAUSES OF POOR RESIN BOLT ANCHORAGE

The two most likely causes of poor anchorage are weak rock and

poor installation.

Weak Rock: Testing has consistently shown that weaker rock

requires a longer grouted length to achieve the same anchorage as

strong rock (9, 11). Table 1 gives typical Grip Factors (and

associated anchorage lengths) obtained from the literature. Short

encapsulation tests are apparently rather rare in the U.S.; the only

available published data were obtained from Peng (15). These test

results imply that in the U.S., the top 20 in or more of a fully

grouted bolt may be required to develop an anchorage force equal

to the breaking strength of the rod. In very weak rock, Grip Factors

can be so low that 6-ft bolts have been pulled from the rock at

14 tons even though they were fully grouted for their entire length

(16)!

Perhaps the most extensive study of resin bolt anchorage in the

U.S. was conducted by the former U.S. Bureau of Mines in the mid80's (17). More than 1,000 pull tests were conducted at 11

underground coal mines throughout the U.S. The tests involved

3

Standard pull tests cannot be used on bolts that are fully grouted for their

entire length. Such a test only measures the strength of the rod, because the

pulling forces seldom extend more than 18-24 in up the resin column (8, 12,

13). 
anchorage lengths of 12, 18, 24, 36 and 48 in. The anchorage

length was considered adequate when 90% of the tested bolts

reached the yield load of the steel. The study found that coal and

shale roofs required an average of 31 in of grouted length to meet

this criterion. Sandstone required 18 in on average, and limestone

needed just 12 in.

Poor Installation Quality: The TGRSS computer program (4)

identifies a number of factors that can result in poor anchorage with

fully grouted bolts. These include:

• Defective grout can result from improper storage (too hot, too

cold, too wet, or shelf life exceeded), or (rarely) from

manufacturing problems.

• Improper mixing can occur if the proper spin time is not

followed. Underspinning can result in inadequate mixing,

while overspinning can destroy the partially cured resin.

Improper mixing can also occur with long bolts where the top

of the hole has less time to mix before the bottom sets up. The

temperature of the resin at the time of installation can also

affect the cure time.

• Improper holes can be too long, too short, too large, or too

smooth. The proper grout cartridge must also be matched to

the hole and the bolt being installed.

• Finger gloving occurs when the plastic cartridge wrapper

remains intact around the hardened resin. It is more likely if

the bolt is not rotated as it is inserted in the hole (18).

DEVELOPMENT OF A U.S. STANDARD SHORT

ENCAPSULATION PULL TEST

The primary goal of this study was to develop SEPT procedures

that could be widely used in U.S. mines. The test focuses primarily

on No. 5 Gr. 60 and No. 6 rebar, which together constitute the great

majority of U.S. roof bolt installations. It is designed to be a simple

“green/yellow” test where:

• Green means that a 12-in encapsulation length achieves the yield

strength of the rebar (at least 9 tons, and often10-11 tons, for

No. 5-Gr. 60 and No. 6-Gr. 40 rebar; and about 14 tons for

No. 6- Gr. 60 rebar), and;

• Yellow means that the anchorage obtained from 12 in of

encapsulation is less than the yield strength of the rebar.

The test is also designed to be quick and simple, and to require

a minimum of specialized equipment. A detailed description of the

test procedure is included in the Appendix.

While simple in concept, international procedures for SEPT have

differed in a number of details: 

• Encapsulation length: The international consensus seems to be

that at least 12 in of the bolt should be grouted to minimize the

effect of the zones of poor mixing at the top and the bottom of

the resin (19). Shorter and longer lengths have sometimes been

used, however.

• Hole depth: In the U.S., production roof bolt holes are often

overdrilled by 1 in (4). The overdrill presumably provides a

space for the resin cartridge wrapping, clips, etc, while also

providing a margin of error against underdrilling. When

conducting an SEPT, however, it is important that correct

encapsulation length be obtained, and overdrilling might result

in a miscalculation of the amount of resin used.

• Hole Reaming: One method to ensure the correct encapsulation

length is to ream the lower portion of the hole to a larger

diameter (figure 4b). Then only the upper, unreamed portion of

the hole is effectively grouted. This method is employed

internationally, though an unreamed test is also allowed in the

U.K. under certain conditions (10, 20). Reamed holes also make

it possible to pull the bolt completely out, so that the resin

anchor can be viewed.

A series of 56 bolts were pulled in the NIOSH Safety Research

Coal Mine (SRCM) at Bruceton to help develop the test. Both No.

5 and No. 6 bolts were tested in 1-in holes, and the effects of hole

depth and hole reaming were evaluated.

The tests were conducted at the ends of a dead-end entry and

adjacent crosscut. Two coreholes were drilled to determine the

most suitable horizon for performing the tests. The first consistent

horizon of sufficient thickness that was not coaly was a weak

claystone about 5.5 ft above the roofline. Bolt lengths were

standardized at 79.5 in, not including the bolt head. The long bolts

and a relatively low mine roof made it necessary to bend the bolts

to install them. This may have had some effect upon resin mixing,

since it may have made the bolts crooked. Since the bends were

several feet below the bolting horizon and since none of the bolts

reached yield, the bends are not believed to have had an effect on

the bolt strength. 

In order to ensure consistent drilling depths, the steels were

marked and checked each day, and all drilling was performed with

the same drill steels. Because the horizons drilled were so soft, bit

wear was found to be minimal during the tests. Bit diameters were

measured regularly, and no significant change in bit diameter was

noted.

The resin used was a 1 minute resin from the same manufacturing

date and lot (January 2002). Cartridges were made up on the day

of the test, with a manufacturer’s clip at the bottom of the cartridge

and a tie wrap at the top. The tie wraps used were all of the same

size. The speed of the bolting machine was determined to be

500 rpm, and the resin manufacturer’s recommendation for 30 to 50

revolutions was followed by setting the spin time at 6 seconds, thus

giving 50 revolutions for each bolt installation. Hold times were

standardized at 54 seconds.

BRUCETON TEST RESULTS

Figure 6 shows a typical load deformation curve for a short

encapsulation pull test in which the anchorage fails (and the rod

does not yield). Initially the load deformation curve is linear.

However, as the resin along the lower portion of the anchor begins

to fail, the load deformation curve deviates from a straight line. As

the applied load approaches and exceeds the peak anchor capacity,

the anchor begins to slip. After the peak the anchor still carries on

average about 70% of the peak load over 1.5-2.0 in of deformation.

Table 2 contains the results of the individual tests. In the “Hole

Depth” column, “E” refers to holes drilled to the exact depth

required to accommodate the bolt (taking into account the bolt plate

and pull collar), while “O” refers to holes overdrilled 1-in deeper.

The resin cartridge lengths were adjusted to account for the

different hole volumes. “Hole Preparation” includes “R” for holes

that were reamed with a 1 3/8 in bit up to the anchorage horizon,

and “S” for standard holes.
0

1

2

3

4

5

6

7

8

9

10

11

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

Displacement, in

Load, tons

Anchor Yield 

Point 

Peak Load

Figure 6. Load deformation curve for typical short

encapsulation pull test.

Table 2. Bolt test data.

Test

# Bolt dia Hole

preparation

Hole

depth

Max load,

(tons)1

Disp at

max load,

(in)

Calculated

Grip Factor

(ton/in)

4 No. 6 R2 E3 8.90 0.381 0.74

6 No. 6 R E 10.00 0.369 0.83

11 No. 6 R E 10.30 0.137 0.86

13 No. 6 R E 7.20 0.455 0.60

19 No. 6 R E 12.70 0.359 1.06

22 No. 6 R E 9.80 0.570 0.82

28 No. 6 R E 8.70 0.293 0.73

30 No. 6 R E 10.50 0.321 0.88

36 No. 6 R E 8.00 0.420 0.74

37 No. 6 R E 9.70 0.262 0.81

42 No. 6 R O 7.40 0.436 0.62

49 No. 6 R O 7.40 0.354 0.62

53 No. 6 R O 6.50 0.263 0.54

59 No. 6 R O 10.00 0.423 0.83

67 No. 6 R O 6.10 0.269 0.51

73 No. 6 R O 7.90 0.325 0.66

43 No. 6 S E 5.90 0.440 0.49

52 No. 6 S E 8.20 0.392 0.68

58 No. 6 S E 7.90 0.285 0.66

63 No. 6 S E 8.30 0.222 0.69

68 No. 6 S E 9.30 0.294 0.78

71 No. 6 S E 11.10 0.485 0.93

46 No. 6 S O 9.60 0.417 0.80

48 No. 6 S O 3.30 0.261 0.28

54 No. 6 S O 7.40 0.336 0.62

60 No. 6 S O 7.50 0.401 0.63

70 No. 6 S O 9.40 0.294 0.78

72 No. 6 S O 7.30 0.282 0.61

2 No. 5 R E 7.70 0.570 0.64

5 No. 5 R E 7.40 0.400 0.62

12 No. 5 R E 7.00 0.431 0.58

16 No. 5 R E 7.5 0.373 0.63

20 No. 5 R E 10.5 0.438 0.88

24 No. 5 R E 8.5 0.491 0.71

25 No. 5 R E 8.3 0.332 0.69

32 No. 5 R E 7.5 0.316 0.63

33 No. 5 R E 7.0 0.363 0.58

38 No. 5 R E 6.9 0.309 0.58

45 No. 5 R O 7.8 0.464 0.65

47 No. 5 R O 10.4 0.650 0.87

57 No. 5 R O 4.7 0.242 0.39

61 No. 5 R O 6.8 0.339 0.57

65 No. 5 R O 8.7 0.540 0.73

74 No. 5 R O 6.3 0.334 0.53

44 No. 5 S E 9.9 0.461 0.83

50 No. 5 S E 7.7 0.420 0.64

56 No. 5 S E 10.5 0.393 0.88

62 No. 5 S E 8.9 0.524 0.74

66 No. 5 S E 8.8 0.296 0.73

76 No. 5 S E 8.7 0.377 0.73

41 No. 5 S O 8.7 0.452 0.73

51 No. 5 S O 6.6 0.501 0.55

55 No. 5 S O 10.4 0.397 0.87

64 No. 5 S O 5.7 0.270 0.48

69 No. 5 S O 10.6 0.540 0.88

75 No. 5 S O 10.0 0.575 0.83

1

Maximum loads rounded to nearest 0.1 ton.

2

R=Hole reamed to bolting horizon, bolt pulled from roof. S=Standard 

 drill hole.

3

E=Hole drilled to exact depth to accommodate bolt length. O=Hole 

 overdrilled by 1-in.

*Spin times 6 seconds for all bolts. Hold times 54 seconds for all bolts.

*All bolts 79.5 in long. All bolts installed using resin from the same lot.

The results of the tests are summarized in Table 3. The average

Grip Factor for all the tests was 0.69 tons/in, which is well below

the “Green” level of about 0.9 ton/in. This result confirms that low

grip factors can be encountered when the rock is extremely weak,

even under optimum installation conditions. In general, the results

were reasonably consistent, with the standard deviation on average

being about one-fifth of the mean.

Table 3. Average test results by group.

Bolt Size Test

Type

Hole

Depth

Mean

Maximum

load (tons)

St.

Dev.,

(tons)

Grip

Factor

(tons/in)

No. 6 R E 9.7 1.4 0.81

No. 5 S E 9.1 1.0 0.76

No. 5 S O 8.7 2.1 0.73

No. 6 S E 8.5 1.7 0.71

No. 5 R E 7.8 1.1 0.65

No. 6 R O 7.6 1.4 0.63

No. 5 R O 7.5 2.0 0.62

No. 6 S O 7.4 2.3 0.62

There was no statistically significant difference between the

No. 5 and the No. 6 rebar in these tests, either in the mean Grip

Factor or the standard deviation. Apparently, the difference in

annulus did not affect these results.

Reaming the hole also had no statistically significant effect on

the test result. The test procedure suggested in the appendix,

therefore, allows either type of hole. Standard holes are more

convenient, but reamed holes can be used if the additional

information that can be obtained from visual inspection is desired.

The only statistically significant difference was between the exact

depth and the overdrilled holes. Surprisingly, the exact depth holes

achieved greater pullout loads, even though the visually inspected

bolts showed that it was common for the top one-half to two inches

of their resin to pull away from them. It seemed that the upper

portion of the resin was weak due to the presence of the bag and

resin clips. In contrast, the overdrilled holes generally appeared to

have solid resin, with no residue from the bag or clips to the top of

the bolt. However, since the effect was relatively small (about

10%), the Appendix suggests that overdrilled holes be used, but

exact depth holes could also be used if that is the normal bolt

installation practice at the mine.
Figure 7. View of a short column grout

anchor recovered from the roof.

As part of the tests, 32 of the anchors were recovered by pulling

the bolts completely out of the reamed holes. Figure 7 shows a

typical anchor that was pulled from the roof. Each anchor was

examined for evidence of the length of the installed grout column

while the length of the grout still attached to the rebar was

measured. This confirmed that the specified grout length was

achieved during the installation. Usually about a 7 to 10 in long

solid length of anchor was still attached to the rebar after being

pulled from the roof. About 2 to 4 in of the grout would break

away from the lower portion of the anchor. This is the portion of

the grout that failed during the pull test.

PRELIMINARY TESTS IN U.S. COAL MINES

As part of this study, short encapsulation pull tests were

conducted in two U.S. coal mines, one in West Virginia and one in

Pennsylvania. The goal was to determine how widespread the

problem of poor anchorage might be, and the mines were selected

because they were encountering extremely weak roof conditions. 

The tests were conducted early in the study, and the procedures

differed in some respects from the final ones contained in the

Appendix. So while the results are not strictly comparable, they do

provide some indication of the anchorage that can be encountered.

The tests involved a variety of No. 5 and No. 6 rebar types.

Table 4 shows that the anchorage was “green” at 2 of the 5 sites,

and borderline at a third. At the other two the grip factor was still

greater than 0.6 tons/in.

IMPROVING THE PERFORMANCE OF FULLY

GROUTED BOLTS

Once short encapsulation tests have confirmed that the anchorage

is poor, there are some things that can be done. The first step is to

check the quality of the installation. It is essential that roof bolt

operators carefully follow the installation instructions provided by

the resin manufacturer. The TGRSS program contains some simple

suggestions for testing the resin and the hole. 

If the grout and the installation procedure are found to be

adequate, then attention should shift to the hole and the bolt.

Because resin grout acts by mechanical interlock, not by adhesion,

rifled holes and rougher bolt profiles result in better anchorage (11,

21, 22). Unfortunately, special bits to drill rifled holes are easier to

find overseas than they are in the U.S. 

Another possibility is to reduce the hole annulus. Numerous

tests over the years have found that optimum difference between the

diameter of the bolt and the diameter of the hole is no greater than

0.25 in, giving an annulus of about 0.125 in (11, 23, 24). For

example, a 0.125-in annulus is obtained by a 3/4 in bolt in a 1-in

hole. The current U.S. trend to replace No.6 rebar with No.5, while

leaving the hole diameter at 1 in, results in slightly larger 0.188 in

annulus.

Larger holes can result in poor resin mixing and a greater

likelihood of finger-gloving, with adverse effects on anchorage.

Studies in the U.S. and Australia have found that the anchorage

improved by more than 50% when the annulus was reduced from

0.35 to 0.1 in (24, 25). Smaller holes, on the other hand, can cause

insertion problems and magnify the effects of resin losses in

oversized holes (26). However, one recent U.S. study found that

annuluses ranging from 0.1-0.25 in all provided acceptable results

in strong rock (27), and the tests described in this paper at the

SRCM found no difference between the anchorage obtained by

No. 5 and No. 6 rebar. Ulrich et al. (24) found no significant

difference in the mean anchorage strength between annuluses of

0.125 and 0.25 in, but the standard deviation was much higher for

the larger annulus.

In very severe conditions, the only way to increase anchorage

may be to increase both the hole diameter and the bar diameter.

This enlarges the area of the grout-rock contact surface, thereby

increasing the total shear resistance (11, 16).

Table 4. Results from Underground Coal Mines.

State Rock Type Grip Factor (tons/inch) Green/Yellow

Pennsylvania site 1 @ 6 ft clay, claystone 0.87 Green

Pennsylvania site 1 @ 8 ft layered dark. gray shale 0.72 Yellow

Pennsylvania site 2 @ 6 ft dark gray fireclay 0.63 Yellow

West Virginia @ 6 ft dark gray shale 1.04 Green

West Virginia @ 1.5 ft thinly banded gray shale 0.83 Borderline
CONCLUSIONS

Since its introduction more than 30 yrs ago, resin grouting has

dramatically improved the effectiveness of roof bolting. One

important advantage of fully grouted resin bolts over conventional

mechanical ones is that resin anchorage generally does not degrade

over time. Another is that resin bolts usually out perform

mechanical anchor bolts in difficult ground.

On the other hand, despite years of research, no practical and

reliable method to routinely test resin bolt installations has ever

been developed. It is very difficult to know whether resin bolts are

performing as well as they could be–whether a mine is truly getting

its money’s worth in support and safety.

This paper has focused on the specific problem of poor

anchorage. When anchorage is poor, roof movements near the top

of the bolt (within the anchorage zone) can pull the bolt out of the

upper portion of the hole at loads less than the yield strength of the

rod. The two most likely causes of poor anchorage are weak rock

and poor installation quality.

The short encapsulation pull test (SEPT) is a relatively simple

technique to test resin bolt anchorage. Step-by-step procedures for

conducting SEPT are described. It is hoped that more widespread

use of the SEPT will aid quality control, improve the effectiveness

of resin bolts, and result in enhanced safety for U.S. mineworkers.

REFERENCES

1. Campoli, A.A. Variables Affecting Resin Anchorage

Performance in United States Roof Bolting Systems.

Proceedings, Roofbolting in Mining, RWTH Aachen, Germany,

2001, pp. 19-28.

2. Dolinar, D.R. and Bhatt, S.K. Trends in Roof Bolt Application.

Proceedings, New Technology for Coal Mine Roof Support.

(NIOSH) Publication No. 2000-151, IC 9453, 2000, pp. 43-52.

3. Mark, C. Analysis of Roof Bolt Systems. Proceedings, 20th

International Conference on Ground Control in Mining.

Morgantown, WV, West Virginia University, 2001, pp. 218-

225.

4. Mazzoni, R.A., Karabin, G.J. and Cybulski, J.A. A troubleshooting Guide for Roof Support Systems. MSHA IR 1237,

1996, 101 pp.

5. Mark, C., Dolinar, D.R. and Mucho, T.P. Summary of Field

Measurements of Roof Bolt Performance. Proceedings, New

Technology for Coal Mine Roof Support, NIOSH Publication

No. 2000-151, IC 9453, 2000, pp. 81-98.

6. Signer, S.P. Load Behavior of Grouted Bolts in Sedimentary

Rock. Proceedings, New Technology for Coal Mine Roof

Support. NIOSH Publication No. 2000-151, IC 9453,2000,

pp. 73-80.

7. Mark, C. Design of Roof Bolt Systems. Proceedings, New

Technology for Coal Mine Roof Support, NIOSH Publication

No. 2000-151, IC 9453, pp. 111-131.

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Mechanics of Fully Grouted Roof Bolts. USBM RI 9135,

1987, 17 pp.

9. Franklin, J.A. and Woodfield, P.R. Comparison of a Polyester

Resin and a Mechanical Rockbolt Anchor. Trans Inst Min

Metall (Section A: Mining Industry), 1971, pp. A91-100.

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Rockbolts to Support Roadways in Coal Mines. U.K., Health

and Safety Executive, Deep Mines Coal Industry Advisory

Committee, 1996, 16 pp..

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Conventional and Resin Bolting Systems. MESA IR 1033,

1976, 22 pp.

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Fully Grouted Roof Bolts. USBM RI 9301, 1990, 13 pp.

13. Tadolini, S.C. and Dyni, R.C. Transfer Mechanics of FullColumn Resin-Grouted Roof Bolts. USBM RI 9336, 1991, 14

pp.

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Newcastle, New South Wales, Australia, 1991, 24 pp.

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Age Magazine, 1998, Dec:32-38.

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Implementation and Evaluation of Roof Bolting in MICARE

Mine II. Proceedings, 16th International Conference on

Ground Control in Mining, Morgantown, WV, West Virginia

University, 1997, pp. 139-148.

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Resin-grouted Roof Bolts. Proceedings, 5th International

Conference on Ground Control in Mining, Morgantown, WV,

West Virginia University, 1986, pp. 6-14.

18. Pettibone, H.C. Avoiding Anchorage Problems with ResinGrouted Roof Bolts. USBM RI 9129, 1987, 28 pp. 

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Roof Bolt Performance. Proceedings, International Conference

on Geomechanics/Ground Control in Mining and Underground

Construction, Wollongong, NSW, 1998, pp. 413-422.

20. Wittenberg, D. and Ruppel, U. Quality Management for

Grouted Rockbolts. Proceedings, 19th International

Conference on Ground Control in Mining, Morgantown, WV,

West Virginia University, 2000, pp. 249-254.

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Study of Shear Loading and Bolt Load Transfer Mechanisms

under Constant Normal Stiffness Conditions. Proceedings,

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Morgantown, WV, West Virginia University, 1999, pp. 239-

247.
Figure A2. Additional material needed to ream hole and

completely remove roof bolt.

Figure A1. Exploded drawing of pull test apparatus.

Tube length (inches) ( Hole D ( Bolt D )

Tube D

= 13 − 2 12 2

2

Figure A3. Preparation of the resin cartridge.

22. Haas, C.J. Analysis of Rockbolting to Prevent Shear Movement

in Fractured Ground. Mining Engineering, 1981, Jun:691-704.

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Laminated Rock. Eng Min J, 1974, Feb:80-90.

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Between Annulus Thickness and the Integrity of Resin Grouted

Bolts. USBM RI 9253, 1989, 13 pp.

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Mechanisms in Reinforcing Tendons. Proceedings, 11th

International Conference on Ground Control in Mining,

Wollongong, New South Wales, Australia, University of

Wollongong, 1992, pp. 212-219.

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Annulus Size Effects on Rebar Bolt Pull Strength and Resin

Loss to Fractured Rock. Proceedings, 18th International

Conference on Ground Control in Mining, Morgantown, WV,

West Virginia University, 1999, pp. 222-231.

27. Tadolini, S. The Effects of Reduced Annulus in Roof Bolting

Performance. Proceedings, 17th International Conference on

Ground Control in Mining. Morgantown, WV, West Virginia

University, 1998, pp. 230-236.

APPENDIX: SHORT ENCAPSULATION PULL TEST

PROCEDURES

Performing a short encapsulation pull test is basically the same

as a standard pull test. The main difference is that the resin-rock

anchorage of a specific horizon in the roof is being tested instead of

overall bolt performance. The equipment needed is the standard

pull gear (fig. A1), plastic tie wraps, and a resin keeper (an O-ring).

The resin column may also be pulled out of the roof for

examination. This requires items listed in fig. A2 and some

additional time.

Step-by-step procedures follow:

Prepare the Resin Cartridge

1. Calculate the one-ft resin tube equivalent length, which for

a 1-in over drilled hole is: 

where D = Diameter in inches.

2. Measure and attach a tie wrap around the resin cartridge at

the calculated length (fig. A3). Add 1/8” to the calculated

length before tightening the tie wrap to allow for

compression of the resin cartridge. Cut away the excess tie

wrap and resin. 

Select Test Horizon and Prepare the Bolt

3. Determine the roof horizon where test is to be performed.

The bolt may be cut to test any area within bolted horizon.

The bolt length should be adjusted to compensate for the

pull collar and roof bolt plate.

4. Attach the resin keeper to the bolt one foot from the top

end. 

Drill the hole

5. Measure the hole length by assembling the bolt, pull collar,

and flat roof bolt plate. The depth of the test hole is the

distance from the plate to end of the roof bolt, plus one inch

(fig. A4).
Figure A4. The Short Encapsulation Pull Test. (A) Normal

hole; (B) Reamed hole.

Figure A5. Using a short length of PVC tubing to

insert the resin cartridge into the top of a reamed hole.

6. Use a new bit to drill the first hole, and measure periodically

to ensure that there has not been excessive wear.

7. (If a reamed hole is not desired then skip to step #8.)

Counter-bore the hole with a 1-3/8- to 1-1/2-in bit to within

thirteen in of distance measured above. It is important not

to go too deep with the counter-bore because it will

compromise the effective length of resin column. A

suggestion for keeping the resin in place while inserting a

bolt through the oversized hole is shown in fig. A5.

Insert the bolt

8. Insert the resin with the plastic tie wrap pointing up toward

top of hole (The factory crimped end of resin cartridge will

be touching the bolt). 

9. Push the bolt up until plate just touches roof, spin for the

resin manufacturer’s recommended time and push the bolt

against the roof. Hold for the recommended time.

10. Allow at least one hour, but no more than 30 hours, before

pulling the bolt.

Conduct pull test

11. Assemble and attach the pull gear as shown in fig. A1

(except for the dial indicator and adjustable rod). Make

sure the claw is well seated on the pull collar and tighten

the nut as much as possible by hand. Load the ram with the

hand pump to 1,000 lbs.

12. Add the dial indicator with adjustable rod in line with the

bolt axis so that as the bolt is pulled it will push directly

down on dial indicator. Adjust the dial on the indicator

face to zero.

13. Begin loading the ram in 1,000-lb increments. Record load

from the gauge on the pump and deformation from the

indicator. Repeat this process until the bolt has reached its

peak load. Then continue pumping and recording the load

for each subsequent 0.05 in of displacement (as measured

on the dial indicator) for an additional 0.25 in (or up to full

extension of the ram). If it is clear before then that the rod

has yielded (as defined in step 15 below) the test may be

ended early.

14. To pull the bolt completely out of a reamed hole, remove

the dial indicator and adjustable rod and continue to pump

the ram until full extension is reached. Release pressure

from the ram and allow the piston to return completely.

Insert slotted plates and or slotted tubes to take up the space

created from ram extension. Do not tighten the nut to take

up this space or the claw may bottom out against bottom of

pull tube. Continue this process until you have pulled the

resin column into the counter bored portion of the test hole.

The entire assembly, including the bolt, must be supported

in some way or it will drop to the floor when the resin

column is pulled into counter bored portion of the test hole.

Data Analysis

15. If the bolt yielded, then the result of the test is “Green,”

meaning that the anchorage is adequate. Bolt yield is

indicated when: (a) The peak load was greater than the

minimum yield strength of the rod (generally 9 tons for

No. 5-Gr. 60 and No. 6-Gr. 40 bolts, and (b) The bolt

continued to deform, but the load remained constant or

increased very slowly (but did not decrease) with additional

pumping.

16. If the bolt did not yield, then the result is “Yellow,”

meaning that the anchorage could be improved. The Grip

Factor (tons per in) should be calculated as:

Grip Factor = Maximum Load/12.