ABSTRACT

More than 80% of U.S. roof bolts are fully-grouted, but about 1500

roof falls are reported each year. Anchorage failure of a fully grouted

bolt can occur when the roof is active near the top of the hole. This

paper reports on an extensive series of short-encapsulation pull tests

(in which the bolts are installed with only 1 ft of resin) that were

conducted in the National Institute for Occupational Safety and Health

(NIOSH) Mine Safety Research Laboratory and operating mines in

WV and PA. The tests confirmed that poor anchorage can be

encountered under some weak rock conditions. Suggestions for

improving anchorage are included.

BACKGROUND

Roof bolts have been the primary roof supports in U.S. coal mines

since the late 1950's. Fully grouted resin bolts were introduced about

a decade later. A survey conducted by NIOSH in 1999 (Dolinar and

Bhatt, 2000) found that the coal mining industry used about 85 million

roof bolts. Of these, approximately 80% were fully grouted. Figure 1

shows the historical trends in roof bolt usage in the U.S. For many

years, the most common fully grouted bolt installation was a 19 mm

(0.75 in) bolt in a 25 mm (1 in) hole. In recent years, however, 16 mm

(5/8 in) bolts have become more popular. The 1999 survey found that

about 80% of all fully grouted bolts were the smaller diameter.

The installed cost of roof bolts has been estimated at more than

$500 million annually (Campoli, 2001). Yet MSHA statistics show that

nearly 1500 non-injury roof falls are reported each year (Pappas et al,

2000). The big majority of these falls extend higher than the

anchorage horizon of the bolts. Each of these large roof collapses

represents a failure of the roof bolting system.

FAILURES OF FULLY GROUTED BOLTS

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

illustrated in figure 2, the movement may be vertical sag, shear along

a bedding plane, or dilation of a roof layer buckled by horizontal stress

(Signer, 2000; Fabjanczyk and Tarrant, 1992). The movements cause

tensile forces in the bolt, usually combined with bending stresses.

Depending on where the roof movements are concentrated, the

bolts can fail in one of three ways, as shown in figure 3 (Serbousek

and Signer, 1987; Mark, 2000):1

1. The head or the plate can fail;

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

tension and bending; or

3. The anchorage may fail.

The anchorage can fail when roof movement occurs near the top

of the hole, as shown in figure 3c. 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 3b). 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 3c and figure 4).

ANCHORAGE MECHANICS OF FULLY GROUTED BOLTS

A fully grouted bolt anchors itself by frictional interlock between

the resin and the rock. The performance of a fullygrouted bolt is

determined by the load-transfer mechanisms between the rock, the

grout, and the bolt. Signer (1990) provides an excellent discussion of

load transfer mechanisms. Good load transfer exists when very high

loads develop in the bolt in response to small ground movements, and

these loads are rapidly dissipated away from the zone of roof

movement.

1 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Preprint 03-138

FIELD PERFORMANCE TESTING OF FULLY GROUTED ROOF BOLTS

C. Mark

C. S. Compton

D. R. Dolinar

D. C. Oyler

Natl. Inst. for Occuptnl. Sfty. & Health

Pittsburgh, PA

1In 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.
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. 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 300 mm (12 in)

of the bolt is grouted (figure 5). The Grip Factor (tons/in) is calculated

as:

Grip Factor = Maximum SEPT Load (tonnes (tons))/(300 mm(12

in))

Figure 6 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 mm or inches) is the bolt’s yield load (Y, in

tonnes or tons) divided by the Grip Factor (GF):

LAnch = Y / GF (1)

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

resistance, as shown in figure 6b. 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 (2)

SEPT have been used since the earliest days of resin bolts

(Franklin and Woodfield, 1971). They are widely employed

2 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 2. Loads in fully grouted roof bolts caused by roof movements. A) Tension resulting from bed dilation or bed separation; B)

Tension and bending caused by slip on a bedding plane.

Figure 1. Trends in U.S. roof bolt usage (source: Dolinar and Bhatt, 2000).

2In the literature, what this paper calls the “Grip Factor” has also been

referred to as the “Bond Factor” or the “Anchorage Factor.”
internationally today, and are even required in the UK (Health and

Safety Executive, 1996). In the US, Karabin and Debevec (1976)

reported some valuable results obtained from SEPT, and

recommended that “pull tests of approximately one ft of grouted

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

is of good quality.”3

Table 1 gives typical anchorage factors and anchorage obtained

from the literature. Short encapsulation tests are apparently rather

rare in the US, and the only available published data was obtained

from Peng (1998). Although the Australian (Yearby, 1991) and UK

(Bigby, 1997) data probably applies to slightly larger bolts, there does

seem to be a clear difference. The implication is that in weak rock in

the U.S., the top 500 mm (20 in) of a fully grouted bolt may not be

providing significant reinforcement to the rock. In such conditions, the

“effective length” of the bolt may be considerably less than its nominal

length.

CAUSES OF POOR RESIN BOLT ANCHORAGE

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

poor installation.

Weak Rock: Table 1 shows that weaker rock requires a longer

3 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 3. 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 4. Photograph showing fully grouted bolts pulled from

their holes in a roof fall.

Figure 5. The Short Encapsulation Pull Test. (A) Normal hole; (B)

Reamed hole.

3Standard 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 450-600 mm

(18-24 in) up the resin column [Serbousek and Signer 1987; Signer,

1990; Tadolini and Dyni, 1991].
grouted length to achieve the same anchorage as strong rock. In very

weak rock, Grip Factors can be so low that 1.8 m (6-ft) bolts have

been pulled from the rock at 12 tonnes (14 tons) even though they

were fully grouted for their entire length (Rico et al. 1997)!

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 [Cincilla 1986]. More than 1000 pull tests were conducted at 11

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

anchorage lengths of 300-1,200 mm (12-48 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 790 mm (31 in) of grouted length to meet this

criterion. Sandstone required 450 mm (18 in) on average, and

limestone needed just 300 mm (12 in).

Poor Installation Quality: The Troubleshooting Guide for Roof

Support Systems (TGRSS) computer program (Mazzoni et al., 1996)

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 (Pettibone, 1987).

Other possible causes of poor anchorage that have been

identified are:

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 6 mm (0.25 in), giving an annulus of

about 3 mm (0.125 in) (Fairhurst and Singh, 1974; Karabin and

Debevic, 1976; Ulrich et al., 1989). For example, a 3mm (0.125 in)

annulus is obtained by a 19 mm (0.75 in) bolt in a 25 mm (1 in) hole.

Larger holes can result in poor resin mixing, a greater likelihood of

“finger-gloving,” and reduced load transfer capability. One Australian

study found that the load transfer improved more than 50% when the

annulus was reduced from 4.5 to 2.5 mm (0.35 to 0.1 in) (Fabjanczyk

and Tarrant, 1992). Smaller holes, on the other hand, can cause

4 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Table 1.—Grip factors for fully grouted resin bolts

Grip Factor, Length for 10 tons of

Rock type Country tons/in (N/mm) anchorage, in (mm) Reference

Coal, shale Australia 0.7-2.1 4-12 Yearby, 1991

300-900 100-300

Hard sandstone, limestone Australia 2.3-5.8 1.4-3.6 Yearby, 1991

1000-2500 35-90

Minimum allowable1 U.K. 1.1 8.9 H&S Exec., 1997

400 N/mm 210

Soft rock U.S.A. 0.5 20 Peng, 1998

220 500

Strong rock U.S.A. 2 5 Peng, 1998

870 125

1Over at least 50% of the bolt length.

Figure 6. 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/ch; (B) Grip Factor = 1.0 tons/in.
insertion problems and magnify the effects of resin losses or

oversized holes (Campoli et al., 1999). However, one recent U.S.

study found that annuli ranging from 2.5-6.5 mm (0.1-0.25 in) all

provided acceptable results in strong rock (Tadolini, 1998), indicating

that annulus may be most important in soft rock. Ulrich et al. (1989)

found no significant difference in the mean anchorage strength

between annuluses of 3 and 6 mm (0.125 and 0.25 in), but the

standard deviation was much higher for the wider annulus.

Hole and bolt profile: Because resin grout acts to transfer load by

mechanical interlock, and not by adhesion, rifled holes and rougher

bolt profiles result in better load transfer (Karabin and Debevic, 1976;

Haas, 1981; Aziz et al., 1999). Reportedly, wet drilled or water-flushed

holes can also improve load transfer (Siddall and Gale, 1992). One

study found that the pullout load of standard rebar was seven times

that of a smooth rod (Fabjanczyk and Tarrant, 1992).

Resin characteristics: Tests in the UK in the late 1980’s

demonstrated that the compressive strength of resin was important to

the performance of grouted roof bolts (British Coal Technical

Department, 1992), and current UK regulations require resin strength

to exceed 80 MPa (11,000 psi). A strength test was recently added to

the U.S. ASTM standards for resin. However, an extensive series of

laboratory “push tests” found little correlation between shear stress

and resin strengths in the 20-60 MPa (3,000-6,000 psi) range

(Fabjanczyk and Tarrant, 1992).

DEVELOPMENT OF A US STANDARD SHORT 

ENCAPSULATION PULL TEST

The primary goal of this study was to develop SEPT procedures

that could be widely used in US mines. The test focuses primarily on

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

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

“green/yellow” test where:

• Green means that a 300 mm (12-in) encapsulation length

achieves the yield strength of the rebar (at least 8 tonnes (9 tons) for

No. 5 Gr. 60 and No. 6 Gr. 40 rebar; and about 12 tonnes (13 tons)

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

• Yellow means that the anchorage obtained from 300 mm (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 has been provided elsewhere (Mark et al., 2002).

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 300 mm (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 (Fabjanczyk et al., 1998). Shorter and longer lengths have

sometimes been used, however.

• Hole depth: In the US, production roof bolt holes are often

overdrilled by 25 mm (1 in) (Mazzoni et al, 1996). The overdrill

presumably provides a space for the resin cartridge wrapping, clips,

etc, while also providing a margin of error against underdrilling. When

conducting a 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 5b). 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 UK under certain conditions

(Health and Safety Executive, 1996; Wittenberg and Ruppel, 2000).

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 300 mm (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

1.70 m (5.5 ft) above the roofline. 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 insure consistent drilling depths, the steels were

marked and checked each day and all drilling was performed by 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 MINE PULL TEST RESULTS

Figure 7 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 pct

of the peak load over 38-50 mm (1.5-2.0 in) of deformation.

The results of the tests are summarized in Table 2. 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 34 mm (1 3/8 in) bit up to the anchorage horizon, and “S” for

standard holes. Details of the individual tests can be found in Mark et

al. (2002).

The average Grip Factor for all the tests was 250 N/mm (0.69

tons/in), which is well below the “Green” level of about 360 N/mm (1

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.

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. Standard holes are more convenient, but reamed holes

can be used if the additional information that can be obtained from

5 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio
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 the anchorages, even though the visually inspected

bolts showed that it was common for the top 12-50 mm (0.5-2 in) 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,

because the effect was relatively small (about 10%), overdrilled holes

should normally be used, unless exact depth holes are the normal

installation practice at the mine.

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

the bolts completely out of the reamed holes. Figure 8 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 70-80% of the installed length of anchor was still attached to

the rebar after being pulled from the roof. The other 50-100 mm (2 to

4 in) of the grout usually broke away from the lower portion of the

anchor. This is the portion of the grout that failed during the pull test.

PRELIMINARY TESTS IN US COAL MINEs

Short Encapsulation Tests were conducted in two operating 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 used in the Bruceton

study. 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 Nos.5 and 6 rebar types.

Table 3 shows that the anchorage was “green” at 2 of the 5 sites,

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

only 220 N/mm (0.6 tons/in).

IMPROVING ANCHORAGE 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, which is available at the

NIOSH Mining Website, 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. Rifled

holes and rougher bolt profiles should result in better anchorage.

Unfortunately, special bits to drill rifled holes are easier to find

6 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 7. Load deformation curve for typical short encapsulation pull test.

Table 2. Average test results by group ordered by hole depth

Mean St. Grip

Bolt Test Hole Maximum Dev., Factor

Size Type Depth load (tons) (tons) (tons/in)

No. 6 R E 9.7 1.4 0.81

No. 6 S E 8.5 1.7 0.71

No. 5 R E 7.8 1.1 0.65

No. 5 S E 9.1 1.0 0.76

No. 6 R O 7.6 1.4 0.63

No. 6 S O 7.4 2.3 0.62

No. 5 R O 7.5 2.0 0.62

No. 5 S O 8.7 2.1 0.73
overseas than they are in the US. 

Another possibility is to reduce the hole annulus. The simplest

way to do this might be to substitute #6 rebar for #5, while keeping the

hole size constant. 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 [Karabin and Debevic,

1976; Rico et al. 1997].

CONCLUSIONS

Since its introduction more than 30 years 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 can provide significant support even when

their anchorage is poor.

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 have been provided elsewhere (Mark et al., 2002).

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 US mineworkers.

REFERENCES

Aziz, N., Indraratna B, Dey, A., and Wang, Y., 1999, “Laboratory

Study of Shear Loading and Bolt Load Transfer Mechanisms under

Constant Normal Stiffness Conditions,” Proceedings of the 18th

International Conference on Ground Control in Mining, Morgantown,

WV, West Virginia University, pp. 239-247.

Bigby, D.N., 1997, “Developments in British Rock Bolting

Technology,”Coal International, Rockbolting Technology, May, pp.

111-114.

Campoli, A.A., Mills, P.A., Todd, P., and Dever, K., 1999, “Resin

Annulus Size Effects on Rebar Bolt Pull Strength and Resin Loss to

Fractured Rock,” Proceedings of the 18th International Conference on

Ground Control in Mining, Morgantown, WV, West Virginia University,

pp. 222-231.

Campoli, A.A., 2001, “Variables Affecting Resin Anchorage

Performance in United States Roof Bolting Systems,” Proceeding of

the Roofbolting in Mining, RWTH Aachen, Germany, pp. 19-28.

Cincilla, W.A., 1986, “Determination of Effective Column Lengths

for Resin-grouted Roof Bolts,” Proceedings of the Fifth International

Conference on Ground Control in Mining, Morgantown, WV, West

Virginia University, pp. 6-14.

Dolinar, D.R. and Bhatt, S.K., 2000, “Trends in Roof Bolt

Application,: Proceeding of the New Technology for Coal Mine Roof

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

Fabjanczyk, M.W. and Tarrant, G.C., 1992, “Load Transfer

Mechanisms in Reinforcing Tendons,” Proceedings of the 11th

International Conference on Ground Control in Mining, Wollongong,

New South Wales, Australia, University of Wollongong, pp. 212-219.

Fabjanczyk, M., Hurt, K. and Hindmarsh, D., 1998, “Optimization

of Roof Bolt Performance,” International Conference on

Geomechanics/Ground Control in Mining and Underground

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

Fairhurst, C. and Singh, B., 1974, “Roof Bolting in Horizontally

Laminated Rock,” Eng Min J Feb, pp. 80-90. 

Franklin, J,A, and Woodfield, P.R., 1971, “Comparison of a

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

Haas, C.J., 1981, “Analysis of Rockbolting to Prevent Shear

Movement in Fractured Ground,” Min Eng Jun, pp. 691-704.

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Mark, C., 2000, “Design of Roof Bolt Systems,” Proceedings of

the New Technology for Coal Mine Roof Support, NIOSH Publication

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

Mark, C., Compton, C.S., Oyler, D.C., Dolinar, D.R,. 2002,

“Anchorage Pull Testing for Fully Grouted Roof Bolts, “Proceedings of

the 21st International Conference on Ground Control in Mining,

7 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Table 3. Results from Underground Coal Mines.

Grip Factor

State Rock Type (tons/in) Green/Yellow

Pennsylvania site 1 @ 6ft clay, claystone 0.87 Green

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

Pennsylvania site 2 @ 6ft dark gray fireclay 0.63 Yellow

West Virginia @ 6ft dark gray shale 1.04 Green

West Virginia @ 1.5ft thinly banded gray shale 0.83 Borderline

Figure 8. View of a short column grout anchor recovered from

the roof.
Morgantown, WV, West Virginia University, pp. 150-113.

Mazzoni, R.A., Karabin, G.J., and Cybulski, J.A., 1996, “A

Trouble-Shooting Guide for Roof Support Systems,” MSHA IR 1237,

101 pp.

Pappas, D.M., Bauer, E.R., and Mark, C., 2000, “Roof and Rib

Fall Incidents and Statistics: a Recent Profile,” Proceeding of the New

Technology for Coal Mine Roof Support, NIOSH Publication No.

2000-151, IC 9453, 3-22.

Peng, S.S., 1998, “Roof Bolting Adds Stability to Weak Strata,”

Coal Age Magazine Dec:32-38.

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

Rico, G.H., Orea, R.R., Mendoza, R.L. and Tadolini, S.C., 1997,

“Implementation and Evaluation of Roof Bolting in MICARE Mine II,

Proceedings of the 16th International Conference on Ground Control

in Mining, Morgantown, WV, West Virginia University, pp. 139-148.

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8 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio