302

ABSTRACT 

 The The National Institute for Occupational Safety ane Health 

(NIOSH) Pittsburgh Research Laboratory (PRL) is continuing to 

investigate the behavior of fully grouted roof bolts in the weak roof 

rock of the Safety Research Coal Mine (SRCM). This paper 

describes the results of three studies: 

• A series of 24 pull tests of bolts installed fully grouted 

and overcored to leave 12 inches of grouted bolt. The 

tests compared the pull-out performance of offset-head 

roof bolts with that of standard 5/8-inch bolts; 

• A second series of 24 tests that compared the pull-out 

loads obtained by bolts installed with 1 ft of resin (a 

Short Encapsulation Pull Test or SEPT) to that of fully 

grouted bolts overcored to leave only 12 inches of 

grouted bolt, and; 

• An investigation into the pressures generated during the 

installation of fully grouted resin bolts. 

 All of the bolts installed and pulled were overcored and 

removed from the roof after the pull tests to allow inspection of the 

resin. 

 The first study found that there was no significant difference in 

pullout load between the offset-head bolts and the standard bolts. 

The second study, comparing the SEPT bolts and the partiallyovercored fully grouted bolts, indicated that SEPT can significantly 

underestimate bolt anchorage grip factor, suggesting that the SEPT 

is a conservative measure of actual bolt performance. 

 Significant loss of resin to cracks in the roof was observed in 

the bolts overcored in the SRCM. The third study explored one 

possible explanation for the resin loss, high pressures generated 

during bolt installation. These tests employed bolts installed in 

strain gauged steel tubes. The results confirmed that significant 

pressures (greater than 4,000 psi) can be generated during bolt 

installation. The results of the pressure tests, combined with the 

observations of resin loss in bolts installed in the SRCM, suggest 

that under some circumstances resin loss and under-encapsulation 

could take place and lead to degraded bolt performance. 

 Glove fingering was observed in all recovered bolts to various 

degrees. The limited comparative data suggested that the effect of 

glove fingering ranged from slight to moderate. Complete mixing 

of the resin was observed on all of the 40 bolts recovered (28 fully 

grouted and 12 SEPT). Australian and U.S. resins were compared 

and the Australian resins were observed to have much smaller 

catalyst compartment areas and lower resin viscosities. These 

differences are suggested as potential explanations for occasional 

resin mixing problems noted in Australian resin bolt installations. 

INTRODUCTION 

 Despite 30 years of experience with their use, resin grouted roof 

bolts remain a major research topic in the U.S. and around the 

world. Some issues of recent interest are: 

• Grip factor (anchorage capacity) in weak rock 

• Measurement of resin bolt performance 

• Importance of resin mixing 

• Significance of glove fingering 

 NIOSH recently developed a procedure to standardize the use of 

the Short Encapsulation Pull Test (SEPT) (Mark, et al., 2002) in the 

U.S. The test can be used as an index to identify when anchorage 

problems might occur, primarily through the comparison of 

regularly performed SEPTs. However, the question remains 

concerning how well a SEPT can actually reflect the real conditions 

at the top of a fully grouted resin bolt. Specifically, is the 

performance of a SEPT different from that of a fully grouted resin 

bolt given the differences in mixing conditions and resin 

confinement? 

 Other issues of recent interest were resin mixing and glove 

fingering. Recently Campbell and Mould (2003) identified 

frequent occurrences of both poor mixing and glove fingering in 

mines in New Zealand, both in newly installed bolts and in bolts 

installed as long as 12 years ago, indicating a long-standing 

problem. The poor mixing and glove fingering were found to take 

place regardless of geologic conditions, installation practice and 

resin manufacturer. More recently Mould, Campbell and 

MacGregor (2004) began looking at solutions to the mixing and 

gloving problems. They looked at a number of bolt modifications 

which seemed to reduce the incidence of both poor mixing and 

glove fingering, and they began to examine the possibility that 

modifying the design of the resin cartridges used in New Zealand 

might mitigate the problems. 

Investigation of Fully Grouted Roof Bolts Installed 

Under In Situ Conditions 

Craig Compton, Engineering Technician 

David Oyler, Mechanical Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA 
303

 Pettibone (1987) reported on a number of bolt installations 

using resin from three manufacturers. All bolts were inspected, but 

only a few were pulled. The occurrence of glove fingering was 

variable, common with one resin and uncommon with the other 

two. Pettibone could not detect any detrimental effect due to glove 

fingering on fully grouted roof bolts, even when 50% of a bolt was 

gloved. However, the technique he used to identify poor bolts was 

pull tests of fully grouted bolts. Fully grouted resin bolts will 

almost always pull to the yield strength of the steel, even when the 

installation is poor and the grip factor might otherwise be 

considered unacceptable. 

 One solution recently proposed to the problem of glove 

fingering and poor resin mixing is the use of offset head bolts 

(Campoli and Adams, 2003). Mould, et al., (2004) mentioned this 

as one of the more successful methods for improving bolt 

performance. Campoli and Shapkoff (2004) performed 1 ft short 

encapsulation tests on three standard bolts and three offset head 

bolts. However, all six bolts failed through yielding of the steel, so 

although all of the bolts performed well, the results of the 

comparison were inconclusive. 

 This NIOSH study was originally designed to evaluate the 

SEPT and offset head bolts. The SEPT procedure followed was the 

one described by Mark, et al. (2002). Both the SEPT and the offset 

head bolts were compared by measuring the bolt pull strength and 

comparing this to a fully grouted bolt overcored to within the short 

encapsulation length (12 inches) of the top of the bolt. This latter 

bolt configuration was referred to as the “Standard” bolt to 

distinguish it from the SEPT and offset head bolts. All three bolt 

treatments were conducted using 4 ft long, 5/8-inch, Grade 60 

bolts. All of the bolts were overcored and recovered so that the 

resin condition could be observed and evaluated. 

 Because all of the bolts were recovered, this provided the 

opportunity to evaluate the degree of resin mixing and glove 

fingering and their effect on the resin grip factor (anchorage 

strength). Evaluations of the degree of resin mixing and glove 

fingering of the bolts are included in the report. No attempt was 

made in this study to influence the occurrence of or to determine 

the factors which might influence the occurrence of glove fingering 

or poor resin mixing. 

 Early in the course of the two comparative tests it was observed 

that resin was being injected into the roof rock during every bolt 

installation. The resin cartridges used provided a 55 inch 

equivalent length of resin and were installed in a 48 inch hole. 

Resin returns should have been seen during every bolt installation 

(except SEPT installations). Instead, typically 30 to 50% of the 

resin from each bolt was lost into either pre-existing cracks or into 

fractures created during the installation of the bolts. A recent study 

conducted in New Zealand (Campbell and Mould, 2003) reported 

the measurement of significant pressures in test bolt installations 

conducted in the laboratory using resins available in Australia. A 

decision was made to conduct a series of tests to determine the 

pressure generated during bolt installation using resins available in 

the U.S. A series of bolt installations were conducted in the 

SRCM, installing bolts in a steel pipe inserted in a hole previously 

drilled in the roof. These tests obtained results similar to tests 

reported by ., et al. (1999) where three sizes of bolts were installed 

in 1-inch inside diameter pipes using two different viscosity resins. 

A 0.25-inch-diameter hole in each pipe was used to simulate 

fractures. The tests measured bolter thrust rather than resin 

pressure, but the thrust values reported suggested high insertion 

pressures, and the thrust magnitudes increased as the bolt diameter 

increased and the annulus decreased. The Campoli, et al. (1999) 

tests measured resin losses of between 35 and 60 pct, depending 

upon bolt diameter and resin viscosity. 

SITE DESCRIPTION 

 The generalized lithology of the roof rock at the site is shown in 

Figure 1. The lithology shown is a composite from a corehole 

located about 15 ft from the site and from a small diameter hole 

drilled at the test site and examined using a video camera. The 

immediate roof at the SRCM consists of a series of weak, 

predominately carbonaceous, shales and coals to the top of the 

bolting horizon. The estimated CMRR for the bolting horizon 

(4 ft) is 35. The study site was mined in the mid-1990’s and was 

chosen because the weak roof rock gave a greater probability of 

bolt failure due to anchorage than to steel yield. 

 The bolting horizons for the SEPT, standard and offset head 

bolts (the latter two after overcoring) all extended from 34.5 to 

46.75 inches above the roof line (this is slightly more than 

12 inches because the length of the bolts varied by about 

0.25 inch). The anchorage length for each bolt was 12 inches and 

consisted of about 6.5 inches of coal on top and 5.5 inches of 

carbonaceous shale below, although the depths of the coal and 

shale varied slightly across the study site. 

GENERAL TEST PROCEDURES 

 Both the comparison tests and the installation pressure 

measurements were conducted at the Safety Research Coal Mine 

(SRCM) using a single boom roof bolting machine. All bolts 

installed were 5/8-inch Grade 60 resin bolts, except for a few 

3⁄4-inch bolts used in bolt installation pressure tests. 

 The pull tests were instrumented with a string pot potentiometer 

to measure bolt displacement, and a pressure transducer and 

calibrated load cell to measure the pull load. Strain gauges were 

mounted on the outside of the 1 inch inside diameter schedule 40 Bolting Horizon

Bony Coal/Shale

Coal

Black shale

Dark gray shale

Gray shale

Coal

Coal

Coal

Coal

Black shale

Black shale

Dark gray shale

Dark gray shale

Gray shale

Bony coal/shale

6

5

4

3

2

1

0

ft

Figure 1. Roof lithology at the bolt installation site, showing 

typical bolting horizon. 
304

pipe used to simulate roof bolt holes, to measure bolt installation 

pressures. A datalogger and a laptop computer were used to collect 

and display the data during the tests. The pull equipment, including 

reaction fixture, pull claw, 20 ton hydraulic ram and 1-inch thread 

bar, is shown in Figure 2. The bolts were loaded using a hand 

pump. 

Offset Head versus Standard Bolt Performance 

Comparison Procedure 

 Both the offset head and standard bolts were installed and 

pulled using the same procedure. The only difference intentionally 

introduced was in the bolt types. The resin used in the installations 

was the resin recommended for use with offset head bolts. The 

resin for the first 15 bolts (8 offset head and 7 standard) tested 

came from the same box of resin. When that box of 20 cartridges 

ran out, a box of similar, but slightly faster setting resin was used. 

In order to reduce the effects of resin in general and from the 

change of resin in particular, bolts were usually tested in pairs, one 

offset head and one standard, and the same resin used for both bolts 

in a pair. 

 Several steps were required to install, pull and recover the bolts. 

The sequence, consisting of 6 separate operations, is shown in 

Figure 3. 

1. A 1-inch diameter bolt hole with 1-inch overdrill was 

drilled using dry vacuum drilling with carbide tipped bits 

(Figure 3a). 

2. A short NX diamond core bit with a pilot centralizer was 

used to drill a shallow (1 to 2 inch) NX (3-inch diameter) 

hole centralized on the bolt hole. This short pilot hole 

was also dry drilled. Dry drilling did not damage the 

core bit because of the soft roof and brief drilling period 

(Figure 3b). 

3. The bolt was then installed without a roof bolt plate. 

Bolts were nominally 4 ft in length, but that length 

included the bolt head which, along with the pull collar, 

had to be subtracted from the drilled depth. The actual 

bolt lengths in the roof were between 46.5 and 46.75 

inches, depending upon bolt length. With the 1-inch 

overdrill, hole depths were between 47.5 and 47.75 

inches (Figure 3c). 

4. A starter core barrel, and later a 4 ft NX core barrel were 

then run to overcore the bolt (Figure 4) to within 

12 inches of the top of the bolt, leaving 12 inches of 

grouted bolt to pull (Figure 3d). 

5. The test fixtures were then installed and the bolt pulled 

(Figure 3e), either to anchorage failure (preferred) or to 

bolt yield, which took place in only a few cases (Table 1). 

6. The 4 ft core barrel was then run again to core the bolt 

out of the hole for inspection (Figure 3f). 

Figure 2. Pull test equipment showing reaction fixture, 20 ton 

hydraulic ram, 1-inch thread bar and string pot displacement 

sensor. 

Figure 3. Bolt installation, pull test and overcoring procedure 

for offset head and standard bolts. (a) 1-inch bolt hole. 

(b) Overcore starter hole drilled using a pilot bit. (c) Bolt 

installation, without roof bolt plate. (d) Bolt overcored to 

within 12-inches of the top of the bolt using an NX (3-inch 

OD) bit. (e) Bolt pull tested. (f) Bolt overcored and removed 

from the roof for inspection. Drawings not to scale. SEPT bolt 

testing procedure similar, except the bolts were installed using 

a 12 inch equivalent resin length, step d was eliminated and the 

entire length of the bolt was overcored in step f. 

a b

c d

e f
305

 The bolt holes were drilled using dry vacuum drilling and a 

1-inch bit. Initially hole diameters were measured using a caliper, 

but after they were found to be very consistent (from 25 to 26 mm 

in diameter), the frequency of the measurements was reduced. 

Holes were drilled between 47.5 and 47.75 inches in depth, 

depending upon bolt length. The required depth was the total bolt 

length less the head length and the height of the pull collar (the top 

of which was set flush with the roof line), plus 1 inch for the 

overdrill. 

 Bolts were installed along with a 43.25-inch by 0.9-inch resin 

cartridge, giving an equivalent resin length of 55 inches in a 

nominal 1-inch hole, with the 1 inch overdrill. Without resin loss 

to the roof rock, this would have been sufficient resin to ensure 

resin return in every bolt installation. Instead no resin returns were 

observed in any installation. Measured hole diameters generally 

ranged from 25 to 26 mm, close to the nominal size. Examination 

of the length of resin on the overcored bolts showed that an average 

of 44% of the volume of each resin cartridge was lost through 

Figure 4. Bolt overcoring equipment; NX core barrel, EW drill injection of the resin into the roof rock (standard deviation 5%). 

rod, water swivel and hex steel adaptors. 

Table 1 – Offset Head versus Standard Bolt Data

Bolt 

ID 

Max Load 

(tons) 

Cause of bolt 

failure Glove fingering1,2 Measured resin 

length (in)3 Resin loss (Pct) Resin set time 

(min) 

OFFSET HEAD BOLTS 

5 7.1 Anchorage M 27.9 49% 34 

6 6.1 Anchorage S 26.8 51% 48 

8 7.5 Anchorage N/A 30.8 44% 31 

10 10.5 Yield L 26.9 50% 31 

12 7.9 Anchorage S 31.3 43% 21 

13 8.5 Anchorage M 29.5 46% 28 

20 8.8 Anchorage S 30.6 44% 22 

24 6.5 Anchorage L 31.8 42% 22 

29 6.5 Anchorage L 37.6 32% 22 

32 6.2 Anchorage L 33.4 39% 30 

34 8.9 Anchorage L 32.7 40% 25 

39 6.1 Anchorage L 26.5 51% 32 

Average: 7.5 30.5 44% 29 

Std Dev: 1.4 3.3 6% 8 

STANDARD BOLTS 

4 10.6 Yield M 30.1 44% 82 

7 8.9 Anchorage S 29.2 46% 26 

9 6.0 Anchorage L 28.5 48% 35 

11 10.3 Yield? L 30.8 44% 24 

14 10.6 Yield? M 31.1 43% 20 

19 9.4 Anchorage L 37.9 31% 43 

23 8.5 Anchorage L 31.8 42% 21 

26 10.7 Yield M 37.2 32% 23 

30 7.7 Anchorage N/A N/A4

 N/A 23 

31 7.5 Anchorage L 31.4 43% 117 

35 6.2 Anchorage L 32.3 41% 24 

38 10.3 Yield? L 29.0 47% 33 

Average: 8.9 31.8 42% 39 

Std Dev: 1.7 3.1 5% 30 

All bolts: 5/8-inch, Grade 60, nominal 4 ft length. Holes overdrilled 1". 

Resin cartridges 43.25" x 0.9", holes nominally fully grouted 

All bolts pulled with 12" resin column. 1

Estimated by area of film at resin surface. Low<33%, Med=33-66%, Severe>66% 2

An offset head bolt, categorized S was dropped from the data for lack of a corresponding standard bolt, but used in estimating 

the glove fingering effect. Pull 3.2 tons 

3

Does not include resin in 1" overdrilled hole. 

4

Not overcored. 
306

 Bolt installations were accompanied by slow rotation as the 

bolts were inserted using the roof bolting machine. Insertion of the 

bolt was stopped when the top of the pull collar reached the roof 

line and the bolt was then spun for 6 seconds. This time was based 

upon the resin manufacturer’s recommendation of 30 to 50 

revolutions for adequate mixing and the measured maximum bolter 

table speed of 510 rpm, giving 50 revolutions of the bolt. 

However, later measurements of bolter rotational speed recorded 

using a torque, thrust and rpm measuring instrument indicated that 

it took about 2 seconds for the bolter to reach maximum speed, so 

the actual number of rotations was estimated to be 40. Since this 

was still within the manufacturer’s recommendation and all bolts 

were spun the same length of time, spin time was not believed to 

have biased the test results. Bolts were held by the bolter for at 

least 1 minute and frequently longer for convenience. Hold times 

were assumed to not introduce any bias into the test results. 

 Because the bolts were being overcored and the alignment of 

the NX core barrel with the bolt was critical to prevent drilling 

through the resin and bolt (which sometimes still happened in the 

early tests), the bolting machine was not moved until a bolt was 

pulled and cored out of the roof. This made it very time consuming 

to let the resin cure for long periods before conducting the pull 

tests. In past work NIOSH has typically opted for a minimum 

curing time of 1 hour before pulling bolts. In this test series the 

curing time was as long as 117 minutes, but in most tests a 

minimum time of 20 minutes was considered acceptable and the 

average curing time was 34 minutes. Curing times for both the 

offset head and standard bolts were similar. The average for the 

offset head bolts was 29 minutes and for the standard bolts 

39 minutes (excluding two standard bolts given unusually long 

curing times the standard bolt average was also 29 minutes). 

 In order to reduce the effects of geologic anomalies on the test 

results, bolts were initially installed in pairs, with a coin toss 

deciding the first bolt of a pair to be installed. The first 11 bolts 

were installed using this procedure. A bolter malfunction 

prevented the completion of the last bolt pair. The remaining 

13 bolts (including a makeup bolt for the one not installed earlier) 

were installed in groups of three, Standard, Offset head and SEPT 

bolt, so that the standard bolt statistics could be used in both sets of 

comparisons. All bolts (including SEPT bolts to be discussed later) 

were installed within a roughly 10 ft by 7 ft area of roof (Figure 5). 

Bolt hole locations were chosen for ease of positioning the roof 

bolting machine rather than for randomization of location, but the 

effect of the bolt placement technique used was, to a great extent, 

to randomize the placement of the bolts. 

SEPT versus Overcored Standard Bolt Performance 

Comparison Procedure 

 The procedure for the SEPT versus overcored fully grouted bolt 

comparison was similar to that of the offset head versus standard 

bolts. The standard bolts were installed and pulled using the 

procedure described in the previous section and both the SEPT and 

overcored bolts were anchored at the same roof horizon. However, 

the procedure for installing and testing SEPT bolts was slightly 

different. Because it was not necessary to overcore the bolts to 

obtain a 12 inch length of grouted bolt, the following changes were 

made to the procedure described in the previous section. Steps 1, 2 

and 3 (bolt installation), were conducted as before, but step 5 (bolt 

pulling), was then performed next. Steps 4 and 6, overcoring the 

bolt, were then combined into one operation. 

 The installation procedure for both the SEPT and standard bolts 

was the same as for the offset head comparison. All bolts were 

rotated slowly during insertion, and when the bolt reached the 

proper depth it was spun at full bolter rotation speed (510 rpm) for 

6 seconds. This is estimated to have produced 40 mixing 

revolutions. All bolts were held stationary for a minimum of 

one minute, and frequently the hold time was extended beyond 

one minute. Bolts were tested after a minimum of 20 minutes. The 

average times were 41 minutes for the SEPT bolts and 43 minutes 

for the standard bolts. Excluding two standard bolts and one SEPT 

bolt tested after long set times, the averages were 28 and 

32 minutes. 

 As was the case in the offset head versus standard bolt tests, 

resin was lost in every standard bolt installation. The average resin 

loss for the standard bolts used in the SEPT versus standard bolt 

comparison was 40% (standard deviation 6%). Some resin was 

also lost from the SEPT bolts, which complicated the test results. 

The bolts were examined and the raw pull loads were corrected for 

the lost resin, as will be discussed in the RESULTS section. 

Installation Pressure Measurement Tests 

 The pressure developed during installation of a fully grouted 

resin bolt was measured by installing bolts in strain gauged 1-inch 

steel pipes (Figure 6) using the single boom roof bolting machine. 

One inch Schedule 40 pipe has a nominal 1.049 inch inside 

diameter, which is only slightly larger than the diameter of a 1 inch 

roof bolt hole. The pipes were cut to a length of 49 inches to allow 

the insertion of 4 ft resin bolts with a 1 inch overdrill. 

 The pipes were instrumented with circumferential strain gauges 

and the gauges were calibrated by pressuring the pipes with oil to 

2,500 psig and recording the pressure and the strain readings1

. The 

gauges were installed at approximately 2, 8, 14 and 20 inches from 

the top of the pipe. The calibration curves were also compared to a 

calculated strain versus internal pressure relationship determined 

from thick wall cylinder theory (Cook and Young, 1985). The 

 

1

An earlier series of pressure measurements were made using ports drilled 

in the pipe, oil-filled tubing and pressure transducers. The results of these 

tests were inconsistent and the measured pressures were low, probably 

because void space at the port had to be filled before pressure was 

transmitted to the pressure transducer. 

Figure 5. Bolt installation site in NIOSH Safety Research Coal 

Mine (SRCM), showing the 3 inch diameter holes from 

overcored bolts and several previously pulled bolts which were 

not removed from the roof 
307

empirical calibration curves obtained from the laboratory agreed to 

within a few percent with the theoretical values, but the empirical 

strain/pressure calibration curves were used to convert the 

measured strain to pressure. 

 Four bolts were installed, three 5/8 inch bolts and one 3⁄4 inch 

bolt. In all four cases the resin used was from the same box of 

cartridges used for the pull tests. The resin cartridges used 

provided a 48.75 inch equivalent resin length in the pipe and for 1 

inch of overdrill length. 

 The pipes were inserted into a 4 ft deep, 3-inch-diameter 

corehole previously drilled in the SRCM roof. A test fixture, held 

in place by two mechanical bolts installed in the roof, was used to 

clamp each pipe in place to prevent its rotation during the bolt 

installation. 

 The strain gauge wires were connected to a datalogger and 

strain readings were recorded at 0.1 second intervals during the 

installation process. Since the strain gauges were not temperature 

compensated the strain readings taken just before bolt installation 

were used to zero the gauges. The pressure readings obtained were 

also probably slightly greater than the actual pressure because of 

heat generated by the resin. The magnitude of the temperature 

error was estimated from pre-test strain readings to be about 

100 psi, which was not considered significant when compared to 

pressures of 5,000 psig or greater. 

 The bolts were installed in the same manner as the bolts 

installed in the two comparison tests. The resin cartridges installed 

insured resin return in the steel pipe. Bolts were slowly rotated 

during installation, and the bolts were pushed into the roof at the 

maximum speed of the bolter head, an estimated 5 inches/sec. The 

bolts were to be pushed to within about 1⁄2 inch of the roof, spun for 

6 seconds and then pushed to contact the pipe. 

RESULTS 

Offset Head versus Standard Bolts

 Table 1 shows the results of the offset head versus standard bolt 

tests. The average pull strength of the 12 offset head bolts was 

7.5 tons, with a 1.4 ton standard deviation. The average pull 

strength of the 12 standard bolts was 8.9 tons with a 1.7-ton 

standard deviation. These results suggest that there is no 

significant difference between the performance of offset head and 

standard roof bolts 

 Observation of the resin in the overcored bolts also indicated 

that adequate mixing took place in all bolts of both types 

(Figure 7). The resin from all bolts recovered appeared hard, with 

no indication of unmixed resin. Four bolts not shown in Tables 1 

and 2 were installed and only spun for one second; or an estimated 

two revolutions. In those four cases the average offset head bolt 

pull load was 1.3 tons and the average standard bolt pull load was 

2.5 tons. All four bolts showed indications that unmixed resin had 

been washed off the bolts by the drilling water during overcoring. 

Some of the remaining resin was solid, but crumbly. This appeared 

to be an indication of partially mixed resin. Neither resin loss due 

to washing of the bolts or crumbly resin was observed in the case of 

bolts spun for 6 seconds. 

 Since all resin on the test bolts had been fully mixed, it was not 

possible to evaluate resin color as an indicator of resin mixing, but 

it was noted that the resin color was frequently affected by the 

lithology the resin was mixed in and by the dust present in the holes 

and mixed into the resin; so it appeared that resin color would not 

be a good indicator of resin mixing in mixed roof lithologies. 

 Glove fingering was observed on every bolt, both offset head 

and standard bolts, to various degrees. In order to quantify the 

effect of glove fingering a visual evaluation of each bolt was 

performed and the bolts categorized into light, moderate and severe 

glove fingering groups, based upon the percentage of cartridge 

polyester film visible on the outside surface of the resin (Figure 7). 

The pull strengths of the lightly and severely gloved offset head 

bolts were then compared to look for a glove fingering effect. Ten 

offset head bolts were categorized as lightly or severely glove 

fingered (four severely and 6 lightly). The average pull capacity 

for the lightly glove fingered bolts was 7.4 tons (standard deviation 

1.8 tons), while the average for the severely glove fingered bolts 

was 6.5 tons (standard deviation 2.4 tons). However, one of the 

four severely glove fingered bolts pulled to 3.2 tons, three standard 

deviations below the mean of the offset head bolts in Table 1. This 

bolt was not included in Table 1. If this outlier bolt is eliminated 

from the set of severely glove fingered bolts the mean and standard 

deviation become 7.6 and 1.4 tons, respectively. If the outlier bolt 

is included in the data, the results suggest a moderate reduction in 

bolt strength due to glove fingering. If this bolt is rejected from the 

data, the data suggest that there is no glove fingering effect. In any 

case no strong conclusions can be drawn from the glove fingering 

results. There were not enough severely glove fingered standard 

bolts to perform this comparison on the standard bolts recovered. 

Figure 6. Four strain gauged 1-inch Schedule 40 pipes for 

measuring bolt insertion pressure. The end caps have not yet been 

put on three of the pipes in the photo. 

Bolt 13

Bolt 32

Bolt 31

Line of extruded resin

Figure 7. Bolting horizons of bolts 13, 32 and 31, showing typical 

good resin mixing and partial glove fingering. Bolt 13 shows a line 

of resin extruded into the coal. Bolts 13 and 32 are offset head 

bolts. Bolt 31 is a standard bolt. Bolt tops at left. 
308

Of the 8 standard bolts categorized as either lightly or severely 

glove fingered, only one was categorized as severe. 

 Almost every overcored bolt showed evidence of resin 

extrusion into coal cleat or induced fractures. The extruded resin 

frequently formed wavy lines about 1/8-inch wide and about 1/8-

inch deep (Figure 7) which remained on the bolts after overcoring. 

The resin could either be soft or well mixed and hardened, typically 

the latter. These observations are in agreement with those made by 

Campbell and Mould (2003), who reported resin injection similar 

to that shown in Figure 7. In two cases resin remaining on the bolts 

was extruded into the fractures at least 1⁄2-inch (Figure 8). In those 

cases the resin was well mixed and hard. During one bolt 

installation resin was observed falling out of an adjacent NX cored 

hole (3-inch-diameter). The holes were approximately 4 to 

5 inches apart, center-to-center, indicating that resin had been 

extruded a distance of 2 or 3 inches through coal cleat or vertical 

fractures. The extruded resin was not recovered, so the exact 

quantity of resin extruded and its degree of mixing could not be 

determined. 

Extruded Resin

1 inch

1/2 inch

Figure 8. Resin extruded into vertical fractures or opened coal 

cleat. Long wavy lines of resin extruded less than 0.1-inch were 

typical. The more deeply (up to 1⁄2 inch) extruded resin was 

unusual. Note that the extruded resin was thoroughly mixed. 

Table 2. SEPT versus Overcored Bolt Data 

Bolt Max load (tons) 

Id Raw Corrected 

Cause of bolt 

failure 

Tested resin 

length (in) 

Resin loss 

(pct) 

Resin set time 

(min) 

SEPT BOLTS 

SC1 5.2 5.8 Anchorage 10.8 10% 188 

SC3 5.1 5.9 Anchorage 10.3 14% 39 

SC5 5.7 6.3 Anchorage 10.9 10% 34 

SC7 4.5 5.4 Anchorage 10.0 17% 26 

SC9 4.0 4.8 Anchorage 10.0 17% 35 

21 4.2 6.6 Anchorage 7.6 36% 22 

22 3.9 4.4 Anchorage 10.8 10% 26 

27 4.0 4.4 Anchorage 11.0 8% 24 

28 5.8 6.4 Anchorage 11.0 9% 22 

33 3.0 3.4 Anchorage 10.5 12% 24 

36 4.6 5.2 Anchorage 10.5 13% 27 

37 5.8 7.4 Anchorage 9.4 22% 25 

Average: 4.6 5.5 10.2 15% 41 

Std Dev: 0.9 1.1 0.9 8% 47 

STANDARD BOLTS 

SC2 8.8 8.8 Anchorage 12 36% 51 

SC4 7.5 7.5 Anchorage 12 38% 82 

SC6 8.9 8.9 Anchorage 12 36% 48 

SC8 5.5 5.5 Anchorage 12 45% 31 

19 9.4 9.4 Anchorage 12 31% 43 

23 8.5 8.5 Anchorage 12 42% 21 

26 10.7 10.7 Yield 12 32% 23 

30 7.7 7.7 Anchorage 12 N/A 23 

31 7.5 7.5 Anchorage 12 43% 117 

35 6.2 6.2 Anchorage 12 41% 24 

38 10.3 10.3 Yield? 12 47% 33 

40 9.9 9.9 Anchorage 12 48% 22 

Average: 8.4 8.4 12 40% 43 

Std Dev: 1.6 1.6 0 6% 29 

All bolts: 5/8-inch, Grade 60, nominal 4 ft length. Holes overdrilled 1". 

Standard resin cartridges 43.25" x0.9", holes nominally fully grouted 

SEPT resin cartridge 10.25"x0.9". Holes overdrilled 1". 

Resin loss did not affect Std tested resin lengths, but did affect SEPT tested length. 
309

SEPT versus Overcored Standard Bolts 

 Table 2 shows the results of the SEPT versus Standard bolt 

tests. The average pull strength of the 12 SEPT bolts was 4.6 tons 

with a standard deviation of 0.9 ton. The average strength of the 12 

Standard bolts was 8.4 tons with a standard deviation of 1.6 tons. 

 One explanation for the poor performance of the SEPT bolts 

was the loss of resin into the roof in the SEPT tests. This also took 

place in all of the standard bolts and in the offset head bolts, but did 

not effect the test results, since the grouted column always 

exceeded 12 inches and overcoring was conducted to reduce the 

effective resin length to the required 12 inches for testing. The 

resin cartridges for SEPT tests were carefully measured to give a 

12 inch resin column. Any resin loss into the formation would 

reduce the actual resin column length to less than 12 inches. 

Inspection of the tested SEPT bolts suggested that some resin was 

lost from all of the SEPT bolts and large amounts of resin were lost 

from some. Further examination of the bolts also suggested that 

some resin was broken off of some bolts after testing due to 

vibration of the bolts during overcoring. This was confirmed by 

examining the standard and offset head bolts and observing that 

about one third of the fully grouted bolts had resin damage. Since 

the SEPT bolts were free over much of their length during 

overcoring, it was expected that a larger proportion of the SEPT 

bolts would sustain resin damage during overcoring. 

 The decision was made to correct the SEPT pull data for the 

reduced resin length, but the resin length during pull testing was 

assumed to include resin which broke off during overcoring, based 

upon resin traces remaining on the bolts. The test statistics were 

then computed using the Standard bolt statistics directly and 

adjusting the SEPT pull strengths to the estimated strength of a full 

12-inch resin column. This increased the average SEPT bolt pull 

strength from 4.6 to 5.5 tons and changed the standard deviation of 

the strengths from 0.9 to 1.1 tons. This correction assumes that the 

anchorage strength is constant over the length of the resin. 

 Corrected for resin loss the pull strengths of the SEPT bolts 

were 5.5 tons compared to 8.4 tons for the overcored bolts. The 

difference between the pull strengths of the two samples is 35%; 

calculated on the overcored bolt average. The difference between 

the two groups of bolts appears to be far too large to be explained 

in any way except by assuming that the overcored bolts produced 

significantly higher pull strengths than SEPT bolts. This 

conclusion would also be reached assuming that all of the SEPT 

bolts had 12 inches of resin; an obviously erroneous assumption. 

Assuming no resin loss during overcoring, and that all the resin loss 

took place prior to testing would give results that would require 

assuming no difference between SEPT and overcored bolt 

performance, but all of the observations indicated that this was also 

an erroneous assumption. The data suggest that there is a 

significant difference between the SEPT pull strength and the 

overcored bolt pull strength and that the SEPT is highly 

conservative. That is, when a SEPT determined anchorage factor is 

found to be acceptable, it appears reasonable to assume that the 

anchorage factor for typical, properly installed bolts will be 

acceptable. However, when a decision is made that the SEPT 

determined anchorage factor is low, it cannot automatically be 

assumed that the anchorage factors for typical bolts are also low. 

This does not necessarily invalidate the SEPT test, which can still 

be used to indicate when a problem might exist and as an index to 

compare anchorage factors under conditions of changing roof 

lithology. 

 One possible explanation for the performance differences 

between the two tests (assuming that the corrections to SEPT pull 

strengths are accurate) is that the lack of confinement of the resin 

during the mixing and hardening process reduces the SEPT bolt 

anchorage strength compared to that of the fully grouted bolts. 

Pressures in the short SEPT resin column were probably much less 

than in the full resin columns. Another possibility is that during 

fully grouted bolt installations resin was being injected into existing 

or newly created cracks and hardening, thus improving the 

mechanical interlock. This could also happen during SEPT, but at 

the expense of the length of the effective resin column. The 

overcored SEPT bolts did not show any sign of resin extrusion. 

Several recovered standard bolts showed signs of hardened resin at 

distances up to 0.5 inch from the bolt hole wall (Figure 8) and 

many standard bolts showed long shallow lines of both hardened 

and unmixed resin injected into cracks in the rock to depths of no 

more than 0.1 inch. If the former explanation is correct, then SEPT 

would generally be expected to be conservative. If the latter is 

correct, the differences between the two test types might be more 

variable, depending more on differences in rock properties, in situ 

horizontal stress, and variations in installation procedure (causing 

differences in resin pressure during the installation). 

 The authors feel that additional work is warranted to confirm 

the difference between SEPT and overcored standard bolt pull tests. 

Future work should be conducted in a more homogeneous roof 

rock, without the mixed coal and shale lithology present at the 

NIOSH SRCM. Tests should also be conducted in the laboratory 

under controlled conditions to look at the effect of resin 

confinement on bolt strength. The preferable site would be a one 

with a uniform roof lithology with weak rock between 2 and 4 ft, 

allowing the use of 3 or 4 ft roof bolts. 

Installation Pressure Measurements 

 All four of the bolt installations produced high pressures in the 

pipes (Figure 9), with the maximum pressures for the 5/8 inch bolts 

ranging from 5,500 to 6,900 psig and a 9,900 psig maximum 

pressure for the 3⁄4 inch bolt. If the bolts are assumed to behave like 

a piston with an area approximately equal to that of the bolt, the 

maximum pressures generated would have been 14,700 and 

10,200 psig, respectively for 5/8 and 3⁄4 inch bolts, given the 

4,500 lbf maximum thrust developed by the single boom bolter. 

The difference may be assumed to be due to leakage of resin 

around the sides of the bolt during insertion. The leakage appears 

to have been significant for the 5/8 inch bolts, reducing the actual 

insertion pressure to less than half of the maximum possible 

pressure. The leakage for the 3⁄4 inch bolts appeared to be much 

less and the maximum insertion pressure was just slightly less than 

the maximum possible pressure. 

 None of the four bolt installations went smoothly. The first 

bolt, a 5/8-inch bolt, buckled before being completely inserted. 

The second 5/8-inch bolt was spun for only 3 instead of 6 seconds, 

and the resin appeared to have fully set up at that time. The third 

bolt, a 3⁄4 inch bolt, could not be completely inserted into the hole 

and was never spun at all. The fourth bolt was only spun for about 

a second before stalling the bolter. The difficulty of bolt insertion 

is partially explained by the higher thrust required to insert a bolt 

into a closed pipe, but this also suggests that in normal bolt 

installations the pressure is being reduced by extrusion of resin into 

the roof rock. The overcored bolts showed that large quantities of 

resin were lost in every bolt installation at the SRCM, so this 

mechanism for reducing the insertion pressures appeared to be at 

work at the SRCM. The difference in ease of bolt insertion 
310

suggests that extrusion of resin significantly reduces the required 

installation thrust. Unfortunately, the bolter thrust was not 

measured during the project, precluding a comparison of normal 

installation thrust with the thrust required during the installation 

pressure measurement tests. 

 Tests were conducted during the following week to determine if 

the difficulty in bolt installation was increased by low temperatures 

at the time of the installation pressure tests. Several bolts were 

installed under three sets of conditions, the first was using resin 

warmed to room temperature, the second cold resin, and the third 

using cold resin, bolts and pipe. No quantitative measurements 

were made, but the installations were carefully observed. Although 

noticeably more thrust was required to install the bolts in cold 

resin, all bolts were installed without difficulty and all bolts were 

spun for 6 seconds. The tests were considered inconclusive, 

although they suggested that temperature had not played a 

significant role in the difficult bolt installations. This leaves the 

high thrust required to insert bolts in the pipes as the most likely 

reason for the installation difficulties. It suggests that either high 

installation pressures or resin extrusion are the normal conditions 

during resin bolt installation and that the high pressures recorded 

during the insertion pressure tests do represent pressures which 

could be reached in a sufficiently high strength rock. 

 Generally, all four strain gauges in each installation read similar 

pressures (Figure 9). Bolt pressures were generally low until about 

half of the length of the bolt was inserted in the pipe. The pressures 

in all gauges would then increase rapidly from a few hundred psig 

or less to around 4,000 to 5,000 psig. Although the travel of the 

bolter table was not measured during the tests, it was known to be 

about 5 inches/sec. The rate of pressure increase was found to be 

slightly lower below the top of the bolt than above it. The time at 

which this slight reduction in the rate of pressure increase took 

place allowed determination of when the bolt top passed each 

gauge. The estimated positions (large circles in Figure 9) agreed 

with estimates based upon the known speed of the bolter table. The 

data suggested that pressures would be slightly higher above the 

top of the bolt, but that pressures would continue to rise along the 

entire length of the bolt as long as bolter thrust was being applied. 

The exact pressure profile to be expected in a real bolt hole could, 

of course, be modified by extrusion of resin into the roof rock. 

 After the bolt was fully inserted and the bolt was spun the 

pressure rapidly dropped off. No attempt was made to determine 

the cause of the pressure reduction, but it was assumed that the 

primary cause was flow of resin around the bolt and out of the tube, 

assisted by the action of rotating the bolt. 

OBSERVATIONS ON RESIN CARTRIDGE GEOMETRY 

 During this project the authors were able to observe the 

condition of the resin and degree of glove fingering taking place on 

33 fully grouted resin bolts, installed using “best practice” methods, 

and 4 fully grouted bolts deliberately installed with short spin 

times. Because significant amounts of unmixed resin were not 

observed on any of the properly installed bolts, an effort was made 

to find an explanation for the difference between the performance 

observed during this study and that reported by Campbell and 

Mould (2003). More recently, Mould, et al. (2004), began testing 

resin cartridges with larger catalyst areas. The results of their work 

in this area have not yet been reported as of early 2005. 

 Discussions in early 2004 with representatives of the two U.S. 

resin manufacturers led to the suspicion that there might be 

significant differences between the resins used in the U.S. and 

Australia/New Zealand. U.S. resins appear to be of much higher 

viscosity, apparently contain much higher quantities of ground 

limestone, and the catalyst component takes up a larger percentage 

of the cartridge area. 

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

0 2 4 6 8 10 12 14 16 18

Time, seconds

Pressure, psig

0

16

32

48

64

80

96

112

Depth of bolt, inches

Top Gauge

6" Gauge

12" Gauge

18" Gauge

Depth of bolt

(estimated)

Start of insertion

Estimated end of 

spin 12.6 seconds. 

Estimated start of spin 9.6 sec.

Figure 9 – Pressure at four locations in a schedule 40 1-inch pipe simulating a bolt hole during resin bolt installation. The straight line 

shows the position of the top of the bolt. 
311

 As part of this study one of the resin cartridges from the SEPT 

tests was sectioned and cartridges from a major Australian 

manufacturer and the other major U.S. manufacturer were also 

obtained and sectioned. The relative areas of the catalyst 

compartments of all three cartridges were determined and are 

shown in Table 3. Because of the flexibility of the cartridges and 

the difficulty of making accurate area estimates the areas have been 

estimated to within 5%. 

Table 3 – Resin cartridge catalyst relative area 

Resin Catalyst relative area (%) 

US A 30-35 

US B 40-45 

Aust A 5-10 

All cartridges approximately 0.9 inch diameter 

 Figure 10 shows photographs of the Australian cartridge and the 

U.S. cartridge with the smallest relative catalyst area. 

Superimposed on the photographs are circles representing the 

diameters of the typical holes the cartridges would be used in and 

the diameters of the typical bolts used in that hole. In the U.S. case 

a 5/8-inch bolt in a 1-inch hole was chosen. In the Australian case 

a 22-mm bolt with a 27-mm diameter hole is shown. In both cases 

the diameter shown includes the ribs on the bolt, since the diameter 

of the bolt at the ribs will control resin mixing. Figure 10 shows 

the worst possible positioning of bolt, resin cartridge and borehole 

for resin mixing. 

 Figure 10 shows that it is highly unlikely that even a 5/8-inch 

bolt could run through a US resin cartridge without tearing the 

polyester film between the two compartments (Note that the two 

U.S. manufacturers included in Table 3 control the entire US roof 

bolt resin market.). In the Australian case tearing the film between 

the compartments appears possible, but does not appear to be as 

certain to take place as in the case of the U.S. resins. 

 Campbell and Mould (2003) reported that the resin along about 

27% of the length of the bolts they investigated was unmixed and 

they noted that the catalyst compartment was often found flattened 

against the side of the borehole and remained unshredded. 

Figure 10 suggests a reason why this might have frequently taken 

place. The geometric observations and the data from this study 

together suggest that unshredded catalyst compartments should not 

be a cause of poor mixing of resin in U.S. mines. 

CONCLUSIONS 

 The offset head versus standard 5/8 inch roof bolt pull tests 

suggest that there is no significant difference between the pullout 

performance of a standard 5/8 inch roof bolt and an offset head roof 

bolt of the same diameter and grade, when both are installed in the 

same manner. 

 The Short Encapsulation Pull Test (SEPT) versus overcored 

standard bolt pull tests indicated that there is a significant 

difference between the two; the results suggest that SEPTs are 

highly conservative. The SEPT test is still useful to confirm 

adequate anchorage and as index test to compare relative anchorage 

strengths. 

 Adequate resin mixing was observed on all of the bolts installed 

in this study. All bolts were spun for the same time period 

(6 seconds) and no unmixed resin was observed on any of the 

45 tested bolts cored and removed from the roof. Several bolts, not 

included in the comparative studies, were installed using 1 second 

spin times, and in these cases poor bolt performance and partially 

unmixed resin were both anticipated and observed. 

 Glove fingering, of varying extent, was observed in almost all 

bolts installed in this study. Comparison of four slightly glove 

fingered offset head bolts to six severely glove fingered offset head 

bolts, based upon visual examination of the amount of polyester 

film visible at the surface of the resin, led to inconclusive results, 

but suggested that the effect of glove fingering ranged from slight 

to moderate. This comparison could not be conducted on standard 

bolts because of a lack of bolts which could be considered severely 

glove fingered. 

 The pressure tests suggested that significant pressures, up to 

5,000 psig, may be generated in the resin during the installation of 

fully grouted resin bolts, unless extrusion of resin into the roof rock 

takes place. Physical examinations of bolts installed at the NIOSH 

Safety Research Coal Mine (SRCM) clearly showed that significant 

quantities of resin (averaging 44%, but up to 50%) were lost in 

weak rock through extrusion of resin into existing fractures or 

fractures created by the installation pressure. In weak roof rock 

resin loss of this magnitude could have a detrimental effect upon 

bolt performance and possibly upon mine roof conditions, 

especially where the lower portions of the bolts are left ungrouted 

by the resin loss. However, the SRCM tests also suggest that resin 

extrusion may be common and minor extrusion of resin, as long as 

sufficient resin is present to insure that bolts remain fully grouted, 

may not have a detrimental effect. 

 Finally, comparison of what are believed to be typical U.S. 

resin cartridges to typical Australian cartridges suggests that the 

large difference in the relative proportions of catalyst to resin 

between U.S. roof bolt resins (30-45% to 70-55%) and Australian 

(10% to 90%) resins may partly explain the relatively high 

incidence of poorly mixed resin reported by Campbell and Mould 

(2003). If this is the case, the problem of poor resin mixing may 

not be common in the U.S., since the manufacturers supplying most 

of the resin used in the U.S. make resin cartridges with large 

catalyst compartments, which are more likely to insure tearing of 

the wall between the two compartments and mixing of the two 

components. 

US Resin Australian Resin

1-inch hole

5/8 bolt

27 mm hole

22 mm bolt

Catalyst

Catalyst

Resin

Resin

US Resin Australian Resin

1-inch hole

5/8 bolt

27 mm hole

22 mm bolt

Catalyst

Catalyst

Resin

Resin

Figure 10 – Sections of typical U.S. and Australian resin cartridges, 

showing catalyst and resin compartments. Large circles are the 

diameter of a typical bolt hole. Small circles are the bolt diameter 

across the ribs. Cartridge diameters approximately 0.9 inch. 
312

REFERENCES 

Campbell, R.N. and Mould, R.J. (2003). Investigation into the 

Extent and Mechanisms of Gloving and Un-mixed Resin Fully 

Encapsulated Roof Bolts. Proceedings, 22nd International 

Conference on Ground Control in Mining, Morgantown, WV, 

Aug 5-7, pp. 256-262. 

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

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, Aug 3-5, pp 222-

231. 

Campoli, A. and Adams, L. (2003). Eclipse Bolting System. 

Proceedings, 22nd International Conference on Ground Control in 

Mining, Morgantown, WV, Aug. 5-7, pp. 246-248. 

Campoli, A. and Shapkoff, S. (2004). Eclipse System Bolting in the 

Illinois Basin. Proceedings, 23rd International Conference on 

Ground Control in Mining, Morgantown, WV, Aug. 3-5, pp. 235-

237. 

Cook, R.D. and Young, W.G. (1985). Advanced Mechanics of 

Materials (Macmillan, New York, USA, ISBN 0-02-324620-0). 

Mark, C., Compton, C.S., Oyler, D.C. and Dolinar, D.R. (2002). 

Anchorage Pull Testing for Fully Grouted Roof Bolts. 

Proceedings, 21st International Conference on Ground Control in 

Mining, Morgantown, WV, Aug. 6-8, pp. 105-113. 

Mould, R.J., Campbell, R.N. and MacGregor, S.A. (2004). Extent 

and Mechanisms of Gloving and Unmixed Resin in Fully 

Encapsulated Roof Bolts and a Review of Recent Developments. 

In: Villaescusa & Potvin, eds. Ground Support in Mining and 

Underground Construction. Proceedings, 5th International 

Symposium on Ground Support, (Perth, Australia, Sept. 28-30, 

London, UK: Taylor & Francis Group, pp. 231-242. 

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