ABSTRACT

The San Juan Coal Mine, located near Farmington, New

Mexico, supplies the San Juan Generating Station with more than 6 

million tons of coal annually. To replace dwindling surface mine 

production, San Juan installed a longwall in 2002. The roof rock in 

much of the underground reserve consists of interbedded coals, 

carbonaceous shales, and mudstones. Roof support consists of 

fully-grouted roof bolts, both tensioned and non-tensioned, 

supplemented by modified cable trusses. 

Short-encapsula tion pull tests conducted by San Juan’s

geotechnical engineering department showed that some of the 

immediate roof layers provided variable bolt anchorage. To ensure 

that the roof bolts were obtaining the necessary anchorage,

additional tests were conducted in a variety of locations. The 

results have been used to determine the optimum support design for 

different areas of the mine. Another series of tests evaluated the 

performance of several bolt and hole diameter combinations. The 

experience at San Juan Mine shows that short -encapsulation pull 

tests can provide the information necessary to improve ground 

control in variable roof conditions. It also illustrates how a pro -

active geotechnical engineering program can be an essential

component of a state of the art underground coal mine.

INTRODUCTION

As part of a long-term effort to provide a cost-competitive fuel 

supply to the San Juan Generating Station, BHP Billiton’s 

Mineral’s San Juan Coal Company (SJCC) recently completed the 

installation of a state of the art longwall mine adjacent to its surface 

operations at Waterflow, near Farmington, New Mexico (figure 1). 

The underground mine was developed only after exhaustive

geotechnical, geological, environmental, and economical

evaluations were completed. 

As part of the program, a small-scale pilot mine was developed 

within the No. 8 Seam on part of the surface mine lease in close 

proximity to the surface operation. The express purpose of the 

pilot mine was to assess the minability of the No. 8 Seam,

including ground control concerns, subsidence, operational issues, 

and equipment selection. Room-and-pillar mining methods were 

employed for geological, geotechnical, and geomechanical

evaluations. Variable roof and floor conditions, coupled with splits

of the No. 8 Seam above the mining horizon, required

experimentation with different mining horizons, opening and pillar 

geometries, and various roof and rib support systems to be

evaluated.

A generalized geological column of the No. 8 Seam and the 

immediate roof is shown in figure 2. During the initial drilling 

program it became apparent to the Company technical staff that the 

roof and floor strata were relatively variable, and potentially

susceptible to water degradation and air slaking. Typical UCS 

values for the immediate roof shales are between 1,000 and 2,000 

psi. The relatively low uniaxial compressive strength, and the 

presence of closely -spaced bedding planes, slickensides and other 

discontinuities in these rocks results in an estimated Coal Mine 

Roof Rating (CMRR) in the 30-35 range. Work is currently being 

undertaken to determine a more systematically derived value. 

In some areas of the property as many as four significant 

sandstone channels may be encountered, three residing above the 

No. 8 Seam and one immediately above the No. 9 Seam. Difficult 

mining conditions can be encountered when the channels migrate 

SHORT-ENCAPSULATION PULL TESTS FOR ROOF BOLT EVALUATION

AT AN OPERATING COAL MINE

James Pile, Geotechnical Engineer

Steve Bessinger, Engineering Manager

BHP Billiton San Juan Coal Company

Waterflow, New Mexico USA

Christopher Mark, Section Chief, Rock Mechanics

Stephen C. Tadolini, Section Chief, Geotechnical Engineering

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

Figure 1. Mine site location, Waterflow, New Mexico
closer to the No. 8 Seam because of differential compaction near 

the channel margins, compactional features below the sandstone 

body, and the potential for more water occurrence. 

Fortunately, the in situ stress regime appears to be relatively 

benign. The depth of cover over the initial reserve ranges between 

approximately 300 and 800 ft, though ultimately the mine may be 

as deep as 1,400 ft. Hydraulic fracturing stress measurements 

indicate that the major principal stress is typically less than 1,000 

psi. The ratio of the major principal horizontal stress äH to the 

minor horizontal stress äh averaged 1.6. No clear indications of 

roof damage caused by horizontal stress have been observed in the 

underground workings.

Concerns about the immediate roof strata, the potential for 

water degradation, and the potential effects of in situ horizontal 

stresses prompted a comprehensive roof support evaluation

program. The goal of the program was to ensure that optimal 

performance would be obtained from the roof bolts. The program 

focused on the anchorage provided by the different roof strata, the 

potential bolt lengths, the required bolt capacities, and other

parameters including the bolt diameter and the hole size. 

ANCHORAGE CAPACITY OF FULLY GROUTED 

BOLTING SYSTEMS

Where the rock is weak, fully grouted roof bolts are prone to 

anchorage failure (1, 5). Resin grouted bolt systems are typically 

used in such conditions, and consist of three components, the bolt, 

the resin, and the host rock. The components have very different 

material properties that can affect the final performance of the 

system. Steel materials are ductile and have high ultimate strengths 

and modulus of elasticity, meaning they can sustain large loads and 

deflections. Resin grouts and the host mine rocks can be brittle and 

are not capable of sustaining high tensile loads. The combinations 

of these two different materials usually result in shear failures at the 

rock and resin interface. However, if the length of the resin 

encapsulation along the bolt length is long enough to complete the 

required load transfer, the full capacity of the bolt can be achieved 

(2).

Grip Factor

The anchorage of a fully grouted bolt is measured by the Grip 

Factor, which is defined as the bolt’s resistance to pullout per inch 

of bolt length. The Grip Factor is measured by loading the upper 

portion of the grouted bolt in a short encapsulation pull test, or 

SEPT. The maximum length of the resin column tested in an SEPT 

is 12-18 inches, depending on the strength of the host rock. This 

length provides enough material to ensure adequate mixing but 

more importantly minimizes the possibility of the bolt yielding 

prior to the bond failure occurring. The Grip Factor (tons per inch) 

is calculated as:

GF = (Load to Slippage)/(Resin Anchor Length)

The “Load to Slippage” is the load applied to the bolt, usually 

expressed in tons, and the “Resin Anchor Length” is the length, 

expressed in inches, of the bolt encased with resin. 

A low Grip Factor (less than approximately 1 ton/inch) means 

that the bolt will have less available resistance to rock movement, 

as shown in Figure 3. Within the anchorage zone (the upper 

portion of the bolt), the bolts available resistance is less than its 

nominal yield strength. As a result, the bolt can be pulled out of 

the roof when rock movement occurs near the top of the bolt. 

Improving the Grip Factor can significantly improve the

performance of a roof bolting system.

Shear Strength and System Stiffness

Other useful properties that can be obtained from SEPT are 

the peak shear stress and system stiffness (3). The shear stress ô 

(psi) can be calculated by using the equation:

Figure 2. Generalized geological column of the mining 

horizon and immediate roof.

Figure 3. Effect of the Grip Factor on the resistance available 

from 10-ton roof bolts to react against roof loads. (left) Grip 

Factor = 0.5 ton/in; (right) Grip Factor = 1.0 ton/in.
where: ΔF = change in force over the encapsulated length, lb

L = final resin length, inch

φ = bolt diameter or borehole diameter, inch.

The system stiffness, k (lb/inch) or the ability of the system 

to resist load, can be determined by applying this equation:

Where: ä = change in length, inch.

The key to using these relationships is that shear failure must 

take place between the resin -bolt, or resin -rock interfaces. In weak 

roof materials the resin -rock interface controls the failure

mechanism. If the rock materials are strong, the failure surface 

may be the resin-bolt interface, which simply tests the resin 

strength and/or the bolt rib profile performance. If failure does not 

occur and the peak shear strength exceeds the applied force, the 

equation can be used to calculate the shear stress for the applied 

force and system stiffness can be calculated for the applied force 

and measured displacement (4).

Factors Affecting Bolt System Performance

A variable that often affects resin bolt performance is the size 

of the resin annulus, defined as the difference between the hole and 

bolt radii. As the annulus increases, the mixing of the resin paste 

and mastic becomes less efficient. Conversely, if the annulus 

mixing is too small, higher installation pressures can force some 

material into cracks or fissures, though this may be compensated 

for with additional cartridge length. 

While a number of studies over the past 25 years have indicated 

that 0.125 inch is the optimum annulus for resin grouted bolts, the 

industry practice is actually different (5, 6). On the other hand, two 

recent US studies did find a significant annulus effect (2, 5). 

The annulus can be reduced either by decreasing the hole 

diameter or by increasing the bolt diameter. However, reducing the 

bolt hole diameter also reduces the contact area between the resin 

and the rock. Figure 4, shows a nominal No. 7 rebar bolt, 0.804 

inch diameter, installed in 1, 1-1/8, 1-3/8 & 1-1/2-inch diameter 

holes. As shown, the outside circumferential contact area between 

the resin and rock interface increases with the hole size. If the 

shear strength of the host material is 725 psi, the shear strength 

capacity of the bolt installed in the 1-inch hole (circumference 

3.1416 inch) would be 725 psi x 3.1416 inch = 2,278 lb/inch. The 

1-1/2-inch diameter hole has a circumference of 4.7 inches

resulting in a shear strength of 3,416 lb/inch, an increase of 50 

percent. This can make a major difference in the final hole and bolt 

selection in weaker roof materials, but often remains unexamined.

Bolt performance can also be affected by bolt surface

roughness, the hole roughness, and the resin characteristics (3, 6, 

7). Rougher bolt profiles generally result in improved anchorage, 

as do rifled holes. Resins with higher compressive strength are also 

generally expected to improve bolt anchorage performance. 

TESTING PROGRAM

The bolt testing program undertaken in the San Juan Mine had 

two goals:

• Optimize the bolt length by determining the best anchorage 

horizon in different parts of the mine, and;

• Optimize the bolt system performance by determining the best 

combination of hole diameter and bolt diameter.

Bolt Length (Roof Horizon) Tests

Tests were conducted at 4 locations to examine the bolt

anchorage at various horizons. At the first three sites (sites B, C, 

and D), the tested bolts were 0.804 inch diameter rebar installed in 

1.125 inch boreholes. The bolt lengths examined were 5, 6, 7, and 

8 ft. The resin cartridges used in the tests provided a nominal 12-

inch anchorage. 

The roof in all of the locations consisted of interbedded coal 

and carbonaceous shale, with a thin rider coal seam overlying it 

known as the 8-5b. The unit above the 8-5b coal consisted of a 

thick, clay-rich shale, or mudstone, as described earlier. 

Site B

Test site B consisted of 24 bolts installed and tested in the East 

Mains at cross-cut 37. The thickness of the carbonaceous shale 

band above the 8-5b coal in this area measured approximately 10 

inches. The bolts were installed and the load required to initiate a 

shear failure at the resin/rock interface was recorded. The grip 

factors for each bolt length are shown in figure 5.

The results show that better anchorage was obtained by the 5 

and 6 ft long bolts. They obtained average grip factors of 0.72 and 

0.65 tons/inch respectively, versus 0.50 and 0.55 tons/inch for the 

7 and 8 ft bolts.

ä

ÄF K =

ðöL

ÄF ô=

Figure 4. The available rock-resin contact surface for the 

respective borehole diameters with a 7/8-inch diameter bolt 

installed.
Site C

The next series of tests was conducted in between cross-cuts 13 

and 14 in the same East Mains, approximately 2,300 ft away from 

Site B. Test Site C consisted of 24 bolt installations. The thickness 

of the shale band above the 8-5b coal in this area measured 

approximately 14 inches. The bolts were installed in the same 

manner and the required force to initiate a shear failure at the 

resin/rock interface was recorded. The grip factors for each bolt 

length are shown in Figure 6. As can be clearly seen, the 7 ft long 

bolts had the highest average grip average of 0.73 tons/inch of 

grout. This value was nearly double the grip factors obtained with 

the 5, 6 and 8 ft bolts, which were 0.38, 0.35 and 0.34 tons/inch, 

respectively.

Site D

The next test site was inby test Site B between cross-cuts 42 

and 43. The 24 bolts were installed and tested using the identical 

procedure. The thickness of the carbonaceous shale band above the 

8-5b coal in this area measured approximately 10 inches, the same 

thickness determined in Site B. The bolts were installed and the 

load required to initiate a shear failure at the resin/rock interface 

was recorded. The grip factors for each bolt length are shown in 

Figure 7.

The average grip factor for the 6 ft long bolts was 1.29

tons/inch, the best value measured at any of the sites. It exceeded 

the averages for the 5, 7, and 8 ft long bolts by more than a factor

of two.

Site E 

The fourth bolt length test site was conducted approximately 

450 ft to the northeast of site B, in cross-cut 40 of the East Mains. 

Only two b bolt lengths, 6 ft and 8 ft, were tested. The bolts had a 

diameter of 0.677 inches and were installed in both 1-1/32 and 1-

1/8-inch diameter holes. The thickness of the carbonaceous shale 

band above the 8-5b coal in this area measured approximately 10 

inches, the same as test area B and D. As shown in Figure 8, the 

grip factors obtained with the 6 ft long bolts averaged 0.75

tons/inch, compared to just 0.43 tons/inch for the 8 ft long bolts. 

Discussion of Bolt Length Tests

Geological columns were obtained near each test area as shown 

in Figure 9. The roof horizon is indicated and potential

“anchorage” horizons are indicated for bolts lengths of 5 to 8 ft. 

The results show that in all four cases the best anchorage horizon 

was probably the shale just above the 8-5b coal. One hypothesis is 

that there is a hard band of stronger shale located there. Another is 

that the top of the coal may actually be “wallowing out” slightly 

during drilling to create a slightly larger hole and resin “keying” 

effect. This keying effect may be increasing the grip factors. 

Figure 5. The grip factors for each bolt length in test site B

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

4 5 6 7 8 9

Bolt length, ft

Grip factor, ton/inch

Figure 6. The grip factors for each bolt length in test site C.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

4 5 6 7 8 9

Bolt length, ft

Grip factor, ton/inch

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

4 5 6 7 8 9

Bolt length, ft

Grip factor, ton/inch

Figure 8. The grip factors for 6 and 8 ft long bolts in test site E.

Figure 7. The grip factors for each bolt length in test site D.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

4 5 6 7 8 9

Bolt length, ft

Grib factor, ton/inch
As is the case with most sedimentary rock formations, the 

immediate roof can be highly variable and have a major impact on 

bolt performance and subsequent roof stability. Site geology from 

sites B, D and E are all specified from a single column that ranges 

in horizontal separation from 225 to 500 ft, indicating how quickly 

the immediate roof geology can change. When comparative testing 

is completed, it is extremely important to classify the roof horizon 

where the tests were performed. 

The complete results are contained in Table 1. Figure 10 shows 

the results using the measured encapsulation length to calculate the 

Grip Factor. Data from the 1.25 inch holes were excluded because 

difficulties with the bits resulted in anomalously low pullout loads. 

The results from 20 tests in which the bolt yielded are included in 

the data. In these cases, the peak load was not sufficient to cause 

the anchorage to fail, meaning that the true Grip Factor must have 

exceeded the calculated one.

Bolt System Performance

Ninety-two tests were performed at Site A, located between 

cross-cuts 31 and 32 of the East Mains, approximately 1,000 ft west 

of site E, to examine the effects of bolt diameter, hole diameter, and 

annulus. Two bolt diameters, 0.804 and 0.914 inch, were installed 

in 5 different diameter holes, resulting in 9 different annuli. The 

borehole diameters were 1.0, 1.125, 1.25, 1.375, and 1.5-inches. 

The final resin column lengths were actually measured and varied 

between 13 and 29-inches. 

Figure 10 shows that the best results were achieved with 

0.804 inch bolts installed in 1.125 holes, with an annulus of

0.16 inches. With this combination, mean Grip Factor was

0.87 tons/inch, with 7 of the ten bolts tested going to yield. 

However, the results for 0.914 inch bolts in 1.125 inch holes 

(annulus = 0.1055 inches) were nearly as good. The mean Grip 

Factor was 0.82 tons/inch, this time with four yielded bolts. On the 

other hand, the smallest annulus (0.804 inch bolts in 1 inch holes) 

resulted in some of the lowest grip factors.

The data for the larger holes shows a generally declining trend 

in grip factor as the annulus increases. Higher grip factors were 

obtained in the 1.375 inch holes than in the 1.5 inch holes, and for a 

given hole size, the larger bar performed somewhat better.

Table 1. Site A Bolt System Performance Test Results

Annulus (in) Bolt Dia (in) Hole Dia (in) Number Yielded 

Bolts Shear Stress (psi) Mean Grip Factor (tons/in)

Nominal Mean Grip 

Factor (tons/in)

0.161 0.804 1.125 7 498 0.88 0.73

0.231 0.914 1.375 4 380 0.82 0.73

0.293 0.914 1.500 3 306 0.72 0.67

0.106 0.914 1.125 4 464 0.82 0.57

0.168 0.914 1.250 0 321 0.63 0.60

0.286 0.804 1.375 0 301 0.65 0.60

0.348 0.804 1.500 0 229 0.54 0.50

0.223 0.804 1.250 0 255 0.50 0.45

0.098 0.804 1.000 0 331 0.52 0.42

Figure 9. Geological columns for test sites A, B, C, D, and E.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 0.1 0.2 0.3 0.4

Annulus (in)

Grip Factor (tons/in)

0.804-in Bolts

0.914-in Bolts

1.5-in Holes

1.375-in

Holes

1.125-in

Holes

1.0-in Holes

Figure 10. Grip Factor calculations for various bolt and hole 

combinations.
Table 2. Site E Bolt System Performance Test Results

Annulus (in) Bolt Dia (in) Hole Dia (in) Shear Stress (psi) Nominal Mean Grip Factor 

(tons/in)

0.161 0.804 1.125 323 0.57

0.192 0.677 1.060 294 0.49

0.224 0.677 1.125 277 0.49

0.286 0.804 1.375 194 0.42

0.106 0.914 1.125 232 0.41

0.168 0.914 1.250 204 0.40

0.223 0.804 1.250 188 0.37

0.231 0.914 1.375 120 0.26

Measurements of the encapsulation length showed that the 

smaller the annulus, the smaller the actual encapsulation length 

when compared with the nominal. This is probably because resin 

losses during insertion have a much greater effect on the

encapsulation length in smaller annuli. If the nominal 

encapsulation length is used to calculate the Grip Factor instead 

of the measured one, the relationships between annulus and grip 

factor are much less pronounced. As Table 1 shows, the Grip 

Factors obtained with the 0.914 inch bolts are fairly uniform, 

with the highest values occurring with 1.375 inch holes. 

A second series of bolt system performance tests were

conducted at site E. The testing matrix for the 8 ft long bolts 

consisted of a sample size of 32 bolts, which evaluated three bar 

diame ters, 0.677, 0.804, and 0.914 inches. The bolts were 

installed in holes that were drilled 1-1/32, 1-1/8, 1-1/4, and 1-3/8-

inches in diameter. The bolts were anchored with resin, allowed 

to cure properly, and subsequently tested using the same

procedures as before. 

The data are contained in Table 2 and figure 11. The actual 

encapsulation length was not measured in these tests, so the 

nominal encapsulation length is used to calculate the Grip Factor. 

Statistical analysis indicates that there were no significant trends 

that could be attributed to hole diameter, bolt diameter, or

annulus. 

The data shows that the shear strength of the resin -rock 

interface decreased as the hole size increased. At site A, for 

example, the strength of the 1.125 inch holes averaged 481 psi, 

while the strength of the 1.5 inch holes was 268 psi. The 

reduction in shear strength more than compensated for the

increased circumferential area.

Discussion of Bolt System Performance Tests

It is important to remember that the Grip Factor is simply an 

index property that provides insight into the anchorage capacity 

and the relationship between the host rock, resin, and bolting 

interfaces. Index property tests are most useful when the data is 

repeatable and confidence in the bolt system tested can be 

obtained. To minimize variables in the basic analysis here are 

some important considerations:

• Use a standard test procedure. The one suggested by Mark et 

al. (5) is a good starting point. 

• An encapsulation length of 12-inches is recommended. This 

ensures there is enough resin material and also minimizes 

the possibility of achieving the bolts yield strength. 

• Characterize the anchorage zone being tested with site

specific core sampling or a color borehole camera. 

• Combinations of various bolts and hole sizes usually

necessitates the introduction of different bits, bolt surface 

profiles, and resin cartridge diameters and products. These 

additional variables can complicate interpretation of the 

results. Once the optimum annulus is determined, bolt 

length tests should all be completed with the same resin and 

bolt combinations.

CONCLUSIONS

The short encapsulation pull test results from the San Juan 

Mine indicate clearly that superior anchorage can be obtained in 

specific roof layers. The tests indicate that the optimum bolt 

length at this mine is probably one that penetrates the highanchorage layers, but does not extend beyond them. Additional 

length would be largely wasted because the anchorage is low in 

the overlying mudstones. Based on these results, the tests are 

now being used routinely to select the best bolt length for

different areas in the mine. 

The most significant result from the bolt system test results 

has been that 0.677-in bolts can achieve the same anchorage as 

larger diameter bolts at the San Juan Mine. Therefore, it should 

be possible to use the smaller bolts without any discernable 

diminution in performance. The annulus results were less

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.10 0.15 0.20 0.25 0.30

Annulus (in)

Grip Factor (tons/in)

0.677-in Bolts

0.804-in Bolts

0.914-in Bolts

Figure 11. The Grip Factor for 8 ft long bolts with various 

annulus distances determined in test site E.
conclusive. While there has been some indication that the 

optimum annulus is approximately 0.125 inches, the evidence is 

inconsistent. It seems that at San Juan Mine, finding the proper 

anchorage horizon may be more important than optimizing the 

annulus.

The proactive research efforts continue at the San Juan mine 

to improve roof bolt performance. Current efforts are focused on 

evaluating the effects of borehole roughness on anchorage

performance, through the use of rifled drill holes. In addition, 

equipment has recently been purchased to obtain geological cores 

from the test areas, and to over-core tested bolts for inspection. 

The goal of the program is to optimize the roof support systems, 

which will directly improve the safety of the work environment.

REFERENCES

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

Implementation and Evaluation of Roof Bolting in MICARE 

Mine II. Proceedings, 16th International Conference on 

Ground Control in Mining, Morgantown, WV, August 5-7, 

1997, pp. 139-148.

2. Tadolini, S.C. The Effect of Reduced Annulus in Roof

Bolting Performance. Proceedings, 17th International

Conference on Ground Control in Mining, Morgantown, WV, 

August 4-6, 1998, pp. 231-236.

3. Fabjanczk, M.W. and Tarrant, G.C. Load Mechanisms in 

Reinforcing Tendons. Proceedings, 11th International

Ground Control in Mining Conference, Wollongong,

Australia, July 7-10, 1992, pp. 1-8.

4. Campoli, A., Mills, P., Todd, P. and Dever, K. Resin 

Annulus Size Effects on Rebar Bolt Pull Strength and Resin 

Loss to Fractured Rock. Proceedings, 18th International 

Ground Control in Mining Conference, Morgantown, WV, 

August 3-5, 1999, pp. 222-231.

5. Mark, C., Compton, C.S. Oyler, D.C. and Dolinar, D.R.

Anchorage Pull Testing for Fully Grouted Roof Bolts.

Proceedings, 21st International Ground Control in Mining 

Confe rence, Morgantown, WV, August 6-8, 2002, pp. 105-

113.

6. Karabin, G.J. and Debevec, W.L. Comparative Evaluation of 

Conventional and Resin Bolting Systems. MESA IR 1033, 

1976, 22 pp.

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

Study of Shear Loading and Bolt Load Transfer Mechanisms 

under Constant Normal Stiffness Conditions. Proceedings, 

18th International Conference on Ground Control in Mining, 

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

247.