25th International Conference on Ground Control in Mining 

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ABSTRACT 

 The International Ground Control Conference in Mining has 

always provided an open forum for the publication, presentation, 

discussion, and often heated debate on roof bolting systems mostly 

with attention to how, when, and why they work. During the last 

24 Conferences at least 170 papers have been presented by 

International Experts on these “simple” devices that range in length 

from 30-inches to 20 ft and from 5/8 to 2 inches in diameter. Roof 

bolts are primarily mechanically anchored, glued, cemented, or 

driven. Bolts are placed vertically, angled, or tied together with 

special fixtures (trusses and slings). They have been termed active, 

passive, stick-slip, and several other descriptive acronyms or 

mining slang expressions. This retrospective paper will present the 

changes in bolt types and usage, point out some of the biggest 

myths and hail the most significant advances (according to the 

authors’ opinions of course). 

INTRODUCTION 

 Ground control, like golf, is an eternal conflict between elation 

and despair. Our favorite response to “How did you shoot?” upon 

returning to the clubhouse after a beautiful day at the Lakeview 

resort is a rousing “I think I found the secret and know what I’ve 

been doing wrong!!” Inevitably, the next day the response is a 

hopeless “I think I lost it again.” Secrets and successes in both golf 

and ground control come and go rapidly. You find one secret and 

lose it between the 8th and 9th hole or in as short of a distance as the 

next break or cross-cut. And even if you rediscover the magic, it 

doesn’t help to have a bunch of secrets stored in your head just as 

you are about to start your backswing or develop a new miner 

section. The brain just can’t handle “straight left arm, keep your 

head down, follow through to the target” all at the same time. “Just 

hit it or just mine it” seems to work the best from both a golfing 

and operational perspective. Unfortunately, as if getting to the 

green isn’t difficult enough, you are often faced with a 5 ft putt 

where things can really go wrong. You push’em, pull’em, and 

often leave them short. Of course every bad shot on a golf course 

usually makes someone in the foursome happy. Unfortunately, 

mistakes made in an effective ground control program can have a 

devastating instant effect and negative repercussions for the safety 

of the miners or the life of an entire operation. 

 If we have learned anything at the International Ground Control 

Conference in Mining (IGCCM) over the past 24 years as we head 

into our Silver Anniversary, it is that some things have changed 

tremendously while many others have simply stayed the same. 

Case study after case study has revealed that the practical or 

commonly used theories show that little can be done with roof bolts 

to prevent rock movements, especially for the elastic part, around 

an underground coal mine opening. Of course, some people have 

argued that massive, preloaded supports, which are difficult (slow) 

to install and can be quite expensive can prevent or at least 

minimize movements. The competent ground control engineer tries 

to install the supports as quickly as the entry is developed to take 

advantage of the movement that will indirectly load the supports 

and prevent or minimize additional downward roof movements. 

Some quick snapshots of bolt usage may provide some insight into 

what developments have been made in the last 25 years. 

ROOF BOLT USAGE TRENDS 

 An analysis of roof bolt usage may provide valuable insight 

into the development of the systems and what has proven to be 

most effective over time. The IGCCM has been the stage for the 

introduction, description, and subsequent analysis of several of 

these bolting or roof support systems. While occurring after the 

beginning of the IGCCM in 1980, an estimated roof bolt count was 

presented that summarized usages in 1988 (Scott, 1989). Scott 

discussed the trend of using higher quality roof bolt systems that 

featured higher strength steels, matched bearing plate strengths, and 

complex anchorage devices that included the use of mechanical 

shells and resin. The picture painted by Dr. Scott is much different 

than the statistical sampling of roof bolt usage presented in 1976 

from reportable Mine Safety and Health Administration (MSHA) 

data (Karabin and Debevec, 1976). That report was quite simple - 

of the estimated 100 million roof bolts used that year 80% of the 

bolts used were mechanical anchored bolts and the remaining 20% 

were fully grouted (resin or cement). Figure 1 shows the 

distribution of roof bolt usage in the U.S. in 1988. In 1988, 

approximately 85 million bolts were used in underground mining 

applications. Resin rebar accounted for 34 million or 40% of all 

roof bolts used. Mechanical anchored bolts (commonly called 

Conventional bolts) accounted for 35% of bolt usage or about 30 

million units. Resin-Assisted Mechanical Anchor bolts are 

mechanically anchored bolts whose ultimate anchorage has been 

enhanced or transformed with the addition of a resin plug used at 

TWENTY-FOUR CONFERENCES; MORE THAN ONE-HUNDRED AND 

SEVENTY PAPERS; UNDERSTANDING ROOF BOLT SELECTION AND 

DESIGN STILL REMAINS PRICELESS

Stephen C. Tadolini, Acting Chief, Rock Safety Engineering Branch 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA, USA 

Raymond A. Mazzoni, Mechanical Engineer

MSHA-Pittsburgh Safety and Health Technology Center 

Pittsburgh, PA, USA 
25th International Conference on Ground Control in Mining 

383 

the top of the bolt. These systems are installed using the same 

techniques as a mechanical bolt with a resin cartridge placed in the 

hole before the bolt is inserted. Resin Assisted Mechanical Anchor 

bolts accounted for about 14% or 12 million bolts. Combination 

bolts, essentially a two-piece bolt, commonly used when the mining 

height is less than the required support height, accounted for 3 

million bolts (3.5 percent), and 3 million tension-rebar bolts were 

used and tallied 3.5 percent of the annual usage that year. The 

remaining 15 million bolts included friction stabilizers (Swellex 

and Split-sets), trusses/slings (both rigid and cable), Dyna-Roc’s, 

Fiberglass and wooden dowels. 

 A similar survey was conducted for this paper to report the 

estimated roof bolt usage in the U.S. for 2005 and results are shown 

in Figure 2. Resin rebar accounted for about 68% of bolt usage, 

which was an increase of 28% from the 1988 survey. Mechanical 

anchored bolts dropped to about 4% of support types used and 

Resin-Assisted bolts also decreased from about 14 to 9%. Tensionrebar bolts experienced an increase from 3.5 to 12%. Friction 

stabilizers, used primarily in hard rock applications, accounted for 

4% of the bolts used in 2005. Cable bolts (about 1 million units) 

and Cable Slings and Trusses account for the remaining 1.5% of 

roof support usages. 

 The most significant difference between the 1988 and 2005 

survey results, besides roof bolt usage trends that will be discussed 

in more detail, is the decrease in the number of bolts used in the 

U.S. underground mining applications. The 1988 survey estimated 

the number of units at 85 million and in 2005 that number was 

reduced to about 68 million bolts. This reduction in roof bolt usage 

can be partially explained if a quick analysis of underground (UG) 

coal mine tonnage is completed. In 1988 the total number of UG 

coal mines was 1,768 and produced 376 million tons. Of the 

1,768 mines, 1,696 were room and pillar operations, which 

accounted for 246 million tons, and 72 were Longwall mines that 

produced 131 million tons or 34.7% of the total UG production. In 

2005, a total of 606 mines were active; 562 room and pillar mines 

accounted for 176 million tons, while 44 Longwall mines produced 

190 million tons or 52% of the total 376 million tons. While the 

total of tons produced in UG coal mines has remained relatively flat, 

longwall production increased 45 percent. Of course roof bolt 

usage is greatly reduced in longwall operations when considering 

the number of bolts per ton of coal. In general, the number of roof 

bolts used is about 4 times higher for room and pillar than longwall 

mining for the same extracted tonnage. The reduction of 70 million 

tons in Room and Pillar operations from 1988 to 2005 off-set by 

Longwall mining would account for about a 10 percent usage 

difference of 8.5 million roof supports. This only accounts for 

about half of the reduced 17 million bolts, and additional theories 

and hypotheses could be presented but are highly speculative and 

beyond the scope of this effort. 

WHAT HAS WORKED, WHAT HAS NOT, WHAT IS 

CURRENTLY CHANGING, WHERE ARE WE NOW? 

Mechanical Anchored Bolts 

 An examination of roof bolt usages does provide some valuable 

insight into what has worked and what has not in mining 

applications, particularly coal. Obviously the reduction of 

mechanical anchored bolts has been dramatic. The mechanical 

anchored bolt, as shown in Figure 3, can be quickly installed and 

anchorage capacities up to 25,000 lbs have been achieved. The 

rock strength, more specifically shear strength, always controls the 

anchorage and the amount of bolt tension that can be applied by the 

mechanical anchor. The real critical issue of the system is the 

“bleed-off” or loss of tension after embedment and mechanical 

relationships of the strata and bolting components occurs was 

examined at the First IGCCM (Mahyera et al., 1981). The report 

examined several techniques to control the thrust and minimize the 

variability in installation torque and subsequent bolt tension, as 

well as all the “frictional” factors that can impact effective 

installations. At the Fourth Conference Maleki described the use of 

hardened washers, lubricated threads, facing the hole surface (a 

complicated process to account for bending), and installing the bolt 

with additional torque so the “predicted” subsequent bleed-off was 

at an acceptable limit. The conclusions stated that installed 

tensions were normally 50% less than those required to ensure 

proper bolt installation and subsequent performance (Maleki et al., 

1985). 

Figure 1. Distribution of roof bolt usage in the U.S. in 1988 (Scott). 

40%

35%

14%

4%

4%

2%

1% Resin Rebar

Mechanical

Resin-Assisted

Combination

Tension-rebar

Friction Stabilizers

Others

68%

4%

9%

1%

12%

4% 2%

Figure 2. Estimated roof bolt usage in the U.S. in 2005. 
25th International Conference on Ground Control in Mining 

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 While several technical concerns have been evaluated and 

solutions developed and published, the decline in the use of 

Mechanical anchored bolts can also be largely attributed to the 

required installation issues and subsequent torque measurement 

requirements cited in the CFR75.204(f)(4) and (5). This regulation 

requires that in each roof bolting cycle, the actual torque or tension 

in the first tensioned roof bolt with each bolter head be measured 

immediately after it is installed. Thereafter, at least one out of four 

bolts is checked after installation and if the place is open for 

24 hours, at least one out of ten bolts is checked from the outby 

corner of the last open crosscut to the face in every advancing 

section. The labor required, coupled with the lower bit costs for a 

1-inch diameter hole, to ensure quality bolt installations simply out 

weighs the costs of alternative supports. This fact, coupled with the 

introduction, performance, and subsequent approval of no. 5 resin 

rebar, led to large decreases in mechanical bolts and the increase in 

resin-rebar usage. 

Resin Rebar Bolts 

 

 At the Fourth IGCCM, Serbousek reaffirmed the concept that 

resin grouted bolts, illustrated in Figure 4, did not anchor because 

of chemical bonds or adhesion. Rather, it is the mechanical 

interlock between the rock-resin-and bolt interface that is the most 

important parameter and develops the reinforcing mechanism used 

for anchorage and bolt strength development today (Sebousek and 

Signer, 1985). This work served as the foundation for research that 

has culminated in the lively and often debated topics of glovefingering, annulus thicknesses, pull-test anchorage capacities, bolt 

stiffness, etc. The system basically works because of superior 

anchorage and stiffness that develops as a result of the full bolt 

length resin anchor. The high stiffness is accomplished because of 

the full contact of the resin anchor and the ability of the resin to 

quickly transfer the developed loads, via the bolt, back into the rock 

mass. Because of the bolt’s superior stiffness, a significant 

resistance to rock movement is developed both axially and laterally. 

Loads that are developed along the bolt, if roof separations occur, 

are quickly transferred back into the rock and the movement is 

resisted at the parting or separation. As mentioned, the annulus or 

the distance between the rock and the bolt has been adequately 

debated. Research results show that effective load transfers can be 

developed with annulus thickness or distances as small as 

0.0625 inch (Tadolini and Hendon, 1998) or optimized at 

0.125 inch which accounts for fluctuations in rebar size, potential 

resin losses due to installation pressures, and some mixing issues 

(Campoli et al., 1999). Researchers fundamentally agree that there 

are two critical aspects to performance of the resin rebar bolts 

based on the resin annulus distance; the correct bolt installation 

(annulus, mixing, hold-times, etc.) and the load transfer along the 

bolt. Both can be negatively impacted by a large annulus size 

which is the distance from the borehole wall to the rebar. As 

mentioned, the no. 5 resin-rebar system (5/8-inch diameter) has 

served as the primary replacement support for mechanical anchored 

bolts. No. 5 rebar bolts are routinely installed in a 1-inch diameter 

hole, the annulus for this specific system is 0.1875 inch. 

Installation solutions and bolt modifications have been introduced 

to ensure adequate mixing of the resin. However, the transfer 

mechanics of the bolt can be slightly diminished and should be 

considered if large roof movements and high loads are expected 

(Rico et al., 1997). Also, this is important, there is a significant 

difference in the shear capacity and system stiffness between a 

no. 5 and the more “ideal” no. 6 rebar systems. 

 No instrument or system has been developed that can be 

routinely used to determine the effectiveness of resin rebar bolt 

installation; not resin mixing, simply the assurance that the bolt is 

fully grouted (if that is the specific application). At the Second 

IGCCM, a device called “The Roof Bolt Bond Tester” developed 

under contract by the U.S. Bureau of Mines, was introduced that 

utilized ultrasonic energy weightings that were turned into green, 

yellow, and red lights that indicated 100%, 50 to 75%, and less than 

50% resin contact to the bolt (Stateham, 1982). Unfortunately, the 

Roof Bolt Bond Tester system later proved to be no more than a 

random color generator and never was modified or successfully 

marketed. Because no systems are available to perform nondestructive testing of resin-grouted bolts, this may still be a 

potential research area. 

 In summary, resin rebar bolts have proven to be extremely 

effective in several applications. Rebar bolts are particularly 

effective for supporting highly stressed, weak roof strata with the 

length being increased for high overburden depths. They are easy 

Figure 3. Mechanical anchored roof bolt (high-tension, low 

anchorage capacity, low system stiffness, low-shear resistance). 

Figure 4. Resin rebar bolt (low-tension, high anchorage 

capacity, high system stiffness, high-shear resistance). 
25th International Conference on Ground Control in Mining 

385 

to install; even in restricted seam heights by bending the bolts, and 

they are cost-effective and provide superior anchorage and support 

stiffness. 

Tension-Rebar Bolts 

 A Tension-rebar bolt is the best of both worlds - it is essentially 

a resin-grouted rebar bolt that is tensioned upon installation. The 

support system consists of rebar that is threaded on the end to allow 

a nut to be installed and tightened against the bearing plate. While 

it is still very common that only the top portion of the bolt be 

anchored in resin, the maximum stiffness or complete benefit of the 

system is attained when two speeds of resin are used to achieve a 

full-column or complete anchorage of the bolt. To achieve a fullcolumn of resin and still be able to tighten or tension the bolt, the 

top portion is anchored with a fast setting resin and the bottom with 

a slower resin. Current resin manufacturing processes limit this 

application to a two cartridge system. As the bolt is inserted into 

both cartridges of resin, a delay mechanism on the nut or reverse 

rotation is used to mix the resin thoroughly. The fast setting resin 

sets and the bolt is tightened before the lower or slower setting 

resin has had a chance to cure. This permits the lower portion of 

the mine roof to be placed in compression, distributing the forces 

over the lower portion or length of the bolt, as shown in Figure 5. 

These systems combine the positive characteristics of a mechanical 

bolt and the superior anchorage developed in a resin rebar bolt. 

Maleki reported that tension-rebar bolts were extremely effective in 

supporting a difficult tailgate and even maintained loads, recorded 

with strain gages along the bolt axis, after the heads had been 

sheared off! The mechanical interlock created along the entire 

length of the bolt was high enough to redistribute the applied 

bearing plate loads (Maleki et al., 1986). Without a bearing plate, 

fully grouted bolts can still function and resist rock movements. 

However, the control provided by the plate and the subsequent 

loads measured between the mine roof and bearing plate indicate 

that plates can add significantly to the roof support. Adequate 

plates help resist roof movements in the lower 2 ft of the roof and 

are an important support element in roof reinforcement (Tadolini 

and Ulrich, 1986). 

Resin-Assisted Bolt 

 Resin-assisted bolts (sometimes called point anchors) are 

essentially mechanical anchor bolts that have been transformed 

with the addition of a resin anchor at the top of the system. They 

can be installed like a mechanical bolt with the addition of a 

cartridge of resin placed in the hole before the bolt. As the bolt is 

inserted, the resin flows around the shell and then the bolt is 

tightened against the mine roof. Because the bolt is installed with 

resin, it experiences less tension bleed-off and a superior final 

anchorage than achievable with a mechanical shell system. A 

typical installation is shown in Figure 6. The bolts are used widely 

and successfully in difficult ground conditions. 

 At the Second IGCCM in 1982, White published and presented 

comprehensive details on several types of resin-assisted systems 

being manufactured and evaluated by the Birmingham Bolt 

Company working closely with Jim Walter Resources. A very 

insightful portion of the conclusion stated, “We must base 

decisions on roof control and roof bolting in particular, largely on 

experience and far too often on few “knowns” and many 

“unknowns”. Point anchors are not, of course, the answer, but are a 

very reliable tool” (White, 1982). 

 These bolting systems are broadly used today and incremental 

improvements have been made and presented throughout the 

history of the Conference. In 1990, Stankus reported on the design 

and development of the Jennmar Corporation “Insta’l 

Compression” system that incorporated anti-friction washers 

between the bearing plate and the bolt head, resin compression 

rings, and a new design of expansion shell. Incremental 

improvements in the systems by several manufacturers and positive 

case studies have been reported (Stankus, 1990). In 1997, Stankus 

presented a new concept and design criteria, utilizing these high 

installed tensions. At that time Stankus introduced the term OBE 

or Optimum Beaming Effect using Resin-Assisted bolting systems 

defining OBE as a roof beam that had no separations within and 

above the beam with the shortest roof bolt possible. The analysis 

was largely generated by using finite-element modeling techniques 

that utilized a one-dimensional truss element with a simulation of 

loads between concentrated points (Stankus and Guo, 1997). While 

Figure 5. Tension rebar bolt (high-tension, high anchorage 

capacity, high system stiffness, high-shear resistance). 

Figure 6. Resin-Assisted bolt (high-tension, high anchorage 

capacity, low system stiffness, low shear resistance). 
25th International Conference on Ground Control in Mining 

386 

the modeling results were reported to be different than the 

magnitudes obtained from field measurements, the trends were 

determined to be the same. Furthermore, although the theory was 

based on the condition that no separations could occur within or 

above the beam, the authors reported that from real practice we 

know that there can be some bed separations while maintaining 

stable roof. Reducing the bolt length in difficult ground conditions 

is contrary to current roof bolt support design criteria. The 

utilization of the OBE design criteria has not been reported on and 

has not gained acceptance. 

Cable Bolts 

 While not included in the 1989 roof support usage estimates, 

cable bolts have become an extremely useful ground control 

technique for controlling mine roofs. High-strength cables are 

being used in two basic applications, slings and vertically, to 

support coal mine roofs. The estimated number of vertical cables 

used in 2005 was one million, a testimonial to the success of the 

support system. At the First IGCCM, Scott presented a paper that 

described the application of the Scott Cable Sling, a one-piece 

strand of cable drive and held into place by Split Set Stabilizers 

(Scott and Castle, 1981). Cement cartridges were placed into the 

borehole prior to insertion and allowed to cure to provide long-term 

anchorage and enhance the performance. The conclusions stated, 

“In many cases it should be considered as a quick-fix “band-aid” 

support to eliminate the ground control hazard so that other, stiffer 

and/or stronger supports may be placed. In other cases, it may 

prove to be cost effective and advantageous to use Scott Cable 

Slings as a complete substitute for traditional timber, steel or truss 

support methods.” 

 Cable slings (often incorrectly called trusses) can be installed 

over the ribs into competent strata to develop the capacity of a 0.5 

or 0.6 inch diameter cable. Unfortunately, field measurements 

have indicated that while it is possible to install a cable sling with 

4-6 tons of load using the bolting machine or hydraulic jacking 

systems, subsequent monitoring indicates that the load is quickly 

dissipated. Cable sling applications should be regarded as purely 

passive supports that perform adequately when the reaction is 

downward into the entry. Cable slings, or traditional trusses for 

that matter, are not as effective when the entry movements are 

horizontal which dissipates the tension, directly reducing the 

support forces. The stiffness of a cable sling is directly related to 

the free-lengths of cable(s) used in the application. Other 

extremely important factors include the installation angle over the 

pillar, the location of the “angle” bolt with respect to the rib-line, 

and pull-in or slippage in the barrel/wedge anchors. Both two and 

three-piece wedge assemblies are only effective when they are 

evenly “buried” deeply into the barrel which permits full-teeth 

penetration into the strand. This position can only be achieved 

when the strand tension is high and delivered at the back of the 

barrel, and the wedges can slide freely into the barrel. Installation 

issues and incremental improvements in sling design and 

application were summarized at the 23rd IGCCM (Pile et al., 2004). 

This specific system, which is closely related to a truss system, 

used bar/bolt systems for the angle bolts but the mechanics and 

cable tightening techniques remained largely unchanged. The 

biggest advantage of cable truss systems is that the angle bolt, 

usually a resin assisted or tension rebar bolt, places the immediate 

roof into compression prior to the tensioning of the cross member. 

This helps establish a “roof beam” which resists downward 

movements during subsequent loading. Cable slings are still used 

in several applications and Operations personnel are familiar with 

the drawbacks (shear failures of individual strands and tension 

losses) and benefits (roof suspension and high deformation 

capabilities) of the systems. Figure 7 shows a typical cable sling 

failure resulting from combinations of vertical and horizontal roof 

movements. 

 Vertical cable bolts have been extremely successful in 

underground mining applications with the paradigm shift coming to 

fruition when traditional bolting equipment and resin grouted 

systems could be used to rapidly install vertical cables that have 

ranged in length from 8 to 24 ft. The first applications of resingrouted cable bolts were reported on at the 13th IGCCM when three 

different systems, passive, stiff, and tensionable, were installed to 

support an active underground longwall tailgate. Of course the 

development work was completed before and concurrently with the 

test area installation by the recognized inventor of resin-grouted 

cable bolts, Doug Gillespie, owner of Rocky Mountain Bolt 

Company and George Seely, Sales Manager of Fosroc (now 

Minova), who are both now happily retired. The development team 

diligently overcame installation problems, resin mixing issues, the 

challenge of developing adequate bolt anchorage, and cable 

insertion problems (Tadolini and Koch, 1994). Since that time, a 

plethora of research has been completed on vertical cable supports 

and applications. The capacities of cables currently used range 

from 20 to 65 tons and are installed with no additional tension, high 

post-tensioning with jacking system, low-tension using expansion 

shell technology and even post grouting of the cables after tension 

is applied. Forty-one technical papers presenting research results 

on cable bolts from around the world have been published at the 

IGCCM since 1991. These reports detail the mechanics of cable 

bolts, present underground case studies and describe laboratory and 

numerical modeling results. The IGCCM has served, and will 

continue to serve, as a technical discussion platform for this 

multifaceted, but highly effective, support system. 

OTHER EAGLES AND BOGEYS 

 While it is difficult to summarize all of the ground control 

support system types and selections in this short retrospective paper, 

a few items remain that are worth mentioning. These include rigid 

truss systems, pull-tests, roof bolt design systems and MSHA’s role 

in the roof support approval process. 

Rigid Truss Systems 

 Rigid roof truss systems have been discussed in 24 papers at the 

IGCCM, often in great detail. The introduction of roof truss 

Figure 7. Cable sling failure in mine entry subjected to vertical 

and horizontal loading. 
25th International Conference on Ground Control in Mining 

387 

systems can be traced back to the patent of C. C. White in 1969 

(Birmingham Bolt Company). Correctly stated, the roof truss 

systems were used primarily in weak roof areas to artificially 

“narrow” the width of the entry span. The recommended 

installation was about 3 to 5 ft from the rib-line (depending on the 

entry width) on both sides that were linked together by using 

hardware attached to a cross-member. At the Second IGCCM, 

Bollier reported that Peabody had purchased and installed over 

180,000 trusses and Birmingham Bolt Co had supplied over 

1 million systems to 50 underground mines in 1975 (Bollier, 1982). 

At the same conference, Mangelsdorf presented the results of an 

extensive laboratory investigation, funded by the U.S. Bureau of 

Mines, presenting component tensions and the complex interactions 

that occur at the bearing plate and angle bolt (Mangelsdorf, 1982). 

As shown in Figure 8, the descriptions of the active or compressive 

forces can be quite misleading (Cox, 2002). In fact, current 

research indicates that tensioned bolts, installed with high 

installation torques that generate high bolt tension, used in some 

systems for the angle bolts, can provide an active compression 

component to a maximum depth of about 36 inches (Zhang and 

Peng, 2004). The compressive forces developed in the roof 

through the tightening of the cross-member remain unproven. Roof 

Truss Systems move in and out of favor depending on incremental 

improvements in component and installation designs. The systems 

are effective in supporting previously supported roof that is 

subjected to downward deflections. No longwall tailgates in the 

U.S., usually the highest loading levels in coal mine applications, 

are currently using roof truss systems solely for secondary roof 

support. 

Pull-Tests 

 Pull-tests, commonly called anchorage capacity tests, are 

performed using pull test equipment. Basically, the system consists 

of a hydraulic jack that pulls the bolt from the developed anchorage 

while the loads and deformations are recorded. Load vs. 

deformation plots are created and the “yield” of the anchorage or 

the bolt is recorded. At the Fifth IGCCM, held in 1986, two papers 

were presented that utilized pull-tests data in very different ways. 

Cincilla calculated the effective column lengths, called ECLs, an 

index property, by performing pull-tests on over 1,000 bolts 

varying the anchorage length between 12 and 48 inches on a 48-

inch long bolt. The obvious conclusion stated that longer lengths 

of resin were required in weaker rock types to meet the established 

criteria of 8 tons of applied load while maintaining less than 

0.2 inch of displacement. The unique conclusion was that there 

appeared to be a relationship between the Rock Mass Rating (RMR) 

and the determined effective column length, a premise that has 

evolved into the routine evaluation of shorter columns of resin (or 

even shells) on tested bolts to determine the tons/inch of anchorage 

for design considerations and final support selection (Cincilla, 

1986). 

 At the same conference, Kempen presented a method to design 

an efficient bolting plan based on simple in-mine measurements 

that included pull-tests (Kempen et al., 1986). The author 

estimated that the pull strength of a typical mechanical anchor was 

about twice the compressive strength of the host material, which 

appears reasonable. He also presented a concept that stated that 

average installation loads and subsequent bolt tensions, combined 

with pull-tests, could be used to determine safe life stand times for 

underground openings. This conclusion was based on the safe life 

peak, which was a function of strength (estimated from the pull-test 

results) of the anchorage horizon. The design technique is complex 

and worth reviewing in detail. However, the use of pull-test or 

anchorage capacity testing results in this manner requires an 

unproven extrapolation of data. The author states, “Standard Unit 

Roof (SUR) control technology is intended to complement, not 

substitute for seasoned judgment and underground experience.” In 

reality, a properly performed pull-test can provide an index value 

that has been mathematically related to the shear strength or 

capacity of a specific bolt, in a specific hole, at a specific horizon. 

Any use beyond that should be considered a statistical crapshoot. 

Roof Bolt Design Systems 

 With regard to roof support selection government regulations 

usually dictate minimum requirements, but offer little guidance for 

determining an optimum support design. In most instances roof 

support is chosen based on past experiences of mines in the area or 

the familiarity of the mine operator with a specific support. 

Numerous papers have been presented at the IGCCM regarding 

support design, but for the most part most of these papers focus 

primarily on the analysis of current roof support efforts at a specific 

mine with the intent of improving support selection for the future 

development of that particular mine. These papers use a variety of 

procedures to achieve their goal including geologic analysis, stress 

evaluation, pull-tests and even monitoring of actual support loading 

utilizing strain gaged roof bolts. Very few of the papers presented 

at the conference have attempted to develop a support design 

methodology that a mine engineer could use to design a support 

system for a new mine or section. One of the first to attempt this 

was presented at the Fifth IGCCM and utilized the Standard Unit 

Roof (SUR) which was previously mentioned (Kempen, 1986). 

However, the reliance on monitoring conventional bolts would 

make this method somewhat dated. 

 At the 8th and 12th IGCCM, Unal and Ozkan outlined the design 

and subsequent verification of a computer program that could be 

used to select support systems (Unal, 1989 and Unal and Ozkan, 

1993). After calculating a modified RMR the authors utilized the 

computer program (Rockbolt 5.0) to evaluate a set of input 

parameters that included the unit weight of the overburden and 

immediate roof, roof span, and horizontal to vertical stress ratio in 

addition to roof geology parameters. The output generated would 

provide the user with support length, diameter, spacing, and grade, 

as well as the anchorage capacity necessary to successfully support 

the roof. Although reporting that their studies demonstrated that 

the design criteria provided a realistic means of determining rockFigure 8. “Theoretical” compression forces produced by a 

roof truss (Cox, 2002). 
25th International Conference on Ground Control in Mining 

388 

reinforcement for the case study, they correctly concluded that 

“further studies and feed back cycles are required to explore the 

effectiveness of the design approach in different mining 

conditions.” 

 The National Institute for Occupational Health and Safety 

(NIOSH) presented the most recent support design procedure at the 

20th IGCCM (Mark et al., 2001). In this approach support selection 

is determined by utilizing a set of empirical formulas that were 

statistically derived from analyzing roof falls in nearly 100 case 

histories under a variety of different mining conditions. Two 

noteworthy findings were a statistical relationship between depth of 

cover (related to stress) and CMRR (a measure of roof quality). 

The roof fall data indicated that when the CMRR was less than 40 

at shallow cover, and less than 45-50 at deeper cover, both 

essentially moderately weak roof conditions, high roof fall rates 

could be encountered even with relatively high roof bolt densities. 

As intersections (sum of the diagonals) in weak roofs increased, the 

likelihood of roof falls also increased. As with the previous two 

papers mentioned related to support design, the authors concluded 

that “much more remains to be learned to further improve the 

efficiency of roof bolt design.” 

MSHA Technical Support 

 The Mine Safety and Health Administration (MSHA), primarily 

the Roof Control Division of Technical Support, has been 

instrumental in performing laboratory and field testing for various 

roof bolt types, resins, and critical support components; such as 

bearing plates, expansion shells, threaded nuts, etc. The role that 

they perform to ensure that roof bolt components meet or exceed 

ASTM Standards, to review installation and use applications, and 

analyze failed materials is critical to the safety of all underground 

miners. There are no ASTM Standards for bolting systems 

comprised of “parts” that met or exceeded ASTM testing 

requirements. However, and it appears in technical papers 

throughout the IGCCM Proceedings, you see statements saying, 

“”ASTM approved” or “MSHA has approved this support” or “this 

bolt has been approved by MSHA,” it is simply not true. All roof 

bolts and subsequent installation applications (spacing, length, 

diameter, etc.) are presented in the mines Roof Control Plan that is 

approved by the MSHA District Manager (where the mine is 

located). MSHA-Technical Support may have indeed tested the 

components of the bolt, witnessed the installations or even 

monitored the performance of the final systems; however, no 

“blanket” approval is provided to the manufacturer or mine 

operator. If you read that, be very skeptical. 

SUMMARY OR THE 19th HOLE 

 In golf, after you have bought all of the latest equipment, taken 

the lessons and joined that golf league your game reaches a certain 

level. Sure you can put together a good game or hit a great shot on 

occasion, but then on the next round the bogeys and double bogeys 

mount up and you just can’t figure out what happened. Golf and 

roof control require a commitment to make it to the next level. In 

order to become a more consistent golfer you have to focus on the 

weak parts of your game and work on them until they have been 

mastered. If you just buy the latest “gadget” club rather than spend 

the time to learn the game you will always be an amateur (aka 

“duffer”). It’s the same with roof support design, it requires focus 

and commitment. Most of the papers presented at the IGCCM over 

the years have one thing in common. They all conclude that further 

research is needed in the particular area that they report on. The 

focus and commitment however, seem to be lacking. Promising 

research is typically rarely continued and the direction of the 

research seems to lack focus. Rather than spend the extra effort 

needed to take promising research to the next level, the authors take 

a totally different direction or report on the latest “gadget” roof 

support. There have been great strides taken in ground support, but 

we’re all still just duffers. We occasionally have that good “game” 

but we definitely haven’t approached “pro” status yet. One 

reviewer reminded us that while the bolting “hardware” has 

progressed nicely, the bolting “software” still lags behind. Papers 

published throughout the conference on bolt monitoring and 

modeling have made valuable contributions toward understanding 

complex support mechanisms, bolt selection and design. Due to 

the complexity of roof geology and the variations of rock properties 

under various stress and physical environments, no universal design 

tools for roof support design has or may ever be developed. 

 The timely vision of Professor Syd S. Peng to establish this 

conference was to promote closer communication among 

researchers, consultants, regulators, manufacturers, and mine 

operators. The cumulative experience that we continue to share, 

discuss and debate with each other is extremely important to attain 

our goals. We believe that the continuation of his vision, combined 

with the hard work and dedication of these groups, will eventually 

result in a comprehensive understanding of roof bolt selection and 

design solutions that improve the safety, productivity, and 

economics of mining throughout the world.....now we have to get 

to the No. 1 tee. 

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