rock bolting

8/10/2019 Rock Bolting

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Coal International, Vol. 249, Issue 1, 30-33 (Jan/Feb2001)

29.03.2007 .

V!"VI!# F $.%. "C& 'I*+.

J. Luo, C. Haycocks, M. Karmis

bstrat

Focuses on mine roof rock bolting practices in the United States.

Advantages of rock bolting over other supporting systems;

Mechanisms underlying the binding effects of rock bolts; Design

of roof bolting.

&eors

MI! roof bolting

A revie" of bolting practices in the United States sho"s that the ma#or emphasis in

bolting development has taken place in increased anchor capacity and length. An

understanding of bolting theory for coal and hard rock mines is no" progressing

beyond $ualitative ideas of minimi%ing inter&strata shearing or direct suspension'

into (ptimum )eaming !ffect *()!+' "hich are leading to improved bolt

applications. ,here has also been an increased understanding ho" bolting can

contribute to or cause roof failure under some conditions. In hard rock

applications' including slopes' interlocking compression %ones are used to stabilise

the rock mass. ,echni$ues related to traditional bolting methods' including such

support systems as cable bolts' trusses and split sets' are e-amined in terms of their

uni$ue applications and comparative support capacities.

For centuries' underground roof support methods "ere e-ternal and passive. In

/01 a metal mine in the U.S. began using a ne" support technology2 very

primitive steel slot&and&"edge rock bolts34. ,his "as the first time that internal

reinforcing stresses "ere applied to roof strata' making the support system active.

In /56' 7eigel 304 proposed the basic concepts of roof bolting as a systematic

method to support "eak roofs. Some of his ideas about roof bolting are still thefoundations of modern bolting theories and application guidelines. ,hese are2

o Support "eak rock belo" the natural arch line *Suspension+;

o )olt "eak' thin strata together to create thicker' stronger strata *)eam building+;

o )olt early in the mining cycle.

In an attempt to reduce the number of accidents caused by roof fall' the U.S.

)ureau of Mines *US)M+ advocated the use of roof bolting technology in /51. In

less than t"o years' more than 088 mines employed this ne" roof support method.


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)y /90 annual roof bolt consumption had reached 09 million bolts. )y /:' 99

million bolts "ere used annually by /0 coal mines' and :8< of coal production

took place under bolted roofs. In the /18=s' a rapid increase in the use of roof bolts

"as triggered by the /:/ U.S. >oal Mine ?ealth and Safety Act' "hich re$uired

that roofs and ribs of all active underground road"ays' travel"ays' and "orking

places be supported or other"ise ade$uately controlled to protect persons from fallof roof or ribs. As a result' most entries in underground coal mines "ere supported

"ith roof bolts. In /5' the US)M estimated that about 08 million roof bolts

"ere used and over /8< of underground coal production took place under bolted

roof.

In addition to the effectiveness of ground control and cost reduction' some

advantages of roof bolting over other supporting systems significantly enhance its

application. ,hese advantages mainly include2

o @educing storage and material handling re$uirements;

o Decreasing the si%e of the opening that is needed to achieve the same given

clearance;

o reventing any appreciable roof deformations by $uick installation after opening

is made;

o Improving ventilation by lo"ering the resistance to the air via elimination of

obstructions' such as cribs' posts' and girders;

o roviding greater freedom for trackless vehicles "ithout risk of dislodging

supports;

o roviding natural supports for hanging pipes' tubes' and electrical cables;

,oday' rock bolting not only is "idely used in underground coal mines to support

entries primarily and secondarily' but also finds applications in surface mining'

hard&rock mining' tunneling' civil engineering' and almost every"here ground

stability is involved.

@ock )olt ,ypes

)olts reinforce the rock by binding the stratified or broken rock layers or blocks

together ,he binding effect is achieved through the friction forces created by the

physical interlocking along the anchor and rock interface. )ased on the basic

anchor types' bolts can be categorised as + point&anchored' and 0+ full&length&

grouted. ,he upper end of a point&anchored bolt is anchored by either a mechanical

device or a short resin column; a bearing plate set bet"een the bolt head androofline serves as the other anchor Usually a certain amount of tension is applied to

the bolt at the time of installation. A full&length&grouted bolt is grouted to the


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borehole by fast&setting' high&strength resin or inorganic cement throughout the

full length of the hole. o pretension is applied to the bolt.

Another "ay to categorise bolts is based on "hether tension is applied to the bolt

at the time of installation. Usually' a point&anchored bolt is tensioned' and it is also

called a tensioned bolt' "hile the full&length&grouted bolt is referred to as anuntensioned bolt because no tension is applied. ,able sho"s the bolt types

commonly used in coal mines' non&coal mines' and surface mines in the U.S..

,ypes of bolts commonly used Underground

It is estimated that' in underground mines' the most popular bolt is a mechanical

bolt' because it is economical' easy to install' and effective for the lifetime of

underground entries' "hich are e-pected to last a fe" months or years. About :8able bolts "ith great fle-ibility completely overcome

this limitation. If bolts are to act mainly in suspension' it should be ensured that the

bolts are long enough to be firmly anchored in a competent rock mass. If the

situation does not allo" suspension' bolt length should be long enough to create

beam building or keying effects. Cang and )ischoff */5+ developed a

relationship bet"een bolt length and roof span *Figure +' "hich can be used as a

guideline to determine bolt length.

In the U.S.' Federal regulation >F@ 68 mandates that coal mines follo" a

systematic pattern of bolting' e.g. 5= 5 ft.' 9=9 ft' or :=: ft.' regardless of bolt length.

Figure / sho"s several roof bolt patterns for stratified roof. Usually' bolts are

installed vertically' sometimes at an incline. In Figure / e' those inclined bolts are

anchored in the roof above the pillars. ,his helps transfer load to the center of thepillars' reducing the possibility of roof failure along the ribside. For inclined bolts'

a special bearing plate must be used such that the plane of the bearing plate is

al"ays perpendicular to the bolt. Factors affecting bolt spacing mainly include

strata thickness' location of "eakness plane' roof condition' and bolt tension.

)olt diameter depends on the yield strength and bolt length and spacing' "hich

determine the load the bolt is supposed to carry 384. In practice' the most

commonly used bolts in underground mines have diameters of 6' 9' 1' or

inches.

For point&anchored bolts' the tension applied at installation plays a crucial role for

creating suspension' beam building' and keying effects. )ased on composite beam

theory and assumption of competent strata "ith full&load transfer' Cang and

)ischoff 3/4 derived an e$uation' as sho"n belo"' to estimate the minimum bolt

tension' "here2

3Multiple line e$uation*s+ cannot be represented in AS>II te-t4

, B minimum bolt tension;


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a B ratio depending on time delay of installation of bolts after e-cavation *8.9 for

active reinforcement and .8 for passive reinforcement+;

g B unit "eight of the rock;

m B tano' "here o is friction angle of the rock mass;

c B apparent cohesion of the rock mass;

B &sin o Hsin o

h B average hori%ontal stress;

C B length of bolt;

A B reinforced area *s- s' "here s B bolt spacing+;

@ B shear radius of rock column *s5+;

D B height of stressed rock above opening *CHs+.

,his e$uation ignores the geology and rock conditions. It can only be applied "hen

the rock mass is moderately competent

)ased on beam building theory' the reinforcement factor *@F+' "hich is used to

measure the degree of reinforcement produced by roof&bolting laminae of e$ualthickness' is defined by anek as 342

@F B H *ef efu+

"here ef and efu are the ma-imum strains in the bolted and unbolted roof'

respectively. After being visuali%ed as Figure 8' the e$uation can be used as a bolt

design tool. (n the other hand' this e$uation indicates that beam building effects

increase "ith decreasing bolt spacing' increasing bolt tension' increasing the

number of bolted laminae' and decreasing roof span.

?o"ever' recent research sho"ed that the beam building effect does not al"ays

increase "ith the bolt length. Stankus *//1+ 304 found that in many cases shorter

bolts "ith higher pretension produce a much stronger beam than long bolts "ith

lo"er tension' and thus proposed a ne" concept' the (ptimum )eaming !ffect

*()!+. ()! is defined as the roof beam that has no separation or acceptable

separation both above and "ithin the bolted range "ith the shortest bolt possible.

,he methodology for designing an optimum beam is sho"n in Figure .

)olting Failure Analysis


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For point&anchored bolts' including mechanical' point&resin&anchored' and

combination bolts' a certain amount of tension is al"ays applied during installation

to help create suspension' beaming' and keying effects. ?o"ever' point anchors'

especially e-pansion shell anchors' produce high stress concentration against the

surrounding rock' resulting in rock fracture and a tendency for the anchor to slip or

creep "ith time. ,he magnitude of concentrated stresses is proportionally related tothe tension applied to the bolts. Generally' the additional fracturing induced by

tensioning is insignificant compared to the additional reinforcement provided by

the bolting system. )ut it has been observed that this type of fracturing passes

through the ends of the deepest bolts' and the fracturing is more likely to propagate

as bolt spacing is reduced. Sometimes the bolting system "orks very "ell in the

range of the supported area' but the entire section of the supported roof caves in as

a unit. ,his fracturing' combined "ith high abutment pressure due to mining' could

be a potential cause of roof failure 364.

)y contrast' full&length&grouted bolts eliminate this problem' since no tension is

re$uired for them to function' or even if tension is applied' the stress induced is

distributed evenly along the "hole bolt length. 7ithout too much disturbance to

the roof' full&length&grouted bolts are more effective than point&anchored bolts in

"eak' highly laminated shales and other soft rocks. !ven though they are more

e-pensive' untensioned full&length&grouted bolts are gaining "ide acceptance in

many U.S. coal mines. ,ensioned full&length&grouted bolts not only can establish

good beam building effects but also produce suspension and keying effects.

,he reinforcement ability of both point&anchored and full&length&grouted bolts isprimarily accomplished by the frictional forces created by the physical interlocking

along the anchorrock interface or along the groutrock contact Coss of fractional

forces is the most common cause of bolt failure and is referred to as anchorage

failure. It could occur because of either the damage to the surrounding rock or the

damage to the anchor itself. For tensioned bolts' anchorage failure results in loss of

bolt tension and conse$uently loss of compressive force bet"een the bolt ends.

Suspension' beam building' and keying effects could be reduced significantly or

sometimes totally lost' resulting in roof fall. Anchorage failure in untensioned full&

length&grouted bolts results in diminished beam building effects' reduced load&carrying capacity' and decreased shear resistance to hori%ontal stresses. ,he main

factors attributed to anchorage failure include2 "eak rocks at the point&anchor area'

improper hole si%e' adverse installation environment such as run&off "ater in the

hole' "ear and damage of hard"are components' and seismic force such as blast

shock @egularly conducting pull tests and tor$ue tests can detect potential

anchorage failure in time' and corresponding measures can be adopted to prevent

anchorage failure from happening.

It seems apparent that sagging and bed separation of roof strata increase "ith time'

although some researchers pointed out that no relationship e-ists bet"een time


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delays and stress distribution 354. In most cases' it is recommended that bolts be

installed as soon as possible after the opening is e-cavated.

For permanent support "ith bolts' "eathering effects should be taken into account

In such cases' coated rebar andor full&length&grouted bolts must be considered.

Special )olts

In reality' some special geological and tectonic settings and high abutment pressure

induced by mining activities often prevent the conventional bolting systems from

functioning effectively. Special bolt types such as cable bolts' trusses' and split sets

are designed to counteract such problems.

>able bolts

In the early years' discarded hoists or slusher ropes "ere still in $uite good shape.,hey "ere reused as bolts instead of rigid steel bar after degreasing. ,he primary

motivation "as to save money' "ithout e-pecting them to "ork better than steel

bars. Fle-ibility allo"s the cable to be packed into a coil' greatly facilitating

material handling; in addition' bolt length is no longer limited by the opening

geometry. >able fle-ibility also allo"s hori%ontal movement' thus reducing the

tendency to shear. >able bolts "ere introduced to the U.S. mining industry in /18

as a method to reinforce ground before mining' as sho"n in Figure 0 394. ,he

presence of cables had the effect of reinforcing the rock mass against the

subse$uent blast shock and stress redistribution. In long"alls' they are "idely usedas secondary support' supplementing or replacing traditional secondary support

systems like "ood cribs and posts' hydraulic #acks' or spot roof bolts 3:4. ,oday

the high&strength cables are about inch in diameter and consist of seven strands' as

sho"n in Figure 0. A cable bolting system is capable of strengthening and

reinforcing roof strata to transfer high pressure into the main roof and onto

supporting structures a"ay from the ribline' enhancing pillar performance. ,his

system also reduces entry convergence.

@oof ,russes

,russes "ere introduced in the /:8=s as an alternative method of supporting

unstable roof "hen conventional roof bolts alone "ere not effective' such as "hen

fallout is fre$uent bet"een bolts or cutter roof is encountered. )asically' a truss

consists of t"o inclined point&anchored bolts' a connection bar' a turnbuckle to

give proper tension to the bar' and an ad#usting "edge bo-' as sho"n in Figure 6.

,he tensioned connection bar keeps the roof in compression and also can yield

do"n"ard as the roof displaces. 7hen used as slings' trusses provide a suspension

effect to the roof' transferring load a"ay from the entry out to the center of the

pillars. In three& and four&"ay intersections' "here tensile stresses are very high'trusses can produce compressive forces to (ffset high mid&intersection roof tensile

stresses. In areas "here roof falls have previously occurred' trusses are a useful


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"ay to stabili%e ne"ly e-posed roof arches. ?o"ever' under certain conditions'

mining&induced high hori%ontal shear stress could affect the rigid inclined bolts. As

an alternative' cable trusses are introduced. ,he fle-ibility of cable allo"s

installation "ithout regard to the level or degree of bearing surface of the

immediate roof. ,he inclined bolt can be installed at the roofrib corner'

transferring load farther into the pillars and thus reducing any sloughage effect.

Split Sets

A split set rock reinforcement system *friction rock stabili%er+ is "idely used in the

U.S. metal mines. It consists of a thin&"all steel tube of inches in diameter' "hich

is forced into a borehole "ith a diameter of inches. ,he spring action of the

compressed steel tube induces a frictional force along the length of the tube and

anchors the tube into the rock. Installation is very $uick and is $uite effective if the

split set is not installed close to a face and the stresses imposed on the tube are not

very high. Appro-imately 6.9 million bolts are used in metal mines per year.

>onclusions

Due to its effectiveness in a variety of geotechnical conditions' bolting has become

the most important support system in mining and civil engineering "orld"ide'

greatly reducing fatal accidents and timber consumption. Structures reinforced by

bolts are typically very reliable and long lasting. A great deal of research' both

analytical studies and laboratory investigations' have been conducted' in an effort

to understand the bolting mechanisms so that bolting technology can be appliedmore efficiently. Despite significant progress' a rational basis for all bolting system

designs has still not been fully achieved. Fortunately' the successful and

unsuccessful bolting practices of the last 98 years provide abundant empirical

e-periences for bolt utili%ation. ,oday' more effective bolting systems are being

designed by combining mathematical computation "ith these case study data. 7ith

the increase of computer po"er' precise and large&scaled numerical modeling of

the real bolting situation' such as finite element models and boundary element

models' "ill continue. It is believed that this research "ill ultimately lead to a

complete understanding of bolting mechanisms.

,able I2 ,ypes of @oof )olts

Cegend for >hart2

A & ,ype of bolt

) & ,ypes of anchor


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> & Suitable strata type

D & >omments

A ) >

D

oint&anchored Slot&and&"edge ?ard rock

bolts *tensioned+

Used in the early stage

!-pansion shell

Most commonly used in the U.S.A.

Standard anchor Medium&strength rock


)ail anchor Soft rock


!-plosive set Co"er&strength rock

Cimited use

@esin grout

Increasing usage recently

ure point anchor

@esin length B 05 in.

>ombination All strata especially

system for "eak rock

@esin length JB 05 in


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>ombination anchor Most strata

*e-pansion shell

and no mi- resin+

Good anchorage "ith Kno mi- resinK

Full&length& >ement Most strata

grouted bolt erfo

*untensioned+ In#ection

>artridge

Disadvantages

. Shrinkage of cement

0. Conger setting time

@esin All strata

In#ection

>artridge

Increased use recently

especially for "eek

strata

@oof truss !-pansion shell Adverse roof

@ecommended for use at intersection

andor heavy pressure area

>able sling >ement anchor and 7eak strata

full&length


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fraction

Substitute for timber'

steel or truss support

Eieldable bolt !-pansion shell Medium&strength

rock

It is an e-pansion&shell bolt "ith

yielding device

umpable bolt @esin 7eak strata

>omple- installation

?elical bolt !-pansion shell Most strata

Split set Full&length 7eak strata

fraction

>heap but need special

installation e$uipment

S"elle- bolt Full&length 7ater&bearing strata

holding

Using high&pressure "ater to

s"ell the steel tube

G@A?2 Figure 2 Increasing trend of rock bolting

DIAG@AM2 Figure 62 Suspension effect of roof bolt

DIAG@AM2 Figure 52 artial suspension effect in rock slope reinforcement

DIAG@AM2 Figure 92 )eam building effect of bolting

DIAG@AM2 Figure :2 eying effect of bolting


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DIAG@AM2 Figure 12 >ompression %one created by keying 314

G@A?2 Figure 2 @elationship bet"en bolt length and roof span

DIAG@AMS2 Figure /2 ,ypical bolting pattern 354

DIAG@AM2 Figure 2 Flo" chart for determining ()!

DIAG@AM2 Figure 02 >able bolts

DIAG@AM2 Figure 62 >ompression %one created by truss 314

ICCUS,@A,I( *)CA> L 7?I,!+

@eferences

34 )olstad' D. and ?ill' . /6' KU.S. )ureau of Mines rock bolting research'K

roceedings of the International Symposium on @ock )olting' Abisko' S"eden'

pp. 66&608.

304 7eigel' 7. /56' K>hannel Iron for @oof >ontrol'K !ngineering And Mining

ournal' v. 55' May' pp. 18&10.

364 )ienia"ski' N. /1' Strata >ontrol in Mineral !ngineering' ohn 7iley L

Sons' Inc.' pp. 0/&91.

354 eng' S. /5' >oal Mine Ground >ontrol' ohn 7iley L Sons' Inc.' pp. 6&

16.

394 eng' S. and )is"as' . //5' K,ailgate Support ractice in U.S. Cong"all

Mines & A Survey'K roceedings of 6th >onference on Ground >ontrol in Mining'

Morganto"n' 7O' pp. 91&:8.

3:4 iteau' D. and Martin' D. /' KMechanics of @ock Slope Failure'K

roceedings of the 6rd International >onference on Stability in Surface Mining' v

6' Oancouver' )>' >anada' pp. 6&:/.

314 Gerrard' >. /6' K@ock bolting in theory'K roceedings of the International

Symposium on @ock )olting' Abisko' S"eden' pp. 6&0/.

34 Smith' 7. //6' K@oof >ontrol Strategies for Underground >oal Mines'K

Information >ircular /69' the U.S. )ureau of Mines. p 1.

3/4 Cang' ,. and )ischoff' . /0' KStabili%ation of @ock !-cavations Using @ock

@einforcementsK roceedings 06rd U.S. Symposium on @ock Mechanics' AIM!'e" Eork' pp. /69&/55.


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384 )iron' >. and Arioglu' !. /6' Design of Supports in Mines' ohn 7iley L

Sons' Inc. pp. /&1.

34 anek' C. /9:' Krinciples of reinforcing )edded Mine @oof "ith )olts'K

U.S. )ureau of Mines @I 99:' p. 09.

304 (ldsen' ' Stankus' ' Guo' S. and hair A. //1' K>ontinuing Development

of Innovative >able Support Systems'K roceedings of :th >onference on Ground

>ontrol in Mining' Morganto"n' 7O' pp. 1&0/.

364 orstad' ,. @ock bolting. M. S. ,hesis' >olorado School of Mines' Golden'

>(. Feb. /:1' p. :.

354 @adcliffe' D. and Stateham' @. /1' K!ffects of ,ime )et"een !-posure and

Support on Mine @oof Stability'K U.S. )ureau of Mines' @I 0/' p. 6.

394 Fuller' @ /6' K>able Support in Mining'K roceedings of the International

Symposium on @ock )olting' Abisko' S"eden' pp. 9&90.

3:4 ,adolini' S. and och' @. //6' K>able Supports for Improved Cong"all

Gateroad Stability'K roceedings of 0th >onference on Ground >ontrol in Mining'

Morganto"n' 7O' pp./&9.

314 hair' A. /6' Khysical and Analytical Modeling of the )ehavior of ,russ

)olted Mine @oofs'K roceedings of the International Symposium on @ock

)olting' Abisko' S"eden' pp. 09&5.

PPPPPPPP

)y . Cuo; >. ?aycocks; M. armis and !. 7estman' Department of Mining and

Minerals !ngineering' Oirginia olytechnic Institute and State University'

)lacksburg' Oirginia' U.S.A

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