ba 80-99 use of rock bolts (dmrb vol 2, section 1, part 7, 1999)

February 1999

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 2 HIGHWAY STRUCTURES:DESIGN(SUBSTRUCTURES ANDSPECIAL STRUCTURES)

SECTION 1 MATERIALSUBSTRUCTURES

PART 7

BA 80/99

USE OF ROCK BOLTS

SUMMARY

This Advice Note covers the design, construction andtesting of rock bolts. Some information provided is alsoapplicable to similar ground support systems such asrock dowels and cable bolts.

INSTRUCTIONS FOR USE

1. This is a new document to be incorporated intothe Manual.

2. Insert BA 80/99 into Volume 2, Section 1, Part 7.

3. Archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.

BA 80/99

Use of Rock Bolts

THE HIGHWAYS AGENCY

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICEY SWYDDFA GYMREIG

THE DEPARTMENT OF THE ENVIRONMENT FORNORTHERN IRELAND


Summary: This Advice Note covers the design, construction and testing of rock bolts.Some information provided is also applicable to similar ground supportsystems as rock dowels and cable bolts.

ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Volume 2 Section 1Part 7 BA 80/99

February 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of

amendments amendments

Registration of Amendments


February 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date of No incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 2 HIGHWAY STRUCTURES:DESIGN(SUBSTRUCTURES ANDSPECIAL STRUCTURES)

SECTION 1 MATERIALSUBSTRUCTURES

PART 7

BA 80/99

USE OF ROCK BOLTS

Contents

Chapter

1. Introduction

2. Uses of Rock Bolts

3. Types of Rock Bolts

4. Design

5. Construction details

6. References

7. Enquiries




Chapter 1Introduction

corrosion protection system sufficient to provide aservice life of 120 years; such a system requires the1. INTRODUCTIONScope

1.1.1 This Advice Note covers the design, constructionand testing of rock bolts. Some of the informationprovided is also applicable to similar ground supportsystems such as rock dowels and cable bolts. It isintended to give the reader an introduction to thepotential uses and benefits of this technique for groundsupport. This document does not cover the design ofstructures supported by rock bolts.

1.1.2 The British Standard Code of Practice for groundanchorages (BS 8081: 1989) provides comprehensiveinformation on the design, construction and testing ofanchorage systems. BD 71 (DMRB 2.1.6) covers the useof ground anchorages for highway works. The range ofapplication of each of these documents excludes rockbolts but they all state that some of the information theyprovide may be applicable to particular elements of thedesign, construction and testing of rock bolts. Indeed BS8081: 1989 has sections that cover the design and use ofrock bolts within the overall context of groundanchorage systems.

Equivalence

1.2.1 The construction of rock bolts will normally becarried out under contracts incorporating theSpecification for Highway Works (MCHW Volume 1).In such cases, products conforming to equivalentstandards of technical specifications of other memberstates of the European Economic Area, and testsundertaken in other states of the European EconomicArea will be acceptable in accordance with the 104 and105 series of clauses of that specification. Any contractnot containing these clauses must contain suitableclauses of mutual recognition having the same effectregarding which advice should be sought.

Rock bolts

1.3.1 A rock bolt is a short, low capacity reinforcementcomprising a bar (or tube) fixed into rock and tensionedto a predetermined load. Some of the components of arock bolt are defined in Figure 1.1. Rock bolts areusually less than 6m long and rarely longer than 10m.Their working load is typically between 150 and 200kNand they would normally be formed from high yield steelbars with diameters up to 32mm. However, butunusually, working loads of up to 300kN may bespecified; typically these would be formed from highyield steel bars having diameters up to 40mm.February 1999 ELECTRONIC COPY NOT FO1.3.2 The types of rock bolt commonly used for civilengineering works include:

(i) Mechanical bolts - typically these have a wedgeshaped shell assembly which, when expanded,anchors them into the drillhole.

(ii) Cement grouted bolts - typically these are formedby inserting the bar into a drillhole filled withgrout.

(iii) Two-speed resin bonded bolts - with these the baris fixed (and then stressed) within a fast settingresin at the distal end and subsequently bondedalong the remainder of its length by a slowersetting resin.

Typical arrangements of these types are shown inFigures 1.2, 1.3 and 1.4 respectively.

1.3.3 Rock bolts are used widely to improve the stabilityand load bearing characteristics of a rock mass. Oftenthey are used to stabilise relatively small blocks of rocksin cuttings, slopes and underground excavations such astunnels, caverns and mines. They can be used on theirown or in conjunction with other support systems suchas ground anchorages.

1.3.4 The proximal end of the bar may be threaded sothat a nut and faceplate can be attached; the plate mayprovide local support to the rock surface and allow theattachment of mesh reinforcement which may berequired for a shotcrete finish.

Rock dowels

1.4.1 A rock dowel comprises a bar which is inserted ina drillhole and fixed along its entire length. Movement ofthe rock surrounding the drillhole is relied upon toinduce tension in the dowel and thereby strengthen themass as a whole.

Cable bolts

1.5.1 Cable bolts utilise bundles of steel wires orfibreglass rods to form a fixed anchorage at depth. Theinherent flexibility of cable bolts allows long unjointedbolts to be installed where there are cramped workingconditions or where access is difficult.

Durability

1.6.1 BD 71 (DMRB 2.1.6) specifies that permanentground anchorages must be provided with a doubleR USE OUTSIDE THE AGENCY. 1/1


Chapter 1Introductionprovision of two physical barriers to protect the steeltendon from corrosion. Details of such systems are givenin BD 71 (DMRB 2.1.6) and BS 8081: 1989. Rock boltsare not provided with a similar standard of protection asgiven to ground anchorages because;

(i) rock bolts are formed from high yield or mildsteels rather than the high tensile prestressinggrades of steel that are commonly used for groundanchorages; the latter are more susceptible tostress corrosion and hydrogen embrittlement,

(ii) rock bolts provide local support at a multitude ofpoints, whereas ground anchorages support muchhigher loads at wider spacings, and thus thefailure of a bolt is much less likely to lead to thecatastrophic collapse of the supported structure.

1.6.2 Rock bolts in new permanent structures shall bedesigned for a service life of 120 years but in structuralmaintenance applications, slope stabilisation andtemporary works the service life should be compatiblewith the needs of those works. Corrosion protectionshould take account of the aggressivity of the groundand groundwater, the required service life and theconsequences of failure.

Definitions

1.7.1 The following definitions apply to common termsused in this Advice Note; other terms are defined as theyarise or in the references quoted. Many of the componentparts of a rock bolt are shown in Figures 1.1 to 1.4.

1.7.2 A Rock bolt is a short, low capacity reinforcementcomprising a bar fixed into rock and subsequentlytensioned to a predetermined load.

1.7.3 The Bolt head usually comprises a faceplate, nutand washer: a cap to the nut may also be included. Ittransmits the load from the tendon to the rock face orstructure requiring support.

1.7.4 Distal the end situated furthest from the bolthead.

1.7.5 Proximal the end situated nearest to the bolthead.

1.7.6 The Tendon, or shank, is that part of the rock boltthat transmits the tensile load from the anchor to the bolthead.

1.7.7 The Design anchor length is the length over whichthe tensile load is designed to be transmitted to thesurrounding ground.ELECTRONIC COPY NOT FO1/21.7.8 The Free length is the distance between theproximal end of the design anchor length and the bolthead.

1.7.9 The Tendon bond length is the length of tendonthat transmits the applied tensile load to the surroundinggrout.

1.7.10 The Free tendon length is the length of tendonthat is decoupled from the surrounding grout.

1.7.11 Primary grout is a thin fluid mortar placed orinjected prior to the stressing of the bolt.

1.7.12 Secondary grout is a thin fluid mortar injectedfollowing the stressing of the bolt.

1.7.13 Debonding is the breakdown of bond at aninterface.

1.7.14 Decoupling is the separation of components toprovide, ideally, a frictionless interface; for example theseparation of the free tendon length from the secondarygrout by a greased sheath.

1.7.15 The Proof load is the maximum pull out load towhich a bolt is subjected during stressing.

1.7.16 The In-service load is that load specified to becarried by a bolt throughout its service life.

1.7.17 The Faceplate is usually a flat steel plate thatdistributes the load from the rock bolt to the rock face orstructure requiring support.

1.7.18 A Cable bolt is a bolt comprising a number ofsteel wires or fibreglass rods formed into a strand orcable.

1.7.19 A Rock dowel is a short, low capacityreinforcement comprising a bar (usually of steel), whichis bonded by grouting over its full length at installation;it is not tensioned to a predetermined load.

Implementation

1.8.1 This Advice Note should be used forthwith on allfuture schemes for the construction, improvement andmaintenance of trunk roads, including motorways. Itshall also apply to schemes currently in preparationprovided that, in the opinion of the OverseeingOrganisation, this would not result in significantadditional expense or delay progress. DesignOrganisations shall confirm its application to particularschemes with the Overseeing Organisation.February 1999R USE OUTSIDE THE AGENCY.


Chapter 1IntroductionFig.1.1 Components of typical rock bolts

Face plate

Bolt head

Nut and washer

End anchorage

b. Mechanical expansion shell rock bolt for temporary worksFProximal end

Drill hole

Bar, tendon or shank(fully bonded after stressing)

Secondary grout

Primary grout

Distal end

Ancho

r lengt

h,

tendo

n bon

d leng

th

Free le

ngth,

free t

endon

length

(prio

r to se

condar

y grou

ting)

a. Fully bonded rock boltebruary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY. 1/3


1

Chapter 1IntroductionExpansion anchor

Tendon

Grout return tube

Face plate

Spherical washerShortcrete lining

Grout tubeFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY./4

Fig 1.2 Schematic view of a typical mechanical rock bolt


February 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY. 1/5

Face plate

Spherical washer

Grout inlettube

Grout seal

Grouttubes

Deformed shankto form anchor

Grout returntube

Self-lockingspherical washerwith integral nut

Resin mixed withhardener by rotationof bar

Slow settingresin

Fast settingresin

Face plate

Fig 1.3 Schematic view of a typical cement grouted rock bolt

Fig 1.4 Schematic view of a typical two-speed resin bonded rock bolt

Chapter 1Introduction



2. USES OF ROCK BOLTS

2/1

Chapter 2Uses of Rock Bolts

Underground excavations

2.1.1 Rock bolts are installed:(i) to support discrete wedges or blocks of rock that

would otherwise be free to fall or slide;(ii) to reinforce the crown or sidewalls of a tunnel:(iii) in older designs rock bolts were used as part of

temporary support, but more recently as part ofthe permanent support system

Typical support schemes are shown schematically inFigures 2.1 to 2.4 and in detail in Figure 2.5.

Rock excavations, slopes and faces

2.2.1 For highway works, rock bolts are predominantlyused to stabilise relatively small instabilities. Rock boltscan give support to discrete unstable blocks bounded bydiscontinuities of various types. Where there iswidespread instability a gridage of rock bolts has beenused to improve the overall integrity and stability of therock mass, sometimes in combination with netting, orwhere bolts/dowels (and cables) have been used to holdrock fall protection (ie at the crest and toe). Commonsituations are shown in Figure 2.6. Future usage couldbe envisaged in areas of maintenance and improvementschemes (ie rock slope protection)A typical stabilisation scheme for a highway cutting isshown in Figure 2.7. As shown here an integratedapproach is commonly used in such works combiningground anchorages and rock bolts with small-scalebuttressing and dental concrete.

Other applications

2.3.1 Rock bolts have been used to restrain lightstructures, such as gantry signs, which are subject tooverturning or tension forces.

2.3.2 Rock bolts have also been used to strengthen orrepair earth retaining walls, see for example Figure 3.12.


February 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.2/2


Fig 2.1 Use of rock bolts to support wedges in underground excavations

(i) For roof support (ii) For side support

(iii) Location of rock wedges requiring support



Fig 2.3 Rock bolting to generate beam effect in layered strata

Fig 2.2 Arrangement of rock bolts to support roof excavation

Joint planes Rock bolts

Asymmetric distribution of rock bolts to take account of variation in structuralfeatures of rock




Fig 2.4 Compression zone or arch formed by tensioned rock bolts

Compression Zone

Arch createdby bolting Limit of loosening




Fig 2.5 Typical outline scheme defining pattern of rock bolting for a tunnelthrough jointed rock, after Hoek and Brown (1980)

Optionalbolt toimprovestabilityof this wall

Possibleoverbreak

Bolts radialfrom this point

Angle boltsacross bedding

Orthogonal jointsaverage spacingabout 1.5m Use spherical

washer to anglebolts - angle not to exceed 15

Bedding inclined at 15,average spacing about 1m

Compression stress in shaded zone

45

8 metres

4m long 25mmbolts on 2 x 2mgrid.Tension to 15 tonnes &grout

11 m

etre

s

4 m

etre

s




Fig 2.6 Use of rock bolts to stabilise blocks on a slope

Cut slope

Potential topplingafter removal of keyblock

Potentially unstablekey block

Bedding planes

Possible alignmentof rock bolt tostabilisekey block

Jointplanes

(i) Key wedge in rock slope

(ii) Bolting of unstable blocks in rock face

(iii) Bolting to resist toppling of blocks of rock





2/7

Fig 2.7 Details of a typical slope stabilisation scheme, Anon (1972)

R.C. retaining wall variableheight & plan line

Rock Bolts

Disturbed or weatheredmaterial to be trimmed off whree necessary

WEATH

ERED Z

ONE

ORIGIN

AL GRO

UND LEV

EL

Jointing normal to bedding - stable

Unfavourable jointing creatingunstable wedges or massesof rock to be supportedby rock bolts

Solution effects in limestone.Infilling removed and replacedby concrete with masonryfacing

Coal measures in Limestone-Dolomite thrust interfacematerial hacked outand replaced withreinforced concretetied with rockbolts and masonry faced Slope of S/B Rock face vaults 1or 11/2 to 2 or 1

Note: Weathered rock zone also effects top of median face

1

2

LIMESTONE BEDDINGDip of limestone varies10 to 40 with strike between 0 and 25to motorway

36

(i) Range of measures used to stabilise a rock slope

Mass concrete retaining wallR C retaining wallAnchored retaining wall

Thrust Crib walls

R C wall tied back to soundrock with rock bolts

Face rock boltedwhere necessary

R C retaining wall

Crib wall

60 Ton ground anchorsat 5ft centres

Coal measurespennant sandstone and shales

80 Ton ground anchorsat 10ft centres

THRU

ST FAU

LT

LIMESTONE

ELEVATION OF CUT VIEWED FROM NORTH SIDE

MOTORWAY IN ROCK CUTTING

(ii) Overall view of a stabilisation scheme


3. TYPES OF ROCK BOLTS

Chapter 3Types of Rock Bolts

of the drillhole.Introduction

3.1.1 Because they are installed for similar purposes,there are inevitably similarities between rock bolts andother types of support devices, such as rock dowels,cable bolts and ground anchorages. Some of theinformation given in this chapter is relevant to dowelsrather than bolts but has been included to provide acomprehensive coverage of the range of low capacityrock support methods.

3.1.2 Details of proprietary devices and materials aregiven in this Advice Note, but the presence or absence ofinformation for a particular device or material shouldnot be taken to imply that it is recommended or notrecommended for use. Further details of the types anduses of rock bolts and similar devices have beenprovided by Hoek and Brown (1980), by Hobst andZajic (1983) and by Stillborg (1994).Types of anchor

3.2.1 Rock bolts and dowels can be divided into threebroad categories, according to how they are anchoredinto the rock mass.

(i) Mechanical - where the load is transferred to therock through some form of mechanical device.Typically this is achieved through the use ofexpanding wedge systems or deformable steeltubes placed in intimate contact with the sides ofthe drillhole.

(ii) Cement grouted - where a cementitious grout isused to anchor the bolt into the rock. Such groutsare usually pumpable, but systems based on theuse of capsules have also been developed.Cementitious grouts are commonly used forsecondary grouting works.

(iii) Resin bonded - typically these employ polyesterresins to anchor the bolt into the rock, but epoxyresins have also been used. In most cases, forconvenience, the grout is supplied in pre-packedsausage-like capsules which contain the resin andhardener in separate compartments (ExchemMining & Construction Ltd). The action ofrotating the tendon during installation ruptures thecapsules and mixes their contents. Resinousgrouts have been used in pumped or poured formsbut these are less common.February 1999 ELECTRONIC COPY NOT FOR3.2.2 Mechanical anchors

(i) Expansion shell anchorWith an expansion shell anchor a wedge attachedto the shank is pulled into a conical shell forcing itto expand against the walls of the drillhole; atypical arrangement is shown in Figure 3.1.This type of bolt can be tensioned immediatelyafter installation and grouted at a later stage whenshort-term movements have ceased. Theexpansion shell anchor has a proven track recordin competent rocks where relatively high boltloads can be sustained but systems have beendeveloped for use in soft rocks by increasing thesurface area of contact through an increase in thelength or diameter of the shells or by the use ofcoupled assemblies. A range of expansion shellsystems is shown in Figure 3.2.Often such bolts are used as a permanent supportand in such cases secondary grouting would beemployed to provide the tendon with someprotection against corrosion. Various means havebeen developed to achieve this, but typically arubber bung is inserted in the collar of thedrillhole to centralise the bolt and act as a sealagainst grout leakage. Alternatively, a rapid setmortar can be used to seal the collar - and often insuch cases the mortar is extended to bed down thefaceplate. Grout can be injected into the drillholeby various arrangements. For upward facingholes, the grout is injected into the collar end andthe return pipe is extended to the base of the hole;grout injection is stopped when all the air hasbeen displaced and grout flows from the returntube. For downward facing holes, grout is pumpedto its base through a full-length injection pipe andexits at its collar. The shank of the bolt can beformed from a tube, as in the Titan injectionanchor (Ischebeck Titan Ltd), which makes it fareasier to inject the secondary grout. In upwardfacing drillholes the centre bore acts as the airbleed and grout relief vent, whereas indownwardly inclined holes it acts as the groutinjection tube. Details of typical arrangements areshown in Figure 3.1.Skilled workmanship and close supervision arerequired to install expansion shell systemscorrectly. It is essential that the size of theexpansion shell anchor is suitable for the diameter USE OUTSIDE THE AGENCY. 3/1



(ii) Full length expansion anchor

A form of this type of anchor, developed byWorley Co. of Philadelphia, is shown in Figure3.3: technical details of the bolt can be obtainedthrough Mine Roof Support Systems. As the nutis tightened against the washer, the ramps alongthe tendon expand the anchor against the sides ofthe drillhole. Undoing the nut and hammering onthe end of the bolt reverses the process and allowsthe anchor to be loosened and removed; providedthat it is not too deformed or corroded the devicemay be reused. Because it cannot be grouted, andhence protected from deterioration, this device isonly suitable for short-term applications.Furthermore the device cannot be tensioned sothat load is transferred to its distal end. It istherefore usually installed as soon as possibleafter the excavation of the drillhole, i.e. beforeany movement of the rock has occurred due torelaxation.

(iii) Split set stabiliserAs shown in Figure 3.4 this device, which wasdeveloped by Scott (1976) for Ingersoll-Rand Co.Ltd, comprises a split steel tube which is driveninto a slightly smaller diameter drillhole. Thespring action of the compressed tube applies aradial force against the rock surface and generatesa frictional resistance along the interface. Thesplit tube cannot be tensioned; its support actionis developed by movement of the rock and so to beeffective it must be installed as soon as possiblefollowing excavation. The effectiveness of thismethod is critically dependent upon the diameterof the drillhole; most of the failures that occurduring installation are due to the diameter beingeither too small or too large. The split tube candeteriorate and therefore the device is unsuitablefor long-term use.The Split set system has found worldwide usagein the mining industry, but is not commonly usedin civil engineering works.

(iv) Swellex frictional anchorIn the Swellex system, which was devised inSweden by Atlas Copco Construction and MiningLtd, a collapsed steel tube is expanded by waterpressure to fit the drillhole. This expansion leadsto some reduction in the length of tube, whicheffectively loads the faceplate and therebyprovides immediate support. Details of the systemare shown in Figure 3.5.ELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONI

3/2Corrosion of the tube can be a problem, andalthough a coated tube is available its long-termperformance is unproven. Whilst commonly usedworldwide for mining applications, the device hasnot found much use in civil engineering works.

(v) Slotted bolt and wedgeAlthough this type of bolt is now rarely used incivil engineering works in the UK, details areincluded for completeness. As shown in Figure3.6, the device comprises a tube with a short cutat its distal end: a wedge is fitted into this slottedend. The wedge is driven into the end of the tubeby pushing or driving the assembly against thebase of the drillhole. The wedge expands the endof the tube thereby anchoring it to the rock.Because of the small contact area between theexpanded section of the tube and the rock, localcrushing of the rock can occur with consequentslip of the anchor; this is a particular problemwhen the intact rock strength is less than about10MPa. The device probably represents theearliest type of mechanically anchored rock bolt,but it has been superseded by the more versatilemechanical expansion shell anchors and capsuleresin systems. Early devices used in the miningindustry were manufactured from timber, butthese have been superseded by ones manufacturedfrom fibreglass and from steel - details of thesehave been given by Hoek and Brown (1980).

3.2.3 Cement grouted anchors

Where time and conditions allow, rock bolts may beformed using pumpable cementitious grouts. Theadvantages of such grouts are their ability to develop agood bond in poorer quality rocks, their flexibility of useand the relative cheapness of the components: methodsof mixing and placing are also likely to be familiar tosite operatives. A disadvantage is the time requiredbetween the installation of the bar, its tensioning, andany subsequent secondary grouting operations: thesedelays limit production rates in tunnelling works forexample.A two-stage grouting process can be adopted wherebythe anchor length is formed and the bolt tensioned whenthe grout has achieved the required strength, followed bysecondary grouting to bond the remainder of the shank.However this is a lengthy process and often is not wellsuited to construction works that require rapid cycletimes, such as the drill, blast, excavate and installsupport cycle of tunnelling operations. Details of a boltinstalled through a two stage process are shown inFigure 1.3. Alternatively the free length of the bolt canFebruary 1999 USE OUTSIDE THE AGENCY.C DOCUMENT ARE UNCONTROLLED


Chapter 3Types of Rock Boltsbe decoupled, for example with a smooth plastic sleeve,and a single stage grouting process adopted as used forthe installation of a low capacity ground anchorage. Insuch cases the free length of the tendon is not bonded tothe surrounding rock, and for permanent works attentionhas to be paid to the corrosion protection of the exposedshank and the threaded connections - corrosion at theselocations would result in the loss of the bolt.Systems have been developed which utilise prefabricateddry cement capsules, which themselves contain waxsealed water micro-capsules. The action of installing androtating the bar crushes the capsules and micro-capsulesto form a rapid hardening cement grout. This systemwas developed by the US Bureau of Mines in the late1970s (Hoppe, 1979), but current usage of the system isunknown. Pre-packaged cement capsules with permeableouter covering have also been used in the miningindustry. The capsules are soaked in water for a fewminutes and the hydrating mix is then injected into thedrillhole, using a purpose built high pressure pistonpump, prior to installing the bar. However to date suchsystems have not been widely used in civil engineeringworks.

(i) Perfobolt systemAs shown in Figure 3.7, this system comprisestwo perforated half tubes which are packed withcement mortar and then wired together andinserted into the drillhole; half tubes of fine-meshed wire have also been used (Kennedy et al,1973). The mortar is extruded as the central baris pushed down the tube. A stiff mortar or groutcan be used to control the volume extruded whenthe device is installed in an upwardly inclineddrillhole.The system has been used mainly to form(untensioned) rock dowels, used for example asroof fixings, but it has also been used to form theanchor of a (tensioned) rock bolt.

(ii) Untensioned grouted dowelThis type of dowel is formed simply by pumping athick grout into the drillhole, using a hand pumpor a mono pump, and pushing a steel bar into thegrout, excess grout being extruded out of themouth of the drillhole. Thixotropic admixturescan be added to the grout to reduce run-out. Afaceplate and nut can be added if required, asshown in Figure 3.8. Because the tendon cannotbe tensioned it must be installed before anysignificant deformation of the rock mass has takenplace. Resins or grouts are increasingly beingused as the fixing medium because these offersupport almost immediately following installation.February 1999 ELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONIThese dowels are cheaper than the Perfoboltsystem described above and are widely used aslow capacity supports, for example, to steel meshand ventilation trunking in tunnels.

3.2.4 Resin grouted anchors

Resin bonded anchor systems most commonly utilisecapsules which contain both the resin and catalyst, thecatalyst being held within the resin in a glass or plasticcontainer. The essential features of this system areshown in Figure 1.4. The capsules are placed within thedrillhole and the bar is then inserted and rotated in thehole thereby breaking the capsules and containers andmixing their contents. In most systems capsulescontaining a fast setting resin are placed at the distal endof the drillhole, so that the bolt can be tensioned within afew minutes of mixing, whilst a slow setting resin mixfills the remainder of the hole to bond the bar aftertensioning.In some applications pre-mixed resins have been injectedon site: the advantages of this are that it requires lessexpensive bulk materials and is more suitable for largerdiameter drillholes (to bulky, encapsulated bolts) whichrequire higher volumes of resin. Such bolts are oftenused in conjunction with secondary grouting techniquesbut close control and supervision are essential with suchtechniques. For large-scale production works, systemshave been used in which both the resin and catalyst aresupplied to the drillhole by a proportioning pump viaseparate delivery lines. A nozzle at the injection pointmixes the components as they are injected into thedrillhole and the bar is inserted into the setting resin.Virtually all currently used resin-based systems employpolyester resins: these have the advantage over epoxybased systems in that the mixing and proportioning ofthe resin and hardener are not so critical to its successfulperformance. Furthermore whilst the setting reactions ofboth types of resin are exothermic, the performance ofepoxy resins are more adversely affected by lowtemperatures at mixing. A disadvantage of polyesterresins is that they exhibit a small reduction in volume oncuring and so they are not usually considered suitablefor smooth sided drillholes such as produced, forexample, by coring techniques; rotary percussive drillingtechniques are usually required for such resins.The gelling time of a resin is considerably affected bytemperature. In hot conditions the curing process isaccelerated so that the shelf life and the availableworking time of a resin are greatly reduced. The limitedshelf life of the materials must be considered whenstoring and rotating stock for a particular site.Conversely the rate of curing is reduced in coldconditions, ceasing altogether at temperatures close tozero and this temperature dependency must be addressedat the work site. USE OUTSIDE THE AGENCY.C DOCUMENT ARE UNCONTROLLED

3/3

Volume 2 Section 1Chapter 3

Types of Rock Bolts

The great advantages of resin-based systems are thatthey are simple to use and they set relatively quicklywhich maintains a rapid cycle time in a constructionsequence where the cost of the material may be arelatively inexpensive part of the process. Dependingupon the quality of the rock, this type of bolt canmobilise high lock-off loads. With appropriate settingtimes, a one-shot installation can produce a fully groutedand tensioned rock bolt, and such bolts are widely usedfor permanent works.

3.2.5 Cable bolts

Cable bolts have been formed from seven-wire steelprestressing strand and also from bundles of individualglass fibre rods. With the former, a single 12.7 or15.2mm steel strand having a breaking load of between200 and 300kN is commonly used. The individual wiresof the strand are unwound from around the king-wireand bushed (bird-caged) over the load transfer lengthinto a series of nodes and antinodes, as shown in Figure3.9. Fibreglass rods are typically 6mm in diameter and 6to 12 of these are formed into a bundle: the bundle isusually fabricated on suitable spacers to form a node/antinode configuration similar to that used for steelstrands.Cable bolts are usually fixed in place with cementitiousgrouts, but pumpable resinous grouts have also beenused. The flexibility of these types of bolt allows them tobe used in long unjointed lengths in areas of restrictedaccess.

Types of tendon

3.3.1 Steel bars

Steel bars are by far the commonest form of tendon usedfor rock bolts. In principle any suitable steel may beutilised but, because of the potential loss in section dueto corrosion, it is uncommon for bars of less than 20mmin diameter to be used, particularly in permanent works.Details of the type, size and yield strengths of steels thathave been used for rock bolts in the UK are presented inTable 3.1.

(i) High yield steelCurrently in the UK, tendons are most commonlyformed from high yield steels (to BS 4449: 1997),which have a characteristic yield strength of460N/mm. The characteristic rupture and yieldstrength for a range of Grade 460 bars are givenin Table 3.2.The bar must be threaded at the proximal end toallow tensioning of the bolt and for a nut andfaceplate to be attached. The distal end may bethreaded for attaching mechanical expansionELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONIC

3/4

shells or for coupling and extending the bars.Part 7 BA 80/99

Such threads are usually formed by machining theribs off the end of the bar and machine-cutting thethread. This removes material and thereforereduces the allowable design strength of the bolt.Table 3.3 gives details of the characteristicrupture and yield strengths for a range of standardcut thread-forms used with Grade 460 bars.Alternatively, rolled threads can be providedwhich, because they do not remove material,retain the full strength of the bar. Such threadscan be formed on a bar after machining off theribs, however a coarse thread can be rolleddirectly onto a ribbed bar to provide a thread asstrong as the parent bar. A common andconvenient alternative to rolled or cut threads is toform a coarse continuous thread-likeconfiguration onto the surface of the deformed barduring the rolling process. Such bars would havecharacteristic yield and ultimate strengths of500N/mm and 550 to 600N/mm respectively.Examples of this form of bar are DywidagGewi-Steel (Dywidag-Systems International Ltd)and Macalloy Mac500 (McCalls Special ProductsLtd): data for these products are given in Tables3.4 and 3.5 respectively. Couplers, end nuts andfittings are available for all the various forms ofthread.

(ii) Mild steelMild steel bars are rarely used for rock boltsmainly because they are more expensive per kN ofload carried than high yield bars. In addition, mildsteel bars are formed with a smooth rather than adeformed surface and hence generate a lower bondstress between the tendon and any surroundinggrout. However, for reference purposes, thecharacteristic rupture and yield strengths for arange of cut and rolled threaded bars are providedin Table 3.6.

(iii) Stainless steelAustenitic stainless steel bars (Grade 302, 304 or316 to BS 970: various parts and dates or BS6744: 1986), with diameters of between 16 and40mm have been used for rock bolts. Cut or rolledthreads can be provided to such bars. Thecharacteristic rupture and yield strengths for arange of products are given in Table 3.7.

(iv) High tensile steelBecause they require protection against corrosion,prestressing quality high tensile steel bars are notcommonly used for rock bolts particularly forpermanent works. Furthermore, for the majorityof diameters available, the bars are over-strongfor most rock bolting applications.February 1999 USE OUTSIDE THE AGENCY. DOCUMENT ARE UNCONTROLLED


Chapter 3The range of standard high tensile bars include theDywidag (Dywidag-Systems International Ltd)and Macalloy (McCalls Special Products Ltd)systems, which are both used extensively forground anchorages, prestressed andpost-tensioned structures. Details of the physicalproperties of these bars are given in Tables 3.8and 3.9 respectively. Stainless steel Macalloybars (McCalls Special Products Ltd) have alsobeen used for some contracts; and data for thesebars are given in Table 3.10.

3.3.2 Glass fibre composites

In recent years, glass fibre reinforced compositematerials (GFRC) have developed to occupy asignificant niche for a range of structural applications,particularly where its non-corrodible property is valued.GFRC and GRP (glass reinforced plastic) rock boltswere originally developed for the coal mining industry tomeet the need for a strong but temporary reinforcementto an advancing face or a sidewall, which could besubsequently excavated by tunnelling and cuttingmachines without damage to the cutting teeth. For thisapplication alone, over half a million GRP bolts havebeen used in the British coal industry. Other higherstrength fibres can be used, such as aramid or carbonfibres, but these materials are usually too expensive fornormal civil engineering usage.A composite bar consists of thousands of continuousglass fibres laid parallel to one another and encased in amatrix of polyester or epoxy resin. Typically thecomposite is manufactured by a continuous pultrusionprocess which produces bars having diameters rangingfrom 1mm to in excess of 20mm. As an example, a7.5mm diameter bar might contain about 64,000individual fibres which have a mean diameter of 25microns (i.e. 25x10-3 mm). The volume of fibre variesfrom between 45% to greater than 75% of thecomposite: the percentage varying according to themanufacturer and application.A range of Polystal GFRC bars (Miesseler and Preis,undated Polystal Composites GmbH), including plainand epoxy-coated bars, and Durglass FL bars round,hollow, flats, Y and patented structural sections, (SiregSpa) have been used for rock bolting and soil nailingapplications: the components for the latter are shown inFigure 3.10. Usually such bars are manufactured with acentral hole which is used as a tremie pipe to allow groutto be pumped to the bottom of the drillhole. GFRC tubesare also manufactured, in this case using glass fibreshaving a diameter of about of 3.5 microns. Data forglass fibre products are provided in Table 3.11 alongwith, for comparative purposes, data for carbon andFebruary 1999 ELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONICTypes of Rock Bolts

aramid fibre composites and high yield steel. However,because GFRC products are currently underdevelopment, it would be prudent to confirm theproperties of a particular product prior to design anduse.One of the difficulties with GFRC bolts is the grippingof the tendon to allow tensioning and load lock-off at thehead; currently the capacity of a GFRC bolt is limitedby the strength of the head. For example, the bolt headassembly shown in Figure 3.10 has a breaking load of160kN and a design working load of 100kN comparedwith a rupture load of 310kN for the tendon (WeidmannAG).The results of pull out tests on GFRC composite barsare reproduced from Faoro (1991) in Figure 3.11. Thetests were carried out on epoxy coated glass fibre barshaving a 30mm bond length. Ultimate bond strengths of9.2N/mm were recorded for the tests with the barsembedded in a sand-cement mortar, and in excess of26.4N/mm with an epoxy mortar. It is necessary toundertake specific tests for the particular GFRC systemunder consideration.By way of example, the use of tensioned GFRC bars institching masonry blocks together is illustrated in Figure3.12, taken from Faoro (1991). For this application thebolts were post tensioned and the force from the bolthead had to be transferred to the masonry facing blocksso that the bolt heads were not visible. In design, a forceof 15kN was assumed to be transferred into the facingblocks over a bond length of 120mm and a factor ofsafety against bond failure of 2.5 was adopted. GFRCbolts, fully bonded with a single speed resin, have beenused as permanent support in the Vereina rail tunnel;details have been provided by Streuli and Klahr (1995).GFRC bolts can be fitted with purpose-made expansionshells that allow a pre-tensioning force of perhaps 20%of the ultimate lock-off load of the bar to be appliedimmediately following installation. Usually in such casesgrout is injected through the centre of the anchor tube tobond the bolt to the rock.

Other components

3.4.1 Faceplates

Usually a faceplate is attached following the installationof the tendon in the drillhole. Load is transferred fromthe tendon via threaded nuts onto the faceplate. In mostcases the plate bears directly onto the rock face orstructure, but on irregular surfaces the faceplate willoften be bedded into a rapid-set mortar.Faceplates are usually square, rectangular or circular inplan but triangular plates have also been used.Faceplates are most usually formed from a flat steelplate, typically 150 to 250mm square, but they can bedomed or dished. Forged, cast or pressed plates can also USE OUTSIDE THE AGENCY. DOCUMENT ARE UNCONTROLLED

3/5


Chapter 3Types of Rock Boltsbe used, and these may be ribbed to increase strength. Aribbed plate deforms as the load in the bolt reaches acertain level and this provides some warning ofoverloading. Faceplates used in conjunction withcorrosion protected rock bolts often have protectionspigot tubes attached, these are similar to thosedescribed in BS 8081: 1989 for ground anchorages.Where the tendon is not perpendicular to the faceplate,the load may be transferred uniformly to the platethrough a hemispherical nut or washer bedded in asuitable tapered seating; alternatively a pair of bevelledwashers can be used. Where the load to the rock bolt isto be applied by a torque wrench, washers are usuallyhardened and lubricated to reduce friction. Where a boltis tensioned by an hydraulic jack, usually a purposemade foot is attached to the jack so that the force actsdirectly through the washer system thereby avoidingeccentric loading between the jack and faceplate.For permanent installations, consideration should begiven to protecting the thread and nut from damage andcorrosion by a cap assembly or by a cover of structuralgrade concrete. When used, a cap should be filled withapproved grease or other corrosion protectioncompound.

3.4.2 Centralisers

Centralisers are attached to the tendon to ensure correctalignment and a minimum cover of grout.

3.4.3 Grouts

(i) Cementitious grouts would usually be formedfrom one of the following;a) Ordinary Portland cement to BS 12: 1996b) Rapid hardening cement to BS 12: 1996c) Portland blast furnace cement to BS 1370:1979d) Low heat Portland cement to BS 1370: 1979e) Sulphate resisting Portland cement to BS 4027:1996f) Low heat Portland blast furnace cement to BS4246: 1996Cement used in grout capsules should complywith BS 12: 1996 or BS 915: 1983.

(ii) Epoxy and polyester resins in pumpable, pourableor capsule form are commonly used for rockbolting: such resins should be designed andrecommended for this application. If appropriate,to suit the conditions of the particular application,the selection of material performance criteriashould be established in conjunction with theELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONI

3/6manufacturer. Laboratory and field tests shouldbe undertaken, or the results of previous testsshould be available, to verify mix times, settingtimes and pull out capacity.

Corrosion protection

3.5.1 For permanent works, or where bolts are installedin a corrosive environment, the rock bolt (including thehead) should be protected from corrosion. The degreeand type of protection depends upon the design life ofthe bolting system, the corrosivity of the environment,and the severity of the consequences of failure. Means ofassessing the aggressivity of a site are given for groundanchorages in BD 71 (DMRB 2.1.6), and for reinforcedsoils in BS 8006: 1995 as implemented through BD 70(DMRB 2.1.5). In these, aggressivity is assessed byallocating weighted values to a range of variables and asimilar system for rock bolts is provided in Table 3.12.This gives a general guide to aggressivity but it isimportant to understand the influence of individualfactors and their affect on the specific installation. Thecorrosion of rock bolts has been discussed by Baxter(1997) and Franzn (1997).

3.5.2 Rock bolts are often considered to need a lowerlevel of corrosion protection than ground anchorages forthe following reasons.

(i) Most rock bolts for permanent works are requiredto be fully bonded after stressing. This is achievedeither by the injection of a secondary grout to thefree length soon after the bolt has been stressed,or by the use of a two-speed resin system.

(ii) The tendons of ground anchorages are usuallyformed from high tensile steels which are far moresusceptible to stress corrosion cracking than thelower grades of steel more commonly used forrock bolts.

(iii) Rock bolts carry much lower individual loadsthan ground anchorages, and usually act inconsort with other measures, such as shotcreting,to provide support. Thus the failure of a boltwould be far less significant to the overall supportsystem than would the failure of a groundanchorage.

(iv) The loss of part of a bolt by corrosion may notlead to the catastrophic detensioning of the entiremember.

Despite the foregoing it is important to remember thatrock bolts are load carrying tensile members installedinto natural ground which is inherently heterogeneous.Furthermore, a bolt may not always be installed entirelyin accordance with the specification. Thus, for all sites,February 1999 USE OUTSIDE THE AGENCY.C DOCUMENT ARE UNCONTROLLED



the installation process and the corrosivity of the ground

within which the bolts are installed must be assessed todetermine the appropriate corrosion protection measures.

3.5.3 Details of various corrosion protection measuresare given below.

(i) Sacrificial thicknessThe specification of sacrificial thickness of steelfor reinforced earth and soil nailing applicationsare covered by BS 8006: 1995 as implementedthrough BD 70 (DMRB 2.1.5). The corrosionallowances for a particular design life varyaccording to the aggressivity of the ground withthe over-riding proviso that unprotected steelshould not be used in highly aggressive conditionsfor permanent works. Such allowances may beadapted for rock bolting works. It should be notedthat the sacrificial thickness (t) is applied to eachexposed surface; thus the diameter of a bar isincreased by 2t.At present, there is no way of predicting corrosionrates to a comfortable level of confidence.Because of this BS 8081: 1989 recommendedthat, as a general rule, permanent groundanchorages should be protected from corrosionbut it proposed that a secondary grout cover mayprovide sufficient protection to low capacitypermanent rock bolts used solely as secondaryreinforcement.Water flow in rock masses, and the correspondingtransport of potentially corrosive fluids, ispredominantly through fissures. Thus the localeffect of water flow upon a particular bolt cannotbe predicted well. Therefore it is recommendedthat permanent rock bolts should not be usedwithout some form of corrosion protection and,where the failure of a bolt could lead to asignificant risk to public safety, a sacrificialthickness of material should not be relied upon toprovide longevity.

(ii) Secondary grouted annulusSecondary grouting can be applied to varioustypes of rock bolts. Where there is a risk that thefailure of a bolt would have serious consequencesit is not recommended that a cement groutannulus, by itself, be deemed adequate protectionfor the most aggressive ground conditions, or inthe most demanding of work conditions. Theexecution of the grouting operations should ensurethat an adequate cover of grout is provided alongFebruary 1999 ELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONICthe full length of a bolt: the inflow of water into adrillhole can reduce severely the effectiveness ofgrouting operations, see for example Azir et al(1992).For upward facing holes, grout is usually injectedat the mouth of the hole and air expelled through ableed tube installed to the distal end of thedrillhole. For downward facing holes grout can beinjected to the bottom of the hole, either through asmall inlet tube or through a central hole withinthe bar. Alternatively a temporary oversized tubecan be placed over the shank to the distal end ofthe free length; this tube is withdrawn as thesecondary grout is injected. The bolt is thentensioned, or retensioned, before the secondarygrout hardens. However secondary groutingtechniques are time-consuming compared with asingle pass two-pack resin capsule system.The effectiveness of the method of grouting mustbe assessed when the drillholes are inclined at lowangles to the horizontal (i.e. about 10). At suchlow angles, bleed or small losses of grout canleave parts of the bolt ungrouted.Attention must also be given to the centring of thebolt within the drillhole. The problem withcentralisers is that they restrict the available borewhich can lead to the formation of groutblockages or air pockets, particularly with smallerdiameter holes. Water flow within the rock massis predominantly through fissures and thepermeability of the intact rock is often relativelylow compared to that of the rock mass. Anappraisal of the properties of the rock mass andthe proposed grout may demonstrate that thefilling of the annulus with a dense homogeneousgrout is of prime importance, and the use ofcentralisers could be counter-productive to thisaim.Where resin grouted rock bolts are fixed in placeusing a capsule system, the resins not only bondthe free length of the bolt but effectively encase itwithin an inert medium. Such systems arecommonly employed for permanent rock bolts,particularly where they form part of the secondarysupport system. For example they were usedextensively at the Pen-y-Clip and Penmaenbachroad tunnels on the A55 North Wales coast road:details have been provided by Littlejohn et al(1987) and Xu et al (1995).

(iii) CoatingsThe most usual coating to the shank of a rock boltis zinc applied through a hot dip process. Inaddition to providing a physical barrier, zinc willcorrode preferentially to the steel substrate. With USE OUTSIDE THE AGENCY. DOCUMENT ARE UNCONTROLLED

3/7


Chapter 3

bolts the free length is grouted following itstensioning. Secondly, the protection to the boltdoes not necessarily extend over the complete barand head assembly. For example as shown inFigure 3.13, the base of the bolt is exposed: boltsfixed by resin capsules require some form ofpuncturing, stirring and mixing device at theirTypes of Rock Bolts

minor mechanical damage, such as a scratch, thezinc adjacent to the breach will reduce the rate ofcorrosion of the underlying steel, and in somecases the resulting corrosion products will inhibitfurther corrosion at the breach. The rates ofcorrosion of a galvanised coating can varymarkedly with both time and position, andcurrently the lifetime of a coating cannot bedetermined with a high degree of confidence. Thusit is not recommended that galvanising be used asthe sole or primary corrosion protection measurewhere long term durability is a primeconsideration. (The density of the galvanising canbe assumed to be equivalent to 7.15g/m2 permicron of coating thickness).Epoxy resins are inert and provide a physicalprotection to the steel surface. The characteristicsand use of these proprietary coatings should bedefined in an approval certificate. Most of theproprietary systems that have been used involvethe factory application of fusion bonded coatings.With these a coating of epoxy powder is depositedonto the pre-heated bars and allowed to cure in aheat catalysed reaction. Such coatings can beapplied to galvanised bars, and the Combi-coat(Ischebeck Titan Ltd) is claimed to enhance theservice life of a galvanised bar by a factor ofabout 2 to 3.The possibility of coatings being damaged duringinstallation must be considered and siteprocedures should ensure that the extent of anysuch damage is minimised. A damage factor couldbe assumed in design which, effectively, wouldlead to the installation of additional rock boltsover and above that required to maintain stability.However this may not be applicable to all sitesand applications, and the selection of anappropriate damage factor is problematic.

(iv) External sheathsDetails of externally protected rock boltassemblies that have been used in practice aregiven in Figures 3.13 and 3.14. Typically, theserequire centring the bar within a corrugatedplastic sleeve which is then filled with acementitious grout, i.e. in a similar manner toground anchorages. Indeed such rock bolts areoften termed double corrosion protected, but notethat neither BS 8081: 1989 nor BD 71 (DMRB2.1.6) recognise grout as being an effectivecorrosion protection barrier.There are important differences between anexternally protected rock bolt and a doublecorrosion protected ground anchorage. Firstly,unlike ground anchorages, most bolts are notELECTRONIC COPY NOT FORPAPER COPIES OF THIS ELECTRONIC

3/8

debonded over their free length: with permanentdistal end. In other cases the primary bond lengthis left exposed with the external sheath coveringonly the free length. In addition, it is notuncommon for the protection provided to the outerhead of permanent ground anchorages to beomitted for rock bolts. For example where thebolts are incorporated into a shotcrete lining, thelining may be deemed to obviate the need forprotection to the cap. This accords with the viewthat the bolts and the shotcrete are part of anintegrated support system.Figure 3.14 shows the arrangement of a doublecorrosion protected rock bolt which meets therequirements of BS 8081: 1989 except that thearrangement at the head is not as complexbecause the bolt is fully bonded after tensioningand is incorporated into shotcrete facing. If thehead of the rock bolt is to be left exposed, thehead assembly should be capped and protected ina similar manner to the requirements of BD 71(DMRB 2.1.6).Figure 3.15 shows details of a rock bolt where aheat-shrink sleeve has been fitted to the free lengthof a coarse threaded bar.External sheaths have been used in combinationwith hot dip galvanised and epoxy coated bolts,see for example Smith (1994).

(v) Corrosion resistant bars and fittingsGlass fibre reinforced composites and stainlesssteels have been used to form the tendons andother components of rock bolts. The use of eitherof these materials bears a cost premium and sotheir use is usually only justified where longevityis a particular concern. It should not be assumedthat such materials will not deteriorate over time,but in most situations they are essentially durable.In many cases they are simpler to install than steelbars that require some form of protection.February 1999 USE OUTSIDE THE AGENCY. DOCUMENT ARE UNCONTROLLED



Type of bar Nominal diameter Characteristic yield(mm) strength (N/mm2)

(mm) (mm2) Rupture Yield

16 201.1 120 9320 314.2 187 145

25 490.9 292 226

32 804.2 478 370

40 1256.6 748 578

50 1963.7 1168 903

Table 3.2 Characteristic rupture and yield loads for Grade 460 high yield reinforcing barsMild steel smooth 20, 25, 32, 40, 50 250

High yield deformed 20, 25, 32, 40, 50 460

Cold worked deformed 20, 25, 32, 40, 50 460

Stainless steel deformed 20, 25, 32, 40 460

Table 3.1 Typical sizes and characteristic yield strengths of steel bars usedfor rock bolts and ground anchorages in the UK

Nominal diameter Nominal cross Characteristic loads (kN)sectional areaFebruary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/9



Nominal ISO Characteristic UNF Characteristic Whitworth Characteristicdiameter metric loads threads* loads threads* loadsNominal bar diameter (mm) 16 20 25 28 32 40 50 63.5

Steel grade (N/mm2) 500/600 500/600 500/600 500/600 500/600 500/600 500/600 555/700

Diameter over threads (mm) 19 23 29 32 36 45 56 69

Mass per unit length (kg/m) 1.58 2.47 3.85 4.83 6.31 9.87 15.40 24.80

Cross sectional area (mm2) 201 314 491 616 804 1260 1960 3167

Characteristic yield load (kN) 100 157 245 308 402 630 980 1758

Minimum ultimate tensile load (kN) 121 188 295 370 482 756 1176 2217

Standard lengths (m) 6 12 12 12 12 12 12 12

Table 3.4 Physical properties of Dywidag Gewi-Steel Grade 500/600 high yield fully threaded bar(Dywidag-Systems International Ltd)threads*(mm) (mm) (inch) (inch)

Rupture Yield Rupture Yield Rupture Yield(kN) (kN) (kN) (kN) (kN) (kN)

16 M142 55 46 51 43 40 34

20 M182.5 92 77 - 83 70 - 68 57

25 M222.5 150 125 - 167 140 - 144 120

32 M303.5 277 231 284 237 1 240 200

40 M364 406 339 1 533 446 450 376

50 M454.5 657 550 - - - 1 607 508

* Thread types defined, for example, in Kempes Engineers Yearbook (1997)

Table 3.3 Typical characteristic rupture and yield loads for machinecut threads on Grade 460 high yield reinforcing barsFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/10


Nominal Maximum Cross Mass per Thread pitch Rupture load Yield loaddiameter diameter sectional area metre length

(mm) (mm) (mm2) (kg/m) (mm) (kN) (kN)

20 21.7 314 2.47 8 173 157

25 27 491 3.85 10 270 246

28 30.1 616 4.83 11 339 308

32 34.6 804 6.31 12.5 442 402

40 42.9 1256 9.87 16 691 628

50 53.1 1963 15.4 20 1080 981

Table 3.5 Physical properties of Macalloy 500 high yield threaded bar (McCalls Special Products Ltd)

Chapter 3Types of Rock BoltsFebruary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/11


Nominal Cut threads Rolled threadsdiameter

(mm)

Thread type* Characteristic load Thread type* Characteristic load(kN) (kN)

Rupture Yield Rupture Yield

12 M12 1.75 29 18" UNF 37 23

Chapter 3Types of Rock Bolts" BSW 29 18

16 M16 2 55 35 M16 2 63 39-" UNF 61 38 -" UNF 65 41-" UNF 50 31 --

20 M20 2.5 87 54 M20 2.5 98 62" UNF 89 56 " UNF 95 59" BSW 75 47 --

25 M24 3 125 78 M24 3 142 891" UNF 158 99 1" UNF 169 1061" BSW 138 86 1" BSW 161 100

32 M30 3.5 201 126 M33 3.5 283 177" UNF 261 163 1" UNF 277 173" BSW 224 140 1" BSW 261 163

40 M39 4 356 222 M42 4.5 458 2861" UNF 388 242 1" UNF 406 2541" BSW 327 204 1" BSW 377 236

50 M48 5 537 336 M52 5 715 4472" BSW 672 420


Table 3.6 Characteristic rupture and yield loads for mild steel bars with cut and with rolled threadsFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/12


Bar type Nominal Nominal cross Characteristic strengths (N/mm2) Characteristic load of parent bar (kN)diameter sectional area

(mm) (mm2)

Rupture Yield Rupture Yield

Chapter 3Types of Rock BoltsRolled M18 x 2.5 107 90 131 109

25 Cut M22 x 2.5 150 125 - 167 140 - 144 120Rolled M24 x 3 195 163 - 179 149 - 168 141

32 Cut M30 x 3.5 277 231 - 284 237 - 240 200Rolled M30 x 3.5 316 264 - 305 255 - 285 238

40 Cut M36 x 4 406 339 533 446 450 376Rolled M39 x 4 547 457 559 467 519 434


Table 3.7(b) Typical characteristic rupture and yield loads for stainless steelhigh yield deformed reinforcing bar with cut threads(0.2% proof stress) (0.2% proof load)

Stairib 304/ 16 201.1 700 525 141 106Stairib 316

20 314.2 700 525 220 165

25 490.9 700 525 344 258

32 804.2 700 525 563 422

40 1256.2 700 525 879 660

Table 3.7(a) Characteristic rupture and yield strengths for Grade 304/316 austeniticstainless steel deformed reinforcing bar (Ancon CCL)

Nominal Thread ISO Characteristic UNF Characteristic Whitworth CharacteristicDiameter process metric load (kN) thread* load (kN) thread* load (kN)

thread*(mm) (mm) (inch) (inch)

Rupture Yield Rupture Yield Rupture Yield

16 Cut M14 x 2 55 46 51 43 40 34Rolled M14 x 2 64 54 - 90 75

20 Cut M18 x 2.5 92 77 - 83 70 - 68 57February 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/13


Nominal bar diameter (mm) 16 20 24

Chapter 3Types of Rock Bolts26.5 1080/1230 678 595 4.48 551 13

32 900/1030 828 724 6.53 804 16

32 1080/1230 989 868 6.53 804 16

36 900/1030 1049 916 8.27 1018 18

36 1080/1230 1252 1099 8.27 1018 18

Modulus of elasticity, E=205kN/mm2 5%

Table 3.8 Physical properties of Dywidag fully threaded steel bars used forprestressing works (Dywidag-Systems International Ltd)Diameter over threads (mm) 16.4 20.1 24.0

Diameter thread root (mm) 13.4 17.1 20.2

Mass per unit length (kg/m) 1.3 2.1 2.9

Characteristic yield load (kN) 91 147 190

Minimum ultimate tensile strength (N/mm2) 750 750 750

Ultimate tensile load (kN) 105 170 219

Standard lengths (m) 3 - 6 3 - 6 3 - 6

Table 3.7(c) Typical physical properties of Grip-Bar stainless steel threadbar:Grade 304 or 316 austenitic stainless steel (Stainless UK Ltd)

Nominal Steel grade Ultimate Yield Mass Cross Pitchdiameter yield/ultimate load load sectional area

strength(mm) (N/mm2) (kN) (kN) (kg/m) (mm2) (mm)

15 900/1100 195 159 1.44 177 10

20 900/1100 345 283 2.56 314 10

26.5 900/1030 568 496 4.48 551 13February 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/14


Chapter 3Types of Rock Bolts25 to 50 1030 835 6 170

70 1030 835 6 205

Table 3.9 Physical properties of standard grade Macalloy steel barsused for prestressing works (McCalls Special Products Ltd)

Nominal diameter Characteristic rupture load Minimum 0.1% proof load(mm) (kN) (kN)

20 314 251

25 491 393

32 804 643

40 1257 1006

Grade Characteristic ultimate Minimum 0.1% Minimum Approximate modulustensile strength proof stress elongation of elasticity

(N/mm2) (N/mm2) (%) (kN/mm2)

Stainless 1000 800 15 210

Table 3.10 Physical properties of Macalloy stainless steel barsused for prestressing works (McCalls Special Products Ltd)Nominal diameter Characteristic rupture load Minimum 0.1% proof load(mm) (kN) (kN)

25 506 410

26.5 569 460

32 828 670

36 1049 850

40 1295 1050

50 2022 1639

70 4311 3495

Nominal diameter Characteristic ultimate Minimum 0.1% Minimum Approximate modulustensile strength proof stress elongation of elasticity

(mm) (N/mm2) (N/mm2) (%) (kN/mm2)February 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/15


Chapter 3Types of Rock BoltsDiameter Ultimate tensile load Ultimate shear load End loading* Normal weight(mm) (kN) (kN) (kN) (kg/m)

Bolts

22 380 120 100 - 250 0.75

25 500 150 125 - 250 0.97

32 820 275 130 - 300 1.67

6 (strand) 145 50 90 0.056

Tubes

22 (OD) 10 (ID) 250 85 90 - 140 0.65

32 (OD) 15 (ID) 495 195 - -

* The higher figure is obtained with high end load fittings

Table 3.11(b) Data for polyester resin Fibregrip products (Weldgrip)Material Tensile Yield Strain at Modulus of Density Range ofstrength strength failure elasticity available

diameters(N/mm2) (N/mm2) (%) (N/mm2) (Mg/m3) (mm)

'Polystal' 1670 - 3.3 51 000 2.0 1 to > 25(68% glass fibres)

'Durglass FL' 1000 - >3 40 000 1.9 5 to 40(70% glass fibres)

'Weidmann' 1200 - - 50 000 2.5 22 OD(75% glass fibres) 10 ID

(tubular)

'Twarun' 2150 - 2.0 125 000 1.45 -(aramid fibres)

Carbon fibres 2800 - 0.7 400 000 1.75 -

High yield steel >500 >420 10 210 000 7.65 20 to 50

Table 3.11(a) Comparison of material values for glass fibre composites and other materials,after Miesseler and Preis (undated), Faoro (1991) and Sireg SpaFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/16

Volume 2 Section 1 Chapter 3

Part 7 BA 80/99 Types of Rock Bolts

Item Measured value Mark

Composition of ground Material containing not more than 10% of particles (by weight) +2passing the 63 micron sieve size with material passing the 425 micronsieve being essentially non-plastic

Material containing not more than 75% and 10% of particles 0(by weight) passing the 63 and 2 micron sieve sizes respectivelyand with the material passing the 425 micron sieve having aplasticity index less than 6

Any grading, material for which the particles passing the 425 micron -2sieve have a plasticity index greater than 6 but less than 15

Any grading, material for which the particles passing the 425 micron -4sieve have a plasticity index of 15 or greater

Material having an organic content of 2% or greater -4

Fill material containing cinder, coke or slag -4

Groundwater Well drained area +1

Poorly drained area -1

Above level of rock bolt -4

Resistivity (ohm-cm) 10,000 or more 010,000 - 3,000 -13,000 - 1,000 -21,000 - 100 -3100 or less -4

pH of ground or Greater than 9 -2groundwater Between 6 and 9.0 0

Less than 6 -2Between 4.5 and 6.0 -4

Soluble sulphate (ppm) 200 or less 0within ground or 200 - 500 -1groundwater 500 - 1000 -2

1000 or more -4

Presence of sulphide None 0and/or hydrogen sulphide Trace -2

Present -3High -4

Chloride ion (ppm) within 50 or less 0ground or groundwater 50 to 250 -1

250 to 500 -2500 or more -4

Table 3.12 Assessment of the aggressivity of a site

Ranking value Aggressivity -3 Low-4 to -6 Medium-7 or less HighFebruary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/17


Grout returntube

Expansion shellanchor

Rubber groutseal

3/

Chapter 3Types of Rock BoltsGrout inlettube

Face plate

Nut

Sphericalwasher

Grout inletand return

tubes attachedto shank

Grout returntube in slot

to shank

Grout returnthrough centre

of shankFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.1 Details of a typical expansion shell rock bolt having the provision for secondary grouting

18


Fe

Chapter 3Types of Rock Bolts(b) Shells for large diameter drillholes or for use in soft rock

Fig 3.2 Details of typical expansion shell rock boltsbrPattin bolt Goldenberg bolt Bail bolt

(a) Standard typesuary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

3/19


Chapter 3Types of Rock BoltsWasher

Expansion anchor

Expansion anchor

Washer

Ramps machinedor cast into boltFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.3 Details of the Worley mechanical rock bolt (Mine Roof Support Systems)

3/20


Fe

Chapter 3Types of Rock BoltsFace plate

Split tube forced into drillhole

Anchor formed by folding end of tube back on itself

Section throughsplit tube

13mm

2.3mmbruary 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.4 Details of the Split set rock bolt (Ingersoll-Rand Co Ltd)

3/21

Volume 2 Section 19

Fig 3.5 Details of the Swellex frictional anchor (Atlas Copco Construction and Mining Ltd)

Chapter 3TypFace plate

Bevelled washers

Hardened washer

Nut Bolt shank

Slot Wedge

Wedge3/2Part 7 BA 80/9

expanded after insertionin drill hole

prior to installation

es of Rock BoltsFebruary 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.6 Details of slotted bolt and wedge device

2


February 1999 ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.8 Details of untensioned grouted rock dowel


3/23

Perforated half tube

Mortar

Halves wiredtogether

Mortar extruded as baris inserted

Bar

Grout

Face plate

Fig 3.7 Details of the "Perfobolt" system, after Hoek and Brown (1980)


February 1999ELECTRONIC COPY NOT FOR USE OUTSIDE THE AGENCY.PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

Fig 3.9 Schematic view of wire cable bolt installation

Grout seal

Distal

Competent strata Weak strata

Locking plateand cone

Breather tube

Proximal

Cable

Injection tube

3/24




Fig 3.10 Details of a typical GFRC bolt head assembly

Dimensions in mmbut not to scale

115.0

130.0

70.0

45.5


3/25



Fig 3.11 Results of bond tests for GFRC bars, taken from Faoro (1991)

10

8

6

4

2

00 1 2 3

deflection (mm)

(i) GFRC bars embedded in sand/cement mortar

t (N/mm2)t

u

=9.2N/mm2

30

20

10

00 0.5 1.0 1.5

deflection (mm)

(ii) GFRC bars embedded in epoxy-acrylate resin

t (N/mm2)

t

u

= 26.4N/mm2tu

= 26.4N/mm2

Type Amortar

Type Bmortar

3/26




Fig 3.12 Use of tensioned GFRC bars for masonry repairs, taken from Faoro (1991)

Masonry facing block

GFRC - bars installed in22mm diameter drill hole

1000mm0


3/27



Fig 3.13 Details of externally protected rock bolts

End nut

Resinous mortar

Corrosion sheathingwith cement grout filling

Stopper and centring plug

Grout tube forsecondary grouting

Vent tube

3/28




Bottom end plug

Corrugated plastic duct

Bar

External centralisers,as required

Cement grout filling tocorrugated duct

Resin retainer/sealer

Internal spacer

Secondary grout venttube

Primary resin grout bleed/vent tube

Outer trumpet assembly

Anchor head assembly nutand washer

Tendon bond lengthPolyester resin(Primary grout)

Free tendon lengthCement grout

(Secondary grout)

Secondary groutinjection tube


3/29

Fig 3.14 Details of a double corrosion protected rock bolt



Resin mortar

Heat shrink withinner lining and corrosion protectioncoating to bar(e.g. epoxy coating)

Shortcrete facing

3/30


Fig 3.15 Details of a rock bolt provided with a heat-shrink sleeve


Chapter 4Design

to a stable anchor zone at some depth. The design ofindividual bolts must address the following aspects.4. DESIGNPrinciples and objectives4.1.1 This section covers the design of rock bolts. Thedesign of rock bolted structures is beyond the scope ofthis Advice Note.

4.1.2 The objective of using rock bolting systems is tocreate a stable rock mass. The design of the supportsystem is dependent upon the physical properties andcharacteristics of the rock mass - including the strengthof the intact rock; the in situ stress field; the spacing,persistence, nature and infill of discontinuities; and theorientation of the discontinuities relative to the face ofthe excavation. In addition, the stress straincharacteristics of the rock mass and the reinforcementsystem should be matched to suit the end productrequired (Gerrard 1983, Barton & Bakhtar 1983).

4.1.3 The methodology behind the design of rock bolts,rock dowels or cable dowels can be divided into twoapproaches, Active and Passive, examples of which aregiven in Figure 4.1Active reinforcing elements are stressed with a designworking load on installation. Active reinforcementlimits strains and therefore prevents, or minimises,movement or deformation of the rock mass. Hence, thistype of reinforcement should be targeted at sites wherestrain must be limited (eg serviceability limits).Tensioned rock bolts are most effective in retaining looseblocks or wedges of rock. The support is only usuallyrequired to hold up the dead weight of loose material.The tensioning of the bolts is required in order to tightenthe loose blocks and provide as much interlocking aspossible between these blocks and their failure plane. Itis by helping the rock to support itself, and byprevention of further unravelling and deterioration of therock mass that tensioned rock bolts provide effectivesupport (Hoek and Wood 1992).Passive reinforcement is so called because thereinforcing elements are not stressed. Passive elementsonly become stressed once deformation/movement takesplace in the rock mass. This type of reinforcementincludes dowels and cable dowels (McMillan 1993).Untensioned dowels are often installed before significantmovement has taken place in the rock mass, and becometensioned through strain of the rock mass (Hoek andWood 1992)

4.1.4 Information on the design of rock bolts is providedin BS 8081: 1989, and references such as Hoek andBrown (1980) and Hobst and Zajic (1983).February 1999 ELECTRONIC COPY NOT FORLoads

4.2.1 Determination of the loads to be resisted by a rockbolting system often requires a complex analysis to takeinto account the large number of factors, many of whichmay be ill-defined. However, the revealing ofunforeseen ground conditions, or poorly executedconstruction techniques, such as blasting, may wellrequire the original analysis to be reworked with newinformation. Thus an important, if not the crucial,ingredient of any rock support work is the ability of theengineering team to adapt the design and installationprogramme to the conditions encountered on site. It is,therefore, common practice for any sizeable bolting orrock stabilising operation to be controlled and refined ona day-to-day basis in the field; in such cases theengineering team must have a clear understanding ofwhat is to be achieved, what options are available, andwhich option should be selected to suit the conditionsencountered.

4.2.2 The orientation of a bolt or dowel is important asit determines the efficiency of action of thereinforcement. For active reinforcement the forcerequired to stabilise a block sliding on a failure planevaries with the angle of inclination between the force andthe failure plane (Figure 4.2a) (Franklin and Dusseaut1989). The example given in Figure 4.2a is a simplecase, but the same principle applies for more complexsituations as shown in Figure 4.2b. For passivereinforcement the shear resistance of the doweleddiscontinuity varies as a function of the angle betweenthe dowel and the discontinuity (Gaziev and Lapin 1983,Ludvig 1983). There is an optimum angle for theinstallation of a dowel. This angle lies in the range 35to 50 to the plane of the discontinuity (Barton andBakhtar 1983).

4.2.3 The loads in individual rock bolts commonly rangefrom 150 to 200kN. Loads of up to 300kN have beenused in practice but they are exceptional for surface, ornear-surface, construction works.

Design of individual bolts

4.3.1 Following the establishment of the overall outlinesupport scheme, the detailed design of the bolting systemmust address the transference of the required supportforces from the bolt head (which applies the stabilisingforce into the structure, excavated face or jointed block) USE OUTSIDE THE AGENCY. 4/1


Chapter 4

Ps = pi d L

u

where u is the ultimate bond strength of the rock/

grout interface and d is the diameter of the bondedDesign

(i) The available bond at the anchor/rock interface.

(ii) The location of a stable anchor zone.

(iii) The available bond at the tendon/anchor interface.

(iv) The available bond at any encapsulation/groutinterface.

(v) The properties of the tendon.

(vi) The adequacy of the bolt head with regard tostrength, stiffness, robustness and durability.

4.3.2 Bond at the anchor/rock interface

The transfer of load from the tendon to the rock surfacein the anchor zone can be achieved by mechanicalmeans, such as the expansion shell anchorage, or bycement or resin bonding.

(i) Mechanical anchorsMechanical rock bolts are tensioned immediatelyafter installation, and so it is unnecessary toprescribe a specific approach to their design.Manufacturers of such types of rock bolt have arange of products to cope with varying rockconditions and give guidance on design for arange of rock qualities and strengths. The act oftensioning the bolt to its required loadingconfirms, at least in the short term, the ability ofthe mechanical bolt to support a particular load.High tensions, where the yield strength of thetendon is approached, can be achieved in goodquality rock, but in poor quality rocks localcrushing of the rock can be generated by the highlocal point loads exerted by an expansion shell,and anchor slip can also occur.

A summary of the results of pull out tests onmechanical anchors is reproduced from Moy(1973) in Table 4.1. The data show thedecreasing pull out resistance of mechanicalexpansion shell type rock bolts in weaker rocks.Although capacity can be increased in such rocksusing special types of malleable expansion shellsand coupled expansion assemblies, such devicesare not commonly used in the civil engineeringindustry in the UK.

(ii) Cement grouted anchorsThe calculation of the design anchor length (L) ofa cement grouted rock bolt generally follows asimilar methodology to that employed for groundanchorages, i.e. the ultimate shaft friction (P

s) is

calculated using an equation of the form;ELECTRONIC COPY NOT FOR US4/2length (the units of the variables should beconsistent).

It should be noted that this equation is based onthe assumption that load is transferred uniformlyover the whole surface area of the design fixedanchor length. Littlejohn (1979) cautions that thisapproach may lead to high concentration of stressat the proximal end of anchorages in weakdeformable rock; nevertheless the assumption of auniform bond stress is common practice.Preliminary design values of ultimate bondstrength

u may be derived from the results of

laboratory or in situ tests, or from previouslypublished values.

(a) Laboratory tests. Values of u are commonly

based on the unconfined compression strength(UCS) of the rock, for example;

u = 0.1UCS

An upper limit of 4000kN/m2 is normally appliedto

u - this commonly being about 10 per cent of

the design characteristic unconfined compressivestrength of a cementitious grout.

(b) Field tests. Correlation between strength andstandard penetration blow counts (N) have beenderived for a range of rock types.

Chalk: For stiff/hard chalk (weathering grades Ito III), Littlejohn (1970) suggested the followingrelation;

u = 10N (kN/m2)

Barley (1988) derived the following from tests onpressure grouted anchorages;

u = 20N to 30N (kN/m2)

similarly, Turner (1980) derived values of u(in kN/m2) equivalent to 16N for chalk grades II

to III.February 1999E OUTSIDE THE AGENCY.


Feb

Chapter 4DesignIt should be noted that a new system forclassifying chalk has been developed since theabove was carried out. Details of the newclassification system can be found in CIRIAProject Report 11, 1994 entitled Foundations inChalk.

Weathered granite: Suzuki et al (1972) gave thefollowing relation for anchorages installed inweathered granite in Japan,

u = 7N + 120 (kN/m2)

Mudrocks: Barley (1988) suggested that thecorrelation for the various types of mudstone fellinto the following range;

u = 2.4N to 6N (kN/m2)

but suggested that a lower bound value of190kN/m2 for

u seemed to be appropriate for a

wide range of mudstones with N values rangingbetween 34 and 95.

Sandstones: Barley (1988) gave the followingrelation for weak sandstones;

u

= 5.5N to 15N (kN/m2)

(c) Published values. A review of bond strengthscompleted by Littlejohn and Bruce (1977) wasreproduced in Tables 24 and 25 of BS 8081:1989. Barley (1988) also tabulated bondstrengths for a wide range of rock types. Turner(1980 and 1995) gave bond strengths derivedfrom a range of tests (undertaken mainly in theUK) and compared them to other parameters suchas rock type, strength, Rock Quality Designationand degree of weathering. All the above data,however, relate to ground anchorages whichusually have larger diameters and pull outcapacities than rock bolts.

The results of pull out tests undertaken on cementgrouted rock bolts are reproduced from Moy(1973) in Table 4.2. In all these tests the yieldstrength of the bar was reached before theultimate strength of the interface was attained.

Cement grouted rock bolts can incorporate amechanical expansion fixing device to provide theinitial load holding capacity: this can besubsequently enhanced by grouting of the fulllength or just the anchor length of the bolt.ruary 1999 ELECTRONIC COPY NOT FOR(iii) Resin bonded anchorsAn estimate of the strength of a resin/rockinterface can be based on previous experience,empirical relations or site tests. The results of onsite pull out tests are reproduced from Whittakeret al (1977) in Table 4.3. They suggested that thedesign anchor length is a major considerationwhen the uniaxial compressive strength of therock is less than 35MN/m2: this is equivalent tothe mid-range of a moderately strong rockaccording to BS 5930: 1981. For weaker rocks,the bond strength of a resin/rock interface oftenhas to be established or confirmed from the resultsof site tests. For stronger rocks the bond betweenthe resin and the bar often controls the pull outcapacity of the bolt. Whittaker et al (1977)suggested that an estimate of the bond length (Lb)for a polyester resin bolt embedded in a strongrock can be derived from;

Lb = (50 + 2.5P)

where Lb is in mm and P is the maximumanticipated design load (in kN).

Franklin and Woodfield (1971) produced a designchart relating a bond factor (in inches/short ton)with rock strength expressed as point load index;their chart is reproduced in Figure 4.3 afterconversion to metric units. They suggested thatthe required bond length (Lb) can be derived fromthe relation;

Lb = (bond factor x P) + safety margin.

Their approach gives bond lengths of typicallyless than 0.5m. Franklin and Woodfield (1971)noted that failure in weak rocks tended to occur atthe resin/rock interface, but with very strongrocks it was likely to occur at the resin/anchorinterface.

In conventional civil engineering practice bondlengths generally range between 1 and 2m; theselonger lengths are adopted to account forvariations in rock quality over a site andconstruction practice.

4.3.3 Uplift or pull out capacity

It is usually necessary to check that the anchor length issufficiently deeply embedded that in the event of the fulldesign force being mobilised (for example to resist anuplift or toppling load for a tensioned structure) failureis not generated by excessive movements. Whilst this USE OUTSIDE THE AGENCY. 4/3


Chapter 4Design

undertaken if such high bond values are proposedfailure criterion may not be critical in many rock boltingapplications, such as in bolting arrays for roof support,it nevertheless must be assessed.

Usually it would be assumed that at failure an invertedcone of rock resists the anchor loads in the mannershown in Figure 4.4. The included ang

ba 80-99 use of rock bolts (dmrb vol 2, section 1, part 7, 1999)

Documents

rock bolts3

cable bolts

electronic copy

types of rock bolts4

electronic document

signature date

design ofstructures

similar ground supportsystems