Design and construction of ground anchors. 2nd edition CIRIA R65.pdf

REPORT 65

SECOND EDITION OCTOBER 1980

Design and construction of ground

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anchors

T. H. HANNA PhD BSc CEng FlCE

i

This work was guided throughout by the Project Steering Group which comprised: R. Clare BSc DIC CEng FICE AMIHE (Chairman)

Sir Robert McAlpine & Sons Ltd

J. B. Burland PhD MSc(Eng) CEng MICE FGS

Building Research Estnblishment

I. W. Ellis CEng FICE

Fondedile Foi~ndationsU d

J. Mitchell DIC CEng MIStructE

Ove Arup & Partners

V. C,. l'owson CEng MIStructE

Department of the Environment, South East Road Construction Unit

B. M. Sadgrove MA CEng MICE

CIRIA

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This research was carried out at the University of Sheffield where Professor Hanna is Head of the Department of Civil and Structural Engineering. Sincc the production of the first edition of this Report in July 1977, a draft British Code of Practice on Ground Anchors* has been produced, and for specific recommendations on anchors the reader is directed to this Code. Also, since the first edition was published, major changes have taken place in the UK in corrosion protection, anchor testing and evaluation of the test data. The revised edition, wluch w a s also produced by Professor Hama, takes these changes into account.

*to he published as a Draft for Devclcpmcnt in 1980

ClRIA Report 65

! !

Contents LIST OF ILLUSTRATIONS

SUMMARY 1.

INTRODUCTION

2.

CLASSIFICATION OF GROUND AIJCHORS

3.

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4.

5.

6.

7.

8.

2.1

Definitions

2.2

Anchor types

CftLC'J LATION OF LOAD CAPACITY

3.'

lsolateo dead anchors

3.2

Anchor grouping

ANCHOR DESIGN CONSIDERATIONS

4.1

Creep and loss of load

4.2

Repeated loading

4.3

Corrosion protection

4.4

The tendon

CONSTRUCTION CONSIDERATIONS

5.1

Ground conditions

5.2

Anchor drilling

5.3

The tendon

5.4

Grout

5.5

Grouting the tendon

5.6

Grout quality control

5.7

Construction limitations

5.8

Replacement anchors

5.9

Safety

ANCHOR STRESSING, TESTING AND EVALUATION

6.1

Stressing

6.2

Anchor testing

6.3

Prestressing o f anchors

CONSIDERATIONS ON THE DESIGN OF AN1:HORED STRUCTURES 7.1

Rotailling walls

7.2

Overall stability

7.3

Examples of anchor supported w:,lls

7.4

Rafts

7.5

Failure of single allchors

7.6

Instrumentation of anchors and anchored structures

CONCLUSIONS

REFERENCES BIBLIOGRAPHY APPENDIX A APPENDIX

B

APPENDIX C

Checklists Anchor testing details Creep determination

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List of illustrations Figure 1

Details of a ground anchor used in vertical and inclined directions

Figure 2

Comparison of a dead and a prestressed anchor

Figure 3

Typical details of a tension and a compression anchor

Figure 4

Illustration of main anchor groups available in UK

Figure 5

Anchor Type I - sequence of construction operations

Figure 6

Anchor Type 2 - sequence of construction operations

Figure 7

Detail of tube Q manchette.for pressure grouting control

Figure 8

Anchor Type 3 - sequence of construction operations

Figure 9

Dimensions for design of anchors in stiff clays

Figure 10

Detail of protection given to permanent anchors (Dy widag Soil Anchor)

Figure 11

Use of hollow hydraulic jack for anchor bar stressing

Figure 12

Wedge grips for strand and multi- wire tendons

Figure 13

Load me8surc:nent using hydraulic jack 8nd reaction chair

Figure 14

Mechanism of failure assumed in Kranz method of stability analysis

Figure 15

Support for excavation for Bank of California excavation in Seattle

Figure 16

Support for Entertainments and Theme Towers Excavation, Lw Angeles

Figure 17

Mechanism of failure essurned for checking overall stability of anchors and anchor groups

Figure 18

Load-displacementdata obtained during anchor load test

Figure 19

Time-displacement relationship for anchor load test

Summary This document is based on current design and construction practices carried out in different countries throughout the world and on a critical review of the literature relevant to the subject. Ground anchors are classified and this is followed by a summary of ultimate load calculation methods. Anchor design considerations include loat! loss and creep, repeated loading, corrosion protection and details of the tendon. In construction c~nsiderations,sections are devoted to site investigation, drilling, groutir~g,construction lin~itations,replacement anchors and site safety. Requirements for anchor stressing, testing and evaluation draw 011 the reported experiences of others as well as codes of practice and other non-mandatory publications. A section is devoted t o the design of anchored structures, particular attention being focused on limitation in methe55 of analyses, construction know-how and performance evaluation.

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Seven checklists arc appended t o aid the designer and/or constructor t o obtain all relevant t a t 2 necessary for an efficient anchoring scheme while appendices detail methods of anchor testing and creep determination. A subject bibliography is also appended.

ClRlA Report 65

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1. Introduction Published work on ground anchors goes back about 40 years although their use in sands, clays, gravels and 'weak' rocks started less than 25 years ago. The lrse of such ground anchors has bcco~irewidespread in the support of temporary and permanent retaining walls as well as in resisting tensile forces in a numbcr of other situations. Dcspite the proven capacity of ?round anchors in the solution of foundation engineering problems, little is yet known about the' mechanism of anchor behaviour and consequently designs must be conservative. Also, the lack of a good understanding of anchoring principles has led, in several instances, to failures wit11 consequent damage to nearby facilities. For these reasons, the present methods of design and construction and understanding of anchor use are very approximate (in some cases bdd practices arc fc~llowed).Few referellces on the subject are readily available. Over the yuars a nu~lrberof types of ground anchor have been developed and many have bcerl slrow~rto he suitnblc for a range of applicatio~ls,although some have practical uses within a restricted rangc ofgror~ndand load conditions only. An incorrect choice may lead to trouble at a later stage with serious financial and otllcr inrplications. The problem of anchor selection for a specific sitc-is not easy hccuusc of the many factors which, if not accounted for, can significantly affect their performance. Ilre errginccr 113:; to quantify desif,n and, with a fair degree of confidence, estimate loadcarryingcapacity ofanclrors not only irt isolation but whe11 forn~ingpart of a structure. Analytical ~ncthodso i load and ~llovernentprediction are desirable, and vcry important dcvclop~ne~lts Ilavc t:iken place although tnuclr still has to be learned. For these reasons, the designer of an anchoring projsct ~llustappreciate the major influence which construction operations can have on the lxrformance of an anchor. Because many of these influences cannot be quantified analytically, ficld testing of both new and existing anchor systems, and monitoring. the bchavioilr ofancllorcd structures are essential. To assist design rrlgincers, contractors and users of ground anchors in understanding the p r i ~ ~ i a rfactors y vtllicll control tllc selection, design and use of a ground anchor scheme, a review is given of tire 111ajo:ity ol'systc~llscurrently available, construction ni~thods,design principles and ~rlethodsof perl'ormancc evaluation and control. In addition, attention is focused on several area!; w!lcre tlrc present state of k~lowledgeis limited. Because of the practical importance of ground a11c11orstherc arc a large nunlbcr of publications available and most of these ere listed. Illis P.cport is r ~ o t3 code on anchor usc, but rather a general survey of ground anchors, particularly in soft ground such as clays, sands, gravels and weak rocks. The subject of rock anchors is llot covcrcd, altl~ouplrrrluctl of tlrc discussion which follows is common to both, except th:~tt l ~ eloads wl~ichcan bc developed in rock strata are tnuch greater than in soils.

a

2. Classification of ground anchors 2.1 DEFINITIONS

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A grocoid atichor is a structural rneniber which transmits an applied tensile force t o 'competent' ground. The shear strength o f the s i ~ r r o ~ ~ n dground ing is used t o resist this tensile force. An anchor niay comprise a tension pile, a rock bolt, a dcadman, b u t today the most common anchor consists of a high strength steel tendon installed at the required inclination t o resist the applied load efficiently (Figure I). The tettsile force in an anchor is that necessary for cquiiibrium betwecn the anchor, the structure and the ground mass system such that the rfiovement o f the structure and the ground mass are kept t o acccptable levels. 771e tmdoti is usua!ly a high strength steel member surrounded by :I grout. Tlie tendon transmits the tensile force t o the surrounding ground via the grout ;mnl.!lls. Tlie tendon must be adequately protected against corrosion. The f a e d a r ~ c h o letrgrh r is that length of the anchor over which the tensile force is transmitted t o the surrounding ground, while over the free aticlror lerrgt11 n o tensile force is tr;,nsmitt:d t o the surrounding ground mass. Anchors may be dead or prcstresscd. Initially, a dead anchor is under n o load, but as the ground moves relative to tlic anchor member, load is rnobilised in it. With such an anchor, relatively large niovclnents niay be required to ~nobilisethe full load-carrying capacity. To reduce such movcliients t o more acccptable levels, ground anchors 3re usually prestressed by initially tensioning the anchor to the structure or t o 2 ground surface slab o r components (Figure 2), and in so doing the anchor system is also subjcctcd t o a structural load test. The level o f pres:rcssirig is usually a pcrccntage o f the dcsign working load (see Section 6). When externally loadcd, the prcstresscd anchor bel~avcsas a much stiffcr mcmbcr than the dead anchor. Ground anchors may bc rcquired for short periods o i use frorii a few days (e.g. reaction for pilc and other load tests) to several months (support o f temporary excavatior~s)or for p c r r n a ~ ~ c n11sc t (c.g. support o f dry dock floor. stabilisatiori of a hillside). Such uses are lefcrrcd t o as s l ~ o r r and - Iotrg-terttr, respcctivcly. Tlic riiccha~~istn o f load niobilisation within thc aiiclloragc zone lnay bc one wlicrc the lo:~dis dcvclopcd from the top of the anchorage g grout in the a~iclioragezonc into tension. T o overcome tensioning and zone, tI111.sp u t t i ~ ~[lie associated cr;lckir~go f the grout \\,ith thc potcriti;~lfor corrosion attack, some more sopl~isticated s y s t c ~ tr:111sli'r ~~s t l ~ c;IIICIIO~ force to tlic bottom end o f tlic anchorage zone and thcsc anchors arc rcfcrrcil to :IS cotir~~rc.ssioti :tiicl~ors(Figure 3). Wit11 s11s11anchors, tlic force is transferred i of the t e l i d o ~via ~ the prcssurc pipe. Usually to tllc 1)riln:lry r,routsd arcs f'rorii the b o t t o ~ i clid a pl:~tcis ; ~ t t a ~ I ~toc t11c d C I I ~of the tendon. Thcsc a1ic11i)rsare seldom used for temporary ~ i t comprzssion anchor has found much works, but whcrc thcrc is a corrosive e ~ i v i r o n l ~ i ctlic favcur, partic111:lrly in Wcst G c r ~ i i a n ~ ( ~ ) .

XAB,

f

TENSILE FORCE

4T'v

STRUCTURE

grout

11 VERTICAL ,ANCHOR

Figure 1

INCLINED

lenglh.&

Fixed

anchor length

4

ANCHOR

Det~ilsof a ground anct~orosed in vertical arid inclined directions ClRlA Report 65

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TENSILE FORCE

DEAD ANCHOR

TENSILE FGRCE

P R E S T' R E S S E D ANCHOR 0

Figure 2

Comparison of a dead and a prestressed anchor

DISPLACEMENT OF ANCHOR

2.2 ANCHOR TYPES The types of anchor con~~nercially available in the UK may be subdivided into three main groups (Figure 4, page 9): I. 2.

3.

a cylinder filled with grout n cylil:?er enlarged by grout injected under a high but controlled pressure, or by peasi;rrd g~avelforced into tllc sides of the anchor llole r cylinder mech3nically snlarged at one or nlorc positions along its length, t s enable a larger load to be mobilised.

.

In generul, cement grout is used to tri~nsferthe tcndon force to the surrounding ground and also to protect thc tendon ag;~instcorrosion. In some cases, a resin may be used to bon'd the tendon to the surrounding ground, while cl~cmicalgrouts nlny be uscd to permeate fine granular soils. Corrosion protection of all ancllors is essential and n~ethodsare referred t o in Section 4.3. Ancl~orTypes 1 . 2 and 3 are suitable for all grobnd conditions where it is possible t o drill a hole wllicl~does not tend to collapse. In the case of cohesionless soils, a Type 2 ancllor 113s to bc used. General details o r c a c l ~of the three rncllor types follow.

2.2.1

Type 1

This anchor tvpe is seldoni uscd In soils, it is rl~ainlyused for rocks. Where tile rock strata are stable, so that the hole does not collapse, a percussive-type drill rig is uscd. Where collapsible solls overlyi~lgrock are encountered, a rotary percussive rig is nornlally used, water or

Anchor

~-r

/&Free

Structure

-I

anchor length TENSION

I

cover

ANCHOR

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n d plate

Figure 3 Typicaldeteilsofa tension and a compression anchor

Corrosion

protection compound (grease)

secondary

grout

~iirnary

grout I

Seal I

COMPRE SSlON ANCHOR conipressed air being used t o reniove the drill cuttings via the hollow drill pipes. It Is usual t o check the pcrnieability of tlie fixed anchor zone by water testing. In this test, the boring is sealed by nlcans of a packer about l m above the top end of the flxzd anchorage zone, water pun~pedin under controlled prcssurc, and the flow of water recorded. The primary purpose of this test is to givc a genernl indication of the structure of the ground and a nleasure o f grout quantities requircd. Wlierc tlie boring fails the water test (see Littlejohn and ~ r u c e ( ~it) )Is grouted under low prcssurc, and after a pcriod of at least 1 2 h the hole is redrilled and retested for water tightness. If this test is succcssful, UIC anchor tendon may be placed. In drawing up criteria for the water test, it is hiportant t o remember that the primary purpose of this test is to check if cement grout could be lost from the fixed anchor length through fissures in the borcliole sides. Cenient particles h w e a finite size, and only opening greater than this size pcrniit grout loss. There nlay be scverd joints or openings along the f i e d anchor icngth. Consequently. in specifying the pcrn~issiblcwater flowrates, it is essential t o take all these factors Into account rathcr than t o !ry to provide a watertight hole. Great care is needed in the execution o f the water test t o ensure that the pncker seals the anchor hole, and that the quantities of flow are correctly nieasurcd. Also, the enginecr requires an understanding of the nature of the ground, particularly fissure frequency and size. This latter feature may be quantified at the site investigation stage by measurcrilcnt of rock quality designation (RQD) values or by use of a borehole TV camera. The ion11 of thc tendon depends on whether it is for teniporury or permanent use. It is usual with te~nporarynnchors to wrap thc free anchorage length with a grease-impregnated tape which gives short-term corrosion protcction yet pern~itsthe tendon t o stretch during stressing; With per~iiancntnnchors, the I'rcc anchorage length is protected either with a polythenc shcnti~surrounding the tendon aucrnbly or each member forniinp the tendon Is grease hipregnated and factory covcrcd with an extruded polypropylene sheath, th;ls giving the tendon perniuncnt protection yct allowing 111ovcnient. In the fixed ~nchorugezone, the tendon is ClRlA Report 65

$

% I--Free TYPE

Primary 'grout

Tendon I

\secondary

1

grout

anchorlength

Length

1 ANCHOR CYLINDER FILLED WITH GROUT

Tendon

Enlarged z o n e f o r m e d by g r o u t

4

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Frel;n;~~hhchor & l e n g t hF i x e d anchor TYPE 2 -

ANCHOR CYLINDER ENLARGED BY GROUT INJECTED UNDER

HIGH BUT CONTROLLED PRESSURE

Figure 4

- a nF cr eheo r length

illustration of main anchorgroups - - available TYPE 3 ANCHOR in UK

-1-

length

A CYLINDER MECHANICALLY

4 ENLARGED AT ONk

OR MORE POSITIONS ALONG ITS LENGTH

stripped and degrcascd, and there are various patented details of the arrangement of the tendon (see Section 4.4). Grouting of the tendon is usually by a trcmie method and may be before or after placement of the tendon. In general, the grout matures for s period o f 7 days before stressing, altllough rapid llardening agents nlay be used to speed u p the process, especially for anchor test work. In some circumstances, wherc the anchor has to be stressed a few hours after installation, epoxy or polyester resins are used. A diagrarnnlatic representation of anchor construction is given in Figure 5 (page 10).

2.2.2

Type 2

ClRlA Report 65

This type o f a n c l ~ o ris used in both cohesive and cohesionless soils. In general, the anchor hole is drilled by a rotary or rotary percussive rig. Different anchor systems employ different methods. but no st use a bit on hollow rods working within and just ahead of an outer casing. Some systenls e~nploya sacrificial drill bit with reaming devices attached to the casing. The net result is a hole 75 to 120 mnl in diameter. During drilling, water or compressed air is used to bring the spoil to thc surface. Tile placcn~cntand subsequent grouting of the fixed ailchor rone is regulated by tile patented anc!lor systenl being used.

Drill percussive dril,' Anchor inclination

-Primary

\C' ~ i x & anchor length Licensed copy:Careys Group PLC, 02/10/2016, Uncontrolled Copy, © CIRIA

\

woter tested to check permeability

1

gnwt anchor strening

Anchor tested and s t r e s s e d after grout hos motured

Tendon being

ogoinst corrosion ~enddn stripped and degreased

Figure 5 Anchor Type I

- sequence of construction operations

Most systenls use a 25-mm diameter grout pipe attached :(.. the tendon, the grout being pressuriscd as the casing is withdrawn. With the correct choice of grout pressure,* the diameter of the grouted zone may be u p to four times that of the original borehole (Figure 6, page 11). rile success of the anchor depends t o a large extent on the grouting pressure used and this, in turn, depends on Ule permeability of the soil t o grout penetration and the overburden pressure. Because of the extreme Importance of carefully controlling the pressure grouting operations, several systenis enlarge thc cylinder with grout under high pressure by means of a tube ci. tnonchette through which the tendon is placed (Figure 7,page 12). This is a sophisticated method of grout placement and control but its chief merit is that several stages of grouting niay be used if necessary. With this method of anchor construction, the hole is filled with grout (sleeve grouting) and the anchor tendon, incorporating a PVC tube dtttnt~clretreand an inflatable packer to seal the f i e d from the free anchor zones, is inserted into the anchor hole. This packer is inflated by injection of quick setting grout. A double packer is inserted into the tube d nlot~clrctte and lowered to the bottom of the anchor zone. Grout is then punlped into the double packer under pressure and escapes by raising the oneway valves (manchc.ttcs), bursting through the 'sleeve grout' and into the surrounding ground. Tliis grouting is repeated in stages over the fixcd anchor length by raising tire double packer within the tubed ~tu)rrhette.At any time after the initial set, the double packer is again introduced and a second sti~geof grouting pcrfornied, usually at a higher pressure than the first. A third (and so~neti~nes a fourth) stage o f grouting may be used to enlarge the grouted 'It nlust be en~phnsisedthat u ~ ~ c o ~ ~ l r ohigh l i e dpressure mny lend to hcave o f the ground and dnmage to adjncant services nnd oncltors. Thus, wittr nll systems, the grout prcuurc ond qunntily of grout Injected must be mensurud nnd c o ~ ~ l r o l l u d .

C I R I A Report

65

Anchor hole drilled with

bit moy be used

Cased hole

placed in cased hole

( Sacrificial bit forced

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from end of casing

a s casing is withdrawn L,

-.--i-

Enlorged onchoroge zone caused by grout permeation

Figure 6

Anchor Type 2 - sequence o f construction operations

zone. Tlrc c o ~ r s i ~ ~ n p tof i o ngrout is carefully monitored and thtr iystem is suitable for a wide range of ground conditions from sands and clays to fissured and jointed rocks. Each p a t e ~ ~ t esystem d of anchor construction is different in detail, although the principle is the sane: the pcr~neationof grout into the voids of colresionless soils or the enlargement of a cylinder by Iligh grout pressure in cohesive type soils, and weak rocks. Most construction systcnrr, do not effectively for111on enlarged cylindcr in very stiff clays or weak rocks, except tl~oscc~nployingtlrr tubed ttwtrckrre grout control system or an equivalent method. In an attempt to cnlargc? the cylinder, some systems eniploy the placement of pea gravel in the anchorage zone. This pea gravel is forced into the walls of the hole by mechanical means prior to grouting. Other mLthods('), which have not had much practicnl success, include void forrriation by use of explosives.

2.2.3

Type 3

With clay strata, the load-carrying capncity of the anchor depends on the strength of the clay available at the anclror/clay interface. Also it is impossible to treat tlre clay strata in the manner described for anol~orsin colrcsionlcss soils (Type 2). The nlnst succcssful anchor unit comprises the drilling uf a cylindrical straft and the ~nechanicalenlargement of this shaft at predetermined positions (Figure 8, puge 13). This is achieved i s follows. The anchor hole is usudly drilled by a rotary rig with a continuous flight auger, although a rotary percussive rig !nay be rcquircd wliere granularvtype soils overlie the clay. With this rig, the hole is advanced througll the granular soil and the hollow drill tubes are sealed into the clay. A conti~ruousIligl~tauger is then used to drill in the clay.

(blGrouting pipe and double packer introduced opposite manchettes and grout injected. The process may be repeated. St rand

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(For details of tube k manchette system see below 1

Grouting head with double packer top and bottom routing pressure disten !s rubber manchette and forces grout through

Figure 7

Detail of tube B manchette for pressure grouting control

Weak grout to seal tube and manchette into hnle

50mm dia, tube

h

manchette

Several patented systeliis arc available for under-ream cutting along the anchorage zone. ~ is lowered to the base of the hole and rotated for With one systeni, a special c x p a r ~ d i nbrush about 10 nlin while water is circu1at.d t!~~.ougllt l ~ urill r rods to remove the soil cuttings. This process forms a bell and is repeated at about 0.8-111 irltervds up t!e anchorage zone t o give the required nuniber of undcr.reams. Today. ~iiostanchoring contractors favour an expanding cutter tool comprising a torpedo in which the expanding tools are housed. They are slowly and progressively opened by positive niecl~nnicalmeans as the torpedo is slowly rotated until the blades nre fully expanded*. Depending on the systeni in use, from one to cight under-reams can be fonned in one operation. With systems which cut two under-reams, the under-reaming procedure can be repeated several tinies t o produce the desired number of under-reams, which niay be up to four times tllc nominal anchor shaft diamcter. During drilling, the soil cuttings are removed by flushing usually with water. After removal of the expanding cutter bit, the tendon is installed and grouted (Figure 8). It is important to ensure that the ~rader-reanisare correctly formed and t o the size specified. Generally, it is not feasible to measure the shape of tt1.1 f i e d anchor zone on every project, but where a new and untried systeni of under-reaming i! being used, the as-cut shape 'It must be cmphndsed tlint the bellinr tool nrust be oponed at a slow and constant rate, o l h e w h e the beU Is not clean. The use of hydrnullc prr,.sure as a nrcons olopenlng tlre to *I Is not recommended, because It & not posslble l o control the rate a t \* rich tho helllng tool Is opened.

ClRlA Report 65

c o n t i n u o u s flight a u g e r

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v

M e c h a n i c a l m e a n s of controlling

opening of under-ream

Under- ream

Figure 8

Anchor Type 3 sequence of construction operations

V

Completed anchor

of the under-reams should be quantified. either by tneans o f special calipers lowered t o thc base o f t l ~ eanchor hole or by exhutning one or more co~npletedanchors. A number ofancllors have been forlncd in stiff clays using the t u b e d ttutrchette system o f grout control. By use of several stages of grouting, it is possible to produce fissures in the walls o f the hole during the first stage and tliese arc filled with grout during the sccond stage. Subsequent stages 1113)' he used to enlarge the 'bulb' produced by the previous two stages. However, the net effect is t o prod~lccan anchornge zone which consists o f a cylinder enlarged at a nu~iibero f localiolls along its length. This cnablcs a much greater unit side friction t o be mobilised. The dimensions o f the cnlargetllent arc not known with any d c g ~ e eof precision.

ClRlA Report 65

The practice of anchor construction in clay strata mentioned above is not universal. In the USA, for example, much larger diameter anchors have been used with a single under-ream at the base. Ttius the anchor is a small diameter belled pile. However, in recent years, the trend has been t o use the smaller diameter anchor systems developed in western Europe. In Germany, the Type 2 anchor with multi-stage youting is preferred(')and gives loadcarrying capacities comparable with the more sophisticated Type 3 anchor. It is difficult t o n relative merits o l both aichor types. With the Type 3 anchor, a mechanical comment * ~the grip is formed in the clay by the undcr-rcams, but wit11 the 'Type 2 anchor, reliance has t o be placed in the multi-stage pressure grouting of the anchorage zone. From the evidence available, there is much t o favour the use of the Type 3 anchor in such clay strata, and usually there is no potential danger of ground heave which niay arise with the Type 2 anchor construction unless care is taken t o strictly control the grouting operations.

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Irrespective of which type of anchor is used in clay type strata, it is imperative that the user be aware of the implications of forming an anchor in these materials and of the need t o prevent damage t o the ground in the vicinity of the anchor, such as softening or fracture.

ClRlA Report 65

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A ground anchor may fail in one o r more o f the following modes:

1. within the ground mass usually along joints and fissures 2. at the ground/grout interface 3. at the grout/tendon interface 4. within the tendon or the attachment of the tendon t o the structure. Nonilally, anchors are used in closely-spaced clusters, and consequently overall stability o f the anchor/ground t~~ass/structure system niust be provided not only from a failure viewpoint but also wvith respect t o movements. This is a c o ~ n p l e xand somewhat inexact problem t o quantify because o f the difficulty of modelling not only ground beliaviour but also construction - long-tern1 stability. operations both with respect to s l ~ o r t and

3.1 ISOLATED DEAD ANCHORS Current dcsig11 tnetliods a:e based o n a blend of semi-empirical rules for pnrticular anchor systclns and ground conditions, thcory hascd on the theory o f elasticity, and extensions o r tlicorics worked out for other fo1111dations(particularly piles). Tlie fixcd anchor Icngt9 Is normally d c t c r ~ n i ~ ~byc de~iipiricalrules a~tpportedby simplifying theoretical assumptions as follows:

3.1.1

Sands and ravels

Thc ultirnate load-carlying capacity, T,,o f an anchor niay he expressed by

P

wlicre L ic *he fixcd anclwr ic~igthin Ilictrcs, 4' is the angle o f internal friction o f the soil, and 11 is a factor depctldent primarily o n the permeability of the soil. For coarse'sands and gravels II = 400 to 600 kN/m. while In nne I.) medium sands n = 150 kN/m. Details are given by Littlcjohn ('I.Extensive tests in Wcst Gernluny ( I ) show that, in general, load-carrying capacity is dependent on relative denzity, uniformity of the soil, ftxed anchorage length, and to n lesser extent on anchor diameter. This work delnonstrates that the most important ractor i~!ccntrollirig carryirlg capacity is the dilatancy o f the soil which results in a very large nor~iialstress being created o n the groutlsoil interface. It should be appreciated h a t the method of anchor construction influences carrying capacity t o an unknown extent. Consequently, tl~esegeneral design criteria IIILISIbe used with care, and with experience o f the ~i \Vl~crcnecessary, field testing nlust be performed t o condifferent anchor c o ~ i s t r u c t i usystems. fin11 ultinrate load-carrying capacity.

3.1.2

14

Stiff clays

With tlrc Type I anchor system, load is ~ilobilisedalong tlie cylindrical shaft in adhesion and in suction at the anchor base. In general, the suction force i; small, unreliable and should be

CIRIA Report 65

neglected. The shaft adhesion value depends to a considerable extent ori construction technique and an average o f 0.3 Cu is appropriate T, = 0.3Cu nDL

(2)

where D is the diameter of the fixed anchor, L is the fixed ancho: length (see Figure 9 ) and C, is the average undrained shear strength of the clay in the anchorage zo:.e area. Where pea gravel is forced into the walls of the anchorage cyli!ider area (Type 2), the shaft adhesion rises from 0.3 t o about 0.6 C,, the effective diameter o f the fixed anchor increases by about 50% and par: of the load capacity is given in end hearing as follows T, = 0 . 6 C u nDL

+ (D' - d2)CUNc

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(3) 4. where D is diameter of the fixed anchor, d is diameter of the shaft, L is fixed anchor length (See Figure 9), Nc is a bearing capacity factor (=9), and C, is undrained shear strength of the clay

Type 1

Anckw

Type 2 Anchor

C L

4

---.

-.

Figure 9

Dimel~sionsfor desip of ~r~chors in stiff clays

?--

- 1 Type 3

Anchor

Witti tiiulti urldcr-rca~lledanchor~ges(Type 31, tlie ultimatc carrying capacity may be expressed by

r,, = nD,,L,,Cu f,

t

a

(D,,~ - d2)cUh1,, +

o.~c,T~~P,

(4) wliere D, is diameter of tlie under-ream, d is diameter of the shaft, L, is length of under-ream zone. P, is lcngtli ol'tlie shaft in the fixed anclior zone (see Figure 9), N, is a bearing capacity I'uctcjr. C, u:ldraincd slleur strengtli of the clay, and f, is an efficiency factor equal to about 0.8 ( ~ a s s e t t ( ~and ) ) dependent on [lie under.reaming technique used. The results of Inany tests reported by 0sternlayer0) support the above approacl~but also stlow .hat carrying capacity can be significantly increased by multi-stage grouting techniques such as tl~osc~ i l e n t i ~ n eearlier d in this Section. Other support to this observation is given ~y ~orge(').

3.1.3

Weak rocks

ClRlA Report 65

There are few rcliahlo and well docu~l~erltcd nietliods of anchor design in weak rocks such as weatllered chalk. Kc111)cr~ilarl,or sliulc. Botli straightsided and nlulti under-reamed anchors are 15

In use. The following general e:cpression(6) has been used t o provide design estimates: T, = nDLC,

(5)

where D is the d i ~ n e t e of r the fixed anchor length, L is the length of the fixed anchor, and C, is the unit value of the shaft adhesion. The value of C, in some cases has been related to the standard penetration test Nvalue, with C, = ION kN/m2 :'or chalks. For anchors in Keuper marl, estimates of ultimate load canacity may be obtained in a similar manner t o that for stiff clays. However, it must be borne ,n mind that very few records are available, and consequently all such estimates must be supported by field testing to failure. A major state 3f the art revieuv on rock anchors is given by Littlejohn and ~ruce(') in which they exanline current practices and limitations in knowledge on the design, construction, ctressing and testing.

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In general, the quality of workmanship during construction greatly influences the performance of thc anchor. T l ~ elocal variations in the rock quality and its struc:ure result, with closely spaced anchors, in variations in anchor performance which cannot be quantified using ground information from a routine site survey programme. It is essential, therefore, that as much care anri detail is given t o rock anchors as anchors in sands and clays, and that each anchor is s1:bjected t o an initial proof load test. The safety and quality of each a~ichorcan be assured by doing this and by carrying out other anchor tests (see Section 6).

3.2 ANCHOR GROUPING Low c ~ p a c i t yanchors have to be spaced at close centres, and consequently the zones of stressed ground around them interact. Little design guidance is available on this topic as the only quantitative work is that based on small-scalc laboratory tests which have shown that the ultimat: capacity of the group is always less than that of an equal,nu~nberof isolated anchors('). For this reason, designers of anchor systems have worked to certain rules of thumb such as a minimum spacing of five anchor dianieters. In many cases, the anchorage zones of adjacent an:hors can be locatcd at different depths or at dif'ferent inclinations in order to overcome the efl'ects o f close spacing. Under special cases, field tests may be rcquired to quantify the grouping behaviour of closely spaced anchors (see Section 6.2). The French code(") gives some general guidance on groupi~ipand recommends either that the anchors be set out so that they do not influelice one anotller or that the allowable load per anchor be reduced. A reduction cunfe, based on nncllor spacing and t l ~ esize of the cone of influence of the stressed ground around tllc anchor is given, the reduction factor varying between 0.5 and 1.0.

ClRlA Report 65

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4. Anchor design considerations 4.1 CREEP AND LOSS OF LOAD Loss of load is !he decrease of load iti the tendon with time while the tendon is heli under constant strain. Creep is tlle movement o f the tendon liead with time under constant tendon load. Both phcnornena occur together, and it is useful to appreciate the significance of each. In general, stress relaxation can cause load losses of 5 to 10% dthough the use of stabilised ) ~annister("). Since wire has led to losses of less than 2%. Details are given by ~ n t i l l ' ~and most of the relaxation occurs during the first few hours of loading, deliberate temporary overloading of the tendon for, say, 15 niin reduces future relaxction losses by eliminating the rapid initial rel~ltation.However, this tecllnique has little value with stabilised strand where long-term losses are sniall. Most uscfi~linformation on relaxation effects has been compiled by Littlejohn and ~ruce('). Under constant load, creep displnce~nentsoccur with atichors in both cohesive and uniforni grained non-cohesive soils. In general, the relationship between creep displacenlent, s, and tittle, I, is an exponential f u ~ l c t i ogiving ~ ~ a straight line on a semi.logarithmic plot. The slope oTthislinc, ds/d(logr), may be defined as the creep coefficient, k,. I t is essential that plastic flow of the soil a r o h d the anchor shaft does not take place. ~ s t e r m n ~ e rclaims ( ~ ) that the crecp coefficient due to relaxation of the tendon steel, psrtial and progressive debonding

CIRIA Report 65

of the steel/grout interface and cleep of the grout is about 0.4 mm. It must be emphasised that creep under constant load and under repeated loading is poorly understood. Until further data are available, particularly for multi under-reamed anchors in clay, the recommendations of ~ s t e r m ; ~ e r ( 'should ) be followed: the creep coefficient should not be greater than 1 mm for 1.5 times the working load. Further comments are given in Section 6 and useful hints are provided by Fenoux and poitier("). In many cases, it is not possible t o determke the creep coefficient, and many users believe that loss of load c ~ p c i t may y be used as an alternative parameter. !oss of load must be considered most carefully and, where this is used as an acceptability criterion, the engineer should be clear that this loss of load is not related t o factors such as the vertical and associated lateral movement of the wall(I2). It must be appreciated that a d e c r e e of load in an anchor can occur which is not caused by creep or relaxation effects. In general, where the load losses are small (less than 5 t o 10%) over a period of, say, 24 h, the anchor may b e considered satisfactory provided that further losses d o not occur (see Appendix B).

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4.2 REPEATED LOADING Structuressubjected t o natural forces (e.g. wind and wave action) often impose uplift loads of fluctuating magnitudes on their foundations. Thus the number of cycles of repeated loading during the lifctime of a structure may be very large. Consequently, the cumulative effects, if any, of a large number of load repetitions should be taken into consideration. NO field data are available, but some laboratory results(I3) suggest that repeated load effects can be serious, particularly where the load rangc is large and where the anchor may be coit~pletelyunloaded during the load cycle. At present, design guidance is not available, and each special case should be carefully assesscd t o decide if a problem exists. Bascd on all the laboratory tests available, the repeated loading of a prestressed anchor should present no serious poblems for the designer, provided that the :oad fluctuations are small in relation to the working load and that the anchor system h i s been correctly evaluated and proven by the field testing (see Section 6).

An unusual case of repeated loading is the influence of nearby blasting. l t t l e data are available, but rccent ficld observations by Littlejohn et U I . ( ~ suggest ~) that prestressed anchors pcrforni well, even when subjected t o nearby blasting.

4.3 CORROSION PROTECTION All steel embedded in the ground tends t o corrode, and the engineering problem is t o decide whether full corrosion protcction is necessary. This depends on the rate of corrosion, which itself dcpcnds on the agressiveness of the anclior environment. At present, there is no absolute method of quantifying the aggressivencss to enable corrosion rates t o be predicted with confidence. The corrosion of prestressing steels is largely electrolytic, and s useful review is given by ~ o r t i c r ( ' ~In ) . providing protcction to anchorages, three main considerations prcdo~iiinnte: I . the working life of thc anchor 2. the e~~vironnicnt 3. tlie possibility of damage to t l ~ eprotection systeni during site operations such as tendon assc~nbly,installatio~iand stressing.

In assessing the aggressiveness of the ground, several factors are i~nportmt.These include cxposurc of anchorages to: 1. 2. 3. 4. 5.

sea water saturated clays with a low oxygen and a high sulphate content ground subjcct to corrosive effluents or a corrosive atnlosphere soils or rocks cotitai~lirigcllloride partly uturated soils, fluctuating ground water tables or strata with different ctremical co~liposition 6. cyclic londi~lgeffects. Useful guidance on this gcricral subject is given by ~ i n g (for ~ ~reinforced ) earth. Wlien trying to

ClRlA Report 65

c- c Flushing tube b b , L ~ d h e s i v etapeL7 rn

Hexaaonal lock rut

Hexagonal nuts Sealing caps

-

1

, I ,

Anchor plate for:I in) dia. and 5 x 16mm ('113 in) dia.

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4 x 16mm ('18

u

Flushing tube

Exterior cement grout

\

Corrugated sheathing

Figure 10 Detail of protection given to permanent anchors (Dywidag Soil Anchor) quantify the a ~ c s s i v c n c s sit. is useful to examine the history of buried metals in the vicinity of the proposed works. Occasionally, s11c1ldata nioy be used as a guide to the degree of protection required. Any protection systelil should providc thc following: 1. durability at least equal t o the proposed life of the anchors 2. free tendon niovcnlent over tlie free anchor length 3. .. resistance to damage during handling. transport t o and installation in the ground 4. resistance t o danlage during anchor stressing.

With all systems of corrosion protection. attenipts arc made to enclose the tendon with a covering or d ~ e a t and l ~ t l ~ u scontrol tlic atniospherc around tlie steel. A widc variety of coatings CIKIA Report 65

and coverings is available, and, in general, different treatment may be necessary for thi! tendon, the fixed anchor length, and the anchor head. Protection may be applied during manufacture of the tendon, and usually comprises a thin layer of grease covered by a polypropylene sleeve. Alternatively, fluld may be plated around the tendon in the borehole or between the protective sheath and the steel. A si~gly protected anchor has only one system, wllerens double protection usually involves both sheathed or coated tendon together with a grout injected around the tendon after homing or i n t c m d y after tendon manufacture. It is important to ensure that the grouts used have suMt;ient flexibility so that they d o not crack, otherwise the physical barrier they provide against corrosion may be lost. Any protection provided should be over the full anchor length, because partial protection may well induce more severe corrosion on the unprotected areas. Because prestressed anchors stress the tendon steel t o very high levels, stress corrosion effects have t o be considered. Once pitting of the tendon steel has started, it is unwise t o use the steel for permanent anchorages.

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For convenience, three areas of protection are considered; 1. the free anchor length 2. the fixed anchor length 3. the anchor head unit. For each of these parts o f t h e anchor unit, systems of protection for both temporary and permanent anchors may be required, and the f o l l o e i ~ gis a very general over-view of acceptable practice. For temporary anchors in a non-corrosive environment, the cement grout protects the f ~ v e danchor length, and the range of gr0u.i cover recommended in the literature varies from 5 to greater than 20 mm. It is particularly important wit11 high capacity anchors t o ensure there is a sufficient thickness of grout t o pr2vent grout crushing during anchor use. This topic is discussed further in Section 5.5. Over t!~efree anchor length, it is comrnon practice t o provide some form of protection. Usually, the tendon is grease coated and c o v e i d with a plastic tape with a minlmuni of 5Wo overlap. The tendon must be greased before wrapping to exclude the atmosphere, and n check made that uhe grease is compatible with the plastic tape. The elements of the nnchor head are usually prefnhricnted. For short-tenn use, little protection, other than a coat of paint, is provided. '

There niay be cases where the te~;!porary anchor is to be used in a corrosive envjronment. The best form of protection for the fixed anchor length is a compression type anchor, while with n tension type anchor, t l ~ grout e cover should be increased. The protection in the free anchor lcngtll Is usually grease and tape. With penllanent anchors, it is assunied that the environment may become aggressive during the life of the anchor. In general, a double protection system is provided. In the fmed ancllor zolle, the protective system must be capahle of transmitting the high tendon stresses t o the ground SLICII that there is neither continuing creep nor cracking of the corrosion protection systems. To meet these two criteria, it is necessary for the double protection system t o be provided. Cement grout is the norrnal nlaterial used to fix the anchor, but, bccause of its brittle nature, both radial and longitudinal cracks occur. Epoxy and polyester resins 1;lay a!:" hused, but are more expensive. It is in~portantto establish the long-term stability of s ~ l , .resins ~ before their use in protecting anchor tendons is authorised. Some detaii: nre given m Figure 1 0 of permanent and temporary anchor systems in use. Tlie free anchor length is csua:ly proticted by a greasepacked plastic sheath placed under factory conditions. The annu11.1~ t2tween the plastic sheath and the borehole is nornldly cement groutcd. One area in need of detailcd consideration is t11c anchor head, because the tendon metal just belund the anchor bearing plate is exposed and liable t o corrosion attack. The usual solution is to enclose the tendon steel within a tube futed in the structure. After the anchor is stressed, filler materials are introduced into this tube wl~ichdisplace any water present. On completion of filling of the void, the external part of the anchor head should be protected by capping, which nornlally entails a grease or epoxy or polyester resin nort tar.

ClRlA Report 65

Over the years, a range o f pre- and post-protection systems h a been developed. The most common pre-protection system is the plastic sheath or greased tape. The greased tape is usually applied to the tendon with 50% overlap. Its main disadvantage is damage susceptibility during tendon assembly and installation. With PVC or polypropylene sheathing, the individual steel wires or strands are usually delivered t o site coated but recently methods of site installation have been used with success(tS). The post-protection system usually comprises a filler placed around the sheath over the free anchor length after stressing. The filler is usually a cement grout although many substances such as oil, bitumen and resin have been used. There are many other more sophisticated corrosion protection systems, and full details are given by 0stermaYer('), porticr(14) and Bureau ~ e c u r i t a s ( ~The ) . draft British Standard on Ground ~ n c h o r s ( ~ discusses ') in detaii corrosion problems and practical methods of dealing with anchor corrosion.'A checklist of corrosion protection considerations is given it1 Appendix A.

4.4 THE TENDON

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Anchor tendons are usually formed of bar, strand or wire. In general, bar is more economic for 1 . 3 load ~ i;pacity anchors and is ea~'!yprotected against corrosion. Wire and strand are t o be favoured for high capacity and long anchors due t o ease of storage, transportation and f a b r i ~ t i o n Tendon . steel characteristics are governed by the following standards: BS 4486: 1969(Ii'. BS 2691: 1969(17), BS 3617: 1971(18), BS 4757: 1971(19) and CP 110: 1972("). Bar anchors are used singly and on occasion in clusters of up to four bars, although such a cluster requires 3 relatively large anchor hole and is difficult t o handle. Some anchors have sophisticated end plates and compression tubes in the anchorage zone (Figure 3). Where prestressing wire is used (usually S t o 8 mm diameter) 10 t o 100 wires have been used per anchor, depending on its load capacity. Strand is a more popular material than wire in the UK. Seven-wire strand is common and usually tendons comprise four t o more tlian 20 strands. At the design stage, allowable tendon stresses and factors of safety must be considered. Over the years, as expcrience has been accumulated, design working stresses have been reduced, and for temporary anchors a factor of safety against failure of at least 1.6 is common (i.e. working stress not greater than 62.5% of characteristic strength). In the design of permanent anchors a safety factor of 2 against tendon failure should be aimed for (i.c. working stress not greater tlian 50% of cliaracteristic strengih). This also permits a larger test overload t o be applied during stressing opcratioris of up t o 1.5 times the working strcss. For anchor tendons comprising several bars, wires or strands, spacer devices are required. Their primary purpose is to centralise the tendon unit in the hole and prevcnt friction generatlon between the tendon coniponcnts. Spacers are placed at about 5 m intervals in the free anchor Icngth. 111the fixcd anchor zone, care must be given t o the spacer layout. Tlleir primary purposes are to ccntralise the tendon in the borehole (and thus give adequate cover of grout for corrosion control) and to provide a positive grip between the tendon and the surrounding grout yet permit the grout to adequately penetrate and completely cover the tendon. The pitch of tlie spacers varics with the aaclior system in usc and is from 0.5 m upwards. This is clearly an area of design where niuch re~naitisto be learned about the influence of spacer design on the efiiciency of load transfer in the anchorage zone. There are nilriierous design details it1 use for the fixed a n c h ~ tendon. r In some cases, the strands are parallel, in others they are waisted at intervals. These details to some 2xtend depend on the method of grout placcnicnt in use. f l e d anchor system which shows merit has been Recently, a pernia~~ently-protected developed and widely used in the UK. Thc protection comprises an inner convoluted steel sheath and a sitiiilar outer plastic shcath wit11 a factory injected grout filing the annulus. These units are produced under factory conditions in standard Ic~igthswhich, when combined, give f ~ v e dlengths in 1111increments. Tlic fixed length is assenibled on site.

ClRlA Report 65

a

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5. Construction considerations A search through [tic rclcvant literature sliows t1131, in general, the construction of ground anchors is still an art, fcw gcncral principles o f constrt~ctionbeing firmly established. Carelessncss durir~gconstrt~ctionhas Isd to a ~itlliibcrof failures at loads wcll bclow the working load ClRlA Report 65

and much damage t o adjacent property has resulted. The factors discussed in the remainder of Section 5 are considered important.

5.1 GROUND CONDITIONS

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In use, ground anchors stress the ground locally t o very high levels. With some systems, the states of stress in the ground around the anchor are altered by high pressure grouting. Other systems cause the permeation of grout or chemicals irito the pore space of the soil near t o the anchor and effectively enlarge the anchor zone. Thus the efficient design of a ground anchor depends t o a great ext2nt o n knowledge and understanding of the ground in which the anchors are t o be placed. T o mbke an efficient design the following ground data are required: 1. soil/rock succt:ssion, including position of 3round water tables and their variations (if any) 2. relative density ;rqd grading of non-cohesive strata; strength of cohesive strata; strength quality of rock str7ta and, in particular, their susceptibility to softening during and after drilling of the a~:rhorhole 3. the presence of fissc:es,ioints and discontinuities 4. the bulk and local pcrmea >ility of nontohesbe strata and the watertightness of rock strata 5. variability of the ground acloss the site both in plan and elevation 6. cementing, if any, of sand and silt strata. Very often, the general site investigation is not designed with a ground anchor construction programme in niind. Consequently much geotechnical information relevant t o anchor choice may be missing. Before proper design work can be begun, further site investigation work may be required perhaps in association with znchor testing trials. On some sites inclined boreholes may be required to reach under adjacent property t o obtain information on the ground in the proposed anchorage zone areas outside the line of the proposed excavation. The ground information required is essentially an accurate description of the soil and rock strata, which is useful when the technique of anchor construction is being decided and the mechanics of load development. For example, in fine grait~edsand strata great care is necessary during sampling t o prevent loss of fines, yet it is these fines which control the choice of grout, if a grout permeation technique is proposed. Also, tlic susceptibility of the sides of a boring to soften very rapidly or even collapse dictates the anchor system necessary in cohesive type ground (e.g. clay weak rock).

5.2 A N C H O R D R I L L I N G In the great lilajority of uses, anchor holes are drilled in a near horizontal through to a vertically downwards direction. Occasionally, holes niay have t o be drilled in an upward direction. In the selection of a nicthod of anchor hole drilling the fnllowing main factors dictate the choice: I, 2. 3. 4.

ground conditions to be drilled through and the anchor system to be installed accessibility arid topography of the site penetration rate dianlctcr, depth and inclination of the ancllo~hole and the presence, if any, of underreams in the fixed anchor zone 5. tllu ~iiediunito be used for borehole flushing 6. tolerances to be worked to, particularly anchor location.

For anchor construction in weak ground, wliere the borellole remains stable during drilling operations, a rotary rig with a contiliuous flight auger is normally used* but tliere are special rcquireliicnts with some of the anchor systems in used. For example, where water-bearing col~csionlcssstrata overly clay or shale, a rotary percussive rig may be used t o advance the hole through the cohesionless strata, and t l ~ ehollow drill tubes are thcn sealcd into the clay or shale. The percussive bit is then replaced with a continuous flight auger to sink the shaft in clay beyond t l ~ ecasing and to tllc required depth. For anchor construction in alluvium, it is essential to case the boring during drilling, and several techniques are in use with and without sacrificial drill bits (Section 2.2). Duri~igdrilling, conlprcsscd nir or wnter is circulated to 7

*Softet~ln a n~d sn1uar111po f I11e nnclior hole rides niuy resull In a dccrense o f nvallnbla

ndhrslon

remove the drilling epoil materials. It is important t o appreciate that the use of compressed air, under some site conditions, may lead to damage through loss of ground, particularly in loose and dry sand strata. In gcner~la borehole of 75- t o 150-mm nominal diameter is roduced, although in some parts of the world much larger anchor shafts are constructed(" .

P

Recently, there has been a trend towards the use ofvibratory driving of casing. In many ground conditions, this technique is satisfactory, but care is essential in loose sand and fill strata where appreciable settlement may result with consequent damage t o property and nearby services. Attention to ground data obtained during a site investigation enables such potentially dangerous uses to be avoided.

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Drilling rates depend t o a large extent on the quality of the drilling machines and their torque capacity. Other factors which are important are site organisation, working space and the method o f hole drilling employed. General details are given by Littlejohn and ~ruce(') with respect t o rock drilling. W ~ t hmulti-belled anchors, the greater part of the total drilling time is taken up with under-reaming in the f i e d anchor zone. The most common flushing materials are water and air, although a bentonite slurry is occasionally used. In general, air is not suitable for dry ground conditions, but in confined spaces precaution must be taken with regard t o the health hazard from dust particles. With water fluslung, many ground conditions are improved. Water flushing can clean the sides of the hole and give a strong bond at the grout/ground interface. In weak rocks (sdch as marts, chalk and fissile shales) the minimum amount of water flushing should be used because of the known susceptibility o f these rocks to softening. It is well known that soils and weak rocks vary locally and useful information is obtained by careful logging of the positions of all ground water tables, the amount offlush return and its variation together with details of the size and length of casing in use. With widely spaced anchor holes (greater than 2-m spacing), alignment of the hole is not usually important. Misalignment results from initial incorrect setting up of the drill and deviation of the l~olefrom the correct initial line. Correct setting up is easily achieved and checked by the use of a spirit level and profile. Misalignment of the drill can occur from causes such as settlement of the drill in soft ground. In such cases, special mats are required. Dcviation during drilling is a mucl~more difficult phenomenon to control. Causes quoted are cxcessivc thrust, presence of fissures and rock discontinuities. In general, there are few problems with short- to nletlium-lengtl~ancllors. Checks can be made with continuous reading boreliole inclinometers. It is usual to quote avcra e deviations of l o (1 in SO), although in the South 8 is mentioned. In general, the few measured deviations African code(") a maxin~umvalue of 2i0 reported are within t l ~ eI in SO values. Care, however, must be taken with very long anchors.

5.3 THE TENDON During storage on sitc, all tendon material must be protected against mechanical damage and corrosion. Ideally ail steel should be stored under cover in a clean, dry area and stacked off the ground. Stcel bars sl~ouldbe stored in parallel in straight lengths and strand in coils. It is i~nperativethat no steel is accessible to weld splash. Protective sheathing is relatively soft and consequently tendons shoilld not be dragged across the ground or over abrasive surfaces. Sharp edges on cut ends sl~ouldalways be removed and threaded ends protected. Rust must be removed before use and srnall dianletcr multi-wire strands should be rejected if pitted. Tendon fabricatio~~ sllould be a well organised site operation. Important points include the efficic~~t c l e a ~ l i ~and ~ g oiling of all bar threads along with full engagement of all couplers, nuts and plates. With ~nulti-wire3 r d n~ulti-strandtendons where the strand has been supplied with a PVC coating, careful dcgreasi~lgand cleaning of the tendon over the fixed anchor length is required. A range of solvents is available and scveral anchor systems have specialist techniques available. Care I I I ~ I Sbc~ exercised in the positive fixing of all spacers, both in the fixed and free anchor Ic~~gths. The homing of a tend011 illto a borehole depends to a large extent on its length and weight. With very hcavy flexible tendol~s,a drum is used from which the tendon is unreeled into the Iiole. I'rior to insertion it is prudcnt to i~lspcctthe tendon for damage or bad fabrication.

Special attention must be given t o centralisers and spacers, used either individually or in combination. The material used should have no deleterious effect on the tendon, particularly with respect t o corrosion. In addition, ensure that the minimum grout cover is maintained around the tendon, irrespective of the angle of inclination of the anchor or its length. Checks on the centralisers and spacers may be made at random b y carefully withdrawing a tendon prior t o groirting. It must be remembered that it is difficult and expensive to extend a hole. Consequently, close liaison should be maintained between drilling and fabrication and the specified lengths of all anchor holes carefully checked.

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5.4 GROUT In the construction of ground anchors, the moz; common grout is neat cement. Usually ordinary Portland cement is :!ifficie:~r, but in some cases sulphate-resisting and rapid-hardening varieties may be required. In geceral, high alumina cement should not be used for long-term anchors but may be u s d fst short-term test anchors. Careful storage of the cements on site must be adhered to and the relevant clauses followed in BS l2(") and BS 915c2'). Water for grouting should be cliemically checked if necessary according to BS 3148("), with particular attention given t o the presence of sulphates and chlorides. The waterlcement ratio must be sufficiently high t o give adequate fluidity t o pump the g r w t into small holes yet it must have little bleeding or shrinkage if it is to possess continuity and act as a waterproofing and anticorrosion medium as well as a structural material. In general, tlie waterlcement ratio should not be greater than 0.45. Occasionally, admixtures are used. They are usually 'chemical', their primary purpose being to control shrinkage, to prevent bleeding, to ensure fluidity and t o control setting times. Tlie most important consideration is to ensure that the chemical admixture is compatible with thc cemcnt,and before use of such adniixtures is permitted study should be made of the relevatlt literature (c.g. CP 1 When designing a new grout mix the following information is desirable:

I . water/cernent ratio 2. admixture concentration 3. flow reading 4. strcngth 5, cxpansion, shrinkage, bleed and setting cliaracteristics In addition to the ncccssary punipa, ility, low bleed cliaracteristics, low waterlcement ratio and slight expansion on setting, a grout must possess the strength necessary t o develop a bond at the groutltcndon and grout/ground lntcrfaces. A reliable measure of this strength capacity is the unconfined con~presslontest, bccausc this provides a measlare of quality control on the grout as mixed. For tilost anchor construction work, unconfined compressive strengths in tlie order of 25 to 30 N/mni2 at 7 days arc in comrnon.use. It is good practice to crush pairs of 50-niti1cilbes at 3, 7, 14 and 28 days for c:tcl~anchor constructed. However, the final the method of niixing and the quality of the grout depends on several P~ctors,partic~llarly method of grouting. Tlierc are severnl good practices which should be followed in all cases:

I . thc ceiiicnt and adniixtures should be nicasured by weight 2. water and any admixtures should be addcd to the mixer before the cement 3. the liiixing time should be such that the grout is uniforlii in composition and of ) on tlie satisfactory strcngtli. Minimum niixing titlies are given by ~ e v i l l e ( ' ~based type of niiser and it is felt that his recomniendations should, in general, be followed. This rcquircs a alinirnum mixing time between 1 and 3 min. Clean, well maintained grouting eqitipnient and pumps are essential. The objective of mixing is to produce a grout of utiifor~nconsistency.

5.5 GROUTING THE TENDON There are two main grouting sequences: 'singlc stage' and 'double stage'. With the single-stage method, the coniplete lengtll of the hole is grouted in a single operation. In general, this d the frce length of the tendon is particularly well greased so that ~netliodis not ~ ~ v o u r eunless tlicre is no friction along it. i : ~ rtliis reason tlie doi~blcstage nlcthrxl is preferred. With tliis

CIRlA Report 65

method the fixed anchor zone is grouted, the tendon tested and finally stressed. Then a second stage o f grout injection is applied to provide corrosion protection t o the free anchor length. In practice, it is normal to extend the grouting of the fixed anchor length 0.5 m or more above the theoretical length of the fixed anchor. The primary purpose is t o control tension crack formation at the top end o f the fixed anchor during stressing. With double stage grout placement, the grout within the fixed anchor zone may be pre-placed and the anchor tendon installed within a few minutes of grouting or it may be post-placed. There are several disadvantages with the two-stage grouting system:

1. there is a potential discontinuity in the grout at the top o f the fixed anchor zone with the potential for corrosion attack

2. it is difficult to estimate and check the quantity of grout required for the fixed

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anchor length zone 3. the two-stage system is time consuming. Prior t o the start of grout placement, the borehole must be thoroughly cleaned by flushing from the bottom upwards, the tremie pipe cleaned and all joints in grouting pipes checked for tightness. Good grout placing practice should be followed, and at no stage should the end o f the grout pipe be lifted above the surface o f the grout or the level o f the grout in the storage tank drop so that air rnight be drawn into the grout. Also, care is necessary t o control grout pressures, otherwise damage to adjacent anr'lors and property may result from hydraulic fracturing of the ground. The pressures t o be used depend to a large extent on the ground conditions and the anchor system and the pressures recommended by the specialist anchoring contractors should be followed but with the requirement that where necessary test trials are performed prior to the construction procedure bcing put into general use. The detailed design of the anchor tendon influences tlie stresses in the grout and the ground. For example, where a compression type anchor is used, very high bursting stresses develop locally, and it is essential that the groun; can accomtnodate such stresses. It is for this reason that sufficient grout cover is provided t o prevent crushing of the ground. The interpretation of an anclior load test indicates if crushing of the borehole walls is taking place. Both of these phenonrena would indicate that the anchorage system is being overloaded.

5.6 G R O U T Q U A L I T Y C O N T R O L Tlle quality o f a grouted a~lclloris regulated by vnriations in the physical properties o f the d by inadequate nixing and variations in the quantity and grout. Tllesc arc c a ~ ~ s cprimarily qunlity of t l ~ c~naterialsbeing used. Tlre usual 11lethod of quality control is to crush one o r nwrc ~111311cubes 31 specified ages. 111addition, checking is necessary after the grout is mixed before it is injected into tlle anchor hole. Two useft11 measurements are grout fluidity by use of a flow~nctcrand 3 cone. The water/cc~nentrntio can bc reliably clleckcd by measuring the specific gravity of the grout using a niud balanc,.. Chemical contamination of the grout may be esti~natedby ~ i i e a s ~ ~ rthe i n gpH value. hluch useful guidance on grout quality control is given by Littlcjolln and l$rucc('), ~ e v i l l c ( ' ~and ) ~owers("). T o produce a high quality grout, it is necessary to use a predctcr~liinedwater/ce~nentratio and weigh batching for all materials, to control t l ~ emixing time and rate, : i d to cnsure that the grout is pumped and injected inlniediately after ~nising.

5.7 C O N S T R U C T I O N L I M I T A T I O N S I t is tlleoretically possiblc to construct ground ancliors in any ground, but load capacity nnd construction costs at present preclude construction in soft, firm and organic clays and other materials of a siniilar co~isistency.In some ground conditions, it is particularly difficult to for111anchors reliably urilcss a particular ancllor construction system has been proven in that ground typc. Tl~eseinclude very line sands and silts, clay sllalcs, niarls and chalks which are s~~sccptible to rapid softeni~rgduring borehole drilling operations. In such ground conditions, tllc anchor hole for~lii~lg method should be such that the maximum roughness is given t o the walls of the hole and th3t the ~llcthodof flusllirlg docs not cause rapid softening of the ground. Thus, i t niay be necessary to trsc special cl~e~irical grouts to pcrnlcate fine grained soils (fine sands and silts), but at prcscnt little field wotk has been done on this topic. A more usual method is to cnlploy tlic ~liulti.stngecontrolled grouting of the fixcd ancllor length by means ClRlA Raport 65

I

of a tube d moncltette device for weak rocks as well as alluvial deposits. With such a construction method and by use of controlled and known grout pressures, a range of anchor capacities can be achieved(3). Very long anchors can become extremely expensive unless the drilling machine has sufficient torque capacity, and some general details on this subject are given by Littlejohn and ~ruce('). Also, with very long anchors the problems of anchor placement increase as well as the difficulty of ensuring the correct alignment of the borehole. Site access should no! be a major obstacle t o anchoring although it can add considerably to the unit costs. This is particularly important when anchoring in very steep landslide zones, and on very conpcsted sites where small drills may be essential. For more sophisticated anchor systems with double corrosion protection, the physical dimension of the anchor unit has increased, yet there is a reluctance to drill a larger diameter borehole t o accommodate this. Consequently, there has been a trend t o provide less grout cover. This is dangerous, and it is suggested that the minimum grout covers mentioned earlier are always used. Fuller details are given in the draft British code@').

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5.8 REPLACEMENT ANCHORS Occasionally anchors fail t o carry their test load satisfactorily. Usually this results in the downgrading of the pcr~nissibleworking load and one or more additional anchors may be necessary t o provide adequate support for, say, a wall panel. Under sucll circumstances it is necessary t o drill an additional anchor hole between the anchors already placed. Factors of importance include: 1. spaclng of tlic anchors already constructed 2. the prcsc~ltstate of construction (usually 7 or morc days aftcr anchor grouting) 3. acccss for the drilling cquipnlcnt to thc lcvcl where the rcmedial anchors have to be placed 4. the neccssity t o cut a holc in tlic wall nlembcr to accept the drill rods. Wlicn co~istructi~lg rc~licdialanchors, great care should be exercised to cnsure that the drilling and grouting tnctlrods do not cause darnagc or distrcss to the already stressed anchors.

5.9 SAFETY R~rlrigall ar~cliorconstruction work, 11igl1standards of safety niust be fo!lowed. In particular, duri~lgtlic fabricati011 of tendons, rougll cdges should be rcmovcd, while during stressing the ancllor llcad sllould bc protcctcd or a strict site rule sliould apply that no personnel are in the line of tllc unclior Ilcad. During the flushing of anchor I~o!cs,considcration niust bc given to thc control of dust associated with dry flusliing. Care is also needed to ensure that wet flushing docs not causc danlagc to ncnrby P~iilitics.Tliis is particularly inlportant in arcas such ns dry docks, wlicre cxtcrisivc danlagc to electronic equipnient is possiblc if care is not exercised in the control of t l ~ cspread of tllc flustring spoil from anchor holes.

.

CIRIA Report 65

During packcr testing of a ~ ~ c l Irolcs ~ o r (see Scction 2.2.2), take care that a packer under pressure is I I O ~projcctcd fro111 thc end of the boreholc.

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6. Anchor stressing, testing and evaluation It is essential 11131 a standard mctllod of anclior stressilig and tcsting is followed for both shortand long-term behoviour, otherwise testing has different meanings and the results of tests d o not ncrcssarily have value unless thcy a r t carried o u t in a standard and clearly understood nianncr. In the past, it llas bccn argued that the prcstrcssing o f an anchor automatically tests the iustallation 311d ql131ltificsto a certain degree IIIC scrviccability of tllc P I I C ~ O THowever, . in use, the dssigncr rnay rcqt~irckuowlcdgc o n long-\cnl.r capacity, crecp cllaracteristics, factor ofsafcty wit11 rcspcct t o p t ~ l l - o \ as ~ t well as tlic pcrfor~nanceof 111sanchor when acting as a unit in thc sltppurt of an e~ipiliccritigstructure.

CIRIA Report 65

6.1 STRESSING

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For ground anchor stressing, the most suitable method is direct pull although torque via a torque wrench has been used to stress low-load capacity bars, normally rock bolts. There are numerous difficulties in torque wrench calibration, and general details are given in Reference 28. The majority of ground anchors can c?nlvbe stressed by direct pull (Figure 1l), using either multi-strand jacks where all the tendon strands are stressed simultaneously or where the individual strands are stressed In turn (Monjacknid. The principles c i anchor stressing are simpler, but there are several practical topics of value to mention. In all stressing work. a bearing plate is placed on the ground or on the structure through which the anchor passes. This plate must be placed central and norma! :D the direction of loading, otherwise fouling of the tendon may result. Stressing usually takes place after several days when the grout has reached a minimum crushing strength, usually 25 N/mm2. Prior t o the start of stressing, it is essential that the tendon wires or strands are not crossed or fouled in the free anchor zone and several spccial guides are available. With bars and single tendon units, few (if any) problems arise, and the single-strand hollow jack is easily fitted on t o the strand. Very often the tendon has t o be tested to a predetermined overload figure, and special precautions may have t o be taken during stressing to allow the design load value t o be finally placed in the anchor. Points worth bearing in mind are the weights of jacks necessitating lifting equipment for high-capacity anchors, and the high operating pressures of modern stressing systems (safety and accuracy), sometimes necessitating the use of motor-driven pumps.

Figure 11 Use of hollo w h y draulic iack for anchor bar stressing (after Littlejohn and t?ruce('))

.-

Nut / /

Rock bolt / / Coupling

Chair \ Hollow hydraulic jack

Single-strand strcssing I n d s favour with tendons up t o about five strands, because the stressing operation is rapid and tnc jack unit is lightweight. However, there are potentlal difficitlties and as ~ i t c h e l l ( ' ~points ) out, after upplication o f a nominal seating load t o each strand, the load sl~oirldbe applied in several increments to each strand in turn in an attempt throughout the tendon. For this rcason (and others), t o avoid non-uniform load distributio~~ n~ulti-strandstressing is favoured for large-capacity anchors, although it is difficult t o guarantee the unifortnity of load in tllc individual strands. Nnrrnally this is not a serious problem unless the ancllor is very short wllen even small variations in stretch represent 8 large variation in load In an individual strand. 'The purpose of anchor stressing is to confirm the competence of the anchor system, especially the capacity of the fixed anchor zone to carry the load with safety. It is essential, therefore, that the conditions of stressing are as representative of the real field conditions of loading as possible. In particular, it is essential that cc-nplctc debonding occurs in the frcz anchor length. Having provided such ssfcguards, it is possible t o obtain a load/extc~~sion relationship whicll can be interpreted from principles of mechanics. It is important t o differentiate bctweetl gross extension and real or net extension of the tendon. The as-measured

CIRIA Report 65

Anchor block

plate

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Anchor block Figure 12 Wedge grips for strand and multi-wire tendons (after Littlejohn and grucec2)I extension before 'lock o f f is the gross extension. O n lock-off, the wedge grips (Figure 12) pull into the strand until equilibrium is reached. In addition, movements o f t h e t o p anchor hearing plate, deflection o f the structure, slip within the fixcd anchorage zone may occur as well as elastic stretch o f the tendon under load. In most strcssing operations, the extension o f the ram is measured. This is an unreliable method because slip o f the strand relativc t o the overestimation o f the tendon extension. wedges occurs with res~lltat~t More reliable ~netllodsinclude the marking of the individual strands and measuring their position relative t o the load-bcaring pla!e by steel scale, the use o f dial gauges mounted o n a fixed datum, and the use o f survey methods. Wl~ereit is necessarv t o know the amount o f fixed anchor lnovement during a test, a tensioned wire o r c x t e n s o m e t c ~may be fixed at the t o p o f the fixcd anchor zone t o record the fixed anchor niovement. At lock-off, the strand wedge pulls into the strand and slip nlovcment occurs, and this movetnent contributes t o prestress loss during which can be significant for short anchor stressing. T t ~ e s cIl~ovclncnts~ n o ybe severil ~nilli~netres, lengths. During strcssing, anchor loads are often measured by the stressing equipment along with a destrcssing bridge or stool. The principle o f load measurement is t o pull the anchor until a thin fcelcr gauge can be inserted under the bearing plate (Figure 13). At this stage, the minimum pump pressure is rccordcd and losd accuracies o f 2% arc possible. A lllore positive method is t o place a load cell between the strcssing jack and bearing plate, and a range of mechanical cells and electrical cells is available capable of at least 1% accuracy. Care should be taken t o centralise the tendon, otherwise cccetltricity errors may result. With a11 load-measuring devices, frequent calibration and caref~llhandling o n site are prerequisites if accuracy is to be achieved.

*

Where load/exte~lsiondata are required during stressing, it is good practice t o obtain readings for several load increments, and a mir,i~llulno f five is reconirnended for routine tests. In addition t o the plotting of load against extension (usually based o n corrected data), all o f the original infotmation should be given. Data of itnportance include the seating load used to take up slack in the system, range o f possible errors, and temperature, especially on time tests where the relaxation of the tendon steel may be significant. The interpretation of the load/extension relatiollsllip re-,uires skill and an appreciation of the n~ecllanislni ~ ~ v o l v cind loatl trarlsfer as well as the properties o f the tendon. The following deserve consideration.

Reinforced

Figure 13

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Load measurement using hydraulic jack and reaction chair

section A I3 1. Where the load/extcnsion curve deviates from linearity in a significant manner, debonding in the fixed anchor zene and fixed anchor movements are most likely. By use o f cyclic loading fixed anchor nlovcrnents can be confirmed if the load extension behaviour of the anchor is not reproducible. Calculation of the effective free lengtll to produce the elongation of the tendon as measured, gives the extent of debonding within tlte fixed anchor length. 2. Fcnoux and ort tier(") discuss ~ l ~ c t h o of d s interpretation and in particular cyclic loading ~ n friction d in the anchorage zone. 3. In general, there are discrepancies between tl~corcticalextension and nlcasured extension. The amount of deviation from the tl~eoreticalline is a matter not fully agreed on but it reflects the allowable anchor movcmcnt which may take place. When the discrepancy is greater than 10%- care should be takcn to establish tile cause. Factors of importance are the n~oduluso f the tendon, w11icl1may be less than that of a short tcst specimen o f strand; tllc frce letlgth of the ancllor: friction along the frce length of the anchor wliere complete delionding witlri~ltllc grcnsc/polypropylcne slxath has not takcn place; and errors in load, particularly friction in rhc jack.

6.2 ANCHOR TESTING Two aspwts ol'a~lcllortesting are considered: testing of the tendon components, and testing of the gro~tndanchor. There ~ i nc11u1nbcrof standard tests available for the former. and in addition it is coninlon to rcqucst, fro111the suppliers of the tendon steel, tcst certificates in the form ofload/extensio~~ curves for each batch of ~lrateria:as well as ultimate strcngth values. The following British S~andardsshould be used: BS 2691("). BS 3617(")), BS 4447("), CP I 1s ( ~ ' )AII . FIP I~ublicatio~i(")contains details of fatigue resistance of steels, anchorages, and tendon stccl testing. Useful inforniation is givcn by Littlcjolln and ~ruce('). tlnving cstablisl~edthe quality of the tendon stccl, the major test effort ni11st be givcn t o the completed anchor. Despite tllc general background information wl~ichis available for yile testing, there is ;I lack u f a w ~ r c ~ ~of c sthe s necessity for properly conducted loud testing of nncl~crs.In other arts of the world tllcrc are codes a ~ standards d in use including ~ z e c l ~ o s l o v a k i,a~( r~a~~ i c e i~~ e) .r ~ ~ l a nSouih ~ ( ~~frica('*), ~), ~ w i t z c r l a n d (and ~ ~ )the USA(~'). Ille following comntents are h:lsed, in part, on reco~i~mendations c ~ n t a i n c din these documents.

Y

In general. tlicrc are tllrcc main classcs o f tcst. First, when a new anchor system is being devclopcd, f~~ndarliental tcsts should be carried out, usually by an i~ldepcndentauthority. The ~ri11iarypulpose of tllcse tcsts is to ensure that the system pcrformc as planned and also t o define features which could give rise to difficulties tvittl respect to load-carrying capacity over ;I long period of ti!lic. Wit11 this type of test. it is prudent to test at lcast tl~recancl~orst o at

ClRlA Report 65

least 1.5 times the allowable working load or to failure and then exhume the anchors for inspection. This Is a nlost Important stage In the test since It permits checking of the quallty of the grouted zone (defccts, cracks, strength, inclusions, dimensions), the position of the tendon within the grout, grout cover, the length of the grouted zone, the shape and regularity of underreams, and the quality of the corrosion protection system. It is essential that all such fundamental tests are very carefully executed and that all details of the test and the site inspection which follows are carefully logged. For the tests to be of general value they may have to be carried out in more than one soil type. Generally, such tests are not carried out by clients, because they are priiitarily of a developn~entnature. However, at present there is no requirement in the UK for such fundamental testiris of ilew anchor systems to be erformed. Excellent guidance on this topic is to be found in the German Code, L\lN 4 1 2 5 ( ~ '. Where a new system of anchoring is proposed, test data of tJiis nature should be inspected tr\ confirm the competence of the anchoring system.

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P

The second type of test is normally carried out before construction work commences. The purpose o f this test is either to verify a design load value or to determine the limiting tensile capacity of the anchor in a particular grol~nd.The number of tests which should be subject t o this suitability test is controlled by several considerations. In general, such tests are only representative if the number, for a particular ground condition, increases with the size of the project. It is important that the anchors on a particular site are divided into groups such as those formed in a particular ground type, those of a particular inclination and those of a particular load capacity. Once the anchors have been divided into specific groups, the minimum number of tests necessary should be established. Criteria similar t o those in use for pile test work could be uscd but it is usual to test a minimum of two anchors per group. Useful guidance is given in the French code@),which relates the nrinimum number of tests to the nulllber of anchors falling within each group starting with two tests for 0 to 200 anchors and up to ninc tcsts for a large project comprising 16030 to 32 000 anchors. This type of test is in many respects similar to a pile test in which each anchor is subjected to load and unloading cycles up to about 1.5 times the working lozd, in order that both elastic and non-elastic deformations can be detcrlnincd. The elastic dcfortnations permit checking of the free length of the tendoll, a11d the plastic dcformatiolls give a measure of the nlovements of the fixed anchor length. In addition, it may be necessary in some ground types, particularly clays, to determine the crecp bcllaviour of the anchor. By taking records of the time/displaccment relationsliip for each load incrctncnt, a crccp coefficient can he cstablisllcd (Appendix C). Current U K practice is to relate thc acccptahility of an anchor in part to its ability to Ilold irs prcstrcss load over a 24-h period. It is usual to rcquire that the load loss is less than tllcrc are cxccptions whcre this value tnay be as high as lo%, especially about 5%, altho~rgl~ wl~erca very flexiblc wall is in use and where tl~ercis collsiderablc uncertainty in the design cart11 prcssilrc cl~velopcbcing uscd. In cases wllere the load change is grcater than 5 to 10% in 24 11, li~rtherol:servatiolls sllould nortnally be made to dctermine the cause of these changes. It sllould bc borne in 111indtllat the main ancllor work should not commence until the suitability tcsts have been carried out and intcrpretcd. This work !nay take 3 wc+:ks or more. Also, thc tests should be sited so t11at they are as rcprcsentativc as possible of the ground cotlditions to which the ~nainat~cl~ors bcIong(i.c. with respect to Icngtll, inclination, ground ty, c arid corrosion protection given). During anchor collstruction, it is necessary to test cach anchor because ground conditions vary locally and installation procedures change sliglltly, giving rise to changes in loadcarrying capacity. The tcst Iuad to be placed in each enchor usually depends on wllether it is a tclnporary or a perlnancnt irtre. With temporary anchors, testing t o I .2 times the working load, with 5 to 10% ol' the ancllors to 1.5 tinlcs the working load, is satisfactory. At tlle start of a largc constructiotl progratnlne, it is prudent to tcst cvcry fiftll anchor to SO% overload progrcss this is gradually rcduced to the 5% figure as quality initially, but will1 co~~structioli control on sitc is cstablislled. In the case of pcrl~lancntancllors, i t is usual to stress each anchor to 1.5 tiules tllc working load. In ordcr not to overstress thc tendon stcel t l ~ cworking stress should be litnitcd to about 5 0 1 of cllaracteristic strcngtll. Wllcrc atlchors arc closcly sp:~ccd,it is possible that the testing of an individual anchor ilf 01at :111cll~lr\\'hen forming part docs 11ot givc a truly rcprcsent3tivc tncasure of tlle bcl~avio~lr

ClRlA Report 65

of a cluster of anchors. Under such circumstances a cluster of three anchors should be tested as a unit. It must be appreciated that the load testing of an isolated anchor does not necessarily provide information on the behaviour of that anchor when.used in the support of a structure. The test does, however, confirm the quality of the individual anchor, and, provided the factor of safety is adequate, it also provides a check on failurc when forming a unit in a cluster of anchors. Specific comments on the above types of test are given in Appendix B.

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6.3 PRESTR[.iSSING OF ANCHORS After all test work is complete, the anchor is finally prestressed and the load locked off in the tendon. This is achieved by the insertion of permanent grips into the anchor block, for wire or strand (Figure 1 3 , while with a bar the nut is secured tightly against the head bearing plate (Figure 11). l i e strand wedge pulls into the tendon steel at lock-off, causing a lock-off load loss. For this reason, some anchoring systems place a load several percent greater than the working load in the anchor t o compensate for these losses. This load loss is not a direct function of the pull-in but is a direct function of the free anchor length. The importance of length is clearly demonstrated by Barron e t o ~ . ( ~ ' ) Occasionally it is essential to recheck the load in an anchor, and a convenient method is shown in Figure 13, in which the anchor tendon is pulled against a reaction chair until it lifts just clear of the bearing plate as determined by the insertion of a feeler gauge. The load is inferred from ram fluid pressure. If this load is considered too small, the anchor may be restressed and, if necessary, steel spacers inserted beneath the anchor block to increase the tendon extension and thus raise the load. During anchor stressing operations, it is essential that all personnel arc fully aware of the necessity for very high standards of safety to be maintained. In particdar, personnel should not stand dircct in line with the end of the anchor tendon, all test equipment should be regularly ~naintaincd,and care slloi~ldbe taken to ensure that the tendon strands are not damaged prior to stressing.

CIKIA Report 65

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7. Considerations on the design of anchored structures Ground a~rcllarshave bccr~uscd in thc solutio~rof a very wide range o f foundation engineering prohlcl~lsf~.onlthe tying back o f nlasses of unstablc hillsidc to the tying down o f hydraulic structures jsucll as dry dock floor;), as well as tllc morc conventional and common uscs such as tying back retaining walls, tying down nrasts and providing reaction t o bl-sit11 loading tests (piles, plates, ctc.). Within tllcsc general areas o f anchor use, many spccilic design problems arise, solnc o f which arc associated with the use of ground anchors. Tlre design philosophy followed in all cascs sllotlld be:

1. dctein~inationo f tlrc load-carrying capacity o f a single anchor whcn used in the particular structure. This entails an appreciation of factors such as group action, pre. stressing a ~ l dthe strain distribution witllin tllr retained nlass comprising the structure, ground anchors a11(1t l ~ cgrou~ld 2. dcter~rrinationo f IIIC positions and geometrical layout o f the anchors 3. cllccking of thc overall stability of thc systenl. At present, thc greatest use o f ground anchors is in the support of temporary and perrnancnt retaining walls for deep excavations. L3ck ofkno\'lcdgc and know-llow have resultcd in a s o ~ ~ ~ e w lirnitcd hat 11scof anchors in stiff clays because of pllenorncna such as crccp, but their use in such strata is increasi~lg.In the futr~re,it Inay be cxpccted that ground anchors will finci uses in 111311)' olllcr arras, particularly in the solution o f offshore engineering prubiems. is 011 retaining wall problems. tlowcver, in this Rcport tlrc riai in c~~lpllasis

ClKlA Report 65

7.1 RETAINING WALLS

Licensed copy:Careys Group PLC, 02/10/2016, Uncontrolled Copy, © CIRIA

Retainings~ructuresare of three main types: massive, flexible and rigid, each type having an influence on the behaviour of the wall/ground/anchor system. The determination of the earth pressure distl.ibution on an earth-retaining structure is relatively straight-forward and requires details of thr soil strength parameters, soil density, position of ground water table, surcharge loading as weh as the mode of wall move~nent.The tie-back system of wall support results in being progressively futed. Consequently, the lateral deformations are the retaining stl-~cture limited t o sueb. an extent that failure within the retained soil is unlikely. It can be argued, and it is s r r p + ~ ~ e by d many laboratory experiments("), that the earth pressure problem is somewhat similar t o that associated with a strutted excavation where the esrth pressure changes with construction progress*. The envelope t o all these earth pressure distributions approximates to a rectangle (Terzaghi and peck@')). The magnitude of the earth pressure is governed by the extent of the lateral yield which can take place. Normally a A', earth pressure coefficient suffices, but many d ~ s i ~ n e r s ( ' ~have * ~ ' tended ) t o use a value between the at-rest, KO,and active, K,, pressure states in attempts to control movements. Many successful designs have been performed using a triangular earth pressure distribution@). However, in view of the mechanism involved(39),it is felt that the rectangular-shaped design envelope is appropriate for most routine design work. Having arrived at the design earth pressure load distribution, the vertical and horizontal spacings of the anchors have to be determined. The simplest nlethod follows tlie procedure in use for strut spacing determination(42) and has given acceptable results. What is claimed to be a more logical approach is the semiempirical design method of James and ~ a c k ( ~ ' )in, which a stage-by-stage analysis simulating field construction progress is used. Each stage of excavation is analysed by assuming an equivalent single tied-wall rncthod of analysis. The results of such an analysis compare favourably with those obtained from model and full-scale field tests. In such calculations, the horizontal co~liponentof the anclior force is used to balance the : rth pressure. Consequent;y, the flatter the anchor inclination, the more efficient the carrying *capacityof the anchor becomes. Analyses and both field and laboratory observatio~lshow that the use of anchors reduces bending monients in the wall tllember, and the verticsl spacing of the anchors is usually determined by consideration of tlle bending stress in the wall. Field observation^(^) clearly demonstlate the reduction in wall bending stresses wit11 anchor insertion, while Cloug11 and ~sui("') have sllown a sil~lilartrend by use of a finite element idealisation. In addition t o showing the reduction in wall bcnding moment caused by anchor use, ~ u r d i ( ~ 'also ) demonstrated the stiffening effects wl~ichanchors have on wall flexibility. However, wl~ileground ancl~ors,correctly spaced, control stresses in the wall t o acceptable levels, tllcy do not necessarily keep lateral and vertical movements of the wall to acceptable levels.

This is particularly the case in very heavily overconsolidated clays and clay shales, where the zone of ground subjected to stress changes frorrl excavation works exterids t o severill times the excavation deptll in the lateral direction. Under such conditions considerable lateral movements result, irrespective of the ancllor support system used.

7.2 OVERALL STABILITY Several ~nodcsof wall hilure are possible. Complete failure niay result from inadequate bearing capacity at wall base lcvel wlle~lthe wall settles vertically, translates and rotates due to tlle interaction of the inclined rows of ancllors. If such a condition occurs, very !a~,gcniovenients with resulting distrcss take place. It is necessary, tllerefore, to assess the bearing capacity of the wall base. Lr)ading arises fro111tile vertical con~ponentof the earth pressure load and thc vertical co~nponentof th? anchor prestress loads. In addition, it should be appreciated that soil is gradually rclnovcd fro111in front of the wall as excavation progresses and therefore the bearing capacity problclll bccolnes more critical as construction progrcsses. At prcsent, there are 110 analytical ~lletllodsof checking bcsring capacity, and the most suitable n~ethvdis t o ~ ' )otllcrs for the bearing capacity of eccentric follow tllc rcconllnendations of M ~ y e r h o ~ or .'One of llle in~vortu~~t d i l l .rcnces is the vurlicnl co~nl~onent of l l ~ anncl~orforces nnd Iha eorth pressure loud nctiny o n ttir wu1\(1394'\.

ClRlA Report 65

and inclined foundations. H'ith steeply inclined anchors, very high unit bearing pressures occur at wall base level, and failure of walls founded in bedrock is not urlcommon where excavation is taken near to wall base level. In cases where the excavatiot~is in firm clays, it is possible that the base of the excavation ) successfully modified the stability factors of Bjerrum and ~ i d e ( ' ~for ) will heave. ~ t i l l e ( ~ 'has braced excavations to allow for the vertical conlponent of the anchor forces. This, in turn, alters the direction and the magliitude of the shear stresses along !he outside and inside faces of the wall. Stille shows that the overall stability of the base of the excavation is very dependent on the magnitude of these shear forces and that the stability nuniber may be considerably less than that associated with a braced excavation. Good agreement is found between his theory and records of base failures of tied wall excavations in Sweden.

Licensed copy:Careys Group PLC, 02/10/2016, Uncontrolled Copy, © CIRIA

Checking the overall stability of the wall/anchor/soil system requires a number of simplifying assumptions to be made. Most methods of analysis are based on a limit state whereby a surface of failure is assunled and the disturbing forces are conlpared with the resisting forces to give the overall factor of safety. With such methods of limit analysis it is not possible to estimate deformation within the retained soil or rock mass. One of the most commonly used methods is that due to ~ranz(") in which a composite failure surface, made up of an active wedge zone behind the anchors, a passive wedge zone it) front of the anchors and a connecting

' ~ r e e anchor ' Length

Assumed continuous deadman or anchor wall I

zone \.-