The Institution of Engineers of lreland

P ilin g in RockMichael Long, 8.E., M.Eng.Sc., CEng., M.I.C.E, M.I.E.I. Uniuersity College Dublin.Fergus Collins, B.8., Eurlng, CEng, F.I.E.I. HMC Construction Co.

l,aper first prescntcd to a joint mecting of the (lorl< I{cgion of the Llr.l, and the (lcotcchnical Socrcty of lrcland,at University Colleec (lork on 30th, N{arch 1999

,

Structuralload

IGround level

Synopsis'l his papcr

tcstingIrasusuall1,llcenllvcclrrr,cntiotla1sraticloading[ltttthcrccx1;cricnce and docunrenrarion is rcquircd bcfore SlMllAI-testing can be acccptcd ls a sttbstitttte for static tests. L)esign ofthe piles isbased on cmpirical mcthods w*rich rvcrc developcd for soft rocks clsovhcrc in thc rvorld. 'l'hc rcstrlts of thirty pilc tests at cightccn sitcs

located throughour thc counrry have bcen backanalyscd using thc hi,pcrboiic transfcr fitnction tcchnique in ordcr to prodttcc estimatcs

of rnobiliscd skrn fricrion and cnd bcaring. Prlc behavioLrr has usLrally [reen vcrv sood, x'itlt thc load scttlement resPonse being linearclastic rvell beyond design load, suggcsting pcrhaps rhar currcnr pilc dcsign practicc is conservrtivc. It is shown that thc cxisting designmethods are nor appropriate for Irish rock socketcd pilcs. Somc gtridancc is givcn on appropriatc dcsign parameters based on experience

and sn the rcsults ofbackanalysis. lt is shorvn rhat the govcrning design criteria may bc thc nced to limit thc concrctc or grotlt stress to:r1 acceptablc valuc and that it rvould sccm that tlicrc is no nced to cxtctr.i thc rock sockct lcngth tty more than four pile dtameters. Ancw design approach which uscs basic rock and pilc param,rtcrs and ellows fbr thc cffccrs of roLtghncss along thc pilc shaft is introduccd.

l)etails offbLrr case historics rvhcrc problcnrs wcrc encountcrcd rvith rock sockct pilcs arc also prescntcd.

l.INTRODUCTION

The recent period of sustained economicgroMh in Ireland has led to an increase inthe use of rock socketed piling due to therequirentents for higher structural loadcapacities and the development ofpreviously marginal sites. The purpose ofthis paper is firstly to describe thedevelopment and current state of the art ofthe techniques available in Ireland and tothen describe the current design methods.The results of pile loading tests frorneighteen sites located widely around thecountry will be analysed in order to examinethe behaviour of the piles and to review thevalidity of the current design process.Behaviour of rock socketed piles hasgenerally been very good bnt someexamples of failures and other problemsassociated with rock socketed oiline will bepresented.

The research work presented in this paper isnot considered to be in any rvay final orfully conclusive. It is hoped that this paperwill stimulate interest in this topic in theindustry which in tum will generate norecase history data. Full scale instrumentedtests are essential to verifr some of theconclusions made here.

2. GEOLOGICAL SETTING

Overburden conditions in ireland aredominated by the rnantle of glacial depositsthat cover the country. These glacialdeposits are complicated and variable innature with randon interlayering of fluvioglacial gravels and boulder clay. Thedeposits are usually competent in strengthterms but often include large cobbles andboulders. Many of the major cities andtowns have developed on the banks of riversor near estuaries where terrace gravels,alluvial clays, silts and peats can be found.Previous development of the cities hasresulted in rnade ground deposits up to 5rnthick.

Bedrock in Ireland is frequently fresh andstrong. The most commonly found rock typeis Carboniferous limestone, which underliesthe main cities of Dublin, Limerick andCork. Strong sandstones (near Cork) andigneous rocks (at Galway for example) arealso found.

3. ROCK SOCKETED PILINGTECHNIQUBS3.1 Small / Medium diameter piles

The techniques described in this paper havebeen developed over the past ten years. Thetwo main criteria which have driven thedevelopment were the need to minimisenoise and vibration, as many of the projectsare located in the inner city areas, and theneed to be able to deal with the variable andoften bouldery overburden. The techniquecan be described as bored vibrationless non- displacement cast in situ piles.

Drilline PlantPiling rigs are either self contained orexcavator mounted hydraulically poweredrigs, such as that shown on Fig. 1. Ahydraulic drifter (i.e. a hydraulic rotationwhich can rotate and hammersimultaneously) or a pure rotation unit isnrounted onto the mast and this is used todrill the piles using one of the following setsof attachments:

1) augers only,2) augers and ten-rporary casing.3) casing and rods,4) augers and down the hole harnmer.5) down the hole hammer only,6) Odex svstem.

Sites free of obstructions or large boulderscan be drilled using augers only. If bouldersare present a cased pile and / or augers anddown the hole hammers can be used as andwhen required. Tlpical drilling attachmentsare shown on Fig. 2a.

The Odex system involves the simultaneousdrilling of a casing with a down the holehammer. An eccentric bit is attached to thehammer which reams out the soil in advanceof the casing following r.nder its self weight.At the required toe level the harnmer isrotated in reverse. the eccentric bit iswithdrawn and the whole drilling assemblycan then be removed, see Figure 2b. Thepile can then be fonned using nonnal tremieconcreting techniques. Once the pile hasbeen formed over the full length, the casingcan be removed and the reinforcement,which consists of a helical cage, can belowered into the concreted pile.

The flushing system used for items 1, 2 and3 is water at high pressure. The highpressure flushing brings all the displacedspoil to the surface and also causes theeffective diameter to be greater than thetheoretical one in the overburden layer,which results in high skin friction. ln items

r{ i\.!

':t,

,qae -=::

Figure 1. Small dia. piling rig

Fignre 2a. Typical drilling bits -

Down the hole hantmer

Figure 2b. Typical drilling bits -

odex

Figure 3. Cluster drill usedfor largedia. piles

ql:-..-g; :ffi;

Figure 1. Reactiort frante used forTentple Bar test

.l and 5 high prcssurc water llushins is alsouscd to clean out thc rock sockct onwithdrawal of thc dorvn thc holc ltarnrner.This cnsures a clcan rock sockct lnd givesrraximur.n possiblc skrn lriction and cndbearin-9 valucs.

Hr-{h pressurc air llushing is used lbr boththc Odcx systern and the clou,n thc holcharnrner system. Thc air is used both toopcrate the hammer and to keep the bore1'lushed clcan.

Crouting ProcedurePilcs of 350 rnm dia. and less arc usuallygroutcd with eithcr a neat ccmcnt collordalgrout or a l: I sand cement colloidal grouthaving a wiiter / cemcnt ratio of less than0.5. Thc basic principlc uscd is that the prlcsare trernie grouted. In order to achicvc aclean base an auger is oficn attachcd to thcdrill string at the end o1'the drillingproccdurc. Crouting then usually takes placethrough the rods or the hollow stcm of thcauger being discharged at thc base ol'thepile thus displacing the spoil upwards to bercplaccd by the grout. lt is normal practiceto allow lbr 1.5 to 2 times the theoreticalvolume ol grout to be usecr.

Basc groutcd piles havc irlso been used inlrcland to ir.nprovc settlemcnt pcrlbrrrance(Higgins and Mason, 1989, Rcl'. l)RcrnlbrccmcntT-vpically thc pilcs arc rcinlbrced wrth onc(of a clustcr ol'trvo ur thrcc) high tensilebars lirr thc lirll lcngth of thc pile. Thcreinlirrcing is clcsi-rlned to resist thc l'ullapplied load shoulcl any nccking ol'the pileshati occur'. Dcpending on the latcralloading recluiremcnts and thc thrckncss o1'loosc ovcrhurden, l short helical cage canalso be usccl. The relnlbrcement is loq,eredinto the prle on thc completion of grouting.

Constructron ControlConstruction control is by good supervision,cxtracting samples of

-qrout fbr cubc tcstingand carciul monitoring ol'grout volurnesusecl. Fully instrumcnted piling rigs. withthc capacity to mcasurc dnlling torque,dcpth, grout volume and pressure etc. aregradually being ernployed.

3.2 Large diameter piles

More recently large rigs rvith rrast heightsof up to l5m have been employed inIre land. Piling with these rigs rnvolvcs the

use of augers, buckets and core barrels totbrn'r rock sockets up to 800 mrr-r in dia. Thedrilling proccss can typically involvefbrming the pile using the casing and augerdown to rock. The casing cuts into the top ofthe rock and the auger is withdrawn. Adou,n the holc hammer of diametcr up to500 mm can then be attachcd, which in turntbrrrs the required length ol'the rock socket.For pile diametcrs of 800 mrn a clustcr drillis requircd. In the casc ol' the 5CD32 drillshown on Figure 3, tlris consists ol'a drill of5 down thc holc hammcrs which is uscd tofbrrn the requirccl rock socket. Pilcs are thenconcrctcd using the standard tremieconcreting techniclucs.

Reinfbrcing of the larger diamcter pilestypically consists of a hclical cage of hightcnsile bars rvhich is lowered into thcconcreted pilc.

4. PILE TESTING4.1 Static tests

Pilc tcstrng is most frcquently by means ofconventlonal static load testing, usingground anchorages or kentledgc forreaction. Prcliminary tests on sacriflcialpiles arc carried out on larger projects orwhere qround conditions are unknown,Prool loading tcsts on contract piles arecarricd out on most jobs, Thc maxrmumload applied is usually L5 trmc the specilied*'orking load (SWL) Thc load is applicd ina numbcr of cyclcs, with cach cyclcconsisting o1' a number of incrcments ol'load. 'fhe load at czrch incrcmcnt ismaintained until the rate ol'scttlcmcnt is lessthan 0,05 mrr rn 30 minutcs.

The largest static pile test carricd out in thecountry to date (to the knowledge of thcauthors) was on a rock socket pile at thcTcmple Bar site (Pile 13e on Table l). Itwas loaded to a maximum o1' 13750 kN.Ground anchorages togcther with standardkcntledge were used to apply the loadthrough a heavy duty rcaction fiame asshown on Figure 4.

4.2 SIMBAT tests

Dynamic testing using the SIMBATapproach is also liequently employed. Thistcchnique involves the dropping of a weight(typically 1 tonne) on the prepared head ofsome selected piles. Thc rcsponse ismonitored using an accelcromcte r and astrain gauge which have been attached tothe pile head. The data is analysed using

i r .,.:.liilli-lr-

Applied load (kN)600 800 1000 1200 1400 1600

0

1tr

yrzoEe3(Jgct.9tE'E'(E-Psgit

o^

7

Figure 5. Comparison of SIMBAT and slatic test results

,/2,.9/ts\..

Cast in - sifu -concrete pile

Socket drilled Jinto rock

Ground level

,///,9//*

SoftOverburden

\\\\\

Rock toundingstratum

-

Side resistance

'-

Structurat toad

Steel Lining

S N-rr - Pile 17 - SIMBAT --

+Pile 17 - Static--tsPile 10 - Static

thc luvc cquation approach, as lur drivcnprlcs, to

-rlivc cstimatcs ol' pilc capacity andIoad scttlcntcnt rcsponsc.

Thcrc lias bcen an incrcasing rcliancc onSIMBAT tcsts ovcr thc past f'crv ycars to thecxtcnt that in sornc cascs thcy havc beenused as a substitutc to static tests. Thcresults ol static and SIMBAT tesrs olt piicslronr lwo srtcs in Dublrn (Pilcs l0 and l7 onTablc 2) arc shown on Fisurc 5. ln bothcilscs thc operator ol' thc SIMUAT tcst hadno knorvlcdgc ol'thc pcrlirrnrancc of'thc pilcurtdcr statrc loud. It cun hc scclr thtit thcSIN4BAT prccliction is vcrv closc to thclctuxi statlc tcst rcsult in hoth cliscs.'['hcrcrs u slight ovcrprcclictior.r ol'pilc hclclscttlcmcnt ancl rcsiclual nloVcntct)t onunloltding. Thcsc rcsuits and rnanr, likc thcrnhur,c increliserl thc conllclcncc in thisploccdurc. llorvcvcr. il ntust hc pointecl outthat thc najoritv ol thc contpar.isons havcbccn ntaclc on pilcs r''hcrc:

ru) thc prlc bchuviour is llirgcly cllstic.h) littlc basc rcsistlirrcc has bccn ntohrlisccl.c) thc luctor ol' salcty against liiilurc is

Irigh.

In lhc opinion ol'thc authors thc tcst clcurlvlll)l)clrrs pronrisinu uncl provirics vcrv usclirlclata, palticularly lirr sitcs rvhcrc tlrc r.csultsItlrr c hct'rt r'lrlihtrlt.tl lrutrirrsl :l.rtr.. (c:ls.llori'cvcr. ntorc clitta is rccluircd on pilc tcstsrvhcrc thc pilcs cxhibit othcr thlin clasticbchlviour lind on prlcs whcrc thc concrctc /srout ts strcssccl to a high Ict,cl bclirrcSIN'lllAT tcsting can hc consir.lcrccl an cclulllltcrnatiIc to stlltic tcsting,

'1.3 Integritv testing

or'"^rburdcn contains silts, pclts or \\,atclbcrrri ng slnds.

5. UNDERSTANDING THE PILEBEHAVIOUR

In this scction a ntodel to cxplain lhcbchaviour ol' rock sockctcd pilcs rvill bcprcscnted and it \\.ill thcn hc asscsscd byrcl'crencc to a largc databrrsc o1' rcal pilcl-rchaviour as obscrvcd in static pilc loadin_utcst s.

Fiqurc (r illustratcs thc rncchanisnr ol'tfirnsltr ol' loutl litlil thc structurc into thcgrourrd ril thc pilc. I-oatl cln bc transl'crrcdin skin ll'iction (or sidc fcsistancc) bct'"vcct.rthc pilc antl thc rtvcrbLtrdcn, in siclcrcsisllncc bct*'ccn thc pilc lnd thc rock anclin hasc rcsistuncc. Analvsis ol-pilc loar.l tcstclata hlts sucqcstcd thut thc llrst c()ntponcntis usuiillv vcrv srnull. 'fhc urnount ol loirdthllt ls translcrrctl intO thc tock llong thcpilc shuli irncl through thc pilc blsc dcpcnclson nrlny lirctors. including thc dilrrrrctcr ol'thc pilc, its lcnglh. rhc prlc rnatcrralstillicss. thc sl|cnslh and sttllltcss ol- thcfock. thc lnctlto(l ol'pilc construction, tlrctinrc dclly bclitlc concrcting thc pilc ctc. lnacklitiorr to this thc drilling proccss usccl toIolnr thc rock sockct nrtrrnally lclvcs sorlclirrtn ol roLrghncss on thc \\,iill ol thc sockct.

It is clcrrr lionr thc abovc that thc loacltrlnslcr rncchltnisrlt is vcry cotnplex. Inorclcr to unclcrstiud it. rt is nccessary toanalysc thc bchavrour. ol' rcul pilcs underIoad ancl to havc itcccss to an analyticalnodcl which can separatc thc various cflectsdiscusscd abovc.

5.1 Database ol'load lests

For thc purposc oi this paper thirty pile testrcsults havc bccn choscn f'ront cightecn sitcsrn Ircland. Thc pilcs wcrc constructcd bythrcc dil'f'crcnt contriictors. The location ol'thc sitcs is shor.vn on Fig. 7 and thcy can bcsccr) to bc sprcacl rvirlcly tlrroughout thccountry. As lvcll irs clivcrsity ol- gcography,gcololy altd construction tcchnicluc, thccritcria lirr chorcc 0l'thc tests Nls blscd onthc lirllowin-r.

L Sitc rnust havc sitc invcstigation rcsults,2. Invcstigltion lnust havc includcd rock

cori ng,3. Rcsults ol'static load tcst availablc,-1.'I'cst carricd out to thc spccillcation

clcscribccl uhove,5. Pilc tcst rcsults arc rcliablc.Thc dctails ol' thc casc historics and pilctcsts arc grvcn in Tablc l. Pilc diarnctersvary bctwecn 0.17 m ancl 0.8 nr and thcirlcnr:ths vary bctwccn 3.0 rn and l9 m.

N

-.

Snlall hanrrncr tvpc intcur.ily tcsting islicqucntly culriccl out lur quirlily ilssufitnccpurposcs. Thc nicthod rclics on l clclnsi-qnal hcine rcccivcd liortr a pilc ri,hrch hlsbccn stluck u,ith it snnll hlinrrncr.. As arcsult lntctprctatiot) ol'thc tcst clat0 isliaucht with clrl'lrcLrlty. Scvcul conltrcnccshavc bccn hcld worldrvidc to cliscuss thcproblem and in oltc casc u scrics ot'clclibcnttcly 11a,'l'cd pilcs \\,crc cot.lstructcdltrtd varioLts tcstin-q houscs',vcrc askcd topinpoint thc cliscontinuitics irr thc pilcs. Thctcsults u,crc rlixccl ltr.rcl oltcn pclor..

At bcst it is possihlc t() sity tllitt thc tcst canrcvcll only largc anorllilics. It is usclirl ltspart ol iln Ovcftrll cluality contKrl packagcrilriclr inclutlcs !(r(r(l sitc srrpcrvisiorr. stlrtictcstlns ilnd / or SIMIIA-l' tcstins. It ciul bcpnrticuf rrrl), usclul on sitcs $'hcrc thc Fignre 7.

scale km0 50 100Site locatiotts

CaseHistory

Location Contractor Avg.Pile

Dia*.

SWL RockSocketLenoth

OverallLength

RockType

Pile Head Mov. (rrun) f Frb 9" (a-C/ q,

(m) (kN) (m) (m) SWL 1.5 swl. 2.5SWL

Unload (kNim') (kN/m') (MN/m')Galway HMC 0.35 700 1.5 7.15 Strong

sranite1.40 2.19 0.19 20r5 0 150 0.013

2a Westport HMC 0.35 1000 2.8 8.3 Mod. stronglimestone

1. l0 t.76 2.37 0.26 750I1500+

0 30/50 0.028

2b Westport HMC 0.35 1000 2.3 7.8 As above l.8l 2.87 0.40 1000 /1880+

0 30/50 0.035

3a Limerick HMC 0.35 1000 J.J 9.3 As above 3.22 4.92 8.81 0.40 1920 0 50 0.038JD Limerick HMC 0.35 1000 2.0 9.0 As above r.94 3.15 0.49 1455 0 50 0.0294 Ringaskiddy HMC 0.225 600 1.0 10.0 Mod. strong/

strong I'mstt.l4 1.78 0.36 3000 0 75 0.040

5 Cobh HMC 0.18 500 3.0 19.0 Strongsandstone

2.62 4.47 r4.15 4.85 200 12500 100 0.002

6 Little Island HMC 0.3 800 3.0 14.0 Stronglimestone

l.76 2.90 0.33 550 0 100 0.006

7 Midleton HMC 0.35 700 1.5 7.5 Mod. stronglimestone

3.7r 5.48 10.19 1.s2 t20 30000 50 0.002

8 Enniscorthy HMC 0.325 650 2.25 9.0 Mod. strongsiltstone

t.o I 2.56 0.22 765 0 50 0.015

9a Dublin CivicOffices

HMC 0.35 r000 2.7 7.1 Mod. stronglimestone

1.58 2.56 4.43 0.80 1620 0 50 0.032

9b Dublin CivicOffices

HMC 0.35 1000 3. 15 t2.65 As above 1.78 2.97 0.40 1270 0 50 0.025

l0 Dublin Ship St HMC 0.35 1000 2.0 8.2 As above 3.88 5.8 r 0.02 1275 0 50 0.026lla Monaghan HMC 0.21 425 u.) I 1.3 Strong

limestone7.44 15.15

l.3swl-7.27 620 1 1500 80 0.008

1lb Monashan HMC 0.19 425 5.5 As above 3.66 6.09 1.07 420 5642s 80 0.005t2 Blanchardstown HMC 0.35 600 0.5 5.2 Mod. strong

limestone1.38 2.t4 0.27 3275 0 50 0.066

l3a Dublin TempleBar

Murphy Int. 0.6 3000 2.0 6.8 Mod. stronglimestone

3.05 4.27 15.54J

SWL

6.38 I 950 14200 50 0.039

l3b Dublin TempleBar

Murphy Int. 0.6 1000 0.2 6.25 As above 2.62 4.67 1.78 0 7050 50

l3c Dublin TempleBar

Murphy Int. 0.6 3000 t.75 9.1 As above 4.19 7.11 0.08 t670 0 50 0.033

l3d Dublin TempleBar

Murphy Int. 0.6 2750 2.1 6.6 As above 4.93 8.53 2.03 500 t4200 50 0.010

13e Dublin TempleBar

Murphy Int. 0.8 5000 1.5 3.0 As above 5.00 7.44 13. 132.75SWL

5.13 470 87000 50 0.009

CaseHistory

Location Contractor Avg.Pile

Dia*

SWL RockSocketLeneth

OverallLengtlr

RockType

Pile Head Mov. (mm) f rb Q"t/ q"

(m) (kN) (m) (m) SWL 1.5 SWL 2.5SWL

Unload (kN/m') (kN/m') (MN/m')

13f Dublin TempleBar

Murphy Int. 0.8 3000 0.2 4.9 As above 2.79 5.00 0.69 2000 19900 50 0.040

l3e Dublin TempleBar

Murphy Int. 0.6 22s0 1.6 6.5 As above 3.33 6.55 r.40 t300 0 50 0.026

I4a DublinTownsend St.

Murphy Int. 0.8 5000 3.0 9.5 V weak m'st/ strong I'mst

bands

4.r9 7.77 2.21 910 4000 l6 0.057

l4b DublinTownsend St.

Murphy Int. 0.6 2500 J.U 13.3 As above 3.2s ).9 I 1.60 975 0 16 0.060

15a DublinNangor Rd.

Murphy Int. 0.8 3000 3.0 7.0 Mod. str.I'mst / weakm'st bands

1.65 2.57 0.30 995 0 25 0.040

t5b DublinNangor Rd.

Murphy Int. 0.8 3000 3.0 7.9 Mod. str.I'mst / weakm'st bands

1.98 3.58 0.81 99s 0 25 0.040

l6 Blanchardstown PJ Edwards 0.45(CFA)

900 0.2 3.5 Mod. stronglimestone

2.1 2.75 0.75 0 15700 50

t7 DublinOliver Bond St.

PJ Edwards 0.19 t000 0.5 8.7 StrongI'mst/weakm'st bands

3.32 5.53 0.64 3000 0 40 0.075

l8 DublinSt. Steohens Gr

PJ Edwards 0.17 400 0.5 12.5 weak / strongI'mst

2.2 4.15 0.22 1500 0 74 0.020

* value quoted is the amount mobilised, actual value available likely to be much larger* for weathered rock / intact rock

Table 1: Details of Case Histories

Figure 8. Civic Offices, Dublin

Avcragc rock sockct lcn-lths rvas 1.8 rl ultrlthc range was 0.2 rn to 3.3 nt. The ntaxintuntpile test load was 13750 kN.

Thc cornpleted building fbr Casc Hisrory 9.the Dublin Civic Ofllccs is shown on Figurc8. This building wiis constructcd on l sitcwhich contained importlnt Vikinc rcnrtins.

5.2 Preliminary assessment ol' pilebehaviour

The behaviour ol.thc piles r','ill Irrst bcillustrated using tr.',,o cxnrrplcs. Figurc 9shows the Iull load tcst rcsult lirr Pilc 31. thcprcliminary tcst pile at rhe Mount Kcnncrrsite in Limerick. lt can bc secn that thc pilcbehaviour was virtually elastic up to ir loadot' 2500 kN, which is 2.5 times rrs SWL.Residual settlcment on unloading rvas only0.4 mm. It is clear thar all ol' thc load isbeing supported by clastrc rcsponsc betwcenthe pile shaft and the rock.

Figure l0 shows thc lull load tcst results fi)rPile l3a, the preliminary resr pilc ar TcmpleBar in Dublin. Thc bchavrour ol'the pilc isvcry dil'f'crent llorn thc Mount Kcnnct.t pilc.It is a "stubbicr" pilc and it rs clcar thntlbllowing an initial clastic respctnsc, withthc load being transl'crrccl into thc rock viathe shali only. load is then transl'erredmostly onto thc pilc basc lbr the retllarndcrof the test. The load scttlenlent curve isrnuch more rounded. Howevcr the pilcbehavcd very wcll with a residual settlcntcntlbllowing removal of 3 SWL of only 6.-lMITI.

An irtspcction o1' ull ol' thc pilc louclin_q tcstrcsults cunlilnts thut thc hch.rr iour hls bccngood irr ail clses. Most pilcs rcspondctlsinrilur to thc \lount Kcnnctt clsc uith pilchcad scltlcntcnt rcsl)onsc bcing llirqclvIine ar clrstic *,cll bcvoncl thc SWL.

Pilc spccilicltions licc;ucntl1, r'crlLrilc thltthc scttlcnrcnt ol'thc piic lt SWI- shoultl rrotcxcc'ccl ltbout l0 tlrn and thlt thc lcsiclutrlscttlcnlcnt orr unloitriing il0rn thc sccOnclloacling cvclc ( I .-5 SWL Lrsuirlll,) should notcxcccd ubout 3 rnnr. It can hc sccn tlnt irrthc cascs urrcicr considcltrtion Itcrc thcscttlc'nrcnt ut SWL oltlv crcccdccl 5 ntnt irronc ciisc urrri uls usrnlll' lbout 2 nrnr. ThcrcsirlLtitl scttlcllcltt orr Lrnlolclinq llont 1.5SWL rras usrrlll\ lcss tltan I ntnt.

Clcarll, thc bchaviour ol'thc rock sockctcclpiles undcr consideration has bccn verygoocl. Howct,cr clocs tlris cxccllcntbchaviour suggcst that pilc dcsign pmcticcis ovcr)y conscrvativc'l. In orclcr toundcrstand thc bchaviour ntorc clearll, it isncccssarv to rclcr to an anulytical rnodcl.Firstly a simplc ana)ytical nroclel, rvhich iscurrcntly uscd in dcsign practice in Irclandu'ill bc prcscntcd trnd thcn Iwo lrorcconrplcx nroclcls *ill bc discLrsscd.5.3 Simple model ol'pile behaviour

Thc dcsign ol rcck sockctcd pilcs in Irclandto clatc has bccn bascd on a sirnplc nrodcl ol'thc prlc belraviour itrrd a scrics of'rulcs asfbllows:

l. Linrit coltcrctc / qrout strcss to 257 ol'its charactcristic strcngth,

2. Llltirnltc pilc clpacitr,{Q,,) is thc sunr ol'rhc ulrinlrrc shlli capacity (e.) and lhcLrltintatc basc cupacity (Qr,). i.c.

Q,, = Q, -l Q,' (])

3 Allorvable pilc capacity (Q.,rr) is -qivcn

NV:

(l)

-1. I-intit scttlcntcnt itt SWL to l0ntnt.

Q. is Lrsullll, rlctcr ntincd lhll tltc lirrrrulu

O:tAor

(3)

a, = (xpq,,A, (J)\\'hc rc.

c = lclhcsi0rr llictot tcliltc(l to rl.,D = llctor to allou lirr rock ll'actLrlc spacinrr.cl. = unconllncd conrprcssivc strcn_qth ol thcr()c K.n, = pilc shalr arcu.

o and B urc usuallv dctcnnincd liom thc.,r'ork ol Willianrs uncl Pclls ( 1981, Rct'. 2),uho lclatcrl thc pilrilntctcr ct to q. Lrsing thcrclutionship sho\ n lrtcr o|t Fig l l.Willranrs lntl Pclls rvork gas lirl thc u'clrksiln(lstoncs. ntuclsloncs tirrd shtrlcs olAustrlliu. rocks q hich be rrr' littlcrcscnrbiancc 1o lltc strong lintcstoncs artdgmnitcs ol'Ircland.

Valucs ol l. bct.,vcen 500 KN/rn: lnd 7.50kN/nrr arc ti'picallv usctl in dcsisn.

Q- rs dcterrttitrcd lrulr) tlrc cxprcs:iolt:

Qt, = .l',,-\A,,

whcrc:

An = pilc birsc arcu11. is choscn (liont cxpcricncc) to bc in thcransc ol'-5 N4Pu to 20 \{Pl. -['ltis *'iclc- rlngcol Values inclicatcs thc unccrtuinty thilt cxistsin thc industly rcgarclinu ln appropriatcdesi-sln r,aluc lor this parantctcr.

In order to unclerstand hrlrv the rcalbehaviour ol'rock sockctcd piles in Ireland

oL5Q,,rr =

(5)

5000

2

4

6

I10

12

14

16

18

20

Figure 9. Mount Kennett, Linterick trial pile (Pile 3a)

2000

Applied load (kN)1000 1500 2000

Applied load (kN)4000 6000

0 2500 3000

EE+,coEo

ooT'(Eo

giT

0 8000 1 00000

2

E4E+rotrg8o

=10oIp(EP14

=16q-18

20

-=*

Figure 11. Hyperbolic transferfunctions

compiires with the Wrlliams and Pcllsmethod it is necessary to resort to l ntorccomplex model so as thc valucs of l. and fl,can be determined.

5.4 "CEMSET" model of pile behaviour

An analysis of the pilc test dara has bccncarried out using t.hc hyperbolic transl'erlunction approach as describcd by Chin(1970, Rel'. 3 and 1983, Rcf. 4) and Flcrning(1992, Ref. 5) Thc intcraction at thc pilcshali - soil, pile shati - rock irnd prle basc -rock ilrc rnalysed using lhrcc scnitratchyperbolic lunctions, as illustratcd onFigure I l. Elastic shortening is also allorvccllbr in thc analysis. In thc casc of thc pilcs inthis study, pile shali - soil intemction wilsfbund to bc vcry smirll rncl hus bccrrignored. Thc rrethod has also bccn uscclsuccesslully to analysc some rock sockctedpiles in Italy as dcscribcd by Carrubba(1997, Ref.6).

The fbrmulac riven by Flernin,u (1992) u,crcinscrted into a spread sheet analysis ancltypical output lbr the analysis of'Tcst l3a atTcrnple Bar, Dublin (sce also Figurc l0) isincluded in Appcndix A of thrs papcr. Thistest is chosen to illustratc thc process as thcpilc load was resistcd by a cornbination olskin ll'iction and cncl bearing, which isunusual lirr thc pilc tcsts unclcrconsidcration.

Fleming (1992) shows that rhc rncrhod isapplicablc lbr a widc rangc ol soils and lbrweak rocks. Is the method also the napplicablc to strong rocks'?. Chin (1983)

sug-qcsts that hyperbolic lirnctions urcin'rportant in thc nrattcr ol' lbundationscttlcment bccausc nrobilisation of stress ina soil q'ith incrcase in strain is ir l'unction ol'tlic incrcasing nunrbcr ol'cl-lcctivc soilcontacts. It may,thcn bc visualisccl thlt a sorlur.tclcr corttprcssivc strcss transrlits load bvintcrnll colunrnirr grain structurc. u,itlt rnorcand rlorc colurr.urs supporting loaci as thcstrcss incrcascs. It is ltlt that this hypothcsisrray also bc applicablc to the nrincnrlstructurc ot locks.

5.5 Results of "CEN'ISET" analvsis

Skin lrictionA sunrnary ol the analysis rcsults in termsol rnobiliscd skin liiction (l-). mobiliscd endhcarintt ( li ) and thc bnck crrlculatcd rr valucsis givcn on Tablc l. Thc q. r'alues wcrctrkcn lls thc ilVcragc Vltluc liitm thclabomtory tcstinq (rnostly point load tcstintbut somc r-rnconlined contprcssivc stren-cthtcsts rlso available) or',r,here no valucs ',vercluvailablc arc bascd on vrsual cstitnates ol'the rock strcngth. B .,vls assur.necl to bc cqualto 1.0 in lll cuscs as cxpcricncc has showntlrrrt llrt'nutrrrc ol jointing rrnrl jointingpatterns in Irish rocks docs not secrn tornl'lucncc the prlc bcaring capacity. Load*'ils ussurricd to be transf-errcd along thc lirlllock socket lcngth in all cascs, althoughcxpcricncc sirlgcsts that in rnany cascs loadis only bcing tnrnslcrrcd llong part oi thesockct. Thus thc valucs of'I, dctcrntined rnaybe conscrvatr \,c,

The l. r'alues are in thc rangc ol 120 kN/m2to 3375 kN/m2 ancl are above 1000 kN/mr infiritny cases. An averagc value ol'about I 335kN/rn2 was obtained. Thesc valucs arcItighcr than thosc which would bc normallyemployed in dcsign.

There are sonrc anornalously Iow rcsults, lbrcxample the Midleton tcst (Pilc 71. T'hcreason fbr this is that this sitc is in a karstarea. Somc voids wcrc encountercd whilcdrilling thc pile irnd ii pcrmancnt lincr had robc placcd ovcr thc majoritv ol'thc pilc shaliin ordcr to prevcnt grout loss. It irlso sccmsthat in cascs r'"'hcrc high cnd bcaring wasntobiliscd, thc r,alues arc rclativcly lo',v. lirrcxamplc Pilcs 5, I Ir. I lb, l3d and l3c.This point ncccls lirrthcr rcscarclr but ntaypossibll,' due to u rccluction in skin l'rictionat largc clisplaccntcnt cluc to l brcakclown inthe concrclc / rock contacts rvith incrcasingst ral n,

Comparison with Wrllilms and Pells ( 198 I )ct valucs dctcrrnincd liorr the analysrs abovchavc bccn plotte'd against qf on Fi-qurc 12.Willrants ancl Pclls'(l98l) pnrposed designlinc has also bccn supennrposed on thcligurc. Althouch thc pattern ol- clccrcusingadhcsion l'actor with tncrcasing unconlrncdc0rnprcssiVc strL.nqth is coltsistcnt r','ithllndin-{s clscwhcrc. it can bc sccn that thc o-vllucs irll llll bclori'thc Willianrs lnd PcllsJinc, suggcsting thlt dcsicn clrricd out usingtlris rncthocl could bc unsal'c. A tcntiltivctrcnrl linc lirr Irish rocks, rvith q clccrcusinsli'orn 0. I at q. equal to l0MPa ro 0.01 at q.cqurl to 100 MPa is also shown on thcllgure .

As an cxirrnplc, lilr thc N'lount Kcnnett tr-ialpilc in Limcrick (Pilc 3a), thc Williams andPclls rrcthod would givc an l* r,aluc of 3000kPa and thc proposed dcsign linc rvouldgivc a valuc o1'1750 kPa. Thc pilc hacl an(CEN'ISET) cstinrated l', r,aluc of 1920 kPaand thc valuc uscd in thc original designwas 7-50 kPa.

Comparison with Kulhawv and Phoon( | 993)Althou-ch the Williarns and Pclls dcsign linchas bccn uscd extensivcly worldwidc, it wesdcvclopcd lilr thc weak slndstoncs,mudstorres and shales o1'Australia andthcrefbre its weakness has becn longrccogniscd. Rowc and Armitagc ( 1984, Rel'.7) cxtcndcd thc original databasc with 67l'urthcr tcsts obtaincd liom sitcs worldwidc.This databasc \\,ils again cxtcnded byKulhawy and Phoon (1993, Rcf. 8) whoadded 47 morc tests. Both of thesc studiesalso extended the correlation to account fbr

rE

CL(E

oo(E

.9ooE

0.20.180.160.140.12

0.10.080.060.040.02

0

weaK ately weak [tf - {strUnconfined compressive strength q" (MPa)

Figure 12. Alpha (a) values tlerived fnttn Williams and Pells ( I98l)

0.01

0.00110 100

9" I 2P'Figure 13. Alplm (a) values derived from Kulhawy and Phoon (1993)

0.1

(E-co-Gjoo(Etr.9oo-trE

zj

(D'=(Eo

t(,t,o.9-oo

=

'100000

90000

80000

70000

60000

50000

40000

30000

20000

1 0000

0

End bering-^tit:^^l

No end beari g usually mob sed

+

a

! I I

12 16Ratio of rock socket length to pile diameter

Figure 14. End bearing resistance versus rock socket length

Design line suggested byWilliams and Pells (1981)

Kulhawy & Phoon (1993) designline for very rough sockets

Trend for lrish rocks

20

socket roughness and produced design linesto cater for different roughness conditions.

The data determined in this study has beenreplotted on Figure 13 together with thedesign lines proposed by Kulhawy andPhoon (1993). Note that the data ispresented in a slightly different form fromthat used by Williams and Pells (1982). Logscales are used on both axes and theurconfined compressive strength has beennormalised by twice p", atmosphericpressure (equal to 100 kN/m21. Again it canbe seen that all the data points fall below thedesign lines suggesting that the strongerIrish rocks do not conform with findineselsewhere.

End resistanceMost of the tests showed that the appliedload is being supported entirely in skinfriction on the pile shaft / rock interface andno load is being shed to the pile base.However ten of the tests did recordmobilised f6 values, which were in the range4000 kN/mz to 87000 kN/m2 with anaverage of about 27250 kN/m2. It should benoted that the ma-.cimum base resistanceactually available would have been muchgreater than this. This is an area of majoruncertainty and more research work isessential here.

It is interesting to examine the rock socketlength to pile diameter ratio in the pileswhere end bearing was mobilised, seeFigure 14. The pile tests in which skinfriction dominated have relatively large rocksocket lengh to pile diameter ratio, being inthe range of 4 to 10. Those in which endbearing was mobilised have a ratio of 4 orless. This is consistent with the findings ofTomlinson (1995, Ref. 9) who repo(s thatthe ratio must be less than 4 if it is desiredto mobilise base resistance in addition toskin friction.

6. PROOF' OF HIGH SIilNFRICTION AVAILABLE

It has long been felt in the piling industrythat the skin friction values being assumedin the design of rock socketed piles inIreland were very conservative. In order toprove this a series of tests were carried out,by PJ Edwards & Co. Ltd, during whichpiles with very short rock socket lengthswere extracted from the ground. Four testswere carried out, one in granite at RossavealCo. Galway and three in various limestonestrata as follows.

Test at Rossaveal Co,. GalwayThe test arrangement at Rossaveal is shownon Figure 15. The pile was initially loadedunder tension to 1600 kN. Elastic movementof about 10 mm only was recorded and itwas fully recovered on unioading. The pilewas then reloaded to 1600 kN, with asimilar response and then on loading to1800 kN a further 20 mm movementoccurred suggesting that full skin frictionhad been mobilised. This load is equivalentto a skin friction (f,) value of about 3,200kN/m'.

Case history I , on Table 1, shows that a skinfriction of about 2000 kN/rn2 was mobilisedon the Galway compression test pile.Allowing for the conservative assumptionthat the full 1.5 rn long socket had beenmobilised in this case, then these skinfriction values are reasonably compatible.The value of about 3000 kN nr2 is nruchhigher than had previously been allowed forGalway granites (typically about 1000kN/m').

Pile reinforced withT32 rebar md 76

dia. steel tube

50 N/mnz grout

Pile sleeved throughold quay wall (215mm dia.)

Peaty clay

.192.mm1.2 m socket in veryfissured weatheredgranlte

Dia.

Figure 15: Test atangement at RossavealCo, Galwav

Tests in limestoneThree similar tests were carried out in threedifferent limestone strata throughout thecountry as shown on Table 2.

The Wood quay test was on a 150 mm long,104 mm dia. socket, which was constructedusing 50 N/mm2 grout. Load was appliedvia. a 36 mm dia. Dwyidag bar. Maxirnumload was about 900 kN and the mover.nentrecorded was mostly elastic and recoverable.Interpretation of the skin friction iscomplicated by the presence of a nut on theend ofthe Dwyidag bar.

Four tests were carried out at Leixlip. Thepile dimensions and load test arrangententwas sirnilar to Wood Quay.

At Shannon, although the rock was termed"fissured", it had average TCR, RQD andSCR values of 100%, 60% and 80%respectively. Loading was again by a similararTangement.

It can be seen that based on a number oftests on different lirnestone strata throughoutthe country { values mobilised are muchgreater than had been allowed for in designin the past.

Table 2: Pullout tests in limestone.

7. PILE DESIGN

Having established the nature of thebehaviour of the rock socketed piles andhaving established some design paran-reters,it is now possible to present some designmethods for Irish rocks.

7.1 Suggested pragmatic approach

Pile bearine capacity - skin frictionThe calculation procedure outlined insection 5.3 above should be used. t valuescan either be determined from the designline shown on Figure 12 or from the valueslisted on Table 3 below for some selected

8.0 rn

Site Rock tlr:e f, 1kN/nr'z)Wood Quay

Dublin(ddh hamner)

Black"calp"

limestone

-7.000

LeixlipCo. Kildare

(4 tests)(odex)

Lirrrestonewith bands

ofmudstone

3,7505,0005,0005,000

ShannonCo. Clare

(odex)

Fissuredlimestone

7,500

areAs whcrc ,good expcricncc cxists. Thcsc

valucs hlve bccn dctcrminccl 1'rorn thccxpCficncc ()l- thC Various contrilctors ilndconsultlng cnginccrs $ho havc bccninvolvcd in rock sockct pilc dcsicn and inthc intcrprctittion ol pilc loircling tcsts.

It ts Itol 1r,rrsihlc lrt tlris:tlrgc lo !l\r,guiclclinc dcsisn values lirl thc Corklirncstoncs. Cork in atiy cvcnt is notcd lirrCFA typc pilcs lirunclcd in thc in lluvio -glacial gravcls. Sorlc, but n(it ucorlprchensivc illlloultt, ol rock sockctcdpilc expcricncc docs cxist rntl this cllilatcncls to bc sornc* lrat I lrrrehlc lncl sitcspccilic.'fhc Cork lintcstoncs arc l'r-cc;ucntlvhclvilv ri'cathcrcd ut thc surllcc itnd luc lrlsoknorvn to bc karstic. IjLrrtlrer rcsciircn \\'()tKis rccluirctl into this topic.

'fahlc 3: Susgcstcrl l. ticsrr:n valtrcs.

Pile bcaring crrpacitv - cnd hculinilIn lirlcstoncs, rt is suggestcd that itrclativcly conscrvlltivc cncl bcuring valuc(lr,) ol' 7,.500 to 20.000 kN/mr bc Lrsccl inclcsrgn to lllou, lirr possiblc unccrturntics inthc qualrty ol lhc pilc basc itncl llso tocn\utc thirt lltc loud \cttlL'nt|'nt trsn0lt\c i\controllccl to lrcccptablc lcvcls. In grlnitcs.sinrillr to thosc cltcountcrccl rn Gllri,ay. 11,valucs up to 50.000 kNirnr coultl bc saltll'iidoptcd. It is likcly that all ol'thesc valuescoLrltl hc incrc;rsccl il'rlcccnt prclirninrrry pilctcst clirta rvas lvailablc ltrr l particular sitc.Factor ol'sal'ctliAn ovcrall l'actor ol' sulety oi' 2.5 should bcapplicd und whcrc possiblc pilcs should hcdcsi-gned such that all ol-thc u'orkrng loacl istakcn rn skin ll'iction and thc pilc busc actsrncrcly to provicic a lactor ol' sllcty luainstlailurc.

Cmut / crThc net resull o1'this study ntay bc that thcgovcrning critcfia in pilc dcsign may bc thcrtccd to linrit thc

_grout / concrctc strcss to iurlucceptable lcvcl, providcd o1'coursc thcrc isa rcasonable sclckct into sound rock. Takc

lol c.xlrnpic thc casc ol'l .l-50 rnnt dia, pilcin Dublin uitl.r u sockct lcngth ol' I.5 m (-1tinrcs thc pilc cliirnrctcr'). Llsrng rn l. r'aluc ol-2000 kN/nrr rreans that thc Q,,11 r'aluc (shalil'rictron alonc) *'ill hc irbout 1..100 kN.Il()\\c\CI. lh\' qIrril( :ttclS \'r'it('r'ir)lt I)tL'ilt)\thlt thc Q,,,, rlluc ntust hc lirnitcd to 1000KN,

Lortd scttlcnrcnt lcsponscl-hcrc arc lnrlnv tcchniclres uvailublc lirrprcclicting pile hcud loacl sL'ttlcnlcntrcspolrsc. Il thc pilc bchaviour is cxpcctccl tobc nrostly cllstic, thc clcsign churtsdcvclopccl bv Poulos und Daris ( 1990. Rcl.l0) can prolidc usclill antl accuriitc rcsults.Irt othcr-cases thc "CE\,lSET" approlchoutlinc(l in Scctirrn 5.J ahovc is consiclcrcclto hc ln cxccllcnt tool lirr prcrlictiorr ol' ptlcloutl scttlcrlcnt hchlr iour.

7.2 "ROCKI'l'l'97" nrethod

[] irckgroundDcspitc thc ability' ol' thc sinrplc nroclcl tosLrcccsslullv prcdict pilc hchaviour ancl itsvulicialion by mlny loltcl tcsts. l clclLrbt cxistsahout tlrc lirntlarncntal mccherrisnr ol' loadtntrtslcr into thc rock. Pilr'/ rock intcrluccsilrc usurtlli, llr lhrnr bcing slllootlt.Inspcction ol' tltc rntcrllicc in thc bccldcdDuhlin linrcstoncs. lirl cxirnrplc. hls shorl'nit to lirlnr u scries ol'horizontal shclves, ri iththc stfor)qcl Iirrrcstortc hlinds protrLrclingovt'r' thc rruclstor.rc buncls into lhc pilc bulc.In tltc Crilulv gnrnitcs bv contrast thc borcis oitcn snrooth cluc to thc strcngth lindr.rni lbrnritt' ol thc rock.

Rockct9T rnoclclThis problcrn \\,as rcco-rnrscd by thcAustraliln rcscarchcrs (Johnston andHlbcrllcld, 1991. Rcl. I I ancl Sciclal andHabcrllcld. 1995, Rcl, l2) at an cally stageancl thct'havc dcvclripcrl a rnodcl to ilccountlirr sockct roushncss. It is illustrllcd onFigulc l(r.

Bclirrc thc pilc is lorclcd thc side o1' thc pilci',ill bc in l!ll contact r',ith thc surrounclingrock as illustratccl in Figurc l6a. Whcn thcstnlctunrl lortd is upplicd it is nornral lbr thcpilc to causc thc sockct to dilatc initialll, asthc pile concrctc sliclcs over thc lockaspcrities as illustratcd in Figure | 6b. As thcdilltion procccds thc arca ol'rock in contirctwith thc concfctc dccreascs until thercsistlncc to uspcrity shearing bccorncs lcssthan tl.rc rcsistancc to sliding. lt lbllou,s thatthc initiully dorlinant slrding rncchanismgivcs su1' to a shcaring nrcchanisnr. It hasbecn sl.rown that a lilndamental aspcct ol'

this approach is the rnodclling o1'therntcrl'acc by ar corrstant normal stillncssboundury condition.

Thc lpprorrch outlined abor,c has bccnintcgftltcd into thc colrrputcr codc Rockct97,rihich *as tlcvclopccl at Monush LJnivcrsityin Australil (Scidal. 1995. Rcl'. Il). Thcsliding / shcarinc rncchlirlsms arc rnoclcllcdusing thc draincd shcilr strcngth ol thc intactrock and thc rcsiduirl sliclin-q liiction anslcol' thc rock / c()ncrctc tntcrlacc. Advanccclnroclcls ol'rcugl.rrrcss. bascd on ll'actalgcornctrv. rrc incOIpuratcd into thc codc,scc cxarnplc orr Figure 17. Vcry goodcortrplirisons havc bccn obtitincd bctw'ccnRockct9T prcrlictions ancl rerl licldbchlr iour in ir varictv ol' rocks, llbcitq,cukcr than thosc cricountclcd in llclancl.

Apnlicution ol'Rockct97 to Irish pilc tcstsRockct9T has bccn usccl to prcdict thchchar rour ol'tuo ol'thc pilc tcsts clctailccl onTlblc I (pilcs l3g rvlrcrc thc skin ftictiondortrinltcd und thc bchavior wus clastrc andpilc Ila rvhcrc load \\'as takcn n ilcornbination ol' skin liiction and cnclncanng ).

Thc input paranrctcrs tirr thc two tcsts arcsutnnlrr-iscd orr Tablc -1. (Othcr plrantctcrsarc givcn on 'l'lblc l ) It can hc secn that ligood rrumhcr ol' pltrartrctcrs arc rccluired,nltnV ol u,hich .,r,crc not llvailirblc lirrnr thcsitc invcstigation ancl hucl to bc clctcrminccll'ronr gcncritl corrcllitrons in thc litcraturc.

Tablc 4: Rockct9T input pararlctcrs.

Full Rockct9T output lbr Pilc l3a is givcn inAppcnclix A ol' this papcr. It can bc scenthat, sirnilur to that dcduccd l'rom thcCEMSET analysis. shati liiction initiallydominates thc bchaviour but is lullymobilised at a movcment of about 8.0 nm,bcyond which thc pilc basc provides the

Locatron/ rocK tvpc

-fcR / RQI)(r'( )

I,I k N/rrr )

DublinI i nrcstonc

>80/>10 l.5002.(XX)

Li nrcnckI i nrcstonc

As abovc 1.t00

Sor,rthDublinqranltc

As aboVc I ..s00

Gnlu'ayrlfitn I tc

As lrbovc r.(xx)

P.trlntr'tcr LJnit Tcst l3Lr 'fest l3qShali

nrodLrlLrsN,IPA I 000 2000

0rli,lir Dcc )1