iC SC uB i';~li SreCBS'.concre';e ii esRequirements, design, loading, inspection and loading

by BENGT H. FELLENIUS, Dr.Tech, PEng, MEIC, MASCE+

Background to the reportThe following paper was presented to aninternational conference on the planningand design of tall buildings, held in

August 1972 at Lehigh University, Bethle-hem, Pennsylvania, USA, and sponsoredby the American Society of Civil Engin-eers (ASCE) and the International Asso-ciation for Bridge and Structural Engineer-ing (ASBSE). The author served as State-of-the-Art Reporter on Precast ConcretePiles together with Mr. G. M. Cornfield,whose Report "Piles for tall buildings"has previously been reprinted by thisJournal (July 1973, Vol 6 no 4). Beforesubmitting the Report to this Journal, ithas been updated and slightly abbreviated.

A presentation is given of the highquality precast concrete pile including therequirements of concrete strength, aggre-gate size, reinforcement and splices. Theeffect of driving is briefly discussed withrespect to compression and tension shockwaves and to cracks in the concrete.General requirements of the structuralstrength of the pile are reviewed. Rules areproposed for design with respect to sound-ness and bending and the influence of,negative skin friction. Inspection and test-ing methods are discussed and case his-tories are presented.

1. IntroductionIn a State-of-the-Art report on Piles and

Caissons by Cornfield (1972) the generalconsiderations have been described,which are relevant to the design of highcapacity pile foundations together withcomments on some of the many differentpile types which are in use. This paperpresents a particular pile type, specificallythe precast concrete pile, as this pilediffers in many aspects from other piletypes and furthermore is unknown tomany designers.

The presented information is mainlybased on experience in Sweden, whereprecast piles are commonly used. In 19702.8 million metres (8.7 million ft) of pre-cast piles were driven, which is equal to65 per cent of all piles installed mSweden that year (Swedish Pile Commis-sion, 1971). Precast concrete piles arenaturally used to a large extent as also in

many other countries. For example, in

Holland in 1970 85 per cent of all piles or6 million metres consisted of precastconcrete (Joustra, 1971).

The precast concrete pile is often con-sidered to be a low standard pile notsuitable for use where large loads andhigh reliability are needed as in the appli-cation for high buildings. On the otherhand, the following shows that precastpiles can well be used for these purposes,provided a sufficiently good quality ofboth pile and pile installation is employed.This presentation is divided into six sec-tions.

'Consultant, Terratech Ltd., Montreal, Quebec,Canada.

28

General descriptionQuality requirementsInstallation procedureDesign and loading requirementsInspection and testing methodsCase histories.

2. General descriptionDuring transportation and handling a

precast pile is submitted to bendingmoments and has to be reinforced accord-ingly. The reinforcement is often also de-pendent on the intended axial loading ofthe pile (see Section 5) with a minimumreinforcement area stipulated (normally1.5 to 2.5 per cent). The largest reinforce-ment stresses however, often occur dur-ing driving due to obstructions in theground, sloping rock surface, eccentric(accidental) driving, and so on. Therefore,regardless of pile length and intendedworking load the piles should be rein-forced so that the bending resistance ofthe pile section is larger than a certainminimum value. For pile sections of nor-mal areas, 500 to 1 000cm"-, the minimumbending strength (ultimate value of failurein bending) should not be less than 5 to10 ton-m, respectively (see Section 5).

The reinforcement consists of longi-tudinal main reinforcement and spiral re-inforcement (stirrups). The longitudinalreinforcement resists the bending stresses.The spiral reinforcement not only preventsbuckling of the longitudinal reinforcementand resists the torsional and shearstresses, but it also decreases the widthand spacing of the cracks in the concrete.

The shape of the cross section of thevarious pile types can vary: square, cir-cular, hexagonal, etc. Square piles arenormally reinforced with four longitudinalbars and the circular or hexagonal withsix bars. The reinforcement can be ordi-nary or pretensional. In the latter smalldiameter wires are sometimes used in-stead of bars.

When precast piles become longer than15-20m, practical problems occur, whichtend to limit their use. Handling stresses

increase considerably with increasinglength, transportation from factory towork site becomes difficult and expen-sive, large and heavy driving rigs arerequired, and so on. However, for severalyears special precast pile systems havebeen in use, where long piles are drivenin segments which are spliced in the field.With these splices the economic lengthof pile segments is 10 to 13 m consider-ing handling stresses, transportation, sizeof driving rigs and cost of splices.

There are many splicing systems on themarket, but most of these cannot meetthe strict demands which must be appliedto high capacity precast piles. The splicein a pile must be equal in strength to theunspliced pile section in bending, tensionand compression. A high quality spliceconsisting of a male and female part,which are cast with the pile, is shown in

fig. 1.For details about various precast pile

systems, see Cederwall (1962), Severins-son (1965), Blomdahl (1968), Moiler(1968), Gerwick (1968 and 1972), Sund-berg (1968) and Fuller (1970).

3. Quality requirementsThe use of precast piles is generally a

question of quality. Unlike most other pre-fabricated precast concrete elements, pre-cast piles are subjected to severe dynamicforces (during the drivtng). The dynamicforces can cause cracks in the pile andspalling of the concrete cover. To ensurea sound pile after driving all parts of apile must be of a high quality and functionwell together. Thus, the concrete strengthshould on no occasions be less than 500kg/cms (7 200 Ib/in'), and preferablyhigher. However, even more importantthan the compressive strength are the ten-sile and fatigue strengths of the concrete.To achieve a good overall strength, theaggregate size should not exceed 15 to

Fig. 1. Male and female steel coupling forsplicing of precast concrete pile, Herkulessystem

la lithe= ="= =

25mm () in to 1 in) depending on pilediameter. In addition, the aggregate par-ticles should not be rounded, but consistof crushed stone from hard rock material.

The pile is submitted to large bendingand tensile stresses during installation.lf the yield stress of the reinforcement isexceeded there will be permanent defor-mations that may cause the pile to bendor to fail during the continued driving.Therefore, the reinforcement must con-sist of deformed bars of high yieldstrength. In Sweden the stipulated mini-mum yield strength of the steel is40 kg/mm'57 000 Ib/in'). In high qualitypiling in Great Britain and Canada, andoccasionally for special cases in Sweden,the manufacturers use a reinforcing steelwith a yield strength of 60

kg/mm'85000lb/in'), a steel which has a moreaccentuated deformation of the bars,apart from the higher yield value, ensuringa better integration with the concrete. Thespiral reinforcement should consist ofsmooth, slender (about 5 mm) bars witha pitch not exceeding 100 mm.

To prevent spalling of the concrete, theconcrete cover should be small. The pri-mary purpose of the cover is to ensure agood interaction between the main rein-forcement and the concrete and to pre-vent corrosion of the reinforcement. Con-sidering the high quality of the concretewhich is being used, these requirementsare met with a cover thickness equal tothe diameter of the largest stone aggre-

gate, which as mentioned is small, i.e.25 mm or less. The National Building Codeof Canada (1970) requires normally a38mm to 50mm (1.5 in to 2.0in) con-crete cover, but for a concrete strengthof minimum 550 kg/cm'7500 lb/in-") thecover requirement is 25 mm (1.0in).

For a square pile the corners at thehead of the pile may spall due to the un-even stress distribution during drivingwhich occurs over a square cross section.Care must be taken to prevent this whendriving as this type of pile will also besubmitted to larger torsional stresses duringdriving, as compared to round or roundedpiles, and these may be harmful to thepile when bending and compression aresuperimposed.

Cracks can often develop in a drivenconcrete pile. However, for a high qualitypile they are normally very small. Theycan be observed in the part of the pilewhich is above the ground after a rainyday as white lines against the dark andmoist pile surface. These hair-line crackshave a width of about 0.1 mm to 0.2 mm(about 0.01 in) and less and are of no con-cern. With careless driving or an inferiorquality of concrete and steel, considerablylarger cracks may develop, which may re-duce the effect of the driving by dampingthe shock wave. Finally, a false refusalmay develop, provided the pile does notbreak beforehand. Particularly when pilesare driven in water a good quality mustbe ensured or disastrous spalling may

Installation of precast hexagonal Herkules piles by a railway embankment in Scotland

occur as cracks open under the effect oftension waves and water is then suckedinto them. At the next blow on the pilethat water is ejected from the crack withgreat force so spalling the concrete. Forfurther discussion on cracks in concretepiles see Hellers and Sahlin (1971).

Splices in precast piles are made up oftwo steel couplings, connected to the pilesegments by reinforcement bars. The splic-ing is performed by locking the jointcoupling by various means. Typical ex-amples are, a bayonet system (Herkulessystem. See fig. 1, and Cederwall, 1962;Severinsson, 1965) or locking pins (ABBsystem, Moiler, 1968). The splices shouldbe equal in strength to the pile. However,this must be the situation even duringand after driving. Splices made up ofjoints that involve holes in the concretefor bolting the joint, welding of reinforce-ment bars and of plates, or whatever, willweaken during driving or even break.

The various splicing systems used inSweden are subject to approval by theSwedish Building Authorities. To be ac-cepted a spliced part of a pile has to betested in tension and bending accordingto a standard procedure. The test speci-men has to be taken from a pile, whichhas been driven in the field under specifiedand controlled conditions. Naturally, thetest specimen should not be madeespecially for the test, but be taken atrandom from the regular line of produc-tion.

As precast piles are normally used ashigh capacity end-bearing piles, the endof the pile has to be protected duringdriving. The minimum protection of thepile tip is a flat shoe of steel. (Frictionpiles driven in soils with small probabilityof encountering boulders do not neces-sarily require shoes, but the shape of thepile point may be important.) Boulderscan often be encountered in the soil andthe bearing layer can consist of dense tillor bedrock. In these situations the pilesare provided with a special rock-shoewhich has a point of hardened steel(rock-tip). Investigations have shown thatthe rock-shoe must be of a special designto ensure the desired effect or the rock-tip may split the pile end (Fellenius,1963). Also the rock-tip must be of aspecial alloy and specially hardened orthe rock-tip itself may deform or crack

Fig. 2. Pile end provided with a rock-shoewith a tip of hardened special alloy steel

/t

I uu

it 34.RS4

Ground Engineering 29

(Rehnman, 1968 and 1970). Provided therock-shoe and rock-tip are of a correct de-sign, the pile end can penetrate into evena hard sloping rock surface. Fig. 2 (Gran-holm, 1967) is a photograph of a pile-endwhich has been excavated after drivingthrough 8 m of clay to sloping rock sur-face and where the rock-tip had pene-trated about 50mm (2 in) into the rock.

4. InstallationPrecast concrete piles are normally in-

stalled as driven displacement piles. InSweden the driving is most usually per-formed by drop hammers weighing 3 to5 tons. To provide a suitable shock wavein the pile and to protect the pile, the pilehead is equipped with a driving helmet tocushion the impact. Normally, a packingof soft wood is placed between the ham-mer and the pile head. The driving rig ismounted on a crawler excavator to per-mit flexible pile locations and alignmentsof the pile.

To prevent damage to the pile thetravel of the hammer must be parallelwith the pile and the impact be concen-tric. The impact creates a shock wave thatpropaqates down the pile and reflects atthe pile end. The reflected shock wavewill be that of a compression wave if thepile enrI stands in firm soil and a tensionwave if it stands in soft soil. Thus near apile end, standing for instance on rock.the stress in the pile can be doubled. Con-versely if the pile end is in soft clay, atension wave is reflected and that canbe almost as large as the initial compres-sion wave.

The largest stress on the pile. which isproportional to the amplitude of the shockwave, is dependent on the height of fallof the hammer. An approximate relation-ship is given by 0 ——30Vh„where ~ isthe stress (kg/cm'-') at the head of thepile and h„ is the effective height of fall(cm) of the hammer (Hellman, 1967). Con-sequently, the stress is approximately in-dependent of the weight of the hammer,which instead governs the length of theshock wave. The length is approximately3Q q/L, where Q is the weight of thehammer and q/L is the weight of the pileper metre (Hellman, 1967). Consequently,two blows of the same energy in terms ofheight of fall times the weight of the ham-mer, where hammers of different weightsare being used, will not have the sameeffect on the pile driving. This is one ofmany reasons why conventional pile driv-ing formulae are not reliable.

In practical pile driving it is essentialwhen the pile end is driven through softsoil that the height of fall is kept low inorder to avoid excessive tension in thepile. The height of fall should also be keptlow when the pile end meets dense till orhard bedrock to prevent overstressing ofthe pile end. A recommended length offall for these applications is 30 to 40cm.A normal length of fall is 50 to 60cm. Ifthe pile cannot be driven down to bear-ing soil layers due to dampening of theshock wave by frictional resistance alongthe pile shaft, the height of fall should notbe increased but the shock wavelengthened by using a heavier hammer.

Often diesel hammers are used in placeof drop hammers as they provide a con-siderably faster driving, particularly in asandy soil. The energy delivered to thepile by a diesel hammer, due to the abovereasons, must be adjusted to match thesoil conditions at the pile end. Particu-larly if the pile end meets a boulder or

bedrock, care should be taken to ensurethat the energy is lowered immediately.Otherwise the reflected stress wave willcause an increase of the energy deliveredby the hammer at the next blow, an ac-celerating process which can damage thepile. (See for instance Davisson andMcDonald, 1969.)

The conclusion that ordinary dynamicpile driving formulae are unreliable inpredicting the bearing capacity of pileshas been shown by many investigations.Yet most building codes recommend adynamic formula in lack of another suit-able method. Formulae based on the waveequation are as a rule more reliable, butrequire both special field measurementsand computerised calculations, and thisprevents their use in practical foundationwork. However, with simplified assump-tions generally valid criteria can be estab-lished. Broms and Hellman (1970) pre-sented a discussion on shock waves inprecast piles and the assumptions behindthe criterion which is included in theSwedish building code for allowable loadson piles and which is based upon thewave theory.

The piling system of today is in allits essentials the same as that whichCaesar used when drivinq piles for atemporary bridge across the Rhine. Ofcourse, power engines have been de-veloped and elaborate driving rigs arebuilt but the installation still involveshuman judgement to a large degree.Lately, development is being made toautomate the operation of the drop ham-mer (Helmfrid, 1971). Work is also underway to eliminate the uncertainties whichlie in the conventional driving helmets.Goble et al (1970), (1971) and (1972)presented an interesting and promisingmethod of predicting pile bearing capa-city by means of dynamic measurements.

5. Design and loading5.1 Soil mechanical approach

The bearing capacity of shaft bearingpiles is limited by the strength of the soil,whereas for end-bearing piles the capa-city is normally limited by the structuralstrength of the pile. General methods forestimating the bearing capacity of pilescan be studied in text books, for instanceTerzaghi and Peck (1967), Chellis (1961)and Teng (1962). Recent findings for pilesin clay are given by de Mello (1969) andBurland (1973). A summary of calcula-tions for pile bearing capacity is presentedby Broms (1966). Also a practical designapproach is summarised by Clark et al(1966). Since this report is limited, furtherreference will not be made to the designof piles from a soil mechanics'oint ofview.

5.2 Structural strength and buildingcode

In almost all building codes the struc-tural strength of piles limiting the allow-able load on single piles is expressed bya formula of the following composition

P = f,A,O, + f,A,(y„where f,, and f, stand for certain constantlimiting factors, A„and A, stand for areaand <„and 0,. stand for allowable com-pressive stress, of concrete and steel re-spectively. In some codes it is mentionedthat the particular formula means that thepile is designed as a short column. Forinstance in the Swedish Building Code(1967), the length to diameter ratio, L/d,is equal to 20. In other words, the struc-tural strength is calculated by a buckling

formula. Yet, though attempts have beenmade to buckle straight standard piles, nobuckling has ever been observed. Buck-ling caused by axial loads may still be anissue in extremely soft or highly organicsoil and in those cases where some partis standing free in water or air. However,it is not logical that these rare instancesshould be deciding all other cases.Even a very soft soil will give enoughlateral support to a pile. For the same rea-son the pile, however strong, will not beable to resist a lateral soil movement asillustrated in fig. 4, after Fellenius (1972a).

It is also very common to limit theallowable load to a maximum total com-pression stress in the pile. Such maximumstresses are normally kept low, i.e. 50 to100 kg/cm'700 to 1 400 lb/in'-'). For thehigh pile quality this paper is referring to,the range is 100 to 200 kg/cm'-, dependingon circumstances. However, the conceptof a maximum total stress in the pile isnot a correct design approach.

Experience has shown that for drivenprecast concrete piles it is the bendingstrength which governs the safe pile in-stallation. It is therefore recommendedthat the allowable load be calculated quitesimply by P = f„,M, where f„, is a certainfactor and M is the bending strengthof the pile. It is of course vital that thetrue strength is being used. The strengththerefore must be evaluated from labora-tory tests on test specimens cut fromdriven piles. Furthermore, these pilesshould be chosen at random from thenormal line of production. Practical valuesof the coefficient f could be 10-15 limit-ing the allowable loading on piles to 50-150 tons depending on the actual struc-tural strength of the piles when they aredesigned for a minimum ultimate bendingstrength of 0.01 ton-m/cm'-'ile cross sec-tional area.

The allowable load on precast concretepiles differs between different countries.In Norway, Great Britain and Canada loadsof 120 tons on hexagonal 800 cm-'ilesare common, while in Finland only a thirdof this value is used on square 625

cm'-'iles.

However, there is a large differencein quality between those two pile types.

In the Swedish Building Code it isrecognised that it is economical to havepiles and piling of different quality. Thusthree piling classes (A, B and C) are de-fined. Class C consists of square 550cm'-'r

730 cm'-'iles with a concrete strengthof only 400 kg/cm'nd an allowable loadof 33 and 45 tons respectively. The re-quirements for investigation of the pilinginstallation are very mild. Class B pileshave the same cross sectional area asclass C piles, but the concrete strengthrequirement is 500 kg/cm'. The allowableloads are 45 and 60 tons. The require-ments of inspection and execution of thepiling is less mild as compared to theclass C. The splices must be of the qualitydescribed in Section 3. The quality ofclass A has to be at least equal to thatof class B. The allowable load is notlimited. This pile class is used when pileloads exceeding 60 tons are required andat sites where the piling conditions aredifficult. As a consequence, the pilingwork must be carefully inspected. Piledriving tests and load tests are oftenperformed.

The allowable loads are also influencedby deviations of the piles from the in-tended locations. For example, theSwedish Building Code (1967) states that

the pile loads should be recalculated afterthe pile driving is completed with respectto the actual locations and lengths of thepiles. The piling is accepted withoutfurther considerations if the recalculatedloads do not exceed the original designload by more than 15 per cent.

5.3 Main pointsEnd-bearing piles are almost always usedfor tall buildings du to the large loadswhich are required. Thus, the structuralstrength of the pile is the limiting factorand the allowable load can be designedby the formula, P=f M, which was re-commended above. However, three mainpoints should be considered and ensuredin the design:

a. That the piles will be sound andnot damaged during the driving.

b. That the pile will be driven straight.c. That the risk of negative skin fric-

tion is checked.The requirement in a means that the

quality of the pile and of the installationwork must be high as outlined in Section3. and 4. Much emphasis should be placedon the inspection as will be discussed inSection 6. A primary requisite, however, isthat a geotechnical field investigation hasbeen devised not only for deciding onthe pile foundation but also for the drivingprocedure of the piles, which has to beadjusted to the soil conditions. The soilsinvestigation must provide detailed in-formation on the depth to and the natureof the bearing layers as well as on thetype of the overlying soil, presence ofboulders, and other considerations.

If the requirement of b is not met,buckling may be an issue. Test resultspublished by Hanna (1968) indicated thatseverely bent H-piles buckled during aload test. The piles had a minimum bend-ing radius of about 60m (180 ft). On theother hand in an investigation by theSwedish Pile Commission (1964) a bent60 m long pile in soft clay was load testedover a period of 15 months. The pile hada minimum bending radius of 177 m cal-culated over 10 m length. During 12months of constant load of 125 tonsequal to 210 kg/cm'3000 Ib/in"-) was ap-plied to the pile head. Inclinometer mea-surements indicated that no additionalbending occurred during the test.

A bent pile will, even if it does notnecessarily buckle under load, be moredifficult to drive to a high end-bearingresistance than a straight pile. Thereforethe bent pile may obtain larger deforma-tion under load than a straight pile. Itshould be evident that it is not the out-of-verticality that is important, but thebending in the soil. However, when is apile bent and when can it be consideredstraight? This question cannot be ans-wered generally as the allowable bendingis dependent on many factors such asthe type of pile, the pile length, theworking load, the type of soil, etc. Tosome extent it is also dependent on thefactors which govern the actual bending;the squareness of the splices, the qualityof the installation work, the slope of bed-rock, the pile spacing, the presence ofboulders, and so on.

In Swedish practice, when judging apile, the bending of the pile segments istreated separately from the bending of thepile over the splices, Fellenius (1971a).For example, for long piles driven in softclay in south-western Sweden the allow-able minimum bending radius of the pilesegment is 300 to 400m. The latter value

32

Fill /// i

Clay J J

neg

''i

neg — u

1

i

i'I

\

i a=3to5I

II

II

A

Rock

i o a o a c ~penmeler3 0 0 OOC) a a a ac

corresponds to the minimum bendingradius for steel piles allowed by Nor-wegian authorities as reported by Bjerrum(1957).

Point c deals with the very complexproblem of negative skin friction. For in-stance, both the Swedish Building Code(1967) and the National Building Code ofCanada (1970) state that negative skinfriction should be considered. However,it is not mentioned just how the negativeskin friction should be considered. Recentinvestigations have shown that large dragloads due to negative skin friction willdevelop on piles in settling soil layers(Johannessen and Bjerrum, 1965, Bjerrumet al, 1969, Bozuzuk and Labrecque, 1968and Bozuzuk, 1970). Also, negative skinfriction can be caused by the reconsolida-tion of clay around a driven pile (Felleniusand Broms, 1969d). However, this dragload is normally small and of little con-cern as it is eliminated when the workingload is applied on the pile ( Fellenius,1971b and 1972b).

Methods of calculating drag loads onpiles have been proposed by severalauthors. Zeevaert (1959) employed a theo-retical approach using the reduction ofeffective vertical pressure in the soilcaused by the drag on the piles. Bjerrumet al. (1969) assumed that the negative Iskin friction is equal to about 0.3 times

Ithe effective vertical pressure in the soil(as found from practical full scale investiga- iitions. Bozuzuk (1971) assumed that the

neaative skin friction corresponds to thehorizontal effective pressure against thepile and a friction factor for the soil actingon the pile. The uncertainty is greater,when the drag load on piles in groups isestimated. Here the method of Terzaghiand Peck (1967) can be used or the ap-proach in fiq. 3 after Broms (1971).

Until additional field measurements areavailable, a conservative method must beused to calculate the drag load. A safeassumption for single piles is to assume

I'hatthe maximum draq load is equal to t

the surface area of the pile times the:jshear strength of the settling soil layer

as determined in a geotechnical investiga-'ion.

This drag load should in the designnaturally be added to the other loads onthe pile. However, it must be rememberedthat loads from negative skin friction aredifferent in nature from ordinary workingloads. This fact is too often neglected in a

design for negative skin friction.When having found that drag loads of

a certain magnitude will or may be impos-ed on a pile, the designer should not addthis load directly to the working load.Nor, as is normally the case, subtract thedrag load from the usually applied allow-able load to obtain a new design load.This design method will naturally giveallowable loads, which are safe from anegative skin friction point of view. How-ever, it will also ensure a maximum costfor the owner for his foundation.

To decide what effect the drag loadmay have a detailed study is needed,where all forces on the piles are con-sidered, each with an appropriate safetyfactor. That is, in design for negativeskin friction one simple overall safety fac-tor cannot be used. Instead, the designmust be carried out using partial factorsof safety. A design approach and simpledesign formulae have been presented byFellenius (1971b) and (1972b).

When considering negative skin frictionin a design, there will be instances whenthe drag load is shown to be too largeto be accepted. Then the negative skinfriction will have to be reduced. Fieldinvestigations have shown that this isachieved efficiently by coating the pilewith a thin (1 to 2 mm) layer of bitumen(Bjerrum et at., 1969 and Walker andDarvall, 1973) .

The bitumen can be chosen among awide range of qualities. The importantfactor is really to ensure that the coat isintact after the installation. Protectionagainst negative skin friction by means ofa bitumen coat is therefore mainly apractical problem. However, the bitumenreduces the positive skin friction asefficiently and it can involve larqe extracost. It is therefore recommended that itnot be used unless proved to be necessaryby a detailed study.

5A Special problemsThere are occasions when problems

arise requiring special considerations. Forinstance, driving in clay may involve adisplacement of the clay toward existingbuildinqs. This effect can be reduced bypredrillinq and commencing the drivingnear the buildinq in danqer and proceedingaway from it. Driving piles in loose sandmay cause large settlements belowneiahbouring buildings.

The excess pore pressure caused by

(a)Case I (b) Case 11 (c) Case 71IFig. 3. Calculation of negative skin friction. Case I, widely spaced piles to be treated asindividual piles. Case II, closely spaced piles and large surcharge on the ground surfacemobilising full friction. Case III, closely spaced piles with moderate surcharge. (AfterBroms, 1971)

W I LDIHG SURCHARGE

Fig. 4. Consequence of surcharge on softsoil outside a building

6. Inspection and testingPile installation work must be inspected.

Inspection infers both the examinationperformed by the inspection officer andthat which is carried out by the con-tractor to ensure that his product meetshis own quality control requirements.However, examination is not only per-formed as a check that certain require-ments are met. Designing involves judg-ments and assumptions, particularly onbearing capacity and on settlements, whichhave to be verified. Inspection so definedis in fact the final most important mea-sure to ensure the safety of the pilingwork.

The inspection of a pile installation canbe divided into inspection a prior, bduring and c after installation of the piles.

For precast piles the inspection prior

driving piles in clay generally dissipateswithin days or weeks. However, in theinterim, they can create stability prob-lems when driven in sloping terrain. Holtzand Boman (1974) described a simplesolution to this problem, where plasticdrain strips were attached to the piles toreduce the pore pressures and to acceler-ate their dissipation.

The driving of displacement piles alsoaffects the shear strength of the soil.Field measurements indicate however thatthe shear strength will decrease only verylocally around the piles and that theaverage shear strength is not affectedappreciably, even if the piles are driveninto a highly sensitive clay (Orrje andBroms, 1967, Torstensson 1973 and Hans-bo et al., 1973). Naturally, the pile spac-ing must not be too close. For this reasonand many others, the minimum spacingcentre to centre is 3 to 4 pile diameters.

Horizontal movements of the soil to-ward the piles must also be considered.Fill placed around a pile supported buildingas illustrated in fig. 4 is such an instance.Discussions and examples of the lattereffect have been presented by Stermacet al. (1968) and Tschebotarioff (1970) .

Corrosion on concrete and on steel isanother point of consideration. Corrosionon the concrete of the quality describedis normally not an issue but that the re-irrfercement and steel couplings is quiteoften discussed. The views on this ques-tion are based mainly on experience fromburied horizontal structures. However,this experience can rarely be directlyapplied on piles. On the contrary, corro-sion on driven piles in natural soils, es-pecially below the ground water level isnormally negligible. See for instance Ro-manoff (1962) and Wynne-Edwards(1968).

to driving consists of ensuring that thesteel and concrete quality is that whichhas been specified and of checking thatthe pile segments have not been damagedduring handling and transport. Also thesteel couplings are checked to make surethat they are cast square with the pile.Here the Swedish Building Code (1967)allows the very liberal out-of-squareness of1:150per coupling.

The impact energy delivered by thehammer during driving is checked. Thedistance of fall of a drop hammer and thepressure in a diesel hammer (as men-tioned in Section 4.) should be keptwithin certain limits depending on the soilconditions. Observations that the hammeracts parallel with the pile and hits con-centrically on the pile head are made con-tinuously. Also checked is that the pilehead is free to twist and move laterally inthe pile driving helmet as prevention ofthese movements can damage the pile.

The inspection after the driving consistsof checking the driving at termination toan established refusal criterion, when alsorebound and set measurements are oftentaken. On some occasions retapping iscarried out. Finally, after completed driv-ing the pile head of each pile is levelledand its horizontal location is determined.In the special cases when the piles areprovided with centre pipes, the piles areplumbed to check that they are sound.A following measure, inspection of pilebending by means of inclinometer mea-surements in the centre pipes, is normallyused only for test piles driven prior tothe driving of the contract piles or for aselected number, e.g. every 10th, of thecontract piles (Fellenius, 1971a).

Testing is a careful inspection of select-ed piles where special measurements areperformed involving more than the ordin-ary passive observations. For example,retapping is an excellent inspectionmethod, which often can reduce the vol-ume of test piles. However, if during theretapping the piles are equipped withspecial gauges measuring the impactenergy and shock wave acceleration inthe pile, this is testing.

By testing most designers refer speci-fically to test piles which are driven priorto the driving on the contract piles and/or load-test piles. There are many differenttesting methods. Naturally, different meth-ods must exist, as the reasons for carry-ing out a test differ. However, the num-ber could be limited and in particular themethod of reporting the results should beuniform. One test is seldom judged alonebut is compared at least indirectly withother test results, depending on the ex-perience of the engineer. Lack of unifor-mity often makes this comparison diffi-cult. In recommendations for pile testingproposed by the Swedish Pile Commis-sion (1970) the value of uniformity hasbeen recognised and the report from apile test is recommended to be drawnup according to a standard procedure ofwhich examples are given. In addition, thedata which should be collected from apile test are listed. For example, fromthe energy delivered to the pile in termsof foot-pounds, but information shouldalso be provided on the height of fall ofa drop hammer or the impact velocity ofa diesel hammer.

Load tests of piles are often used forchecking that the test pile has at leasta minimum bearing capacity. The interna-tionally most common method is the main-tained-load method (M.L.-method) recom-

mended by ASTM (1969). In USA andCanada the test is carried up to 2.0 timesthe design load, in Great Britain only to1.5 times the design load. By the NationalBuilding Code of Canada (1970) a pileis approved of if at a test:

1. the gross settlement under full testload is not more than 1.50 in (38mm)

2. the gross settlement under workingload is not more than 0.75 in (19mm)

3. the net settlement after final re-bound is not more than 0.75 in(19mm)

These requirements for the gross set-tlement do not consider the influence ofthe elastic deformations of the pile, whichin the case of a long pile can be sub-stantial. Also, the net settlement can beinfluenced by negative skin resistance,preventing the pile from rebounding fully.In fact, this test method and interpreta-tion approach give a poor indication of thetrue bearing capacity of a pile. On theother hand, the Canadian rules for ap-proval of the pile are liberal and veryspecific. The Swedish Building Code(1967), for instance, merely states thata pile test should be brought to failureor at least to three times the design load,omitting all specifications on load-testmethod and interpretation of the testresults.

If a routine load testing of piles isdesigned just to verify that the pile bear-ing capacity meets certain simple require-ments, it can be carried out by severalloading methods. In Sweden the constant-rate-of-penetration method (C.R.P.-method) is used for this purpose, accord-ing to a standard procedure recommendedby the Swedish Pile Commission (1970).

The C.R.P, method has the advantageof being simple to perform, fast, economi-cal and providing normally a result thatis easy to interpret. It provides no infor-mation about expected settlements for theactual pile foundation at working load,but neither does any other load-testmethod. Separate settlement analyses haveto be performed by soil mechanical rea-sonings, possibly by the aid of a longterm load test, which must be carriedout for a period of several weeks,preferably months.

When the aim of the load test is toanswer more specific questions concern-ing for instance distribution of skin fric-tion and end resistance, a question whicharises in connection with negative skinfriction, no generally used test is directlyapplicable as it is, at least not for longpiles. To obtain sufficient informationfrom the test, the minimum requirementis to measure the tip movement of thepile, preferably also the deformation ofthe lower portion of the pile.

To measure the deformation of a precastconcrete pile a centre pipe is used forinserting tell tales in the pile. Simplestandard gauges are available, which en-able the measurements to be taken withpractically no delay or extra costs (Bromsand Hellman, 1968). The measured defor-mation in the pile can be transferred toaverage loads in the pile between themeasuring locations by use of the elasticmodulus of the piles. However, the valuesof elastic modulus of the pile materialvaries. For precast piles the range is 300,000 to 400,000 kg/cm'- (4.3 to 5.7 . 10"Ib/in"-) (Fellenius, 1969b). On some occa-sions the appropriate value of the moduluscan be evaluated from the test results,

Ground Engineering 33

on othefs the mo'holus must be determinedin a labortory test. Even in this case, thecalculated loads are average values. If adefinite value is required of the loadat a certain point in the pile, the pilemust be instrumented with gauges thatare placed in the pile before the driving(Fellenius and Haagen, 1969c).

If the tip movement cannot be measuredan approximate estimate of distributionof skin friction and end resistance of apile can be obtained by carrying out acyclic load test and plotting a curve of theload and the net settlement of the pile.(Weele, 1957; Trow, 1967; Hanna, 1971.)

When arranging for and carrying out aload test great care must be taken thatthe accuracy which is used is real. Davi-sson (1970) has published a valuablereview of mistakes which can be made inmeasurements of pile behaviour and givenrecommendations for the proper arrange-ment of pile-load tests.

is

7. Case historiesTwo short case histories are given in

this section as examples of the use ofhigh capacity precast concrete piles. Athird case shows the importance of re-tapping of piles in sand.

Case 1Precast piles were used in 1967 for Cana-dian International Paper Co, plant Gatineau,Quebec. The pile lengths were about30 m (100 ft) and the piles met refusal ina dense glacial till with boulders andcobbles in a matrix of sand. The overlyingsoil consisted of about 20m of stiff clayfollowed by 10m of stÃf to very stiff claywith dense sandy layers. The piles, 30cm(12 in) diameter Herkules Type 800, weredriven with a 3.4 ton drop hammer falling60cm (2ft) to a final penetration of 10mm (3/Bin) for the last 50 blows. Twopiles were load tested"to 225 tons (ASTM-method). The gross settlements were25 mm (0.98 in) and 25 mm (1.01 in) andthe net settlements were 6.5 mm (0.26 in)and 8.0 mm (0.32 in) . respectively. Theload-settlement curves .showed a straightline indicating that the failure load waswell above the maximum test load. Thepiles were approved for a working loadof 112.5 tons (Brodeur, 1971.)

e,st en san

:~~,i i~t

Installation of Herku(es hexagonal precast concrete piles for tank farm extension

LOAD TESTSETTLEMENT OF PILE HEAD FROM 100 TONS LOAD

Case 2In 1970 a large number of piles were

driven for Borgia Urbart. Renewal Develop-ment, Sudbury, Ontario„Canada. The soilconsisted of silt, sand and soft clay ona sloping bed-rock to which the depthvaried from a few feet to 47m (127 ft).The piles, 30cm (12 in) and 21 cm (8.5in) dia hexaqonal Herkules piles TypeH800 and H420, respectively, were drivenby diesel hammers. The piles were pro-vided with a rock-tip of the type showedin fig. 2 to ensure that the pile endwould be set safely in the sloping bed-rock. Five piles were selected for loadtesting. The results are tabulated in Table 1.

10-

15-

20-

200 300LOAD

(tons)

A: STOPPED ABOUT 1m ABOVEREFUSAL LEVEL

B,C: DRIVEN ORDINARtLY

D'HE LAST PILE TOBE DRIVEN RETAPPED

Table 1—Results from tests at Sudbury

GrossPile Length intended settlement Nettype in soil working at 2 work settlement(ft) load (ton) load (in) (in)

30-

35-

incremental tending testCR.R - test

H800 52.0 131 0.59H800 136.7 112 1.03H420 92.4 64 0.54H420 108.2 76 1.02H420 122.0 64 1.43

0.040.110.020.020.42

SETTLlm

The piles were approved for the intendedworking loads (Brodeur; 1971).

Fig. 5. Results from load test on four piles of approximately equal length (28 m ) drivenin the vicinity of each other

Case 3]n 1968 piles were driven to support a

60m (180 ft) high silo building at Koping,Sweden. The soil consisted of some 3 toSm of clay underlain by sand to largedepths. Circular 33 cm pretensional piles(Sundberg, 1968) were driven to a depthof about 28m (90ft) where they met agradual refusal. Four piles were load test-ed. One pile (A) was driven to about 1 mabove final refusal level, two of the testpiles (B and C) were driven according tothe requirements of the contract and thefourth pile (D) was driven to refusal andretapped. Pile B and C were not re-tapped.

The results from load tests on the fourpiles are shown in fig. 5. Pile D behavedas an end-bearing pile with a failure loadwell exceeding 300 tons and piles A, Band C behaved as friction piles with vary-ing end-resistance. Settlement measure-ments taken during the driving of the pilesshowed that considerable settlementsoccurred in the sand layers around andbelow the pile reducing the end-bearingcapacity of previously driven piles in theneighbourhood. The load test on pile Dindicates that this reduction could havebeen eliminated by retapping of the piles.However, in this particular case, the in-tended working load was low, 65 tons,and even pile A, having a failure load ofabout 220 tons, was approved of. (Fel-lenius, 19710.)

8. ClosureDuring the past 20 years the technique

of precast piling has been steadily de-veloping toward stronger piles and largerallowable loads. This has been made pos-sible by a continuous increase in under-standing of the problem involved in thedesign of piles and pile foundation. It isevident that present research and develop-ment in the field and in laboratories invarious parts of the world will continuethe developmont. However, the necessityof high quality of pile material and pileinstallation must not be overlooked. Also,it must be remembered that all phasesof the operations must be carefully in-spected by experienced engineers.

9. AcknowledgementThis Report was originally written whenthe Author was employed by the SwedishGeotechnical Institute, Stockholm, whokindly allowed this reprinting.

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28 Fellenius, B. H., 1972e: "Buckling of piles dueto lateral soil movements", Proc. 5th EuropeanConf, on Soil Mech. and Found. Engng.,Madrid, Vol. 2, pp. 282-284.

29 FaHenius, B. H., 1972b: "Downdrag on pilesin clay due to negative skin friction", Can.Geotech. J., Vol. 9, No. 4, pp. 323-337.

30 Fuller, F. M., 1970: "Methods and equipmentfor the installation of piles in foreign coun-tries", Design and installation of pile founda-tions and cellular structures, Envo Publ. Co.ed H-Y Fang, pp. 109-144.

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35 Goble, G. G., Moses, F. and Rausche, F., 1972:"Soil resistance predictions from pile dy-namics", ASCE J.S.M. Div., Vol. 98, SM 9,pp. 917-937.

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Ground Engineering 37