ultimate bearing capacity of friction piles calculated

Ultimate bearing capacity of friction pilescalculated from load tests on pilot pilesby Dr. Eng.FERNANDO LIZZI, MASCE

IntroductionTHE PROBLEM OF calculating theload-bearing capacity for large diameterfriction piles (known also as "drilledshafts", "caissons", etc.) is of the greatestinterest, considering the widespread use ofsuch piles in modern foundation engin-eering.

The theoretical approaches cannot beconsidered as fully reliable especially in theheterogeneous soils which form the major-ity of cases, whereas direct load tests arevery expensive.

This is why there is a general tendencyto perform restricted load tests on pileswhich will be incorporated in a futurefoundation, without resorting to piles con-structed for testing purposes only. Con-sequently, in the majority of cases, thepiles are not tested to failure because thatcould render them unsuitable for perm-anent use.

Thus the ultimate bearing capacity of thepile remains unknown and the workingload, corresponding to a chosen safety fac-tor, cannot be correctly assessed.

This Paper proposes the use of dataobtained from smaller diameter piles (pilotpiles), tested to failure, for the calculationof the corresponding ultimate capacity forany larger diameter pile of the same mat-erial, same length, cast (or driven) in thesame soil with the same technique.

The Paper refers generally to long piles,whose load-bearing capacity is derivedessentially from skin friction.

Previous approachesIn two previous Papers'2,the author

described a method for obtaining theload/settlement (P/e) diagrams for cast-in-place piles of any diameter from a directload test carried out on an instrumentedsmaller diameter pile (pilot pile).

The calculation is developed through asystem of "congruence" equations in

which the absolute and relative settlementsof the piles with respect to the soil, to-gether with the elastic shortening of thepile shaft, are introduced.

One must remember that for largediameter cast-in-place piles the magnitudeof the pile's settlement in the yvorking loadrange (that is before the failure phase) isgenerally of the same order of magnitudeas the elastic shortening. In this stage thepile can be correctly considered as anelastic body moving in a partially elasticmedium.

From the congruence equations appliedto the pilot pile, it is possible to derive thebasic charts by which the P/e diagram canbe easily calculated for piles of anydiameter and with the same length.

Some specific load tests, carried out in

'Technical Director of Fondedile S.p.A., Via Verdi 35,80133 Naples, Italy

the field on both pilot piles and the cor-responding prototype piles, have demon-strated a reasonably good confirmation ofthe proposed scheme of calculations.

The failure phaseThe failure phase, for friction piles, is

characterised by progressive, very largesettlements under practically constantload; the pile sinks in the soil.

In this stage, the amount of thesettlement greatly exceeds any possibleelastic shortening of the pile and thereforethe pile behaves like a rigid body moving in

a medium which offers a uniform frictionalresistance. These conditions are complet-ely different to those before failure, whenthe values of the friction resistance of thedifferent layers of soil are variable anddepend on the value of the load P appliedto the pile head, and must satisfy a con-gruence relationship which only a moni-tored pile can investigate.

Therefore:—where a full P/e diagram is desired, the

values for the prototype pile can beobtained through the data obtained fromthe instrumented pilot pile using thecongruence equations, as explained in

the earlier Papers '2;—where only the ultimate bearing capacity

is desired, the failure value for theprototype (or prototypes) can be directlycalculated from the value of the ultimateload of the pilot pile tested to failure; thisis the aspect dealt with in the presentPaper.

From the pilot pile to theprototype (failure phase)

Friction piles derive their load-bearingcapacity essentially from the side resis-tance of the soil against the outer surfaceof the pile. In addition, end-bearing (point)resistance is sometimes available.

On the other hand, when the pointresistance is high with respect to skin

friction, the pile behaves like a column(end-bearing pile). This case is outside thescope of the present Paper which dealsessentially with long friction piles.

As stated before, in the failure phase afriction pile behaves like a rigid bodymoving in a medium of uniform resistance;this resistance derives from:—side friction resistance (the greater part);—point resistance, if any.

NOTATION

The following symbols are used in this Paper

d = diameter of the pilot pileD = diameter of the prototypeP = vertical axial load on the pilePle = load vs. settlement diagram

When progressing from the pilot pile to alarge diameter prototype of the same mat-erial and length, it seems logical topresume that the side resistance of thebigger pile is in the ratio of the correspond-ing outer surfaces, i.e. for cylindrical piles,in the ratio of Dld, where D is the diameterof the prototype pile and d is the diameterof the pilot pile.

As far as the point resistance is con-cerned, three hypotheses are possible:(aj It could be nil, because in the presence

of very large settlements the soil doesnot offer substantial resistance to therigid body moving downward.

I'bj It could be in the ratio of thecross-sectional areas of the piles(D2/d2), as in the case of loads actingon plates laid on the surface of a soilbehaving like an elastic medium, beforeany rupture of the soil occurs. This isnot the case in the failure phase of apile because there is actual rupture ofthe soil. Therefore, such an hypothesisis considered to be very exaggerated.

(cj It could be in the ratio of the diameters(D/d) as for the side resistance. Thishypothesis seems to be closer to reality.

In favour of hypothesis (c) are, forinstance, the tests performed by Kerisel 3

which show that when all other conditionsremain constant, the point unit pressure atfailure decreases as the diameter of the pileincreases.

If hypothesis (c) above is consideredacceptable, then the ultimate bearingcapacity of the prototype is obtaineddirectly from the failure load of the pilotpile by multiplying it by the linear ratio Dld.

In the opinion of the author, such a quickcalculation is considered to be reasonablyacceptable, particularly when one con-siders that there is always a large scatter ofvalues of load-bearing capacity for piles ofthe same diameter and length belonging tothe same foundation. Indeed, any pileexpert is familiar with the not insignificantdifferences that can result from load testscarried out on piles presumed to be iden-tical. These differences depend on the soiland on the construction techniques, whichare still far from being fully standardised,as well as on the skill of the site operatives.

It must be stressed that the above quickcalculation relates to the ultimate bearingcapacity only.

If the full Ple diagram is desired, then thepilot pile must be appropriately monitoredand the calculations carried out throughthe congruence equations, since, prior tofailure, the elastic and non-elastic defor-mation of the pile and the soil cannot beneglected.

Field testsIn order to confirm that the ultimate bear-

ing capacities of the prototype and the

November 1983 41

Load, kN

0 500 1 000 1 500 2 000 2 500 3 0000 I I

0 cm

8-EE 9C

E 10-

III 11

d = 25 cm(a(

I1II

\ II III I

II

(1) Fig. 1 illustrates the results of loadtests carried out on some experimentalpiles, within the framework of the abovementioned studies on congruenceequations 2.

The piles, pilot and prototype, were ofthe same length and were of thecast-in-place type, bored using the sametechnology. The soil consisted of a toplayer of fill to the water table (-4.50),followed by a very loose alluvial soil ofvolcanic origin over a peat layer (-11.50to14.50).At the base was a natural volcanicsoil (pozzolana and pumiceous materials).

Load, kN

0 5000 10000 15000 20000 250000 I

10200 cm((1J

Fig. t. Results of load tests on cast-in-placeiles

from Lizzi, 1981)

pilot pile are in the ratio of D/d, some exam-ples are examined below. They are takenfrom direct load tests, actually carried outfor other purposes.

The smaller diameter piles are assumedto be the pilot piles, whereas the larger dia-meter piles are assumed to be theprototypes.

The figure shows:—the P/e diagram for the pilot pile (d =25cm; L = 20.00m), as derived by adirect load test —Curve (8 ).—the P /e diagram for the prototype (D =80cm; L = 20.00m), as derived by adirect load test —Curve (b).—the P /e diagram for the prototypeanalytically calculated from the P/ediagram of the pilot pile, according tothe ratio: Dld = 80/25 = 3.20 —Curve(c).With regard to the ultimate bearing

capacity, the difference between theexperimental and the calculated values isapproximately 7% and can probably beattributed to the fact that the prototypewas only just entering its failure phase. In

any event, a difference of 7% is well withinthe normal range of scatter expected forsuch piles.

(2) Fig. 2 illustrates another example takenfrom an extensive programme of load testscarried out for the design of the founda-tions for the New Law Courts Building inNaples.

Two cast-in-place piles, 42m long, withdiameters of, respectively, d = 150cm andD = 200cm were tested.

The site was not very far from the site ofthe tests indicated in Fig. 1; i.e. after aswampy layer up to a depth of approx-imately 15m, there was volcanic soil(pozzolana and pumiceous materials). Boththe test piles were constructed in the samearea, and the tests were carried to failure.

Fig. 2 shows:—the P/e diagram for the pile, d= 150cm

derived by direct load test —assumed tobe the pilot —Curve (a).—the P/e diagram for the pile, D =200cm, derived by direct loadtest —assumed to be the prototype—Curve (b).—the P /e diagram for the prototypeanalytically calculated from the P/ediagram of the pilot pile, using the ratioD/d = 2001150 = 1.33—Curve (c).The agreement between the calculated

and the experimental values of the failureload for the pile, D = 200cm, is quite good.

(3) A third example is taken from a recentPaper, by Meyerhof et al 9, in which the

results of load tests on two precast con-crete driven piles are reported, in order tocompare some theoretical design app-roaches with experimental results.

For the purpose of the present Paper,only the experimental data is utilised. Thepiles, driven through till, were both loadedto failure. The pile characteristics were:—a Herkules 420 hexagonal pile (12.7cm

each side; full perimeter = 76.2cm)driven to a depth of 12m,—a Herkules 800 hexagonal pile (17.8cmeach side; full perimeter = 106.8cm)driven to a depth of 13m.In Fig. 3 the following load settlement

curves are shown:—the P/e diagram for the Herkules 420,

derived by direct load test, as reported inthe original Paper —Curve (a)—assumedto be the pilot pile.—the P /e diagram for the Herkules 800,derived by direct load test, as reported inthe original Paper — Curve (b)assumed to be the prototype.—the P /e diagram for the Herkules 800analytically calculated from the P/ediagram of the Herkules 420, accordingto the linear ratio 106.8/76.2 = 1.40—Curve (c).

It can be seen that the calculated failureload of the H. 800 pile is somewhat smallerthan the experimental value (curves (c) and(b), respectively). This may be due to thefact that the assumed pilot pile is actuallyshorter than the assumed prototype (12magainst 13m).

This difference is taken into account incurve (d) which represents the P /e"diagram for the Herkules 800 analyticallycalculated from the P/e diagram of theHerkules 420, taking into account also theratio between the full lengths of the ass-umed prototype and the assumed pilot pile,i.e. 13/12 = 1.08.

The failure load from curve (d) ispractically coincident with theexperimental value of curve (b).

Additional notesFor the sake of exactness, the following

should be noted.(af The practical coincidence between theexperimental and calculated values should,

20

300

0

Load, kN1 000 1 500 2 000 2 500

4010

ark u(es 800((rl

70 (cl

Fig.3. (rightl Results ofload tests onprecastdriven piles(from MeyeIhof,)981) 30

E 80E

C

E 90

100

Fig.2. Results of load tests on large diametercast-in-place piles(Naples, 1 981)

EE

50C

42 Ground Engineering

in principle, only relate to the failure phasewhere the pile behaves like a rigid bodyand should not be extended to the full Plediagram where the pile should be con-sidered as an elastic body moving in apartially elastic medium.

(b) The first two examples relate tocast-in-place bored piles whose construc-tion technology does not entail appreciableremoulding of the soil, whereas the third

example refers to driven piles, wheremarked shaking and remoulding of the soiloccul's.

Nevertheless, the hypothesis put for-ward in the present Paper on the linear rela-

tionship between the ultimate bearing

capacity of different diameter piles seemsto be applicable to both types of pile.

It is the opinion of the author that any

remoulding of the subsoil creates a temp-orary state of unstable equilibrium which,as in the third example, especially in clayeysoils, cannot last for ever. The soil, in thelong run, has the tendency to gradually

regain its original state of equilibrium and

compactness, thereby undoing the effectsof remoulding, temporary overstressing,etc.

The mechanism of failure for both typesof piles, is therefore a shear rupture,

occurring not at the soil/pile interface but in

the soil surrounding the pile close to thepile's outer surface. The resistance to thepile's further downward movement is due

to some sort of rolling friction. All thestresses generated in the soil surrounding

the pile during the previous phases of load-

ing completely disappear, and what is left

is the soil/soil interaction only.

(c) The linear extrapolation of the pilot pile

failure values to prototypes of different

length, as in case No. 3 above, can only beaccepted as a rough estimate, provided the

quality of the soil does not change in

depth, and the difference in length is notvery large.

The problem of the crushingresistance of the pilot pile

Generally speaking, the efficient andconvenient use of a concrete foundationpile is most economically achieved whenthe ultimate bearing capacity (failure) isreached at the same time as the com-pressive stress on the concrete cross-section of the pile head approaches itscrushing limit; otherwise, the fullload-bearing capacity of the pile is notcompletely utilised.

When moving from the pilot pile to theprototype, the cross-sectional areaincreases in accordance with the ratio ofthe square of the diameters (D 2/d2).Therefore, for unreinforced piles, if thelimiting crushing stress has been reachedon the pilot pile, the same limit cannot bereached on the prototype.

On the other hand, if the pilot pile isstrongly reinforced, in percentages greatlyin excess of those of the prototype, it ispossible to reduce the gap between thecrushing stresses on the pilot pile and onthe prototype.

It is therefore suggested that the pilotpile is over-reinforced (at least in its upperpart), in order to eliminate the risk ofcrushing of the concrete before the failure

limit for the pile is reached.

Summary and conclusionsFor cast-in-place friction piles, load tests

carried to failure on smaller diameter piles(pilot piles) can be usefully used forassessing the corresponding failure valuesfor larger diameter piles (prototypes) of thesame type and length. The economicaladvantage is evident.

If the only information required is theultimate bearing capacity (failure), noinstrumentation of the pilot pile isnecessary.

If, on the other hand, a full knowledge ofthe load/settlement diagram for theprototype is desired, some instrumentationis necessary in order that calculationsbased on the congruence equations, asdescribed in the earlier Papers i 2 can beapplied.

The diagram obtained from the linear(Dld) extrapolation of the pilot pile's Piediagram does not consider the deformationof both the pile and the soil which takesplace before the failure phase. This resultsin a steeper slope to the diagram than thatobtained experimentally.

For the majority of cases the method can. be considered to be satisfactory and

adequate for practical purposes where onlythe assessment of the ultimate bearingcapacity is required.

Theoretical approaches to the problemof the load-bearing capacity of piles cangive quite good results in many cases 3.

However, the great importance andresponsibility involved in a piled foundationrequires some definitive field confirmation.Such direct confirmation is very expensivefor large diameter piles. The proposedmethod, based on the use of pilot piles, is apractical alternative, at a reduced cost.

References1. Lizzi, F. (I 980): "An experimental approach to the

design of large diameter floating piles - the similitudemethod". Ground Engineering, Vol.13, No.2, March,pp. 17-19,25

2. Lizzi, F. (1981): "The design of large diametercast-in-place bored piles using a pilot pile andcongruence equations". Ground Engineering, Vol.14,No.4 May pp. 24-33

3. Kerisel, J. (1961):"Fondations profondes en milieuxsableux", Vth ICSMFE, Paris, Vol.ll, pp 73-82

4. Kerisel, J., L'Herminier, R Bi Tscheng, Y. (1965):"Resistance de pointe en milieux pulverulents deserrages divers", Vlth ICSMFE, Montreal, Vol. II, pp.265-269

5. Meyerhof, G.G., Brown, JD. Bi Mouland, GD (1981):"Prediction of friction pile capacity in a till". XthICSMFE, Stockholm

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imately 10m, Bm being below the watertable.

Khansaheb-Sykes, a subsidiary of HenrySykes, were appointed sub-contractors forthe project, responsible for the supply, inst-allation and maintenance of the wellpoint

dewatering system under a contract valuedat approximately f500 000. Because ofthe depth of excavation, a multiple-stagewellpoint system would normally havebeen employed to obtain the necessarydraw-down. However, severe limitations on

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Two-stage dewateringfor Dubai underpassGROUND WATER CONTROL has been animportant factor in maintaining progress in

the f 37.5m underpass contract beingundertaken in Dubai by the Arabic/Frenchconsortium, Al-Ashram/EPTO/LTPA, forHis Excellency Sheik Rashid Bin Saeed AI

Maktoum, Vice President and Prime Mini-ster of the UAE and Ruler of Dubai. Thescheme, which is intended to help ease therapidly increasing traffic congestion in thecity, involves the construction of five under-passes at four separate locations. Thesevary in length from 300m to 420m, andextend to a maximum depth of approx-

el

S Ay es pumps, header pipes and disposable wellpoints attached to the sheet piles formingthe wa//s of the underpass

November 1983 45