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ACTA GEOTECHNICA SLOVENICA, 2005/1 45.

SUGGESTION ABOUT DETERMINATION OF THEBEARING CAPACITY OF PILES ON THE BASIS

OF CPT SOUNDING TESTSJZSEF PUSZTAI

About the authors

Pusztai JzseBudapest University o echnology and Economics,Department o GeotechnicsMegyetem rkp. 3. K. m 1., 1521 Budapest, Hungary

E-mail: [email protected]

Abstract

Te Cone Penetration est (CP) is a well-recognizedtool for the calculation of the ultimate bearing capacity ofpiles. Within the Hungarian physiographic territory, theCP and Static Pile Load ests of the bored (ContinuousFlight Auger - CFA, protective tube) and driven (Franki)piles installed in different soils (gravel, sand and clay) werecompared to determine the ultimate bearing capacity ofpiles by using new formulae.

Keywords

bearing capacity o piles, cone penetration test

1 BACKGROUND

Both international and Hungarian proessional literature[1-8] deal intensively with the topic o the load bearingcapacity o piles, determined on the basis o in situexploration methods. Tis is a result o the rapid andextended prolieration o new exploration technologieswhich reveal more inormation about the undergroundcondition on the spot (CP and CPu) than traditionalboring methods. Having gained in this way substan-tive additional knowledge about the soil throughnew parameters, engineers try to develop appropriateormulae or equations that enable more efficient designand construction o structures. Tis also means thatmore reliable predictions can be made about the bearingcapacity o a pile already in the design stage.

Te relevant proessional literature arrived to the unani-

mous conclusion that presently the most inormative

method or the determination o bearing capacities opiles in granular soils is the CP (Cone Penetration est)probing technology, because it differentiates between thecone resistance (q

c) and the local sleeve riction (

s). Te

equipment produces a diagram describing separately

these two resistances as a unction o depth. An exampleis shown in Fig. 1.

In the Netherlands the design code [3] comprises therules derived via innumerable cone tests and experi-ments or capacity calculations.

Te load bearing capacity o the pile is determined romthe cone resistance (q

c) o the CP test. Tis is because

the cone resistance values are more sensitive to variationin soil density than the sleeve riction, and the identica-tion o the soil type rom the ratio o q

cto

sis not always

clear-cut.

Consequently, in the traditional manner, the ultimatebearing capacity o a single pile (Q

u) is calculated as the

sum o the ultimate resistance o the base (Qb) and the

ultimate resistance o the shaf (Qs) capacities:

Q Q Q A q U Lu b s b c s= + = + (1)

where:

Ab

= nominal plan area o the base o the pile [m2]U = length o the piles periphery [m]

L = length o the pile [m]qc

= average cone resistance in the zone o the pile toe[MPa]

s = average ultimate skin riction along the pile shaf

[MPa].

Based on experience, Meigh [3] suggested using theollowing correlation between pile skin riction and coneresistances (able 1).

Te values given in able 1 reer to piles that are exposedto static loads. Meigh [3] proposes to take the ultimate

skin riction to 0,12 MPa at the utmost.

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ACTA GEOTECHNICA SLOVENICA, 2005/146.

JZSEF PUSZTAI: SUGGESTION ABOUT DETERMINATION OF THE BEARING CAPACITY OF PILES ON THE BASIS OF CPT SOUNDING TESTS

Te average resistance against the progress o the cone,

or penetration ( qc ), can be derived using the ormula:

qq q

c

c c=

+ 1 2

2 (2)

In the Netherlands, in accordance with the advice oMeigh [3], the method generally applied is the one in

which the average cone resistance ( qc1 ) is determined

to the depth o our times the pile diameter (4D) below

Figure 1.Measurement results o a Cone Penetration est.

Table 1.Correlation between skin riction and coneresistance.

Pile typeUltimate unit

skin riction (s)

imber 0,012 qc

Precast concrete 0,012 qc

Steel displacement 0,012 qc

Open ended steel tube + H-section 0,008 qcOpen ended steel tube driven into ne tomedium sand

0,0033 qc

the toe, and the average cone resistance ( qc2 ) to thedepth o eight times the pile diameter (8D) above thepile toe.

Regarding the 4D 8D method, it is important to notethat:

minor peak depressions have to be ignored in thecalculation; supposedly, they do not reer to thin

weak strata, and the q

c> 30 Mpa values will be also ignored in this

interval.

Obviously, there are also methods other than the 4D 8D method; in use, however, they only differ in thecalculated depth below the pile toe (or example bytaking 2D, instead o the 4D suggested above).

e Kamp [9] preerred to suggest the saety actorspresented in able 2, or the calculation o limitingcapacity in the Netherlands, when the 4D 8D method

is used:

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Table 2.Factor o saety or piles.

Pile type Factor o saety

imber

Precast concrete, straight shafPrecast concrete, enlarged shaf

1,7

2,02,5

Because o the disturbance and loosening o the soilvia the boring tool, the Codes advise not to use coneresistance values when the skin resistance o bored pilesis calculated.

Te relationship established or Dutch soil conditionsis not necessarily applicable to cohesionless soilseverywhere. Te yielding and rupture o the soil causedby pushing a cone into the ground are different rom

those resulting rom driving a pile by hammer ollowedby static loading. Te work o Vesic [10] has shown theimportance o the state o preconsolidation and mineral-ogy o the soil grains in any correlation o in-situ condi-tions with pile resistance. By coincidence, static coneresistance in the Netherlands (and Belgium) was oundto be equal to pile base resistance. Elsewhere, Gregersenet al. [11] ound the pile base resistance to be only onehal o the cone resistance or loose medium to coarsesands in Norway, and Gruteman et al. [12] reported thata actor o 0.75 was applied to the cone resistance toobtain the ultimate base resistance o piles in silty sandsin Russia.

2 IN-SITU TESTS

Te analysis o in-situ test data can result in betterdesign parameter estimates which will affect the ultimate

bearing capacity Quo piles. A comparison o the Static

Pile Load est and CP measurements in Hungariansoils was undertaken to better dene mechanismsaffecting Q

uand to create ormulae that are appropriate

or Hungarian soils and that also consider constructionmethods.

In this sense, the author selected the results o domesti-cally perormed Static Pile Load ests where the resultso the CP tests were also available. Altogether, datarom seven CFA tests, three tests with protective tubes,and 26 Franki piles were gathered (able 3).

3 SUGGESTED FORMULAE TOCALCULATE THE ULTIMATEBEARING CAPACITY OF PILES

Te derived ormula with the values in Fig. 2 relates to theailure load o a single pile. In deriving the ormulae, thecustomary static basis has been used as a starting point,whereby the ultimate bearing capacity o a single pile (Q

u)

is the sum o the ultimate resistance o the base (Qb) and

the ultimate resistance o the shaf (Qs) capacities:

Q Q Qu s b= + (3)

Figure 2.Concept or the estimation o the ailure load o a single pile.

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Te rst part of the formula(Qs) depends on the total

surace area o the shaf, the earth pressure actingthereon, the interactive orces between the surroundingsoil and the shaf, and the technology o abrication.

Tese make:

Q U L U As s s s fs= = (4)

where:

U = length o the piles periphery [m]L = length o the pile [m]A

fs = area o the plotted

scurve rom the CP probe

test (explained in Fig. 2) [MPam]

s = empiric actor with view on the applied piling

technology; it expresses the shaf resistance [-]

s = average ultimate skin riction along the pile shaf[MPa].

Te second part of the formula(Qb) depends on the

extension o the surace area where the pile toe rests, thespecic resistance o the soil in the zone o the pile base,and on the applied piling technology. Tese make:

Q A qb b b c=

cos60

2

(5)

where:

Ab = nominal plan area o the base o the pile [m2

]

b = empiric actor with view on the applied piling

technology; it expresses the base resistance [-]

qc= average value o the cone resistance below the pile

toe (explained in Fig. 1) [MPa]. [It has been observed that the depth (nD) below

the pile toe is strongly inuenced by the appliedpiling technology, which has to be accounted orwhen the average q

cvalue is derived].

4 ANALYSIS OF THE ULTIMATECAPACITY OF PILES USINGTHE STATIC PILE LOAD TEST

Te pile load tests were perormed according to thestandard loading procedure described in the HungarianStandards, MSZ 15005-1:1989 and MI 04.190:1984.

All pile load tests have been carried out until the ailureload was reached.

5 RESULTS OF THE COMPARISONOF THE MEASURED (STATICPILE LOAD TEST) ANDCALCULATED (CPT) ULTIMATEBEARING CAPACITIES OF PILES

Te results o the recommended CP method used toestimate the ultimate bearing capacity o the selectedpiles discussed in the in-situ tests section were comparedwith the results o the Static pile Load ests. Te ndingscan be seen in able 3.

Location

(ap = motorway)Pile type

Length[m]

Diameter[m]

Soil belowthe toe

From CP using the ormulaeStatic PileLoad ests Differ-

ences

b

n Qu, calculated

Qu, measured

[1] [1] [1] [kN] [kN] [%]

M3 ap./B 2

Franki

7,00 0,60 Gravel

1,40 1,70 3

3 694 3 650 -1%

M3 ap./B 3 5,00 0,60 Gravel 3 176 3 750 18%

M3 ap./H 29 7,00 0,60 Gravel 2 275 2 350 3%

M30 ap./1 9,50 0,60 Gravel 4 974 4 550 -9%M30 ap./4 7,00 0,60 Sand 5 011 4 375 -13%

M3 ap./H 30 4,00 0,60 Sand 3 243 3 720 15%

M3 ap./B 9 6,50 0,60 Clay

2,40 2,70 3

3 820 4 350 14%

M3 ap./B 6 7,00 0,60 Clay 2 190 2 050 -6%

M3 ap./B 7 9,00 0,60 Clay 3 637 2 980 -18%M3 ap./B 11 9,00 0,60 Clay 2 809 3 240 15%

M3 ap./B 13Protective

tube

23,00 1,00 Clay 0,45 0,05 1

2 867 2 600 -9%M3 ap./B 14 17,80 1,00 Clay 2 116 2 100 -1%

M3 ap./H 32 20,60 1,00 Clay 2 890 3 250 12%

M3 ap./HB 44

CFA

15,50 0,80 Sand

0,75 0,75 2

1 740 1 780 2%M3 ap./HB 46 14,50 0,80 Sand 3 225 2 760 -14%

M3 ap./HB 47 13,50 0,80 Sand 2 665 3 050 14%

M3 ap./H 35 14,60 0,80 Sand 3 125 3 050 -2%

M3 ap./HB 42 15,80 0,80 Fine sand 2 160 1 927 -11%

M30 ap./2 13,80 0,80 Clay 4 023 3 900 -3%

Table 3. Comparison o calculated and measured bearingcapacities o piles.

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Based on the results o the perormed calculations, theregression coefficient (r) or each piling technology is asollows:

For D = 60 cm diameter Franki piles: r = 0.87,

For D = 100 cm diameter piles boredin protective tubes: r = 0.84,

For D = 80 cm diameter piles boredwith CFA technology: r = 0.94.

On the basis o piling technologies and pre-calculationsthe assumptions used and conclusions are as ollows:

In the course o the calculations the bulb diameteror the Franki piles was assumed to be equal to the

trunk diameter; so the expansion o the bulb is inclu-ded in the actor b.

o account or the densication o the soil in the caseo driven D = 60 cm diameter Franki piles in granu-lar soils, it is recommended to use the values

s= 1.40 and

b= 1.70 (higher than or the

Ko= 1-sinequilibrium pressure), as well as 3D

zone-depth, in the calculations. For D = 60 cm diameter Franki piles in cohesive

soils, it is recommended to use s= 2.40,

b= 2.70,

and 3D zone-depth.Te values

s= 2.40 and

b= 2.70 are just one unit

higher (because o the pore-water pressure) than in

the case o granular soils. In the case o D = 100 cm diameter piles bored in

protective tubes presumably due to the accumu-lated pulverised sediment at the bottom o the hole it is recommended to use

b= 0.05 or the base

resistance and s= 0.45 or the shaf resistance (lower

than Ko= 1-sin), as well as 1D depth-zone.

For D = 80 cm diameter piles bored with the CFAtechnology, it is recommended to use

b=

s= 0.75

and 2D depth-zone.

6 SUMMARY

Tis study presents the evaluation o a new method inpredicting the ultimate bearing capacity o different piles(Franki piles, piles bored in protective tubes and pilesbored with the CFA technology) driven into differentsoils in Hungary.

Tirty six pile load test reports with CP soundingsadjacent to the test pile were collected. Te predic-tion o pile capacity was perormed or each pile;

however, the statistical analysis and the evaluation o the

suggested prediction method were based on the resultso the nineteen piles (presented in this paper) thatplunged (ailed) during pile load tests.

An evaluation scheme was executed to evaluate theCP values based on the ability to predict the measuredultimate bearing capacity. Different values (

s,

band

nD) were suggested or different piling technologies orthe evaluation scheme.

Based on the results o this study, the suggested ormulaeusing the results o the CP testing are given to predictthe ultimate load bearing capacity o the piles.

While one may not expect that any calculation carriedout based on the result o either the CP test or anyother probing test will lead in all cases straight to the

determination o the exact ultimate bearing capacityo piles derived by using static loading test results, theperormed study proves that more accurate approachescan be ound to replace traditional static ormulae in thedesign stage.

REFERENCES

[1] Chen, B.S.-Y. and Mayne (1995). ype 1 and 2

piezocone evaluations o overconsolidation ratioin clays. Proceedings, International Symposium onCone Penetration esting (CP 95), Vol. 2., Swed-ish Geotechnical Society Report No. 3:95, Linkop-ing, 143-148.

[2] De Beer, E. (1963). Scale effect in the transpositiono the results o deep sounding tests on ultimatebearing capacity o piles and caisson oundations.Geotechnique 23, 39.

[3] Meigh, A. C. (1987). Cone penetration testing,CIRIA-Butterworth.

[4] Meyerho, G. G. (1976). Bearing capacity andsettlement o pile oundations.Journ. Geot. Eng.

Div., ASCE (102) G 3, 195 - 228.[5] Poulos, H. G. (1989). Pile behavior-theory and

application. Geotechnique, Vol. 39, No. 3, 365 - 415.[6] iti, H. H. (1999). Evaluation o bearing capacity

o piles rom cone penetration test data. Louisianaransportation Research Center, LA.

[7] omlinson, M. J. (2001). Foundation Design andConstruction.7th ed., Pearson Education Ltd,Essex, 99 - 154.

[8] omlinson, M. J. (1971). Some effects o pile driv-ing on skin riction, Proceedings of the Conferenceon the Behavior of Piles, Institution o Civil Engi-

neers, London, 107-14.

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[9] e Kamp, W. C. (1977). Sondern end underingenop palen in zand. Fugro Sounding Symposium,Utrecht.

[10] Vesic, A. S. (1977). Design o pile oundations.

NCHRP Synthesis 42, ransportation ResearchBoard, Washington D. C.

[11] Gregersen, O. S., AAS, G. and Dibiagio, E. (1973).Load tests on riction piles in loose sand. Proceed-ings of the 8th International Conference, ISSMFE,Moscow, Vol. 2.1., 21-5.

[12] Gruteman, M. S. et al. (1973). Determination opile resistance by means o large-scale probes andpile oundation analysis based on allowable settle-ments. Proceedings of the 8th International Confer-ence, ISSMFE, Moscow, Vol. 2.1., 131-6.

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