Acceptance Criterion and Driving

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Acceptance criterion and driving procedure of open-end pilesCritère d’acceptation et procédure de battage de pieux tubulaires

Luc Maertens Besix Engineering Department (Brussels) and Catholic University of Leuven

ABSTRACTOpen-end pile foundations are largely used in near shore projects, such as jetties for loading and unloading of Oil, LPG, etc. Since theconstruction of these offshore structures implies very heavy and expensive marine equipment, any delay in construction time has to beavoided. The pile installation is often the main part of the critical path in the execution schedule and no construction interruption canbe accepted due to acceptance problems of installed piles. It is therefore mandatory that a clear installation procedure, a simple accep-tance criterion as well as a fast remedial program for non accepted piles is available for each part of the foundation. This paper dealswith the procedure of fixing these criteria.

RÉSUMÉLes pieux tubulaires sont largement utilisés pour des projets marins, tels que des jetées pour chargement ou déchargement de Pétrole,

Gaz liquide, etc. Comme ces structures marines nécessitent un matériel très lourd et coûteux, il va de soi que tout retard doit être évi-té. L’installation des pieux gouverne souvent le chemin critique dans le planning des travaux et il est dès lors inacceptable d’avoir desinterruptions des travaux dues à des problèmes d’acceptation des pieux réalisés. Pour cette raison, il est absolument nécessaire de dis-poser de procédures d’installation très claires, de critères d’acceptations simples, mais également de méthodes très rapides pour y re-médier dans le cas où un pieu ne répondrait pas aux critères imposés. Cette contribution expose les procédures qui peuvent être sui-vies pour fixer ces critères.

1 INTRODUCTION

Near-shore projects must be designed to enable the contractorfor a fast track execution, in order to minimise the utilisationtime of expensive marine equipment such as Self ElevatingPlatforms (SEP-see figure 1), tugs, working vessels, dredgingequipment, etc.

Figure 1

The construction time is in general governed by the executiontime of the pile foundations. The standard foundation type forthis type of structures consists in open-end steel piles becausethese piles can be applied in ALL types of soil: sandy soils,clayey soils, cemented soils, weathered rocks and even soundrocks. Of course, in some cases driving will not longer workand drilling will be needed.To avoid unnecessary delay due to pile installation, it is neededto provide a method statement for piling, which makes the linkbetween the design to the execution through a clear installation

procedure, which includes an acceptance criterion together witha fast remedial program if ever the pile is not acceptable.This paper deals with the methodology to define the processfrom the design to the acceptance of open-end piles.

2 METHODOLOGY

The following steps are to be followed in the foundation engi-neering procedure:• Geotechnical design of the piles (1)• Selection of the driving equipment (hammer)• Driving records on trial piles• Dynamic tests (PDA) on trial piles• Maintained load test on trial piles• Analyses of results• Acceptance criterion report (including remedial pro-

gram for not accepted piles)• PDA on working piles (with feed back to the pro-

posed acceptance criterion)

(1) Geotechnical design is not treated in this paper.

3 SELECTION OF DRIVING EQUIPMENT

An accurate selection of driving hammer(s) is one of the mainelements for a successful pile installation.Selection of the hammer is to find the best compromise betweenthe hammer weight (linked to the crane capacity), the hammercapacity (linked to the ability of driving the pile to the requireddepth and/or resistance) and the installed driving energy (linkedto avoid damaging the pile).In a first approach, one can assume that the weight of the ham-mer (kN) is 1.2 times the hammer energy (kJ). The hammerhelmet is not included (+/- 60% of the hammer weight).



Appropriated software (for example PDP-Wave, developed byDelft University) can help to predict for a known soil profileand a given pile the blow count, the stresses in the pile duringdriving and the expected SRD (Static Resistance During Driv-ing) for a given hammer.

Figure 2

Figure 3

As an example, one can see here above the picture of a IHCS200 hammer (figure2) and the calculated blow count, SRD andpile stresses for a given soil profile (figure3).

4 DRIVING RECORDS ON TRIAL PILES

For all trial piles, the blow count and the energy per blow arerecorded. On top, all trial piles are continuously tested using aPDA (Pile Dynamic Analyser) (figure4).

0

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6

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10

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14

16

18

20

0 10 20 30 40 50 60 70 80 90 100 110

Penetration [m]

Blow count [bl/25cm]

Blow count

Figure 4

This leads to a graph as given here below:Libya WAFA Test Pile 2002 (Test 2 Pile 2 ) - Hammer S150 and S200

0

25

50

75

100

125

150

175

200

225

250

275

0,00 5,00 10,00 15,00 20,00

Penetration (m)

B l o w c o u n t ( B l p e r 2 5 0 m m ) / E n e r g y ( k J )

C a s e S t a t i c R e s i s t a n c e x 1 0 ( M N )

H a m m e r E n e r g y

S 1 5 0

S 2 0 0

B l o w c o u n t

C a s e S t a t i c

R e s i s t a n c e

0

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0 10 20

Penetration [m]

Driving Resistance [MN]

SRD during driving

30

Figure 5

On this chart, various information is given for differentdepths:(figure 5):

• Hammer Energy• Blow count• Static resistance by PDA (CASE)

On top of this, we changed from a S150 hammer to a S200hammer for the last meter.By doing this for a series of trial piles, and by working with dif-ferent hammers, one can prepare a graph, giving the relationshipbetween the blow count and the SRD for 2 hammers for theconsidered soil profile (for the same example) (figure 6):

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170 180 190 200 210

Penetration [m]

Stress [MPa]

Maximum compressive stress in the pile

Libya-WAFA- Refusal Condition for Piles in Section A and B

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30

0 20 40 60 80 100 120 140 160 180

Number of blows per 250 mm

" C A S E " S t a t i c R e s i s t a n c e ( M N )

Hammer 150 (tests 1001/2001)Hammer 200 (tests 1002/2001/2002)

0.85*Ham150

0.85*Ham200

Required Static Resistance

Log. (Hammer 150 (tests 1001/2001))

Log. (Hammer 200 (tests 1002/2001/2002))

We conclude for Section Aand B:

Hammer: Min Blowcount

over last 500 mm:

S200 : 20 blows/250 mm

(175to 200 KJ) or max 12.5 mm/blow

S150 : 50 blows/250 mm

(125to 200 KJ) or max 5 mm/blow

Figure 6



Given that 85% of the SRD is a “trustable” value, one is able todefine the blow count which is needed for each consideredhammer to guarantee in a safe way the required pile capacity.

5 MAINTAINED LOAD TEST

Depending on the needs of the design, different types of main-tained load tests can be carried out: Compression Tests, TensileTests or Lateral Tests.The aim of these tests is not only to confirm the ultimate designcapacity but also to validate the foundation stiffnesses which are

used in the computer models for the design.These stiffnesses are defined by applying specialised softwarepackages such as ENSOFT (by N. Reese).Maintained load tests are performed offshore (figure 7):

Figure 7

The maintained load test on the trial pile given under point 4 isperformed from the SEP. The two other piles are referencepiles.The SEP can NOT be considered as a reference, since it is mov-ing under the test loads. The result of this test is given below:

LOCATION 2 PILE 2 - COMPRESSIONAL MAINTAINED LOAD TEST

SETTLEMENT VS LOADING

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60

0 2.000 4.000 6.000 8.000 10.000 12.000 14.000

LOAD (kN)

S E T T L

E M E N T ( m m )

Statical Pile Test Design (ENSOFT-A-17.5m) Design(ENSOFT-B-23m)

Figure 8

6 ACCEPTANCE CRITERIA

The acceptance criteria have to be formulated as clear as possi-ble to allow for the piling operator to decide very fast what has

to be done, especially when two types of hammers are used onthe spot.For the piles described under point 4, following criteria weregiven:• Design penetration has to be reached• Maximal penetration per blow over the last 500 mm:

o S 150 hammer: 5 mmo S 200 hammer: 12.5 mm

In most of the cases, the penetration is reached using the ad-vised hammer and the blow count criterion also (to guaranteethe required bearing capacity).If penetration is not reached, or if the blow count criterion is not

met, remedial actions have to be taken as given in the chartgiven hereafter (figure 9):

Select Hammer

S 150 or S 200

Drive pile torequired

penetration (1)

Required depthreached? (2)

Refusualcondition

Dynamic test

In accordance withworking load (3)

- Lengthen pile if needed

- Continue driving to requiredrefusal

HammerS150?

Performdynamic test

Evaluation by

SIXCO

Technicalproposal by

SIXCO

Continue drivingwith S 200

No

Yes

No

Yes

END

OK

Not OK

END

Yes

No

Figure 9

(1) See execution drawings(2) See criteria above(3) To be defined by SIXCO (=the Engineering Company)

Furthermore, the results on all the working piles are collectedand placed in a chart which gives a statistically more trustablegraph due to the large number of results available. The chartbelow gives the results of such analyses for another section of the same site above (figure 10).

WAFA JETTY

PILE REFUSAL AND BE ARING CAPACITY : S-200 HAMMER

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35

0 50 100 150 200 250

Blows/250mm in last metre

S t a t i c b e a r i n g c a p a c i t y f r o m C A P W A P a n a l y s i s

( M N )

Profile C; L=20.0 or 21.0 m; 75KJ/blow; Minimum

Profile C; L=23.0 or 24.0 m; 75kJ/blow; Minimum

Target bearing capacity for piles L=20.0 or 21.0 m

Contractor Proposeds Refusal Criterion for piles

L=20.0 or 21.0 mTarget bearing capacity for piles L=23.0 or 24.0 m

Contractor Proposeds Refusal Criterion for piles

L=23.0 or 24.0 mN=30

Log. (Profile C; L=23.0 or 24.0 m; 75kJ/blow;

Minimum)Log. (Profile C; L=20.0 or 21.0 m; 75KJ/blow;

Minimum)

30

Figure 10



7 INSTALLATION PROCEDURE FOR SOCKETED PILES[1] [2]

When piles are driven to a rock layer, with little overburden,tensile forces can not be supported by friction on the shaft of theopen-end piles, and installation of bored sockets or anchors ismandatory. Since sockets are installed by boring equipmentwhich has to operate through the pile, no damage of the pile tipcan be accepted.A compromise has to be found between driving the pile to a suf-ficient penetration to guarantee the capacity of the pile undercompression, and in the mean time to prevent for damaging the

pile. Note that damage at the pile tip for compression piles hasalso to be limited to avoid driving problems.In Dabhol in India, piles have to be driven in weathered basaltand the following procedure was set up to avoid damaging thepile as shown in the picture below (figure 11):

Figure 11

Driving analysis by TNO-WAVE (PDP Wave) can predict forthe considered soil profile the SRD (Static Resistance duringDriving) as well as the stress in the pile during driving, for dif-ferent Hammer Energy levels and different penetrations perblow.

In the figure 12 the results of this analysis are shown for a com-

pression pile (760*16 mm).It shows that the stress during driving decreases significantlywhen the hammer energy is reduced. For a S90 hammer (hy-draulic hammer from IHC), one can see that for an SRD valueof 5250 kN, the driving stress is 350 MPa for a full energy set-ting of 90 kJ and is reduced to 260 MPa when the setting of theenergy is reduced to 45 kJ.On the other hand, the number of blows is increased from 28 to108 blows per 100-mm penetration. This means that the drivingtime is almost 4 times longer as the blow rate remains 50 blowsper minute.Field test on full hammer capacity are showing damages be-tween 0.1 and 0.5 m for a driving stress of 380 MPa, close tothe yield stress (415 MPa). In fact these maximum drivingstresses are computed with the assumption that the stresses areuniformly distributed over the entire cross section. This is of

course never true in reality, and an appropriate safety factor hasto be used in the definition of the refusal criteria.Final installation criteria to guarantee the required SRD aregoverned by in-depth stress and damage analyses. It was con-cluded to allow 80% of the yield stress (= 332 MPa) for com-pression piles and 55% (= 225 MPa) for tension piles, since ten-sion piles need a socket.

This criterion was checked by installing two additional rakingpiles on the test location onshore. After inspection, no damageat pile tip was observed as shown in the picture below (figure13).

Definition of refusal for compression pile 16 mm

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9000

10000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Blows per 100 mm penetration

S R D ( k N )

Maximum SRD = 2 Maximum Working Load

= 2 * 2620 kN = 5250 kN

67,5 kJ

45 kJ

150

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300

350

400

450

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Blows per 100 mm penetration

M a x i m u m D r i v i n g S t r e s s ( M P a )

Yield stress = 415 MPa

Allowable stress = 332 MPa

90 kJ

Figure 12

Figure 13

8 CONCLUSIONS

Acceptance criteria can be defined based on driving records,dynamic (PDA) tests during driving and maintained load testson trial piles. Feed back by PDA testing on working piles al-lows increasing the accuracy of the criterion.

A clear acceptance criterion together with an easy remedial pro-cedure for non accepted piles can prevent for unnecessary delayduring construction of offshore structures.

REFERENCES

[1] L. Maertens “DESIGN AND INSTALLATION OF STEEL OPENEND PILES IN WEATHERED BASALT.”International Deep Foundations Congress A.S.C.E. – Orlando, 14-16/02/2002

[2] L. Maertens «BORED SOCKETS IN WEATHERED BASALT.”4th International Geotechnical Seminar on Deep Foundations onBored and Auger Piles, Ghent, 2-4/06/2003.

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Acceptance Criterion and Driving