Axial Bearing Capacity Of Driven Piles In Accordance With .
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012Axial Bearing Capacity of Driven Piles in Accordance with API andDNVS. Jegandan1INTECSEA (UK)N. I. Thusyanthan2CAPE GROUPD. J. Robert3Monash University1,2,3Formerly KW Ltd, UKAbstractPiles are fundamental part of most offshore structures. Thus assessment of pile capacity is critical for the design and installation of offshore structures. Currently, there are various codes that provide guideline on thepile capacity assessment. The most common ones are API and DNV. Each code provides slightly different design approach and utilises different safety factors. Thus, it is often not easy to directly compare the pile designs of different code. Furthermore, most appropriate design methodology is often chosen based on the available input parameters. i.e. geotechnical properties or CPT results. For a single design case, adapting differentcodes can result in different pile length requirements. This difference in pile length requirement is purely dueto different methodology and associated safety factors used in codes. This paper aims to provide an overviewof all common pile design methodologies from different codes and present a comparison of design pilelengths.1. IntroductionAssessment of axial bearing capacity of pile varies indifferent codes in terms of methodology and safetyfactors. As a result, the outcome of the pile lengthassessment differs from one code to another.Nevertheless, axial bearing capacity of the pile is asingle value perhaps has an offset from the resultsobtained from bearing capacity assessment based onvarious methods outlined in different codes. Theobjective of this paper is to present the variation ofpile length for a single compressive load based onmethodologies presented in the codes above.2. MethodologyAPI and DNV codes describe slightly differentapproaches to assess the axial bearing capacity of apile. These codes provide guidline for the calculationof pile length in common soil conditions such as clay(cohesive) or sand (cohesionless). The assessmentalso depends on the type of soil information availablei.e. laboratory test results showing soil propertiessuch as undrained shear strength and friction angle orthe in situ Cone Penetration Test (CPT) data from thefield tests. Thus, the suitable design approach ischosen based on the available soil data as shown inFigure 1.APIBased on Soil PropertiesLRFDWSDDNVBased on CPT DataSimplifiedICP -05OffshoreUWA -05Fugro -05Based on Soil PropertiesNGI -05Figure 1 Design approaches accoding to API and DNV codesLRFDWSD
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 20122.1 API:2.1.1 Working Stress Design (WSD) MethodUpon availability of soil properties such asundrained shear strength (Su) or friction angle ( ' ),API 2A-WSD, Ref [1], presents the followingmethodology for pile capacity assessment:In cohesive soils, unit skin friction ( f ) can beassessed by f c . Where α is a dimensionlessfactor and c is the undrained shear strength of thesoil at the point in question.α can be computed by 1 .0 0.5 0.5 1 .0 0.5 0.25With the constraint that, 1.0 , where c / p0 'for the point in questionp0 ' is effective overburden pressure at the point inquestion.In cohesionless soil, unit skin friction ( f ) can becomputed by f p0 ' . Where is dimensionlessskin friction factor and p0 ' is the effectiveoverburden pressure at the depth in question. values for open-ended piles driven unplugged aregiven in Table 1.Unit end bearing ( q ) is assessed by q 9c forcohesive soils and it is assessed by q N q p0 ' incohesionless soils. Where, N q is dimensionlessbearing capacity factor (From Table 1) and p0 ' iseffective overburden pressure at the depth inquestion.Relative densityTable 1 Design parameters for cohesionless soil, API, Ref [1]Soil descriptionShaft frictionLimiting shaft frictionEnd bearingvalues (kPa)factor Nqfactor Very LooseLooseLooseMedium DenseDenseMedium DenseMedium DenseDenseDenseVery DenseVery ltSandSand-SiltSandNot ApplicableNot ApplicableNotApplicableNot 61155012000The ultimate pile capacity is assessed by adding totalskin friction and total end bearing as show below:QD Q f QP fAS qAPWhere: Q f – Skin friction resistance, in force unitsQPfASqAP–––––Limiting end bearingvalues (kPa)Total end bearing, in force unitsUnit skin friction capacity, in stress unitsSide surface area of pileUnit end bearing capacity, in stress unitsGross end area of pileTable 2 Safety factor for allowable pile capacity, API, Ref [1]Load conditionFactorof SafetyDesign environmental conditions with appropri1.5ate drilling loadsOperating environmental conditions during drill2.0ing operationsDesign environmental conditions with appropri1.5ate producing loadsOperating environmental conditions during pro2.0ducing operationsDesign environmental conditions with minimum1.5loads (for pullout)The allowable pile capacity is determined bydividing the ultimate pile capacity by safety factorrelevant to loading type on pile. Safety factorscorresponding to various loading types are presentedin Table 2.2.1.2 Load Resistance Factor Design (LRFD)MethodUnlike WSD method, safety factors are used toaccount for uncertainty in loading in pile resistancein LRFD method. According to API RP 2A- LRFD,Ref [2], the axial pile resistance should satisfy thefollowing conditions:PDE PE QDPDO POQDWhere:PDE (or PDO ) – Axial pile load for extreme (oroperational) environmental conditions determinedfrom a coupled linear structure and nonlinearfoundation model using factored loads.
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012 PE–Pile resistance factor for extremeenvironmental conditions ( 0.8) PO –Pile resistance factor for operatingenvironmental conditions ( 0.7)QD – Ultimate axial pile capacity, which isdetermined by adding total skin friction and total endbearing as shown in WSD method.Load facors on gravity loads:Q 1.3D1 1.3D2 1.5L1 1.5L2D1 – Self weight of the structure.D2 – Dead load imposed on the platform byweight of equipments and other objects.L1 – Live load 1 includes the weight ofconsumable supplies and fluids in pipes.L2 – Short duration force exerted on the structurefrom operations like lifting, drilling.Load factors on wind, wave and current loads:Under extreme condition the following factors areused:Q 1.1D1 1.1D2 1.1L1 1.35(We 1.25Dn )We – The force applied to the structure due to thecombined action of the extreme wave (typically 100year return period) and associated current and wind.Under operating condition the following factors areused:Q 1.3D1 1.3D2 1.5L1 1.5L2 1.2(We 1.25Dn )2.1.1 CPT based MethodsThe CPT based methods are based on directcorrelations of pile unit friction and end bearing datawith cone tip resistance (qc) values from conepentration tests. According to API RP 2A-WSD, Ref[1], the CPT based methods are preferred to themethods based on soil parameters as these methodshave shown statistically closer predictions of pileload test results.The four recommended CPT-basedconsidered here for cohesionless soil are:1.2.3.4.methodsSimplified ICP-05Offshore UWA-05Fugro-05NGI-05Details of the pile capacity assessment based onthese methods are given in detail in API RP 2A-WSD, Ref [1] and hence not reproduced in thispaper. These methodologies have been followed forthe pile capacity assessment based on CPT data.2.2 DNVSimilar to API, DNV also provides WSD and LRFDmethods for pile capacity assessment but withdifferent safety factors.2.2.1 WSD MethodDNV OS C201 code, Ref.[4] reports structuraldesign of offshore units according to WSD method.But this code does not provide a specific foundationdesign method. Instead, it states that the foundationdesign shall be carried out according to either LRFDmethod (as descibed in section 2.2.2) or inaccordance with DNV CN 30.4, Ref. [5], or otheracceptable standards. In this paper, in order tocompare the resulting pile legnths from each designmethod, the axial pile capacity assessment inaccordance to WSD method has been carried outbased on DNV CN 30.4. According to DNV CN30.4, Ref. [5], the compression capacity of pile issum of cuumulated skin friction and end resistanceas in API WSD method presented in section 2.1.1.Method to assess unit skin friction and endresistance for cohesion soil is exactly same as in APIWSD method with same coefficients and limits.Similarly, the method to assess unit skin friction andend bearing in cohesionless soil is same as in APImethod, except unit skin friction, which is defined asfs Kpo’tanδ f1.Where K is the lateral earth pressure, taken as 0.8and δ is the soil-pile interface friction angle.Also, the factors and limits are slightly differentfrom API WSD method presented in Table 1. Theseare shown in Table 3. It must be emphasised that theDNV CN 30.4 is based on API RP 2A (1987) andstill in its first version published 1992 and it is stillreferred in DNV OS C201, Ref. [4] for foundationdesign.The unit end resistance of plugged piles in cohesionless soil, qp, may be taken as qp po’Nq qlWhere po’ is the effective overburden pressure at thepile tip elevation, Nq is bearing capacity factor and qlis limiting end bearing as given in Table 3. In pilecapacity assessment, safety factor of 1.5 (as in Table1) was used to compare required pile length according to DNV WSD method with API WSD.
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012Relative densityTable 3 Design parameters for cohesionless soil, DNV CN 30.4, Ref [5]Soil description δ (degrees)f1 (kPa)End bearingfactor NqVery ry DenseDenseVery ltSandSand-SiltGravelSandLimiting end bearingvalues 02.2.2 LRFD MethodPile capacity design guidelines according to LRRDmethod is described in DNV OS C101, Ref [3].For determination of design soil resistance againstaxial pile loads in ULS design, a materialcoefficient m 1.3 shall be applied to allcharacteristic values of soil resistance, e.g. to skinfriction and tip resistance.For pile foundations of structures where there areno or small possibilities for redistribution of loadsfrom one pile to another, or from one group ofpiles to another group of piles, larger materialcoefficients than those given above shall be used.This may for example apply to pile foundations fortension leg platforms or to deep draught floaters. Insuch cases the material coefficient shall not betaken less than m 1.7 for ULS design.For calculation of design lateral resistance, thefollowing material coefficients shall be applied tocharacteristic soil shear strength parameters forUltimate limit state (ULS) design: m 1.2 for effective stress analysis m 1.3 for total stress analysisFor accidental limit state (ALS) and serviceabilitylimit state (SLS) design, the material coefficient mmay be taken equal to 1.0.Load factors:For analysis of ULS, two sets of load combinationsshall be used when combining design loads asdefined in Table 4 below. The combinationsdenoted (a) and (b) shall be considered in bothoperating and temporary conditions. The loadfactors are generally applicable for all types ofstructures, but other values may be specified in therespective object standards.Table 4 Load factors for different combinations, DNV OSC101, Ref [3]Combination of designLoad categoriesloadsEDQG(a)(b)Load categories are:G – permanent loadQ – variable functional loadE – Environmental loadD – deformation load1.31.01.31.00.71.31.01.0The code further states the following aspects whenconsidering load factor:1. When permanent loads ( G ) and variablefunctional loads ( Q ) are well defined, e.g.hydrostatic pressure, a load factor of 1.2 may beused in combination (a) for these load categories.2. If a load factor f 1.0 on G and Q loads incombination (a) results in higher design loadeffect, the load factor of 1.0 shall be used.3. Based on a safety assessment considering therisk for both human life and the environment, theload factor f for environmental loads may bereduced to 1.15 in combination (b) if the structureis unmanned during extreme environmentalconditions.3. Pile Capacity AssessmentsIn order to demostrate the differences in resultingpenetration depth requirement from differentdesign-codes considered in this paper, required pilepenetration for a given design senario was assessed
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012in accodance with the codes. Outcome of thedesign methods of codes were compared in termsof pile length required to carry an unfactored axialload of 2000kN which comprises dead load of1000kN, live load of 600kN and environmentalload of 400kN. Since the aim is to focus on the piledesign methods in codes, scour around the pile andother secondary aspects have not been consideredin the assessment.Soil data from an offshore platform location hasbeen used in the pile capacity assessments.Samples were taken from the site and the requiredsoil properties were obtained from onshorelaboratory tests. These soil data is then used forLRFD and WSD methods defined in API and DNVcodes. CPT data from offshore survey in the samelocation has been used for CPT based methodsrecommended by API. These soil data and the CPTdata are shown in Table 5 and Figure 2respectively.Table 5 Soil properties obtained from laboratory testsDepthSoilSubmerged Friction an- Undrained(m)unit weight gle (degrees) shear -15Sand1015-25Clay11.315032.53284. ResultsOpen end pile with outer diameter of 24” (610mm)and wall thickness of 19mm was considered in theanalysis. It has been assumed that the pile can beinstalled to the desired penetration depth withoutrefusal or any fatigue issues.Pile capacity was assessed from the design methods based on soil properties and the results are presented in Figure 3. Both plugged and unpluggedstates of the pile have been shown by separatecurves where appropriate. External skin frictionand the end bearing of the total pile cross sectionwere summed to evaluate the ultimate capacity ofthe pile in plugged condition. In unplugged state,internal and external skin frictions were added tothe end bearing of the pile annulus area to calculatethe ultimate capacity of the pile.Punch through efffect due to presence of weakersoil layers was considered with the depth ofinfluence zone of 2.5 times pile diameter in soilproperties based methods.Pile capacity results from CPT data based methodsare presented in Figure 4. Both unplugged andplugged pile capacity curves are plotted only forNGI-05 method. However, pipe piles are generallyplugged as stated in API, Ref [1].The required load capacity is shown by a verticalred dotted line in all cases. This load requirementis either factored or un-factored depending on design method. Safety factor of 1.5 has been used forall the CPT based design methods to evaluate theallowable pile capacity.Figure 2 Cone resistance from CPT dataIn WSD methods, safety factor of 1.5 has beenused in calculations to derive the allowable pilecapacities which are then compared with workingload of 2000kN. In API LRFD method, load factors of 1.3, 1.5 and 1.35 were used for dead load,live load and environmental load respectively andmaterial resistance factor of 0.8 has been used inline with API, Ref [2]. In DNV LRFD method,load factor of 1.3 was used for both dead load andlive load. Load factor of 0.7 was used for environmental load along with material safety factor of 1.3which is based on DNV guidelines, Ref [4]. Material safety factors in LRFD methods have been applied on load capacity of the pile.
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012(a) API WSD(b) API-LRFD(c) DNV-WSD (DNV CN 30.4 with safety factor 1.5)(d) DNV-LRFDFigure 3 Pile capacity results based on soil properties (a) API WSD (b) API LRFD (c) DNV WSD (d) DNV LRFD
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012(a) Simplified ICP-05(b)Offshore UWA-05(d)NGI-05(c) Fugro-05Figure 4 Pile capacity results based on soil properties (a) Simplified ICP-05 (b) Offshore UWA-05 (c) Fugro-05 (d) NGI-055. DiscussionThe primary difference between WSD and LRFDmethods is on how the uncertainty of loading isconsidered in the design. In LRFD, a partial safetyfactor is incorporated with each type of loading toaccount for uncertainty in the loading and partialsafety factors are used to account for materialuncertainties. On the other hand, no safety factor isconsidered for loading in WSD method; instead, acombined safety factor of larger value is consideredto evaluate the allowable capacity. Thus, different
Published in 7th International Conference Offshore Site Investigation and Geotechnics.UK. 2012loading types will not make any difference in therequired pile capacity in WSD method as only thetotal load on the pile is considered in the assessment.The minimum required penetration depth inaccordance with each design method are summarisedin Table 6. According to this table and as shown infigure 4 and 5, there is a slight difference betweenthe outcome of the assessment in accordance withdifferent methods presented in API and DNV codes.However the minimum required penetration depthfor a given load at a particular site would be singlevalue. In other words, pile driven to a certain depthhas compression capacity of a single value.Therefore the difference noticed in the results aboveis purely due to the differences in design guidelinessuch as the adopted safety factors adopted andempirical coefficients. Some design methodsincorporate conservatism in design method toovercome uncertainty in load and soil properties.Nevertheless it is difficult to point out where theconservatism is in each design approach withoutmeasurement of pile capacity from field tests, whichis beyond the scope of this paper.Among the results from methods based on soilproperties, WSD methods shows slightly lesspenetration requirement compared to LRFDmethods. As the assessment of ultimate pile capacitybeing the same among these two methods, thedifference in required pile penetration depth is due tothe difference in safety factors used in thesemethods.Table 6 Summary of minimum required penetration depthresultsDesign methodRequired penetrationdepth (m)API – WSD20.3API – LRFD21.6DNV – WSD20.3DNV – LRFD20.4CPT based - Simplified ICP-0518.4CPT based -Offshore UWA-0518.2CPT based -Fugro-0517.8CPT based -NGI-0517.6When the pile capacity assessment methods based onsoil properties and CPT data are compared, it isclearly evident that longer pile penetration isrequired if assessment is carried out based on soilproperties compared to design methods based onCPT data. The primary reason behind the differencebetween these methods can be associated with thelimits enforced on unit skin frction and end bearingin methods based on soil properties. Though, theselimits have been provided to ensure safe design, theycan be conservative compared to other designmethods. Even though CPT based design methodsresult in comparatively smaller penetration depths,design codes like API emphasise that the methodsbased on CPT data must be used only byexperieneced engineers. This caution can be duewith several aspects. The first thing is variation insoil properties at a particular site is not captured inCPT data. This will lead to failure to account for soilstrength variation in pile design. Another importatntaspect is that the CPT based design methods arerelatively new and calibration from field test islimited for these methods. On the other h
API and DNV codes describe slightly different approaches to assess the axial bearing capacity of a pile. These codes provide guidline for the calculation of pile length in common soil conditions such as clay (cohesive) or sand (cohesionless). The assessment also depends on the type of soil information available i.e. laboratory test results showing soil properties such as undrained shear .
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