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When designing a founda-tion system, engineershave many choices,

including the ultimate load per

pile and pile size (type, length anddiameter). The ultimate pile capac-ity must exceed the applied loads bya sufficient margin or the founda-tion will have unacceptable settle-ments. The required pile capacityalso depends on the test method forverification of the pile capacity andthe frequency of testing. This papercompares safety factors contained inthe PDCAs Recommended DesignSpecifications for Driven BearingPiles with factors from other pub-

lished codes.Safety factors are assigned to

lower the risk of foundation failure.They compensate for uncertainties

from unknown loads or loading con-ditions, from site variations and frominaccuracies in load determinationmethods. Additional geotechnical

considerations like consolidationin compressible layers and negativefriction are beyond the scope of thisdiscussion. Statistical methods canbe employed to assess risk. Thesestatistical methods form the basis forthe safety factors proposed by mod-ern codes such as the PDCA code.

Safety factors are either (a)global for allowable stress designs(ASD), or (b) partial for load andresistance factor (LRFD) designs. Inallowable stress design, the ultimate

pile capacity is divided by a globalsafety factor to find the allowableor working load on the pile. Thusall uncertainty is lumped into thissingle factor.

LRFD design recognizes that dif-ferent loading conditions have differ-ent uncertainty and therefore assignsdifferent applied load factors. Forexample, the structures dead weightis known while the applied live load-ings due to wind, earthquake or tem-porary loads can be highly variable.

Thus, load factors for dead weightsare lower than for the less certain

Safety Factorsfor Pile Test ingBy Garland Likins

34

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live loads. LRFD methods assign dif-ferent strength factors (often calledresistance factors with values lessthan unity) which relate to the verifi-cation procedure reliability. The gen-eral expression for LRFD design is

iQi k Rk

Where i

is the load factor forthe load Q

iof the ith load type (e.g.

1might be 1.4 for the dead load Q

1,

and2might be 1.7 for the live load

Q2), and

kis the resistance factor

for the resistanceRkfor the kth limit

state (e.g. might be 0.80 for a static

load test R on 1% of the piles). Inconcept, for a given set of load and

resistance factors, an equivalentglobal safety factor can be calculatedfrom the load factor divided by the

resistance factor (e.g. in the aboveexamples, the equivalent globalsafety factor is 1.94 for a 50% deadload situation). Further mentionof LRFD in this article will usecomputed equivalent global factors.

The risk of foundation failuresmakes capacity evaluation necessary.Logically, less testing increases the

risk of a failed foundation, whilemore testing reduces risk. Similarly,more accurate test methods reducerisk, while less accurate methodsincrease risk. The goal is anacceptably low probability of failure.Piles can potentially fail either dueto structural failure or geotechnicalfailure (e.g. soil strength). Generally,driven piles rarely fail structurally(drilled or augered piles have a higherprobability of structural failure and

thus usually have higher associatedsafety factors, or lower factors on

the structural strength conditions).Static Load Testing has

traditionally been the standard forevaluating soil strength and ultimatepile capacity. Prior to about 1970, pileswere loaded using a slow maintainedload procedure over several days totwice the design load, as specifiedin ASTM D1143. Generally, onlyone static test was performed persite and these proof tests rarely

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failed. The traditional safety factorof 2.0 was thus established becauseof this loading to only twice design,

even though actual safety factorswere larger since the pile did not fail.Common failure load evaluationswere determined by some pile topmovement limit (typically 0.75 to1.5 inches), or a net movement limit(typically 0.25 to 0.75 inches) afterload removal. Due to recent emphasisby the FHWA, the quick procedurestatic test method detailed in A STMD1143 is becoming common, theevaluation for failure or ultimate usesthe offset yield line method, and theloads are often carried to failure or toat least three times design in a testtaking only a few hours. The PDCAcode follows this guidance.

When the ultimate failure loadcan be determined, rather than onlya proof load, foundation costs canbe potentially reduced. For largeprojects, special preconstruction testprograms are effective. Fewer pilesare required when higher loads areproven, or shorter piles can be used.

For smaller projects, the first produc-tion piles serve as test piles andsome driving criteria adjustment and

cost savings are possible if the pilescan be shortened. Production pilesare driven to the test pile criteria.

However, it is not practical tostatically test every pile because oftime and cost constraints. Therefore,static testing is usually limited to avery small sample of piles on any site(typically 1% or less on large proj-ects, or often only one per site, ifany, for small projects).

When static testing is performedproperly, the measuring accuracyshould be within 20% of the truevalue. The reliability of results isimproved if a recently calibrated loadcell is specified. However, interpreta-tion of the resulting load-settlementgraph can give several different ulti-mate loads depending on the evalu-ation method (e.g. Davisson, Chin,Butler-Hoy, double tangent, slope,D/10, etc).

In the extreme case where everypile is tested with a very accurate

Notes

a dynamic testing requires signal matching

b requires correlating static test

c dynamic formula for sands only - not clays

d 1%static or >3%dynamic

f higher SF if 3%static, and extensive

site investigation with careful construction control

h depends on pile type, site variability, load conditions, etc.

j not specifically addressed

NA - not applicable

NR - not recommended

Global Safety Factors - Allowable Stress Design ValuesAustralia

Code PDCA AASHTO IBC 2000 AS2159-95 ASCE (20-96) for driven pile types ASCEyear 2001 1992 2000 1995 1996 (non-driven piles)

design loads >40T 16 to 40 T 40 to 100T >100T >100T

static analysis 3.50 3.50 6.00 2.12 to 3.44 NA NA NA NA

notes:

dynamic formula 3.50 3.50 NA 2.50 to 3.06 2.0 to 2.4 NR NR NAnotes: c h

wave equation 2.50 2.75 NA 2.50 to 3.06 1.8 to 2.2 1.9 to 2.3 NR NAnotes: h h

dynamic testing 1.9 to 2.1 2.25 2.00 1.72 to 2.12 1.6 to 2.0 1.7 to 2.0 2.0 to 2.4 2.6 to 3.6notes: a b a, f, g h h h h

static testing 1.8 to 2.0 2.00 2.00 1.53 to 1.93 1.5 to 1.8 1.6 to 1.9 1.8 to 2.2 2.3 to 3.2notes: d f, g h h h hstatic & dynamic 1.65 to 1.9 1.90

notes: a, b, e j j j j j j

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method (e.g. static load test) witha conservative failure definition,the safety factor can be significantlyreduced because the risk is reduced.

The offset yield line criteria recom-mended by the PDCA code is amongthe most conservative of failure cri-teria and thus justifies lower safetyfactors.

The PDCA code awards lowersafety factors for testing more piles,because the uncertainty is reduced.For testing only 0.5% of the piles, asafety factor of 2.0 is suggested, whileif 5% of the piles are tested, then thesafety factor can be reduced to 1.65.Piles are selected so site variability isadequately addressed, and adequatehammer performance is periodicallyverified. Lower safety factors meansthe pile load can be increased, result-

ing in fewer piles, or that the drivingcriteria can be relaxed, thus reduc-ing production pile installation timeand costs. The extra testing costs aremore than compensated by reducedfoundation costs.

Dynamic Pile Testing is a rou-tine pile capacity evaluation meth-od. Dynamic testing requires mea-suring pile force and velocity duringhammer impact and subjecting thisdata to a signal matching analy-sis to determine the soil behavior.

Extensive correlations between staticand dynamic testing have verifiedthe methods reliability. A fter cor-relating the static and dynamic tests,the PDCA code allows substitutionof three dynamic tests for one statictest in determining the quantity offurther testing. Thus, with at leastone successful correlation, then thePDCA suggested 5% static testingcan be translated into testing 15%of the piles dynamically, for thesame suggested safety factor of 1.65.

The large number of tests allows sitevariability and hammer performanceconsistency to be properly assessed.

In many cases, dynamic pile test-ing has completely replaced statictesting. In this case, no site-specificcorrelation is established and thusthere is a higher risk, since the cor-relation depends upon past experi-ence of the signal matching analysisaccuracy. This extra risk requiresan increased safety factor compared

with static testing methods. In thiscase, the safety factor can vary from2.1 with only 2% of the piles testeddynamically down to 1.9 when atleast 10% of the piles are testeddynamically.

To obtain a reliable ultimatecapacity from dynamic pile testing,some very basic guidelines must be

followed. The hammer input must besufficiently large to produce a mini-mum set per blow so the soil is load-ed plastically and thus mobilizes thefull soil strength. In cases where theset per blow is very small (e.g. largeblow count), the dynamic pile testwill only activate a portion of the fullsoil strength and thus will under-pre-dict the true ultimate capacity (thisis analogous to a static proof test),so the result is conservative. Finally,

the pile capacity often changeswith time after installation (usuallyincreases due to setup, although insome cases reduction due to relax-ation are found). To measure timedependent capacity effects, the pileshould be tested by restrike after anappropriate waiting time. Restriketests are recommended standardpractice for capacity evaluation bydynamic pile testing.

Dynamic testing provides otherbenefits. Dynamic pile testing pro-

vides valuable additional informa-tion on driving stresses which, if toolarge, can result in pile damage. Pileintegrity can be evaluated dynami-cally for both location and extentof damage, if any. Proper hammerperformance is extremely importantfor driven piles because engineersrely on the blow count (or set perblow) as a driving criteria for pileacceptance, thus implicitly assum-ing that the hammer is performingproperly. By monitoring periodicallythroughout larger projects, it can beassured that the hammer is perform-ing properly and consistently duringthe entire project so that the sameinitial driving criteria can be used forall piles with confidence. Periodictesting can check site variability andinvestigate the cause of piles that aretoo short or too long or that haveunusual blow count records to deter-mine if the cause is the hammer orthe pile or the soil. These guidelines

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for checking site variability and peri-odic hammer verifications are men-tioned in the PDCA code.

Wave Equation Analysis isa computer simulation of the piledriving process. A numerical modelis constructed for the hammer, forthe pile, and for the soil. Numerousassumptions are made, such as ham-

mer performance and soil responsebehavior. A ssumed ultimate capaci-ties are entered, a one dimensionalwave propagation analysis is made,and the resulting blow counts arepredicted. A series of assumed resis-tances and associated predicted blowcounts produce a bearing graph toestablish a suggested driving criteria.However, because of the increaseduncertainty associated with theassumptions, the risk is increased and

thus the safety factor in the PDCAcode is suggested as 2.5.Dynamic Formula were devel-

oped over 100 years ago to estimatepile capacity by simple energy con-siderations. Some engineers still use

them today to make a preliminaryselection of hammer size. However,these methods are very simplistic.Numerous studies have concludedthat their prediction accuracy is poorand, to minimize risk, large safetyfactors are necessary. The standardENR formula, for example, has abuilt-in safety factor of 6. Recent

studies have shown that the Gatesformula is statistically the best forprediction. The Gates formula is theonly formula currently recognizedby the PDCA, AASHTO, and theFHWA (although FHWA stronglyrecommends that dynamic formulabe replaced by wave equation analy-sis). Since accuracy is relatively poorand risk increased, the recommendedsafety factor by PDCA for the Gatesformula is 3.5.

Static Analysis estimates pilecapacity from soil strength estimatesobtained from site soil investigations.Numerous correlations and empiricalcorrection factors for soil strengthwere developed for SPT, CPT, or

other soil sampling tools. However,there generally is considerable scat-ter of strength prediction results andlocal experience does not transfer todiffering conditions or differing sam-pling methods. Numerous predictionevents have demonstrated that suchpredictions are generally highly inac-curate. Because of large inherent risk

due to poor prediction accuracy, thePDCA code requires a safety factorof 3.5 for piles installed to a staticanalysis criteria only for an accept-able level of risk.

Comparison of the PDCA Codewith Other Codes is summarized inthe accompanying table and providesan interesting platform for discus-sion. The PDCA code origin startedwith the AASHTO Standard ASDcode from 1992.

AASHTO (American Associ-ation of State Highway andTransportation Officials) representsthe 50 state highway departmentsplus the FHWA. Subsequently,AASHTO is moving toward LRFD

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but the result is still under development. Because of simi-larities of origin, factors for static analysis and dynamicformula are identical to the PDCA code. AASHTOrecognizes that wave equation analysis is more reli-able than dynamic formula so the safety factor is setat 2.75. Dynamic testing does not specifically mentionsignal matching and thus may partially account forthe relatively high factor 2.25 for dynamic testing. Statictesting alone has the traditional standard factor of 2.0.

Testing both statically and dynamically results in a lowersafety factor of 1.9. Generally, the AASHTO code doesnot address the amount of testing to be performed.

IBC 2000, from the International Building Code, isan effort of the three USA regional building codes to forma single national code. The foundation section comesoriginally from the Southern Building Code which has itsbase from the 1940s with an update in 1982 to cover a fewnew technology items missing from the original code(e.g. prestressed piles, et al), but nothing new relating tosafety factors. The IBC did provides for dynamic pile test-ing (as per ASTM D4945) as a new inclusion of this new

code. This SBC code is obviously the oldest and gener-ally reflects older practice requirements. For piles withdesign loads under 40 tons, capacity is determined by anapproved driving formula or by static analysis, with noload testing required. The static analysis uses either a soilsinvestigation or a safety factor of 6 referenced to a chart

of conservative soil strengths.For loads of 40 tons or higher,wave equation analysis is speci-fied to estimate the driving crite-ria, and the load is to be verifiedby either static or dynamic test-ing (dynamic testing in ASTMD4945 indirectly implies at leastone correlating static test).

In contrast to IBC 2000, theAustralian Code AS2159-1995is perhaps one of the most pro-gressive in the world. AS2159is an LRFD code and the globalfactors shown here for compari-son are computed from an equalweighting of live and dead loads(having 1.5 and 1.25 load fac-tors respectively). The rangeof safety factors in the code isgiven with some guidance by the

code. The dynamic formula fac-tors are to be applied to sandysoils only; dynamic formula areprohibited for clay soils. Factorsfor static analysis are based onthe soil exploration method(e.g. SPT or CPT; CPT meth-ods are given higher confidenceand thus lower safety factors).

The dynamic testing factorsrequire signal matching. Lowersafety factors for dynamic testingrequire at least 15% of the piles

to be dynamically tested (and also comprehensive siteinvestigations and careful construction control), whilehigher factors result when less than 3% of the piles aredynamically tested. The lowest static testing safety factorscome from statically testing more than 3% of the piles,while higher factors apply when less than 1% of the pilesare statically tested.

The ASCE 20-96 is the Standard Guidelines for the

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Design and Installation of PileFoundations. This code is quitedifferent from others in that thesafety factor is defined by threeparts (capacity determinationmethod, design axial load levels,and structural pile type). Thecapacity determination methodis the only common criteria

with other codes. The latter twocriteria have come under somerecent criticism (ProposedOverhaul of Deep FoundationProvisions of the InternationalBuilding Code by Len Cobb,presented at the ASCE Geo-Institute Deep Foundations 2002Conference, Feb 2002). Becauseof more structural uncertainty,this code requires significantlyhigher safety factors for non-

driven piles. Determination ofcapacity solely on static analy-sis is not permitted. Except forlightly loaded piles, dynamicformula are not recommendedand no factors are even sug-gested (factors for lightly loadedpiles are unrealistically small forthe associated risk). The factorsfor dynamic and static testingare generally similar to PDCAvalues for lower pile loads, butthe factors are higher than the

PDCA values for piles with design loads of 40 tons ormore. (This code is currently in a revision process andsafety factors are likely to be reduced for the higher loadcases).

As a common practice, static analysis methods aregenerally only used to estimate pile lengths in the designprocess. Rarely are pile installations governed by thismethod, so whether a code has a factor or not for static

analysis is almost a non-issue. Dynamic formula are alsodecreasing in usage. They remain mainly a tool for pre-liminary hammer selection. In most cases, actual use ofdynamic formula to govern pile installation are perhapslimited to light design loads. From a practical view, awave equation analysis is almost as fast and simple asa dynamic formula. Generally, some other more precisemethod (wave equation, dynamic testing, or static test-ing) is also specified on most projects, particularly proj-

ects with design loads above 40 tons, so the lower safetyfactor and improved reliability of the more accuratemethods would then govern the project anyway.

In summary, keeping the risk of foundation failurebelow an acceptable level is the goal for any founda-tion. To accomplish this, safety factors are applied tothe ultimate pile capacity to calculate an acceptabledesign or working load for the piles. The risk of failurecan be reduced by testing more piles, or using evaluationmethods that are more accurate. A reduced risk of failure

justifies lower safety factors. The safety factors recom-mended by several newer codes generally give a range

of safety factors depending on the type and amount oftesting performed on site, and result in factors less thanthe traditional factor of 2.0. These more modern testingmethods, combined with a higher frequency of testingand the resulting lower safety factors, can reduce thetotal foundation costs.

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