PDA testing - data acquisition Obtain other information

Pile name, project name (if BOR, EOD date)Hammer description (name, weight, drop H) Pile material strength (e.g. f’c, Fy)Capacity requirements (ultimate or design)Unusual occurrencesSoilsSet per blow (or blow count)Static load test results (and date), if any

PDA testing - data acquisitionObtain other information

PDA Testing

Data AcquisitionData QualityData CalibrationData InterpretationData Communication

Problems generally come frombad cablesbad sensorsbad attachmentbad radios (low battery)bad pilesbad calibrationsbad judgmentbad luck

Main problems#1

PDA testing - data quality

How to spot Problems?Data not proportional

calibrations?

Wavespeed? ( ε = v/c )pile non-uniform?transducers near ground?diesel hammer? (precompression)long rise time? (soft cushion)

Data not consistent blow to blowResults not reasonableView PDA “warnings” & individual signals

Important

Proportionality of Strain or FT1 with VT1

(for downward going waves)

ε = v / c (for upward going waves)

ε = - v / c Impedance “EA/c” = ρ c A = Mc/L = “Z”

Important


F↓ = {F + ZV} / 2 = WD

F↑ = {F - ZV} / 2 = WU

Important

F = F↓ + F↑

V = V↓ + V↑

ZV = ZV↓ + ZV↑

ZV = F↓ - F↑


Importance of WD and WU:

• separates out input from reflections

• allows direct interpretation of one curve (WU)

• resistance distribution

• pile damage

• wave speed


Is this F and V data “proportional”?


Look at Wave Up data

Smooth gradual rise after 1st rise time marker

Is this F and V data “proportional”?Yes?

Check with CAPWAP for resistance distribution


Is it “proportional” ?

(steel pile)Yes, is proportional. Rise time >2L/c getting reflections from toe before T1


Which is proportional ?

NO

“Step” in Wave Up is wrong

YES – rapid rise of F and V at time T1 are similar.

No “step” in Wave Up”

Hydraulic hammers (variable stroke)“air” hammer

“diesel” hammerExternal combustion hammers

40 msec pretrigger buffer

10 times scale

ECH HAMMER (air, drop, hydraulic)

DIESEL HAMMER

PAX, 8G have user selected pretrigger buffer for diesels. PAK has fixed 20ms buffer

for all hammers

35+ msec pretriggerbuffer is recommended

for diesel hammers

velocity above or below force prior to T1 for diesel hammers

depends on capacity

F2 strain is loose


Loose V2 accelerometer


Loose V2 accelerometer

OK with one accelerometer


F1 strain has “spike” (probable sensor or

cable failure)

BN 301

damage? damage? BN 327

Real damage has progressed

not great




Smart Sensors avoid this problem


PDA testingdata quality

F1 does not return to zeroPlastic yielding

Event finished

F1

This data should NOT be used for

any analysis

How to fix Problems?Replace sensorsReplace cablesRepair defective sensors and cables

Make sure attachment is correctLoose bolt or anchors?Alignment/Orientation along pile axis?Always 2 strains to compensate bendingProper instructions to crew


PDA TestingData AcquisitionData QualityData CalibrationData InterpretationData Communication

PDA testingstrain calibration

Accuracy to about 1 or 2 % study says must tighten to 60 inch-pounds (6.7 N-m)

traceable to calibrated micrometer

PDA testingaccelerometer calibration

X Target Ram

“Hopkinson’s Bar” calibration

Measure:unknown acc

Compare with:known accknown strain

PDA testingcalibration summary

• Strain accuracy to within 2 %

• Accelerometer accuracy to within 2%(NIST shock calibration to only 5%)

•Therefore, minor adjustments to your data for CAPWAP are reasonable and justifiable.

• Recalibrate old sensors (ASTM D4945 recommends every 2 years)

PDA TestingData AcquisitionData QualityData CalibrationData Interpretation Data Communication

R

R / 2Important

PDA testing - data interpretationShaft resistance

Wave up gives information on Rs distribution

higher slope equals higher unit friction

Important

PDA testing - data interpretationShaft resistance

Do not interpret unless data is good(garbage in, garbage out)

Make sure inputs are correct (LE, AR ..)Wrong WS

(3800 m/s)

proportionality?Arrival time?

Correct WS (3300 m/s)

• Wavespeed and Modulus are critical for proper interpretation of concrete piles

PDA testing - data interpretation

Most Sites Have “Set-up” (defined as capacity gain with time)

• Caused by reduced effective stresses in soil due to pile driving (temporary)– Pore pressure effects (clay: log time)– Arching (sand: linear time)

• “Cookie cutters” (pile points, end plates: any soil)– Soil structure (cemented: linear time)

• Measure it by Dynamic Tests on Both End of Drive and Restrike

0.001 0.01 0.1 1 10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2Aucilla, Dynamic TestAucilla, Static Test

A = (mS / QS0) = 0.30R2 = 0.99

1 min

15 min

60 min

Elapsed Time Ratio, ( t / t0 ) with t0 = 1 day

Pile

Sid

e Sh

ear R

atio

, (Q

s / Q

s0 )

Pile Setup Example18” PSC, O’Cell at bottom Side in clay and silty clay in FL

+30%

+30%

+30%

Σ = +90% in 1 day(or 9x EOD capacity)

EOD Capacity plotted at ~1 min

Bullock, Schmertmann, McVay, Townsend. “Side Shear Setup”, ASCE Geotechnical Journal March 2005

1-28d +43% orabout half ofEOD-1d change(1-7d – 25%)(1-3d – 14%)

Capacity versus time

Important to restrike driven pilesBetter correlation with SLTMore economic result

Early restrikes (15 min to a few hours) are useful to establish the set-up trend

(particularly for clay soils)

Must wait for concrete to cure for drilled / bored piles

Aug 11 EOD

Sept 16

24 x 0.5 inch c.e.pipe, ICE 120S

St. John’s River Bridge – $20 million saved

25 bpf

150 bpf

Ref: Scales & Wolcott, FDOT, presentation at PDCA Roundtable Orlando 2004

ST Johns River Bridge

increased loads by 33% with substantially shorter piles

(set-up considered)

Total project:

• $130 million (estimate)

• $110 million (actual)

• $20 million (savings)

Ref: Scales & Wolcott, FDOT, presentation at PDCA Roundtable Orlando 2004

Dynamic Pile Testing

– Pile stresses– Pile integrity– Hammer performance

Last three items detect or prevent problems;

discussed in later session

For each blow determine– Capacity

at time of testing

Codes: Allowable Driving StressesUSA (AASHTO)

Steel piles90% of yield strength for steel

Timber pilesSouthern Pine 3.2 ksi 19 MPaDouglas Fir 3.5 ksi 21 MPa

Concrete pilesCompression: (85% f’c) - prestressTension : prestress + (50% of t.strength)

fpe + 3 sqrt (f’c) [f’c, fpe in psi]fpe + 0.25 sqrt (f’c) [f’c, fpe in MPa]


Integrity - BTA method for uniform pilesLooks for local decrease in wave-upβ < 80 major damageβ < 60 complete break

Possible false causespurposely non-uniform pilebending, “noise”, phase (VT)wrong 2L/c (2 x length / wavespeed)

ALWAYS confirm by visually looking at dataCapacity is meaningless for broken piles

top

toe

brea

k


Applications of dynamic testing

PDA Testing – save money, reduce riskLarge Projects• Pre-bid special test program optimizes design

• Pile length and size, pile type• Early production pile tests (different hammer?)

• Establish driving criteria• Evaluate hammer and procedures

• Periodic production pile tests• Monitors hammer consistency• Evaluate site variability

• Evaluate “problem piles”Small Projects• Test early production piles

• Establish driving criteria• Evaluate hammer and procedures• Evaluate site variability

• Evaluate “problem piles”

8

• Static testing

• Dynamic Testing

• Wave Equation

• Dynamic Formula

• Static Analysis

Better verification methods (i.e. testing) results in lower S.F.

and therefore less cost.

Testing reduces uncertainty in capacity

Testing(measurements)

no testing(assumptions)

Design Concepts •Allowable Stress Design

•Qd < Ru / F.S. (F.S. = Factor of Safety)•σ < σa; σa = σy/F.S.

•Load and Resistance Factor Design (LRFD)•φ Ru > fDLD + fLLL + fiLi + …•φ σy > (1/A){fDLD + fLLL + fiLi + …}

Factor of Safety split into Load and Resistance factors F.S. ~ L / φ)

245

Total load to support (D + L) 40,000 kNFactored load = (1.25 D + 1.75 L) = 55,000 kN (D/L = 3)Ultimate capacity per pile 2,000 kNLRFD: Ult. capacity * Φ ≤ Factored load

Determination method Φ EquivF.S.

Factored resistance kN / pile

# piles required

Gates formula 0.40 3.44 800 69Wave equation 0.50 2.75 1000 55Dynamic test (min.2% or 2#) 0.65 2.12 1,300 43Static or 100% Dynamic test 0.75 1.83 1,500 37Static and >2% Dynamic test 0.80 1.72 1,600 35

Benefits of Testing - (AASHTO ASD – after 2007)

PDA Testing 2% of pilesAASHTO • Resistance factor 0.65 • Equivalent F.S. 2.12 (assumes D/L = 3)

Ohio DOT• Resistance factor 0.70 • Equivalent F.S. 1.96 (still very reasonable)

• Many private sector projects designed with F.S. 2.0• e.g. IBC uses 2.0

• Dynamic tests statistically conservative• More dynamic tests – better site coverage

Year Driven Pile Cost Testing Cost2005 $10,705,041 $305,9212006 $18,836,927 $313,3152007 $15,948,151 $379,7502008 $26,591,945 $587,8822009 $25,308,605 $450,8632010 $26,211,622 $518,557Total $123,602,290 $2,556,288

$2.5M / $123.6 M = 2%Test / driven pile cost

Peter NarsavageOhio Dept of Transportation2011 PDCA seminar, Orlando

Method AASHTO PHI(LRFD)

Relative cost of piles Savings

Formula (Gates) 0.40 1.00 0%

Wave Equation 0.50 0.80 20%

2% PDA 0.65 0.62 38%

2 # PDA Ohio DOT 0.70 0.57 43%100%PDA or SLT 0.75 0.53 47%

PDA + SLT 0.80 0.50 50%

Savings $92,700,000

Advantages of dynamic testing• More information faster

• Supplement or replace static tests• Capacity & distribution from CAPWAP• Detect bad hammers• Know driving stresses (compression-tension)

• Develop better installation procedures• Detect pile damage; Lowers risk of failures

• Lower overall foundation costs• Main cost of foundation is the piles• Optimize foundation design• Testing saves much more than it costs

• More favorable resistance factors (S.F.)

PDA TestingData AcquisitionData QualityData CalibrationData InterpretationData Communication

Do not leave site until sure you have good and sufficient data

Some results usually given on site(requires little interpretation)

DamageStressesEnergy

Communicate capacity only when sure, or at least indicate result is preliminary

( do CAPWAP in field ? )

PDA testingdata communication

Reports - ASTM D4945• Graph of data ( F & V vs. time (PDA or CAPWAP)

• Each pile ( EOD and/or BOR )• Summary of Results ( from PDIPLOT )

• Graphical and/or Tabular ( with Statistics )• CAPWAP results ( plots and tables )• Descriptions

• Pile, Hammer, Soils, Goals of test • Results, Conclusions, or Recommendations

• Stresses, Integrity, Energy, Capacity• Driving Criteria ( if requested )

Reports - ASTM D4945

Reports - ASTM D4945“ PDIPLOT ” program

Very useful tool !

Reports - ASTM D4945

Summary: Good PDA Test• Safety• Cooperation (on time, think ahead, courteous)• Quality data (ASTM D4945)• Correct inputs (AR, WS, LE..)• Correct interpretation (including CAPWAP)• Report (ASTM D4945)

– Collect site info (soils, BLCT, SLT, dates)– Curves, PDIPLOT, CAPWAP, narrative– Submitted promptly

Likins & Rausche, Sept. 2008. “What Constitutes a Good PDA Test?”, Proc, of the Eighth Int’l Conf. on the Application of Stress Wave Theory to Piles,

Lisbon, Portugal

www.pile.comGreat resource for PDA users•Seminar/workshop announcements•Reference papers•Sample project specifications•Equipment specifications•Brochures•PDI newsletter archive

(brief interesting technical articles)•Contact information•FAQ section

Thank you for your attention

Ryan Allin, P.E.

[email protected]

Pile Dynamics, Inc.

www.pile.com

mailto:[email protected]

http://www.pile.com/

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PDA testing - data acquisition Obtain other information