PDA testing - data acquisition Obtain other information
Transcript of PDA testing - data acquisition Obtain other information
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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
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PDA Testing
Data AcquisitionData QualityData CalibrationData InterpretationData Communication
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Problems generally come frombad cablesbad sensorsbad attachmentbad radios (low battery)bad pilesbad calibrationsbad judgmentbad luck
Main problems#1
PDA testing - data quality
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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
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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
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F↓ = {F + ZV} / 2 = WD
F↑ = {F - ZV} / 2 = WU
Important
F = F↓ + F↑
V = V↓ + V↑
ZV = ZV↓ + ZV↑
ZV = F↓ - F↑
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Importance of WD and WU:
• separates out input from reflections
• allows direct interpretation of one curve (WU)
• resistance distribution
• pile damage
• wave speed
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Is this F and V data “proportional”?
PDA testing - data quality
Look at Wave Up data
Smooth gradual rise after 1st rise time marker
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Is this F and V data “proportional”?Yes?
Check with CAPWAP for resistance distribution
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Is it “proportional” ?
(steel pile)Yes, is proportional. Rise time >2L/c getting reflections from toe before T1
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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”
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Hydraulic hammers (variable stroke)“air” hammer
“diesel” hammerExternal combustion hammers
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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
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velocity above or below force prior to T1 for diesel hammers
depends on capacity
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F2 strain is loose
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Loose V2 accelerometer
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Loose V2 accelerometer
OK with one accelerometer
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F1 strain has “spike” (probable sensor or
cable failure)
BN 301
damage? damage? BN 327
Real damage has progressed
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not great
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PDA testing - data quality
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PDA testing - data quality
Smart Sensors avoid this problem
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PDA testing - data quality
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PDA testingdata quality
F1 does not return to zeroPlastic yielding
Event finished
F1
This data should NOT be used for
any analysis
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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
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PDA TestingData AcquisitionData QualityData CalibrationData InterpretationData Communication
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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
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PDA testingaccelerometer calibration
X Target Ram
“Hopkinson’s Bar” calibration
Measure:unknown acc
Compare with:known accknown strain
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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)
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PDA TestingData AcquisitionData QualityData CalibrationData Interpretation Data Communication
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R
R / 2Important
PDA testing - data interpretationShaft resistance
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Wave up gives information on Rs distribution
higher slope equals higher unit friction
Important
PDA testing - data interpretationShaft resistance
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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
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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
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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%)
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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
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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
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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
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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
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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]
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PDA testing - data interpretation
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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
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Applications of dynamic testing
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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”
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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)
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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
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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)
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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
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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
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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.)
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PDA TestingData AcquisitionData QualityData CalibrationData InterpretationData Communication
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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
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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 )
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Reports - ASTM D4945
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Reports - ASTM D4945“ PDIPLOT ” program
Very useful tool !
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Reports - ASTM D4945
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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
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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
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Thank you for your attention
Ryan Allin, P.E.
Pile Dynamics, Inc.
www.pile.com