Download - Spt energy calibration frank rausche
4/30/2015
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SPT
SPT Energy Measurements
... or how to calibrate SPT equipment to obtain normalized SPT N-values
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SPT
SPT Energy Measurements
Outline Introduction
Instrumentation
Processing Equipment
Examples
Summary
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SPT
Introduction
• 1902 Charles Gow of Gow Construction (Boston) used 1 inch dia. drive samplers driven by 110-lb hammer
• mid 1920’s split spoon sampler introduced by Sprague & Henwood of Scranton PA (2.0 to 3.5 inch diameters)
• 1927 Gow used 2 inch split spoon sampler, recording blows to drive 12 inches for 140 lb hammer and 30 inch drop
• 1947 Terzaghi christened the Raymond Sampler as the “Standard Penetration Test” at 7th Conf. on Soil Mechanics and Foundation Eng.
• 1948 Terzaghi and Peck publish first SPT correlations
• 1958 ASTM adopted ASTM D1586
Ref: “Subsurface Exploration Using the Standard Penetration Test and the Cone Penetration Test” by David Rogers; Environmental & Engineering Geoscience, Vol XII No.2, May 2006.
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SPT
Introduction
SPT equipment has standard ram weight and drop height and, therefore, supposedly the same rated energy: ER = Wh
With W = 140 lbs and h = 2.5 ft we get ER-SPT = 350 ft-lbs
We can measure EMX, the energy transferred to the drive rod
EMX values range from 30 to 95%
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SPT
Introduction
Historically and on average, transferred energy, EMX, has been 60% (typical for safety hammers with cathead and rope)
In order to maintain context with data bases, N-values should be adjusted based on measured transferred energy EMX (see ASTM 4633-05) to the expected value of 60% of ER-SPT
N60 = N * (EMX / 0.6 ER )
0.6 ER = 210 ft-lbs
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• N-value for
Soil strength, E, G, …
Liquefaction potential
• Soil Type from sample
Grain size
Why SPT?
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“Standard” Penetration Testing“Non-standard” variables“Standard” Penetration Testing“Non-standard” variables
Hammers Safety
Cathead-rope
Cathead diameter
Automatic Spooling Winch
Chain Driven
Donut
Operators Experienced
Non-Experienced
Concerned
Negligent
• Drill Rods• Size
• Shape
• Length
• Drill Methods• Hollow Stem Augers
• Drilling Fluids
• Split Tube Sampler• Shape
• Liners
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SPT
SPT Equipment is not standard
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Donut hammers: EMX as low as 30% of Er-SPT
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SPT
SPT Equipment is not standard
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Safety hammers typicall 60%, automatic hammers 80 to 90% ofEr-SPT
Standardization of SPT N-Value
“Non-standard” SPT systems deliver highly variable energy values to drive rod. Energy transfer affects N - value
Soil strength estimated from N-value based on experience, i.e. on average N-value
Obtain normalized, N60, value for more reliable static soil analysis
Also: Liquefaction potential estimated from N60 (ASTM D 6066)
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Normalized N-Value: N60
N60 = Nm
EMX
Wh (60%)
Nm, measured N-value
EMX, measured transferred energy
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What Energy?
Potential, Wrh Measure weight, Wr (0.140 kips or 0.623 kN)
Estimate stroke, h (2.5 ft or 0.762 m)
Potential Energy, Wrh (0.350 ft-kips or 0.474 kJ)
Kinetic, ½(Wr/g) vi2
Measure vi with HPA
vi = √(2 g h) (8.96 ft/s or 2.73 m/s)
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WP
mR
hWRvi
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SPT
Transferred EnergyTransferred Energy
Energy = Sum of Force times Displacement
ER(t) = ∫ F du; but v = du/dt
EFV(t) = ∫ Fv dt; transferred energy
EMX = max[EFV(t)]
ηT = EMX / ER-SPT ; transfer ratio
Energy = Sum of Force times Displacement
ER(t) = ∫ F du; but v = du/dt
EFV(t) = ∫ Fv dt; transferred energy
EMX = max[EFV(t)]
ηT = EMX / ER-SPT ; transfer ratio
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F,v
WRvi
SPT
ASTM D4633 – earlier versionsASTM D4633 – earlier versions
Since
EFV = ∫ F v dt and
F = Z v (in a downward traveling wave)
Z = EA/c ... Pile impedance; E ... Young’s modulus,
A ... Cross sectional area; c ... Stress wave speed
Then
EF2 = Z ∫ F 2 dt (only requires force measurement)
But ONLY if there are no forces due to wave reflections; thus, this method is inherently incorrect and obsolete!
Since
EFV = ∫ F v dt and
F = Z v (in a downward traveling wave)
Z = EA/c ... Pile impedance; E ... Young’s modulus,
A ... Cross sectional area; c ... Stress wave speed
Then
EF2 = Z ∫ F 2 dt (only requires force measurement)
But ONLY if there are no forces due to wave reflections; thus, this method is inherently incorrect and obsolete!
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EF2 = 209 N-m
η = 44%
EFV = 0.281 N-mη = 59%
Safety Hammer, Cathead, PE = 0.475 kN-m
EF2 Short L corrections
EF2corr = EF2(1.17)(1.45)(1/1.36)
= 260 N-m (η = 55% )
1.17 due to energy in rod above sensors
1.45 due to short rod length1.36 due to c ratio
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ASTM D4633 – earlier versionsASTM D4633 – earlier versions
Loose Joint Effects
EMX = .232 k-ft
η = 66%
EF2 = .146 k-ft
Safety Hammer with Cathead on AW rodSPT 16
Second loose joint(BTA = 30%)
First loose joint
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SPT
•Choose rod section matching the rod used during test
•Attach strain gages for 2 full bridge strain circuits and 2 accelerometers
•Needs PR accelerometers
•Cancel bending effects and provide backup measurements
•Perform traceable calibration
Measuring F and v
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SPT
Instrumentation
Instrumented section with calibration tag
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Calibration of Force Sensors
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Force Measurement
Strain Measurement
SPT
Pile Driving Analyzer® - Model PAK
Processing Equipment
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SPT
Pile Driving Analyzer - Model PAX
Processing Equipment
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SPT
SPT Analyzer
Processing Equipment
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SPT
Pile Driving Analyzer - Model PAX
Processing Equipment
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SPT
SPT hammers are uncushioned which requires special accelerometers and some higher frequency data processing.
ASTM 4633 requires digitizing frequency
• ≥ 20,000 sps for analog integration
• ≥ 50,000 sps for digital integration
EC7 requires digitizing frequency
• ≥ 100,000 sps for digital integration
May require special software in PDA or an SPT Analyzer
Processing Equipment
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SPT
Example: Spooling Winch on AW RodExample: Spooling Winch on AW Rod
EMX = .135 k-ft
η = 39%
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SPT
Safety Hammer + Cathead on AW Rod with Loose Joint
Safety Hammer + Cathead on AW Rod with Loose Joint
EMX = .232 k-ft
η = .232/.35 = 66%
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Florida DOT SPT Energy Study
“Standard Penetration Test Energy Calibrations”
performed by University of Florida, Gainesville by Dr. John Davidson,
assisted by John Maultsby and Kimberly Spoor
report issued January 31, 1999 report number WPI 0510859
contract number BB-261
Florida state project 99700-3557-119
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58 SPT Hammers tested with SPT Analyzer 44 Safety Hammers
14 Automatic hammers
13 Different drill rig Acker (1)
Florida DOT SPT Energy Study
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SPT
Florida SPT Energy results
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Note Scatter!
Florida DOT SPT Energy Study
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SPT
Utah State University StudyUtah State University Study
Hammer Type
EFV avg %
C.O.V. %
One std Two std
%
Samples
Cathead-rope 63 12 55 – 71 47 – 79 15
CME automatic 75 9 67 – 83 59 – 91 10
Spooling winch 35 8
31 – 39 3 Hydraulic auto 69 15
59 – 79 5 Donut 43 22
34 – 52 3 Other Auto 49 13
42 - 56 6
GRL data compiled by Utah State University
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Comparison of Studies
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Energy similar with 1.25 to 2.25 rope turns on cathead
Extra 10% energy loss for 2.75 rope turns; should be avoided (per ASTM D1586)
Rod type no major effect in energy transfer (AW or NW)
Conclusions from Florida DOT SPT Energy Study
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Energy higher for automatic hammers (80%) than for safety hammers (66%)
Short rods (<40’) have lower energy transfer
SPT energy data is “useful in spotting performance problems of a system”
Conclusions from Florida DOT SPT Energy Study
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“SPT Analyzer may be useful in assessing sites where data appear suspect”
“On large or critical projects, energy testing may verify SPT performance to allow for increased design confidence and economy”
Conclusions from Florida DOT SPT Energy Study
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Significance
Assume measured Nm = 20
Automatic Hammer (assume 80% efficient)
N60 = 20 (80/60) = 27
Donut Hammer (assume 35% efficient)
N60 = 20 (35/60) = 12
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SPT
SUMMARY
SPT rigs and rods are not truly standardized and transferred energy values vary greatly
Energy is important quantity when assessing strength of soil and/or liquefaction potential from N-value
Force and velocity measurements can be evaluated for transferred energy in real time by PDA or SPT Analyzer according to ASTM 4633-05
N-value is then corrected as per energy ratio
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SPT
SPT Energy Considerations
Questions?
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• Measure F, v with PDA
• Calculate soil resistance against sampler or special toe plate or cone
• Measure Torque
• Measure static uplift
Rausche, et al., 1990. Determination of Pile Driveability and Capacity from Penetration Tests, FHWA Research Report
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Using PDA on SPT to Predict Pile Capacity
• 1996 Research: SPT toe configurations
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Using PDA on SPT to Predict Pile Capacity
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Using PDA on SPT to Predict Pile Capacity
Torque Measurements
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Using PDA on SPT to Predict Pile Capacity
Static Uplift Measurements
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Pile top F and v
measured and from GRLWEAP
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Using PDA on SPT to Predict Pile Capacity
Pile top F and v
Measured and from GRLWEAP
Pile bottom F and v calculated from Measurement and GRLWEAP
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Using PDA on SPT to Predict Pile Capacity
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SPT 45
• Integrate v to bottom displacement
• Plot Force vs displacement at bottom
• Compare with Uplift test
Using PDA on SPT to Predict Pile Capacity
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Using PDA on SPT to Predict Pile Capacity
• Integrate v to bottom displacement
• Plot Force vs displacement at bottom
• Compare with Compression test
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Using PDA on SPT to Predict Pile Capacity
Based on SPT measurements, compare calculated capacities from:
• Wave equation
• CAPWAP
With static test
Conclusions from additional SPT measurements
Potential to determine soil properties with a CAPWAP type analysis
For static design implications
For dynamic driveability predictions
More testing and research are needed!
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The End
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