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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 1920s 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 D1586Ref: 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-lbsWe 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 TestingNon-standard variablesStandard Penetration TestingNon-standard variables

Hammers Safety

Cathead-rope Cathead diameter

Automatic Spooling Winch Chain Driven

DonutOperators

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 = NmEMX

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, (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 DisplacementER(t) = F du; but v = du/dtEFV(t) = Fv dt; transferred energyEMX = max[EFV(t)]T = EMX / ER-SPT ; transfer ratio

Energy = Sum of Force times DisplacementER(t) = F du; but v = du/dtEFV(t) = Fv dt; transferred energyEMX = max[EFV(t)]T = 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 ... Youngs 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 ... Youngs 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

= 44%

EFV = 0.281 N-m = 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 ( = 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

= 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

= 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

= .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 (

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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,etal.,1990.DeterminationofPileDriveability andCapacityfromPenetrationTests,FHWAResearchReport

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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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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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