3.6 spt energy calibration -gray

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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

SPT

SPT Equipment is not standard

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Safety hammers typicall 60%, automatic hammers 80 to 90% ofEr-SPT

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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

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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

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


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SPT

Pile Driving Analyzer - Model PAX


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SPT

SPT Analyzer


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SPT

Pile Driving Analyzer - Model PAX


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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


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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)


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SPT

Florida SPT Energy results

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Note Scatter!


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SPT

Utah State University StudyUtah State University Study

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”


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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”


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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

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• 1996 Research: SPT toe configurations

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Torque Measurements

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Static Uplift Measurements

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Pile top F and v

measured and from GRLWEAP

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Pile top F and v

Measured and from GRLWEAP

Pile bottom F and v calculated from Measurement and GRLWEAP

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• Integrate v to bottom displacement

• Plot Force vs displacement at bottom

• Compare with Uplift test


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SPT 46


• Integrate v to bottom displacement

• Plot Force vs displacement at bottom

• Compare with Compression test

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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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