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Standard Penetration Test examined with digital technology
Dr Burt G. Look
Foundation Specialists Pty Ltd, Australia
ISC5 (2016)
Content
SPT circa 1940s
• Counting Blows
• Corrections Required
PDM circa 2010
• Set
• Energy
PDM +SPT
• Test Sites
• Results
Key findings
• Blow count @ 150mm is an estimate and not
“factual”. Seating is not an “exact” 150mm
• University trained supervisors are unable to
count above 20 accurately
• Energy varies
• with ground conditions
• with each blow
• with depth
ISC5 (2016)
Which is your favourite Correction Factors reference ?
ISC5 (2016)Tables from Agour and Radding (2001)
Introduction – PDM the basics
ISC5 (2016)
The Pile Driving Monitoring (PDM) is based on LED opto-
electronic technology and measures pile set and temporary
compression by non-contact measurement from a safe
distance. The peak pile velocity can also be calculated.
+ Energy
2 PDMs + PDA SPT analyser
ISC5 (2016)
Measurements by eye and digitally compared
ISC5 (2016)
150mm is a target. It is
not actually measured
Chalk Mark Technology vs Digital Measurements
ISC5 (2016)
Energy Transfer
ISC5 (2016)
N60 E60 = N SPT ESPT
Australian Standards do not currently specify energy requirements in test
ERGO energy is not measured.
Any correction (if applied) is based on the international literature.
There are many corrections factors to be applied to convert the in-situ N–value to a
useful design value. Energy is widely considered the key correction factor. Energy
transfer is affected by the type of drill rigs, hammers used, operator skills as well as the
ground conditions.
Are you happy with an SPT that is currently close enough?
ISC5 (2016)
http://favoritememes.com/news/close_enough/2014-07-27-342
N60
N (Field value)
SPT Energy Efficiency Measurements with correction factors
ISC5 (2016)
5m • Correction Factor 1.3 (0.87)
7.5m • Correct. Factor 1.45 (0.97)
10m • Correction Factor 1.5 (1.0)
3 Drill Rigs @ 1 Site
ISC5 (2016)
Case Study
ISC5 (2016)
UnitOriginal
(N)
Energy
Corrected (N60)
Net
ChangeDetails
%
Changed
Very Loose (<4) 45 43 -2 2 "Very Loose" changed to "Loose" 4%
Loose (<10) 179 162 -172 "Very Loose" changed to "Loose"
19 "Loose" changed to "Medium Dense"11%
Medium Dense (<30) 446 354 -9219 "Loose" changed to "Medium Dense"
111 "Medium Dense" changed to "Dense"25%
Dense (<50) 77 174 +97111 "Medium Dense" changed to "Dense"
14 "Dense" changed to "Very Dense"18%
Very Dense (≥ 50)
(Excl. Rock)50 64 +14 14 "Dense" changed to "Very Dense" N/A
Granular: 146 category changes out of 797 SPT Tests (18%) Cohesive: 56 Category changes out of 323 SPT Tests (17%) TOTAL: 202 Category changes out of 1120 SPT Tests (18%)
The Final Word
ISC5 (2016)
Perhaps it is time for the ubiquitous SPT (1940s procedure) to enter the
digital age (2010+).
Visually counting values in 150mm increments is shown to vary and is an
“interpretative” number.
The field N-Values are useful to show relative change. If Energy is not
measured, the value is questionable as a design value.
“Guess” from 80% to 172% correction if a trip hammer used (different for
other hammers).
In example, 18% of 1120 SPT results would change classification
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