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