The Standard Penetration Test: Its origin, evolution and future

STUART WAGSTAFF

DIRECTOR – SOIL CONSULTANTS LIMITED

The Standard Penetration Test: Its origin, evolution and future

Since the start of its use as a design tool back in the 1940’s, where does this leave us in 2020 and beyond?

1. a highly sensitive tool from which we obtain highly accurate soil parameters we then use to plug into highly sophisticated software to produce highly accurate predictions or ……. 2. just something we routinely do because that is what we have always done as long as the investigation is cheap and we have reliance on the poor old SI contractors PI policy? The issue we have is that it is considered a rapid, inexpensive, qualitative tool and

can provide data when other sampling / testing techniques are not viable or cannot be justified financially

The Standard Penetration Test: Its origin, evolution and future

Results are variable depending on things like borehole diameter, water level and depth (not to mention the drillers!)

Seems to be an appreciation that there is better reliability in clay

But in reality does anyone ‘standardise’ these results before design?

Skempton (1986) sought to introduce the N60 to iron out differences in the energy efficiency

Surprise!!!! - ‘re-introduced’ with the Eurocodes

The Standard Penetration Test: Its origin, evolution and future

Can we use the SPT as a reliable tool? Terzaghi + Peck (1947/1948/1967) – had a database to determine allowable bearing pressure Stroud (1974) – SPTs in Insensitive clays Stroud & Butler (1975) – correlation between N and Cu Simpson et al (1979) – ratio of undrained Young’s Modulus to undrained shear strength (CIRIA

153) Jardine et al (1984) – tests on London Clay matching well to the Simpson Model (CIRIA 153) Clayton (1995) – can appropriate correlations be made and is the data set large enough. Warns

of sensitivity and an N value design approach not being appropriate (CIRIA 153)

The Standard Penetration Test: Its origin, evolution and future

So how reliable is the SPT today?

SPT calibration:

Year 2012 2013 2013 2014 2015 2016 2017 2018 2019Hammer Re-test

Test house A A Er % 65 64 70 54

Test house B B Er % 69 66 59

Test house C C Er % 75 65 59 70 72 47 56

Test house D D Er % 66 65 66 70 72 77 75 74

Test house E E Er % 70 70 68 73 66 75 73 79

The Standard Penetration Test: Its origin, evolution and future

Looked at London Clay and compared Cu derived from:

UT100’s, Steel and Plastic U100’s, Rotary Core, Pressure Meter and SPT’s; both uncorrected and corrected for N60

The Standard Penetration Test: Its origin, evolution and future

Rotary Cored sample U100 Sample

[Thick Wall Steel Tube]

The Standard Penetration Test: Its origin, evolution and future

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400 450 500

m B

GL

Undrained cohesion kPa

Self Boring Pressure Meter Cu Metal U100 Cu ROTARY CoreCu Plastic U100 SPT [uncorrected] 1 SPT [uncorrected] 2SPT [uncorrected] 3 SPT N60 [corrected] Average

The Standard Penetration Test: Its origin, evolution and future

UT100 to 15m Thick Wall Steel U100 below

Plastic U100

The Standard Penetration Test: Its origin, evolution and future

Plastic U100

The Standard Penetration Test: Its origin, evolution and future

The Standard Penetration Test: Its origin, evolution and future

Do the results show some form of rudimentary uniformity to justify the use of SPT?

Is the SPT any worse than other techniques?

Would an increased data set provide a more reliable average?

If we truly standardised the SPT, would we have the reliability in what was measured?

Should we do more work to ‘standardise’ the technique and correlate this against back analysis to provide a more up to date correlation?

Should we continue as we are? [not much has changed in the last 10 years!]

The Standard Penetration Test: Its origin, evolution and future

WHERE DO WE GO FROM HERE?

The Standard Penetration Test: Its origin, evolution and future

References:

SKEMPTON A.W (1986) Standard Penetration Test procedures and the effects in sand of overburden pressure, relative density, particle size, ageing and over consolidation, Geotechnique 36, 3, 425-47

TERZAGHI K (1947) Recent trends in sub-soil exploration. Proc 7th Texas Conf. Soil Mechanics and Foundation Engineering, 1-15

TERZAGHI K & PECK R.B (1948) Soil Mechanics in Engineering Practice, John Wiley, New York 1st Edition

TERZAGHI K & PECK R.B (1967) Soil Mechanics in Engineering Practice, John Wiley, New York 2nd Edition

STROUD M.A (1974) The standard penetration test in insensetive clay and soft rocks, Proc. Eur. Symp. on penetration testing (ESOPT I) 367-75

STROUD M.A & BUTLER F.G (1975) The Standard Penetration test and the engineering properties of glacial materials. Proc Symp. On engineering properties of glacial materials, Midlands Geotechnical Society, Birmingham 117-128

CLAYTON C (1995) CIRIA 143 The standard penetration test (SPT): Methods and Use

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