SPT to Su

[moe333]

Hi Guys,
I know this is not a reliable correlation, but...does anyone know of something published that correlates N60 to Su? These are very stiff clays with blow counts > 30. References would be great.
Thanks!

 

[BigH]

I will post something tomorrow when I am back at the site.

 

[Mccoy]

MOE, I'm goign to list some but as you are going to see, a correlation factor of 0.06 to 0.07 su/pa is the most used. I also like the Hara correlation which, unlike the other ones, is non linear. I'm curious about the correlations proposed by BigH. Here in Italy 0.06 su/pa as per Terzaghi formula rounded down is the most used.

N60 TO Su CORRELATIONS
1. Terzaghi & Peck, 1948: by far the most popular correlation here, validated in other studies as well (
a. Su=6.4 N60 [kPa]
2. Sowers, 1980
a. Su=7.26 N60 [kPa]
3. Hara et al., 1974 (based on 180 lab samples in 25 sites in Japan with UU tests)
a. Su=28.4 N60^0.72 [kPa]
4. Sivrikaya & Togol, 2007 (based on 411 samples in Turkey)
a. Su=6.4 N60 [kPa] for silt and silty clays
b. Su=4.93 N60 [kPa] for low plasticity clays
c. Su=6.82 N60 [kPa] for high plasticity clays
By studying the plots you can have a feel on the data dispersion and their reliability, the data clustered in the range of N60=2-20 in Hara et al, recent validation of Terzaghi & Peck in Italy has data clustered in the range N60=2-10, in these clusters, due to the number of data, correlations tend to be more reliable

Texas cone correlations in clays are very similar to the above

 

[moe333]

Mccoy,
Very impressive/extensive list, Thanks!
Moe

 

[BigH]

I forgot to bring the graph with me to work - it is at home and I don't have internet access anywhere but at the site office. I did develop a chart of my own for a site in India - have posted it now and will do the other later. Can't say that any measurement to show N60 occurred but it was normal old style crude SPT work . . .

 

[BigH]

There's also this chart from Sowers . . . .

 

[moe333]

Thanks BigH

 

[Mccoy]

BigH, how has Su been determined in the Indian sites?
Interesting to note that the final result is similar to the classic Terzaghi correlation

 

[BigH]

Mccoy - we did some vane tests (uncorrected to Bowles) and also some consolidated undrained tests (without pwp measurements) if I remember correctly. The T&P values, or the one N=2, Su=12.5 kPa; N=4, Su=25; N=8, Su=50; Su=15, Su=100; N=30, Su=200 has always worked well for me.

I enclose a graph presented by Dr. Seah (Thai consultant - MIT Doctorate) at a "short course on pile design" I attended on 4th July in Kuala Lumpur. Pretty basic, but it was good to get out and hear about something more than fill placement and concrete placement. Remember that cu = Su and that ton/m2 is tonne (metric tons, not US short tons). t/m2 ~ 100 kPa. The (2/3)N in t/m2 is one that he uses most of the time in his Thailand practice. I've always taken the N to be uncorrected as this is what the old charts were geared towards.

 

[dgillette]

One caution is that different correlations are referenced to different types of test. I believe the ones ref'd to VST (Houston, Howrah West Bengal) give generally higher strength than the ones based on unconfined compression. I consider the unconfined to be more of an index property than a strength test, because it is so strongly affected by sample disturbance, drying, etc.

I was surprised at how similar the two VST correlations were.

Cheers!
DRG

 

[BigH]

@dgillette - point taken - such correlations are always "fuzzy" and one needs to have judgment. Some of the Howrah data was consolidated quick triaxial - the soil was too soft for unconfined compression. Dr Seah - who gave the presentation on pile design and construction (MIT Doctorate) based on their experience in Bangkok/Thailand, is a strong proponent of the SPT test even for clays although this gets a lot of flack from North Americans . . .