shear wave velocity vs Su

[999Paul999]

Hello, I would like to ask you is it possible to correlate shear wave velocity Vs ( for instance, obtained from SDMT ) with undrained shear strength of cohesive soils. Maybe You know some articles or papers about it ?. I would be greatful for replies.
Paul

 

[dgillette]

Hi Paul. I would expect SOME correlation, but with a huge band of variability/uncertainty. Vs and Su are both driven by a lot of different factors, and probably not in a consistent way, so an apparently good correlation would be purely fortuitous. Vs is measured at very small strains, whereas peak Su typically requires 2 to 10 percent strain.

If you are looking for Su, there are other cheap ways to fly: CPT, VST, SPT, or just oedometer results with an assumed typical shear strength ratio.

Oops. Fire alarm. Gotta go.

 

[msucog]

if you don't mind me asking, is there any particular reason for the question?

if you have enough data point for a particular soil, then you could always do a generalized statistical analysis. but as dgillette notes there can be significant variability doing this. i've done such a thing with Vs and N-values in different geologies with similar results. however, there was significant variability overall since the Vs is a "broader" test while the spt is only across the 12" of the ss. but for very general comparisons it works well.

 

[jdonville]

Do you want this for a Seismic Site Class determination? If so, perform the field shear wave test and have done with it. You should have done at least on test boring to 100 ft, so plan to do it right and spend a little more money to complete the job.

I recently saw a SPT-based Site Class determination performed with a rope and cathead apparatus. Talk about blowing the standard of care to the client...

J

 

[msucog]

let's not knock spt testing too much. it's only the most versatile and practical field testing readily available to us. however, it does have conservatism (potentially significant amounts depending on your geology) built in to it when determining the seismic site class because of how much the specific material can influence the values. besides, you can always perform downhole seismic along with spt to get the best of both worlds. but i personally prefer surface performed shear wave methods after spt testing has already been performed. again, can always perform downhole seismic along side the spt if you choose. the cost of shear wave testing "usually" will pay for itself and possibly the entire cost of the exploration depending on the structure size, specific location, geology, etc etc

 

[coneboy]

Paul,

The NCHRP Synthesis 368 guide that was issued in 2007 listed a method for correlating the Vs and qt values from a CPTU test. (for clay like soils)

The formula:
Vs (m/sec) = 175(qt)exp 0.627 (qt in kPa)

To convert your qt values to Su we commonly use the this formula:

Su (kPa) = (qt - overburden stress)/Nkt (all units in kPa)

We have found that a Nkt value of 13 produces believable results.

I think the NCHRP formula was created in order to take into account most clayey soils on a global basis.
We have found that this formula can have up to 20% error for the soft, NC silty clays that we have in our of the west coast.

Best of luck with your estimates

Coneboy