undrained shear strength from Shear Vane Tests and CPT Tests

[JoyceBCD]

Hi all,

I just finished a project, and we did both shear vane tests and CPT tests on site. The test locations are relatively close, but the shear vane results (Su) and the CPT interpreted Su are various a lot. Say the Su from the shear vane tests is around 150kPa and the Su interpreted from CPT is only around 40-50kPa. I can see there are a little bit sand within the areas we tested, but I don't think that minor portion of sand actually can make that much difference.

Anybody has any idea what's going on there?

Thanks.

 

[L0k]

Hi Joyce
How to get the Su value from CPT? How to carry out the shear vane? if it was carried out at the base of hole, there may be a different soil due to geological reason.

 

[JoyceBCD]

Hi L0k,

the shear vane tests were conducted throughout the entire hole, therefore, the CPTs and the shear vanes should test the same material.

Su can be interpreted from CPT tests, normal they will send you the Su diagram, with Su maximum and Su minimum on it.

 

[moe333]

Typically you would calibrate the Su obtained from the CPT with the corrected vane shear Su. The Su from CPT is calibrated with the the Nkt factor.

 

[L0k]

Hi moe333
You are right, the JoyceBCD problem is the the Su shear vane=150kPa but the Su CPT= 40-50kPa.
I thought there was a friction in the shear vane rods, but I do not know how they did the vane testing.

 

[JoyceBCD]

Thanks moe333,

Yes, you are right. I fully understand that, but normally the difference between those two are not that big.

so my question really is focused on, any possible reason to cause such a big difference between those two sets of results?

Thanks.

 

[moe333]

Either the material is different at the 2 locations or there is an error in either the CPT or Vane. CPT is typically pretty standardized so I would look at potential problems with the vane such as speed, gravel, maybe try correlate with SPT if there were any to see if vane seems too high.

 

[JoyceBCD]

Thanks moe333,

The material should be same, because the testing locations are very close (less than 1m), and that happened to all five CPTs and Shear Vanes on site. We did hand auger with shear vanes next to each CPTs.

So you think the CPT should be the more accurate one, I am a little bit confused, because normally the field test actually govern the adoption of the design parameters.

regards,

 

[coneboy]

I agree that 50 and 150 kPa are way too big of difference.
What Nkt value did you use to convert the CPT data to shear strengths? 12 - 14 is common.
The most common Su method involves qt, overburden stress and an Nkt value....did you use that formula?
Did the CPT test show a positive pore pressure reaction in the clayey soils.....it most likely should.
Were the vane tests conducted with a "Nilcon" or was it a standard field vane with a torque wrench?
Torque wrench style vane tests have to be done carefully with the drillers in order to get good data....but so do Nilcon vane tests.
Double check the vane testing procedures with your staff that were on site.
Double check the casing depths and the vane testing depths and see if the vane tests were too close to the casing (soil disturbance).
I bet the 50 to 150 kPa issue lies somewhere in the details.

Coneboy

 

[DougHole]

what was the Ic value in the CPT? If you are dealing with a material that is close to the boundary of cohesive and granular (check the Ic), it might give less reliable results. i would have thought the CPT-Su correlations are based on a traditional "pure clay". i thought an Nkt of 15 was usually used. also have you corrected the qc (raw value from the CPT) to a qt value. this depends on the net area ratio calibrations that should've been done by the CPT contractor. Ask for their a-value so you can calculate it. recommend getting Robertons manual.it is free online.