undrained shear strength from Shear Vane Tests and CPT Tests
undrained shear strength from Shear Vane Tests and CPT Tests
(OP)
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.
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.





RE: undrained shear strength from Shear Vane Tests and CPT Tests
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.
RE: undrained shear strength from Shear Vane Tests and CPT Tests
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.
RE: undrained shear strength from Shear Vane Tests and CPT Tests
RE: undrained shear strength from Shear Vane Tests and CPT Tests
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.
RE: undrained shear strength from Shear Vane Tests and CPT Tests
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.
RE: undrained shear strength from Shear Vane Tests and CPT Tests
RE: undrained shear strength from Shear Vane Tests and CPT Tests
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,
RE: undrained shear strength from Shear Vane Tests and CPT Tests
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
RE: undrained shear strength from Shear Vane Tests and CPT Tests