Undrained shear strength from vane data versus cone data

[Gregoir]

I was reviewing some data from a recent site investigation program and noticed that the calculated undrained shear strength for our lakebed sediment is giving us values 10-20x the readings of our vane data from the same unit. We have completed 15 holes and have found the same results at various locations across the site. I went back through and confirmed that the cone company values calculations were correct.

We have completed gradation on this material which on average is 90% passing #200, 60-70% silt, 20-30% clay.

The field engineer noted that the vane rods were hard to push by hand, but if left would slowly sink under their own weight.

My only thought so far was that this material is behaving in a Newtonian fluid manner, as if there is some relationship between the undrained shear strength and the rate of penetration/shearing.

Thoughts? Any idea of some lab testing that might be helpful to assess this situation?

 

[BigH]

First - please, if possible, identify where you are located. Would your location be such that sensitive clays might be found? The driller's comments on the pushing of the rods being "hard" and then soft . . . just a thought in that you might be getting with your vanes a partially remoulded value. What was the "peak" and remoulded vane values?

 

[Ron]

Agree with BigH. The material sounds like it is thixotropic. This can happen in certain marls and other clay-silt mixes.

If you had let the vane sit undisturbed in place for a while (unfortunately maybe 24 hours or so) and then checked again, you would find the values would go up (remolded value). You tested in its "time-dependent" viscous state.

 

[Gregoir]

Thanks for the replies.

BigH - The site was located in the Northwest Territories in Canada. There are a number of other sites within the area that have had cone/vane info done (gradation wise, similar materials)and it did not show this drastic difference between the CPT/Vane. It had higher vane data than CPT. Average peak over the 26 vanes completed was around 19 kPa and residual was ~8 kPa.

Ron - Due to time constraints we were unable to stay for that long at a hole.

Can either of you think of any sort of laboratory testing that I could have completed on this material to help support the idea of this as a thixotropic material? We have about 30 disturbed samples of this material from various parts of the lake we could reconstitute if required.