Anisotropy in Triaxial Testing

[moe333]

I am interested to hear how geotechnical engineers in this forum are using results from isotropic compression triaxial testing; both CU and UU with respect to anisotropy.

I have recently reviewed reports where engineers attempt to correct isotropic tests to model what they believe are anisotropic conditions. They do this by multiplying the consolidation stress used in an isotropic CU test by 1.5 to come up with a Su/P’ ratio. This is meant to model a mean effective stress with Ko=0.5 in two horizontal directions and a vertical effective stress. These tests were performed on existing compacted clayey sand and the samples were consolidated to about 1.5 to 5 times the existing vertical overburden pressure; so that they may be normally consolidated. The samples were obtained from about 20 to 40 feet below an embankment crest and groundwater was at approximately 15 feet.

I'm trying to decide if this is a reasonable methodology to come up with an Su/P’ ratio. I think it may be reasonable if Ko=0.5 and if the samples are truly normally consolidated in their in-place state. I don't have the ability to run consolidation tests. Since it is a compacted fill, I'm wondering how the compaction may affect the Ko and the pre-consolidation pressure? Any ideas on this?

I imagine a similar methodology could be applied to UU tests. I don't ever see triaxial shear strengths on granular soils being corrected for anisotropic conditions. I would be interested to hear how others handle anisotropy with triaxial testing for both existing and proposed conditions, granular and clayey soils, natural deposits and compacted fills.

 

[dgillette]

Hi Moe. Pinned up on my bulletin board (can't remember why) is a plot that I got from a Paul Mayne paper. It shows undrained strength ratio (Su/Sigma'vo), for NC only, as a fn. of phi' and type of test. It comes from a Wroth-Prevost model.

Here's the ref. I might have downloaded it from his web page Ga. Tech.

"Stress-strain-strength-flow parameters from enhanced in-situ tests"
Paul W. Mayne

Georgia Institute of Technology, Atlanta, Georgia USA

Proceedings, International Conference on In-Situ Measurement of Soil Properties & Case Histories [In-Situ 2001],
Bali,Indonesia, May 21-24, 2001, pp. 27-48.


The highest curve in the figure is for CIUC. At 30 degrees, it is about 0.35. CKoUC at 30 degrees is right about 0.30, and DSS is more like 0.2. At 25 degrees or below, the difference gets pretty minor.

Compacted fill could have a much higher Ko than natural soil until it's loaded up by pretty high overburden. Acts sort of like OC.

Regards,
DRG

 

[moe333]

Thanks DRG, I'll have a look.

A lot of the work I see uses test results from CIUC, only a few use CKoUC. Probably because not many labs run CKoUC, they are expensive, and you would need to have a good handle on Ko is. It seems CIUC would likely over-estimate strength in many cases.

Do you know of any good references for estimating Ko of compacted fill? How about estimating pseudo OCR of compacted fill (without running consol. tests)

 

[dgillette]

I sure don't know of anything about Ko, but I wouldn't be surprised if it exists in some USACE or USBR research report. The pseudo-precon pressure is probably available easily in old USBR reports, but unfortunately, this is a bad time for me to hunt. I have a lot to get done between now and 12:00 eastern daylight time.

An old USBR lab guy told me that thirty feet of compacted fill was typically about equal to the pseudo-precon pressure for 98% of std., placed at optimum or slightly dry, as was typical USBR practice when he was around there (early 1940s to early 1990s - like I said, he was an OLD lab guy).

You got the stuff that I sent by owl yesterday?

 

[BigH]

Years ago, anisotropic strengths was a big deal in many papers submitted in the Canadian Geotechnical Journal. You might wish to search for some of them. They developed strengths vs various angles from "horizontal". Sorry I can't remember the exact papers - but they probably were from the 70s. Didn't Bishop write a State of the Art paper on compacted clay fills back in the 1960 Boulder Conference? - if my memory serves me right.