Undrained Shear Strength of Compacted Clay

[moe333]

Does anyone know of a correlation for Undrained Shear Strength of Compacted Clay (95% of std. proctor). PI=8, 55% fines, 20% clay. I'm not sure that an Su/p ratio would be appropriate, clay will be placed up to 100 feet thick for an embankment. Triax. will be performed shortly, trying to come up with prelim. values.

Thanks

 

[dgillette]

Hi Moe333. What are you analyzing, end-of-construction stability? This is actually a damned hard and complicated question, with different answers in different parts of the fill. (Effective stress still applies, but the effective stress is governed by capillarity, compaction moisture, overburden, time-dependent dissipation of excess PWP if any, etc.) For large dams, people sometimes get into pretty high-level analyses using consolidation and UU and CU tests with samples at compaction moisture, if it would have a big effect on stability. (If the dam's core is thin, it wouldn't matter much, but with a broad core, it may matter.) I don't have any good refs right at hand to suggest, but Del Fredlund and others (at U of Alberta?) have published a fair amount in the Canadian Geotechnical Journal.

I suspect that, under 100 feet of fill or perhaps a lot less, the clay will be saturated (unless placed well dry of optimum) and behave as though normally consolidated (contractive). Even if you don't do something like storing a reservoir against the embankment to get it wetter, the compression may be enough to eat up all the air voids and make it saturated. In the lower part of the fill, therefore, Su/p' might make some sense (for construction stability, but not long-term stability), with the caveat that there may be some undissipated excess PWP from construction that would affect the value of p'. In the upper part, where the fill would remain unsaturated, the strength envelope is likely to be helped by a sizable pseudocohesion due to capillary tension, which makes effective stress higher than total overburden stress. This is strongly affected by degree of saturation. With overburden pressure, the air voids get compressed, degree of saturation goes up, and capillary tension goes down. For samples tested AT COMPACTION MOISTURE, this typically results in a low phi (not phi') and a high cohesion. USBR's Design of Small Dams shows typical values of 25 degrees and 10 PSI for CL, but I think those numbers come from a mixture of different types and of back-pressured triax results and older ones without back pressure, which is more similar to what's in a new embankment.

Avoiding high construction pore pressure is one argument for placing fill 1-2% dry of optimum in the lower part of a high fill.

 

[moe333]

dgillette, thanks for your reply. Your comments always leave me with more questions than I initially had, but that's a good thing!

I will eventually be analyzing most or all stability conditions including end of construction, drawdown, steady-state seepage, and seismic stability. We are currently in a feasibility level study so we are not doing a mountain of lab testing and analysis, keeping it fairly simple at this stage. I am currently running CU with pore pressure on the compacted clay to be used in the core. It is being compacted to 3% over optimum at 95% standard. I was planning on using the CU strength for both the saturated and unsaturated portions of the core (based on seepage analysis) and thought this would be conservative for the unsaturated zones. I wanted to run some preliminary analyses prior to obtaining the test results just to see how close a correlation would come to the actual results.

I realize it is much more complicated when you consider unsaturated zones and I am not sure I will even get into that at this stage of the analysis. So far I have picked some numbers out of the air for the estimate (started at 1500 PSF and increased the strength 20 PSF per foot to a maximum of 4000 PSF). I am interested to know if there are any correlations out there.

Thanks

 

[dgillette]

For long-term, unsaturated, you could probably use 25-30 degrees and a couple hundred PSF without being radically off the mark. C is probably a lot higher than that at compaction moisture, but it might get rained on and wetted further. Or, it might dry out. For short term, you're dealing with a very strong soil that can probably be cut vertical 25 feet high.

Afraid i don't have any good correlations to offer for the part of the embankment that really matters, unless you are somehow able to estimate the end-of-construction effective stresses in the saturated portions of the core.

 

[BigH]

Might I suggest that you look at Bishop's Paper in the ASCE Specialty Conference in Boulder Colorado, 1960 (I believe). He had a state of the art paper on compacted clays - would be a good place to start.