Rock Fill Dam - laboratory testing

[moe333]

Hello all,

I am putting together a laboratory testing program for a FERC regulated rock fill, clay core dam, about 100 feet high, and have not been able to find complete criteria on their requirements. I do have their September 2006 manual "Chapter IV", embankment dams, but it is not comprehensive. I also have the 2007 USSD paper "Strength of Materials for Embankment Dams" which gives criteria for different agencies.

dgillette, I thought you may have some insight on this in particular:

To develop a composite shear strength envelope with the R-bar, and S tests for either steady seepage or earthquake loading, would it be permissible to use the direct shear test for the S test? I'm trying to avoid using the drained triaxial test on clay material if possible.

Also, does anyone know what the compaction requirements are for clay core and filter material? Is it standard or modified proctor? Is this referenced somewhere by FERC?

Thanks for your replies.

 

[GeoPaveTraffic]

Let me start by saying that I'm not familular with FERC requirements. However, for the agencies that I have worked with, using either direct shear or consolidated undrained traixial tests with pore pressurement measurements for clays have been execptable. Consolidated undrained traixial tests with pore pressurement measurements are also an easy way to get the R strength often required in the analysis.

As for compaction criteria. For the clay core, compaction wet of optimum to about 95% or 100% of standard Proctor at OMC +1 to +3% is what I have seen. As for filters, I would use either 100% standard or 95% modified Proctor.

 

[dgillette]

Aw dangit. I just wrote a long reply, then lost it when I clicked the wrong button. I'll try again.

To get the S parameters of clay, I would not try to use a drained test. Takes too stinkin' long, and you never know if you really have the material saturated. I'd use an R test with back pressure and pore-pressure measurement - it can be the same test you use for the S parameters.

For clay core, modified proctor is almost never used, because it pushes you toward low water contents and stiff, brittle material; use standard, perhaps 98%, with 95% of all tests passing. (I have seen a modified-modified proctor used for a very large dam about 15 years ago. The lower part of the embankment needed to go in dry to minimize post-construction settlement, and to improve construction-case stability with a thick core.) There are two schools of thought on compaction moisture: USBR placing dry, and USACE placing wet. I heard Dr. Peck give a talk on the differences and the rationale for each without actually saying one was preferred. Placing dry of optimum (0-2%) reduces construction pore pressures. It was, at least in part, a concession to building dams in deserts where water is difficult to come by. Wet of optimum (0 to 2 or 3% wet) makes the material more plastic (in the engineering mechanics sense of the word), so it is less prone to cracking and can compact better against structures, and irregular abutments in places where there would not be high confining pressure. Wet also gives lower permeability, which may or may not matter significantly. Depending on your situation, there could be some rationale for using dry in the lower part, under maybe 80 feet of fill, and wetter higher up where there wouldn't be so much confining pressure.

For filters and drains, density is generally controlled by RD, rather than proctor, because decent filter and drain materials are too clean for proctor to work well. Generally shoot for 70%, because any denser than that tends to break down particles without providing any benefit. Often, one can rely on a procedure spec (e.g., 2' lifts with 4 passes of a single-drum 10-ton vibratory smooth-drum roller), taking record tests only.

Recommend Reclamation's "Design of Small Dams," which is sorta misnamed unless you consider over 100' to be "small." Used to be able to buy it from Reclamation or NTIS hardbound for nickles and dimes, but they don't seem to sell it that way anymore.

http://www.ntis.gov/search/product.aspx?ABBR=PB95176368

For FERC guidelines:

http://www.ferc.gov/industries/hydropower/safety/guidelines.asp

For basics of earthquake analysis:

http://www.fema.gov/library/viewRecord.do?id=1573

I could go on like this for a long time if I didn't have to get back to work.

Regards,
DRG

 

[moe333]

Thanks for the replies,

DRG, you don't think a direct shear test would be useful for the S test on clay material? It may be interesting to see how it compares to the CU with pore pressure.

 

[dgillette]

No, I don't believe I would do a direct shear test on clay.

1. To assure dissipation of excess PWP, the test has to be run quite slowly.

2. If you are starting with a freshly compacted clayey specimen (at ~w-opt), it will take a very long time to soak up water and reach a uniform water content. It takes time in a triax specimen also, but you can push water into it from the bottom with a fairly high gradient (much faster).

3. Even if you keep the sample flooded, you still won't have any assurance that it's saturated and free of capillary tension, which can mislead you, unconservatively, by raising the strength with respect to the real S effective-stress envelope. With triax, you can back-pressure and test the B factor to be sure it is close to saturated.

If this is a new hydro dam, don't let the owner nickel-and-dime you on testing. The cost of a good thorough testing program would be just a pimple on the elephant of the whole design, permitting, and construction cost. A good design-phase testing program might even pay for itself by letting you design a cheaper embankment. Plus, there's that public safety thing that FERC is so concerned with. ;-)

 

[moe333]

I can see your points regarding the use of the direct shear test on clay. There are ways to achieve saturation such as letting the sample wick-up moisture from the bottom of the sample. But I think there are also issues regarding stress concentrations and rotations of principal stresses, etc. with this test.

We are currently doing feasibility level work so the lab testing is relatively light compared to what will be done as the project progresses.

Thanks