Phi angle estimate - Fractured Basalt

[ORUSA]

thread255-253888

Looking to perform a preliminary (recon level) slope stability analysis for a through-fill held up by a wall on top of a ridge of moderately weathered fractured basalt. Fractures would be 4-inches to 12-inches wide, closed, with more weathering along the fractures than within the clasts. This material can be excavated with effort by a g-pick. My idea is to treat the basalt as a Mohr/columb "soil". I am assuming (presently) no major through-going seam or joint pattern. No subsurface exploration yet. I am thinking that this would have a friction angle considerably better than that of a well graded, dense compacted gravel, say over 45 deg.

Does anyone out there have guidance on what value to use? Any accepted publications or tables?

 

[oldestguy]

This sounds like a form of testing or evaluation as would be done for a dam. The US Bureau of Reclamation has a library catalog of 6 pages. In there I see references to several types of shear evaluations, probably in-place.

http://69.63.217.22/B10113/OPAC/Sea...=512045&TitleListPageSize=50&SavedSearch=true

Otherwise I was thinking of some in place shear strength tests I used to run with a shear box 12 inches square. To provide a normal loading I used an operating back-hoe bucket sitting on a post near the test as a reaction and applied a vertical load through a proving ring using a screw jack. To push the shear box I used a portable hydraulic cylinder, trade name "Porta Power" with a pressure gage on it calibrated in a testing machine. Vertical load mechanism also was on a roller system, to allow for some movement.

Could you run such a test in one of those fractured zones?

Maybe a cone type test?

I suspect getting good undisturbed samples for large triaxial or other test may not work well.

 

[ORUSA]

That link didn't work for me.... I really don't think testing on any type of disturbed sample will get me anything. Once the material is disturbed it can't be reassembled into its original shape to be meaningful. Any testing at that point would return a minimum phi and i already pretty much know that (45deg).

Not having done an insitu test like you are talking about it is hard to imagine that being useful. Think of clearing off a flat spot to conduct the test. You have weathered bedrock, fractured, uneven, some clasts loose on top (gravel to cobble-sized material), some keyed into the formation...... How could a shear box be properly placed in contact with that surface with sharp edges of some cobbles sticking up and cavities around those where a clast had been removed?

Your idea of a large in-situ test is interesting though, perhaps in another context.....

 

[oldestguy]

In response to you, the link was to the catalog of documents that are available from the US Bureau of Reclamation. I got there by starting at their home page and following titles until those pages came up. Several references deal with testing in place with a cone to relate that to shear strength. That may well be an option for you.

As to the problems you bring up for the material you have, I can only say we have to adapt the test to best fit the site and the material. For instance if the fracture zone is inclined vertically, well we then come up with a way to do it on that inclination and it takes some "mechanical ingenuity" I suppose. If there are difficulties in getting a neat square area, well, the area can be adjusted by filling voids in with plaster of paris or other setting up material considered stronger than the tested material or some of the fines of the site compacted. A flat area is not needed. All you need is a mound of undisturbed material. Seldom can the sample sides be left without adding some filler there. You make the area inside the "box" flat for the application of the normal load with plaster of paris or similar if necessary. I've run this type of test on many different materials, such as paper mill waste, power plant ashes, gravely glacial till, etc. Of course I have had to make due with what ever reactions I could lay my hands on also. Load measuring devices also have varied to as crude as a spring scale at some remote places where I brought the gear there by air. Give it some thought before giving up. First off of course an estimate of loading equipment needed is made by estimating how strong the stuff is. And of course evaluate how representative the test site size is for the overall site strengths.

It helps to have a machine shop and welding experience.

 

[Mccoy]

Orusa, I concur that oldestguy's method would be the best since you would have direct measures, providing the setup is representative enough of the rock mass conditions.

The way such problems are treated by means of field structural analysis are the following:

IF the rock mass can be considered istotropic at the scale of interest, then you can apply the Roclab method which yields the mohr coulomb strenght parameters or the Hoek-bray parameters. You only need measures or estimates of intact rock compressivve strenght and a visual assessment of rock mass conditions. Roclab is a freeware distributed by Rocscience.

IF the rock mass is anisotropic and failure would be governed by specific joint sets, then you'll have to define strenght parameters along those joint sets, by the Barton-Bandis method for example.

The above methods (especially so the 1st one) only require field testing and observations, although you can carry out lab tests on the intact rock, or maybe you can take away the Whole joint with the intact rock mass around and test it in the lab.

If you have different areas of alteration and rupture, as you hinted, then you must include in the analysis more than one homogeneous soil or rock types (you should build up a suitable geotechnical model).