ASTM Gradation for Very Large Diameter Material

[scottm8]

Hello,

I am working on a project that involves relocation of a waste rock pile consiting of material with diameters up to 6-12 inches. Most of the material is very large, only a small fraction appear to be fines.

Is there a way to determine the grain size distribution of this material for use in correlating D10 with internal friction? Most of the material would not fit in a standard ASTM D422 aparatus. I would also be interested in any other ways to determine internal friction (for use in modeling slope stability)... box shear? Any other ideas? Thanks!

 

[philu]

Hi Scottm8

A good visual investigation will may help you better then implementation of few tests (difficult to realize if you haven't any suitable devices, like in stone pit or career devices), and that will be then used in correlation , and not provide you a direct final value (friction angle - Phi).

In your case if most of the material is really composed of small rock blocs from 6 to 12 inches, then the global behaviour of this material is surely granular. Moreover if blocs mainly present multiples angles the friction angle will easily rise up to 40° - 42° or more

If you have easy and direct access to the site, a second way of investigation is to note the tilt of the slope of some existing heap of this material. This natural angle will be close to the effective friction angle.

As this predictive value of Phi is relatively high (40 - 42 ° ?), this make it not very sensitive for your computation, (such a granular material get a strong strength, that won't change lot of ratio in your stability computation results), so you can accept a certain error range on this parameter, and consider for safety, parameter coming from the lower part of your uncertainty range.

This depends on the risk related to your project accordingly to your design. Up to you to appreciate it. If the associated risk is low or normal, trusting on your material description, I think that friction angle will rise easily to 40°

Picture of the material could help to get a remote judgment.

Regards

 

[Erdbau]

Try to find "Review of Shearing Strength of Rockfill" by Thomas Leps in July 1970 ASCE Journal of Soil Mechanics and Foundations. It contains a review of large-scale triaxial tests. That will at least give you a justifiable basis.

No idea of what your budget is, but there are few projects with budgets in which large-scale testing will be practical.

Even if you were able to get a particle sizer analysis done (and you would need A LOT of material) I would be cautious about using a correlation to D10. I don't know the exact correlation you are talking about, if it is for soils like sands, are you significantly extrapolating the correlation?

 

[emmgjld]

I have run size analysis on samples with some boulders as large as 3 feet, (It takes a very full dump truck) and have run a number of direct shears on the -#4.
In my experience, with my materials, it appears that most 'mature' river deposits, the -#4 is about the same degree of 'shape' (round to sub-round) as the coarse fraction and the shear strength comparisons to final construction appeared good.
In my experience, with my materials, it appears that for most colluvial & debris fan deposits, the -#4 is not the same degree of 'shape' (angular to sub-angular for the larger sizes & sub-rounded to sub-angular for the -#4) and the shear strength comparisons to final construction appeared conservative.