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# Friction angle of crushed rock2

## Friction angle of crushed rock

(OP)
There's a thread I couldn't respond to on this subject - https://www.eng-tips.com/viewthread.cfm?qid=196797 where the general opinion was 40 degrees was as high as you should go. FHWA did some large scale direct shear testing and got more than 50 degrees - FHWA Publication No.: FHWA-HRT-13-068 https://www.fhwa.dot.gov/publications/research/inf...

My own experience with clean crushed 3/4 inch rock is that it has a very large angle of repose.

I'm wondering if there's been any change in opinions on this subject since 2007?

### RE: Friction angle of crushed rock

#### Quote (howardoark)

...general opinion was 40 degrees was as high as you should go.
I'm wondering if there's been any change in opinions on this subject since 2007?

Per the FHWA Publication in the link, seems the answer is "No":

The graphs and figures in the publication may cause confusion since they show the "Peak Friction Angle" which is in the 49o+ range. The "fine-print" in the recommendations clarifies this by explaining the peak friction angle is at the top of a statistical distribution and "two standard deviations" above the suggested 39o friction angle.

### RE: Friction angle of crushed rock

It falls off with increased confining stress. I think even 50+ is possible at low confining stress, but the envelope is highly non linear and you wouldn't want to do a model or calculation using a linear mohr-coloumb failure envelope based on a friction angle measured near the start of the graph

>My own experience with clean crushed 3/4 inch rock is that it has a very large angle of repose.

Angle of repose does not equal friction angle. Angle of repose measured on a stockpile of material is going to be dominated by 3D effects (which could be a factor of 1.5 to 2.0), and the influence of low confining stresses (and perhaps suction / partially saturated soil effects depending on the material).

### RE: Friction angle of crushed rock

Depends mostly on compaction and effective stress, in dam design you sometimes see a strength envelope for the rockfill which decreases with an increase in effective stress for very tall dams, or they just use 37 to 42 degrees which doesn't usually drive the results as the problem is with the foundation materials. Included below are snippets of the effect of shear strength vs relative density and shear strength vs confining stress as well as a table which has the references of the researchers looking at the different factors.

### RE: Friction angle of crushed rock

https://www.researchgate.net/publication/343971963...

Some newer research, with an even higher max for some material types

Note that these are all from large-scale triaxial testing not angle of repose - it would be interesting to compare the two. But I would be very hesitant about relying on angle of repose for anything, however appealing of an argument it might be on a construction site

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