properties of granular soil

[qwedsa]

Has anybody got information about effect of grain size of sand or gravel on internal angle of friction or angle of repose?

revaha

 

[UKENG7]

Generally, the greater the particle size, the higher the angle of repose.
For instance dry Sand particles of 0.5mm in diameter may have an angle of repose of approx 30 degrees, small gravels (12mm diameter) will have an angle of repose of about 40 degrees with larger gravels (25mm diameter) being around 50 degrees.
Angular gravels will have an increased angle of repose by upto 15 degrees when compared to smooth gravels of similar size.
Moist (NOT Saturated) sand could have an angle of repose almost double that of dry sands.

 

[GeoPaveTraffic]

I have to disagree with UKENG7. My research over the last 20 years has indicated that there is no direct correlation between grain size and angle of internal friction. Many of the apparent increases are due to specimen size effects during the testing. Other tests results have been tainted by the removal of "large" particles so the samples could be tested in standard size machines. In this case "large" depends on the test. However, many times this results in particles of different mineral compositions and strength being removed. If a series of tests using different maximum particle sizes are ran, then the strength increases as the larger particles are added. However, what was really being measured was the higher strength of the larger particles.

 

[Focht3]

I concur. Particle strength and angularity, as well as soil density, are the key factors in determining "the" angle of internal friction, &[ignore]Phi[/ignore];. I put "the" in quotes because &[ignore]Phi[/ignore]; is not an intrinsic soil property, but rather a numerical convenience.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[dirtsqueezer]

Just in the way of moderation, UKENG7 was talking about angle of repose, not angle of internal friction. There is no for angle of repose. It's just a measurement.

Otherwise...interesting. GeoPave, it sounds like these test method results reflect their own shortcomings rather than the actual soil properties. Is there one that will cover most soil types, or are reliable results limited to just a select few for friction angle. I know when our lab tests soils w/ a high nominal aggregate size, we just use a 'bigger' test, like the vibratory table in place of the D1557. Just interested, it sounds like you're experienced.

 

[GeoPaveTraffic]

dirtsqueezer,

In some ways you are right, the particle size is a limiting factor in triaxial or direct shear tests. But if your goal is to measure the strength of the material with a laboratory test, then you are stuck with one of those methods. Your options become removing the larger particles or using a larger test apparatus. The largest one that I can recall seeing results from was 3 or 4 feet in diameter and was special built for a dam project in central or south america. This problem becomes particularly relevant to the construction of rock fill dams.

With respect to angle of repose and angle of internal friction, I assumed that qwedsa was using the terms interchangeably. While not technically correct, it is common in many areas.

 

[KofK]

GeoPaveTraffic and others,

Can you please provide some published papers (by yourself or others) on this subject?

Thanks