Lowest Possible Friction Angle for Sand

[nbr1]

What would be the lowest practical friction angle one could expect in a laboratory for a f-m sand?

 

[Ron]

Possibly as low as 30 deg, but more likely 33 to 35.

 

[GeoPaveTraffic]

Generally agree with Ron, but if the grains were rounded and you really tried you might get down to 28 degrees.

Mike Lambert

 

[theCorkster]

Run the angle of repose test on the sand - then you'll know the answer!

 

[ebenozen]

For very clean fine sand (SP), I've seen direct-shear test results around 27-28 deg for moisture contents from 3-15%.

From what I can put together, it seems that a more well graded distribution of fine to coarse sand with some fines is needed to achieve the interlocking and frictional resistance that results in the commonly reported 32 deg phi for sands. I'm curious as to what others think.

 

[oldestguy]

Keep in mind moisture content. With a few percent of moisture content you have an apparent cohesion also. It can be significant, especially for rapid excavations where slopping of banks is restricted.

 

[Mccoy]

The lowest practical is phi_cv, or phi constant volume also called phi critical state (phi_cs), the friction angle for a sand at its lowest state of packing.
There are lots of tables on such value, a function of mineralogy, angularity, shape, grading (interlocking as cited) and more, its most common range for clean sands is 30-34°.
A very useful table for sands and gravel has been published by a BS 8002,1994 regulation I'm attaching it here, I find it very useful especially when dealing with gravels, even though not siliceous (please notehi_cv = A+B in the attached table.

Salgado et al. have shown that some fine content (silt) in sand tends to increase phi_cv because it increases interlocking, so if phi_cv of a clean quartz sand is 33°, for example, the same sand with 15% silt may exhibit 35° and over (I just remember the values were pretty high, more than one may have thought).

 

[nbr1]

Based on the above posts, I am assuming a friction angle less than 15 degrees (which was reported from our lab) for a silty fine to medium sand would be unreasonable (tested in direct shear); assuming a unit wt. of about 110 pcf and a moisture of about 7% in-situ. SPT N-values were very low (1 to 3 blows/ft). According to our lab, the samples were not remolded.

 

[GeoPaveTraffic]

Yes 15 degrees would be unreasonable for any sand.

Did the lab indicate how they managed to test these samples without remolding? I've seen silty sands hold together for a minute or two, but never long enough to get in a direct shear; and even if they appeared to hold togehter they would be distrubed.

Looks like you need to remold the samples and try again.

Mike Lambert

 

[fattdad]

A silty sand could have 49 percent MH fines. If it's an undisturbed sample and run in a direct shear test at a rate that does not allow for pore pressure dissipation, then you could get 15 degrees from the test. Now I'm not saying that the 15 degrees is correct, I'm just saying your lab test could return that value.

f-d

¡papá gordo ain’t no madre flaca!

 

[Mccoy]

Do you have a granulometric curve of the tested sample?

 

[ebenozen]

There's also the possibility that clay aggregates are contributing to the sand fraction in which case, you have an incorrect sieve analysis and USCS code. I've seen this repeatedly. Look at the sieve material to check.