Better Angle of Friction

[butch81385]

Hello,

I am currently working on a design to resist horizontal loads through friction and passive pressure. I have gotten the system to work except that the soil is failing in shear. We are going to be using 3/4" crushed aggregate. If it is helpful I may be able to specify a certain type of AASHTO aggregate, though I was unable to find friction angles for any of them.

I need a friction angle between 33.5-39.7 (depending on how much they let me rely on the piles for horizontal resistance).

The system has concrete ballast blocks that provide a load of 1.46 ksf along the surface.

My questions:

Is there a calculation to see how much the angle of friction will increase due to overburden? (Field or lab testing is out as they are requiring full calcs for everything first).

Is there any way to specify gravel with a higher angle of friction? (AASHTO has a chart from the US Dept. of the Navy which gives gravel an angle of friction of 29-31)

Thanks in advance

 

[DRC1]

If you are using crushed clean hard stone such as traprock,
the phi angle should be around 40 degrees. This may vary based on local material and condition of the stone (clean, process,stone & natural soil mix) I would ask a local geotech. As for increasing the phi angle by icreasing the overburden that does not happen. The phi angle is a material property. The passive resitance of the soil is a combination of the phi angle and overburden pressure.

 

[butch81385]

But would the overburden (especially 1.46ksf)add to the compaction of the stone, making it "heavily compacted"? I know that sand increses its friction angle based on compaction and figured that stone would do the same.


I did find out, though, that my original question is moot as the passive pressure does not intersect the next plate and therefore it will not fail in shear as I had calculated.

Thanks for the help.

 

[civilperson]

If the aggregate is angular with fractured faces and compacted in level lifts and then the slope cut with a excavator, (rather than trying to build a slope on the slant), Phi angles up to 42 degrees can be obtained with a 57B gradation.

 

[BigH]

Don't know how the "picture" of your system works - but in the past I have "replaced" the soil in front of a pile cap within and outside the zone of passive "failure" plane using a well graded road base (crushed agg) material - thereby forcing the passive to be completely in engineered fill. I agree that crushed stone in this manner would definitely be greater than 35 degee, I do caution on the use of the passive as you consider larger friction angles due to the exponential rise of "doubling" a value for only a degree or so increase - there is always the rub of using values of phi greater than about 37 to 38 degrees.