soil friction angle question

[Lion06]

I am hoping someone can help me out a little with this.
I need to confirm that the soil friction angle will equal the angle that the failure wedge makes with the horizontal. Specifically, say I have a retaining wall with soil on the left and a friction angle of 30 degrees. I am now going to draw the soil failure plane from the bottom of the wall to the top of the soil. I draw a line that extends from the bottom of the wall (up and to the left) until it reaches the soil. The acute angle formed between the horizontal at the base of the wall and the failure plane I just drew will be 30 degrees.
Is this a correct statement?
I may be looking in the wrong places, but I can't find it in my geotech book, or foundations book, and I don't have my soils notes at work.

 

[shin25]

For active earth pressure with cohesionless soil, the angle that you are describing (failure with horizontal) will be=
45+phi/2. And phi being the angle of friction.

The situation that you are describing is related to slope stabilty without retaining wall.

 

[UcfSE]

It is not correct. The failure wedge for active earth pressure will depend on the active pressure theory you use. Rankine for instance, assumes a failure wedge forms at 45 + [φ]/2 for the active case, 45 - [φ]/2 for the passive, where [φ] is the soil friction angle. Coulomb will be different.

Look up Culmann's graphical solution also.

This information can be found in Principles of Geotechnical Engineering by Das.

Why do you need to draw failure planes, out of curiosity?

 

[miecz]

You can find the solution on sheet 7.2-62 in NAVFAC DM-7.2. available for free at

http://www.ce.washington.edu/~geotech/courses/cee523/manuals/NAVFAC72.pdf

 

[Lion06]

I have a retaining wall that is 6' from rock and I need to take this into account because a sub bulit a wall to span vertically when it designed to span horizontally. The loading conditions are much different now and I need to check it out. Anyway, it is restrained so I need to use tehe at-rest pressure.
I am assuming the 45+-phi/2 is not valid for at-rest pressure? What would you use for at-rest pressure failure plane?

 

[shin25]

Do you mean that the soil is restrained by the retaining wall that is why the soil is at rest? If the soil is at rest then there will be no failure, eventhough the soil mass will exert lateral pressure.

The classical retaining wall system must consider active earth pressure. This is due to the fact that it takes just a little bit of lateral movement to mobilize active condition in the soil mass. And the amount of deflection in all retaining wall system (that at least we design) deflect enougn to create active condition.

 

[msquared48]

Let the sub pay for getting the help of a local geotech to get the correct loading in the siturtion he has created.

DO NOT take the liability yourself.

Mike McCann
McCann Engineering

 

[UcfSE]

Agree with shin and Mike.