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RE: friction angle of saturated soil.

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Thanks Bill,


Just the info that I was looking for.  I currently have a factor of safety of 0.97.   I am looking for a factor of 1.1.  under  very rare circumstances.   Taking into account the angle of friction of the selected backfill  can get a F.O.S of 1.18.  






De : Bill Allen [mailto:t.w.allen(--nospam--at)]
Envoyé : December 12, 2008 1:25 PM
À : seaint(--nospam--at)
Objet : RE: friction angle of saturated soil.


Based on some recent research I've done due to a project I'm working on (a submerged retaining wall), I didn't find anything quantitative but I did find something qualitative. Based on this research, I found that the angle of internal friction (φ) is reduced by about half for granular materials. If that's the case, then the angle of repose would be 45o- φ/2/2. So, let's say φ = 26 o. The angle which would resist uplift would be 90-26/2 or 77 o from horizontal. Don't forget, the resisting weight won't be as much either when you consider buoyancy. All in all, I don't think you will gain that much resistance, but if you are not that far off in the first place, maybe it's enough.


Good luck,



T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509

-----Original Message-----
From: Michael Laplante [mailto:Michael.Laplante(--nospam--at)]
Sent: Thursday, December 11, 2008 12:09 PM
To: seaint(--nospam--at)
Subject: friction angle of saturated soil.


I am evaluating an underground oil water separator.  Basically a large submerged rectangular concrete tank.   While  considering the dead load contribution of the of the overlying  saturated soil on the foundation to resist the hydrostatic pressure, what angle of friction (if any) can I use in order to increase the volume.  Currently I am only taking the vertical  projected area of saturated soil that is  directly above the exterior projection of the foundation.