Footing on Slope and on Top of Slope 4

[pelelo]

Engineers,

NAVFAC Design Manual DM 7.2 provides approaches of computing Ultimate Bearing Capacities for footings on slopes and on top of slopes.

For footings on top of slopes, if the footing is located "far" (e.g b is high) from the slope, bearing capacity can be computed using the traditional bearing capacity equations. See Attached.

My question is, what is the value of b (b = the offset between the edge of the slope and the footing) to consider a footing as "far" enough to use the traditional BC equations?

This is not clear to me.

 

[Lomarandil]

The theoretical answer of course would be to solve both, and find b sufficient that the slope case no longer governs the bearing capacity. I think slope stability (which is what we're really talking about here) is a complex enough issue that it's not easily boiled down to a quick formula for a critical value of b.

I would expect that for most slopes, that's between b=0.8*h and b=1.5*h. (h being slope height). If I needed a quick and dirty estimate, I'd put down something between 1.2h and 2h depending on conditions and magnitude of load.

----
just call me Lo.

 

[GeoEnvGuy]

You can draw out the failure plane using the entry angle of phi the rotation depth of the footing width and exit angle of phi/2. Where the exit comes out before the edge of slope you are to close.

 

[jdonville]

Run a batch of limit equilibrium slope stability runs varying the distance if it's critical...

 

[Guest090822]

These old charts are terrible. AASHTO 8th Edition, Chapter 10 has new tables that are much easier to follow. The tables are based on more recent research. The only drawback is there is no footing embedment included. This is a big problem in construction engineering. 9 out of 10 times when a contractor is working under a bridge near an abutment there is a slope. Their engineers always use presumptive bearing capacity for any temporary shoring foundations, scaffolding, etc. and I constantly have to tell them that can't because they are on a slope. Good think geotechnical applications have a high factor of safety across the board!