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cawunsch (Structural)
17 Mar 11 14:06
When incorporating a footing 'key' into a cantilever retaining wall footing, is it appropriate to increase the coefficient of friction to a 'soil/soil' interface value as opposed to the 'soil/concrete' value for sliding reisitance?  If so, what is a reasonable value to use, (tangent of soil friction angle, phi)? Also, if using the higher coefficient, should it only be applied to the area 'behind' the key while using the 'soil/concrete' value for the area in front of the key?
ishvaaag (Structural)
18 Mar 11 6:59
I have not made recntly one of these but I would think that the common practice would be to count only on concrete-soil friction and add a stabilizing passive force from the key, particulary if the key is buried (not at the toe) and so in a position where the stabilizing ground is unlikely to be removed whilst the wall is functional.

All this, of course, when no overall instability taking the whole wall and some ground around is expected.
JoshPlum (Structural)
18 Mar 11 12:28
I cannot answer as to what the "common practice" is.  At least not definitively.  But, what I have done in these situations is the following:

1) Keep the friction coefficient the same.  

2) Calculate the sliding and overturning resistance assuming 100% of friction and 30% of the passive pressure resistance.  

3) Calculate the sliding and overturning resistance assuming 30% of the friction and 100% of the passive pressure resistance.

4) Use the values from 2 and 3 which give me the MOST resistance when I calculation my total sliding resistance.  Do the same thing for my overturning resistance.  

I'm not 100% sure where I got this procedure from....a PE or SE review book maybe.  Or, maybe it was from one of my colleagues when I was a newbie.  But, it always made intuitive sense to me.  You shouldn't can't count 100% sliding and 100% passive together becuase those resistances occur at different deflection levels for the wall.  But, at the same time it would probably be overly conservative to assume that they don't interact at all.  

 
cawunsch (Structural)
18 Mar 11 12:42
Thanks for the advice guys.  I've been designing walls for a long time, and I've recently seen a couple of failed walls here in the las vegas valley.  It definitely got me thinking about whether or not I have been 'conservative' enough in my approach.  I like the idea of the 100% / 30% method you described.
ishvaaag (Structural)
20 Mar 11 10:22
Certainly, Josh, to extrapolate what one has seen, read or practiced as "the" "common practice" it is a common (at least potential) error from subjectivity to what most humans remain ... "subject".

Yet it is part of the engineering process that who has to decide how to proceed take at some point some decision, the root of which may be even outside the training in engineering and reach psychological appreciations of safety, personal ability, etc ...

With age one becomes more and more philosophical and aware of the relativity of even the most cherished knowledge one very dearly reached to attain. One reads more and more opinions and has more and more experience and sees that the alternative paths to fulfillment of some intent, or akin intents, have had in fact alternative paths open along the time. One chose typically what then thought was the correct one, but can't deny your mind telling you there were other ways along.
Mccoy (Geotechnical)
4 Apr 11 6:03
In my thinking, the friction angle you would use for your footing key would correctly be a phi_peak angle, as opposed to the phi_cv angle related to the concrete-soil interface.

This if the soil where the key has been posed is in its undisturbed state.

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