Passive Resistance for Overturning

[aseeng]

thread255-243275

The thread referenced was interesting and I'm curious of others more recent opinions. For my case consider a residential stem wall extending down to frost depth. Slab on grade on the interior, soil 6" below on the exterior. Frost depth 4'-6", 24" footing, 8" wall. We all know this works - right? Now run the numbers with Ko = 55 pcf. Overturning does not work without considering some passive resistance.

Now consider the same wall has to extend down an additional 4 ft to get to good bearing soils. 8'-6" of soil on the high side - 8'-0" of soil on the low side. If we ignore passive then we have a big retaining wall situation. I have a hard time envisioning the overturning problem here. I'm inclined to keep the design as is because I think the passive side will hold the wall up and it will not overturn. Maybe the failure is a small amount or rotation that affects finishes on the stud wall above?

Thanks for any opinions.

 

[msquared48]

Can you post a diagram of the wall?

Mike McCann, PE, SE (WA)

 

[aseeng]

Yes, see attached sketch. I think most everyone would agree that the top condition of a shallow "frost" wall is not really a retaining wall problem in the traditional sense. My question is if the footing has to be extended lower to reach good bearing conditions does it become a retaining wall problem? If we ignore passive pressure on the resisting side per typical engineering practice and run the numbers for an equivalant fluid pressure of say 55 pcf, then neither wall works. While I understand that we need to move something in order to mobilize the passive pressure I am not necessarily convinced that either wall will actually move. Can you have active or at rest pressures on both sides counter acting each other?

 

[BigH]

Can't for the life of me figure out why, with soil on both sides that you would consider this a retaining wall problem/condition. You'll dig a trench (be careful for OSHA trench rules), construct your footing and wall, then backfill both sides of the wall. Make sure that you backfill "evenly" so that you don't have unbalanced height (and hence, no retaining wall problem), i.e., do not backfill one side only, then the other. Do four or five 6 inch layers on one side, then the same on the other side. I am presuming from your sketch that you have "level" ground conditions.

 

[wannabeSE]

From the link, I think the OP's reference are about the passive pressure on footing, not the wall itself.

Referring to the usual references (Bowles, Coduto, Das), it is customary to neglect the passive pressure developed on the front face (toe side) of the cantilever wall's footing. The explanation given is that " The bottom of the footing may move slightly rearward if any overturning failure were to occur. Therefore, the resistance offered by the passive resistance acting on the front of the footing is not reliable, and should be neglected in the overturning analysis."

 

[aseeng]

Yes, I see that now WannebeSE. The practice of ignoring passive on the footing carries on the wall as well. I only noticed this while reviewing a RetainPro output last week and running some different conditions with high soil in front of the wall. I don't use passive as a rule when designing retaining walls but there seems to be a limit where something else plays into the stability of the wall.

 

[BigH]

Again - with soil on both sides of the wall/footing system, passive and active pressures are a non-issue so long as during backfilling after construction you do not fill in an unbalanced manner. With soil on both sides, which side is "active" and which side is "passive"?

 

[Ron]

Aw come on BigH....don't throw common sense into the mix!

 

[CConrad]

The key here is the difference between a frost wall and a retaining wall. The practice of neglecting the passive resistance to overturning is specific to "retaining walls". That's why RetainPro applies that practice, because it is a program that is specifically intended to design true "retaining walls".

A wall with a 6" grade differential is not a "retaining wall", it is a "frost wall".

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ENERCALC, Inc.
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