Retaining Wall - Footing Width

[CBSE]

An architect specified a typical footing with a 3'-6" stemwall and now he wants an 8'-0" wall for a garage. Not a big deal, other than the footing is already poured.

I'm running through retaining wall designs to see just where this thing needs to be...see attached.

When I play with the height of the footing and vary it in depth, the deeper I go, the larger footing I need. This isn't making sense to me. The location of the soil and garage slab relative to the top of the wall are staying the same, however, when I run the calc with an 8ft wall, I need almost a 4ft footing. When I run the calc with basically a 5ft wall, I need a 2ft footing.

Any ideas? Other than the deeper you go the more pressure is on the footing. But in this case, that intuitively does not make sense to me.

 

[oldestguy]

Right. Try depth at 3 times the height you now have and you look at a sheet pile situation. No footing needed there.

 

[KootK]

Yeah, something's not right here. As your footing gets deeper, the gross allowable bearing pressure should be increasing. And that should completely offset the effect of additional overburden. Given that your footing is detailed to resist only vertical load, I would expect that it should be just fine at 2' wide. Or 1'-4" for that matter. I agree with oldestguy in that the mechanism for overturning resistance here essentially is sheet pile action.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CBSE]

Thanks.

Agreed, it just doesn't intuitively make sense to me. I tried my own retaining wall spreadsheets, ENERCALC, and RetainPro and got basically the same weird result with all of them. It isn't inherently different that a typical garage wall with a footing inside so it's kind of strange.

It makes me wonder if retaining wall design guidance is slightly flawed in that if you have a really deep footing but high soil heights on both sides it doesn't now what to do with it. Strange though.

 

[hokie66]

Is it a retaining wall or not? If the footing is not relied on to resist overturning, then the footing width should not change. What does resist the overturning? Passive pressure?

 

[CBSE]

Well, I'm not really sure it is a retaining wall based on the height of soil over the toe. To me, the soil inside, even though slightly higher, doesn't add that much pressure to the outside soil.

In short, it's confusing me.

 

[hokie66]

Why does the architect want an 8' wall? Is he anticipating excavation near the wall at some future stage? How did the extra 4.5' eventuate? Is the site being filled that much? There doesn't seem to be a logical reason to build 4.5' of buried wall.

 

[KootK]

In the past, I've used a couple of approaches:

1) Modified sheet pile. Assume lateral restraint at the footing elevation and deal with overturning via resisting earth pressure on wall.

2) True retaining wall. Design as though the footing were at grade then shift footing down to desired level without changing anything else.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jeffhed]

Using my retaining wall spreadsheet the footings work fine. Are you modeling the wall with passive pressure on the front? Your comment about the deeper you go the larger footing you need sounds like the passive pressure on the front of the wall is not being included in your design. I don't know what your soil parameters are but I used some typical values for my area with an at rest condition. Overturning and sliding aren't an issue, I even put a surcharge on the slab and it worked out.

 

[hokie66]

KootK,
Your solution 2) doesn't sound like you. More like me.

 

[KootK]

Your solution 2) doesn't sound like you. More like me.

Well, maybe there's hope for me turning a profit yet.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CBSE]

I have included passive pressures in the design.

@jeffhed...you're saying the footings work fine at 24" width? To me, it makes sense that they work at a 24" width, but my dang programs are really confusing me...even my spreadsheet is confusing me on this one.

 

[CBSE]

Running an analysis like it is a conventional footing shows that it is fine. I'm sticking with 24" wide footing and 8" stemwall is okay.