Foundation sizes for short retaining walls 1

[bnickeson]

Hello all, I'm dealing with a design issue that crops up numerous times each year and I'm curious on your opinions. We have a situation where a load-bearing building perimeter stem wall needs to act as a cantilevered retaining wall and laterally resist about 18" of net exterior grade difference. That's it. Just a very short wall to retain a little soil. The problem with this is that the analysis for foundation widths when designing retaining walls only takes into account the overturning moment of the soil versus the resisting moments of the footing and any dead loads on the wall. It does not include any passive pressures against the foundation. In our area of the country, we use 3'-0" deep grade beams for pretty much everything, so the lateral soil pressure acts against 3'-0" of grade beam plus a 2'-0" retaining wall. The footing size you get for this is over 3'-6" wide in my case, which seems completely ridiculous for retaining 18" of net soil.

To illustrate the ridiculousness of standard retaining wall analysis on a short wall (using Retain Pro in this case), if the top of soil on one side of a wall was 100'-1" and the height of soil on the opposite side was 100'-0", it would require a 2'-4" wide footing. Obviously that is not realistic.

For these types of short walls where you have significantly more depth of foundation than net height of retaining soil, do you analyze them differently? Do you just look at the lateral soil pressure on the wall above the footing? Or allow the passive pressure against the footing to act as a resisting moment? I'm afraid the architect or contractor will kill me for putting in a 3'-8" footing for retaining 18" of soil.

Thanks.

 

[JAE]

BAretired - I've seen 1/2" EJ's done thousands of times in my practice. A need to separate the "floating" slab on grade with a perimeter "fixed" wall on a foundation.
In fact, in high expansive clay areas this is a must.

The joint should, of course, be sealed with an appropriate sealant (and maintained).
In the OP's case it is an interior slab so not that much water exposure, if any.

 

[BAretired]

I've seen 1/2" EJ's done thousands of times in my practice.

Really? That is interesting, JAE. I do not recollect ever having seen that detail. We do have expansive clay in Alberta, but perhaps not highly expansive clay. With such a large gap between slab and wall, it may be better to assume that the slab does not contribute to lateral resistance, because 1/2" seems to be too much movement to permit at the level shown.

It is not a detail that I would use because removal of the 1/2" thick form between slab and wall before applying the sealant would be difficult. If it is not removed, there will not be a 1/2" gap.

 

[JAE]

No you leave the expansion joint material in the joint, but hold it down perhaps a 1/4" - then sealant over the top. Very VERY typical in the central US anyway - can't speak about your area in Alberta.

 

[BAretired]

It sounds to me that we do pretty much the same thing, JAE. I leave 3/8" AIFB in the joint as well. It's my understanding that AIFB is, in itself, a sealant, so I don't specify additional sealant on top. It is possible that some local engineers do.

This means there is no gap between slab and wall, other than the small amount the AIFB compresses, so I come back to my original argument that the slab resists the horizontal reaction from external soil pressure. It also means that, using either your detail or mine, the Retain Pro analysis is not consistent with the behavior of the as built structure.

 

[JAE]

OK sounds like we are similar.
But the OP was asking about this behavior and design assuming that the interior slab wasn’t there so relying on a non-existent slab doesn’t help the main question.

 

[Tomfh]

Slab restraint aside, this works with 3 foot embedment, due to passive resistance alone.

 

[DoubleStud]

You should have perimeter drain on the side and maybe use 55 pcf at rest pressure? 105 pcf active seems too high for this kind of design. If that is happening, the house will be flooded with water. If this was my project, I probably have 1 ft thick, 1.5 ft heel, 4" toe.