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What to call this type of foundation.

What to call this type of foundation.

What to call this type of foundation.

What do you call the following type of foundation? This is an 8' tall perimeter masonry wall that has been backfilled. It has a concrete slab floor that is poured on the backfill. I do not call it a simple Slab on Grade (SOG) because it is taller than 4'. For lack of a better name, I refer to it as a Reverse Basement even though it is not a basement at all. In a basement, you are under the floor and the backfill is outside the wall. In this situation, the backfill is under the floor and you are outside. I do not call it an elevated slab, since that tends to indicate you can walk under it.

The building code provides no guidance on how to build this. For this reason, I find these all the time where they are treated and built just like a 4' tall one. I have even seen one of these 14' tall.

So I am just trying to find a reasonable name to call it. So far I only have Reverse Basement or Elevated Slab on Grade. Any other ideas?

RE: What to call this type of foundation.

Slab on raised/elevated grade?

RE: What to call this type of foundation.

Conc. slab on partially retained deep engineered fill. That’s a CSPRDEF slab, just one more dumb assed meaningless name to try to remember. I would think your engineering design and proper detailing and plan notes should show what it is without some made-up silly name.

RE: What to call this type of foundation.

The wall itself is just a masonry retaining wall.

RE: What to call this type of foundation.

It is not my design. It was not engineered either. The contractor built it however they thought it would work. The reason for trying to get at least a reasonable name for it has to do with writing a report that others (including engineers) may understand. The wall has noticeable cracks in it in multiple locations. For years I made up my own name-Reverse Basement. I do realize my name is dumb when it is not even a basement. Can't win.

The wall needs to function as a cantilevered retaining wall until the slab is poured and cured. At that point, it is a propped cantilever. Any new load distributes based on a propped cantilever while the stresses and strains prior to the slab will remain in the cantilever wall. At the same time, it is Slab on Grade. So, it is a retaining wall as MiStruct stated, it is a SOG and it is what dhengr stated other than the "engineered fill", it is more likely uncompacted fill.

dhengr, I am with you on all the names we try to create. It gets aggravating. At the same time, I run into these a lot in the areas I work. I fear referring to it as a retaining wall because that tends to imply certain features a retaining wall has that this most likely does not have. Same problem with using "basement".

I have not done any deep investigation but I bet the following will be true whenever it gets done.
-The block set on an 2' max wide foundation; possibly 18"
-There are no dowels in the footing
-The wall has no connection to the footing; just sets on it.
-The block are 8" wide
-The block are hollow or poorly filled; no vertical reinforcing at all
-The backfill was not engineered but it is probably un-compacted sand.
-water from the front yard migrates into the fill material to some degree; the property slopes to the house in the front.

RE: What to call this type of foundation.

I'd worry that the support from the block wall is far different than that provided on the interior given the propensity of fill to settle. This would put a lot more compression into the block wall, reducing its capacity to contain the fill. Am I barking up the wrong tree?

RE: What to call this type of foundation.

I doubt that any prescriptive building code will tell you how to build this. That is what engineers are for. The wall could be a cantilevered retaining wall, or it could be a propped cantilever, with the slab on ground tied in to restrain the top.

RE: What to call this type of foundation.

Slab on Contained Fill sounds much better than reverse basement. So I am now down to "Elevated Slab on grade" or "Slab on contained fill". Contained fill points to where the problem generally is-the foundation fails to contain the fill.

Hokie, I have never seen a code tell how to build it. I do see in the code that foundations over 4' are to be engineered. Problem is, where I am, they do not enforce that rule. This is especially true on residential. So I wind up looking at a bunch of these on steep sloped lots (especially the lake). I design them as cantilevered walls and then do the propped thing. But builders here are free to do as they please. I am not complaining, I make more money checking out the mess and designing the repair than designing one from scratch. I literally have seen one of these 14' tall and cracked everywhere.

Check out these 2 pictures of an 11' one I did not design but had to look at the failure. This is an 8" thick "lightly" reinforced concrete one, but the contractor felt he did not need to tie the slab to the top of wall. Then he poured the slab but did not build the garage for weeks. Rain got into the fill. In one picture, that is a brick between the wall and slab. The footing is 2' wide with the wall in teh center of it

RE: What to call this type of foundation.

IFRS-you are correct. This is the bathroom door near the area being investigated. The door is fully closed in the picture. What contractors do is "sprinkle" the fill sand in rather than compact it. This gets them their paycheck. Some of them get the bucket as close as possible before they drop the load to minimize compaction. I had one "explain" to me how you successfully build one of these. Unfortunately , his was one we were reviewing that had a 2" depression across 10'

RE: What to call this type of foundation.

Well…., you’ve got the ‘what’s wrong with this foundation design (?) and construction’ part of your report half written when you elaborate on the seven items you’ve already listed; and explain why they are wrong, and don’t work well, with some calcs. and details of the existing conditions. I agree with you, I’d bet they all exist in that found. wall. Then, show a proper design, calcs. and details, with important notes for those general grade conditions, and explain why your general design and thinking are proper, according to code intent (real code intent, maybe not a para. #) and rational engineering principles, and a little common sense and good engineering judgement. The code doesn’t cover every possible dumb contractor idea and detail in the universe. Don’t forget the notes on proper fill and the need for wall bracing until the slab is in place and cured for some time.

RE: What to call this type of foundation.

Well, for me, the winner is "Elevated Slab on Confined Fill". Only half a mouthful and the name indicates the 2 most common issues, the height and ability to confine the soil.

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