unusual house foundation - what?

A house up the street is being built and they've done something that doesn't seem to be logical.
The lot slopes a bit. No basement per se, but they had to remove soil in ~20' length x 30' width, sloping from 5' high to 0 along the 20'.
They then poured footers - along the perimeter, and several interior ones as well.
Then, they built CMU walls about 5' high in the area described above (20'x30'). In doing so, they "made" a couple rooms - without doors. Not small ones, either as the one is about 20' x 15'. One is probably 8x8.
These are on the front side of the house. The back side of this section will be at walk out level w/back yard, and (presumably) have regular frame walls.
Now a dump truck came along and dumped dirt, partially into this "room sans doors. The next day, the "room" was totally filled in with dirt. Haven't gotten around to seeing if the smaller room is also filled in, but I would guess so.
What are they doing?
Why not just do regular footers (30" deep, and stepped, or w/basement area), instead of digging a hole, then filling it back in - after going to the trouble of building walls that are going to be buried (but, presumably, serve as the base for walls above)?
Thanx Renata
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on 9/13/2007 9:28 AM Renata said the following:

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Bill
In Hamptonburgh, NY
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on 9/13/2007 9:51 AM willshak said the following:

also could be for a garage slab, and an entrance lobby slab.
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Bill
In Hamptonburgh, NY
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wrote:

Garage is on the other side of the house. A small part may, indeed, be an entrance slab.
I just don't understand why they went to the effort of digging a hole, only to fill it back in. Why not go ahead and make a basement. A garage, of course, would make sense, but I now for a fact, that the garage is (was?) on the other side. Unless they modified the plans...
Plus, I'm not sure the soil has been compacted. But, they went ahead and poured some concrete - kinda sloppily though.
Renata
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Sounds like slab on grade with frost footings. It is often a debate in areas where the frost depth is deep--if you go down 5 feet, why not go down 8 or 9 and make a basement? Well, some people don't like basements. Some sites have too much potential for moisture issues. Basements do cost money. As to why they don't just dig the footings, I don't know that either, but there could be many reasons. Masons need room to work--a lot easier for them to have the entire hole excavated. Also, when running an excavator, it is a challenge to get the holes in the right place. Might be a better bet just to dig a hole that is surely in the right place than digging footings, finding out they are in the wrong place, coming back, etc. Thats what our excavator does. Maybe it was old fill that they wanted to get out of there. Maybe the architect just specced it because he thought it was a good idea. Usually the soil is usually compacted--how do you know it wasn't compacted? Alternatively, the concrete could be spanning from footing to footing so settling isn't an issue. Can't really tell from here if it's sloppy or not. What do you mean by sloppy? rough finish? Maybe it's going to be covered with tile or carpet. Concrete is slopped around? so what? Maybe they are complete hacks, I don't know, but they might know exactly what they are doing, too. Why not ask them instead of us?
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HA! Thats the standard way in SW FL. They dig out the existing soil to get approved tampable material installed to accept a concrete slab. The reason they don't make a basement is the height of the water table. If you dig down in undistrubed soil 3' deep it will be full of water an hour later.
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