No Soil Under Part of Basement Foundation Near Sump/Sewage Pumps - Normal? (engineered, smell)

[vexious]

Good day all,

Looking for some fellow SE feedback. Just had a home inspection done on a home in Denver, CO with a basement. The utility room has a sump pit and a sewage pump right next to each other. Looking into the sewage pump, you can tell water overflowed into it's void from the sump at one point. The dirt has the consistency of peanut butter. The seller just fixed the sump.

However, the inspector thinks it washed away about a foot deep and 10x10ft of soil under the concrete basement slab. Does this seem right? When I looks at the pictures, I don't really see signs that the foam had dirt directly under it to begin with, but even still, that much dirt being washed away?. It does look like the concrete is just floating and the void does not seem normal? It goes > 4-5 inches of concrete, 8-10 inches of foam, 8 inch gap, dirt. Apparently the seller had a soil and structural engineer OK this from a past inspection, waiting on their contact info. Is there any case where this would have been normal?

See Images:

https://imgur.com/a/kCkEGUn

[gemstone1]

It would be normal....if....the concrete pour had been designed and engineered to be self supporting. Parking decks are poured that way, but that is by design, not by accident. If you really want to discover why, you'd have to go back to the concrete contractor.
I'd suggest that it would be very unusual for a house foundation/slab to be intentionally poured that way.
Regards
Gemstone1

[chet everett]

Basement floors, or "slabs", absolutely can be designed in a variety of ways. Some of the methods of pouring the slab in areas where soil conditions are less than ideal transfer the load to the footings under the vertical walls and/or "pads" that are poured prior to the floor. In such cases the lack of fill under the slab is of no concern.


The situation you describe, of evidence that the water undermined the slab and/or overflowed the sump pit and sewage effluent basin is a different matter. The primary issue is NOT structural but rather the fact that an effluent basin is designed to be both water-tight and contain all noxious gases that go along with sanitary sewers. https://www.grainger.com/category/su.../N-hv8Z1yzilsj The thought of that contaminated waste being pumped as sump water into your yard and the potential for odors festering under the slab eventually filling your home is literally sickening and could even be potentially explosive. Watch this Cautionary Explosive Sewer Gas Smell Video Steps should be taken to ensure that this is not a health issue.


The further fact that there is visible soil "the consistency of peanut butter" strongly suggest that the either the sump pit or effluent basin are improperly installed / have failed. The standard procedure is to backfill such items with clean crushed gravel or appropriately sized pea gravel. Merely shoving the excavated material around the pit / basin is NOT acceptable practice. The whole system should finally be cemented into place / appropriated anchored so that no 'floating' or displacement by groundwater occurs -- http://www.toppindustries.com/upload...tion_guide.pdf It sounds that the situation described by the OP will require repairs / remediation. The extent of such work can only be assessed by someone familiar with local conditions / code / custom.

[MrRational]

Originally Posted by vexious

...the inspector thinks it washed awayIs there any case where this would have been normal?

No.

The SELLER (or owner) needs to go back and remove the question marks here.
Break up and remove the concrete in the area... then rebuild as required.
Take lots of pictures and measurements.