Over-excavated basement

[Hoboneer]

I was called to a row house with party-line common walls where a contractor has over-excavated the basement, undercutting the foundation walls/footings by about a foot. Naturally, he got a stop work order from the inspector. The bottom of new slab isn't going to be below the existing footing level. I've never thought about this, but is it possible to just put the soil back, properly compacted in 6" lifts? Is there a code stipulation? Or is underpinning the only option now? And how to you safely underpin now that all of it has already been over-excavated?

 

[msquared48]

Can you just form and do a CDF pour?

Mike McCann, PE, SE (WA, HI)

 

[PEinc]

I get one of these identical, row house situations almost every year in the Philadelphia area. The safest thing to do is temporarily place a berm of soil or crushed stone against the 1' vertical cut. Then, you start underpinning the 4 walls with individual, spaced-out, concrete, underpinning piers until the walls are supported as deep as you need to go. The hard part is finding a contractor and engineer who really know how to design and install underpinning properly. They all claim to know how; few really do. Those who do know often are not interested in such a small (to them) job.

http://www.PeirceEngineering.com

 

[Hoboneer]

Thanks for the answers.
But what is the theory on just putting the soil back? Let's say, hypothetically, that the final excavation does not need to be lower than the footing. Is there a problem with putting the over-excavated soil back and compacting?

 

[PEinc]

If, as you say, the final excavation does not need to be lower than the existing footing, why was the basement dug 1 foot below the existing foundation? Most older row houses with low basement ceiling heights need to be excavated below the existing bottom of foundation in order to increase ceiling height. These older houses often have foundation walls, often rubble stone, without wall footings and where the bottom of the existing foundation walls is only inches below the bottom of the original floor slab. Therefore, excavation is usually required below the existing foundation wall in order to increase ceiling height. I can't see you getting any significant ceiling height increase unless the house has a block or concrete foundation wall with a wall footing that is wider than the wall and extends to more than about 1.5 feet below the original top of floor slab level. In that case, you may not need to dig below the wall footing, but the new slab would probably then be even with the side of the wall footing and that's not a desirable detail in a basement. Call me if you want to talk about this.

http://www.PeirceEngineering.com

 

[Hoboneer]

Why was it over-excavated to begin with? Have you not been around your share of fly-by-night contractors who have no idea what they're doing?
In all seriousness, in this particular case, it probably will have to be underpinned. I appreciate the input and I do understand all the details.
However, when thinking about his, I came up with my own question, out of my own curiosity, that's why I said: "hypothetically"
Whatever the reason may be: let's say the owner finds it too cost prohibitive, or he has a crazy neighbor that won't allow it and he wants to abandon the project altogether, can you just put the soil back and compact?

 

[MTNClimber]

a contractor has over-excavated the basement, undercutting the foundation walls/footings by about a foot.

But what is the theory on just putting the soil back?

You can't get a compactor to sit on the fill in order to properly compact soil with a footing or wall in the way. Even if you could defy physics, you would still end up with a gap between the soil and foundation element. You're better off placing flow fill or concrete.

 

[PEinc]

This is a row house with party walls. The owner of the house is part owner of the party walls. While local code may require that the neighbors be notified of the intended underpinning work, the neighbors should not have the right to stop the underpinning. However, the underpinning should also not extend into the neighbors' basements beyond the inside face of the foundation wall (or wall footing, if there is a footing).

If the basement slab can be lowered enough to give the desired ceiling height, without needing its subgrade below the bottom of existing foundation, I see no reason why the excavated soil could not be properly replaced. (I just can't picture that the ceiling height could be increased enough to make the effort worthwhile economically.) If the excavated soils is not on site or easy to bring back for replacement, crushed stone could be placed instead of the original soil material. If crushed stone is used for building the new slab subgrade up to where it needs to be, I would recommend that a geotextile fabric be placed between the vertical face of the excavated soil under the foundation wall and the crushed stone fill in order to prevent loss of fines and possible wall settlement.

Whatever course of action is chosen, I recommend that the party walls be survey monitored for movement until after construction is finished. I also recommend that a photographic survey be done ASAP so that it can be referred to when the neighbors claim damage.

http://www.PeirceEngineering.com

 

[oldestguy]

In compacting the soil, it might take some horizontal compaction under the footing to get some load transfer onto the new compacted fill.