Lowering basement slab in a rowhouse 1

[RagnarSTS]

Is this concerning? I am lowering my basement. My contractor dug few inches below footing and that contractor says that this will be reinforced with gravel and concrete slab, and no need for underpinning.

 

[KevinChez]

I would not undermine the footing. In lieu of underpinning you can do a concrete "bench".

 

[KevinChez]

here is the attachment.

 

[RagnarSTS]

Interesting. Why can’t most projects use the concrete bench method? What are the downsides of following the contractor’s suggestion? Wouldn’t the new slab act as a reinforcement/bench?

 

[KevinChez]

The bench projects into the space. Many clients/architects do not want to lose the space. I have done a double bench on deeper excavations where the cellar is for storage. Underpinning is preferred.

I would not excavate the entire basement, undermining the footings. You may get away with it depending on the soil stiffness but I would not design it this way. If it is determined the slab will brace the bottom of the footing then maybe you could install the slab in a sequence so the entire footing is not undermined. But I doubt a contractor would want to waste time doing that.

 

[SWComposites]

Ragnar - you need to hire local geotech and structural engineers to evaluate the situation. preferably before the contractor does any more digging.

 

[RagnarSTS]

Thanks, everyone. The goal was to lower the slab up to the footing, and he went a few inches beyond that. This particular contractor has completed many similar projects in the DMV area, so I assume he knows what he’s doing. The basement is already excavated, and this is the maximum depth—slightly more than initially planned. His crew is supposed to pour the gravel, rebar, and slab next week. I will get an engineer there just in case. My architect said this should be fine from the initial look, though he is not an engineer but has seen many underpinned projects