A guy who works downstairs from my office is buying a pole building up in NH, intends to make it into a ski cottage. It’s 18×24, sits on a shallow foundation. He’s going to add an 18×24′ addition, and he’s insisting that the addition must have a basement. (storage, mechanicals, etc).
I’m telling him that he’s going to complicate his life tremendously if he wants to do this, because the shallow footing on the existing building will be undermined by the excavation for the basement. As a compromise, he thinks he can live with a smaller basement – per the attached sketch(es). The stairway will still require a dig right at the existing foundation.
I’m curious if anyone has any great ideas for shoring the existing foundation, or some cautionary thoughts, or alternative solutions. Please have a look and post back if you can shed some light on this situation.
Thanks in advance!
Replies
The engineer's going to require that he deepen the shallow foundation where's it's undermined by the new dig, either by pouring more concrete under it, or by pouring next to it and further down (huh?). Usually you have to maintain a minimum 45 degree relationship between the bottom of the existing foundation and any digging done adjacent. Depends a lot on the soil. Your friend is creating a major amount of dirt work for himself, but hey, he'll have a basement.
are you sure the polebuilding is on a monolithic slab? Or is the slab poured around poles set deeper?
I expecthe will have to sink piers under the existing.
The other Q is ground water. If he has it and sets perimeter drains to keep this basement dry, he will be draining the spoil and causing the existing to settle possibly unevenly and resiuulting in all sorts of government action ( another phrase for unintended consequences)
He should havea competent soils enfgineer before he turns over a shovelful of either dirt or money.
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real good question Piffin - about the posts. I'll check the plans tomorrow morning.
when you say sink piers under the existing - what exactly do you have in mind. can you elaborate?
Assuming that it is a mono with post and beam setuyp above ground, therre are stress points at the posts. sinkhydro jack piers und at those points before excavating the new.
Or, if that system is unavailable there, dig one point at a time, maybe working with cason setup, to place pier under the existing slab.
The details would have to be between the contractor and the engineere. Thgis idea may only be worth what you paid for it.
But definitely find out about the poles as existing now.
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OK I'm following you better now. The frame is sitting on the slab and not penetrating thru. THanks for the clarification; this guy's got his work cut out for him. Site is probably full of granite, too!
Knowing a little about New Hampshire, I presumed that might be why the original was built without an excavation.
Dynamite - hmmm....
We'll be hearing more about this basement won't we?
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