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Underpinning? - Do I need it to put a deeper basement in addition?
2

Underpinning? - Do I need it to put a deeper basement in addition?

Underpinning? - Do I need it to put a deeper basement in addition?

(OP)
Hi,

I have spent a few days on this website trying to find a topic for this, but have yet to find this exact scenario.

I have a 35 year old two story house with an 8 foot deep basement. I will be putting a two story addition onto the back of the home, and would like to have at least a 9 or 10 foot deep basement under the new addition.

My question is this:  what is the easiest most cost effective way to dig the basement lower in the addition, as I will be exposing the outside of the existing footer when excavating deeper.

1) Do I need to do a underpinnning?;
 
or

2) can I do a drop down footer method, meaning, can I have the first 4 or 6 feet of the new basement addition be at the 8' level (so as not to disturb the existing footer), then drop down to the 9 or 10 foot level? If so, is 4 or 6 feet far enough away from the existing footer so as to not cause any stability problems; or, could I do a drop down footer even closer to the existing footer.

Thanks in advance for any help,

Jeff

RE: Underpinning? - Do I need it to put a deeper basement in addition?

2
Option 2 is the best bet.  Underpinning is very expensive and you still risk cracks in your walls above.

RE: Underpinning? - Do I need it to put a deeper basement in addition?

(OP)
Yes, I thought Option 2 would be easier/cheaper, but how to you determine a "safe" distance away from the existing footer, before you "step down"?

Thanks

RE: Underpinning? - Do I need it to put a deeper basement in addition?

From the bottom corner of the footing, draw an imaginary line downward and away from the house - at a 45 degree angle.  Any excavation below this line may damage the support of the footing.  As long as you stay above, you should be fine.


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