A customer is asking to put an addition with a full depth basement on a house with only a crawl space depth foundation. How is that done without undermining the existing foundation?
Termite30
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It's NOT done without undermining the existing foundation- you need to underpin the existing wall to extend it down to the new basement level. You do this by removing the soil in say a 5' wide area of the wall, removing the existing footing, pouring a new footing at the lower elevation, and then extending the wall down to the new footing. You then move along to another location, do the same thing for 5' more, and so on, and so on, until the wall has been dropped along it's entire length.
Not to sound harsh, but if you have no experience with underpinning (which I'm assuming you don't since you asked the question), either walk away from the job or find yourself a structural engineer and a concrete contractor who are very experienced with it. You're taking a lot of risk of building collapse (read: someone getting killed) if you do it incorrectly.
Bob