I have opened a 7′ section on a 20′ concrete foundation wall for a patio door. Where there was once a 36″ door. Wall is 10″ thick, 3’6″ above grade and 22″ below, to footing.
To transition from inside out (step down) have to lower the outside grade by 4″, leaving me with 18 below grade. I live in SW CT, frost line is 42″.
So I have dug a 30″ wide and 12′ long, 16″ deep hole which I plan to pour concrete to a height of 4″ below grade. Leaving 4″ for finish concrete in the Spring.
Existing door was added to this 50 yo house some 30 or more years ago and the foundation never moved. No underpinning or any kind of support. I think this is because the wall faces North and there is little freezing & thawing.
Questions:
Make form from 2″ insulation, bottom too. line with plastic and leave until spring?
Tie into existing foundation wall with rebar, size, length, spacing?
Dig hole deeper, wider, longer?