augmenting existing continuous foundation

[jeng123]

I want to augment an existing typical 10x20 continuous residential concrete foundation w/ masonry stemwall for additional loading. I am evaluating the following two approaches:

1.Pour an additional strip footing width (say 10 to 18 inches wide) w/ same thickness and at same elevation as the existing, adjacent to the existing foundation with dowels epoxied into the existing foundation. Load transfer would have to be entirely achieved through the dowel action. Any thoughts on how to quantify this? I don't think aci318 appendix d really applies here?

2.Same as #1 but provide a concrete key/lug underpin say 6"x6" under the existing foundation. The expanded foundation area would be 6" thicker and below the existing foundation and would extend 6" under the existing foundation. The idea hear is that I would be utilizing concrete shear strength for load transfer.

Any thoughts on these approaches? Any other suggestions?

 

[JedClampett]

The first thing I would suggest is to hire a Geotechnical Engineer to evaluate the situation. You don't say why you think the foundation capacity needs improving, but maybe nothing needs to be done. It's also possible that just increasing the footprint is not the most efficient way to augment your foundation capacity.
My experience with scoping Geotechnical studies is they're actually pretty cheap, especially when compared with reinforcing, concrete and labor.

 

[aeoliantexan]

I would consider underpinning with intermittent rectangular footings constructed underneath the existing continuous footing. Spacing would be limited by the beam capacity of the existing footing/stemwall combination. This keeps the support centered, avoiding creating moment in the stemwall. They can probably be earth formed.