I'm digging a drainage trench near my foundation, and have hit the bottom edge of that foundation. In this case it's a T style foundation at the edge of a slab, but elsewhere there's no slab. From the drainage point of view I'd like to dig deeper. In this case the soil is heavy almost impermeable clay and the area is not subject to hard freeze. Thusly:
What are guidelines for how much soil must be left undistributed adjacent to and below a foundation?
A related question is: How close should a french drain be to a foundation? which addresses drains near deeper foundations.