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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.

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It would depend on soil type and the load on the foundation. As a very rough rule of thumb, the soil actively supporting a foundation spreads at a 45 degree angle downwards from the outer lower edges of the foundation. You should be able to dig outside this section with impunity. If you encroach upon it, if you only excavate something like 15 foot sections at a time, assuming the wall is longer than this, you shouldn't encounter any issues unless the foundation is extremely heavily loaded, such as supporting a large concentrated load.

When backfilling, be sure the soil within the support prism is very well compacted, but be careful not to collapse the pipe.

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