A.Dave Crosby, an excavation contractor in Santa Fe, N.M., responds: Because undisturbed soil is usually more reliable than backfill, I typically make the extra effort to dig down to it, especially if the backfill contains soft soil (anything I can easily dig without a pick or bar). However, if the undisturbed soil is a long way below the frostline or has a lot of silt and clay — and I’m confident that I have good, compacted fill — I may dig only to the frostline. In either case, whether the soil is disturbed or undisturbed, what I’m looking for is adequate load-bearing capacity.

Bearing strength is a function of the soil’s composition and density. Dense (because it’s either undisturbed or has been compacted), well-graded soil that is properly drained and has little or no expansive potential should easily support a deck with properly sized footings. The age of the house and the surrounding soil is irrelevant, because no...

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