The trench is 18 inches from the face of the footing and its bottom is 14 inches below the bottom of the footing, and the sewer line is already laid in it. What is the concern the inspector must address?
- ANone; a trench inside the building line does not affect a footing because the footing is loaded from above rather than from the side
A footing is supported by a wedge of soil that extends outward as well as downward. Excavating beside it removes part of that wedge, which is why a trench nearby can undermine a footing it never touches.
- BNone; the concern applies only to trenches that physically undercut the footing, and this trench is clear of it by 18 inches
Undercutting is the extreme case, but the wedge extends well beyond the footing edge. Judging only by whether the trench touches the footing misses the geometry that governs.
- The trench removes the soil that supports the footing, since it drops below the plane sloping down and out from the footing edge
- DThe trench must be at least 24 inches from the footing, which is the fixed clearance required for any parallel excavation
There is no single fixed clearance dimension, because the answer depends on how deep the trench goes as well as how far away it is. The 45-degree plane relates the two, which a flat number cannot do.
Why C is correct
Load from a footing spreads into the soil at roughly 45 degrees, and a trench that drops below that plane removes the material carrying the load. At 18 inches away and 14 inches below the footing, this trench cuts into the bearing wedge and must be addressed by compacted backfill or underpinning.
What this question is testing
Whether the candidate understands that footing loads spread into the soil at an angle, can apply the 45-degree bearing plane to a measured trench position and depth, and recognizes that no single clearance dimension answers the question.
On the job
Plumbers dig these trenches after the foundation is in and often after the slab is poured, and the trench is usually backfilled and covered before anyone sees it. Inspectors on a plumbing rough in an existing basement look for spoil piles along a wall and ask how deep the trench went. Settlement from this shows up as a crack in the wall above the trench line, sometimes years later.
Memory technique
The footing leans on soil beside it, not just under it.
Exam tip
Picture a 45-degree line down and out from the footing edge. Nothing digs below it.
Where to look it up
IRC Chapter 4, foundations, footing support requirements, read with the excavation and trenching provisions in the plumbing chapters.