The plans size the wall thickness and reinforcement from a table indexed by unbalanced backfill height. With an interior concrete slab on grade present, where does the inspector measure that height from and to?
- AFrom the top of the foundation wall down to the exterior finished grade, because that is the portion of the wall exposed above the soil.
This measures the exposed part of the wall rather than the soil bearing on it. Unbalanced backfill is the depth of earth pushing on one side with nothing balancing it on the other.
- BFrom the exterior finished grade down to the bottom of the footing that supports the wall, because the footing restrains the base of it.
Running the measurement to the bottom of the footing adds the footing depth to the height and moves you into a heavier row of the table. The restraint is the slab, which sits above the footing.
- From the exterior finished grade down to the top of the interior slab on grade, because the slab braces the base of the wall.
- DFrom the top of the wall down to the top of the interior slab, because the slab is what stops the wall moving at its base.
Starting at the top of the wall rather than the exterior grade counts wall that has no soil against it. The measurement begins where the earth begins, at the exterior finished ground level.
Why C is correct
The measurement captures the height of soil that is pushing on the wall with nothing pushing back. The top of the measurement is the exterior finished ground level, and the bottom is the level at which the inside of the wall is restrained. An interior slab on grade provides that restraint at its top surface, so the height is measured to the slab, which is why finishing the slab before backfill can reduce the design height.
What this question is testing
Whether the candidate can define unbalanced backfill height precisely enough to use the prescriptive foundation wall tables, including which interior element establishes the lower point of the measurement and how a slab on grade changes it.
On the job
Plan review sets the wall from the table, but the inspector is the one who confirms the site matches the assumption. Final grading frequently ends up higher than the plan showed, especially where a walkout drops away on one side and the fill rises on another, and a wall designed for 6 feet of unbalanced fill can end up with 8. When you see the grading contractor raising the exterior, go back to the table row the plan used and confirm the wall is still adequate.
Memory technique
Outside dirt down to inside floor: the slab is where the wall stops needing help.
Exam tip
Unbalanced fill is measured from the outside ground down to whatever holds the inside face of the wall, never to the footing bottom.
Where to look it up
IRC Chapter 4, the definition used with the concrete and masonry foundation wall tables in R404, and the backfill provisions in the same section.