The approved plan reproduces the plain masonry wall table for this soil: an 8-inch nominal wall to 4 feet of unbalanced backfill, a 10-inch wall to 5 feet, a 12-inch wall to 6 feet. The rear stem wall is 8-inch block with 5 feet 4 inches of unbalanced fill. What is the finding?
- AAccept it; the table gives values for basement walls, and a crawl space stem wall is not covered by those limits
The tables apply to foundation walls retaining earth, whether they enclose a basement or a crawl space. The soil pressure does not know what is on the other side of the wall.
- Reject it; at 5 feet 4 inches of fill the 8-inch wall is past its tabulated limit and needs a thicker wall or a design
- CAccept it; the 8-inch wall is acceptable once the cores are filled solid with mortar as the mason parges the exterior
Mortar in the cores is not grout and does not provide the reinforced section a taller wall needs. Parging is a dampproofing step, and neither addresses the bending the wall is being asked to resist.
- DReject it; unbalanced backfill against a masonry wall is limited to 4 feet in all cases, whatever the wall thickness
Four feet is the limit for the thinnest wall in the table, not a universal cap. Thicker walls carry more, which is the whole point of a table indexed by thickness.
Why B is correct
The table limits an 8-inch plain masonry wall to 4 feet of unbalanced backfill, and this wall carries 5 feet 4 inches. The remedies are a 12-inch wall, a designed reinforced wall, or regrading to reduce the backfill height. The wall as built is outside the prescriptive provisions.
What this question is testing
Whether the candidate reads a masonry foundation wall table by both thickness and backfill height, applies it to a crawl space stem wall, and knows the available remedies when the table is exceeded.
On the job
Crawl space stem walls on sloping lots are where this appears, because the wall grows tall at one corner while the crew builds the same 8-inch block all the way around. Inspectors measure the fill height at the tallest point rather than at the average, since that is where the wall fails. The failure mode is a horizontal crack along a bed joint at about mid-height, with the wall bowing inward.
Memory technique
Thin block, shallow fill.
Exam tip
Plain masonry walls have short backfill limits. Measure the fill at the tallest point.
Where to look it up
IRC Chapter 4, foundations, the plain masonry foundation wall tables indexed by thickness and unbalanced backfill.