A residential foundation wall in Michigan is 8 feet high with 2 feet above grade. The backfill exerts lateral pressure of 30 psf per foot of depth. What is the maximum lateral pressure at the bottom of the foundation wall?
Correct Answer
B) 180 psf
The backfill depth is 6 feet (8 ft total height - 2 ft above grade). Maximum lateral pressure = 30 psf × 6 ft = 180 psf at the bottom of the wall.
Why This Is the Correct Answer
The lateral pressure calculation requires determining the backfill depth first. With an 8-foot total foundation wall height and 2 feet above grade, the backfill depth is 6 feet (8 - 2 = 6). The maximum lateral pressure occurs at the bottom of the wall where the full backfill depth applies. Using the given pressure rate of 30 psf per foot of depth: 30 psf × 6 feet = 180 psf.
Why the Other Options Are Wrong
Option A: 210 psf
This answer of 210 psf incorrectly uses 7 feet of backfill depth instead of the correct 6 feet. This suggests a calculation error where someone might have subtracted only 1 foot from the total height rather than the full 2 feet that are above grade.
Option C: 240 psf
This answer of 240 psf incorrectly uses the full 8-foot foundation height without accounting for the 2 feet above grade. This is a common error where the backfill depth is confused with the total wall height, resulting in 30 psf × 8 feet = 240 psf.
Option D: 270 psf
This answer of 270 psf uses 9 feet of depth, which exceeds even the total foundation wall height. This suggests a fundamental misunderstanding of the problem setup or a significant calculation error in determining the backfill depth.
Memory Technique
Remember 'Below Ground = Backfill Bound' - only the portion of the foundation wall below grade experiences lateral soil pressure. Always subtract the above-grade height from total wall height to find backfill depth.
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