CSLB General Building (B)Site Work, Foundations and ConcreteMedium
A soils report identifies the site as having highly expansive clay soil with significant seasonal swell and shrinkage. Which foundation design approach is most appropriate to mitigate this condition?
- AA gravel spread footing with minimal reinforcement
- BA shallow footing poured directly on the expansive clay without treatment
- CA post-tensioned slab or deepened perimeter footings with proper moisture control
- DA standard slab-on-grade with no additional reinforcement
Show answer & explanationAnswer & explanation
Correct answer: C. A post-tensioned slab or deepened perimeter footings with proper moisture control
Expansive clay soils swell when wet and shrink when dry, causing foundation movement and cracking. Mitigation typically involves post-tensioned slabs, deepened footings below the active zone, and moisture barriers or drainage to reduce seasonal moisture fluctuation.
Why the other options are wrong
- A. Gravel spread footings do not address the swelling/shrinking behavior of expansive clay.
- B. Pouring directly on untreated expansive clay risks severe foundation heave and cracking.
- D. A standard unreinforced slab is highly vulnerable to cracking on expansive soils.
Expansive Soil Mitigation
Design techniques such as post-tensioned slabs, deepened footings, and moisture control used to counter swelling/shrinking clay soils.
- Expansive clay swells when wet, shrinks when dry
- Post-tensioning resists differential movement
- Moisture barriers and proper drainage reduce seasonal soil moisture change
Memory trick: Swelling clay needs a strong, tensioned slab to stay.