Residential Building Inspector B1Footings and FoundationsHard
A builder is constructing a residential slab-on-ground foundation in an area prone to expansive soils. Which of the following measures is MOST effective in mitigating potential damage caused by soil expansion?
- AUtilizing a post-tensioned slab design or structural slab over void form.
- BPlacing a 6-mil polyethylene vapor retarder directly under the slab.
- CEnsuring the slab is at least 6 inches thick with 1/2 inch rebar at 12 inches on center.
- DIncreasing the concrete compressive strength to 4,000 psi.
Show answer & explanationAnswer & explanation
Correct answer: A. Utilizing a post-tensioned slab design or structural slab over void form.
Expansive soils swell when wet and shrink when dry, exerting significant forces on foundations. A post-tensioned slab or a structural slab over void form is specifically designed to resist these differential movements by either keeping the slab in compression or allowing soil movement without direct contact with the slab, making them the most effective mitigation strategies.
Why the other options are wrong
- B. A vapor retarder prevents moisture from entering the slab, but does not prevent the soil itself from expanding and contracting.
- C. While good practice for slab durability, increasing thickness and rebar alone may not withstand the significant, differential forces of expansive soils without specific engineering for those forces.
- D. Increasing compressive strength helps with vertical loads but does not directly address differential movement from expansive soils.
Expansive Soil Mitigation
Mitigating expansive soil damage involves strategies to isolate the foundation from soil movement or design the foundation to resist the forces generated by soil expansion and contraction.
- Expansive soils swell when wet, shrink when dry.
- Differential movement causes significant stress on foundations.
- Engineered solutions like post-tensioning or void forms are highly effective.
Memory trick: Soil's big move needs a strong proof.