A contractor is building on expansive clay soil with a plasticity index of 25. According to CBC, what type of foundation system is most appropriate?
Correct Answer
D) Post-tensioned slab on grade
Post-tensioned slabs are recommended for expansive soils with high plasticity index as they can accommodate soil movement without cracking.
Why This Is the Correct Answer
Post-tensioned slabs (option D) are the standard solution for expansive clay soils per CBC. The post-tensioning cables create a continuous biaxial compression across the slab, allowing it to move as a rigid unit when the soil swells or shrinks rather than cracking under differential movement. A plasticity index of 25 falls in the 'medium' to 'high' expansive range, triggering engineered foundation requirements.
Why the Other Options Are Wrong
Option A: Standard concrete slab with vapor barrier
A standard concrete slab with vapor barrier addresses moisture intrusion but provides no structural resistance to the upward heaving forces generated by expansive clay. The slab would crack under differential soil movement.
Option B: Shallow spread footings
Shallow spread footings are appropriate for stable, non-expansive soils. On expansive clay, they would be subject to differential heaving at each individual footing, causing uneven structural movement and potential building damage.
Option C: Crawl space with pier and beam
Pier and beam crawl space construction can work on some problem soils but does not address expansive clay as effectively as post-tensioned slabs. Piers must be drilled deep below the active zone, and the design is more complex and costly without the broad protective benefit of a PT slab.
Memory Technique
Think of 'PT slab = PRETENSION against the PRESSURE' — the post-tensioned cables pre-compress the slab so expansive soil pressure cannot crack it. High plasticity index = high expansion potential = need a foundation that moves as one rigid piece.
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