You discover expansive clay soil conditions during excavation for a residential foundation in Kona. The plasticity index is 28. What foundation system is most appropriate?
Correct Answer
B) Drilled pier foundation system
With a plasticity index of 28, the soil is highly expansive. Drilled piers extending below the active zone provide the most stable foundation solution.
Why This Is the Correct Answer
A plasticity index of 28 indicates highly expansive clay soil that undergoes significant volume changes with moisture fluctuations. Drilled pier foundations extend deep below the active zone where moisture content remains stable, typically 8-15 feet deep in Hawaii's volcanic soils. This system transfers structural loads to stable soil layers, preventing foundation movement from clay expansion and contraction cycles that could cause severe structural damage.
Why the Other Options Are Wrong
Option A: Crawl space with stem walls
Crawl space with stem walls relies on shallow footings that remain within the active zone of expansive clay. The stem walls and footings would experience differential movement as the clay expands and contracts, leading to cracking, settlement, and structural instability.
Option C: Shallow spread footings on native soil
Shallow spread footings placed directly on expansive clay with PI of 28 would be subject to significant heaving and settlement as the soil volume changes with moisture. This creates uneven foundation movement, structural cracking, and potential failure of the building frame.
Option D: Slab-on-grade with vapor barrier
Slab-on-grade foundations sit directly on the expansive clay surface within the active zone. Despite vapor barriers controlling moisture from below, the clay will still expand and contract from seasonal moisture changes, causing the slab to crack, heave, and settle unevenly.
Memory Technique
Remember 'PI over 25, Piers Survive' - when plasticity index exceeds 25, only deep pier foundations can survive the expansive soil movement by reaching stable depths.
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