In volcanic soil areas of the Big Island, what is the recommended foundation type for single-family residences?
Correct Answer
B) Post-and-pier with grade beams
Post-and-pier with grade beams allows for irregular volcanic soil conditions while providing structural stability and accommodating minor settlement variations.
Why This Is the Correct Answer
Post-and-pier with grade beams is the recommended foundation system for volcanic soil on the Big Island because volcanic rock and soil is highly irregular — it may have voids, varying hardness, and unpredictable bearing capacity. Post-and-pier allows load transfer to sound bearing points while grade beams tie the system together and accommodate settlement variations without cracking a continuous slab.
Why the Other Options Are Wrong
Option A: Deep pile foundations
Deep pile foundations are used in very soft soils (like marine clays or loose fill) where surface soils cannot bear load. Volcanic rock is generally hard and competent at shallow depths, making deep piles unnecessary and difficult to install through basalt.
Option C: Slab-on-grade with thickened edges
Slab-on-grade with thickened edges requires a relatively uniform, level, and stable subgrade. Volcanic terrain is too irregular and variable for a continuous slab to perform reliably without risk of differential settlement and cracking.
Option D: Shallow spread footings
Shallow spread footings assume consistent, uniform bearing capacity across the footprint. Volcanic soil's irregular cavities and varying density make uniform bearing unreliable, ruling out standard shallow spread footings.
Memory Technique
Lava flows create uneven ground. Picture piers 'stepping' across the uneven volcanic rock like stilts, connected by grade beams that keep everything level. Stilts on lava = post-and-pier.
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