What is the minimum depth requirement for footings in volcanic ash soil (andisol) commonly found on the Big Island?
Correct Answer
D) 36 inches
Volcanic ash soils require deeper footings (36 inches minimum) due to their expansive properties and potential for significant settlement.
Why This Is the Correct Answer
Andisol (volcanic ash soil) is highly expansive and compressible. It has high water retention, and when saturated, it can lose significant bearing capacity. A minimum footing depth of 36 inches is required to reach more stable soil layers below the active zone and to provide adequate resistance against the settlement that andisol soils are prone to under load.
Why the Other Options Are Wrong
Option A: 12 inches
12 inches is a standard minimum for frost-free mainland locations with stable soils. This depth is completely inadequate for andisol, which is expansive and compressible. A 12-inch footing in volcanic ash soil would be at high risk of differential settlement.
Option B: 24 inches
24 inches is deeper than the standard minimum but still insufficient for the problematic properties of andisol on the Big Island. This depth may not penetrate through the most active expansion zone of the volcanic ash layer.
Option C: 18 inches
18 inches is deeper than the absolute minimum for frost-free zones but still inadequate for andisol conditions. The expansive and settlement-prone nature of volcanic ash soil requires the footings to reach significantly deeper stable layers.
Memory Technique
Think of 36 inches as '3 feet deep to beat the ash' — volcanic ash soils require going three full feet down to find stable support. 'Three feet for the ash beneath your feet.'
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