What is the primary concern when constructing foundations on recent volcanic soil (pahoehoe) in Hawaii?
Correct Answer
B) Uneven bearing capacity and voids
Recent volcanic soil, especially pahoehoe, contains irregular voids and varying densities that create uneven bearing capacity, requiring special foundation design considerations.
Why This Is the Correct Answer
Pahoehoe (smooth, ropy lava) and other recent volcanic soils in Hawaii are characterized by highly irregular subsurface conditions. Lava flows harden unevenly, creating air pockets, tubes, and voids beneath an apparently solid surface crust. These voids result in dramatically uneven bearing capacity across short distances, making standard foundation designs unreliable. Geotechnical investigation and specialized foundation solutions (such as drilled piers to competent rock) are required.
Why the Other Options Are Wrong
Option A: Poor drainage characteristics
Pahoehoe lava actually drains water rapidly due to its porous, fractured structure — poor drainage is the opposite of the actual concern. Drainage problems are more associated with clay-heavy soils or areas with impermeable layers, not fresh lava fields.
Option C: Excessive soil acidity
While volcanic soils can have slightly elevated mineral content, excessive soil acidity is not a primary structural concern for foundations on pahoehoe. Acidity may affect metal components over time but is not the dominant geotechnical hazard.
Option D: High water table
A high water table is a concern in coastal or low-lying areas of Hawaii, but pahoehoe lava fields — particularly in elevated areas like Kona or South Kona — are generally well-drained due to the porous nature of lava. A high water table is not the defining characteristic of pahoehoe foundation risk.
Memory Technique
Pahoehoe = Swiss Cheese underground. The smooth surface hides hollow tubes and voids. A foundation sitting over a void has nothing to bear on — uneven bearing capacity is the threat. Imagine stepping on a hollow chocolate Easter egg: fine until it collapses.
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