You are constructing a single-story home on the Big Island where volcanic soil conditions exist. The soil bearing capacity is 2,000 psf. If the total building load is 180,000 lbs, what is the minimum foundation area required?
Correct Answer
A) 90 sq ft
Foundation area = Total load ÷ Soil bearing capacity = 180,000 lbs ÷ 2,000 psf = 90 sq ft minimum required.
Why This Is the Correct Answer
Foundation area is calculated by dividing total load by soil bearing capacity: 180,000 lbs ÷ 2,000 psf = 90 sq ft. This is the fundamental geotechnical formula ensuring the soil is not overloaded beyond its capacity to support the structure without settlement or failure.
Why the Other Options Are Wrong
Option B: 110 sq ft
110 sq ft exceeds the minimum required, which is not wrong from a safety standpoint, but it is not the calculated minimum. On an exam asking for the minimum, 110 sq ft represents an over-designed answer that does not reflect the direct calculation result.
Option C: 125 sq ft
125 sq ft is even further above the calculated minimum of 90 sq ft. While a larger foundation could be used, this answer misapplies the formula or uses an incorrect bearing capacity value, and is not the minimum code-required area.
Option D: 75 sq ft
75 sq ft is below the required minimum. A 75 sq ft foundation under 180,000 lbs produces 180,000 ÷ 75 = 2,400 psf bearing pressure, which exceeds the 2,000 psf soil capacity and would cause soil failure and settlement.
Memory Technique
Use the formula triangle: Area = Load ÷ Bearing. Picture a house (load) sitting on soil (bearing) — the soil must spread the weight over an area large enough to stay within its capacity. 180,000 ÷ 2,000 = 90, as easy as 18 ÷ 2 = 9, then add the zero.
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