A concrete slab foundation is being poured on Oahu volcanic soil with a bearing capacity of 2,000 psf. What is the minimum thickness required for a slab supporting a total load of 150 psf over a 20' x 30' area?
Correct Answer
B) 6 inches
For volcanic soils in Hawaii, minimum slab thickness is typically 6 inches to handle differential settlement and provide adequate structural integrity, regardless of the load calculation.
Why This Is the Correct Answer
Even though the bearing capacity calculation might suggest a thinner slab is structurally adequate for the given load, Hawaii's volcanic soils require a minimum 6-inch slab thickness as a code baseline. Volcanic soils are subject to differential settlement due to irregular rock formations and voids, so the minimum thickness provides the necessary structural continuity and rigidity to bridge inconsistencies in the substrate.
Why the Other Options Are Wrong
Option A: 4 inches
A 4-inch slab is the minimum for light-duty applications on uniform, stable soil conditions. Hawaii's volcanic soil conditions and the code's recognition of differential settlement risk require a greater minimum thickness regardless of load calculations.
Option C: 10 inches
10 inches exceeds the minimum requirement for the given conditions. While heavier loads or weaker soils might justify 10 inches, the scenario with 2,000 psf bearing capacity does not warrant this thickness as the minimum.
Option D: 8 inches
8 inches is greater than the required minimum of 6 inches for this condition. It may be appropriate for heavier loads or specific structural requirements, but it is not the minimum mandated for this scenario.
Memory Technique
In Hawaii, think 'Six-inch Shield' — the 6-inch slab acts as a shield against the unpredictable voids and differential settlement in volcanic soil. Six is the floor, not the ceiling.
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