During a site layout, the benchmark elevation is 100.00 feet. If the finish floor elevation is specified as 102.50 feet and the slab thickness is 4 inches, what should be the elevation of the subgrade?
Correct Answer
B) 101.83 feet
Subgrade elevation = Finish floor elevation - slab thickness. Convert 4 inches to feet (4/12 = 0.33 feet). Subgrade = 102.50 - 0.33 - 0.33 (for base) = 101.83 feet.
Why This Is the Correct Answer
To find the subgrade elevation, you must work backwards from the finish floor elevation by subtracting all materials that will be placed on top of the subgrade. The 4-inch concrete slab converts to 0.33 feet (4÷12), and typically a 4-inch base/aggregate layer is also required beneath the slab, which adds another 0.33 feet to subtract. Therefore: 102.50 - 0.33 (slab) - 0.33 (base) = 101.83 feet.
Why the Other Options Are Wrong
Option C: 102.33 feet
This answer (102.17 feet) only accounts for subtracting the slab thickness (0.33 feet) but fails to include the required base layer beneath the slab, resulting in a subgrade that would be too high.
Option D: 102.17 feet
This answer (102.33 feet) appears to only subtract the slab thickness without proper conversion or without accounting for the base layer, making the subgrade elevation too high for proper construction.
Memory Technique
Remember 'Top Down Thinking' - start at the finish floor and work your way down through each layer (slab, then base) to reach the subgrade level.
Reference Hint
Look up site work and excavation sections in construction reference materials, particularly chapters covering site grading, elevation calculations, and concrete slab construction details.
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