Subgrade should be compacted to at least what percentage?

• A. 85%
• B. 95% modified standard proctor
• C. 100%
• D. 90%

Answer: (C)

Full density of the subgrade is needed to provide a uniform, stable foundation for the concrete slab and the tilt-up elements. The 100% target means compacting the soil to the maximum dry density obtained in the laboratory Proctor test. Achieving that level minimizes voids, increases bearing capacity, and reduces the potential for settlement under loads over time. If you compact to less than 100%, extra settlement can occur, leading to cracking or uneven support in the slab and walls. Therefore, reaching 100% of the maximum dry density ensures the most reliable performance of the subgrade under typical tilt-up construction. Field testing confirms this density to verify the specification is met.