Which statement about subgrade compaction using 90%–95% modified standard proctor is true?

• A. It increases moisture content only
• B. It reduces the need for footings
• C. It reduces strength
• D. It improves soil density and strength

Answer: (D)

Compacting the subgrade to 90–95% of the Modified Proctor maximum dry density densifies the soil, packing the particles more tightly and reducing air voids. This higher density translates directly into greater bearing capacity and stronger stiffness, which means the soil can support loads with less settlement and better resistance to deformation under a tilt-up structure. The moisture content is used to reach that target density, but the key outcome of this compaction level is the improved density and strength of the soil. While good compaction can influence footing performance, the primary, true effect described here is the improvement in soil density and strength.