Why is proper subgrade drainage important for slabs on grade?

• A. To improve surface finish.
• B. To maximize load-bearing capacity.
• C. To reduce moisture-related problems and differential settlement.
• D. To accelerate curing.

Answer: (C)

Moisture in the soil beneath a slab-on-grade directly affects how evenly and how strongly it can be supported. If water collects or the moisture content changes, the subgrade can change volume or lose strength, and different areas can settle differently. This differential settlement causes cracks, uneven floors, and joint problems. Proper drainage keeps water moving away and stabilizes soil moisture, so bearing capacity stays more uniform and shrink/swell or pumping issues are minimized. That’s why drainage is essential for slab-on-grade performance. The other options aren’t tied to drainage in the same direct way: surface finish, curing, or just increasing apparent load capacity aren’t the primary benefits of good subgrade drainage.