What is a common direct consequence of inadequate subgrade drainage for slabs on grade?

• A. Improved surface finish.
• B. Increased workability.
• C. Moisture-related problems and differential settlement.
• D. Faster curing.

Answer: (C)

When slabs are placed on grade, the soil beneath must provide uniform, solid support. If subgrade drainage is inadequate, water sits in the soil and the subgrade loses stiffness, making it softer and more variable in strength. This moisture leads to uneven support across the slab, so different areas settle by different amounts. The result is moisture-related problems such as cracking, curling, and other defects, along with differential settlement where some zones sink more than others. Keeping the subgrade drier and more uniform helps prevent these issues and promotes a stable, even slab surface.