Rutting due to subgrade or base issue is primarily caused by which factors?

• A. Weathering of the entire surface.
• B. Insufficient compaction, weakened by moisture, insufficient thickness.
• C. Low voids, excess sand, excess asphalt.
• D. Route and seal, patch.

Answer: (B)

Rutting from subgrade or base problems happens when the pavement doesn't have enough structural strength to support repeated wheel loads. If the base and subgrade aren’t compacted to the proper density, voids remain and the layer becomes less stiff, so it can deform permanently under traffic. Moisture makes this worse by reducing the shear strength and stiffness of the subgrade and base, so loads push and squeeze the material more easily. If the base or asphalt layer isn’t thick enough, there isn’t enough stiffness to spread the load to the deeper, stronger material, causing excessive deformation at the surface. Together, insufficient compaction, moisture weakening, and inadequate thickness are the main drivers of rutting caused by subgrade or base issues. Weathering or patching addresses different distress types, not the fundamental structural weakness that leads to rutting.