Which layer is the bottommost in a standard runway structure?

Which layer is the bottommost in a standard runway structure?

• A. Surface Course
• B. Base Course
• C. Sub-base Course
• D. Overlay

Answer: (C)

In a standard runway pavement structure, the layers from top to bottom are the surface course (the worn, riding surface), then the base course, then the sub-base course, and finally the subgrade (the natural soil). The bottommost among the given options is the sub-base course because it sits directly on the subgrade and provides the essential transition between the natural ground and the upper, load-bearing layers above it. The surface course is the top layer, the base course lies above the sub-base, and an overlay is a repair layer placed on the surface, not part of the underlying stack.

In a standard runway pavement structure, the layers from top to bottom are the surface course (the worn, riding surface), then the base course, then the sub-base course, and finally the subgrade (the natural soil). The bottommost among the given options is the sub-base course because it sits directly on the subgrade and provides the essential transition between the natural ground and the upper, load-bearing layers above it. The surface course is the top layer, the base course lies above the sub-base, and an overlay is a repair layer placed on the surface, not part of the underlying stack.