To what depth below the base should plastic material be removed?

• A. 24 inches (2 ft)
• B. 12 inches (1 ft)
• C. 36 inches (3 ft)
• D. 6 inches (0.5 ft)

Answer: (A)

Plastic soils are problematic under pavements because they become soft and unstable when wet and can consolidate and swell, leading to differential settlement of the pavement above. To create a reliable foundation, the layer containing that unstable, high-plasticity material must be cut out to a depth where the remaining soil is non-plastic enough to support loads when compacted.

In this case, the specified depth to remove below the base is 24 inches. This depth ensures that the troublesome plastic material is removed and replaced with suitable, well-compacted material that provides adequate bearing capacity and minimizes future settlement. Removing only a shallower amount (such as 6 or 12 inches) wouldn't reach the unstable layer and could leave the subgrade prone to movement, while removing more than necessary (like 36 inches) adds cost without additional benefit.