What type of soil would be an appropriate roadbed subgrade?

What type of soil would be an appropriate roadbed subgrade?

• A. General Purpose
• B. SP
• C. SM
• D. CH

Answer: (A)

The key idea is that a roadbed subgrade needs a soil that can be compacted into a stable, bearing-capable layer with minimal volume change as moisture fluctuates. A general-purpose soil fits this role because it represents a versatile, workable material that can be compacted to the required density and stabilized as needed to support pavement loads. In contrast, poorly graded sand (SP) has little fines for interlock and strength, and tends to drain quickly, which can lead to insufficient bearing capacity under traffic. Silty sand (SM) contains fines that can make the material susceptible to moisture-related strength loss and frost effects, complicating stability. High-plasticity clay (CH) swells and shrinks significantly with moisture changes, causing substantial pavement distress unless extensive stabilization is used. So, among these options, the general-purpose soil gives the most reliable balance of compactability and stability for a roadbed subgrade.

The key idea is that a roadbed subgrade needs a soil that can be compacted into a stable, bearing-capable layer with minimal volume change as moisture fluctuates. A general-purpose soil fits this role because it represents a versatile, workable material that can be compacted to the required density and stabilized as needed to support pavement loads.

In contrast, poorly graded sand (SP) has little fines for interlock and strength, and tends to drain quickly, which can lead to insufficient bearing capacity under traffic. Silty sand (SM) contains fines that can make the material susceptible to moisture-related strength loss and frost effects, complicating stability. High-plasticity clay (CH) swells and shrinks significantly with moisture changes, causing substantial pavement distress unless extensive stabilization is used.

So, among these options, the general-purpose soil gives the most reliable balance of compactability and stability for a roadbed subgrade.