SW soil class is defined as which?

• A. SP: Poorly graded sand
• B. ML silt with low permeability
• C. SW: Well graded sand, which is poorly sorted (wide variety of grain sizes), less permeable than SP but still more permeable than silt or clay
• D. CL: High plastic clay with high cohesion

Answer: (C)

Understanding how the USCS labels soils helps here: SW means well-graded sand. Well-graded indicates a broad mix of particle sizes, so the sand contains a range of grain sizes rather than being uniform. That varied gradation allows the particles to pack together more densely and creates a sand with characteristic flow and strength behavior that sits between coarse, uniform sands and the finer-grained soils. In terms of permeability, well-graded sand generally has lower permeability than a poorly graded sand (which has larger, more connected voids) but higher permeability than silt or clay, which have much finer particles and much slower flow. The other descriptions refer to different soil types—poorly graded sand, silt with low plasticity, and clay—so they aren’t SW.