Which sediment type has the highest permeability according to the given descriptions?

• A. ML
• B. CL
• C. SP
• D. SW

Answer: (C)

Permeability is controlled by how easily fluids can move through the pore spaces, which closely tracks grain size and how the grains are packed. Coarse-grained materials like sands tend to have much higher permeability than silts or clays because their larger pore throats let water flow more freely.

Among sands, poorly graded sand has relatively uniform grain sizes and tends to form a more open pore structure with fewer fines to clog the flow paths. This combination favors easier fluid movement, giving it higher permeability. In contrast, well-graded sand contains a mix of sizes, and the smaller grains can fill gaps between larger ones, creating a more obstructed pore network that can reduce flow compared with poorly graded sand.

Clays and silts have much finer particles and much smaller pore throats, so their permeability is much lower. So the sediment type with the highest permeability is the poorly graded sand.