Which of the following is a cohesive soil?

• A. Rock
• B. Silt
• C. Sand
• D. Clay

Answer: (D)

Cohesive soils stick together because their fine clay particles bind with each other through electrostatic forces and the presence of water, creating cohesion and plasticity. Clay, with its tiny, plate-like particles, forms strong interparticle bonds that give it high cohesion and the ability to deform without breaking. Sand has larger grains that don’t bond, so it’s non-cohesive and relies mainly on friction. Silt is fine-grained but doesn’t develop the same strong bonding as clay, so it’s not as cohesive. Rock isn’t a soil at all in this context; it’s a consolidated material. So the cohesive soil here is clay.