Which factor contributes to reduced shear strength in a potentially unstable slope?

Which factor contributes to reduced shear strength in a potentially unstable slope?

• A. Increased cementation
• B. Disturbance of sensitive materials
• C. Uniform grain size distribution
• D. Drying of the surface layer

Answer: (B)

Disturbance of sensitive materials reduces shear strength because these soils have a delicate internal structure that holds grains together. When the fabric of sensitive clays or similar materials is disturbed—by vibration, loading changes, or mixing during excavation—the cohesive bonds break and the soil particles realign, sharply lowering both cohesion and friction resistance. If the soil is saturated, disturbance can also cause a temporary rise in pore pressure, further reducing effective stress and weakening the soil, which makes slope failure more likely. The other factors don’t inherently cause the same pronounced loss of strength: increasing cementation adds bonding and strengthens the soil; a uniform grain size distribution doesn’t inherently weaken the fabric; and drying the surface layer can even increase some resistance due to reduced moisture, depending on conditions.

Disturbance of sensitive materials reduces shear strength because these soils have a delicate internal structure that holds grains together. When the fabric of sensitive clays or similar materials is disturbed—by vibration, loading changes, or mixing during excavation—the cohesive bonds break and the soil particles realign, sharply lowering both cohesion and friction resistance. If the soil is saturated, disturbance can also cause a temporary rise in pore pressure, further reducing effective stress and weakening the soil, which makes slope failure more likely. The other factors don’t inherently cause the same pronounced loss of strength: increasing cementation adds bonding and strengthens the soil; a uniform grain size distribution doesn’t inherently weaken the fabric; and drying the surface layer can even increase some resistance due to reduced moisture, depending on conditions.