Which contributed to reduced shear strength in a potentially unstable slope?

• A. Increase in gravitational force
• B. Lower pore pressure
• C. Disturbance of sensitive materials
• D. Wetting front

Answer: (C)

Disturbing sensitive materials directly lowers the shear strength of the soil. These materials, such as sensitive clays or certain silts, hold their strength through a delicate fabric and cementation between particles. When this fabric is disturbed—by loading, vibration, excavation, or rapid loading—the bonds break and the particle arrangement reorganizes into a weaker state. That loss of cohesion and friction reduces resisting shear strength, making slopes more prone to failure.

Increasing gravitational force mainly affects the driving forces along a potential slip surface and does not inherently diminish the material’s shear strength. Lower pore pressure increases effective stress and tends to strengthen a slope, while a wetting front can raise pore pressure and reduce effective stress, promoting instability—but the most direct and reliable cause among the options is the disturbance of sensitive materials, which intrinsically weakens the soil’s shear strength.