NICET - Highway Construction Inspection Practice Exam

What happens to soil's cohesion when the moisture exceeds the optimum level?

1. Improves cohesion and compactness

2. Reduces cohesion and eases particle displacement

3. Does not affect cohesion

4. Enhances cohesion slightly

The correct answer is: Reduces cohesion and eases particle displacement

When moisture exceeds the optimum level in soil, the excess water creates a situation where the soil particles can lose their inter-particle forces and bonding that contribute to cohesion. This happens because as water saturates the soil, the water fills the voids between the particles, leading to the formation of a condition known as "pore pressure." When pore pressure increases, it can effectively reduce the effective stress acting on the soil particles, thereby weakening the friction and adhesion between them. In such conditions, the soil becomes more susceptible to movement and displacement because the cohesive forces that normally help to hold soil particles together diminish. This results in a reduction in the soil's structural integrity, making it easier for particles to shift and leading to potential failures such as sliding or erosion. Therefore, the state of excess moisture undeniably contributes to a reduction in cohesion, facilitating easier displacement of soil particles.