Question

In a soil saturated with water

• A. Surface tension increases
• B. Surface tension decreases
• C. Adhesion decreases
• D. Adhesion increases

Hint:

At soil saturation, matric suction is zero because there are no curved air-water interfaces (menisci) to generate capillary tension.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil water behavior in saturated and unsaturated states is governed by physical forces, primarily adhesion (attraction between soil solids and water molecules) and cohesion (attraction between water molecules).
Capillary forces and suction (tension) occur due to the formation of air-water-solid curved interfaces (menisci) in unsaturated soils.
Detailed Explanation:
When a soil is completely saturated, all soil pores (both micropores and macropores) are entirely filled with water.
Because there are no empty pore spaces, the air-water-solid interfaces are absent.
The absence of these curved boundaries means that the capillary meniscus is flat, and the effective capillary suction or tension is zero.
Consequently, the physical effect of surface tension in holding water under suction is minimized or decreases (B).
Additionally, in an unsaturated soil, water is held tightly as thin films over the soil particles due to intense adhesive forces.
Under saturated conditions, the soil particles are surrounded by a large volume of free water, which means the average adhesive force acting per unit volume of water is significantly lower compared to the tightly bound water films.
Thus, the effective adhesion holding the bulk water decreases (C).
This explains why both effective surface tension effects and relative adhesion forces decrease at saturation.

Step 2: Final Answer:
The correct answer is B and C only, which corresponds to Option (D).