Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: When the water moving downward through a fine textured soil encounters a layer of course sand, the downward movement is temporarily increased until the soil suction in wetting front is sufficiently reduced
Reason R: Presence of large macrospores in the coarse sand are too large to take in water from the unsaturated microspores of the soil rapidly.
In light of the above statements, choose the most appropriate answer from the options given below

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

This is the principle of a capillary barrier. Water will not cross from a fine-pored soil into a coarse-pored soil (like sand or gravel) until the fine soil is nearly saturated. This property is used to design soil covers for landfills to prevent water infiltration.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Water movement in unsaturated soils is driven by matric suction (capillary forces).
Fine-textured soils have small pores (micropores) that exert high suction forces, while coarse-textured soils (like sand) have large pores (macropores) that exert very low suction forces.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion A: "When the water moving downward through a fine textured soil encounters a layer of coarse sand, the downward movement is temporarily increased..."
This statement is incorrect.
When a wetting front moving through a fine-textured soil encounters a layer of coarse sand, the downward movement actually stops or is delayed temporarily.
This phenomenon is known as a capillary barrier.
Because the micropores in the fine soil have a high affinity for water, the water will not move into the larger macropores of the coarse sand until the fine soil layer directly above the sand becomes nearly saturated (reducing its matric suction to near zero). Only then can water flow into the sand.
- Reason R: "Presence of large macropores in the coarse sand are too large to take in water from the unsaturated micropores of the soil rapidly."
This statement is correct.
Under unsaturated conditions, the matric suction of the micropores in the fine soil is much higher than that of the macropores in the coarse sand.
The large pores in the sand cannot exert enough suction to pull water out of the smaller pores of the fine soil, preventing rapid water movement across the boundary.
Therefore, Assertion A is false, but Reason R is true.

Step 3: Final Answer:
The correct option is (D).