Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Quick sand is not a type of sand but it is a condition arising in a sand mass.
Reason (R): When the upward seepage pressure becomes equal to the pressure due to submerged weight of a soil, the effective pressure becomes zero.
In the 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:

Quick condition only occurs in cohesionless soils. Cohesive soils (clays) have "C" (cohesion) which provides strength even when effective stress is zero.

Answer 1

The Correct Option is A

Concept: Quick sand is a hydraulic condition in which the effective stress of a soil mass becomes zero due to upward seepage.

Step 1: Evaluate Assertion (A).
Quick sand is not a specific mineralogical "type" of sand. It is a physical state that occurs in cohesionless soils (like fine sand or silt) when water flows upwards through it with enough pressure to counteract gravity. In this state, the soil loses all shear strength and behaves like a liquid. Thus, (A) is correct.

Step 2: Evaluate Reason (R).
The effective stress ($\sigma'$) is defined as $\sigma' = \sigma - u$, where $\sigma$ is total stress and $u$ is pore water pressure. When water flows upwards, $u$ increases. At a critical gradient, the upward seepage force equals the submerged weight of the soil ($i \gamma_w = \gamma'$). At this point, $\sigma'$ becomes zero. This is the fundamental mechanism behind the quick condition. Thus, (R) is correct.

Step 3: Check for explanation.
Reason (R) describes the exact scientific process that results in the condition described in Assertion (A). Therefore, (R) is the correct explanation.