Assertion (A) Analysis: Quicksand Condition

Assertion (A) states that quicksand is a condition, not a type of sand. This is correct. Quicksand describes a phenomenon where saturated soil loses its strength and behaves like a liquid due to the upward flow of water. It is a state or condition the soil mass enters, not a geological classification.

Reason (R) Analysis: Effective Pressure and Seepage

Reason (R) explains the principle behind the quicksand condition. In soil mechanics, effective pressure (or effective stress), denoted as $\sigma'$, is the pressure carried by the soil skeleton. It is calculated as the total stress ($\sigma$) minus the pore water pressure ($u$):

$ \sigma' = \sigma - u $

When water flows upwards through the soil (seepage), it exerts an upward drag force on the soil particles, increasing the pore water pressure ($u$). The quicksand condition occurs when this upward seepage pressure ($u$) becomes equal to the total vertical stress ($\sigma_v$) exerted by the soil mass above. At this point:

$ u = \sigma_v $

Therefore, the effective vertical stress becomes:

$ \sigma_v' = \sigma_v - u = 0 $

A zero effective stress means the soil particles are no longer pressing against each other, causing the soil mass to lose its shear strength and behave like a fluid. Thus, Reason (R) is a correct statement explaining the physics of the phenomenon.

Assertion (A) and Reason (R) Relationship

The condition described in Reason (R) – zero effective pressure due to equal upward seepage pressure and total overburden pressure – directly causes the phenomenon described in Assertion (A) – quicksand, which is a condition where soil behaves like a liquid.

Therefore, both statements are correct, and Reason (R) provides the accurate scientific explanation for Assertion (A).

Final Answer Selection

Based on the analysis:

• Assertion (A) is correct.
• Reason (R) is correct.
• Reason (R) correctly explains Assertion (A).

This corresponds to Option 1.