Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Terzaghi's theory of consolidation considers only primary consolidation.
Reason (R): Secondary consolidation takes place only at the end of the primary consolidation.
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:

Primary consolidation is "water-controlled" (hydrodynamic), while secondary consolidation is "skeleton-controlled" (viscous). Terzaghi’s theory handles only the hydrodynamic phase.

Answer 1

The Correct Option is C

Concept: Consolidation in saturated clays involves the expulsion of pore water (primary) and the plastic readjustment of soil grains (secondary). Terzaghi’s classical mathematical model focuses exclusively on the hydrodynamic part of this process.

Step 1: Evaluate Assertion (A).
Terzaghi's 1D consolidation theory is based on the assumption that soil is fully saturated and that the reduction in volume occurs solely due to the squeeze-out of pore water under a pressure gradient. It does not account for the creep or viscous deformation of the soil skeleton. Thus, (A) is correct.

Step 2: Evaluate Reason (R).
The traditional "stage" view suggests secondary consolidation follows primary. However, modern soil mechanics shows that secondary consolidation (creep) actually begins as soon as the load is applied and occurs simultaneously with primary consolidation. It only becomes dominant and measurable after the excess pore water pressure has dissipated. Thus, saying it takes place "only" at the end is technically inaccurate. Therefore, (R) is not correct.