Consolidation MCQ with answer and solution

Concept

Initial excess pore water pressure in saturated soils under undrained conditions equals the applied total stress increment.

Formula

Δu = Δσ

Step-by-step solution

1. Understand Terzaghi's Principle of Effective Stress: Total stress (σ) = Effective stress (σ') + Pore water pressure (u). 2. Immediately after loading (time t = 0), drainage is zero due to low permeability of saturated clay. 3. Since water is assumed incompressible, the entire applied load (Δσ) is carried by the pore water as excess pore water pressure (Δu). 4. Therefore, initial excess pore water pressure equals the applied load, i.e., Δu = Δσ.

Explanation

According to Terzaghi's theory of one-dimensional consolidation, when a saturated clay layer is subjected to an external total stress (applied load) in undrained conditions (immediately after loading), the entire load is initially taken up by the incompressible pore water because water cannot escape instantly. Therefore, the excess pore water pressure generated is equal to the applied total stress.

Why the other options are wrong

B: Zero excess pore pressure occurs at the end of consolidation. C: Intermediate values occur during the process of consolidation as excess pore pressure dissipates. D: Infinite is physically incorrect and violates load equilibrium.

Shortcut

In saturated soils, undrained loading means load goes entirely to water first (Skempton's pore pressure parameter A depends on soil type, but initial total pore pressure increase equals applied total stress).

Common mistake

Assuming effective stress takes up the load immediately, which only happens at the end of consolidation when excess pore pressure dissipates completely.