Effective Stress Principle MCQ with answer and solution

Concept

Terzaghi's effective stress principle states that total stress is the sum of effective stress and pore water pressure. In saturated soils under undrained conditions (immediately after loading), any added total stress is initially carried entirely by pore water pressure.

Formula

Δσ = Δσ_bar + Δu

Step-by-step solution

1. Understand Terzaghi's effective stress equation: σ = σ_bar + u. 2. When a load (fill) is instantaneously added, the total stress (σ) increases by Δσ. 3. Because saturated clay has very low permeability, water cannot escape immediately (undrained condition). 4. Therefore, the entire applied stress is carried by the pore water as excess pore water pressure (Δu = Δσ), meaning the change in effective stress (Δσ_bar) is zero immediately after dumping. 5. Thus, the effective stress remains unchanged initially.

Explanation

Immediately after the instantaneous application of a load on saturated clay, the load is entirely taken up by the pore water as excess pore water pressure due to the low permeability and undrained condition. Therefore, the effective stress remains unchanged immediately after loading.

Why the other options are wrong

A: Effective stress does not increase immediately after loading in saturated clay. B: Effective stress does not decrease; it remains constant initially. D: Option C is correct.

Shortcut

In saturated clay, immediately (t = 0) after applying a load, pore pressure takes 100% of the load; effective stress does not change.

Common mistake

Adding the stress of the fill directly to the effective stress immediately, forgetting that saturated clay behaves in an undrained manner initially.