Consolidation MCQ with answer and solution

Concept

Preloading accelerates the consolidation of cohesive soils by increasing the effective stress, thereby forcing out pore water and increasing soil strength.

Step-by-step solution

1. Identify the soil type: Cohesive soil (clay/silt) has low permeability. 2. Evaluate the objective: Removing water (dewatering/consolidation) cost-effectively. 3. Compare methods: Preloading uses gravity and static weight; Electroosmosis uses electricity (expensive); Wellpoints are ineffective in low-permeability cohesive soils; Vibroflotation is for cohesionless soils.

Explanation

Preloading involves placing a surcharge load (like a soil embankment) over the cohesive soil to squeeze out pore water through consolidation. It is considered the simplest and most cost-effective method for large areas of soft clay compared to mechanical or electrical methods. While it takes time, the materials used (soil) are often readily available and reusable.

Why the other options are wrong

B: Wellpoints are ineffective in cohesive soils due to very low permeability. C: Electroosmosis is effective for silts/clays but is very expensive and complex to implement. D: Vibroflotation is used for densifying cohesionless (sandy) soils, not for removing water from cohesive soils.

Shortcut

For cohesive soils, think 'Consolidation' (Preloading/Sand Drains). For cohesionless soils, think 'Vibration' (Vibroflotation).

Common mistake

Choosing Electroosmosis because it is technically effective for clay, but forgetting the 'cost-effective' and 'simplest' constraints of the question.