In saturated soils, the initial consolidation is mainly due to ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 04/12/2023 (Morning)
Detailed Explanation
When a load is placed on a clay layer, the total settlement is usually split into three stages: initial consolidation (the small, almost instantaneous reduction in volume as soon as the load is applied), primary consolidation (the slow volume change as excess pore water drains out) and secondary consolidation (creep after the excess pore pressure has gone).
In a partially saturated soil, the initial stage is mostly the compression and expulsion of pore air. But the question is about a saturated soil, where every void is full of water and there is no air at all.
Why “compression of solid particles” Is Correct
With no air in the voids, and water unable to drain out in that first instant, the only thing that can give a small immediate volume change is elastic compression of the soil solids themselves (the mineral grains, with a very small contribution from the water). That is why textbooks state that, in saturated soils, initial consolidation is mainly due to compression of solid particles. The amount is very small compared with primary consolidation.
Why the Other Options Are Wrong
- A. expulsion of air in voids: Expulsion of air is the main cause of initial consolidation in partially saturated soils, but a saturated soil has no air to expel.
- C. compression of air in voids: Compression of air also only applies to partially saturated soils.
- D. compression of water molecules: Water is nearly incompressible; its compression is negligible and is not described as the main cause.
Key Point
Partially saturated soil: initial consolidation mainly from air. Saturated soil: mainly from compression of the solid particles. Primary consolidation is always from expulsion of pore water.
Key Concepts for Students
- Total settlement = immediate (initial) + primary consolidation + secondary compression.
- Primary consolidation is the stage described by Terzaghi's theory; it is controlled by the permeability of the clay.
- Secondary consolidation (creep) is caused by plastic readjustment of particles and is significant in organic soils and highly plastic clays.
Practice more: this is question 1 in Soil Consolidation MCQs – Part 1. Try the whole set and check every answer instantly.