Piston and spring analogy method was demonstrated by Terzaghi for which of the following processes?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 28/10/2020 (Evening)
Detailed Explanation
To explain consolidation simply, Terzaghi imagined a cylinder filled with water with a spring inside and a piston on top. The piston has a tiny hole (valve) that lets water escape slowly.
- The spring represents the soil skeleton (the grains in contact).
- The water represents the pore water.
- The small hole represents the low permeability of clay.
Why “Consolidation” Is Correct
When a load is placed on the piston, the water cannot escape instantly, so at first it carries the whole load as excess pressure and the spring does not compress. As water slowly leaks out, the piston moves down, the spring compresses and takes a growing share of the load. Finally the excess water pressure is zero and the spring carries everything. This is exactly how load is transferred from excess pore water pressure to effective stress during consolidation.
Why the Other Options Are Wrong
- A. Soil particle gradation: Particle gradation (grain size distribution) is found from sieve and hydrometer analysis; it involves no load transfer.
- C. Permeability: Permeability is described by Darcy's law and measured with permeameters, not by this analogy.
- D. Compaction: Compaction expels air by mechanical energy; there is no slow transfer of load from pore water to the skeleton.
Key Point
Spring = soil skeleton (effective stress), water = excess pore pressure, valve size = permeability. Total stress = σ′ + u at every instant.
Key Concepts for Students
- A smaller valve (less permeable clay) makes the process slower but does not change the final compression.
- At t = 0: u = Δσ, σ′ = 0. At t = ∞: u = 0, σ′ = Δσ.
- This is a model of primary consolidation only.
Practice more: this is question 11 in Soil Consolidation MCQs – Part 1. Try the whole set and check every answer instantly.