Define term consolidation & Explain with sketch Terzaghi’s One Dimensional Consolidation using Spring Analogy

What is  Consolidation ?

 When the soil is fully saturated, compression of soil occurs mainely due to explusion of water from the voids under static pressure. This process is known as consolidation.

Terzaghi’s One Dimensional Consolidation using Spring Analogy:

The process of primary consolidation can be explained with the help of the spring analogy given by terzaghi.

THE SPRING ANALOGY

    Fig shows a spring with a piston on its top. Let the length of the spring be `z_{0}` under  a pressure of  10 units. Ifg 5 units of pressure are added to its top, the spring wil be compressed immediately to a length `z_{1}`. A further application of load will result in further decrease in the length of spring.

   If this spring and piston is  placed in a cylinder containing water up to the bottom of the piston , 10 units of pressure applied with valve open, water will be free of stress, since the whole load is carried by the spring alone.

     If the pressure on the piston is increased to 15 units, and the valve is closed, the spring can not deform si9nce water is incompressible. Hence the additional pressure of 5 units is entirely born by water.

        Let,

             σ   = total pressure

             σ'  =  pressure in the spring 

             ū   = pressure in water

            σ = σ' + ū

           15= 10 + 5

        Now, let the valve be opened slightly so that some water escapes and then valve is closed. Due to escape of some water, the piston moves down, the spring is compressed and hence some pressure out  of 5 units borne by water is now transferred to the spring.

      ∴  15 = (10 + Δσ') + (5 - Δσ')

    Where,

       Δσ' = transfer of pressure from water to spring

      

      If the valve is completely opened , sufficient water will escape till the length of the spring is reduced to `z_{1}`. Thus, the whole 5 units of pressure is transferred from water to the spring. The water becomes free of pressure and spring carries the whole pressure.

      This analogy can be applied to the consolidation process of soil mass containing soil- water system. The solid particles of the soil represents the spring while the voids filled with the water represen by the cyliender. The valve opening is represented by the permeability of the soil mass.

    I hope that you have understand Terzaghi’s One Dimensional Consolidation using Spring Analogy theory very well.