Wednesday, April 11, 2018

CONSOLIDATION OF SOIL


CONSOLIDATION OF SOIL

COMPRESSIBILITY OF SOIL
      When a soil is subjected to compressive force, its volume decreases. This is compressibility of soil.
      It is caused because of following reasons
      Compression of soil particles and water in voids
      Compression and expulsion of air in voids
      Expulsion of water present in the voids

Explanation of cause of compressibility
      Compression of soil particles is negligible and water compression is also extremely small for stress considered in Soil Engineering. So, it is not much significant.
      Compression and expulsion of water in voids occur when soil is not fully saturated. But here we will be dealing with saturated soil only. So, it is also not significant.
      As saturated soil contains only water in voids, expulsion of water from the voids is possible. Therefore, this is significant cause.

Definition of Consolidation of soil
      It is compression of saturated soil under steady static pressure
      It is entirely due to expulsion of water from voids
      When compressive force is applied on saturated soil, water escapes and shifting of soil particles take place.
       Decrease in volume occurs not due to compression of solids or water but due to shifting of position of particles as the water escapes.

Settlement of structure
      It is vertical, downward movement due to a volume decrease of the soil on which it is built
      It is sinking of structure due to compression of soil below

Consolidation vs compaction
      Consolidation occurs only in saturated soil due to expulsion of water present in the voids
      Compaction occurs in unsaturated soil due to expulsion of air present in the voids
      But both process compresses the soil

Type of consolidation
      Initial consolidation
      Primary consolidation
      Secondary consolidation

Initial consolidation
      When load is applied to partially saturated soil, a decrease in volume occurs due to expulsion and compression of air in voids
      Small decrease in volume also occurs due to compression of solid particles
      The reduction in volume of soil just after the application of the load is known as initial consolidation
      No change in moisture content occurs

Primary consolidation
      After initial consolidation, further reduction in volume occurs due to expulsion of water from voids
      When saturated soil is subjected to a pressure, initially all applied pressure is taken up by water as an excess pore water pressure, as water is incompressible
      Hydraulic gradient develops and water starts flowing out and hence decrease in volume occurs
      It is time dependent
      In fine-grained soil, it takes long time due to less permeability
      In coarse-grained soil, it is quick due to high permeability

Secondary consolidation
      Even after primary consolidation, there is decrease in volume of soil at slow rate
      This additional reduction of volume of soil is called secondary consolidation
      Its cause is not fully established
      It occurs due to plastic adjustment of soil fabrics

Most important Consolidation
      Primary consolidation is the most important of all.
  
Normally consolidated
      A normally consolidated is soil which has not been subjected to a pressure greater than the present existing pressure
      Settlement in such soils are more

Over-consolidated
      A soil is said to be over-consolidated if it had been subjected in the past to a pressure in excess of the present pressure.
      Settlement in such soils are small

Over consolidation ratio (OCR)
      It is the ratio of highest stress experienced by the soil to the current stress
      OCR = highest stress experienced/current stress
      For normally consolidated soil, current stress is the highest stress as those soils have never been subjected to pressure greater than the current pressure so OCR = 1.

Pre-consolidated pressure
      It is also known as over-consolidation pressure
      It is defined as the maximum pressure to which an over consolidated soil had been subjected in the past


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