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In a compaction test, if the compacting effort is increased, it will result in
1. increase in maximum dry density and in OMC
2. increase in maximum dry density and OMC remains unchanged
3. increase in maximum dry density and decrease in OMC
4. no change in maximum dry density but decrease in OMC

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Correct Answer - Option 3 : increase in maximum dry density and decrease in OMC


Factor affecting compaction:

  • Water content:
    • At lower water content soil is relatively stiff, hence offer higher resistance against compaction and does not pack into a denser state of contact resulting in lower dry unit weight.
    • With the increase in water content more and more layers of water are found surrounding soil solids which induce the lubrication effect, under which soil solid can be easily worked around in a denser state of contact resulting in higher dry unit weight.
    • This increase in dry unit weight takes place up to a particular water content termed as OMC (Optimum moisture content) at which lubrication effect is maximum, beyond which if water content increases there is no change in lubrication but water starts replacing solid 
  • Compactive effort:
    • For a given type of compaction, the higher the compactive effort, the higher the maximum dry unit weight and lower the optimum moisture content.
  • Type of soil:
    • Coarse-grained soils, well-graded, compact to high dry unit weight, especially if they contain some fines.
    • However, if the quantity of fines is excessive the maximum dry unit weight decreases.
    • Poorly graded or uniform sand leads to lowest dry unit weight value.
    • In clay soils, maximum dry unit weight tends to decreases as plasticity increases.
    • Cohesive soils have generally high value of OMC
    • Heavy clays with high plasticity have a very low value of the maximum dry unit weight and very high value of OMC.
  • Method of compaction:
    • Since the field compaction is essentially a kneading or rolling type compaction and the laboratory test is dynamic-impact type compaction, therefore, laboratory compaction test has more value of maximum dry unit weight.

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