Theory of Soil Compaction

During compaction the reduction in volume is mainly due to expulsion of pore air and rearrangement of particles resulting in their closer packing. Compaction of a soil mass results in increase in dry density. The dry density attained depends on water content, amount and type of compaction. The amount and type of compaction determine the compactive effect. For a specific amount of compactive energy applied on soil, the mass attains maximum dry density at particular water content. This water content is referred to as optimum water content.

Effect of Compaction on Properties of Soil

When a soil is compacted, it changes its engineering properties and thereby behaves differently. Some of the engineering properties which changes on application of compactive effort is briefly described below.

1. Permeability

The effect of compaction is to decrease the permeability. In the case of fine grained soils it has been found that for the same dry density soil compacted wet of optimum will be less permeable than that of compacted dry of optimum.

2. Compressibility

In case of soil samples initially saturated and having same void ratio, it has been found that in low pressure range a wet side compacted soil is more compressible than a dry side compacted soil, and vice versa in high pressure range.

3. Pore Pressure

In undrained shear test conducted on saturated samples of clay it has been found that lower pore pressures develop at low strains when the sample is compacted dry of optimum, compared to the case when the sample is compacted wet of optimum. But at high strains in both types of samples the development of pore pressure is same for same density and water content.

4. Stress-Strain Relation

Samples compacted dry of optimum produce much steeper stress-starin curves with peaks at low strains, whereas samples compacted wet of optimum, having the same density, produce much flatter stress-strain curves with increase in stress even at high strains.

5. Shrinkage and Swelling

At same density a soil compacted dry of optimum shrinks appreciably less than that of compacted wet of optimum. Also the sol compacted dry of optimum exhibits greater swelling characteristics than samples o f the same density compacted wet of optimum.

Also Read: Factors affecting compaction of soil on site

Also Read: Difference between soil compaction and consolidation

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