Compaction, measured in terms of its dry unit weight, increases the strength characteristics of soils, thereby increasing the bearing capacity of “foundations” constructed over them. Maximum dry density refers to the density at which the volume of air at a specific energy application is kept to a minimum, implying that the soil particles are rearranged to give a minimum volume of air at the compaction energy. Soil compaction results in:

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• Higher Strength / Bearing capacity of soils.

• Reduced Settlement of soils under future service loads.

• Reduced Permeability of soils.

 

 

FACTORS AFFECTING COMPACTION OF SOILS

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Moisture Content

Compaction, generally, is the densification of the soil by removal of air through mechanical energy and the degree of compaction is measured in terms of the soil’s dry unit weight.

As the moisture content increases with the same compactive effort, the weight of the soil solids in a unit volume gradually increases. Beyond a certain moisture content, any increase in the moisture content tends to reduce the dry unit weight because water takes up the spaces that would have been occupied by the solid particles. The moisture content at which the maximum dry unit weight is attained is generally referred to as the optimum moisture content, when the degree of saturation equals 100%
 

Soil Type

The soil type, namely the grain size, the shape of the soil grains, the amount and type of clay minerals present and the specific gravity of soil solids, has a great influence on the maximum dry unit weight and optimum moisture content. The curve for the poorly graded sand indicates that the dry unit weight has an initial tendency to decrease as the moisture content increases, and then to increase to a maximum value with further increase in moisture. The reason for this is that at lower moisture contents, the capillary tension in the pore water inhibits the tendency of the soil particles to move around and be compacted.
 

Compactive Effort

The compaction energy per unit volume (E) used for the standard proctor test is described as follows: E = [ (No of blows/layer) x (No of layers) x (weight of hammer) x (drop height of hammer) ] / Volume of the mould. If the compactive effort per unit volume is altered, the moisture unit weight curve is also altered. The figure below demonstrates the effect of compaction energy on the compaction of sandy clay. It clearly indicates that as the compaction effort increases, the maximum dry unit weight of compaction also increases. In addition, the increase in compaction effort also causes the optimum moisture content to decrease to some extent.

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