What is consolidation optimum moisture content and maximum dry density?

Answer 1

Consolidation:

Consolidation is the process where pore water pressures dissipate from a fine grained low permeability soil over time which has a load applied to it and is free to drain (the drained condition is soil mechanics).

As the material is of low permeability, as load is applied and the pore water pressures cannot dissipate, they increase in a proportional manner to the load and so resist deformation. As the water slowly escapes from the soil pore spaces however, the pore pressure reduces and the soil skeleton can deform as the pore water in the pore spaces is no longer holding the pores open. Once the pore pressures have completely dissipated (which may take tens of years in very low permeability clays), the pore spaces will have totally closed and the soil has undergone consolidation. This can result in significant deformation and is something that must be taken into account during geotechnical design.

Optimum Moisture Content and Maximum Dry Density

These terms relate to compaction of soils. This is a process where a load is applied to the soil to "squash out" the air in the pore spaces. This acts to decrease the soils volume and so increase it's density (which has the added benefit of increasing the soil's shear strength).

Moisture content is a way of describing the amount of water in a soil and is defined in a number of differing ways.

The degree of saturation is used to describe the ratio of volume of water (VW) in the soil compared to the volume of available pore space (VV).

Sw = VW / VV

As can be seen from the equation ,if the pore spaces are totally filled with water then the volume of water equals the volume of void or pore space and the degree of saturation is 1 (100 % saturated).

As per the consolidation process above, if the moisture content is too high, when you apply a load to a soil, the pore pressures prevent the soil from being compacted as they hold the pore spaces open. As the moisture content decreases the soil becomes more easily deformed (less stiff), however once you decrease the moisture content beyond a certain limit you are increasing the total stress within the soil which in clays acts to increase the shear strength making them more difficult to compact and in courser soils leads to the formation of soil suction (negative pore water pressures) which also acts to increase the soil strength making the process of compaction more difficult.

As such the optimum moisture content is that at which compaction is easiest for a given soil, which then produces the highest (or maximum) density. This is termed the dry density as it is measured based on the change in mass of a soil sample after the water is removed by drying of the sample.