Compression and Consolidation process of Soil

When any soil layer is subjected to vertical stress, volume change can be take place through rearrangement of soil grains or particles, and few amount of soil grain fracture may also take place. The volume of soil grains at such condition remains constant, so change in total volume of soil mass is due to change in volume of water content. In the saturated soil mass, this can happen only if water is pushed out of the voids portion. The liner movement of water takes time and is controlled by the permeability properties of the soil and the locations of free draining of water over boundary surfaces.

It is very much necessary to determine both the magnitude of volume change (or the settlement value) and the time period which is required for the volume change . The value of magnitude of settlement of soil is totally dependent on the magnitude of applied soil stress, thickness of the soil layer wise, and the compressibility index of the soil mass.

When the soil mass is loaded under underlined condition, the pore pressure increases. When the excess pore pressure dissipates and water leaves from the soil, settlement is takes place. This process takes few time, and the rate of settlement of soil decreases over time period. In case of coarse soils (sands and gravels), the volume change occurs immediately as pore water pressures are dissipated rapidly due to high permeability value of soil. In case of fine soils (silt and clay) particle, slow seepage phenomena occurs due to low permeability index.

Components of Total Settlement:
The total consolidated settlement of a loaded soil has generally three components:

Elastic type settlement,

primary consolidation, and

secondary compression.

Elastic settlement is defined of change in shape at constant volume of soil mass, i.e. due to vertical compression and lateral expansion of soil. 

Primary consolidation(or simple consolidation) is defined the flow of water from the voids, and is a primary function of the permeability and compressibility index of soil. 

Secondary compression is defined as the creep- behaviour of soil.

Primary consolidation is one of the major component and it can be reasonably estimated by scientifically. A generalized conceptual theory for consolidation, which incorporating three-dimensional flow is more complicated and only applicable to a very limited range of raised problems in geotechnical engineering. For the case of vast majority of practical settlement problems, it is much more sufficient to consider that both seepage condition and strain in soil mass take place in one side direction only, as one-dimensional consolidation occur  in the vertical direction only.