The compressibility of soil is defined as the capability of soil to decrease its volume under mechanical loads, whereas the consolidation is the compression of saturated soil under steady pressure and it is occurred as a result of water expulsion from the soil voids. Compressibility and consolidation of soil are crucial parameters required in the design considerations.
The consolidation of soil is divided into three stages including initial consolidation, primary consolidation, and secondary consolidation. The consolidation of soil is time-dependent and its analysis is usually based on Terzaghi's theory.
It is important to know the rate of consolidation as well as the total consolidation to be expected for the design of structures. In this way, the designer would be able to set necessary precautions and design considerations to keep the settlement to tolerable limit, otherwise the desire to use the structure may be impaired and the design life of the structure may be reduced.
Compressibility of Soil
When a soil mass is subjected to a compressive force, its volume decreases i.e. giving amount of settlement. The property of the soil due to which a decrease in volume occurs under compressive force is known as the compressibility of soil.
The compression of soil can occur due to compression of solid particles and water in the voids, compression and expulsion of air in the voids, expulsion of water in the voids.
Consolidation of Soil
The compression of saturated soil under steady static pressure is termed as consolidation which is completely due to expulsion of water from the voids. Consolidation is generally related to fine-grained soils such as silts and clays.
Coarse-grained soils, such as sands and gravels, also undergo consolidation but at a much faster rate due to their high permeability. Saturated clays consolidate at a much slower rate due to their low permeability.
The process of consolidation is often confused with the process of compaction. Compaction increases the density of an unsaturated soil by reducing the volume of air in the voids. However, consolidation is a time-related process of increasing the density of a saturated soil by draining some of the water out of the voids.
Consolidation theory is required for the prediction of both the magnitude and the rate of consolidation settlements to ensure the serviceability of structures founded on a compressible soil layer.
Consolidation of soil is composed of three components which include initial consolidation, primary consolidation, and secondary consolidation:
1. Initial Consolidation
When a load is applied to a partially saturated soil, a decrease in volume occurs due to expulsion and compression of air in the voids. A small decrease in volume occurs due to compression of solid particles.
The reduction in volume of the soil just after the application of the load is known as initial consolidation or initial compression. For saturated soils, the initial consolidation is mainly due to compression of solid particles.
2. Primary Consolidation
After initial consolidation, further reduction in volume occurs due to expulsion of water from the voids. When a saturated soil is subjected to a pressure, initially all the applied pressure is taken up by water as an excess pore water pressure. A hydraulic gradient would develop and the water starts flowing out and a decrease in volume occurs.
This reduction in volume is called as the primary consolidation of soil. In fine grain soils, the primary consolidation occurs, over a long time. However, in coarse grained soils, the primary consolidation occurs rather quickly because of high permeability.
3. Secondary Consolidation
The reduction in volume continues at a very slow rate even after the excess hydrostatic pressure developed by the applied pressure is fully dissipated and the primary consolidation is complete.
The additional reduction in the volume is called as the secondary consolidation. Secondary consolidation becomes important for certain types of soil, such as peats and soft organic clays.