Settlement and ConsolidationSettlement of a structure, which may be caused by several factors, is due to the compression of the soil strata supporting that structure. The construction of a new structure imposes a large amount of force on the subsoil and hence is always associated with some settlement. Increased stress in the subsoil causes compression of the subsoil strata and hence the settlement. Once the construction is over the settlement comes to a halt in majority of the cases. The compression of a soil mass is a combination of:Immediate compressionPrimary consolidationSecondary consolidationThe immediate compression occurs as soon as the load from the building is applied on the foundation soil and is a result of two phenomena. The first phenomenon is due to the compression of air, water and soil grains; the compression of water and the soil grains is usually very small. The second is due to the deflection of the soil mass which is similar to the deflection of a beam under a load.The force applied on the soil strata also increases the pressure in the pore water (water present in a soil mass). This is higher as compared to the pore water pressure in the surrounding soil, thus a hydraulic gradient is developed causing the flow of water to the surrounding areas. This process can be explained by an analogy, as shown in Figure App6.1. The spring represents the soil grains and the cylinder filled with water represents the voids containing pore water. When a force is applied on the tight fitting piston, with the valve closed, the force is immediately resisted by water as it is incompressible (Figure App6.1a). On opening the valve, the water starts to drain and at this moment the force is resisted partly by the springs (or soil grains) and partly by water (Figure App6.1b). Eventually when the excess water pressure is relieved due to the drainage of sufficient amount of water, the force is completely resisted by the springs (Figure App6.1c). This process, when applied to the soils, is known as the primary consolidation.

The time taken for the expulsion of pore water from a soil depends on the permeability of that soil. In sands this happens in a comparatively short period of time, but in a fine grained soil like clay (low permeability) the expulsion of pore water can take a considerable period of time. The soil compression may not stop there. In many soils the compression continues even after the primary consolidation is over, albeit at a much slower rate. This is known as the secondary consolidation and is due to the plastic readjustment of the soil grains, and other causes.If the soil strata compresses evenly on all sides of the structure, then the settlement will be uniform. In a building this will lower the floor levels and may cause damage to the services, but the building itself would remain intact. Differential settlement occurs when the compression of the soil strata on one side of a building is less or more than the compression on the other side/sides, causing cracking of the building.The consolidation characteristics of a soil are determined from the consolidation test. The results of the test are plotted as void ratio (e) versus applied pressure (p) or void ratio versus log p. The compression index (Cc) represents the slope of the pressure-void ratio curve and is given as:

Cc = Where e0 and p0 are the initial void ratio and initial pressure respectively,e is the void ratio at increased pressure p.The settlement due to consolidation (S) can be calculated from the following formula:

S = Cc Where H is the thickness of the soil strata,and p = p0 + p(p is the consolidation or additional pressure)