- Settlement is the vertical downward movement of the loaded base.
- As a result of this the original depth of soil mass decreases due to soil grains coming closer together.
- The settlement of the base is mainly due to straining of the soil mass either by elastic distortion or by volume change.
- The amount of settlement is different for different type of soil or rock.
- For example there is no or very less settlement for rocky strata whereas it is very large in case of marshy land.
- It is a time dependent process i.e in case of clayey soil settlement is very gradual, continues for a long time and is more.
Effects of settlement:
- Effects of Settlements are as follows:
- Cracks in superstructure walls and plaster.
- Damage to water supply and drainage lines.
- Inclination of walls and columns.
Permissible Settlement values:
Permissible values for different footings are as follows:
|Sr.no.||Type of Soil||Total Settlement|
|1.||Clayey soil||65 mm for isolated footing and 65 to 100 mm for raft footing.|
|2.||Sandy soil||40 mm for isolated footing and 40 to 65 mm for raft foundation.|
- To prevent the settlement of structure, soil strata should be hard so as to withstand the load of structure.
- The entire load of the structure is transmitted to the underlying soil by the substructure.
- The structure will be safe if the underlying soil carries the load safely i.e. not failing under shear. The shear failure may result in sinking or tilting of the loaded mass i.e. the transmitting medium and in turn the entire structure may collapse due to unequal settlements .
- Thus it is important to know the maximum load carrying capacity of the underlying soil.
- The maximum average intensity of applied pressure that a finite loaded area can carry (intensity of pressure x area = load) before the underlying material (soil / rock) fails in shear is known as the ultimate bearing capacity of the underlying material (soil/rock).
- For design purpose, however a lower value of bearing capacity (safe bearing capacity) is considered to keep loading well below the maximum or ultimate load to keep the settlement within permissible limit.
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