Foundation

DEPTH OF FOUNDATION

1.

Minimum depth of foundation as per IS 1080-1982 is 500mm from Ground Level.

2.

As per Rankine’s theory Minimum depth of foundation =p/w (1-sin Φ /1+sin Φ) 2 where p=gross bearing capacity w =density of soil Φ =angle of response of soil

3.

But in practice, the foundation depth is kept at 0.90m or even more.

4.

It is customary practice to place the foundation of a “simple footing” at a minimum depth of 1.50m from GL or at least 1.50 times the width of footing. In cold climates the depth is Kept at a minimum of 1.50m below surface because of possible frost action.

5.

For building of six storey or more, it may be kept at 1.5m to 2.0m. For tall isolated Structures, like water towers, the foundation depth may be at 3.0m also.

6.

A few practical requirements also may have to be considered while deciding the depth of foundation such as the existing foundation of nearby building, the possible influence of future expansion etc. If the height of building is more, the horizontal forces acting on the building such as wind force are large. As a thumb rule, minimum depth of foundation may be selected as 5% to 10% of the height of building.

7.

Where the moisture content may vary and cause shrinkage, the depth must be considered with the minimum moisture content variation(1.50m to 2.0m).In case of black cotton soils of expansive nature, the zone of movement may be as deep as 3.0m to 3.50m. This is why ,the under reamed piles in expansive clays are taken to minimum depth of 3.50m.