For economic consideration, the depth at which the foundation is located (i.e. Df) is kept as small as possible. Typically it is in the range of 0.5 to 1.5 m below the ground surface for buildings that do not have a basement. For those building having a basement, Df can be 3.5 m or more.
In determining Df, the soil profile is carefully studied and the influence of the following factors accounted for.
1. Presence of Loose Fill
Often one encounters pockets of loose fill of recently dumped soil or construction waste at or near the ground surface. Foundations should be placed below such loose fills.
2. Depth of Water Table
Wherein possible, shallow foundations are not placed below the ground water level to avoid expensive de-watering costs during foundation construction.
3. Lateral Variability
Usually all foundations are placed at the same Df. there are soil profiles that calls for a different Df for different footings. For example, consider a profile in which rock exist at shallow depth and is gently sloping in some direction. In an attempt to take advantage of the high allowable soil pressure associated with placing the foundation on rock, we will have a different Df for each footings so that each can be placed on the rock.
4. Zones of Volume Change
In cold regions where temperature changes cause soil near the ground surface to go through cycles of freezing and thawing with consequent changes in soil volume, foundation are placed below the zone so affected.
Similarly in swelling soils, there is a zone that undergoes volume change due to wetting and drying cycles. Df is selected such that it is more than the thickness of this zone.
When shallow foundations are designed to be placed below the river bed for river crossing structures, one must recognize that the elevation of the river bed changes on account of scour that occurs when the water flows at high velocity such as during floods.
The scour depth increases due to construction that occurs when piers of river crossing structures are put in place. For such foundations Df must be greater than the deepest scour level anticipated.