For economic consideration, the depth at which the foundation is situated (ie DF) is kept as short as possible. Usually this is 0.5 to 1.5 m below the ground surface for buildings that do not have a basement. For buildings with basements, DF Can be 3.5 meters or more.
in determining DF, The soil profile is carefully studied and the influence of the following factors is taken into account.
1. presence of loose filler
Often one encounters loose fillings of recently dumped soil or construction waste on or near the ground surface. The foundation should be laid under such loose filler.
2. water table depth
Where possible, shallow foundations are not placed below groundwater level to avoid costly de-watering costs during foundation construction.
3. lateral variability
Usually all the foundations are laid on the same DF, There are soil profiles that demand a different DF for different footings. For example, consider a profile in which rock is present at a shallow depth and is sloping gently in some direction. In an attempt to take advantage of the high allowable soil pressure associated with placing the foundation on a rock, we will have a different DF for each notch so that each can be placed on the rock.
4. volume change area
In cold regions where soil near the ground surface undergoes a cycle of freezing and melting due to temperature changes, resulting in changes in soil volume, foundations are placed below the affected area.
Similarly, in swollen soils, there is an area that undergoes a volume change due to cycles of wetting and drying. DF is chosen in such a way that it exceeds the thickness of the region.
5. start investigating
When shallow foundations are designed for river crossing structures below a river bed, one must recognize that the elevation is caused by changes in the river bed, which occurs when water flows at high velocity. Like during a flood.
The depth of the score increases due to the construction of piers of river crossing structures. for such a foundation DF Should be higher than the expected depth.