Selection of foundation types:
Foundation have to transmit the load of the structure on to the sufficient area of the soil. It resists the load of the superstructure and safe structure from collapse. Hence, in whole structural design process, the selection of the most suitable and efficient type of foundation for the particular types of the foundation is an important and trick step. Small error during the construction of footing may leads the heavy loss of money and effort made for superstructure. Here we discuss about the selection of foundation types.
Factor affecting on selection of foundation type:
There are numbers of factors that should be considered for the section of particular types of foundation.
- Bearing capacity of soil
Bearing capacity of the soil should be enough to resist all the load and safe against bearing capacity failure.
2. Adequate strength (Loading intensity)
The foundation should be designed with sufficient strength so that it resist all superstructure loads without fracture or break apart. The soil should not be over-stressed. If not, it may lead to shear failure resulting in the sliding of the soil along the plane of rupture and thus result in the collapse of structure.
The settlement of the footing should be consider because it cause damage on structure. The settlement of the soil depends on various factors like the intensity of the loading, the quality of the soil and the depth below the surface level.
4. Adequate Depth
The depth of the footing should be greater than that obtained from the formula as,
h = (Pu/ϒ)*((1-sinՓ)/(1+sinՓ))2
But depth should also be adequate to prevent from the effect of frost damage.
5. Quality of soil
It would give better result if the quality of soil is same in all base area. If there is different, then there may be problem of differential settlement. It must be able to resist long-term adverse soil changes and any deterioration such as from sulfate attack.
6. Seismic forces
The foundation should be able to support the structure without any lateral movement or excessive settlement during the time of earthquake.
7. Adjoining property line
8. Ground water and underground defects
Based on the analysis of all above factors, an engineer will recommend specific type of foundation.
Requirement for selection of type of foundation:
There are some pre-requisites that are needed for the selection of types of foundation. These pre-requisites are mentioned below:
1.Load and loading pattern
Load and loading pattern includes the intensity and nature of load acted on the foundation from superstructure and substructure. Sometime same intensity of load but the different nature of load may cause the difference in selection of types of foundation. The dynamic nature of load may increase the moment drastically on both axis.
2. Bearing capacity of soil
It simply means capacity of the soil to withstand the load acting on it. The bearing capacity is calculated from different techniques of soil exploration. We will discuss about the calculation of bearing capacity in coming date.
If we have the data of loading condition [axial force and bending moment] and bearing capacity of soil, we can move on further steps of selection of types of foundation.
To make the structure economical, a structural engineer should always try for shallow foundation. If the shallow foundation can’t resist the load then only deep foundation should be preferred.
It is better to know about the TYPES OF FOUNDATION before entering to further steps.
The priority order of selection of types of foundation in descending order as,
- Isolated/spread footing
- Combined footing
- Mat/Raft foundation
- Deep foundation (pile)
Strip and strap footing are excluded from this list. The strip footing is used for line load that is wall load. This simple means if we have to resist the wall load then strip footing is best and it is enough to resist the wall load. But in worst case, the strip footing may have to erect above the pile foundation. Strap footing is normally used for the footing nearby the boundary line and other footings are of spread type. If the strap footing can’t resist the load then mat/raft will be selected.
Isolated footing is the most economical types of column footing among all types of footing. So our first choice is isolated footing while selecting the types of foundation. When the two or more isolated footing are too close or overlap, then it will be better to go for combined footing.
When series of column in a row or column required combined footing due to overlapping of isolated footing, then this types of footing combination can be also termed as strip footing. Strip footing is normally used to support wall load.
It will be economical to choose mat foundation when more than 50% of total base area is covered by isolated/combined footing. Besides this, if there is possibility of differential settlement then mat foundation will be the better option.
If the raft foundation can’t resist the loading of the structure at shallow depth, then pile foundation become compulsory. It becomes expensive to use pile foundation. For residential building, pile foundation is not used because it costs times more than that of superstructure. And raft foundation is also compulsory for the pile foundation case. Raft foundation of thickness greater than 300 mm is necessary to transmit loads of superstructure to the pile foundation.
Note: At edge of reinforced and plane concrete footing, the thickness shall not less than 150 mm for footing on soils not less than 300 mm above the tops of piles for footings on piles.