- We Construct buildings, bridges or any other civil Structure. All these structures rest on earth surfaces through substructure. Finally, all structures transfer their load to the earth and earth support that load. It means the earth’s surface provides stability to the structure.
- The soil on which these structures transfer its load is a very week material when it compared with other construction material. Hence more area is required to transfer the load. So that the stress in the soil is within the allowable bearing capacity of the soil.
- A structural foundation or substructure is a load transfer device between the soil and structure. A substructure is designed in such a way that it transfers the load/stress within the allowable bearing capacity of the soil and reduces the chances of differential settlements. As we know that differential settlement is the main cause of cracks in the building.
- As we know that structure is a man-made thing and soil is made by natural process. Hence the type of load transfer devices depends upon characteristics of the structure and the properties of the soil.
- we design the foundation with the worse combination of the forces that are likely to act during its life span. the possible forces on the structure may be due to wind, heat, water, earthquake, ice, frost, explosive blast, etc.
- Foundation is also called substructure of the building, as it is constructed below the ground level.
According to “Father of Modern Soil Mechanics” Karl Von Terzaghi, It is classified into two types.
If the depth of footing in-ground is equal to or less than its width of the footing, It is categorized as a shallow foundation. This is the most common type of substructure. It can be constructed using open excavation by providing a natural slope to all sides. Such type of foundation is practicable up to 5 m depending on the type of starta and water table. Examples of footing are wall footing, strap footing, combined footing, continuous footing, isolated footing, etc
If the depth of footing in-ground is greater than its width of the footing, it is categorized as a deep foundation. Such type of foundation is constructed when the strata immediately below the structure are not capable to support the load with tolarable settlement Examples of footing are piles, piers, and caissons.
Types of footing
- Wall footing
- Isolated footing
- Combined footing
- Inverted arch footing
- Continuous footing
- Grillage footing
- Raft footing
- Eccentric Footing
This is a simple type of footing in which the brick wall is constructed on a rectangular concrete block on which a simple brick wall or stepped brick wall can be constructed. It can be used for light structures. it is desirable that, the base width of the concrete base course should be equal to twice
the width of the wall. The depth of the concrete bed is at least twice the
it is constructed to support individual columns. It can be constructed as simple spread footing, stepped spread footing, and slopped footing depending upon the design. it is the most commonly used footing.
A combined footing supports two or more columns. It is usually constructed when some portion of the footing of the two-column overlap. The location of the center of gravity of column loads and centroid of the footing should coincide.
Inverted Arch Footing
This type of construction is used on soft soils to reduce the depth of foundation loads above an opening and is transmitted from supporting walls through inverted arches to the soil. In this type, the end columns must be stable enough to resist the outward pressure caused by arch actions.
In this type of footing a single continuous R.C slab is produced as a foundation of two or three or more columns in a row. Such type of footing is suitable at locations liable for earthquake activities. Combined footing prevents differential settlement in the structure. In order to have better stability, a deeper beam is constructed in between the columns
such type of footing is very suitable to transmit heavy loads from steel columns to foundation soils having low bearing capacity. This type of foundation avoids deep excavation and provides necessary areas at the base to reduce the intensity of the pressure of the foundation. it is suitable where there is a pore water pressure.
The raft is a combined footing that covers the entire area below the structure and supports all the columns. It is constructed when the allowable soil pressure is low and the structural loads are high. Raft foundation is suitable where there is a compressible soil and it is very difficult to control differential settlement.
It is a type of footing where the Center of gravity of footing does not coincide with the center of gravity of the column. It is suitable in urban areas where the construction plot is surrounded by the buildings from its three sides and we want to use complete land for construction
Pile foundation is a form of deep foundation. It is a long, slender member that is driven into the ground or cast at the site. Today pile foundation is most commonly used in flyovers than any other type of deep foundation. It is especially used where the shallow foundation is not suitable. The size of the cross-section of the pile is typically 30 cm
Drilled Piers and caissons
The basic difference between piles and piers or caissons is their cross-sectional dimensions with depth relation. as a cross-section of the pile may be typically 30 cm but for caissons or piers cross-section may be up to 1 m. hence pile bends when subjected to horizontal load. But in the case of the pier it undergoes rotation when subjected to horizontal forces. Caissons are mostly used as foundations for bridge piers and abutments in lakes, rivers, and seas, etc
Such type of foundation is used for centuries in India. It is a deep foundation below water. For example, the famous Taj Mahal of Agra constructed on well foundations. Well, foundations are similar to open caissons. The construction of well foundations is based on the conventional wells sunk for obtaining underground water. It derives its name owing to this construction technique otherwise there is no difference between caissons and well foundations.
footings for home construction
- The choice of footing depends upon the following
- The function of the structure and the loads it is subjected
- The subsurface condition of the soil
- The cost of the foundation in comparison with the cost of the superstructure.