Any building structure consists of two main structural parts. One is the sub-structure and the other is the superstructure. The superstructure is the structural component constructed above the ground and the sub-structure is the structural component built below the ground.
|Image Courtesy: https://www.labc.co.uk|
If so, the sub-structure plays the objective of transferring the load coming from the superstructure to be uniformly distributed on the ground below without any differential settlement. Foundation is the sub-structure component of any building construction that transfers the loads coming from the superstructure to the below soil in a uniform and safe manner.
Purpose of a Foundation in ConstructionThe main three objectives or purposes of foundation in constructing any structure are:
- To distribute the load coming from the superstructure to the soil uniformly
- To provide a level surface in order to construct the above masonry or concrete structure.
- To prevent moisture penetration to the superstructure thus preventing the chances of cracks.
- The bottom portion of the foundation is called the footing.
- The component between the sub-structure and the super-structure is called a plinth.
Important Foundation Design Features
The main parameters that are studied in the design of the foundation for any structure are:
- Bearing Capacity of Soil
- Safe Bearing Capacity of Soil
- Ultimate Bearing Capacity of Soil
- Allowable Bearing Capacity of Soil
- Factor of Safety
- Settlement of Foundation
We will discuss the above-mentioned parameters in a simple form.
- The supporting power of the given soil is called the bearing capacity of the soil. This tells how strong the soil is in order to support a given load.
- Safe Bearing capacity is the load that can be applied to the soil so that it does not undergo any shear failure. ( Differential settlement is irrespective).
- Ultimate Bearing capacity is defined as the load at which the soil loses its resistance to the applied load. It is the maximum load a given soil can take
- Allowable Bearing pressure is defined as the load at which the soil neither fails under sear nor settlement.
- The factor of safety is defined as the ratio of ultimate bearing capacity to the safe bearing capacity of the soil.
- Settlement of foundation is an issue when it is a differential settlement. The maximum differential settlement for sandy soil must not be greater than 25 mm and for clayey soil, it must not exceed 40mm.
Types of Foundations in ConstructionThe building foundation is the structural component that connects the building superstructure to the ground and transfers the load coming from the buildings safely to the ground. The two major classifications of building foundations are shallow and deep.
|Types of Foundation|
Image Credits: Dutch Foundations
The major types of footings coming under the shallow and deep foundations are given in the flow chart below.
The different types of foundations used in construction are:
- Shallow Foundation
- Deep Foundation
1. Shallow Foundation
A shallow foundation is a foundation system that transmits the loads to the near-surface strata. These foundation units are placed at a shallow depth. If Df is the vertical distance from the ground surface to the bottom of the structural base, then for shallow foundation Df/B < 1. The level at this depth is the foundation level.
|Fig.2. Shallow Foundation|
Shallow Foundation is of the following types:
- Isolated foundation or Spread foundation
- Wall foundation
- Combined footing
- Continuous foundation
- Cantilever or Strap Footings
- Raft or Mat Foundation
- Grillage foundation
Continue Reading: Types of Shallow Foundation
2. Deep Foundation
Deep foundations are foundations whose Df/B ratio is greater than 1. The major types of deep foundations used in building construction are: