Foundation and Its Various Types
Superstructures are those structures which are above ground and visible such as dams, bridges, buildings, roads etc. These structures apply load on the soil when they rest. Therefore, if superstructures are placed in direct contact with the soil it will lead to the soil getting overstressed and will not be able to provide a safe support. So as to allow the safe transfer of the load of structure on soil certain part of the structure are made to be placed under the ground.
The structure which is placed in below the ground is called the sub-structure. Therefore, structural elements which help in connecting superstructures like bridges, buildings etc. to the ground are known as foundations. The foundation plays a crucial role for every structure as the safety and reliability of the structure depends highly on the foundation.
From the superstructure to the sub-structure the load is transmitted. The foundation helps in distributing the load to the soil in such a way that the soil is able to withstand the load.
Types of Foundations:
1. Shallow Foundations
Foundations which helps in transmitting the load to soil at a shallow depth it is known as shallow foundations. A shallow foundation is used in places where the shallow depth has the capacity to hold the structure and the load that is placed on it. In the case of shallow foundations, the ratio of the depth of foundation to the width of foundation is less than or equal to 1. Examples of shallow foundations- spread footing and mat.
2. Deep Foundations
Foundations which help in transmitting the load to deeper soil is known as deep foundations. When the soil at shallow depth is unable to take the load and a hard stratum is found at a larger depth that is when deep foundations are used. In case of deep foundations, the ratio of depth of foundation to the width of foundation is more than 1. Examples of deep foundations- piles and caissons.
Given below is the list of the various types of shallow foundations:
A. Spread Footing
It is also known as individual column footing. The spread footing shape is often that of the column, it can be square, rectangular or circular. Reinforced concrete is used to make it. The reason why it is called spread footing is because it spreads the load of the structure over a certain area of the soil.
B. Strap Footing
Strap footing is when the isolated spread footings of two column is made to connect by a beam. The beam in this case is called strap beam. No load is transferred to the soil by the strap beam as it is not in any contact with the soil.
C. Combined Footing
Combined footing is when a spread footing is supported by two or more columns. The construction of combined footing takes place when two or three columns are located close to one another. In case the two columns carry equal load then the shape of combined footing will be rectangular and in case they each carry different loads then it maybe trapezoidal in shape.
D. Continuous Footing
Strip footing is the other name of continuous footing. For a load bearing wall continuous footing is used. Bricks are used to make continuous footing. In continuous footing, the length is much longer than the width of the footing.
E. Mat or Raft Footing
Mat footing is when a continuous footing is able to cover the entire area below the structure of walls and columns. Mat footing is an economical option if more than 50% of the building’s plan area is covered by combined by area of isolated footing.As mat footing is structurally rigid and settles uniformly as a monolithic entity because of which the chances of differential settlement is less. This type of footing is chosen when the soil conditions is extremely erratic and the structures are heavy for spread footing to hold.
What affects the depth of shallow foundations?
- Thickness of top Layer: The foundation is to be placed below the thickness of the organic matter which might contain a good percentage of vegetables and other organic decaying matter. This is because the soil undergoes a shift in volume by the placement of external load.
- Ground Freezing: Shallow foundations is affected by ground freezing which takes place when the air temperature goes below 0◦C which leads to a drop in the temperature of the soil which turns the pour water into ice. The formation of ice underground creates an upward movement of ground which is called ice lens. It is important to place the foundations below the depth of frost penetration so as to avoid any damage to the structure.
- Depth of volume change: Due to alternate wetting and drying the soil can undergo volume changes, for example in the case of black cotton soil. Therefore, in such cases the foundation must be placed below the area of volume change.
- Depth of scour: It is important to consider the depth of scour in cases where the structure is adjoining rivers or streams where the structure is faced with the scouring action of water. The depth of the foundation should be, therefore, below the potential scour depth.
- Underground utilities and defects: When the decision of depth of foundation is taken the underground facilities must be kept in mind. It is also important to consider underground defects like cavities, abandoned mines etc. as well while planning for the foundation.
- Property line and adjoining structures: It is extremely important to keep in mind that the foundation where located must not extend itself to the adjacent property line which can lead to legal disputes. In case a new foundation is to be laid near an existing structure, a minimum distance must be maintained away from the old structure.
- Sloping ground: In case of a sloping ground where the foundation must be laid then the depth of foundation must be such that if a line id drawn from the bottom edge of the foundation at an angle of 30◦ and from a horizontal distance of 90cm then they must not intersect.
Types of Deep Foundations:
The various types of deep foundations depending upon mechanism to shape and its material are stated below:
A long and slender structural element which is able to transfer the load of a structure from firm stratum to a great amount of depth below ground surface is called pile foundation.
The large diameter piles which can distribute the load of the structure along the whole depth of soils where it is located is called pier foundation.
Well or Caisson Foundation
A large size hollow box or well which is made to drown into the ground so as to support heavy loads are the well and caisson. They can carry large number of loads. With end bearing and skin friction they transfer the load from the superstructures to the soil. These foundations are usually used in bridge.