Foundation is one of the most important parts of the structure. Foundation is defined as that part of the structure that transfers the load from the structure as well as its own weight over a large area of soil in such a way that the load does not exceed the ultimate bearing capacity of the soil and the settlement of the total structure remains within a tolerable limit. Foundation is the part of a structure on which the building stands. The solid ground on which the foundation rests is called the foundation bed.
Why a Foundation is Provided
Foundation should fulfill the following objectives:
- Distribute the weight of the structure over a large area of soil.
- Avoid unequal settlement.
- Prevent the lateral movement of the structure.
- Increase structural stability.
Why There are Different Types of Foundation
As we know that there are different types of soil and bearing capacity of the soil is different for each individual type of soil. So depending on the soil profile, size and load of the structure, engineers chose different types of foundation.
Types of Foundation
In general, all foundations are divided into two categories, such as SHALLOW FOUNDATIONS and DEEP FOUNDATIONS. The terms Shallow Foundation and Deep Foundation refer to the depth of the soil at which the foundation is placed. Generally, if the width of the foundation is greater than the depth of the foundation it is labeled as “Shallow Foundation” and if the width of the foundation is smaller than the depth of the foundation it is called as “Deep Foundation.” However, shallow and deep foundations can be further classified as shown in the following chart.
Main aspects of different types of foundation along with their images are given below. As economic feasibility is one of the main factors in foundation type selection, it is also discussed briefly with each type of foundation. To know other factors which affect the selection of foundation read: Factors Considered for Selection of Foundation.
Several types of shallow foundations are discussed below:
1. Isolated Spread Footing
This is the most common and simplest type of foundation as this is the most economical type of foundation. They are generally used for ordinary buildings (Generally up to five stories).
Isolated footing type foundation consists of footing at the base of the column. This type of foundations is independent footings. Usually, each column has its own footing. The footing directly transfers the loads form the column to the soil. The footings may be rectangular, square or circular in shape. The size of the footing can be roughly calculated by dividing the total load at the column base by the allowable bearing capacity of the soil.
Isolated Spread Footing is economical when:
- The load of the structure is relatively low.
- Columns are not closely placed.
- Bearing capacity of the soil is high at a shallow depth.
2. Wall Footing or Strip footing
This type of footing is used to distribute loads of structural load-bearing walls to the ground. Wall foundation runs along the direction of the wall. The width of the wall foundation is generally 2-3 times of the width of the wall.
The wall footing is a continuous slab strip along the length of the wall. Stone, brick, reinforced concrete etc. is used for the construction of wall foundations.
Wall footing is economical when:
- Loads to be transmitted are of small magnitude.
- The footing is placed on dense sand and gravels.
3. Combined Footing
The combined footing is very similar to the isolated footing. When the columns of the structure are closely placed, or the bearing capacity of the soil is low and their footing overlap each other, combined footing is provided.
The foundations which are made common to more than one column are called combined footings. They may be rectangular, tee-shaped or trapezoidal in shape. The main objective is the uniform distribution of loads under the entire area of footing. For this is necessary to coincide the center of gravity of the footing area with the center of gravity of the total loads.
Combined foundations are economic when:
- The columns are placed close to each other.
- When the column is close to the property line and the isolated footing would cross the property line or become eccentric.
- Dimensions of one side of the footing are restricted to some lower value.
4. Cantilever or Strap Footing
Strap footings are similar to combined footings and reasons for considering or choosing strap footing is similar to the combined footing.
In strap footing, the footings under the columns are built individually and connected by strap beam. Generally, when the edge of the footing cannot be extended beyond the property line the exterior footing is connected by strap beam with interior footing.
5. Raft or Mat Foundation
Raft or Mat foundations are used where other shallow foundation or pile foundation is not suitable. It is also recommended in situations where the bearing capacity of the soil is poor, the load of the structure is to be distributed over a large area or structure is constantly subjected to shocks or jerks.
Raft or Mat foundation consists of a reinforced concrete slab or T-beam slab placed over the entire area of the structure. In this type of foundation, the entire basement floor slab acts as the foundation. The total load of the structure is spread evenly over the entire area of the structure. This is called Raft because in this case, the building seems like a vessel which floats on a sea of soil.
Raft foundations are economic when:
- The soil is weak and the load has to be spread over a large area.
- The structure includes a basement.
- Columns are closely placed.
- Other kinds of foundations are not feasible.
- Differential settlement is to be prevented.
Several Types of Deep Foundations Are Discussed Below:
1. Pile Foundation
Pile foundation is a common type of deep foundation. They are used to reduce cost and when as per soil condition considerations, it is desirable to transmit loads to soil strata which are beyond the reach of shallow foundations.
Pile is a slender member with a small cross-sectional area compared to its length. It is used to transmit foundation loads to a deeper soil or rock strata when the bearing capacity of soil near the surface is relatively low. Pile transmits load either by skin friction or bearing. Piles are also used for resist structures against uplift and provide structures stability against lateral and overturning forces.
Pile foundations are economic using pile foundation when
- Soil with great bearing capacity is at a greater depth.
- When there are chances of construction of irrigation canals in the nearby area.
- When it is very expensive to provide raft or grillage foundations.
- When the foundation is subjected to a heavy concentrated load.
- In marshy places.
- When the topsoil layer is compressible in nature. • In case of bridges when the scouring is more in the river bed.
Pile foundations can again be classified based on its material and its mechanism of load transfer or function. Several types of pile foundations are shown in the following chart.
3. Pier Foundation
Pier is an underground structure that transmits heavier load which cannot be transmitted by shallow foundations. It is usually shallower than pile foundations.
Pier foundation is a cylindrical structural member which transfer heavy load from superstructure to the soil by end bearing. Unlike pile, it can only transfer load by bearing and by not skin friction.
Pier Foundation is economic when:
- Sound rock strata lie under a decomposed rock layer at the top.
- The topsoil is stiff clay which resists driving the bearing pile.
- When a heavy load is to be transferred to the soil.
4. Caisson Foundation
Caisson foundation is a watertight retaining structure used as a bridge pier, construction of dam etc. It is generally used in structures which requires foundation beneath a river or similar water bodies. Reason for choosing caisson foundation is that it can be floated to the desired location and then sunk into place.
Caisson foundation is a ready-made hollow cylinder depressed into the soil up to the desired level and then filled with concrete which ultimately converts to a foundation. It is mostly used as bridge piers. Caissons are sensitive to construction procedures and lack construction expertise.
There are several types of caisson foundation such as:
- Box Caissons
- Floating Caissons
- Pneumatic Caissons
- Open Caissons
- Sheeted Caissons
- Excavated Caissons
Caisson foundations are economic when:
- Pile cap requirement is to be minimized
- Noise and vibration needed to be reduced
- Foundation has to be placed beneath water bodies.
- Highly lateral and axial loading capacity is required.
To conclude, the foundation is a structural supporting member which transfer the total load form slab, beam, column, wall etc. The main objective of the foundation is to provide stability to the total structure and safely transfer the total load from the structure to the soil at an optimum cost.