Foundation in Civil Engineering
In the language of Civil Engineers, the term “Foundation” denotes, “Structure at Founding level”. The Foundation or Sub-structure is the lowermost portion of the building or any other structure, which is used to transfer different types of loads acting on it, to the subsoil below, on which it rests.
Among all the other construction elements, the foundation is the most important one, it determines the structural stability of the building, and the number of years the building can serve its purpose effectively
The importance of the Foundation has necessitated a separate branch of Civil Engineering, named “Foundation Engineering”, where the design techniques and detail knowledge of this element are given.
The Design of Foundation is very complex, and it requires intelligent scrutiny from one professional’s experienced engineer in this field.
Design of the foundation includes detailed sub-soil investigation, choosing the right type of foundation depending upon the reports of subsoil investigation, bearing capacities of soil(essentially the safe & ultimate bearing capacity) and thorough checks to identify if the subsoil can carry the loads expected to come from the structure, without any critical failure.
The value of the nominal dimensions of a Foundation is given below, less than which the foundation is said to be insecure when the loads are being applied to it.
- Height of the foundation is determined using Rankine’s Formulae, having it is a minimum value of 0.9 meter
- The breadth of the Foundation should not be less than 0.45 to 0.5 meter.
What Is Purpose of Providing Foundation?
There are numerous reasons a foundation is provided, some of which are:
- The most crucial purpose of providing Foundation is Structural Stability. Strength of the foundation determines the stability of the structure be constructed.
- A properly designed and the constructed foundation provides an even surface for the development of superstructure at a proper level at over a firm bed.
- A well-designed foundation prevents the lateral movement of the supporting material(which is the soil in this case) and thus ensuring the safety of the superstructure from the detrimental effects of the lateral movements of soil.
- The foundation serves the purpose of completely distributing the loads from the structure to a large base area, and then the soil underneath. This uniform transfer of loads helps in avoiding unequal settlement of the building, which is one of the detrimental defects in building construction.
Types of Foundation
The foundation can be broadly divided in two categories, these are:
- Shallow Foundation
- Deep Foundation
1. Shallow Foundation
These types of foundation are used in the places where soil hard stratum are found at a shallow depth from the ground surface.
In this type of foundation, the depth of the foundation is less as compared to the breadth, which is why it is also called a spread foundation.
Shallow foundations are very popular choices of foundation, where the safe bearing capacity of soil exceptionally high.
A shallow foundation is suitable when the hard stratum exists at a depth of 2-3 meters.
Advantages of Shallow Foundation
- These are the simplest form of foundation which is used to uniformly transfer loads from the superstructure to the subsoil below it.
- In the case of a shallow foundation, the water table exists at a considerable depth, so there are very little chances of foundation to encounter water table, thus saving the close of application of different waterproofing techniques to the foundation.
- Compared to other types of foundation, the design for this type of foundation can be quickly done, thus saving time & cost.
Shallow foundation can be broadly classified into four categories:
- Isolated Footing.
- Combined Footing.
- Cantilever Footing.
- Raft or Mat Footing.
1. Isolated Footing:
- In this type of Shallow foundation, a stepped footing is provided below each column of the structure.
- The dimensions of the steps are generally determined by design, generally, each step projecting 50-75mm from the previous step. These are generally three stepped courses provided.
- This is the most common types of footing which is being used in low-cost buildings, where the soil condition is very good.
2. Combined Footing:
- A combined footing is that in which a single footing is common to two columns.
- These are used in conditions where the clear span between two columns are very low, which results in overlapping of stepped courses if isolated column footings are provided.
- There are two types If combined footing, Rectangular footing(used when the loads on the two columns are same) and Trapezoidal Footing(used when the columns carry unequal loads).
3. Cantilever Footing:
- In simple words, Cantilever footing is the combined footing connected by a tie beam, which is of lower width and greater depth.
- This type of footing is necessary for conditions where there is a strict restriction of space and it is not possible to project the footing further to it’s column face.
- The tie beam helps in stabilizing the columns, thus making a uniform distribution of loads beneath the footing.
4. Raft or Mat Footing:
- In places where the soil is of inferior quality and the safe bearing capacity of the soil is lower compared to ordinary soil, a common footing is provided beneath each column.
- This type of footing is known as Raft Footing. Raft Footing is used in places where the distance between each column are very less, thus making it unsuitable for Isolated footing.
- Where the columns of Raft footing are connected by a Tie beam, it is known as Mat Foundation.
2. Deep Foundation
These type of foundation is used where the soil hard stratum exists at a considerable depth and the soil is of inferior quality.
These types of foundation are suitable where the hard stratum does not exist 3 meters from the ground surface. As the name indicates, the height of the deep foundations is much greater than it’s the width.
These type of foundation are used where the safe bearing capacity of a soil is low and the soil can not withstand high loading.
Advantages of Deep Foundation
- Deep foundation can be used at places where is soil is only strong enough to withstand intense loads, thus this type of foundation is suitable for all places.
- A deep foundation is the only type of foundation which can be used at places where the water table prevails at a lower depth from the ground surface.
- For the construction of marine and advanced structures, a deep foundation is used, due to the high stability it provides to structures from the effects of settlements and overturning.
One major drawback of Deep foundation is that the design procedure is complex, thus it needs more time & money to spend in it.
Deep foundation is generally subdivided into two categories:
1. Pile Foundation:
This is the most commonly used deep foundation for building projects. Pile foundation can be again divided into the following types:
- Depending upon nature of pile: Compaction Pile, Uplift pile, Batter pile, Sheet file, Anchor pile or fendor pile.
- Depending upon material: Concrete Pile, Steel Pile, Timber pile, Composite Pile etc.
- Depending upon load transfer: End Bearing pile(transfers the load by bearing) and Friction pile(transfers the load by friction between the pile and the surrounding soil-suitable when hard stratum is situated at a great depth)significant.
2. Well Foundation:
- This type of foundation is suitable for the construction of bridges, and at marine structures, where the soil present at river beds are weak and consists of stones, rubble, forming an uneven surface.
- Well foundation is also classified in Cassions. There are generally three cassions, Open, Box and Pneumatic Cassion, within which, the Pneumatic cassions ate being used the most.
Functions of Foundation
It provides lateral stability to the structure. Thus there is little chances of structural failure of structures, making it more durable in adverse conditions.
It protects the structure from natural disasters such as earthquake, flood and storms. A well-designed foundation protects the structure as a whole.
Load distribution of the structure is carried out evenly. The loads from the superstructure are transferred to a wider area in the foundation, and the foundation transfers it to the subsoil, thus making an equal distribution of the roads, and preventing the differential settlement of the soil.
Foundation provides safety to the structure from the effects of undermining and scouring due to the animals, flood water etc.
The foundation helps to prevent or to minimize cracks due to moisture movement in case of weak soils or soils with low bearing capacity.
Purpose of Foundation
- Foundations provide the structure’s stability from the ground: To distribute the weight of the structure over a large area in order to avoid overloading the underlying soil (possibly causing unequal settlement).
- The foundation serves the purpose of completely distributing the load from the structure over a large base area and then to the soil underneath. This load transferred to the soil should be within the allowable bearing capacity of the soil.