What is Foundation in Civil Engineering?
The lowest part of structure on which the whole structure is built on is called Foundation. This is also called Sub-structure of building. There’s two types of foundation shallow and deep foundation.
In construction work there are various types of footing and foundations used in for different structures. All the foundations and footings are suggest and designed according to the project worth, load factors, and the soil condition.
Foundation/Footing is the key part of structure on which all the load of structure depends. Foundation is a sub-structure part of structure which carry all the load transferred by column members and smoothly transfer it on soil.
There are two main parts of building structure the sub-structure and the superstructure. The sub-structure only include foundation, plinth beam and the damp proof course, except all are superstructure parts.
Difference between footing and foundation?
Footing is actually a part of foundation. Its a brick work with rebar or a one inch concrete work on a trench to provide smooth and clean surface for foundation.
Basically a trench is not smooth, the surface of trench is wavy, to provide a smooth and flat surface for foundation there is to pour 3 or 6 inches of lean concrete on soil surface to make it smoother & flat.
Afterwards there is the steps of brick work to fill the trench and make the foundation up from ground level. Up Next to footing there is a real foundation built on, on which the building or a structure is constructed.
What is the purpose of foundation?
- To transfer structure load on more surface area. To insure the load transfer on per unit area will not exceed to the bearing capacity of soil. (If the load on per unit area will increase, the surface of soil will not be capable to bear the structure load and the structure will collapse).
- To transfer structure load on surface at constant rate. (If the load suddenly increase and decrease usually at time of earthquakes, so the foundation bear and transfer it to ground at constant rate).
- To insure the columns of builds do not slip off. (There should be a monolithic reinforcement to prevent the slip of columns).
- To provide a smooth and stable surface for structure to build.
- To provide stability to the structure. (It really help in earthquakes).
What is settlement of foundation?
Usually the surface soil of ground is soft which includes organic matters and some quantity of water element which cause the settlement of foundation and failure of structure.
That’s why firstly dig a trench to get sufficient hard soil to construct a foundation.Basically when the loads transfer onto foundation, foundation transfer it on ground soil and the side soil , if the soil is hard enough, it will bear the load.
Otherwise it will push the side soil upward and the walls or columns of structure will settle down.
Factors to be consider in foundation designing?
There are the many factors that effect the types of foundation selection but the most important these factors to be considered.
Loads & Its Nature:
Sometimes there is equally constant load of structure on foundation sometimes its dynamic and varies at some points. Sometimes there’s also moving load on foundation cause of wind pressure or earthquakes.
If the load intensity is low there can be a spread foundation used. In case of dynamic load there should be a Raft foundation.
At the point where there is high intensity load or whole structure loads act on one point there should be Grillage or Pile foundation to be considered.
Location & Situation:
The situation is really considered while constructing a foundation. If the foundation is to be built in deep water (for bridges and ports) the Cassion foundation is better.
The spread foundation is better to construct because its economical and most common if the surface is normal and dry. If the soil is weak there should be pile foundation.
The bearing capacity of soil isn’t the same at all points. At some point there is clay soil, sandy soil and at some points there’s hard surface or hard rock.
At all the different points the design and type of foundation will be different. On sandy soil and hard rock you can use shallow foundation while on clay and weak soil there should be deep foundation to be considered.
How to design foundation for building structure?
In foundation/footing designing, there’s two thing to calculate . Width of foundation and depth of foundation.
Calculation of width of foundation?
What is width of foundtion? What should be a width of foundation? The width of foundation should be that much, to safely divide the total load on bearing capacity of soil.
There is a thumb rule to find a width of common foundation for a common type of soil.
Width = 2T + 30…..in cm
Width = 2T + 12…..in inches
• where T is thickness of wall.
If the building is more than two storeys.
Width = P/Pb
• where P= Total load acting on foundation
• Pb = Safe bearing capacity of soil
Calculation of depth of foundation:
Depth = Pb/w (1-sinϕ/1+sinϕ)2
where Pb = safe bearing capacity of soil
W = unit weight of the soil
ϕ = Angle of repose of the soil
Read also High rise construction.
Types of foundations?
There are several types of foundations in construction according to nature of work and soil sampling it is to be consider which type of foundation to be construct.
Basically there are two types of foundations shallow and deep foundation further these foundations have different types according to shape and structure.
What is shallow foundation? A foundation which has less depth to two times of its width is Shallow foundation.
Shallow foundation is most commonly used in building construction. It carry load from columns and transfer it to the near ground not like deep foundation.
Shallow foundation is not located more than 5-6 feet below the ground.
These foundations are provided where bearing capacity of soil is enough near the ground and it has sufficient capacity of strata not to result in settlement. Their construction is really simple.
Shallow Foundation is most commonly used in building construction because its really economical.
Shallow types of foundation:
Spread or Isolated Footing:
This type of foundation is used for walls and can be also use separately for one column or two or more than two columns.
There’s two types of foundation in spread:
In construction of spread foundation the thickness of wall is increased in the bottom step by step equal to the foundation width as per designed.
Usually the lowest part of foundation has 3 or 4 inches lean or ordinary concrete mix of grade M7.5 or M10 (1:3:6 or 1:4:8). which provides the smoother surface.
Usually there is 30inches (76.2cm) one lean concrete step at bottom, upon lean concrete step there’s 24 inches (61cm) step of brick work.
Then there’s two steps of 18 inches (45.72cm) of bricks, then two steps of 13 inches (33cm) of brick work, and then 9 inches (22.76cm) of wall at the top end.
Isolated footing for column:
Isolated foundations are built separately for columns. It is used when the distance of columns are more and the load is less.
Strap or Cantilever Footing:
This type of foundation is same as combined footing but in this type, the footing for columns in built separately and then joint the both columns footing with a strap beam.
Strap foundation is made with R.C.C. In this type of foundation the size of footing for columns can be same or different according to load.
Continuous types of foundation:
When the columns of structure are in the series/queue then there is a long beam provided and columns are built on that beam. This is Continuous footing.
If the load ratio is less there is no need to provide reinforcement, but if there is more load the beam and columns are designed accordingly.
Mat or Raft types of foundation:
It is used when soil bearing capacity is less and load is more, then there should construct a R.C.C foundation bed on whole area of plot rather then increasing the size of isolated foundations.
It is to transfer load on more area to decrease the load on per unit area. There are different types of raft foundation in construction according to nature of project.
This type of foundation is suitable for multistory buildings. If there is large area to be covered for raft foundation the ready mix concrete is used mostly.
To construct a a raft foundation dig a required trench, compact it then put a lean concrete mix grade M7.5 (1:4:8). After that put two traps of reinforcement.
First trap of reinforcement should be at height of 6 inches (15 cm) from bottom and the second trap should be 6 inches (15 cm) down from the required foundation height. At the end Fill it with the concrete mix grade M15 (1:2:4).
After the raft foundation completion the required columns and walls points are marked on and built up on.
Deep foundation is a type of foundation which is use to transfer the building loads to more farther or deeper inside the ground surface.
It cost more because of deep down excavation for piling and rebar reinforcement concrete. Sometimes for heavy projects it goes more than 100 feet down pile and at the top there is raft foundation cap.
Deep foundation usage?
May you wonder if the deep foundation is really costly the why it is build on? What is the the use of deep foundation?
Beside cost it is to insure that building load is transfer smoothly to the ground surface. If its not safe, if the foundation is not safe the super structure will collapse.
Deep Foundation is used where the bearing capacity of soil is not sufficient to bear the structure load , so it goes deep down till the strong rock or hard layer of soil reached, where the load can be transferred.
Deep types of foundations:
Following are the most used deep foundations.
The foundation which are constructed on pillers/columns are called pile foundation. Basically these pillars are known as piles. These piles can be made of concrete, wooden and of steel.
In a pile foundation there a pillar of concrete/steel/wood as deep as hard rock or hard soil achieved or may be according to design. After piling required piles there is a pile cap same as raft foundation.
Pile cap is a reinforcement concrete of (M15 (1:2:4) or may be according to the design) bed on top of the pile.
According to use pile has following different sub types of foundation.
- Bearing pile
- Friction pile
- Bearing cum friction pile
- Batter pile
- Sheet pile
- Guide pile
Which types of foundation used for bridge?
Pile is the foundation which is used for bridges, flyover and heavy structures because in this foundation, there is a pile which transfer the load of the structure deep down to the hard rock.
Well foundations are mostly constructed near water places like sea bed, ocean bed, bridges etc. The shape of well foundation is like well. These foundations are hollow inside. Construction of well foundations can be made up of bricks if there is more load then it is made of R.C.C.
In this foundation firstly drill a pile with piling machine/well kerb. Make a hollow well foundation with brick or concrete. Then at the bottom M7.5 (1:4:8) concrete mix is provided which is called bottom plug, and it close it from bottom.
Then at the top there is a layers of sand till the end, at the end there is M15 (1:2:4) concrete mix grade is provide to close it from top and its called top plug.
Caisson foundation is the type of deep foundation, this is same as well foundation. In this type sometimes its parts are pre made and put it in the Caisson if the size of Caisson is more then it is built on site.