v FOUNDATION :- The lower most part of the building or any structure below the ground level that transfers the building load in subsoil without any settlement.
v OBJECTS:-
Ø To distribute the total load coming on structure on a larger area.
Ø To support the structure.
Ø To give enough lateral stability to the structure against horizontal forces such as wind, rain, & earthquake.
Ø To prepare level & hard surface for concreting & masonry works.
Ø To distribute the non-uniform load of structure evenly to the soil.
Ø To provide structural safety against scouring due to flood water.
v REQUIREMENTS OF GOOD FOUNDATION:-
Ø Location - It should be so located that it is able to resist any future influence which may adversely affect its performance.
Ø Stability– It should be stable against any possible failure. The foundation based should be rigid enough to bring down the differential settlement to a minimum extent specially when the super imposed load are unevenly distributed.
Ø Settlement– It should not settle to such an extent that it becomes necessary to impair its usefulness or stability of the building.
v CLASSIFICATION:-
It is mainly classified in two categories :
Ø Shallow foundations
Ø Deep foundations
They are further classified as below :
v Shallow Foundation
Ø Spread footing
Ø Pad foundation
Ø Combined footing
Ø Continuous or Strip foundation
Ø Grillage foundation
Ø Raft foundation
Ø Inverted arch foundation.
Ø Stepped footing.
v Deep Foundation
- Pile foundation.
v SHALLOW FOUNDATION.
Ø Spread Footing : - A spread is provided under the base of wall or column. This spread is known as footing and such foundation is known as spread foundation.
Ø Pad Foundation : - It supports an individual point load such as column. It is square or rectangular in the plan. The thickness of pad foundation is may be of varying nature or it may be uniform.
Ø Combined Footing: - A common footing provided for two or more column is known as combined footing. It is useful when an exterior column is situated near boundary line of the plot & it is not possible to projects its footing symmetrically on both the sides. The shape of combined footing is rectangular or trapezoidal.
Ø Continuous or strip foundation: -It consist of concrete strip placed centrally under load bearing walls. It is used for retaining walls, Cable trenches, closely spaced columns in a row, R.C.C. drains etc.
Ø Grillage Foundation –It is used when heavy structural loads are to be transfer to sub soil of low bearing capacity. It is comparatively light in weight & economical. The depth of foundation is kept limited to 1 to 1.5 m & width is considerably increased to bring the pressure on soil within permissible limits. It consist of the steel joist known as grillage beams. They are provided in double tier placed at right angle to each other. This steel work is thoroughly embedded in the concrete work. Concrete does not carry any load but it only hold grillage beams in the position.
Ø Raft foundation : - In this type a R.C.C. slab of suitable thickness & necessary reinforcement is provided. Sometimes raft with the beams may also be provided. The slab extends over entire loaded area. The raft is designed as an inverted R.C.C. roof with uniformly distributed load of soil pressure & supported with walls, columns & beams. It reduces the differential settlement & used where the soil is of soft nature or loose.
Ø Inverted arch foundation : It consist of constructing the inverted arched between the piers. The rise of inverted arch is about 1/ 5 to 1/10 of the span & built in the half brick ring. The end piers is designed to withstand the outward pressure caused by arch action. The arch is positioned along or across the row of piers. Depth of foundation is reduced & it is economical.
Ø Stepped foundation : When the ground is sloping the correct levels of ground are taken & a longitudinal section is prepared. The overlap between two layers of foundation should be equal to the depth of foundation. A minimum depth of 800 mm should be provided at all the points. The distance of the sloping ground from the lower edge of footing should not be 0.6 M for rock & 1 M for soil.
v DEEP FOUNDATION
- Pile foundation: The term pile foundation denotes a construction for the foundation of wall or pier which is supported on the piles. They are relatively long slender members that transmits foundation load to deeper soil having high bearing capacity. The main components of the pile foundation are pile cap & piles. The pile caps are thick slabs used to tie a group of piles together to support & transmit column loads to the piles.
Pile foundation is required when,
- Top layers of soil are highly compressible for it to support the structural loads through the shallow foundations.
- Lateral forces are relatively prominent.
- In presence of expansive & collapsible soil.
- For off shore structures.
- Strong uplift forces on shallow foundation due to shallow water table can be partly transmitted to the piles.
- For structures near flowing water (Bridge, Abutment etc.) to avoid problems due to erosion.
Types :
(1) Based on function
- End bearing piles.
- Friction piles.
(2) Based on material
- Timber piles
- Concrete piles
- Steel piles
(3) Based on method of installation
- Displacement piles
- Non displacement piles
- CONSTRUCTION (Shallow foundation)
The construction process includes following steps,
- Excavation
- P.C.C.
- Reinforcement
- Formwork
- Concrete
- Curing
1. Excavation: The area at site is layout as per the drawing requirement. Top soil containing organic matter is removed. Excavation is done up to required depth at which the necessary bearing capacity is met. A variety of machines are used for excavation purpose. The sides of excavation should be protected by benched excavation, sheeting or bracing etc.
2. P.C.C.: After excavation the bottom layer is dressed & levelled. The size of foundation is imprinted with the help of lime powder & shuttering is placed. Top level of P.C.C. is marked & concreting is done. It provides a dry working platform & stops the entry of ground water.
3. Reinforcement: Centre line of footing is marked over the P.C.C. surface using the yellow paint. Cutting & bending of reinforcement is done as mentioned in drawing. The placing & binding of reinforcement is done keeping the bottom layer on cover blocks to maintain the gap between reinf. & P.C.C. surface. Reinforcement of the column is placed in accordance with the centre line.
4. Formwork: When the concrete is placed, it is in the plastic state. So it requires to be supported by temporary support & casing known as formwork or simply forms. The sides of footings are made of shutters using ply, battens, & nails etc. The inner surface of ply is coated with oil. To make the shutters firm against forces of concrete supports in the form of Pipes, U jacks, Props etc are provided. The joints of shutters shall be sealed by applying putty or cement paste in order to stop the leakage.
5. Concrete: The concrete of required grade is prepared using batching plants, concrete mixtures or other similar arrangement. Way of pouring the concrete depends upon the size of foundation. For small foundations the concreting is done manually & for big sized foundation it may be done using boom placer or concrete pumps. The concrete shall not be poured from height more than 2 M. The top surface of concrete is smoothly finished using trowel & float.
6. Curing: The concrete must be cured for at least 7 days , 3 times in a day.
CAUSES OF FAILURE :
- Unequal settlement of the sub soil. :
- Unequal settlement of the masonry.
- Withdrawal of the moisture from the subsoil.
- Lateral pressure on the super structure.
- Horizontal movement of the earth.
- Transpiration of trees & shrubs.
- Atmospheric action.
Amazing blog! You are describing all the details about the foundation and its type very well. This information is very important to civil engineers. It is a very important part of the construction because the foundation is the base of a building, house, etc.
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