Shallow Foundation Of Different Types And Their Selection

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Hi it has been long time since I’ve posted on my Techno Genome. It was due to my busy schedule, but now I’m back with bag full of things for every Civil Engineers out there.
Today I will discuss about Foundation, Specifically about Shallow Foundation. After follwing this post the readers will be able to answer and understand about the followings :-
  • What is Foundation?
  • What is the Deep Foundation and Shallow Foundation?
  • What are the different types of Shallow Foundation?
  • What type of Shallow Foundation is to be provided in which case?

First of all let us understand what is Foundation and why these are required to be provide. To be precise a foundation can be defined as the lower most integrated part of a structure, which is in direct contact with the soil, and which safely transmits the whole load of the structure with the full stability of the structure to the soil beneath. So what does this all means? Its functions are as same like as that of our feet. A structure cannot be suspended in the air by it’s own, can it? so we have to give opposite and equal reaction to the direction from which the loads are coming. This done by providing the foundation, which transfers all the loads coming from the structure to the hard soil layer at certain depth, A foundation also gives lateral stability to the structure so that it does not Slide or Overturn.
There are two types of Foundation mainly, a) Shallow Foundation b) Deep Foundation. This two type of foundation is based on a criteria that is as follows, If the Breadth of the lower most portion of the foundation is grater than the depth of the lower most portion of the foundation from the top of the soil, that is Ground Level then it is called as a Shallow Foundation. And is the Breadth of the lower most portion of the foundation is lesser than the depth of the lower most portion of the foundation from the top of the soil, that is Ground Level then it is called as a Deep Foundation.
In most of the cases of light and moderately loaded structures like Residential Buildings up to 5 storied, and other general type of works shallow foundation is to be provided, but the soil must be not of poor quality with highly compressible soil and low bearing capacity. and in case of heavily loaded structures like High Rise Buildings, Bridges and Flyovers etc. deep foundation is to be provided. Deep foundation are the only most suitable method of providing foundation in case of poor soil condition. In this article I will only discuss about the Shallow Foundation, On a article sooner I’ll discuss about the Deep Foundation so keep visiting Techno Genome.
Actually there are many a types of shallow foundation which are to be provided  depending upon the site condition and type of loading and pattern. Namely :-

  1. Isolated Column Footing :- Isolated Column Footing is a very common type of foundation, and is provided in case of Framed Structure and Combined Framed and Load Bearing Structure for providing the foundation of a single column only.For each and every individual column a seperate Isolated Column Footing is to be provided with. This type of foundations can be of two types as a footing having uniform depth at all points of the footing, and a footing being slopped from the face of the column towards the edge of the footing. Sloped footing are much economical than footing with uniform depth.
  2. Combined Footing :- This footing are used when the footing area of one column overlaps the footing area of the other column. In this type of foundation Two or more Columns are provided within a single Footing, which maybe Rectangular or Trapezoidal depending upon the loading pattern on the columns to be provided with Combined Footing. If all columns are having almost equal load on them then Rectangular Footing is used and if the loading differs largely then Trapezoidal Footings are used, the width being greater to smaller from the side of column having greater load the column supporting comparatively lesser Loads.
  3. Continuous Strip Footing :- This is the oldest type of footing I think. This type of foundation is used for providing foundation to Wall, which may or may not be load bearing wall. Continuous Strip Footing are of two types, one made of P.C.C. and the Other is made of R.C.C. If you are planning to provide a continuous strip footing in the form of stepped from main wall thickness at plinth to the foundation width at the bottom of footing then P.C.C. are most economical, but in case of slightly compressible soil and higher loading intensities, R.C.C. Continuous Strip Footing should be used
  4. Strap Footing :- Strap Footing are nothing but an improvised type of Isolated Column Footing, in this type of foundation each isolated column footings are connected at the pedestal level with the adjacent isolated column footings on all four sides by Strap Beams. by connecting all isolated column footings with strap beams they all become bonded together hence they will resist the Differential  Settlements due to the compressibility or lowering of the pore water pressure in the subsoil. 
  5. Raft Foundation :- Raft Foundation are one of the best type of shallow foundation, and which are very effective where the bearing capacity of soil is low and there are chances of Differential Settlements. This type of foundation is generally provided for framed structures, and where the load on each column are are almost same. In this type of foundation a Hard solid base is prepared over which Primary and Secondary Beams are constructed according to the column position, so that the primary and secondary beams intersects each other at the positions where columns are located, and thus the columns are erected at the Nodal Points or points of intersection of the primary and secondary beams.
  6. Mat Foundation :- Mat Foundation are similar to that of the Raft Foundation, this is suitable where the soil is compressible and having low bearing capacity and there are very risk of differential settlement. The load on each column should be almost equal. In this type of foundation an Inverted Slab Beam is constructed monolithically, this means that the slab is at the bottom most part and above it the beam is casted at the same time. As in the case of the Raft the beams are position as primary and secondary beams as required and the columns are then erected from the Nodal Points.
  7. Grillage Foundation :- This type of Foundation are used generally only for providing foundation to the Steel Column, Column Under Huge Loading, and in the case of the Machine Foundation. In this Foundation R.S.J. and Concreting is done.
  8. Inverted Arch Type Foundation :- This type of foundation are mostly suitable for providing foundation for Arcade. In this, Brick or Concrete Arches are provided having Convexity Downwards, between all adjacent Columns. The Foundation Keeps its stability under the Arch Action.
  9. Cantilever Type Foundation :- This type of foundation is used when no sufficient space is available to provide the foundation for a column directly beneath it, but the column must placed in that particular position. In this case a cantilever beam is projected from the nearby column and at the free end of that Cantilever Beam The Column is Supported. Sometimes a Balancing Load is also applied on the other end without fixing the beam with the nearby Column.


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