Types of building foundation its requirements and uses.

Foundation:-

 It is the lowermost part of structure which provide the base for the superstructure and transmit the load of superstructure to the subsoil without any failure.

Purpose of foundation :-

1. To distribute the load of the superstructure over wide area.
2. To provide level surface for building operation .
3. To take the structures deep into the ground to provide stability and to prevent overturning.
4. To minimize the chance of unequal settlement by distributing the load over wide area.

Requirements of good foundation:-

• To distribute the weight of structure over large area ,so as to avoid overloading of the soil beneath.
• To avoid unequal settlement of the structures
• To provide a levelled surface for building operation.
• The foundation take the structure deep into the ground ,thus increase the stability of building and prevents overturning.

Types of foundation:-

  

A) shallow foundation.

B) Deep foundation.

A) Shallow foundation :-

1. Wall footing.*
2. Isolated footing.*
3. Combined footing.*
4. Cantilever footing.
5. Inverted arch footing.
6. Grillage footing.
7. Raft or mat footing.*

B) Deep foundation:-

1.  Pile foundation.
2. Well foundation.

A) Shallow foundation :-

  When the foundation is placed immediately below the lower part of structure ,it is called as shallow foundation.

1) Wall footing :-

  It can be achieved by constructing several courses of bricks widening the base ; the lowest course is usually twice the breadth of wall.

 In case of stone wall , the size of offset is slightly more than the brick wall footing .

  Lean concrete bed normally P.C.C(1:4:8) OR P.C.C (1:5:10) is spread over the entire length of the wall the minimum thickness 15 cm and the projection on either side normally 10 to 15 cm.

fig.1 Wall footing

2) Isolated footing:-

It is provided under a column or other similar member for the distribution of concentrated load in the form of uniformly distributed load on the soil below.
  It shape may be square, rectangular , or circular , in plan .
  It may be constructed in either in brick masonary , stone masonry , or R.C.C .

fig.2 Isolated footing.

3) Combined footing:-
  
Combined column footing is provided as a common footing for two or more columns.
  This type of footing helps in reducing the depth of foundation and unequal settlement can be prevented.
  In this type of footing , a common P.C.C bed is provided on which two or more column rest.
  Shear force calculation is important in combined footing.
   Combined footing may be rectangular , square, trapezoidal ,footing are most common.

fig.3 Combined footing

4) Raft or mat footing:-
  
When the bearing capacity of the supporting soil is very low and the heavy concentrated structural load are to be supported in case of  marshy land or land made up of ground or soft clay - under all such cases mat or raft foundation provided
   It proves to be economical under water logged area where pile foundation cannot be used advantageously and independent column footing becomes impracticable.
  Raft is acting as a floor consisting of thick reinforced concrete slab covering the entire area of the bottom of the structure.
   The slab normally projects 30 to 40 cm on all the side outer wall .The excavation is made to a required depth , the entire area is well compacted .
  When this surface is dry then the slab is laid.

fig.4 Raft foundation.

B) Deep foundation :-

 When the depth of foundation is more than its width , the foundation is called as deep foundation .
  It can be also defined as " if the depth of foundation is more than 2.00 m ,it is called as deep foundation.
  If the strata of good bearing capacity is not available near the ground and the foundation of the structure where shallow foundation cannot be used , under all such cases deep foundation is used .

Types of deep foundation :-
  
1) pile foundation 
2) well or cassion foundation .

1) pile foundation :-
    It is foundation system that transfer loads to a deeper and component soil layer .
  Generally , pile foundation is used if the structure is situated on sea shore or river bed and if the top soil has poor bearing capacity .
 
Pile foundation is used :-
 
1. when there is inadequate bearing capacity of shallow foundation .

2.    To prevent uplift forces .
3.    To reduce excessive settlement.

2) Well or cassion foundation :-
  
 Cassion is a water tight structure made up of wood , steel or reinforced concrete constructed in connection with excavation for foundation of bridge , pier in rivers , lake , dock structure etc .
   Cassion remains in its position and ultimately becomes an integral part of the permanent structure and normally used for deep foundation under water level for bridge and other structure.

Types of cassion  foundation :-
   
1) open Cassion .
2) Box cassion .
3) Pneumatic cassion .