1. soil below foundation does not fail in shear

2. settlements are within permissible limits.

Foundations may classified as 

1. Shallow Foundation : A Shallow Foundation is a structure used to transmit loads to smaller depths. It can also we said as the foundation which is laid at Df (depth of foundation) ≤ Bf (width of foundation).

2. Deep Foundation : A Deep Foundation is a structure used to transmit loads to larger depths. A foundation is said as Deep, if it is laid at  i.e. rock at Df (depth of foundation) ≥ Bf (width of foundation). It is adopted where the soil at or near the ground surface is not capable of supporting a structure.

Types of Shallow Foundations :

1) Strip Footing : A Strip Footing is provided to support row of columns which are closely spaced that their spread footings overlap or nearly touch each other. It is economical to provide Strip Footing than number of Spread Footings. A Strip Footing is also called Continuous Footing.

2) Spread or Isolated Footing : This footing is provided to support an individual column. It can be a circular, square or rectangular slab of uniform thickness.

3) Combined Footing : A Combined Footing is provided to support a pair of columns. It is used when columns are close to each other such that their individual footings overlap. 

4) Strap or Cantilever Footing : A Strap (or Isolated) Footing consists of two isolated footings connected with a structural strap. The strap is connected in such way that it behaves as a single unit. It is provided where the allowable soil pressure is relatively high and the distance between the columns is large.

5) Mat or Raft Foundation : A Mat or Raft Foundation is a large slab supporting large number of columns and walls under or over the structure. A Raft Foundation is required where allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat Foundation is used for reducing Differential Settlements.

Types of Shallow Foundations

Types of Deep Foundations :

1) Pile Foundation : A Pile is slender structural member made of steel, wood or concrete which is either driven in soil or formed in-situ by excavating a hole and filling it with concrete. A Pile Foundation is used to transmit loads of structure to deeper strata. Pile Foundations are also used to resist horizontal forces in addition to support the vertical loads in earth retaining structures or tall structures that are subjected to horizontal forces due to wind  and earthquake. The size of Pile is generally ≤ 0.6 m.

2) Piers : A Pier is a vertical column of relatively larger cross-section than pile which is installed in a dry area by excavating to the desired depth and then backfilling it with concrete. The size of Piers are generally > 0.6 m .

Types of Deep Foundations

3) Caissons : A Caisson is a type of foundation of hollow prismatic box, which is built above the ground level and then sunk to the required depth as single unit. It is a water tight structural chamber used for laying foundations under water such as, rivers, lakes, harbours etc. Caissons are of two types : 

    a) Pneumatic Caissons : Pneumatic Caissons are closed at the top but open at the bottom. It has a working chamber at its bottom in which compressed air is maintained at required pressure to prevent     entry of water into chamber.

    b) Floating Caissons : Floating Caissons are open at top and closed at bottom. These are constructed on land and then transported  to the desired location at site and floated to the place where they are to be installed. These are then sunk to that place by filling them  with sand, ballast, water or concrete to a       bearing surface level.

4) Well Foundation : Well Foundations are similar to open caissons that are constructed on dry bed or after making a sand island. These Wells can also be fabricated on the river bank and then floated to the final position and grounded,  where depth of water is greater than 5 m to 6 m and the velocity of water is high. Well Foundation is used for providing foundations to water monuments, bridges and aqueducts. A Well Foundation may sink into the river bed by 50 to 60 cm under its own weight.