Types Of Bridge Foundations And Their Suitability:
Depending upon nature and depth, bridge foundations can be classified into the following types:
1. Spread or open foundation.
2. Raft foundation.
3. Grillage foundation.
4. Inverted arch foundation.
5. Pile foundation.
6. Well foundation
7. Caisson foundation.
1. Spread Or Open Foundation:
This type of foundation is suitable for bridges of moderate height, to be built on dry ground which is sufficiently firm to support the bridge structure. It is best suited in situations where hard soil is available within 1.5 to 3 m below the bed level of water course and when the scouring is minimum.
2. Raft Foundation:
This type of foundation is suitable when the bed of the water course consists of soft clay and silt and the hard soil is not available within reasonable depth (1.5 to 2.5 m) below the river bed.
It is best suited in situations where the allowable bearing capacity of the soil is low or when the bridge loads are heavy. Raft foundation is also suitable when the soil bed has some soft pockets which are compressible and there is possibility of unequal settlement.
3. Grillage Foundation:
This type of bridge foundation is suitable for heavy and isolated footings of piers where deep foundations are to be avoided. It is best suited where sub-soil conditions are poor.
4. Inverted Arch Foundation:
This type of bridge foundation is suitable when the depth of excavation for foundation is less. It is best suited where the bearing capacity of the soil is less, especially when the sub-soil has some soft pockets and their is possibility of unequal settlement.
5. Pile Foundation:
This type of bridge foundation is suitable when the soil is very soft and the hard strata are not available at a reasonable depth below the bed level of the river. It is best suited where heavy scouring of river bed is expected and heavy concentrated loads are expected to be taken by the foundation. It is also suitable when it is much expensive to provide raft or grillage foundation.
6. Well Foundation:
This type of bridge foundation is suitable where good soil is available at about 3 to 4 m below the bed level of the river and the bed consists of sandy soil. It is best suited to soft soils or sandy beds, where heavy scouring is liable to occur due to excessive velocity of approach of the river water.
7. Caisson Foundation:
Caisson foundation is suitable when a hard strata is available near to the river bed but the depth of water is excessive and it is not economically possible to exclude water from a dry bed for sinking the wells to provide well foundation.