Causes of Failure of Foundation

Foundation are the lower part of a structure which provides a base for the super-structure proper. This term includes the portion of the structure below the ground level as well as artificial arrangement of concrete block, piles, raft, grillage, etc. provided to transmit the loads on the structure including the dead weight of the structure itself to the soil below.Causes of failure

The causes of failure of foundations may be summerised under the following heads :

1. Unequal settlement of the sub-soil.

2. Unequal settlement of masonry.

3. Horizontal movement of the soil adjoining the structure.

4. Shrinkage due to withdrawl of moisture from the soil below the foundation.

5. Lateral pressure tending to over-turn the structure.

6. Action of atmosphere.

7. Lateral escape of the soil below the foundation.

1. Unequal settlement of the sub-soil.

Unequal settlement of fundation results in dangerous cracks which ultimately lead to the failure of the structure. If the nature of soil and the loading conditions are uniform over the entire site of the building, the distribution of the pressure can be safely assumed to be uniform and the danger of unequal settlement is minimised. In practise, however, it is frequently found that different conditions exist under part of the building. If some portion of the building is higher, the soil below the higher portion will be subjected to greater than the soil below the lower partion of the building. Or one section of a building may rest on rock and the adjacent section on compressible clayey soil. In either of the cases, the soil is subjected to unequal settlement, and as such in designing the foundation the pressure on soil should be reduced to a such value that the difference in settlement between the two portions of the structure is reduced to minimum. In no case, the pressure on the soil should exceed the safe-bearing capacity of soil. Materials used for construction should be durable so as to avoid danger of the disintigration of the foundations. As per as possible foundation should be so proportionate that the C.G of load from structure coincides with the center of gravity of the foundation. By adopting these precautions, the danger to strucrures on account of unequal settlement of sub-soil can be minimised.

2. Unequal settlement of masonry.

Mortar used as binding material in the masonry construction shrinks and gets compressed when loaded excessively before it has fully set. This defect may lead to the unequal settlement of masonry. The following measures should be adopted to avoid unequal settlement of masonry:

a. the consistency of mortar should be such that it provides easuy workability. The mortar should neither be very lean nor very stiff.