CAUSES OF FOUNDATIONS FAILURE

 The foundations may fail due to the following reasons:

1. Unequal settlements of sub-soil.
2. Unequal settlements of masonry.
3. Lateral pressure on the wall.
4. Lateral movement of the sub-soil.
5. Sub-soil moisture movement.
6. Atmospheric action.
7. Weathering of sub-soil due to plants.

1. Unequal Settlements of Sub-soil

Unequal settlements of sub-soil is a major causes leads to foundation failure because it leads to cracks in the structural components and rotation thereof.Unequal settlement of sub-soil may be due to:

• Eccentric loading.
• Unequal load distribution on the soil strata.
• Non-uniform nature of sub-soil throughout the foundation.

2. Unequal settlements of masonry

The portion of masonry situated between the ground level and concrete footing has mortar joints which may either shrink or compress, leading to unequal settlement of masonry.Due to this the super structure will also have cracks which leads to unequal settlements of masonry.

3. Lateral Pressure on the Wall

The wall transmitting the load to the foundation may be subjected to lateral pressure from a pitched roof or an arch or wind action.Due to this, the foundation will subjected to a moment .If the foundation has not been designed for such a situation it may fail by either overturning or by generation of tensile stresses on one side and high compressive stresses on the other side of the footing.

4. Lateral Movement of Sub-soil

Lateral movement of sub-soil is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads specially at the locations where the ground is sloping.Such a situation may arise in granular soils where a big pit is excavated in the near vicinity of the foundations.Due to such movement excessive settlements takes place or structure may even collapse. 

5. Sub-soil Moisture Movement

Sub-soil moisture movement is a major causes of failure of foundation on cohesive soil, where the sub-soil water level fluctuates.When water table drops down, shrinkage of sub-soil takes place.Due to this there is a lack of sub-soil support to the footing which leads to cracks in the buildings.

6. Atmospheric Action

The behaviour of foundation may be adversely affected due to atmospheric agents such as sun, wind and rains.If the depth of foundation is shallow, moisture movements due to rains or drought may cause trouble.If the building lies in a low lying area, foundation may even be scoured.If the water remains stagnant near the foundation it will remains constantly damp resulting in the decrease in the strength of footing or foundation wall.

7. Weathering of Sub-soil due to Plants

Sometime plants like small trees, shrubs is grown very near to the wall.The roots of these shrubs absorb moisture from the foundation soil, resulting in reduction of their voids and even weathering.Due to this the ground near the wall depresses down.If the roots penetrates below the level of footing, settlements may increase resulting in foundation cracks.