Monday, October 29, 2012


The foundations may fail due to the following reasons:

1. Unequal settlement of sub-soil. Unequal settlement of the sub-soil may lead to cracks in the structural components and rotation thereof. Unequal settlement of sub-soil may be due to (i) non-uniform nature of sub-soil throughout the foundation, (ii) unequal load distribution of the soil strata, and (iii) eccentric loading. The failures of foundation due to unequal settlement can be checked by : (i) resting the foundation on rigid strata, such as rock or hard moorum, (ii) proper design of the base of footing, so that it can resist cracking, (iii) limiting the pressure in the soil, and (iv)avoiding eccentric loading.

2. Unequal settlement of masonry. As stated earlier, foundation includes the portion of the structure which is below ground level. This portion of masonry, situated between the ground level and concrete footing(base) has mortar joints which may either shrink or compress, leading to unequal settlement of masoray. Due to this, the superstructure will also have cracks. This could be checked by (i) using mortar of proper strength, (ii) using thin mortar joints, (iii) restricting the height of masonry to 1 m per day if lime mortar is used and 1.5 m per day if cement mortar is used, and (iv) properly watering the masonry.

3. Sub-soil moisture movement. This is one of the major causes of failures of footings 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 lack of sub-soil support to the footings which crack, resulting in the cracks in the building.

During upward movement of moisture, the soil (specially if it is expansive) swells resulting in high swelling pressure. If the foundation and superstructure is unable to resist the swelling pressure, cracks are induced.

4. Lateral pressure on the walts. The walls transmitling the load to the foundation may be subjected to lateral pressure or thrust from a pitched roof or an arch or wind action. Due to this, the foundation will be subjected to a moment (or resultant eccentric load). 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.

5. Lateral Movement of sub-soil This is applicable to very soft soil which are liable to move out or squeeze out laterally under vertical loads, specially at locations where the ground is sloping. Such a situation may also arise in granular soils where a big pit is excavated in the near vicinity of the foundation. Due to such movement, excessive settlements take place, or the structure may even collapse. If such a situation exists, sheet piles should be driven to prevent the lateral movement or escape of the soil.

6. Weathering of sub-soil due to trees and shrubs. Sometimes, small trees, shrubs or hedge 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.

7. Atmospheric action. The behaviour of foundation may be adversely affected due to atmospheric agents such as sun, wind, and rains. If the depth of foundaion 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 remain constantly damp, resulting in the decrease in the strength of footing or foundation wall. Hence it is always recommended to provide suitable plinth protection along the external walls by (i) filling back the foundation trenches with good soil and compacting it, (ii) providing gentle ground slope away from the wall and (iii) providing a narrow, sloping strip of impervious material (such as of lime or lean cement concrete) along the exterior walls.

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