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The main causes of failure of foundations are as follows: 
a. Unequal settlement of the sub-soil
b. Unequal settlement of the masonry 
c. Withdrawal of moisture from the sub- soil
d. Lateral pressure on the superstructures
e. Horizontal movement of the earth
f. Transpiration of trees and shrubs
g. Atmospheric action.

Unequal Settlement of the Sub-soil

This occurs due to various reasons such as unequal distribution of load on the foundation, varying bearing power of the sub-soil, eccentricity of the loaded. Due to unequal settlement of the sub-soil, cracks are formed in the buildings, which in future, leads to serious defects. Following are the measures to be adapted to prevent such failure:
Causes of Failures of Foundations and Preventive Measures
Unequal settlement of sub-soil
  • Foundation should rest on rock or hard moorum.
  • Design of foundation should be appropriate to the nature of sub-soil.
  • It should be seen that the allowable bearing pressure on the soil is not exceeded, even under the worst conditions.
  • Proper attention should be given to the eccentricity of the load on the foundations and design should be accordingly modified.
Unequal Settlement of the Masonry

Mortar joints may shrink and compress which may lead to unequal settlement of the masonry. Measures to avoid such situations are as follows:
  • Mortar to be used in the masonry should be stiff and in line with the workability desired.
  • The masonry work should be raised evenly. 
  • The height of wall to be raised per day should be limited to one metre, if lime mortar is used and to 1 -50 metres, if cement mortar is used. 
  • Proper watering should be done to the masonry work.
Withdrawal of Moisture from the Sub- soil

This occurs at places where there is considerable variation in the height of water table. When water table falls, the soil particles lose cohesion and hence, there is shrinkage of soil, resulting in the cracks to the buildings. The precaution to be taken to avoid such failure would be to drive piles unto the hard rock.

Lateral Pressure on the Superstructure

The thrust of a pitched roof or arch or wind action on the superstructure causes wall to overturn. Remedial measures to prevent this failure would be to provide a sufficient wide base and to design the foundations for the worst conditions.

Horizontal Movement of the Earth

Very soft soil is liable to give way under the action of load especially at places such as sloping ground, river banks, etc. hence, in such cases, it is desirable to construct retaining walls or to drive sheet piles to prevent the escape of the earth.
Transpiration of Trees and Shrubs

The roots of trees planted near a building may extend up to the foundation level and may absorb the moisture. This effect is seen in the form of a depression on the ground and it may lead to cracks in the building. Remedial measures are as follows:
  • Foundations should be taken sufficiently deep. A minimum depth of one meter is required for this purpose.
  • Fast growing and water seeking trees should not be planted near the building with a minimum distance of 8 meters.
Atmospheric Action

Rain and sun are the main atmospheric agents to seriously affect the foundations of a building. Heavy rains or considerable variation in temperature or frost action may damage the foundations. Rain water may create pockets near the walls and while descending, it may carry certain chemicals and salts obtained from sewage, animal dung, etc. These chemicals and salts may react with the materials used for the foundation work and turn them into powder. Remedial measures to be taken are as follows:
  • Foundation should be taken beyond the depth up to which rain water can reach.Ingress of rain water and harms associated with it can therefore be avoided.
  • Suitable underground drains should be provided to maintain the water table at a definite level. 
  • After the masonry work is completed, the sides of trenches should be carefully filled with earth and well consolidated. A gentle slope should be provided so as to keep rain water away from the wall.
  • Plinth protection should be provided and given due consideration.









 
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