Thursday, May 19, 2016

Why foundations Fails and How to prevent such failures



Causes
1. Unequal settlement of sub-soil. All soils, except rock and muram, yield under the load of the structure. If this yielding is uniform and within a particular limit, it is harmless, but unequal yielding will cause cracks in the structure and ultimately cause the failure of the structure and the foundation.
2. Unequal settlement of masonry. This is caused by shrinkage and compressibility of mortar joints and the poor quality material used in the foundation.
3. Withdrawal of sub-soil moisture below the foundations. This occurs when the soil is wet. In dry weather when the moisture of the soil is evaporated, the soil becomes compressed and cracked and thereby causes the failure of the structure.
4. Lateral pressure on the super-structure. When the structure is subjected to lateral pressure due to wind or the lateral thrust of arches or a sloping roof or wide cantilever projections, a tendency of tilting or overturning is developed, which causes eccentricity of the loads due to which foundations can fail.
5. Lateral escape of soft soil from undernealh the foundation. When the building is situated near a deep cutting or a river or a nallah, the soil below the foundation may escape laterally and cause the settlement of the foundation. Similarly, if the depth of the foundation is small or the sub-soil below the foundation is of a sliding nature, it may slip away under the loads and cause the failure of the foundation.
Precautions
1. To prevent the unequal settlement of sub soil, a right type of foundation, based on the knowledge of soil conditions and all possible loads liable to come on the structure, should be carefully designed and constructed. The intensity of load should always be kept below the safe bearing capacity of tie soil. The materials to be used for foundation should be strong and durable.
2. The unequal settlement of masonry can be prevented by using very stiff mortar, raising the masonry uniformly and the wall should not be constructed more than 1.5 m in a day.
3. The sub-soil water during hot weather may dry up and cause shrinkage, This can be avoided by driving piles upto a hard stratum or such sites should be avoided.
4. The lateral pressure on the super-structure can cause failure of foundation. This can be prevented by providing a suffi­cient base area below the walls and columns.
5. The lateral escape of the supporting material can be pre­vented by confining to soil by driving Sheet Piles around the foundation.

Sources from Other Sites.