Causes of Failure Of Foundations

Basics in Building Construction 4(2+2)

Lesson 03 : Deep Foundation

Causes of Failure Of Foundations

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

  1. Unequal settlement of the sub-soil:
    Unequal settlement of foundation 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 pressure can be safely assumed to be uniform and the danger of unequal settlement is minimized. In practice, however, it is frequently found that different conditions exist under different parts of the building. If some portion of a building is higher, the soil below the higher portion will be subjected to greater pressure than the soil below the lower portion 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 such a value that the difference in settlement between the two portions of structure is reduced to minimum. In no case the pressure on the soil should exceed the safe-bearing capacity of soil. Material used for construction should be durable so as to avoid danger of the disintegration of the foundations. As far as possible foundation should be so proportioned that the centre of load gravity on structure coincides with the centre of gravity of the foundation. By adopting these precautions, the danger to structures on account of unequal settlement of sub-soil can be minimized.

  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
    1. The consistency of mortar should be such that it provides easy workability. The mortar should neither be very lean nor very stiff.
    2. As far as possible the execution of work should be so planned that the machinery is raised to the same uniform level throughout.
    3. The progress of construction should be such that not more than 1.5 m high masonry is constructed in a day.
    4. Masonry should be properly cured for a period of atleast 10 days to ensure development of adequate strength of the mortar joints.

  3. Horizontal movement of the soil adjoining the structure:
    This defect is very common in clayey and black cotton soils. Such types of shrinkable soils have volumetric changes with the change of atmospheric conditions. They swell excessively when wet and shrink excessively when dry. The differential settlement caused by the movement of the ground on account of alternate swelling and shrinkage of soil result in formation of cracks in the structure. Following precautions are necessary to be observed to minimize the effect of soil movement:
    1. To limit the loads on the soil to 5.5 tonnes/m2 . If water is liable to find an access to the foundation, the limit of load on the soil should be restricted to 4900 kg/ m2.
    2. To take the foundation to such depths where the cracks cease to extend.
    3. To prevent intimate contact of such soils with the part of the structure below ground level. This is achieved by digging trenches on either side of the foundation and filling them with granular material like sand or moorum. Similarly, a layer of sand or moorum is provided below the concrete block or bottom face of masonry foundation.
  4. For important structures, R.C.C raft foundation or pile foundation should be used.

  5. Shrinkage due to withdrawl of moisture from the soil below the foundations:
    The root of an isolated tree generally acquires land equal to the circumference of a circle whose radius is more than the height of the tree. In some soils it has been found to cause significant dying effect. The suction of sub-soil moisture by the roots, specially in abnormal spells of dry weather, affects the foundation adversely. On account of the differential shrinkage below the foundation, cracks are produced in the foundation, which can ultimately lead to the failure of structure. To avoid the damage due to the trees, certain safe distance should always be maintained between the trees and the foundations. It is considered reasonable to keep the foundations away from mature single tree and rows of trees. For mature trees the minimum distance should preferably be 15m and for young trees, the distance should be suitably increased. In addition, the foundation of the structure should be taken to a depth of at least 90cm. to minimize the cracking action.

    This defect is also liable to occur in structure founded on damp soils overlying a band of gravel or sand or some other porous material. An abnormally dry weather may cause the sub-soil water level to descend to such a depth that the soil immediately below the foundations may loose its moisture. This will result in formation of cracks in the building on account of differential settlement. Pile foundations or raft foundations are found to be safe for such place.

  6. Lateral pressure tending to over-turn the structure:
    This defect may be caused by a force which acts on a wall in such a way that the wall develops a tendency to tilt or overturn. The force may be the thrust due to a sloped roof, or an arch pressure or due to a violent storm or the effect of a wide cantilever projection. This defect can cause the foundation of the wall to yield due to the entire load being concentrated near the foundation edge. In such cases, the foundation of the wall should be so shaped and proportioned that the necessary conditions of stability are achieved.

  7. Action of atmosphere:
    A part of rain water finds its way inside the ground. The rain water during its passage downwards sometimes brings salts from the surface which reacts chemically with the material of foundation and causes it to disintegrate. Also if the foundation is not taken deep inside the ground, rain water may scour the soil above the foundation and expose it. The changes in the sub-soil water table as a result of seasonal change result in expansion or shrinkage of the soil which cause cracks in foundations. Following precautions are necessary to avoid the danger of cracks.
    1. The foundations should be taken to such a depth that the adverse effects of atmosphere are nullified.
    2. There should be adequate provision for the drainage of the sub-soil water when it rises up and causes danger to the foundation.
    3. If necessary, dense cement concrete, 1:2:4 or stone masonry should be adopted in the foundations at sites where the ground water and soil contain excessive sulphates or other injurious compounds.

    The sides of the foundation trenches should be well-filled and consolidated. At ground level, the outer surface adjacent to the wall should be given a good slope, so as to allow the rain water to flow away from the wall. A 60cm. to 90cm. width plinth protection serves the purpose more efficiently.

  8. Lateral escape of the soil below the foundation:
    This defect is liable to occur when the building is situated near a river bank or deep cutting where there is danger of the soft or loose soil below the foundation getting disturbed. In case of such sites sheet piles of timber or steel, should be driven to confine the soil and avoid the danger.
Index
Previous
Home
Next
Last modified: Tuesday, 28 February 2012, 9:58 AM