CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES

CAUSES OF FAILURES OF FOUNDATIONS AND REMEDIAL MEASURES

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

a)   non-uniform nature of sub-soil throughout the foundation,

b)  unequal load distribution of the soil strata, and

c)   Eccentric loading.

Remedy

(i)          Resting the foundation on rigid

(ii)         Strata, such as rock or hard moorum,

(iii)         Proper design of the base of footing, so that it can resist cracking,

(iv)        Limiting the pressure in the soil, and

(v)         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 masonry. Due to this, the superstructure will also have cracks.

Remedy

(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. 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.

Remedy

(1)Remove the tree close to the building to reduce the possibility of changes in ground moisture content of the building and also to avoid more penetration to the building.

(2)Any leaking drains, water main or broken downpipes are the cause of the subsidence, simply repairing them will be all that is needed to stabilize the building.

(3)If there are cracks or holes in the foundation wall, it can be remedied by breaking the wall and replace it with new ones in other to avoid excessive cracks. In other ways, it can also be done by breaking the defected area, cut some reinforcement steel and place it horizontally inside the wall and put concrete on it for more stability.

4. 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 remain constantly damp, resulting in the decrease in the strength of footing or foundation wall.

Remedy

(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.

5. Mining Subsidence in Foundation

This is a situation where settlement has taken place due to the mining work undertaken several years back and as a result of this any building erected on that place will definitely collapse.

If mining has taken place in certain areas and storey building is built on it without knowing that mining work had been taken place, as a result of these, the load of the building imposed on the place, there is a tendency that the building will collapse when settled. When such case happened, the ones to watch out for are small crack usually running diagonally, that appear quite suddenly in plaster and also on exterior brick work, around doors and windows in particular place. Cracks there indicate that there may be subsidence are usually thicker than a couple of millimeters and are wider at the top. Distortion of the building will also cause doors and windows to stick or fail to close. Another sign is rippled wallpaper, with no signs of damp to attribute this.

 Remedy

     i.        Underpinning: Underpinning a house prevents further movement by increasing the dept and support of the existing foundations. The standard way of doing this is to dig out short sections underneath the existing foundations and fill them in with concrete. Underpinning is a slow process, with each section having to be inspected before work can continue. It is also an expensive process, costing around 500 per linear metre.

    ii.        Restrained the ground by using retaining wall to reduce the amount of settlement of the area where mining occurred.