Foundation failures - Causes and Precautions / Remedies

FOUNDATION FAILURES - Causes and Precautions / Remedies

1. Unequal Settlement of Sub-Soil: Soils are all compressible. Hence, settlement of soil cannot be prevented. Unequal settlement of sub-soil is due to  (i) Unequal resistance of the soil,  (ii) Unequal loading per unit area of the foundation soil or (iii) Eccentric loading on the soil. This results in formation of cracks in the structure.

Precautions / Remedies

(i) Pressure intensity under the foundation shall not exceed safe bearing capacity of soil.

(ii) Line of action of the resultant load should act on the centroid of the foundation area.

(iii) Foundation has to be placed on a firm soil or hard rock.

2. Horizontal Movement of Earth: This may occur under the action of load when the soil is very soft and particularly in the river banks.

Precaution: To prevent horizontal movement of earth, retaining walls may be constructed.

3. Shrinkage of Sub-Soil: A structure might have been constructed over a soil in which the sub-soil is saturated with water. After the construction, if the water is drained away due to seasonal variation, the sub-soil will shrink. This results in a considerable settlement.

Precautions: Foundation level should extend deeper stratum away from the zone of seasonal variation.

4. Sliding of the Soil: Sliding of the soil occurs, when a building is built on a sloping ground. The weight of the building may cause the sloping strata to become detached and to slide.

Precaution: Sliding of sloping soil strata should be prevented by providing retaining walls.

5. Compression of Mortar Joints of Foundation Masonry: Mortar joints of the foundation masonry may sink or compress. This causes unequal compression of masonry. Due to this, the super structure is subjected to cracks.

Precautions: Mortar should be stiff and consistent with workability. Other remedies are: Uniform construction height of masonry per day and Curing of mortar joints for at least two weeks.

6. Excessive Settlement of Sub-Soil: Foundations placed on slopes and that are subjected to movement of water may laterally yield. It causes excessive settlement of sub-soil.

Precautions: Adequate lateral support by Driving Piles should be provided. Proper drainage arrangements may also be provided.

7. Roots of Trees and Shrubs: Roots of trees and shrubs may consume more underground moisture causing settlement and weathering.

Precaution: Trees or shrubs may be grown away from the building.

8. Atmospheric Actions: Atmospheric agents such as sun, wind and rain may cause moisture movements resulting in foundation settlement.

Precaution: Foundation settlement is avoided by providing adequate drainage arrangements and sun shades in buildings.