The major causes of foundation failure are as follows:
- unequal settlement of sub-soil
- uneven arrangement of masonry
- Presence of spilled soil under foundation
- presence of mature trees
- seasonal change
1. uneven settlement of sub-soil
This is one of the most important causes of foundation failure. Uneven settlement causes cracks in the foundation and eventually leads to failure of the entire structure.
Unequal settlement of foundations generally occurs due to the following reasons:
- The sub soil may not be uniform throughout the site. This means if some part of the site has compressive soil (eg clay) and some part has hard crust (eg rock).
- The load coming from the structure may not be uniform
- It can also happen on a building built on sloping ground.
- The foundation should be designed in such a way that the pressure on the soil at different parts of the structure and the associated differential settlement is minimized.
- The load from the super structure should be within the safe bearing capacity of the soil.
- The foundation should be constructed with the right type of material so as to avoid the disintegration of the foundation due to the presence of harmful chemical f of soil or water.
- The ratio of the foundation should be such that the center of gravity of the load from the structure coincides with the center of gravity of the foundation.
Also read: How To Increase The Durability Of Foundation Material?
2. uneven arrangement of masonry
Generally the foundation of the wall of the building is made of brick masonry or stone masonry. Different types of mortar are used to connect these masonry units. During construction when mortar is not set properly, if subjected to excessive loads, it tends to shrink. This shrinkage of the mortar can lead to uneven settling of the masonry and cause cracks in the wall.
- Workable mortar should be used during construction. The workability of a mortar is measured by conducting a mortar consistency test. The mortar used in construction should not be too lean or too tight.
- The maximum height of one wall to be constructed in a day should be limited to 1.5 metres.
- Masonry should cure for at least 10 days. This will help the mortar joints to gain their required strength.
Read also: How to determine the consistency of masonry mortar?
Also read: Recommended Values Of Consistency For Various Functions
3. Presence of wide soil under foundation
The presence of vast soils (such as black cotton soils) has a tendency to show large variation with the change in moisture content. This type of soil shrinks and swells excessively and thus causes differential settlement of the foundation.
- Maximum load on black cotton soil should be limited to 5 t/m2, If water is likely to come into contact with the foundation, the load should be limited to 9 t/m2,
- The foundation should be laid at such a depth where the cracks stop spreading. The minimum depth of the foundation should be at least 1.5 m.
- try the Avoid direct contact of black cotton soil with foundation material, This can be achieved by creating a wide trench for the foundation and filling the spaces on either side of the foundation with sand or mortar.
Also read: How To Avoid Foundation Failure In Black Cotton Soil?
4. seasonal change
- During the rainy season a part of the rain water goes into the ground. This water brings with it salt in some form or the other. It degrades when it comes in contact with salt foundation material.
- Heavy rains can cause soil erosion or roughness and can affect the foundation
- Seasonal change also changes the depth of the water table resulting in swelling and shrinkage of the extended soil. This can lead to cracks in the foundation.
- The depth of the foundation should be chosen in such a way that the adverse effects of seasonal changes are minimized.
- Provide adequate drainage if the water level is likely to rise to foundation level.
- Always use dense cement concrete or stone masonry where there is a possibility of sulfate attack on the foundation.
- Provide a proper slope at ground level, near the wall surface, to allow rainwater to drain away from the wall.
Also read: How To Calculate The Depth Of Shallow Foundation?
Also read: How To Protect Concrete From Sulfate Attack?
5. presence of mature trees
Where there are trees in clay soil, the problem is more severe. Trees and heavy vegetation draw significant amounts of water from the ground during the growing season. A mature poplar takes up to 1000 liters of water per week. In long hot summers with little or no rain, the tree will continue to draw moisture from the ground and the soil will shrink. This, of course, is in addition to the seasonal drying described above. If buildings are placed individually or near groups of trees, severe cracks in the walls can result from ground movement.
Where trees have been removed from clay soil, the opposite problem occurs. As the soil gradually gains moisture, it will expand and can persist for a period of up to 10 years. The pressure that dry soil develops while reabsorbing moisture is likely to be greater than the building load and the upward movement of the structure. If houses are built on the site before this land extension is completed, cracks will appear in the walls and foundation; The swelling will be uneven because it will be concentrated around the tree that has been removed.
- They should be deeper than the tree roots to prevent this movement from affecting the strip foundation. One option, of course, is to thoroughly clear the buildings of trees.
- It is considered appropriate to place the foundation at least a tree height (h) away from a mature single tree and about 0.5 h away from the row of trees.
- The minimum safe distance preferably 15 meters for mature trees and for young trees the distance should be increased suitably.