BUILDING FOUNDATIONS-2

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WHY MOST FOUNDATIONS FAIL?


Building foundations are the lowest parts of a building (sub structure) that provide necessary support for structures. Foundations provide resistance to the structural loads that act directly on them. Foundations ae also the stability of a structure as it directly transmits the structural lateral loads to the ground while providing resistance against the forces exerted by the subsoil.

Though they are the stability of every structure, they are also subject to failures if not critically planned for before construction. This explains why geotechnical investigations are carried out before any constructions are done. Foundation failures are most likely to occur as a result of overloading on the foundation, due to negligence when choosing the right location for foundation or rather poor supervision of the construction

1. Uneven soil settlement

This usually happens when a structure is constructed in area with different soil types for example an area with both sandy and clayey soil properties. Clay soils expansive in nature in that they tend to expand when wet and stiffen when dry. Sand soils on the other hand have gravel particles that allow water to penetrate to its downer layers without affecting its settlement behavior. The uneven settlement of these two different soils with adversely affect the bearing capacity of the ground leading to the instability to the building foundation. In such cases, failure of a structure is most likely to happen.

 

 

 

 

 

 

 

 

 

2. Uneven load distribution of a structure

The uneven loading of the structure can lead to uneven stresses, which cause differential settlement at areas where columns and walls transfer loads to the foundation. This could be a result of poorly analyzing a structure or rather improperly supervised construction of a structure. For an object to be stable, the center of gravity must remain intact. In this scenario, a structure with imbalanced loads is most likely to have  a displaced center of gravity which could easily lead to its collapse.

3. Ground water fluctuations

This can happen in areas where the groundwater changes more frequently especially in areas that have cohesive soils predominantly. The rise and fall of water tables can lead to intermittent soil compression and subsoil swell leading to differing pressures exerted on the ground. If the foundation benefits is unable to resist such pressures, it will fail. in most areas, construction is discouraged in swampy areas this being one of the reasons. If permitted, restrictions on the building specifications will be dictated by the concerned authorites.

4. Insufficient compaction

When constructing, it is advisable that compaction be done effectively. In most projects, a structural engineer dictates the thickness of layers and MDD content to be considered during compaction. If the site has different soil types, different parts of the foundation will definitely settle on the different soil types which can lead to differential settlement and finally, a foundation failure.

This is why a geotechnical investigation is crucial especially in construction projects. Insufficient/ineffective compaction of the soil layers can yield air voids which when filled with water begin to swell. The contraction and increased pressure within the swelling soils onto the foundation can definitely lead to its failure when unable to hold the lateral loads from the structure.

5. Erosion of soils.

Every foundation rests on soil. Any unexpected movement or erosion of these soils is likely weaken the foundation of a building. The illustration below shows the structural risk associated with moving or eroded soils to the stability of a structure.

 

 

 

 

 

 

 

 

 

6. Heave

A heave is an upward movement of soil beneath the building as a reason of  soil expanding. Structures are more stable when the supporting soils are in compression. While a footing is loaded, the supporting soil reacts by compression to provide resistance and this takes place rapidly in the case of granular soils but much more slowly for clays. When under compression, the structure remains stable, because the foundation no longer settles. This stability therefore depends on the soil areas directly below the footing, or, in case of piles, on the soil near the pile tip. If the soil below the footing is removed or disturbed, settlement or lateral movement can be induced.

When the soil area near to footing is loaded by a new structure, it causes new compression in the soil volume leading to unexpected new settlement of the previously stable building. If the new building is not separated from the existing construction, the settlement caused by the loads from the new structure will overload the previously stable structure leading to its failure.

 

 

 

 

 

 

 

 

 

7. Vibration from other construction sites or natural factors like earthquakes

Structures especially tall buildings are usually analyzed against seismic waves such that their stability can withstand the seismic waves incase of an earthquake. Unfortunately, some building foundations are greatly affected by these underground forces in poorly designed leading to  foundation failures. The constant vibrations collectively distort the different members of a structure from which the lateral loads are exerted thus  weakening the foundation.

 

 

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Posted on: December 15th, 2021 by London Building Contractors No Comments

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