There are several reasons for the collapse of foundations, including poor soil conditions, structural faults, and more. Below are some of the major causes of foundation collapse:
- Collapse of foundation due to exceeding the bearing capacity of the soil.
- Collapse of foundation due to sudden withdrawal of moisture from the soil.
- Collapse of foundation due to differential or uneven settlement.
- Collapse of foundation due to inconsistent construction progress and patterns.
- Collapse of foundation due to movement of adjacent soil.
1. Collapse of Foundation Due to Exceeding the Bearing Capacity of the Soil
The ultimate bearing capacity refers to the maximum load intensity that soil can bear without undergoing shear failure. The size of the footing (length, width, and diameter) is primarily determined based on the soil’s bearing capacity.
The foundation or footing of a structure must be adequately sized so that the load intensity transferred through the footing from the structure to the soil does not exceed the soil’s bearing capacity. If the soil’s bearing capacity is low, or if the applied load is high and the footing size is smaller than required, the load intensity (load per unit area, e.g., KN/m²) imposed on the soil may surpass its bearing capacity. This results in shear failure of the soil and, consequently, the collapse of the foundation.
To ensure proper footing size, the safe bearing capacity should be used instead of the ultimate bearing capacity. The safe bearing capacity is calculated by dividing the ultimate bearing capacity by an appropriate factor of safety, typically ranging from 2 to 4. This provides a margin of safety against unexpected loads or abnormal soil behavior due to variations in its bearing capacity.
2. Collapse of Foundation Due to Sudden Withdrawal of Moisture from the Soil
This is one of the most critical factors that should not be overlooked. Moisture withdrawal from the soil can occur in the following ways:
a) Pumping water from a nearby well.
b) Draining water from an adjacent pond.
c) Lowering the groundwater table near the site.
Soil generally consists of soil solids, water, and air. The structural load on the soil is supported by both the soil solids and the pore water. When the pore water is withdrawn, voids are created within the soil system around and beneath the foundation. These voids lead to subsidence of the soil and the structure it supports, ultimately resulting in the collapse of the foundation.
3. Collapse of Foundation Due to Differential or Uneven Settlement
Differential or uneven settlement occurs when different parts of the foundation settle at varying rates. This is often caused by variations in soil properties, differences in load distribution, or inadequate soil compaction beneath the foundation. When one part of the foundation settles more than another, it creates stress within the structure, leading to cracks in walls, floors, and beams. Over time, these stresses can weaken the structural integrity of the building, ultimately causing the foundation to collapse.
To prevent this, proper soil testing and preparation must be conducted before construction. It is also essential to ensure uniform load distribution across the foundation and to use appropriate construction techniques to minimize settlement.
4. Collapse of Foundation Due to Inconsistent Construction Progress and Patterns
Inconsistent construction progress and patterns can lead to uneven load distribution on the foundation during the construction process. For example, if one part of a building is constructed faster than another, the foundation beneath the completed section may experience higher stress while the other sections remain unloaded. This imbalance can cause uneven settlement or even structural failure.
Additionally, irregular construction patterns, such as varying column spacing or improper alignment, can result in localized stress concentrations, further destabilizing the foundation. To avoid such issues, construction should follow a systematic and well-planned schedule, ensuring uniform progress across all sections of the structure.
5. Collapse of Foundation Due to Movement of Adjacent Soil
The movement of adjacent soil, also known as lateral soil movement, can pose a significant threat to the stability of a foundation. This can occur due to several factors, such as excavation activities near the foundation, heavy construction nearby, or natural events like landslides and soil erosion. When the soil surrounding the foundation shifts, it can create voids or exert lateral pressure on the foundation, leading to instability and eventual collapse.
To mitigate this risk, retaining walls or other soil stabilization techniques should be used to prevent soil movement. Additionally, care must be taken during nearby excavation or construction activities to ensure that the foundation is not compromised by external forces.
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