What is the general value of factor of safety range to find the bearing capacity (BC) of soil?
🔬 Understanding Bearing Capacity and Safety
In geotechnical engineering, we calculate two main types of bearing capacity. The relationship between them is defined by the Factor of Safety.
Ultimate Bearing Capacity (qᵤ): This is the theoretical maximum pressure that the soil can support at the base of the foundation just before it fails in shear. Designing a foundation to this limit would be extremely dangerous.
Safe Bearing Capacity (qₛ): This is the maximum pressure the soil is allowed to be subjected to in the actual design. It is a reduced value of the ultimate capacity, calculated to provide a margin of safety.
⚖️ Detailed Analysis of the Factor of Safety (FOS)
The Factor of Safety is a crucial number that bridges the gap between the ultimate failure load and the safe working load. It accounts for uncertainties in soil properties, load calculations, and construction quality.
The relationship is given by the formula:
Safe Bearing Capacity = Ultimate Bearing Capacity / Factor of Safety
(d) 2 - 3
Why it's correct: For determining the safe bearing capacity of soil for standard building foundations, a Factor of Safety in the range of 2 to 3 is generally adopted.
• A factor of 2.5 is very common for cases where soil properties are reasonably well-known.
• A factor of 3 might be used if there is more uncertainty about the soil conditions or the applied loads.
This range provides a sufficient buffer to ensure the foundation is safe against a shear failure.
Analysis of Other Options
- (a), (b), and (c): All these options represent a Factor of Safety of 1 or less. A factor of 1 means designing the foundation to be right at the point of failure. A factor less than 1 means designing the foundation to fail under the applied load. These are illogical and unsafe, and therefore incorrect.