Factor of Safety and Allowable Pressure
The allowable bearing pressure for a foundation is determined by considering the shear strength of the soil. The allowable bearing pressure is defined by the following equation:
qallowable = (qultimate / Fs) + gDf
Here, qultimate is the ultimate bearing capacity of the soil, Fs is the factor of safety against shear failure, and gDf is the effective overburden pressure at the foundation depth Df.
The factor of safety against shear failure, Fs, is used to ensure that the foundation will not fail due to shear forces. The higher the value of Fs, the safer the foundation will be.
The effective overburden pressure, gDf, represents the weight of the soil above the foundation at a given depth. It is calculated by multiplying the unit weight of the soil, g, by the depth of the foundation, Df.
By considering both the ultimate bearing capacity and the shear strength of the soil, the allowable bearing pressure can be calculated. This value is used to determine the maximum load that the foundation can safely support without causing failure.
Important Features Affecting Factor of Safety
The required factor of safety for a structure is influenced by several factors, including the type of structure, whether it is permanent or temporary, the sensitivity of the structure, the extent of soil exploration, the nature of loading considered, and the assumptions made during the design process. Additionally, the extent of quality control during construction also plays a role in determining the factor of safety required.
For permanent structures built in relatively homogeneous soil conditions, it is recommended that the factor of safety be between 2 and 4. This range serves as a guide to ensure the structural integrity of the building. However, it is important to consider the specific factors influencing the required factor of safety for each project, such as the type of loading the structure will endure and the level of quality control during construction. By taking all these factors into account, engineers and designers can ensure that the structure will be safe and durable.
Factor of Safety for Shallow Foundations
Table 1, based on the work of Vesic in 1970, provides the minimum values of factor of safety that should be used for designing shallow foundations. These values represent the minimum safety margins that should be maintained to ensure the stability and safety of the foundation. It is important to note that these values are only minimums and that higher safety factors may be required in certain circumstances. The table serves as a guideline for engineers and designers to ensure that their designs meet a minimum level of safety and stability.
| Typical Structure | Characteristics of the category | Soil Exploration | |
| Thorough | Limited | ||
| Railway bridge, Warehouses, blast furnaces, silos, hydraulic retaining walls | Maximum design load likely to occur often, consequence of failure disastrous | 3.0 | 4.0 |
| Highway bridge, light industrial and public buildings | Maximum design load may occur occasionally, consequence of failure serious | 2.5 | 3.5 |
| Apartments and office buildings | Maximum design load unlikely to occur | 2.0 | 3.0 |
Note:
Before determining the Factor of Safety for a structure, it is essential to thoroughly evaluate the reliability of all the parameters that govern its structure. These parameters will include the loads, strength, and deformation properties of the soil mass. Only after this evaluation can the Factor of Safety be accurately determined.
For temporary structures, the values of the loads, strength, and deformation properties must be reduced to 75% of their original values, provided that the resulting value is not less than 2. This reduction is necessary to ensure the safety of temporary structures.
In the case of tall structures such as chimneys, towers, and common tall buildings where there is a risk of progressive collapse, it is recommended to increase the Factor of Safety by 20 to 50%. This increase is crucial to ensure that these structures remain safe and secure even in the face of potential collapses.
It is also crucial to consider the effects of scour and excavation overburden when determining the Factor of Safety. Adequate consideration must be given to these factors to ensure that the structure remains stable and secure, even under adverse conditions.
Factor of Safety and Allowable Capacity of Pile Foundation
In pile foundations, the factor of safety is an important concept that is used to ensure the stability and reliability of the structure. The factor of safety is a ratio of the ultimate load that a pile can withstand to the actual load that is applied on it. This ratio is calculated to ensure that the pile can safely support the applied load without experiencing any significant deformation or failure.
The factor of safety for downward loading is calculated by dividing the ultimate load capacity of the pile by the maximum expected load that will be applied on it in the downward direction. This ensures that the pile can safely support the weight of the structure that is resting on it without sinking or settling.
Similarly, the factor of safety for upward loading is calculated by dividing the ultimate load capacity of the pile by the maximum expected load that will be applied on it in the upward direction. This ensures that the pile can safely resist the upward force that may be exerted on it by soil movement or other external factors.
By calculating and maintaining appropriate factors of safety for both upward and downward loading, engineers and builders can ensure the stability and durability of the pile foundation and the entire structure that is built on it.
1. For downward loading
The factor of safety (Fs) should be 2.5 when both end bearing and shaft resistance are taken into consideration. However, if a sufficient number of pile load tests are conducted to ensure that Fs never falls below 2, this value can be reduced to 2. When calculating Fs, the factor of safety for shaft resistance should be 1.5 and for end bearing, it should be 3. It is important that the Fs based on both end bearing and shaft resistance is greater than 2, and more importance should be given to the first approach.
When estimating the allowable load based on the first approach, it is necessary to ensure that it is not lower than the estimation obtained by the second approach. It is crucial to have a higher factor of safety in end bearing than in shaft resistance, and to obtain a factor of safety above the given value when negative stress friction is acting. If negative friction is present, the factor of safety should not be less than 2.
2. For uplift conditions
In the given context, it is mentioned that if the capacity of a pile can be established by conducting a pull-out test, then the factor of safety (Fs) can be reduced from 2.5 to 2. This implies that the pile would be able to sustain a higher load capacity, and therefore, the factor of safety can be decreased.
Furthermore, it is stated that the factor of safety can be reduced to 1.5 if the weight of the pile itself is at least 0.75 times the uplift force. This indicates that the weight of the pile plays a crucial role in ensuring the stability of the structure. If the weight of the pile is not sufficient, then the factor of safety needs to be increased to maintain stability.
Overall, it is essential to establish the capacity of the pile through a pull-out test and ensure that the weight of the pile is adequate to sustain the uplift force. These measures would help in reducing the factor of safety and ensuring the stability of the structure.
Factor of Safety For Embankments & Earth dams
The factor of safety is an important consideration when designing earthwork structures such as earth dams, embankments, and dams. However, these structures typically require a lower factor of safety compared to other types of structures. Providing a very high factor of safety may lead to uneconomical sections of the structure. Therefore, it is satisfactory to provide lower values of safety for these types of structures. Table 2 shows the recommended factor of safety for various earthwork structures.
| Description of earthwork | Safety Factors |
| Embankments, end of construction | 1.0 to 1.2 |
| *cuts, end of construction | 1.2 and over |
| Embankments, long term stability | 1.2 to 1.4 |
| *cuts, long term stability | 1.2 to 1.4 |
| Earthdams | 1.5 and above |
| Earthdams – extreme conditions of loading (i.e. severe flood followed by sudden drawdown) | 1.1 to 1.25 |
Note:
The safety factor for cuts is generally higher at the end of construction compared to some future time.