Factor of safety and allowable pressure:
Allowable bearing pressure, from shear strength consideration, is only a fraction of the ultimate bearing capacity and is defined by the equation
Where, = factor of safety against shear failure
= effective overburden pressure at foundation depth
The required factor of safety depends upon:
- Type of structure permanent or temporary
- Sensitivity of structure
- Extent of soil exploration
- Nature of loading considered and assumption made in the design
- Extent of quality control during construction.
It is recommended that the factor of safety should be between 2 and 4. The following table may be used as a guide for permanent structures in reasonably homogeneous soil conditions.
Minimum value of safety factor for design of shallow foundations (as per Vesic, 1970)
Characteristics of the category
Railway bridge, Warehouses, blast furnaces, silos, hydraulic retaining walls
Maximum design load likely to occur often, consequence of failure disastrous
Highway bridge, light industrial and public buildings
Maximum design load may occur occasionally, consequence of failure serious
Apartments and office buildings
Maximum design load unlikely to occur
- The selection of factor of safety for design can not be made properly without assessing the degree of reliability of all other parameters that enter into design, such as design loads, strength and deformation characteristics of the soil mass etc. Each case should therefore be considered on its merits.
- For temporary structures, above values should be reduced to 75% with the minimum not less than 2.0
- For exceptionally tall buildings, such as chimneys and towers, or generally whenever progressive bearing capacity failure may be feared, the values should be increased by 20 to 50%.
- The possibility of flooding of foundation soil and / or removal of existing overburden by scour or excavation should be given adequate consideration.
- It is advisable to check both the short term (end of construction) and long term stability, unless one of the two conditions is clearly less favourable.
- It is understood that all the foundations will be analysed also with respect to maximum tolerable total and differential settlement. If settlement governs the design, higher factor of safety may be used.
FACTOR OF SAFETY AND ALLOWABLE CAPACITY OF PILE
(A) For downward loading
i. (Factor of safety) = 2.5 – when both end bearing and shaft resistance are considered. This value should be reduced upto 2.0 if sufficient number of pile load tests are conducted to ensure that will never fall below 2.
ii. = 1.5 for shaft resistance and =3 for end bearing. However, based on both end bearing and shaft resistance should be greater than 2.
Approach (ii) should be given greater weightage. Further, even when allowable load is estimated based on (i) above, it is essential to ensure that it is not less than estimated by (ii). It is essential to have higher factor of safety in end bearing than in shaft resistance. With negative stress friction acting, factor of safety as above should be obtained. With negative friction, the factor of safety should not be less than 2.
(B) For uplift conditions
could be reduced to 2, if the capacity of pile could be established by a pull-out test. could be further reduced to 1.5 if the weight of the pile itself is 0.75 times the uplift force or greater.
FACTOR OF SAFETY FOR EMBANKMENTS ETC
The value of factor of safety considered in the case of earthwork, i.e., cuts, embankments, dams are much lower than those considered usually adopted in design of other structures. High safety factors wil result in uneconomical design of earthwork structures. It is found from practices that lower values of safety factors are quite satisfactory in earthwork. The following table gives values of factor of safety for different earthwork.
Description of earthwork
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
1.5 and above
Earthdams – extreme conditions of loading (i.e. severe flood followed by sudden drawdown)
1.1 to 1.25
*For cuts, safety factor has usually higher values at end of construction rather than at some future time.