factor of safety for different foundations

FACTOR OF SAFETY FOR DIFFERENTFOUNDATIONS

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 temporary1.

Sensitivity of structure2.

Extent of soil exploration3.

Nature of loading considered and assumption made in the design4.

Extent of quality control during construction.5.

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)

Category Typical Structure Characteristics of

the category

Soil Exploration

FACTOR OF SAFETY FOR DIFFERENT FOUNDATIONS h0p://theconstructor.org/geotechnical/factor-of-safety-for-di...

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Thorough

A Railway bridge,

Warehouses, blast

furnaces, silos, hydraulic

retaining walls

Maximum design load

likely to occur often,

consequence of failure

disastrous

3.0

B Highway bridge, light

industrial and public

buildings

Maximum design load

may occur

occasionally,

consequence of failure

serious

2.5

C Apartments and office

buildings

Maximum design load

unlikely to occur

2.0

Note:


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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.

1.

For temporary structures, above values should be reduced to 75% with the minimum not less

than 2.0

2.

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%.

3.

The possibility of flooding of foundation soil and / or removal of existing overburden by scour

or excavation should be given adequate consideration.

4.

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.

5.

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.

6.

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.


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(B) For uplift conditions

= 2.5

could be reduced to 2, if the capacity of pile could be established by a pull-out test.

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 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

*For cuts, safety factor has usually higher values at end of construction rather than at some


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future time.

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factor of safety for different foundations

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