factor of safety for different foundations

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)

Foundation Design Foundations Foundation Types

Search

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

1 of 5 4/1/2015 10:38 AM

Category

Typical Structure

Characteristics of

the category

Soil Exploration

Thorough Limited

A

Railway bridge,

Warehouses, blast

furnaces, silos,

hydraulic retaining

walls

Maximum design

load likely to occur

often, consequence

of failure disastrous

3.0 4.0

B

Highway bridge, light

industrial and public

buildings

Maximum design

load may occur

occasionally,

consequence of

failure serious

2.5 3.5

C

Apartments and office

buildings

Maximum design

load unlikely to

occur

2.0 3.0

Note:

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.


2 of 5 4/1/2015 10:38 AM

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

= 2.5

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

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


3 of 5 4/1/2015 10:38 AM

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

future time.

Marble FromConcrete

"Marble made of Concrete" New

construction technology.Enquire

Reinforced Beam DesignComplete and totally integrated solution for RC beam design

Facebook social plugin

Also post on Facebook Posting as Thiha Kyaw

Saddam Hosain BSc In Civil Engineering'14

there is no explain,why we used different factor of safety for end bearing pile

Reply Like Follow Post February 18, 2014 at 12:19pm

Rama Sarma Andhra University

Good &useful article with technical; features -Very good!!!

Reply Like Follow Post December 4, 2013 at 12:01pm


4 of 5 4/1/2015 10:38 AM

factor of safety for different foundations

Documents