The foundation of all Civil Engineering structures transmits the loads to the soil beneath. The load per unit area of soil should be such that the resistance of soil is never exceeded. If it exceeds settlement or failure of soil in shear will takes place leads to collapse of structure.
Bearing capacity of a soil is defined as the largest intensity of pressure which may be applied by a structural member to soil which supports it without any unequal settlement or danger of failure of soil in beating I shear. f Net sake bea
Importance of bearing Capacity in Foundation Design
Bearing capacity of soil is important in design of foundations to ensure safety to the structures. By knowing the bearing capacity of soil, the soil is never loaded beyond its capacity to withstand the loads. The bearing capacity of foundation is needed for dimensioning the foundation for any structure.
Presumptive bearing Capacity (Values only): As per IS 6403: 1981
1 .Ultimate Bearing Capacity: The intensity of loading at the base of the foundation which would cause shear failure of the soil support.
2. Safe Bearing Capacity: Maximum intensity of loading that the foundation will safely carry without the risk of shear failure of the soil irrespective of any settlement that may occur.
3. Allowable Bearing Capacity: The net intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding net safe bearing capacity.
The bearing capacities of various soils as per IS 1904: 1978 are given in table.
Code of Practice for Structural Safety Building Foundations:
Code equation for computing ultimate net bearing capacity: Three types of failure of soil support beneath the foundations have been recognised, depending upon the deformations associated with the load and the extent of development of failure of surface are
1. General shear failure
2. Local shear failure
3. Punching shear failure
The formulae for the first two types of failures are given below for calculating the ultimate net bearing capacity in the case of strip footings.
(a) In case of general shear failure
q{d} = cNc+ q(Nq – 1) + 1/2 * Bγ *Nγ
(b) In case of local shear failure
q¹{d} =1/2 cN¹c+ q(N¹q – 1) + 1/2 Bγ *N¹γ
The values of Nc,N¹c,Nq,N¹q,Nγ,Nγ may be obtained from the table given below.
