Ultimate bearing capacity (qf) :
It is the maximum pressure that causes the sinking of footing into the soil as a result of shear failure.
Net ultimate bearing capacity( qnf) :
It is a maximum net pressure causing shear failure.
qnf = qf – γD
where γ = unit weight of soil.
D = depth of foundation.
Safe bearing capacity of soil (qs) :
It is the maximum pressure which the soil can carry safely without risk of shear failure.
qs = qnf / F + γD
where F = factor of safety
Allowable Bearing Capacity of Soil :
Allowable bearing capacity is defined as the pressure intensity which is considered safe both with respect to shear failure and settlement.
Rankine’s minimum depth of foundation :
where q = intensity of loading
φ’= effective angle of shearing
γ = density of soil solids
Ultimate bearing capacity of soil for shallow foundations :
The ultimate bearing capacity of soil for a footing with center load generally depends upon the following factors:
- Unit weight, shear strength, and deformation characteristics of the soil.
- Size, shape, depth, and roughness of footing.
- Depth of water table and initial stress condition in the soil.
Three Principal Models of Shear Failure Under Footing :
General shear failure.
It is a characteristic of the narrow footing of shallow depths resting on strong dense soil that is relatively incompressible.
Local shear failure.
For weaker, more compressible soil and wider or deeper footings, the failure may be taken as local shear failure or punching shear failure. Local shear failure is characterized by well-defined slip lines below the footing but extending only a short distance into the soil mass.
Punching shear failure.
In highly compressible soils the punching shear mode of failure results. This is characterized by the lack of a well-defined slip line below the footing. The vertical movement of footing is primarily due to the compression of soil below the footing with the soil on the side not being involved.