Bearing Capacity
Bearing Capacity
Bearing Capacity
BEARING CAPACITY
The load-carrying capacity of foundation soil or rock which enables it to bear and transmit loads from
a structure.
Ultimate bearing capacity: Maximum pressure which a foundation can withstand without the occurrence of shear failure of the foundation.
1. Terzaghi's bearing capacity theory 2. The general bearing capacity equation 3. Field tests
Vesic (1973) classified shear failure of soil under a foundation base into three categories depending on the type of soil & location of foundation.
The load - Settlement curve in case of footing resting on surface of dense sand or stiff clays shows pronounced peak & failure occurs at very small stain. The shearing strength is fully mobilized all along the slip surface & hence failure planes are well defined. The failure occurs at very small vertical strains accompanied by large (i) Strip footing resting on surface (ii)Load settlement curve lateral strains.
The foundation movement is accompanied by sudden jerks. The failure surface gradually extend out wards from the foundation.
The shear strength of soil is not fully mobilized along planes & hence Failure planes are not defined clearly.
The failure occurs at large vertical strain & very small lateral strains.
The loaded base sinks into soil like a punch. The failure surface do not extend up to the ground surface.
Large vertical strains are involved with practically no lateral deformation. Failure planes are difficult to locate.
Mechanism of Failure
The zones of plastic equilibrium is divided into:
1 . Zone I of elastic
equilibrium
Contribution of:
Shear Strength
The first term in the equation is related to cohesion of the soil . The second term is related to the depth of the footing and overburden pressure. The third term is related to the width of the footing and the length of shear stress area.
Karl von Terzaghi was the first to present a comprehensive theory for the evaluation of the ultimate bearing capacity of rough shallow foundations.
This theory states that a foundation is shallow if its depth is less than or equal to its width.
It is a method for determining bearing capacity for the general shear failure .
Nc=cotf(Nq 1) Nq=e2(3p/4-f/2)tanf / [2 cos2(45+f/2)] Ng=(1/2) tanf( Kpr /cos2 f -1) Kpr=passive pressure coefficient. The differences in the bearing capacity values arising out of differences in the size of the footing and in the shape of the footing are termed size effects and shape-effects, respectively.
Important points :
Terzaghis Bearing Capacity equation is applicable for general shear failure. Terzaghi has suggested following empirical reduction to actual c & in case of local shear failure Mobilised cohesion Cm = 2/3 C
Based on the experimental results,Terzaghis suggested following equations for UBC Square footing qu = 1.2c Nc + Df Nq + 0.4 BNr Circular footing qu = 1.2cNc + Df Nq + 0.3 BNr