The bearing capacity of soil is defined as the capacity of the soil to bear the loads coming from the foundation. The pressure which the soil can easily withstand against load is called allowable bearing pressure.
Types of Bearing Capacity of SoilFollowing are some types of bearing capacity of soil:
1. Ultimate bearing capacity (qu)The gross pressure at the base of the foundation at which soil fails is called ultimate bearing capacity.
2. Net ultimate bearing capacity (qnu)By neglecting the overburden pressure from ultimate bearing capacity we will get net ultimate bearing capacity.
3. Net safe bearing capacity (qns)By considering only shear failure, net ultimate bearing capacity is divided by certain factor of safety will give the net safe bearing capacity.
qns = qnu/ FWhere F = factor of safety = 3 (usual value)
4. Gross safe bearing capacity (qs)When ultimate bearing capacity is divided by factor of safety it will give gross safe bearing capacity.
qs = qu/F
5. Net safe settlement pressure (qnp)The pressure with which the soil can carry without exceeding the allowable settlement is called net safe settlement pressure.
6. Net allowable bearing pressure (qna)This is the pressure we can used for the design of foundations. This is equal to net safe bearing pressure if qnp > qns. In the reverse case it is equal to net safe settlement pressure.
Calculation of Bearing CapacityFor the calculation of bearing capacity of soil, there are so many theories. But all the theories are superseded by Terzaghi’s bearing capacity theory.
1. Terzaghi’s bearing capacity theoryTerzaghi’s bearing capacity theory is useful to determine the bearing capacity of soils under a strip footing. This theory is only applicable to shallow foundations. He considered some assumptions which are as follows.
- The base of the strip footing is rough.
- The depth of footing is less than or equal to its breadth i.e., shallow footing.
- He neglected the shear strength of soil above the base of footing and replaced it with uniform surcharge. (
- The load acting on the footing is uniformly distributed and is acting in vertical direction.
- He assumed that the length of the footing is infinite.
- He considered Mohr-coulomb equation as a governing factor for the shear strength of soil.
Load from footing x weight of wedge = passive pressure + cohesion x CB sin
qu = c’Nc +
qu = 1.2 c’Nc +
qu = 1.2 c’Nc +
2. Hansen’s bearing capacity theoryFor cohesive soils, Values obtained by Terzaghi’s bearing capacity theory are more than the experimental values. But however it is showing same values for cohesionless soils. So Hansen modified the equation by considering shape, depth and inclination factors. According to Hansen’s
qu = c’Nc Sc dc ic +
|Shape of footing||Sc||Sq||Sy|
|ic||1 – [H/(2 c B L)]|
|iq||1 – 1.5 (H/V)|