What is Bearing Capacity of SOIL
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.
Following are some types of bearing capacity of soil:
Ultimate bearing capacity of soil (qu)
The gross pressure at the base of the foundation at which soil fails is called ultimate bearing capacity.
Net ultimate bearing capacity (qnu)
By neglecting the overburden pressure from ultimate bearing capacity we will get net ultimate bearing capacity.
Where = unit weight of soil, Df = depth of foundation
Net safe bearing capacity of soil (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/ F
Where F = factor of safety = 3 (usual value)
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
Net safe settlement pressure (qnp)
The pressure with which the soil can carry without exceeding the allowable settlement is called net safe settlement pressure.
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.
Example of Bearing Capacity of SOIL
The ultimate bearing capacity of soil is 300 KN/m2. The depth of foundation is 1.00 m and unit weight of soil is 20 KN/m3. Choosing a factor of safety of 2.5, the net safe bearing capacity is
Given ultimate bearing capacity of soil = 300 KN/m2
depth of foundation Df = 1.00 m
unit weight of soil γ = 20 KN/m3
factor of safety = 2.5
Net safe bearing capacity of soil = (Net ultimate Bearing Capacity of soil) / factor of safety
Net ultimate Bearing Capacity of soil = Ultimate Bearing capacity of soil – γ*Df
= 300-20 = 280 KN/m2
Net safe bearing capacity of soil = 280/2.5 = 112 KN/m2