### WHAT IS SAFE BEARING CAPACITY OF SOIL AND HOW IT CALCULATE?

SAFE BEARING CAPACITY

The capacity of the soil which allow to bear loads coming from the structure is called bearing capacity of the soil. The total load at the base of the foundation at which soil fails to bear load is called ultimate bearing capacity. By considering different failure conditions, the ultimate bearing capacity is divided by factor of safety and the resultant  value is called safe bearing capacity of the soil.
There are many types of soils are present so we have to know the soil type before designing the foundation. It can be determine by different soil test.we have some values of safe bearing capacity for different type of soil which are given below.

 Sr no Type of soil Safe bearing capacity Cohessive soil 1 Hard and stiff clay of deep bed 440 KN/sqm 2 Medium clay 250 KN/sqm 3 Moist clay and sand clay mixture 150 KN/sqm 4 Black cotton soil 50% saturated 130 KN/sqm 5 Soft clay indented by thumb pressure 100 KN/sqm 6 Very soft clay penetrate by thumb upto few centimeter 50 KN/sqm Cohesionless soil 7 Compact gravel and sand mixture which not penetrate by machine 440 KN/sqm 8 Compact and dry course sand 440 KN/sqm 9 Compact and dry medium sand 250 KN/sqm 10 Fine sand and silt with dry lumps 150 KN/sqm 11 Loose and dry fine sand 100 KN/sqm Rocks 12 Hard rock like granite, trap etc 3240 KN/sqm 13 Calcareous stones like sandstone and limestone 1620 KN/sqm 14 Soft rocks 440 KN/sqm

A quick field test will give you a rough idea of bearing capacity of soil.

### PROCEDURE

1. Excavate a pit of required depth. (preferably equal to the depth of foundation)
2. Take a solid ball or square cube of known weight and dimension.
3. Drop the ball or square cube several times, from a known height on to the bottom surface of excavated pit.
4. Calculate the average depth of impression made several times on the bottom surface of the excavated pit. Let “d” is the average depth of impression.

## CALCULATION

Calculate the ultimate resistance of soil ( R ) using the formula given below.
• R = (w * h) / d

Where,
• R = Ultimate resistance of soil (in kg)
• d = Average depth of impression (in cm)
• w = Weight of the solid ball or square cube (in kg)
• h = Height of fall of solid ball or cube (in cm)

If “A” is the cross-sectional area of the solid steel ball or cube, then resistance of soil per unit area is calculated using following formula.
Resistance of soil per unit area (in kg/cm2) = R / A
Safe bearing capacity (in kg/cm2) = R / (A * F.O.S)
Where,
F.O.S = Factor of safety.

We can take F.O.S 2 or 3.