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
 Excavate a pit of required depth. (preferably equal to the depth of foundation)
 Take a solid ball or square cube of known weight and dimension.
 Drop the ball or square cube several times, from a known height on to the bottom surface of excavated pit.
 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 crosssectional 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.
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